IMPROVED METHODS FOR DIAGNOSTIC IMAGING USING A CONTRAST AGENT AND A VASODILATOR

Abstract: A method for providing an image of an internal region of a patient has been developed. The method reduces imaging artifacts by providing enhanced contrast between tissue and blood during imaging. The method comprises administering to a patient a contrast agent in combination with a renal vasodilator and performing ultrasound imaging of the region. Renal disease, including renal arterial stenosis, may be diagnosed using the method.

Description: IMPROVED METHODS FOR DIAGNOSTIC IMAGING USING A CONTRAST AGENT AND A VASODILATOR Related Application This application is a continuation-in-part of copending application U S Serial Number 08/712, 173 , filed September 11, 1996, the disclosures of which are hereby incorporated herein by reference in their entirety Field of the Invention The present invention relates to improved methods for diagnostic imagmg More particularly, the present invention relates to improved methods for diagnostic imagmg which involve administering to a patient a renal vasodilator and a contrast agent Background of the Invention Ultrasound is a valuable diagnostic imaging technique for studying vaπous areas of the body including, for example, the vasculature, such as tissue microvasculature Ultrasound provides certain advantages over other diagnostic techniques For example, diagnostic techniques involvmg nuclear medicine and X-rays generally result in exposure of the patient to ionizing electron radiation Such radiation can cause damage to subcellular material, including deoxyπbonucleic acid (DNA), πbonucleic acid (RNA) and protems Ultrasound does not involve such potentially damaging radiation In addition, ultrasound is relatively inexpensive as compared to other diagnostic techniques, such as magnetic resonance imaging (MRI), which can require elaborate and expensive equipment Ultrasound involves the exposure of a patient to sound waves Generally, the sound waves dissipate due to absorption by body tissue, penetrate through the tissue, or reflect off the tissue The reflection of sound waves off tissue, generally referred to as backscatter or reflectivity, forms the basis for developing an ultrasound image This is because sound waves reflect differentially from different body tissues This differential reflection is due to vaπous factors, including the constituents and the density of the particular tissue being observed The differentially reflected waves are then detected, usually with a transducer which can detect sound waves having a frequency of from 1 megahertz (MHZ) to 10 MHZ The detected waves are integrated, quantitated and converted into an image of the tissue being studied Imaging vasculaπzed tissue generally involves an analysis of difference in the acoustic properties between blood and tissues Therefore, attempts have been made to develop contrast agents which serve to increase the acoustic difference between blood and surrounding tissues This may also permit the measurement of blood flow, thereby improving detection of diseases associated with changes in blood flow Contrast agents can serve to improve the quality and usefulness of images which are obtained with ultrasound Certain exemplary contrast agents include, for example, suspensions of solid particles and emulsified liquid droplets The reflection of sound from a liquid-gas interface is extremely efficient Accordingly, certain bubbles, including certain gas-filled bubbles, can be highly useful as contrast agents The term "bubbles", as used herein, refers to vesicles which are generally characteπzed by the presence of one or more membranes or walls surrounding an internal void that is filled with a gas or precursor thereto Exemplary bubbles include, for example, vesicles which are surrounded by monolayers and or bilayers to form, for example, urulamellar, oligolamellar and/or multilamellar vesicles, such as hposomes, micelles and the like As discussed more fully hereinafter, the effectiveness of bubbles as contrast agents depends upon vaπous factors, including, for example, the size and/or elasticity of the bubble The effectiveness of bubbles as contrast agents depends upon vaπous factors, including, for example, the size of the bubble As known to the skilled artisan, the signal which is in the range of diagnostic ultrasound frequencies and which can be reflected off of a bubble is a function of the radius (r 6 ) of the bubble (Rayleigh Scatterer) Thus, a bubble having a diameter of about 4 micrometer (μm) possesses about 64 times the scattering ability of a bubble having a diameter of about 2 μm Thus, generally speaking, the larger the bubble, the greater the reflected signal However, bubble size is limited by the diameter of capillaries through which the bubbles must pass Generally, contrast agents which compπse bubbles having a diameter of greater than about 10 μm can be dangerous since microvessels may be occluded Accordingly, it is preferred that greater than about 90% of the bubbles in a contrast agent have a diameter of less than about 10 μm, with greater than about 95% being more preferred, and greater than about 98% being even more preferred Mean bubble diameter is important also, and should be greater than about 1 μm, with greater than about 2 μm being prefeπed The volume weighted mean diameter of the bubbles should be about 7 to about 20 μm The viability of currently available ultrasound contrast agents and methods involving their use is highly dependent on a vaπety of factors, including the particular region being imaged In certain circumstances, diagnostic artifacts may render a diagnostic image substantially unusable In addition to ultrasound, computed tomography (CT) is a valuable diagnostic imaging technique for studying vaπous areas of the body In CT, the radiodensity (electron density) of matter is measured and is expressed in terms of Hounsefield Units (HU) Hounsefϊeld Units, named after the inventor of the first CT scanner, are an indication of the relative absorption of CT X-rays by matter, the absorption being directly proportional to the electron density of that matter Water, for example, has a value of 0 HU, air a value of - 1000 HU, and dense cortical bone a value of 1000 HU Because of the similaπty m the densities of various tissues in the body, however, it has been necessary to develop contrast agents which can be used to change the relative densities of different tissues This has resulted in an overall improvement in the diagnostic efficacy of CT In the search for contrast agents for CT, researchers have generally sought to develop agents that will increase electron density in certain areas of a region of the body (positive contrast agents) Barium and lodme compounds, for example, have been developed for this purpose For the gastrointestinal tract, barium sulfate is used extensively to mcrease the radiodensiry of the bowel lumen on CT scans Iodinated water- soluble contrast media are also used to increase density within the gastrointestinal tract, but are not used as commonly as the barium compounds, primarily because the iodine preparations are more expensive than barium and are generally less effective in increasing radiodensity within this region of the body The use of low density microspheres as CT contrast agents has also been reported See, e g , Unger, U S Patent No 5,205,290 As discussed above in connection with diagnostic methods for ultrasound, the viability of currently available CT contrast agents and methods involving their use for imaging the heart region is highly dependent on the flow of blood through the chambers of the heart relative to the flow of blood in the blood vessels of the heart tissue itself Magnetic resonance imagmg (MRI) is another diagnostic imagmg technique which may be used for producing cross-sectional images of the body in a variety of scanning planes such as, for example, axial, coronal, sagittal or orthogonal MRI employs a magnetic field, radio frequency energy and magnetic field gradients to make images of the body The contrast or signal intensity differences between tissues mainly reflect the Tl (longitudinal) and T2 (transverse) relaxation values and the proton density, which generally corresponds to the free water content, of the tissues To change the signal intensity m a region of a patient by the use of a contrast medium, several possible approaches are available For example, a contrast medium may be designed to change Tl, T2, or the proton density Generally speaking, MRI requires the use of contrast agents If MRI is performed without employmg a contrast agent, differentiation of the tissue of interest from the surrounding tissues in the resulting image may be difficult In the past, attention has focused pnmanly on paramagnetic contrast agents for MRI Paramagnetic contrast agents involve mateπals which contain unpaired electrons The unpaired electrons act as small magnets within the mam magnetic field to increase the rate of longitudinal (Tl) and transverse (T2) relaxation Paramagnetic contrast agents typically compπse metal ions, for example, transition metal ions, which provide a source of unpaired electrons However, these metal ions are also generally highly toxic In an effort to decrease toxicity, the metal ions are typically chelated with hgands Metal oxides, most notably iron oxides, have also been employed as MRI contrast agents While small particles of iron oxide, for example, particles having a diameter of less than about 20 nm, may have desirable paramagnetic relaxation properties, their predominant effect is through bulk susceptibility Nitroxides are another class of MRI contrast agent which are also paramagnetic These have relatively low relaxivity and are generally less effective than paramagnetic ions The existing MRI contrast agents suffer from a number of limitations For example, increased image noise may be associated with certain contrast agents, including contrast agents involving chelated metals This noise generally aπses out of intrinsic peπstaltic motions and motions from respiration or cardiovascular action ln addition, the signal intensity for contrast agents generally depends upon the concentration of the agent as well as the pulse sequence employed Absorption of contrast agents can complicate interpretation of the images, particularly in the distal portion of the small intestine, unless sufficiently high concentrations of the paramagnetic species are used See, e g , Kornmesser et al , tvϊagnetic Resonance Imaging, 6 124 (1988) Other contrast agents may be less sensitive to vaπations in pulse sequence and may provide more consistent contrast However, high concentrations of particulates, such as ferπtes, can cause magnetic susceptibility artifacts which are particularly evident, for example, in the colon where the absorption of intestinal fluid occurs and the superparamagnetic mateπal may be concentrated Toxicity is another problem which is generally associated with currently available contrast agents for MRI For example, ferπtes often cause symptoms of nausea after oral administration, as well as flatulence and a transient πse in serum iron The gadolinium ion, which is complexed in Gd-DTPA, is highly toxic in free form The vaπous environments of the gastromtestmal tract, including mcreased acidity (lower pH) in the stomach and mcreased alkalinity (higher pH) m the intestines, may increase the likelihood of decoupling and separation of the free ion from the complex Blood flow may affect the quality of images obtained in MRI For example, coronary vasodilators have been used m connection with thallium 201 ( 201 T1) in an effort to improve the visualization of viable myocardial tissue in nuclear medicme Vasodilators can improve visualization by increasing blood flow to the myocardium which enables the 20I T1 to be taken up more efficiently into viable myocardial cells Coronary vasodilators have also been used in combination with Gd-DTPA to improve myocardial tissue imaging in MRI imaging Although Gd-DTPA may be used as an indicator of blood flow, relaxation measurements (Tl and T2) may lack the necessary sensitivity to aid in the quantitative measurement of flow In addition, MRI prior art contrast agents generally possess relatively low molecular weights which permits their diffusion through the vasculature This may render the quantification of blood flow through the vasculature based on pharmacokinetics difficult Patients with renal vascular hypertension usually have a narrowing of one of the arteries to the kidneys, known as renal artery stenosis Detection of renal vascular hypertension and distmguishmg from the more common essential hypertension is critical, because renal artery hypertension does not respond to standard medical treatments administered for hypertension Renal artery hypertension may be treated by angiotensin converting enzyme inhibitors or surgical management Essential hypertension, m contrast, is usually treated with diuretics, beta or alpha blockers, afterload reducers, pre-load reducers and occasionally ganghonic blockers Diagnostic imagmg may be used to detect the renal stenosis associated with renal hypertension Imaging techniques used for imaging of the renal region include radionuclide scintigraphic methods and radionuchde nuclear medical methods Captopπl, an ACE inhibitor, has been used combination with radiologic and radioscintigraphic procedures to detect stenosis m the renal artery See, for example, Nally, et al , Sem Nucl Med XXπ. 85-97 (1992), Itoh, et al , Clin Nucl Med 18, 463-471 (1993) and Dondi, et al , J Nucl Med 33. , 2040-2044 (1992) Nuclear medicine, however, suffers from poor spatial resolution, high expense, and as discussed below, the undesirable necessity of employing radioactive mateπals Angiography is preferred to nuclear methods for detection of renal hypertension, but it also is expensive and invasive Attempts to use ultrasound as a diagnostic tool for renal hypertension so far have generally provided poor results See for example, Postma, et al , Br J Radiol 65,857-860 (1992) and Kliewer, et al . Radiol 189,779-787 (1993) Accordingly, new and/or improved diagnostic imagmg methods, particularly for imagmg the renal region, are needed New and/or better diagnostic imagmg methods which permit the quantification of blood flow m the renal region are also needed The present invention is directed to these, as well as other, important ends Summarv of the Invention The present invention is directed, in part, to new and/or improved methods for diagnostic imaging Specifically, in one embodiment, there is provided a method for providing an image of the renal region of a patient, comprising administering to the patient a vesicle composition comprising lipid, protein or polymer vesicles and a gas or gaseous precursor, in combmation with a renal vasodilator The patient is then scanned using diagnostic imaging to produce a visible image of the renal region of the patient Another embodiment of the invention also relates to a method for providing an image of the renal region of a patient The method comprises admmistermg to the patient a composition comprising a lipid, protein, or polymer and a gas or gaseous precursor, in combmation with a renal vasodilator The patient is then scanned using diagnostic imagmg to obtain a visible image of the renal region A further embodiment of the invention provides a method for diagnosing the presence of diseased tissue in the renal region of a patient compπs g (I) administeπng to the patient a vesicle composition comprising lipid, protein or polymer vesicles and a gas or gaseous precursor, m combination with a renal vasodilator, and (n) scanning the patient using diagnostic imagmg to obtain a visible image of any diseased tissue in the patient An even further embodiment of the invention provides a method for diagnosing the presence of diseased tissue m the renal region of a patient compnsmg (I) admmistermg to the patient a composition comprising a lipid, protein, or polymer and a gas or gaseous precursor, in combmation with a renal vasodilator, and (π) scanning the patient using diagnostic imagmg to obtain a visible image of any diseased tissue in the patient Still another embodiment provides a method for substantially eliminating diagnostic artifacts in a diagnostic image of the renal region of a patient, compπsmg admmistenng to the patient, m combmation with a contrast agent, an agent which induces vasodilation of a renal artery In a further embodiment of the invention there is provided a contrast agent for diagnostic imagmg compnsmg a vesicle composition which compπses lipid, protem or polymer vesicles and a gas or gaseous precursor, in combmation with a renal vasodilator In a still further embodiment of the invention there is provided a contrast agent for diagnostic imagmg comprising a lipid, protein, or polymer and a gas or gaseous precursor, m combmation with a renal vasodilator In an even further embodiment of the invention, there is provided a method for measuπng blood flow m the renal region of a patient The method comprises (I) admmistermg to the patient a vesicle composition which comprises lipid, protein or polymer vesicles and a gas or gaseous precursor, (n) scanning the patient with diagnostic imagmg to obtain a visible image of the renal region, (in) administeπng to the patient a renal vasodilator, (iv) continuing said scanning, and (v) determining the blood flow from a videodensity versus time relationship in the images obtained in steps (n) through (iv) In yet another embodiment of the invention, there is provided a method for measuring blood flow in the renal region of a patient, comprising (1) admmistermg to the patient a composition compπsmg a lipid, protein or polymer and a gas or gaseous precursor, (n) scannmg the patient with diagnostic imaging to obtain a visible image of the renal region, (in) administering to the patient a renal vasodilator, (iv) continumg said scanning, and (v) determining the blood flow from a videodensity versus time relationship in the images obtained m steps (n) through (iv) These and other aspects of the invention will become more apparent from the remaining discussions herein Brief Description of the Figures Figure 1 is a plot of data obtained from pulse ultrasound imagmg performed on a kidney, as described in Example 3 On the y axis are normalized videodensity values (y) against the ratio of reappearance of videodensity signal to peak videodensity signal Each plot represents a different time interval (t) of 1 through 5 seconds between ultrasound pulses applied to rupture gas-filled vesicles which are used as an ultrasound contrast agent The data illustrate the ability to quantitatively measure blood flow using pulsed ultrasound imaging and continuous admmistration of a renal vasodilator Detailed Description of the Invention As employed above and throughout the disclosure, the following terms, unless otherwise indicated, shall be understood to have the following meanings "Lipid" refers to a naturally-occurring, synthetic, or semi-synthetic (also referred to as "modified natural") compound which is generally amphipathic The lipids typically compnse a hydrophilic component and a hydrophobic component Exemplary lipids include, for example, fatty acids, neutral fats, phosphatides, oils, glyco pids, surface-active agents (surfactants), aliphatic alcohols, waxes, terpenes and steroids "Polymer", or "polymeric", as used herem, refers to molecules formed from the chemical union of two or more repeating units Accordingly, included within the term "polymer" may be, for example, dimers, tπmers and oligomers The polymer may be synthetic, natural ly-occurπng or semisynthetic In preferred form, the term "polymer" refers to molecules which comprise 10 or more repeating units "Protein", as used herein, refers to molecules compnsmg, and preferably consisting essentially of, α-amino acids in peptide linkages Included within the term "protein" are globular protems, such as albumins, globulins and histones, and fibrous proteins such as collagens, elastins and keratins Also included are "compound protems", where a protem molecule is united with a nonprotem molecule, such as nucleproteins, mucoproteins, hpoproteins, and metalloproteins "Lipid composition", "polymer composition", and "protein composition" refer to a composition which compπses a lipid, polymer, or protein compound, respectively, typically in an aqueous medium Exemplary compositions mclude suspensions, emulsions and vesicle compositions The compositions described herein may also comprise a bioactive agent "Vesicle" refers to an entity which is generally characterized by the presence of one or more walls or membranes which form one or more internal voids Vesicles may be formulated, for example, from lipids, including the various lipids descπbed herem, or polymeπc mateπals, including the various polymenc materials listed herem, or protems, including the vaπous protems listed herein The lipids, polymers, and or proteins may be natural, synthetic or semi-synthetic Prefeπed vesicles are those which compnse walls or membranes formulated from lipids The walls or membranes may be concentπc or otherwise In the preferred vesicles, the lipids may be in the form of a monolayer or bilayer, and the mono- or bilayer lipids may be used to form one or more mono- or bilayers In the case of more than one mono- or bilayer, the mono- or bilayers may be concentπc, if desired Lipids may be used to form a unilamellar vesicle (comprised of one monolayer or bilayer), an ohgolamellar vesicle (comprised of about two or about three monolayers or bilayers) or a multilamellar vesicle (compnsed of more than about three monolayers or bilayers) Similarly, the vesicles prepared from polymers or proteins may comprise one or more walls or membranes, concentric or otherwise The walls or membranes of vesicles prepared from lipids, polymers, or protems may be substantially solid (uniform), or they may be porous or semi-porous The vesicles described herein include such entities commonly referred to as, for example, hposomes, micelles, bubbles, microbubbles, microspheres, lipid-, protem-and/or polymer-coated bubbles, microbubbles and/or microspheres, microballoons, microcapsules, aerogels, clathrate bound vesicles, hexagonal H II phase structures, and the like The internal void of the vesicles may be filled with a liquid (including, for example, an aqueous liquid), a gas, a gaseous precursor, and or a solid or solute mateπal, including, for example, a vasodilator and/or bioactive agent, as desired The vesicles may also comprise, if desired, a targeting ligand The application of high energy ultrasound, radio frequency, optical energy, such as, for example, laser light, and or heat, can be used, if desired, to rupture the vesicles in vivo and thereby promote release of the entrapped gas and/or gaseous precursor and bioactive agent Thus, vesicular formulations permit the controlled release of a bioactive agent, such as a renal vasodilator, in vivo The use of ultrasound energy in rupturing vesicles, thereby allowing release of a bioactive agent, is discussed in U S Patent No 5,558,092, the disclosures of which are hereby incorporated by reference their entirety "Vesicle composition" refers to a composition, typically in an aqueous medium, which compπses vesicles "Vesicle formulation" refers to a vesicle composition which also comprises a bioactive agent Suitable vesicles or vesicle species for use in vesicle formulations include, for example, gas filled vesicles and gaseous precursor filled vesicles "Liposome" refers to a generally spheπcal or spheroidal cluster or aggregate of amphipathic compounds, including lipid compounds, typically m the form of one or more concentπc layers, for example, bilayers They may also be refeπed to herem as lipid vesicles The hposomes may be formulated, for example, from ionic lipids and/or non-iomc lipids Liposomes which are formulated from non-ionic lipids may also be referred to as "niosomes " "Micelle" refers to colloidal entities formulated from lipids In certain preferred embodiments, the micelles comprise a monolayer or hexagonal H II phase configuration In other prefeπed embodiments, the micelles may comprise a bilayer configuration "Aerogel" refers to generally spherical or spheroidal entities which are characterized by a plurality of small internal voids The aerogels may be formulated from synthetic materials (for example, a foam prepared from baking resorcmol and formaldehyde), as well as natural materials, such as polysaccharides or protems "Clathrate" refers to a solid, semi-porous or porous particle which may be associated with vesicles In preferred form, the clathrates may form a cage-like structure containing cavities which comprise the vesicles One or more vesicles may be bound to the clathrate A stabilizing material may, if desired, be associated with the clathrate to promote the association of the vesicle with the clathrate Suitable materials from which clathrates may be formulated include, for example, porous apatites, such as calcium hydroxyapatite, and precipitates of polymers and metal ions, such as alginic acid precipitates with calcium salts "Emulsion" refers to a mixture of two or more generally immiscible liquids and is generally in the form of a colloid The mixture may be of lipids, which may be heterogeneously or homogeneously dispersed throughout the emulsion Alternatively, the lipids may be aggregated in the form of, for example, clusters or layers, including mono- or bilayers "Suspension", or "dispersion", refers to a mixture, preferably finely divided, of two or more phases (solid, liquid, or gas), such as, for example, liquid m liquid, solid in liquid, gas in liquid, etc , which preferably can remain stable for extended periods of time "Hexagonal H II phase structure" refers to a generally tubular aggregation of lipids in liquid media, for example, aqueous media, in which the hydrophilic portιon(s) of the lipids generally face inwardly in association with an aqueous liquid environment inside the tube The hydrophobic portion(s) of the lipids generally radiate outwardly and the complex assumes the shape of a hexagonal tube A plurality of tubes is generally packed together m the hexagonal phase structure The vesicles employed in the methods of the present invention preferably contain a gas or gaseous precursor "Gas filled vesicle" refers to vesicles in which there is encapsulated a gas "Gaseous precursor filled vesicle" refers to vesicles m which there is encapsulated a gaseous precursor In certain preferred embodiments, the vesicles may be substantially (including completely) filled with the gas and/or gaseous precursor The term "substantially", as used in reference to the gas and/or gaseous precursor filled vesicles, means that greater than about 50% of the internal void volume of the vesicle consists of a gas and or gaseous precursor Preferably, greater than about 60% of the mtemal void of the substantially filled vesicles consists of a gas and or gaseous precursor, with greater than about 70% being more preferred Even more preferably, greater than about 80% of the internal void of the substantially filled vesicles consists of a gas and/or gaseous precursor, with greater than about 90% being stall more preferred In particularly preferred embodiments, greater than about 95% of the internal void of the vesicles consists of a gas and/or gaseous precursor If desired, the substantially filled vesicle may be completely filled (i e filled with about 100% gas and/or gaseous precursor) Although not considered a preferred embodiment of the present invention, the vesicles may also contain, if desired, no or substantially no, l e , less than about 50%, gas and/or gaseous precursor The compositions and or formulations of the invention may be administered to a patient As used herem, "patient" refers to animals, including mammals, and preferably humans The phrases "internal region of a patient" and "region of interest" refer to the entire patient or to a particular area or portion of the patient Internal regions of a patient and regions of interest may mclude, for example, areas being imaged with diagnostic imagmg and or areas being treated with a bioactive agent The phrase "renal region of a patient" refers to the region of the patient defined by the kidney and the vasculature leadmg directly to and from the kidney, and includes the abdominal aorta The phrase "vasculature," as used herem, denotes the blood vessels (including artenes, ve s and the like) m the body or m an organ or part of the body The phrase "circulatory system" refers to the cardiovascular region and the entire vasculature "Bioactive agent" refers to a substance which may be used m connection with an application that is therapeutic or diagnostic in nature, such as m methods for diagnosing the presence or absence of a disease m a patient and/or in methods for the treatment of disease m a patient As used herein, "bioactive agent" refers also to substances which are capable of exertmg a biological effect m vitro and/or m vivo The bioactive agents may be neutral or positively or negatively charged Examples of suitable bioactive agents include diagnostic and pharmaceutical agents (including vasodilators includmg renal vasodilators), synthetic or natural organic or inorganic molecules, including proteins, peptides, vitamins, steroids, steroid analogs, antitumor agents, hormones, anti-inflammatory agents, chemotherapeutic agents, and genetic mateπal, includmg nucleosides, nucleotides and polynucleotides Preferably, the bioactive agent compπses a pharmaceutical agent "Pharmaceutical agent" or "drug" refers to any therapeutic or prophylactic agent which may be used in the treatment (including the prevention, diagnosis, alleviation, or cure) of a malady, affliction, disease or injury m a patient Therapeutically useful peptides, polypeptides and polynucleotides may be included within the meaning of the term pharmaceutical or drug Prefeπed pharmaceuticals and/or drugs are renal vasodilators "Diagnostic agent" refers to any agent which may be used in connection with methods for imagmg an internal region of a patient, including the renal region, and/or in methods for diagnosing the presence or absence of a disease in a patient, especially diseases of the heart includmg, for example, myocardial ischemia and myocardial infarction Exemplary diagnostic agents mclude, for example, contrast agents for use in connection with ultrasound, magnetic resonance imagmg or computed tomography of a patient including, for example, the lipid and or vesicle compositions descnbed herem "Genetic mateπal" refers generally to nucleotides and polynucleotides, includmg deoxyπbonucleic acid (DNA) and πbonucleic acid (RNA) The genetic mateπal may be made by synthetic chemical methodology known to one of ordinary skill in the art, or by the use of recombinant technology, or by a combmation of the two The DNA and RNA may optionally comprise unnatural nucleotides and may be smgle or double stranded "Genetic mateπal" refers also to sense and anti-sense DNA and RNA, that is, a nucleotide sequence which is complementary to a specific sequence of nucleotides in DNA and/or RNA "Thickening agent" refers to any of a variety of generally hydrophilic mateπals which, when incorporated in the lipid and/or vesicle compositions descπbed herein, may act as viscosity modifying agents, emulsifying and/or solubilizmg agents, suspending agents, and tonicity raising agents It is contemplated that the thickening agents may be capable of aiding in maintaining the stability of the compositions due to such properties "Dispersing agent" refers to a surface-active agent which, when added to a suspending medium of colloidal particles, including, for example, certain of the lipid and or vesicle compositions described herein, may promote uniform separation of particles In certain preferred embodiments, the dispersing agent may comprise a polymeric siloxane compound "Diagnostic artifact" refers generally to an imperfection, defect and/or flaw in a diagnostic image including, for example, ultrasound, computed tomography and magnetic resonance images, which may hamper and/or prevent visualization of a region of mterest Diagnostic artifacts may be manifested as undesired darkening and/or shadowing in the diagnostic image "Ultrasound artifact," "computed tomography artifact" and "MRI artifact" refer respectively to diagnostic artifacts associated with ultrasound, computed tomography and MRI "Echogenic vesicle" refers to a vesicle which may be capable of reflecting sound waves, includmg, for example, ultrasound waves Echogenic vesicles may be particularly useful as contrast agents to alter, for example, the acoustic properties of an internal region of a patient, thereby resulting m improved contrast m diagnostic imagmg techniques, such as ultrasound, computed tomography, and magnetic resonance imagmg In preferred form, the echogenic vesicles may comprise gas filled vesicles Alternatively, the echogenic vesicles may compnse vesicles which contain no or substantially no gas or gaseous precursor and which, together with bubbles or globules of a gas or a gaseous precursor, are suspended in a liquid medium m divided form In these latter embodiments, it is contemplated that echogenicity and/or an alteration in the acoustic properties of an mternal region of a patient arises, at least in part, from the presence of the divided gas or gaseous precursor "Videodensity" or "Videodensitometry" refers to the backscatter intensity of an ultrasound image, and may be used to estimate the concentration of contrast agent, especially contrast agents based on vesicles, in a tissue, for example, renal tissue Generally speaking, videodensitometπc analysis involves the use of a computer system which is capable of digitizing a full region of analog video data mto a digital image having 512 x 512 pixels (picture elements) Each pixel may be represented by one of a total of about 256 gray levels which have numerical values of from 0 to about 255, 0 being white (no contrast) and 255 being black (complete contrast) These gray levels may also be referred to herein as videodenistometry units (VDUs) "Brightness" refers to the level of contrast in a diagnostic image including, for example, ultrasound, computed tomography and magnetic resonance images, of a region of mterest Thus, in connection with diagnostic imagmg of the renal region, the term "brightness" refers to the level of contrast of a diagnostic image of the renal region, includmg the kidney tissue and the vasculature associated therewith "Enhanced diagnostic image" refers to a diagnostic image which may be improved relative to diagnostic images obtained using one or more methods of the pπor art and which may be obtained with the methods of the present invention Enhanced diagnostic images may be manifested by an increase brightness in the diagnostic image, substantial elimination of diagnostic artifacts in the diagnostic image, and/or the like Thus, in connection with diagnostic imagmg of the renal region, includmg the kidney tissue and the vasculature associated therewith, an enhanced diagnostic image may be manifested, for example, by increased bπghtness in the diagnostic image of the renal region and/or a substantial elimination the occuπence of diagnostic artifacts m the diagnostic image of the renal region "Increased bnghtness" refers to an increase in the brightness of a diagnostic image which may be obtained with the methods of the present invention Preferably, the increases m brightness provided by the methods of the present invention are at least discermble to the naked eye With particular reference to the gray scale (about 0 to about 255 VDUs or gray levels) identified above, there is preferably provided with the methods of the present invention an increase in the level of bπghtness of at least about 10 VDUs (gray levels) More preferably, in accordance with the embodiment descπbed herein, the methods of the present mvention provide an increased bπghtness of greater than about 10 VDUs, for example, about 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 VDUs In certain other embodiments, the present methods may provide an increased brightness of greater than about 100 VDUs, for example, about 105, 110, 115, 120, 125, 130, 135, 140, 145 or 150 VDUs In still other embodiments, the methods of the present invention may provide an increased brightness of greater than about 1 0 VDUs, for example, about 155, 160, 165, 170, 175, 180, 185, 190, 195 or 200 VDUs Yet in other embodiments, the present methods may provide an increased brightness of greater than about 200 VDUs, for example about 205, 210, 215, 220, 225, 230, 235, 240, 245, 250 or 255 VDUs "Substantial elimination" refers to the prevention or substantial prevention of the occuπence of diagnostic artifacts in a diagnostic image The term "substantial prevention" means that at least about 50% of the artifacts may be eliminated by the methods of the present mvention as compared to at least one prior art diagnostic method Preferably, at least about 60% of the artifacts may be eliminated by the methods of the present mvention as compared to at least one prior art diagnostic method, with the elimination of at least about 70% of the artifacts being more preferred Even more preferably, at least about 80% of the artifacts may be eliminated by the methods of the present invention as compared to at least one prior art diagnostic method, with the elimination of at least about 90% of the artifacts being still more preferred Yet more preferably, at least about 95% of the artifacts may be eliminated by the methods of the present invention as compared to at least one pnor art diagnostic method, with the elimination of at least about 100% bemg still more preferred The terms "administered" and "administration" refer generally to the administration to a patient of a biocompatible material, including, for example, lipid, polymer or protein and or vesicle compositions and/or formulations descπbed herem "Biocompatible" refers to materials which are generally not injurious to biological functions and which will not result m any degree of unacceptable toxicity, includmg allergemc responses and disease states The compositions and components thereof (such as lipids, protein, polymers, gases, gaseous precursors, vasodilators, etc ) employed m the present invention are typically biocompatible "In combination with" refers to the co-administration of a bioactive agent, such as a renal vasodilator, with a lipid, polymer or protein, and/or vesicle composition The term "co-administration" means that the bioactive agent may be administered before, during, or after the administration of the composition In embodiments m which the bioactive agent is included in the composition, as m a formulation, the bioactive agent may be combined with the vesicle composition in any of a variety of different ways For example, in the case of vesicle compositions, the bioactive agent may be entrapped within the internal void of the vesicle In addition, the bioactive agent may be integrated within the layer(s) or wall(s) of the vesicle, for example, by being interspersed among the lipids, prote s or polymers that compnse the vesicle layer(s) or wall(s) In the case of non- vesicular lipid, polymer and/or protein compositions, the bioactive agent may be entrapped between or among the lipid, polymer and/or protein components It is also contemplated that the bioactive agent may be located on the surface of the vesicle or the non-vesicular lipid, polymer and or protein In this case, the bioactive agent may interact chemically with the inner or outer surface of the vesicle or the non-vesicular lipid, polymer and/or protein and remain substantially adhered thereto Such interaction may take the form of, for example, electrostatic interactions, hydrogen bonding, van der Waal's forces, covalent bondmg, or other interaction Also, the bioactive agent may interact with the inner or outer surface of the vesicle or the non- vesicular lipid, polymer or protein in a limited manner Such limited interaction would permit migration of the bioactive agent, for example, from the surface of a first vesicle to the surface of a second vesicle, or from the surface of a first non-vesicular lipid, polymer, or protein to the surface of a second non- vesicular lipid, polymer or protein "Renal vasodilator" refers to a bioactive agent which, when administered to a patient, causes or mamtams dilation of the vasculature in the renal region It is meant to include agents which act directly on the renal vasculature, as well as agents which act indirectly by activating or producing, or causing to be activated or produced, in vivo another agent which acts to cause or maintain dilation of the vasculature in the renal region "Ischemic" denotes a deficiency of blood due to functional constriction or actual obstruction of a blood vessel The present invention is directed, in part, to improved methods for diagnostic imaging, including, for example, improved methods for providing an image of an internal region of a patient, especially the renal region of a patient The improved methods of the present invention may provide enhanced diagnostic images, as evidenced, for example, by increased brightness in the diagnostic image and/or a substantial elimination of diagnostic artifacts in the diagnostic image Embodiments of the present invention involve the administration to the patient of a contrast agent in the form of a lipid, polymer or protein composition comprising a lipid, polymer or protein and a gas or gaseous precursor Embodiments of the present invention also involve the administration to the patient of a contrast agent in the form of a vesicle composition comprising vesicles and a gas or gaseous precursor The patient is scanned using diagnostic imaging, preferably ultrasound, to obtain a visible image of the region An important feature of the methods of the present invention is that, in addition to a contrast agent, a renal vasodilator is admmistered to the patient It is contemplated that the renal vasodilator provides umque advantages in connection with methods for diagnostic imaging, especially ultrasound, relative to methods for diagnostic imaging which have been available heretofore In this connection, renal vasodilators are capable of dilating blood vessels in the renal region of a patient This, m turn, may produce an alteration in the patient's blood flow in the renal region It has been surpnsingly and unexpectedly found that this alteration m the flow of blood in the renal region may produce profound changes m the quality of diagnostic images of the renal region The present invention is directed, at least in part, to exploiting this surprising and unexpected discovery to provide improved methods for diagnostic imagmg of the renal region The present invention provides a simple and effective means for obtaining improved images of the renal region of a patient Diagnostic imagmg, such as ultrasound imagmg of the renal region, may involve the use of a contrast agent, for example, a contrast agent compnsmg gas filled vesicles, which is administered intravenously After injection, the contrast agent may be carried m the bloodstream to the renal region It is understood, of course, that in the normal course of the circulation of blood throughout the circulatory system, blood which contains the contrast agent will flow through the kidneys and associated vasculature Energy, for example, ultrasound, may be applied, and a diagnostic image of the renal region may be generated The basis for the ability to detect disease in the kidney using ultrasound is the difference m acoustic properties between blood and tissues Contrast agents are expected to increase the acoustic difference between the blood and the surrounding tissues, thereby improving the detectabi ty of blood flow and as a result, the detection of diseases involving changes in blood flow In contrast with nuclear medicine, wherein radionuclides aid in assessing kidney function, ultrasound assesses blood flow and the structure of the renal vasculature It has been suφrisingly and unexpectedly found that, connection with methods for the diagnostic imagmg of the renal region which involve the administration to a patient of a contrast agent comprising lipids, polymers, or proteins and a gas or gaseous precursor, and especially a contrast agent comprising hpid, polymer, or protein vesicles and a gas or gaseous precursor, the additional administration of a renal vasodilator can desirably enhance the resulting diagnostic image While the inventors do not wish to be bound by any theory or theoπes of operation, it is believed that the enhanced diagnostic images may be due to an alteration in renal blood flow caused by the renal vasodilator This alteration in the flow of blood may provide enhanced diagnostic images, which may be manifested, for example, by increased brightness and/or substantial elimination of diagnostic artifacts in the diagnostic image It is believed that the administration to a patient of a renal vasodilator may increase blood flow and, therefore, the concentration of contrast agent in the renal tissue This mcrease m contrast agent in the renal tissue may provide mcreased bnghtness in diagnostic images of the renal region and renal tissue, which may result in improved tissue visualization It is believed also that the administration of a renal vasodilator may mcrease the flow of blood (and concentration of contrast agent) in the renal tissue relative to the flow of blood (and concentration of contrast agent) in the associated vasculature This may result in a reduction or substantial elimination of diagnostic artifacts in images of the renal region The measurable physical manifestations of blood flow through the renal artery are affected by factors which dilate or constrict the artery Furthermore, it is expected that branching of an artery into smaller arteπoles, as occurs in some patients, results m a decreased arteπal volume and an associated mcrease m blood pressure It is believed that renal vasodilators reverse this effect, thereby increasing blood volume When a renal artery is branched, one or more branches may be stenotic and a segment of a kidney may be lschemic The methods of the present invention may be used m diagnosing such conditions Another effect of a renal vasodilator is to mcrease the differential blood flow between an lschemic kidney and a non-ischemic kidney This in turn increases the difference m the videodensitometry between the lschemic kidney and the non-ischemic kidney in diagnostic imagmg As discussed below, the dosage level of renal vasodilator is determined by a number of factors including the patient's body weight, but is preferably a level sufficient to increase blood flow in a non-ischemic kidney In preferred embodiments of the present invention, a contrast agent is administered to a patient, ultrasound imaging of the renal region is performed, a renal vasodilator is administered, and ultrasound imaging is repeated It is expected that a non- lschemic kidney will exhibit a more noticeable increased blood flow due to the renal vasodilator than will an lschemic kidney In effect, the increased blood flow caused by the renal vasodilator enhances the difference in the videodensitometry between an lschemic kidney and a non-ischemic kidney The effect of the contrast agent is thereby enhanced by the use of the renal vasodilator The resulting renograms may be analyzed by comparing the intensities of the imagmg signals of the kidneys and renal region before and after the administration of the renal vasodilator The degree of mcrease in renal blood flow which may be provided usmg the methods of the present invention may vary and depends, for example, on the particular hpid and/or vesicle composition admmistered to the patient, the particular renal vasodilator administered to the patient, the respective dosages of the hpid/vesicle composition and renal vasodilator admmistered to the patient, and the like Generally speaking, any mcrease in blood flow which is obtained usmg the methods of the present invention may provide enhanced diagnostic images In accordance with prefeπed embodiments of the present mvention, enhanced diagnostic images may be obtained by increasing blood flow by greater than about 10%, for example, about 20, 30, 40 or 50% Preferably, enhanced diagnostic images may be obtained by increasing blood flow by greater than about 50%, for example, about 60, 70, 80, 90 or 100%, with increased blood flow of greater than about 100%, for example, about 110, 120, 130, 140 or 150%, bemg more preferred Even more preferably, enhanced diagnostic images may be obtained by lncreasmg blood flow by greater than about 150%, for example, about 160, 170, 180 or 190%, with an mcrease of about 200% being still more prefeπed In certain particularly preferred embodiments, enhanced diagnostic images may be obtained by increasing blood flow by greater than about 200% In accordance with the present invention, the foregoing alterations in the flow of blood may be achieved by the administration to the patient of a renal vasodilator Thus, m prefeπed form, the methods of the present invention involve the administration to a patient of a contrast agent in combmation with a renal vasodilator Generally speaking, the renal vasodilator employed in the methods of the present invention may be administered before, during and/or after the administration of contrast agent As would be apparent to one of ordinary skill m the art, once armed with the present disclosure, the order of administration of the contrast agent and renal vasodilator, as well as the manner in which they are admmistered may be vaned, as desired, as a means of modulating the resulting diagnostic image Thus, the present methods enable the skilled artisan to obtain the desired information about the patient, including, for example, the condition of the tissue being imaged, especially renal tissue, and the rate of blood flow m the renal region, which may provide information concerning the overall health and well-being of the patient For example, it is contemplated that the present methods may initially involve a continuous administration or infusion of contrast agent to the patient In this manner, there may be provided the renal region of the patient a substantially constant flow or concentration of contrast agent Administration of a renal vasodilator may provide an increased flow of blood in the renal region of the patient which, m turn, may provide mcreased bnghtness in the diagnostic image of the renal tissue In embodiments which employ a constant administration or infusion of contrast agent, the rate of response of the patient to the renal vasodilator may also provide mformation concemmg the presence or absence of disease, mcludmg renal disease, in the patient Specifically, the time required for the patient to respond to the renal vasodilator as evidenced, for example, by the rate of increase in renal blood flow, may provide information concemmg the mtegnty of the patient's renal artery Thus, a patient having substantially no renal artery stenosis may respond substantially rapidly to the renal vasodilator, as measured by an mcrease in the renal blood flow Conversely, a patient having severe renal artery stenosis may respond to the renal vasodilator at a rate which is slower than the rate at which a patient having no stenosis responds In embodiments of the present invention, the contrast agents may comprise a hpid composition comprising a hp d and a gas or gaseous precursor In connection with hpid compositions, and especially hpid compositions in the form of vesicle compositions, it may be advantageous to prepare the lipid compositions at a temperature below the gel to liquid crystalline phase transition temperature of the involved lipids This phase transition temperature is the temperature at which a hpid bilayer will convert from a gel state to a liquid crystalline state See, for example, Chapman et al , J Biol Chem 1974 249, 2512- 2521 It is generally believed that vesicles which are prepared from lipids that possess higher gel state to liquid crystalline state phase transition temperatures tend to have enhanced impermeability at any given temperature See Derek Marsh, CRC .Handbook ofLipid Bilayers (CRC Press, Boca Raton, FL 1990), at p 139 for main chain meltmg transitions of saturated dιacyl-sn-glycero-3-phosphocholmes The gel state to liquid crystalline state phase transition temperatures of vaπous lipids will be readily apparent to those skilled in the art and are descπbed, for example, in Gregoπadis, ed , Liposome Technology, Vol I, 1-18 (CRC Press, 1984) The following table lists some of the representative lipids and their phase transition temperatures TABLE 1 Saturated Diacyl-sn-Giycero-3-Phosphocholines: Main Chain Melting Transition Temperatures Main Phase Number of Carbons in Acyl Transition Chains Temperature (° C) 1,2-02 0) -1 0 1,2-03 0) 13 7 1,2-04 0) 23 5 TABLE 1 Saturated Diacyl-sn-Glycero-3-Phosphocholines : Main Chain Melting Transition Temperatures Main Phase Number of Carbons Acyl Transition Chains Temperature (° C) 1,2-(15 0) 34 5 1 ,2-06 0) 41 4 1,2-07 0) 48 2 1 ,2-(18 0) 55 1 1,2-09 0) 61 8 l,2-(20 0) 64 5 1,2-(21 0) 71 1 l,2-(22 0) 74 0 l,2-(23 0) 79 5 l,2-(24 0) 80 1 See, for example, Derek Marsh, CRC Handbook ofLipid Bilayers, p 139 (CRC Press, Boca Raton, FL 1990) It may be possible to enhance the stability of vesicles formulated from lipids by incorporating in the hpid compositions at least a minor amount, for example, about 1 to about 10 mole percent, based on the total amount of pid employed, of a negatively charged hpid Suitable negatively charged lipids include, for example, phosphatidylserme, phosphatidic acid, and fatty acids Without intending to be bound by any theory or theones of operation, it is contemplated that such negatively charged lipids may provide added stability by counteracting the tendency of vesicles to rupture by fusing together Thus, the negatively charged lipids may act to establish a uniform negatively charged layer on the outer surface of the vesicle, which will be repulsed bv a similarly charged outer layer on other vesicles which may be proximate thereto In this way, the vesicles may be less prone to come into touchmg proximity with each other, which may lead to a rupture of the membrane or skin of the respective vesicles and consolidation or fusion of the contacting vesicles into a smgle, larger vesicle A continuation of this process of consolidation will, of course, lead to significant degradation of the vesicles The pid materials used in certain of the compositions described herein, especially in connection with vesicle compositions based on lipids, are also preferably flexible This means that, for example, in the case of vesicle compositions based on lipids, the vesicles can alter their shape, for example, to pass through an opening having a diameter that is smaller than the diameter of the vesicle A wide variety of lipids are believed to be suitable for incorporation in the hpid and or vesicle compositions With particular reference to vesicle compositions, for example, micelles and/or hposomes, any of the materials or combinations thereof which are known to those skilled m the art as suitable for their preparation may be used The lipids used may be of natural, synthetic or semi-synthetic ongm As noted above, suitable lipids generally include, for example, fatty acids, neutral fats, phosphatides, glycolipids, aliphatic alcohols and waxes, terpenes and steroids Exemplary lipids which may be used to prepare pid compositions include, for example, fatty acids, lysohpids, phosphochohnes, phosphatidylcholine with both saturated and unsaturated lipids, including dioleoylphosphatidylcholine, dimyπstoylphosphatidylcholme, dipentadecanoylphosphatidylchohne, dilauroyl- phosphatidylchol e, dipalmitoylphosphatidylchohne (DPPC), distearoylphosphatidyl- cho ne (DSPC), and diarachidonylphosphatidylcholine (DAPC), phosphatidylethanol- amines, such as dioleoylphosphatidylethanolamine, dipalmitoylphosphatidylethanol-amme (DPPE) and distearoylphosphatidylethanolamine (DSPE), phosphatidylsenne, phosphatidylglycerols, mcludmg distearoylphosphatidylglycerol (DSPG), phosphatidylinositol, sphmgohpids, such as sphingomyehn, glycolipids, such as gang oside GM1 and GM2, glucolφids, sulfatides, glycosphingohpids, phosphatidic acids, such as dipalmitoylphosphatidic acid (DPP A) and distearoylphosphatidic acid (DSPA), palmitic acid, steanc acid, arachidonic acid, oleic acid, lipids bearmg biocompatible polymers, such as chiton, hyaluronic acid, polyvinylpyrrolidone or polyethylene glycol (PEG), the latter being also referred to herein as "pegylated lipids", with preferred lipids bearmg polymers including DPPE-PEG, which refers to the pid DPPE having a PEG polymer attached thereto, mcludmg, for example, DPPE-PEG5000, which refers to DPPE having attached thereto a PEG polymer having a mean average molecular weight of about 5000, lipids bearing sulfonated mono-, di-, o go- or polysacchaπdes, cholesterol, cholesterol sulfate and cholesterol hemisuccmate, tocopherol hemisuccmate, lipids with ether and ester-linked fatty acids, polymenzed lipids (a wide variety of which are well known in the art), diacetyl phosphate, dicetyl phosphate, stearylamine, cardiolipin, phosphohpids with short chain fatty acids of about 6 to about 8 carbons m length, synthetic phosphohpids with asymmetric acyl chains, such as, for example, one acyl chain of about 6 carbons and another acyl chain of about 12 carbons, ceramides, non-ionic hposomes mcludmg niosomes such as polyoxyethylene fatty acid esters, polyoxyethylene fatty alcohols, polyoxyethylene fatty alcohol ethers, polyoxyethylated sorbitan fatty acid esters, glycerol polyethylene glycol oxystearate, glycerol polyethylene glycol πcinoleate, ethoxylated soybean sterols, ethoxylated castor oil, polyoxyethylene-polyoxypropylene polymers and polyoxyethylene fatty acid stearates, sterol aliphatic acid esters, mcludmg cholesterol sulfate, cholesterol butyrate, cholesterol iso-butyrate, cholesterol palmitate, cholesterol stearate, lanosterol acetate, ergosterol palmitate and phytosterol n-butyrate, sterol esters of sugar acids including cholesterol glucuronide, lanosterol glucuronide, 7-dehydrocholesterol glucuromde, ergosterol glucuromde, cholesterol gluconate, lanosterol gluconate and ergosterol gluconate, esters of sugar acids and alcohols includmg lauryl glucuromde, stearoyl glucuromde, myπstoyl glucuronide, lauryl gluconate, mynstoyl gluconate and stearoyl gluconate, esters of sugars and aliphatic acids, mcludmg sucrose laurate, fructose laurate, sucrose palmitate, sucrose stearate, glucuromc acid, gluconic acid and polyuronic acid, sapomns, mcludmg sarsasapogemn, smilagenin, hederagemn, oleanolic acid and digitoxigenin, glycerols, including glycerol dilaurate, glycerol tnlaurate, glycerol dipalmitate, glycerol and glycerol esters, such as glycerol tπpalmitate, glycerol distearate, glycerol tπstearate, glycerol dimynstate and glycerol tnmyπstate, long chain alcohols, including n-decyl alcohol, lauryl alcohol, mynstyl alcohol, cetyl alcohol and n-octadecyl alcohol, 6-(5-cholesten-3β- yloxy)- 1 -thio-β-D-galactopyranoside, digalactosyldiglyceπde, 6-(5-cholesten-3 β- yloxy)hexyl-6-ammo-6-deoxy- 1 -thio-β-D-galactopyranoside, 6-(5-cholesten-3β- yloxy)hexyl-6-amιno-6-deoxyl- 1 -thio-α-D-mannopyranoside, 12-(((7'- dιethylamιnocoumarιn-3-yl)carbonyl)-methylammo)-octadecanoιc acid, N-[ 12-(((7'- dιeuιylamιnocoumarιn-3-yl)carbonyl)-methylamιno)-octadecanoyl]-2-amιnopalmιtιc acid, cholesteryl)4'-tπmethyl-ammonιo)butanoate, N-succinyldioleoylphosphatidylethanol- amme, 1 ,2-dιoleoyl-sn-glycerol, l,2-dιpalmιtoyl-sn-3-succmylglycerol, l,3-dιpalmιtoyl-2- succinylglycerol, 1 -hexadecyl-2-palmιtoyl-glycerophosphoethanolamme and palmitoylhomocysteme, and or combinations thereof If desired, a cationic hpid may be used, such as, for example, N-[l -(2,3- dιoleoyloxy)propyl]-N,N,N-tπmethylammomum chloride (DOTMA), l,2-dιoleoyloxy-3- (tnmethylammonιo)propane (DOTAP), and l,2-dιoleoyl-3-(4'-tπmethylammonιo)- butanoyl-sn-glycerol (DOTB) If a cationic hpid is employed in the hpid compositions, the molar ratio of cationic hpid to non-cationic pid may be, for example, from about 1 1000 to about 1 100 Preferably, the molar ratio of cationic hpid to non-cationic hpid may be from about 1 2 to about 1 10, with a ratio of from about 1 1 to about 1 2 5 being prefeπed Even more preferably, the molar ratio of cationic hpid to non-cationic hpid may be about 1 1 In the case of hpid compositions which contain both cationic and non- cationic lipids, a wide vaπety of lipids may be employed as the non-catiomc hpid Preferably, this non-catiomc hpid comprises one or more of DPPC, DPPE and dioleoylphosphatidylethanolamme In lieu of the cationic lipids listed above, lipids bearmg cationic polymers, such as polylysine or polyarginine, as well as alkyl phosphonates, alkyl phosphinates and alkyl phosphites, may also be used m the hpid compositions In preferred embodiments, the pid compositions compnse phosphohpids, particularly one or more of DPPC, DPPE, DPPA, DSPC, DSPE, and DAPC (20 carbons), with DPPC, DPPE and/or DPPA being especially prefeπed Saturated and unsaturated fatty acids may also be employed the hpid compositions described herein and may include molecules that preferably contain from about 12 carbons to about 22 carbons, in linear or branched form Hydrocarbon groups consisting of isoprenoid units and or prenyl groups can be used as well Examples of saturated fatty acids that are suitable include, for example, lauric, mynstic, palmitic and stearic acids Suitable unsaturated fatty acids that may be used include, for example, lauroleic, physetenc, mynstoleic, hnoleic, nolenic, palmitoleic, petrosehmc and oleic acids Examples of branched fatty acids that may be used include, for example, isolauric, lsomynstic, lsopalmitic and isostearic acids The methods of the present invention may also involve compositions and vesicles formulated from proteins or denvatives thereof Vesicles which are formulated from protems and which would be suitable for use in the methods of the present invention are descnbed, for example, m Feinstein, U S Patent Nos 4,572,203, 4,718,433, and 4,774,958, and Cerny et al , U S Patent No 4,957,656, all of the disclosures of which are hereby incorporated by reference m their entirety Other protein-based vesicles, rn addition to those descnbed in the aforementioned patents, would be apparent to one of ordinary skill m the art, once armed with the present disclosure In addition to pid compositions and or vesicles formulated from lipids and or protems, the methods of the present invention may also involve compositions and or vesicles formulated from polymers which may be of natural, semi-synthetic (modified natural) or synthetic ongin, with semi-synthetic and synthetic polymers bemg prefeπed As used herein, the term polymer denotes a compound comprised of two or more repeating monomenc units, and preferably 10 or more repeating monomenc units The phrase semi-synthetic polymer (or modified natural polymer), as employed herein, denotes a natural polymer that has been chemically modified in some fashion Exemplary natural polymers suitable for use in the present invention include naturally occurnng polysacchaπdes Such polysaccharides include, for example, arabmans, fructans, fucans, galactans, galacturonans, glucans, mannans, xylans (such as, for example, inulin), levan, fucoidan, carrageenan, galatocarolose, pectic acid, pectins, mcludmg amylose, pullulan, glycogen, amylopectin, cellulose, dextran, dextrin, dextrose, polydextrose, pustulan, chitm, agarose, keratan, chondroitan, dermatan, hyaluronic acid, alginic acid, xanthan gum, starch and vanous other natural homopolymer or heteropolymers, such as those containing one or more of the following aldoses, ketoses, acids or amines erythrose, threose, ribose, arabinose, xylose, Iyxose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, erythrulose, πbulose, xylulose, psicose, fructose, sorbose, tagatose, mannitol, sorbitol, lactose, sucrose, trehalose, maltose, cellobiose, glucuronic acid, gluco c acid, glucaπc acid, galacturonic acid, mannuronic acid, glucosamme, galactosamine, and neurammic acid, and naturally occurring derivatives thereof Exemplary semi-synthetic polymers include carboxymethylcellulose, hydroxymethylcellulose, hydroxypropylmethyl-cellulose, methylcellulose, and methoxycellulose Exemplary synthetic polymers suitable for use in the present mvention include polyethylenes (such as, for example, polyethylene glycol, polyoxyethylene, and polyethylene terephthlate), polypropylenes (such as, for example, polypropylene glycol), polyurethanes (such as, for example, polyvmyl alcohol (PVA), polyvinylchloπde and polyvinylpyπolidone), polyamides including nylon, polystyrene, polylactic acids, fluonnated hydrocarbons, fluonnated carbons (such as, for example, polytetrafluoroethylene), and polymethylmethacrylate, and derivatives thereof Prefeπed are biocompatible synthetic polymers or copolymers prepared from monomers, such as acrylic acid, methacry c acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate (HEM), lactic acid, glycolic acid, e- caprolactone, acrolem, cyanoacτylate, cyanomethacrylate, bisphenol A, epichlorhydnn, hydroxyalkyl-acrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxyalkyl-methacrylates, N-substituted acrylamides, N-substituted methacrylamides, N-vτnyl-2-pyπohdone, 2,4-pentadιene-l-o , vinyl acetate, acrylonitrile, styrene, p-amino- styrene, p-ammo-benzyl-styrene, sodium styrene sulfonate, sodium 2- sulfoxyethylmethacrylate, vinyl pyndine, ammoethyl methacrylates, 2-methacryloyloxy- tnmethylammomum chloride, and polyvinylidene, as well polyftinctional cross nkmg monomers such as N,N'-methylenebιsacrylamιde, ethylene glycol dimethacrylates, 2,2'-(p- phenylenedιoxy)-dιethyl dimethacrylate, divinylbenzene, tnallylamine and methylenebis- (4-phenyI-ιsocyanate), including combinations thereof Preferable polymers include polyacrylic acid, polyethyleneimine, polymethacryhc acid, polymethylmethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly(e-caprolactone), epoxy resm, poly(ethylene oxide), poly( ethylene glycol), and polyamide (nylon) polymers Preferable copolymers include polyvinylidene-polyacrylomtπle, polyvinylidene-polyacrylonitrile- polymethyl-methacrylate, polystyrene-polyacrylonitπle and poly d,l lactide co-glycohde polymers A preferred copolymer is polyvinylidene-polyacrylomtnle Other suitable biocompatible monomers and polymers will be readily apparent to those skilled in the art, once armed with the present disclosure Methods for the preparation of vesicles compnsmg polymers will be readily apparent to those skilled in the art, once armed with the present disclosure, when the present disclosure is coupled with information known in the art, such as that described and referred to in Unger, U S Patent No 5,205,290, the disclosures of which are hereby incorporated herein by reference m their entirety Vesicles derived from lipids, proteins, or polymers for use in the methods of the present invention are preferably low density The term "low density" refers to vesicles which have an internal void (cavity) volume which is at least about 75% of the total volume of the vesicle Preferably, the vesicles have a void volume of at least about 80%, more preferably at least about 85%, even more preferably at least about 90%, still more preferably at least about 95%, and yet more preferably about 100% of the total volume of the vesicles As noted above, the pid, protein, and polymer compositions, and or vesicles, employed in the present methods may also comprise a gas, such as an mert gas The gas may provide the pid, protein, or polymer compositions and/or vesicles with enhanced imaging capabilities, such as reflectivity or ultrasound, particularly in connection with vesicle compositions which the gas is entrapped within the vesicles This may increase the effectiveness of the vesicle compositions as contrast agents Prefeπed gases are gases which are inert and which are biocompatible, that is, gases which are not mjunous to biological function Preferred gases include those selected from the group consisting of air, noble gases, such as helium, rubidium hyperpolanzed xenon, hyperpolanzed argon, hyperpolanzed helium, neon, argon, xenon, carbon dioxide, nitrogen, fluorine, oxygen, sulfur-based gases, such as sulfur hexafluonde and sulfur tetrafluonde, fluonnated gases, mcludmg, for example, partially fluonnated gases or completely fluonnated gases Exemplary fluonnated gases mclude the fluorocarbon gases, such as the perfluorocarbon gases, and mixtures thereof Paramagnetic gases, such as 17 O 2 , may also be used in the hpid and/or vesicle compositions. In preferred embodiments, the gas utilized m the compositions descπbed herein comprises a fluonnated gas. Such fluonnated gases include matenals which contain at least one, or more than one, fluorine atom Preferred are gases which contain more than one fluorine atom, with perfluorocarbons (that is, fully fluonnated fluorocarbons, wherein all hydrogen atoms in the corresponding hydrocarbon are replaced by fluorine atoms) being more preferred Preferably, the perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluoropentane, perfluorohexane, perfluorocyclobutane and mixtures thereof More preferably, the perfluorocarbon gas is perfluoropropane or perfluorobutane, with perfluoropropane bemg particularly preferred Another preferable gas is sulfur hexafluoride. Yet another preferable gas is heptafluoropropane, including 1,1,1,2,3,3,3- heptafluoropropane and its isomer, 1 ,1 ,2,2,3, 3,3-heptafluoropropane It is contemplated that mixtures of different types of gases, such as mixtures of a perfluorocarbon gas and another type of gas, such as air, can also be used in the compositions employed in the methods of the present invention Other gases, including the gases exemplified above, would be readily apparent to one skilled in the art based on the present disclosure In certain prefeπed embodiments, a gas, for example, air or a perfluorocarbon gas, is combined with a liquid perfluorocarbon, such as perfluoropentane, perfluorohexane, perfluoroheptane, perfluorooctylbromide (PFOB), perfluorodecalin, perfluorododecalin, perfluorooctyliodide, perfluorotnpropylamine and perfluorotributylamine. It may also be desirable to incorporate in the lipid and/or vesicle compositions a precursor to a gaseous substance Such precursors include materials that are capable of being converted to a gas in vivo Preferably, the gaseous precursor is biocompatible, and the gas produced in vivo is biocompatible also Among the gaseous precursors which are suitable for use in the hpid and/or vesicle compositions described herein are agents which are sensitive to pH These agents mclude materials that are capable of evolving gas, for example, upon being exposed to a pH that is neutral or acidic. Examples of such pH sensitive agents include salts of an acid which is selected from the group consisting of inorganic acids, organic acids and mixtures thereof Carbonic acid (H 2 CO 3 ) is an example of a suitable inorganic acid, and aminomalonic acid is an example of a suitable organic acid Other acids, includmg inorganic and organic acids, would be readily apparent to one skilled in the art based on the present disclosure Gaseous precursors which are derived form salts are preferably selected from the group consisting of alkali metal salts, ammonium salts and mixtures thereof More preferably, the salt is selected from the group consisting of carbonate, bicarbonate, sesquecarbonate, aminomalonate and mixtures thereof Examples of suitable gaseous precursor materials which are derived from salts include, for example, lithium carbonate, sodium carbonate, potassium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, magnesium carbonate, calcium carbonate, magnesium bicarbonate, ammonium carbonate, ammonium bicarbonate, ammonium sesquecarbonate, sodium sesquecarbonate, sodium aminomalonate and ammonium aminomalonate .Aminomalonate is well known m the art, and its preparation is described, for example, in Thanassi, Biochemis try, Vol 9, no 3, pp 525-532 (1970), Fitzpatπck et al , Inorganic Chemistry, Vol 13, no 3 pp 568-574 (1974), and Stelmashok et al , Koordinatsionnaya Khimiya, Vol 3, no 4, pp 524-527 (1977) The disclosures of these publications are hereby incorporated herein by reference, in their entirety In addition to, or instead of bemg sensitive to changes in pH, the gaseous precursor materials may also compnse compounds which are sensitive to changes in temperature Exemplary of suitable gaseous precursors which are sensitive to changes in temperature are perfluorocarbons, hydrofluorocarbons, hydrocarbon ethers, hydrofluorocarbon ethers, and perfluorocarbon ethers These latter gaseous precursors are exemplifed, for example, by anesthetic and anesthetic-like gases includmg, for example, halothane, enflurane, lsoflurane, desflurane and sevoflurane As the artisan will appreciate, a particular perfluorocarbon may exist m the liquid state when the pid and/or vesicle compositions are first made, and thus may be used as a gaseous precursor Alternatively, the perfluorocarbon may exist in the gaseous state when the hpid and/or vesicle compositions are made, and thus may be used directly as a gas Whether the perfluorocarbon is used as a liquid or a gas generally depends on its liquid/gas phase transition temperature, or boiling point For example, a prefeπed perfluorocarbon, perfluoropentane, has a liquid/gas phase transition temperature (boiling point) of 29 5°C This means that perfluoropentane is generally a liquid at room temperature (about 25 °C), but is converted to a gas within the human body, the normal temperature of which is about 37°C, this temperature bemg above the transition temperature of perfluoropentane Thus, under normal circumstances, perfluoropentane is a gaseous precursor As a further example, there are the homologs of perfluoropentane, namely perfluorobutane and perfluorohexane The liquid/gas transition of perfluorobutane is 4 ° C and that of perfluorohexane is 57° C Thus, perfluorobutane may be useful as a gaseous precursor, although more likely as a gas, whereas perfluorohexane may be useful as a gaseous precursor because of its relatively high boiling point As known to one of ordinary skill in the art, the effective boiling point of a substance may be related to die pressure to which that substance is exposed This relationship is exemplified by the ideal gas law PV = nRT, where P is pressure, V is volume, n is moles of substance, R is the gas constant, and T is temperature The ideal gas law mdicates that as pressure increases, the effective boiling point increases also Conversely, as pressure decreases, the effective boiling pomt decreases A wide variety of mateπals may be used as temperature-sensitive gaseous precursors the compositions descπbed herem It is only required that the material be capable of undergoing a phase transition to the gas phase upon passing through the appropnate temperature Suitable gaseous precursors mclude, for example, hexafluoroacetone, isopropyl acetylene, allene, tetrafluoroallene, boron tnfluoπde, 1,2-butadιene, 2,3-butadιene, 1,3 -butadiene, l,2,3-tπchloro-2-fluoro-l,3-butadιene, 2-methyl-l, 3-butadιene, hexafluoro- 1,3 -butadiene, butadiyne, 1-fluorobutane, 2-methylbutane, perfluorobutane, 1-butene, 2-butene, 2-methyl-l -butene, 3 -methyl- 1- butene, perfluoro-1 -butene, perfluoro-2 -butene, 4-phenyl-3-butene-2-one, 2-methyl-l - butene-3-yne, butyl nitrate, 1-butyne, 2-butyne, 2-chloro-l,l,l,4,4,4-hexafluorobutyne, 3- methyl-1-butyne, perfluoro-2-butyne, 2-bromobutyraldehyde, carbonyl sulfide, crotonomtπle, cyclobutane, methylcyclobutane, octafluorocyclobutane, perfluorocyclo- butene, 3-chlorocyclopentene, perfluorocyclopentane, octafluorocyclopentene, cyclopropane, perfluorocyclopropane, 1 ,2-dιmethyl-cyclopropane, 1,1 -dimethyl- cyclopropane, 1,2-dιmethylcyclopropane, ethylcyclopropane, methylcyclopropane, diacetylene, 3-ethyl-3-methyl diaziπdine, 1,1,1-tπfluorodιazoethane, dimethyl amme, hexafluorodimethylamine, dimethylethylamine, bιs(dιmethylphosphme)amme, perfluorohexane, perfluoroheptane, perfluorooctane, 2,3-dιmethyl-2-norbornane, perfluorodimethylamme, dimethyloxonium chloπde, l,3-dιoxolane-2-one, 4-methyl- 1,1,1,2-tetrafluoroethane, 1,1,1-tπfluoroethane, 1,1,2,2-tetrafluoroethane, 1,1,2-tπchloro- 1 ,2,2-trιfluoroethane, 1,1-dιchloroethane, 1,1 -dichloro- 1,2,2,2-tetra- fluoroethane, 1,2-dιfluoroethane, l-chloro-l,l,2,2,2-pentafluoroethane, 2-chloro-l,] - difluoroethane, l,l-dιchloro-2-fluoroethane, l-chloro-l,l,2,2-tetrafluoroethane, 2-chloro- 1,1-dιfluoroethane, chloroethane, chloropentafluoroethane, dichlorotπfluoro- ethane, fluoroethane, perfluoroethane, nitiOpentafluoroethane, nitrosopentafluoroethane, perfluoroethylamme, ethyl vinyl ether, 1 , 1 -dichloroethane, 1 , 1 -dichloro- 1 ,2- difluoroethane, 1,2-dιfluoroethane, methane, tnfluoromethanesulfonyl chloride, tnfluoromethanesulfonylfluoride, bromodifluoronitrosomethane, bromofluoro- methane, bromochlorofluoromethane, bromotπfluoromethane, chlorodifluoromtro- methane, chlorodmitromethane, chlorofluoromethane, chlorotπfluoromethane, chlorodifluoromethane, dibromodifluoromethane, dichlorodifluoromethane, dichloro- fluoromethane, difluoromethane, difluoroiodomethane, disilanomethane, fluoromethane, lodomethane, lodotπfluoromethane, nitrotπfluoromethane, nitrosotπfluoromethane, tetrafluoromethane, tnchlorofluoromethane, tπfluoromethane, 2-methylbutane, methyl ether, methyl isopropyl ether, methyllactate, methylnitπte, methylsulfide, methyl vinyl ether, neopentane, nitrous oxide, 1,2,3-nonadecanetπcarboxylιc acid 2-hydroxy- tπmethyl ester, l-nonene-3-yne, 1 ,4-pentadιene, «-pentane, perfluoropentane, 4-ammo-4- methylpentan-2-one, 1-pentene, 2-pentene (cis and trans), 3-bromopent-l-ene, perfluoropent-1-ene, tetrachlorophthalic acid, 2,3,6-tπmethyl-pιpeπdme, propane, 1,1,1,2,2,3-hexafluoropropane, 1 ,2-epoxypropane, 2,2-dιfluoropropane, 2-ammopropane, 2-chloropropane, heptafluoro-1-mtropropane, heptafluoro-1-nιtrosopropane, perfluoropropane, propene, hexafluoropropane, 1,1, 1,2,3, 3-hexafluoro-2,3- dichloropropane, 1 -chloropropane, chloropropane-(trans), 2-chloropropane, 3-fluoropropane, propyne, 3,3,3-tπfluoropropyne, 3-fluorostyrene, sulfur (dι)-decafluoπde (S 2 F 10 ), 2,4-dιammotoluene, tπfluoroacetomtnle, tπfluoromethyl peroxide, tπfluoromethyl sulfide, tungsten hexafluoπde, vinyl acetylene and vinyl ether Perfluorocarbons are both prefeπed gases and prefeπed gaseous precursors for use in connection with the compositions employed in the methods of the present mvention Included among such perfluorocarbons are saturated perfluorocarbons, unsaturated perfluorocarbons, and cyclic perfluorocarbons The saturated perfluorocarbons, which are usually prefeπed, have the formula C n F 2r 2 , where n is from 1 to about 12, preferably about 2 to about 10, more preferably about 3 to about 8, and even more preferably about 3 to about 6 Suitable perfluorocarbons include, for example, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane, perfluoroheptane, perfluorooctane and perfluorononane Preferably, the perfluorocarbon is selected from the group consisting of perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane, perfluoroheptane and perfluorooctane, with perfluoropropane being particularly preferred Cyclic perfluorocarbons, which have the formula C n F 2n , where n is from 3 to 8, preferably 3 to 6, may also be preferred, and include, for example, hexafluorocyclopropane, octafluorocyclobutane, and decafluorocyclopentane Generally speaking, perfluorocarbons containing about 4 carbons or less are gases at room temperature, whereas perfluorocarbons contammg from about 5 to about 12 carbons are liquids at room temperature In either case, both liquid and/or gaseous perfluorocarbons may be incorporated in the compositions of the present invention In addition to the perfluorocarbons, it may be desirable to utilize stable fluorocarbons which are not completely fluonnated Such fluorocarbons mclude heptafluoropropane, for example, 1,1,1,2,3,3,3-heptafluoropropane and its isomer, 1 , 1 ,2,2,3 ,3 ,3 -heptafluoropropane The gaseous precursor materials may be also photoactivated materials, such as diazonium ion and aminomalonate Certain hpid and/or vesicle compositions, and particularly vesicle compositions, may be formulated so that gas can be formed at the target tissue or by the action of sound on the composition Examples of gaseous precursors are described, for example, in U S Patent Nos 5,088,499 and 5,149,319, the disclosures of which are hereby incorporated herein by reference, in their entirety Other gaseous precursors, in addition to those exemplified above, will be apparent to one skilled in the art based on the present disclosure The gaseous substances and/or gaseous precursors are preferably incorporated in the pid and or vesicle compositions lnespective of the physical nature of the composition Thus, it is contemplated that the gaseous substances and/or precursors thereto may be incoφorated, for example, in hpid compositions m which the lipids are aggregated randomly, as well as in vesicle compositions, including vesicle compositions which are formulated from lipids, such as micelles and hposomes Incoφoration of the gaseous substances and/or precursors thereto in the lipid and/or vesicle compositions may be achieved by using any of a number of methods For example, in the case of vesicles based on lipids, the formation of gas filled vesicles can be achieved by shaking or otherwise agitating an aqueous mixture which compnses a gas or gaseous precursor and one or more lipids This promotes the formation of stabilized vesicles within which the gas or gaseous precursor is encapsulated In addition, a gas may be bubbled directly into an aqueous mixture of pid and/or vesicle-forming compounds Alternatively, a gas instillation method can be used as disclosed, for example, in U S Patent Nos 5,352,435 and 5,228,446, the disclosures of which are hereby incoφorated herem by reference, in their entirety Suitable methods for mcoφorating the gas or gaseous precursor m cationic hpid compositions are disclosed also in U S Patent No 4,865,836, the disclosures of which are hereby incoφorated herein by reference in their entirety Other methods would be apparent to one skilled in the art based on the present disclosure Preferably, the gas may be instilled in the hpid and/or vesicle compositions after or during the addition of the stabilizing material and/or during formation of vesicles In prefeπed embodiments, the gaseous substances and/or gaseous precursor matenals are incoφorated in vesicle compositions, with micelles and hposomes being prefeπed As discussed in detail below, vesicles in which a gas or gaseous precursor or both are encapsulated are advantageous m that they provide improved reflectivity in vivo As discussed more fully hereinafter, it is preferred that the hpid compositions, and especially the vesicle compositions, be formulated from lipids and optional stabilizing compounds to promote the formation of stable vesicles In addition, it is also preferred that the hpid and/or vesicle compositions comprise a highly stable gas as well The phrase "highly stable gas" refers to a gas which has limited solubility and diffusabihty m aqueous media Exemplary highly stable gases include perfluorocarbons since they are generally less diffusible and relatively insoluble m aqueous media Accordmgly, their use may promote the formation of highly stable vesicles In certain embodiments, it may be desirable to use a fluonnated compound, especially a perfluorocarbon compound, which may be in the liquid state at the temperature of use of the hpid and/or vesicle compositions, including, for example, the in vivo temperature of the human body, to assist or enhance the stability of the pid and/or vesicle compositions, and especially, the gas filled vesicles Suitable fluonnated compounds include, for example, fluonnated surfactants, such as fluonnated surfactants which are commercially available as ZONYL® surfactants (the DuPont Company, Wilmington, DE), as well as liquid perfluorocarbons, such as for example, perfluoro- octylbromide (PFOB), perfluorodecahn, perfluorododecalin, perfluorooctyl iodide, perfluorotπpropylamine, and perfluorotπbutylamme In general, perfluorocarbons comprising about six or more carbon atoms will be liquids at normal human body temperature Among these perfluorocarbons, perfluorooctylbromide and perfluorohexane, which are liquids at room temperature, are preferred The gas which is present may be, for example, nitrogen or perfluoropropane, or may be denved from a gaseous precursor, which may also be a perfluorocarbon, for example, perfluoropentane In the latter case, the pid and/or vesicle compositions may be prepared from a mixture of perfluorocarbons, which for the examples given, would be perfluoropropane (gas) or perfluoropentane (gaseous precursor) and perfluorooctylbromide (liquid) Although not intending to be bound by any theory or theoπes of operation, it is believed that, in the case of vesicle compositions, the liquid fluonnated compound may be situated at the mterface between the gas and the membrane or wall surface of the vesicle There may be thus formed a further stabilizing layer of liquid fluonnated compound on the internal surface of the stabilizing compound, for example, a biocompatible hpid used to form the vesicle, and this perfluorocarbon layer may also prevent the gas from diffusing through the vesicle membrane A gaseous precursor, within the context of the present mvention, is a liquid at the temperature of manufacture and/or storage, but becomes a gas at least at or during the time of use Thus, it has been discovered that a liquid fluonnated compound, such as a perfluorocarbon, when combmed with a gas or gaseous precursor ordmaπly used to make the hpid and/or vesicle compositions descnbed herem, may confer an added degree of stability not otherwise obtainable with the gas or gaseous precursor alone Thus, it is within the scope of the present invention to utilize a gas or gaseous precursor, such as a perfluorocarbon gaseous precursor, for example, perfluoropentane, together with a perfluorocarbon which remains liquid after administration to a patient, that is, whose liquid to gas phase transition temperature is above the body temperature of the patient, for example, perfluorooctylbromide Perfluoπnated surfactants, such as ZONYL® fluonnated surfactants, may be used to stabilize the hpid and/or vesicle compositions, and to act, for example, as a coating for vesicles Prefeπed perfluonnated surfactants are the partially fluonnated phosphocho ne surfactants In these preferred fluonnated surfactants, the dual alkyl compounds may be fluonnated at the terminal alkyl chains and the proximal carbons may be hydrogenated These fluonnated phosphochohne surfactants may be used for making the targeted pid and/or vesicle compositions employed in the methods of the present invention In connection with embodiments involving vesicle compositions, the size of the vesicles can be adjusted for the particular intended end use including, for example, diagnostic and/or therapeutic use The size of the vesicles may preferably range from about 30 nanometers (nm) to about 100 micrometers (μm) in diameter, and all combinations and subcombinations of ranges therein More preferably, the vesicles have diameters of from about 100 nm to about 10 μm, with diameters of from about 200 nm to about 7 μm bemg even more preferred In connection with particular uses, for example, lntravascular use, mcludmg magnetic resonance imagmg of the vasculature, it may be prefeπed that the vesicles be no larger that about 30 μm in diameter, with smaller vesicles being prefeπed, for example, vesicles of no larger than about 12 μm in diameter In certain prefeπed embodiments, the diameter of the vesicles may be about 7 μm or less, with vesicles having a mean diameter of about 5 μm or less being more prefeπed, and vesicles having a mean diameter of about 3 μm or less being even more prefeπed It is contemplated that these smaller vesicles may perfuse small vascular channels, such as the microvasculature, while at the same time providing enough space or room within the vascular channel to permit red blood cells to slide past the vesicles It is contemplated also that these smaller vesicles may be capable of travelmg throughout the vasculature at about the same rate of flow as the blood and thus do not impede or substantially impede normal blood flow The size of the gas filled vesicles can be adjusted, if desired, by a variety of procedures cludmg, for example, shaking, microemulsification, vortexing, extrusion, filtration, sonication, homogenization, repeated freezing and thawing cycles, extrusion under pressure through pores of defined size, and similar methods. As noted above, compositions employed herem may also include, with respect to their preparation, formation and use, gaseous precursors that can be activated to change from a liquid or solid state into a gas by temperature, pH, light, and energy (such as ultrasound). The gaseous precursors may be made into gas by storing the precursors at reduced pressure. For example, a vial stored under reduced pressure may create a headspace of perfluoropentane or perfluorohexane gas, useful for creating a preformed gas prior to injection. Preferably, the gaseous precursors may be activated by temperature. Set forth below is a table listing a series of gaseous precursors which undergo phase transitions from liquid to gaseous states at relatively close to normal body temperature (37°C) or below, and the size of the emulsified droplets that would be required to form a vesicle of a maximum size of 10 μm. TABLE 2 Physical Characteristics of Gaseous Precursors and Diameter of Emulsified Droplet to Form a 10 μm Vesicle' Diameter (μm) of emulsified droplet Molecular Boiling Point to make 10 micron Compound Weight (° C) Density vesicle perfluoro 288.04 29.5 1.7326 2.9 pentane 1- 76.11 32.5 0.67789 1.2 fluorobutane 2-methyl 72.15 27.8 0.6201 2.6 butane (isopentane) TABLE 2 Physical Characteristics of Gaseous Precursors and Diameter of Emulsified Droplet to Form a 10 μm Vesicle * Diameter (μm) of emulsified droplet Molecular Boiling Point to make 10 micron Compound Weight (° C) Density vesicle 2-methyl 1- 70.13 31.2 0.6504 2.5 butene 2-methyl-2- 70.13 38.6 0.6623 2.5 butene l-butene-3- 66.10 34.0 0.6801 2.4 yne-2-methyl 3 -methyl- 1- 68.12 29.5 0.6660 2.5 butyne octafluoro 200.04 -5.8 1.48 2.8 cyclobutane decafluoro 238.04 -2 1.517 3.0 butane hexafluoro 138.01 -78.1 1.607 2.7 ethane "Source: Chemical Rubber Company Handbook of Chemistry and Physics, Robert C. Weast and David R. Lide, eds., CRC Press, Inc. Boca Raton, Florida (1989-1990). The perfluorocarbons, as already indicated, are prefeπed for use as the gas or gaseous precursors, as well as additional stabilizing components. As noted above, it is prefeπed to optimize the utility of the lipid and/or vesicle compositions, especially vesicle compositions formulated from lipids, by using gases of limited solubility. The phrase "limited solubility" refers to the ability of the gas to diffuse out of the vesicles by virtue of its solubility in the surrounding aqueous medium A greater solubility in the aqueous medium imposes a gradient with the gas in the vesicle such that the gas may have a tendency to diffuse out of the vesicle A lesser solubility in the aqueous milieu, may, on the other hand, decrease or eliminate the gradient between the vesicle and the mterface such that diffusion of the gas out of the vesicle may be impeded Preferably, the gas entrapped in the vesicle has a solubility less than that of oxygen, that is, about 1 part gas in about 32 parts water See Matheson Gas Data Book, 1966, Matheson Company Inc More preferably, the gas entrapped in the vesicle possesses a solubility in water less than that of air, and even more preferably, the gas entrapped in the vesicle possesses a solubility m water less than that of nitrogen It may be desirable, in certain embodiments, to formulate vesicles from substantially impermeable polymeric materials In these embodiments, it is generally unnecessary to employ a gas which is highly insoluble also For example, stable vesicle compositions which comprise substantially impermeable polymeric materials may be formulated with gases having higher solubilities, for example, air or nitrogen In addition to, or instead of, the hpid and or polymeric compounds discussed above, the compositions descπbed herein may comprise one or more stabilizing materials Exemplary of such stabilizing materials are, for example, biocompatible polymers The stabilizing matenals may be employed to desirably assist in the formation of vesicles and/or to assure substantial encapsulation of the gases or gaseous precursors Even for relatively insoluble, non-diffusible gases, such as perfluoropropane or sulfiir hexafluonde, improved vesicle compositions may be obtained when one or more stabilizing matenals are utilized in the formation of the gas and gaseous precursor filled vesicles These compounds may help improve the stability and the integnty of the vesicles with regard to their size, shape and/or other attπbutes The teπns "stable" or "stabilized", as used herein, means that the vesicles may be substantially resistant to degradation, mcludmg, for example, loss of vesicle structure or encapsulated gas or gaseous precursor, for a useful period of tome Typically, the vesicles employed in the present invention have a desirable shelf life, often retaining at least about 90 % by volume of its original structure for a period of at least about two to three weeks under normal ambient conditions In prefeπed foπn, the vesicles are desirably stable for a period of time of at least about 1 month, more preferably at least about 2 months, even more preferably at least about 6 months, still more preferably about eighteen months, and yet more preferably up to about 3 years The vesicles descnbed herem, includmg gas and gaseous precursor filled vesicles, may also be stable even under adverse conditions, such as temperatures and pressures which are above or below those experienced under normal ambient conditions The stability of the vesicles descπbed herein may be attributable, at least in part, to the materials from which the vesicles are made, including, for example, the lipids, protems, and/or polymers descnbed above, and it is often not necessary to employ additional stabilizing materials, although it is optional and may be preferred to do so Such additional stabilizing materials and their charactenstics are described more fully hereinafter The materials from which the vesicles are constructed are preferably biocompatible hpid, protein, and/or polymer materials, and of these, the biocompatible lipids are prefeπed In addition, because of the ease of formulation, mcludmg the capability of prepaπng vesicles immediately prior to administration, these vesicles may be conveniently made on site Particularly preferred embodiments of the present invention involve vesicles which comprise three components (1) a neutral hpid, for example, a nonionic or zwitteπonic pid, (2) a negatively charged pid, and (3) a pid bearmg a stabilizing mateπal, for example, a hydrophilic polymer Preferably, the amount of the negatively charged pid will be greater than about 1 mole percent of the total hpid present, and the amount of hpid bearing a hydrophilic polymer will be greater than about 1 mole percent of the total hpid present Exemplary and prefeπed negatively charged lipids mclude phosphatidic acids The hpid bearmg a hydrophilic polymer will desirably be a hpid covalently linked to the polymer, and the polymer will preferably have a weight average molecular weight of from about 400 to about 100,000 Suitable hydrophilic polymers are preferably selected from the group consisting of polyethylene glycol (PEG), polypropylene glycol, polyvmyl alcohol, and polyvmylpyrrolidone and copolymers thereof, with PEG polymers being prefeπed Preferably, the PEG polymer has a molecular weight of from about 1000 to about 7500, with molecular weights of from about 2000 to about 5000 bemg more prefeπed The PEG or other polymer may be bound to the hpid, for example, DPPE, through a covalent bond, such as an amide, carbamate or amme lmkage In addition, the PEG or other polymer may be l ked to a targeting ligand, or other phosphohpids, with a covalent bond including, for example, amide, ester, ether, thioester, thioamide or disulfide bonds Where the hydrophilic polymer is PEG, a hpid bearmg such a polymer will be said to be "pegylated " In preferred form, the hpid bearmg a hydrophilic polymer may be DPPE-PEG, including, for example, DPPE-PEG5000, which refers to DPPE having a polyethylene glycol polymer of a mean weight average molecular weight of about 5000 attached thereto (DPPE-PEG5000) Another suitable pegylated hpid is distearoylphosphatidylethanolamine-polyethylene glycol 5000 (DSPE- PEG5000) In certain prefeπed embodiments of the present invention, the pid compositions may include about 77 5 mole % DPPC, 12 5 mole % of DPPA, and 10 mole % of DPPE-PEG5000 Also preferred are compositions which comprise about 80 to about 90 mole % DPPC, about 5 to about 15 mole % DPPA and about 5 to about 15 mole % DPPE-PEG5000 Especially preferred are compositions which compnse DPPC, DPPA and DPPE-PEG5000 in a mole % ratio of 82 10 8, respectively DPPC is substantially neutral, since the phosphatidyl portion is negatively charged and the choline portion is positively charged Consequently, DPPA, which is negatively charged, may be added to enhance stabilization in accordance with the mechanism described above DPPE-PEG provides a pegylated mateπal bound to the hpid membrane or skm of the vesicle by the DPPE moiety, with the PEG moiety free to suπound the vesicle membrane or skin, and thereby form a physical barner to various enzymatic and other endogenous agents in the body whose function is to degrade such foreign materials The DPPE-PEG may provide more vesicles of a smaller size which are safe and stable to pressure when combined with other lipids, such as DPPC and DPPA, m the given ratios It is also theonzed that the pegylated material, because of its structural similarity to water, may be able to defeat the action of the macrophages of the human immune system, which would otherwise tend to surround and remove the foreign object The result is an increase in the time durmg which the stabilized vesicles may function as diagnostic imagmg contrast media The vesicle compositions may be prepared from other materials, in addition to the materials descnbed above, provided that the vesicles so prepared meet the stability and other cntena set forth herein These materials may be basic and fundamental, and form the pnmary basis for creating or establishing the stabilized gas and gaseous precursor filled vesicles On the other hand, they may be auxiliary, and act as subsidiary or supplementary agents which can enhance the functioning of the basic stabilizing material or materials, or contribute some desired property m addition to that afforded by the basic stabilizing mateπal However, it is not always possible to determine whether a given material is a basic or an auxiliary agent, smce the functioning of the matenal in question is determined empirically, for example, by the results produced with respect to producing stabilized vesicles As examples of how these basic and auxiliary materials may function, it has been observed that the simple combmation of a biocompatible hpid and water or saline when shaken will often give a cloudy solution subsequent to autoclavmg for sterilization Such a cloudy solution may function as a contrast agent, but is aesthetically objectionable and may imply instability m the form of undissolved or undispersed hpid particles Cloudy solutions may be also undesirable where the undissolved particulate matter has a diameter of greater than about 7 μm, and especially greater than about 10 μm Manufacturing steps, such as sterile filtration, may also be problematic with solutions which contain undissolved particulate matter Thus, propylene glycol may be added to remove this cloudiness by facilitating dispersion or dissolution of the pid particles The propylene glycol may also function as a wetting agent which can improve vesicle formation and stabilization by increasing the surface tension on the vesicle membrane or skin It is possible that the propylene glycol can also function as an additional layer that may coat the membrane or skm of the vesicle, thus providing additional stabilization As examples of such further basic or auxiliary stabilizing matenals, there are conventional surfactants which may be used, see D'.Aπιgo U S Patents Nos 4,684,479 and 5,215,680 Additional auxiliary and basic stabilizing matenals mclude such agents as peanut oil, canola oil, olive oil, safflower oil, com oil, or any other oil commonly known to be ingestible which is suitable for use as a stabilizing compound m accordance with the teachings herein Vaπous auxiliary and basic stabilizing matenals are disclosed, for example, in U S application Serial No 08/444,574, filed May 19, 1995, the disclosures of which are incoφorated herem by reference, in their entirety In addition, compounds used to make mixed micelle systems may be suitable for use as basic or auxiliary stabilizing matenals, and these mclude, for example, lauryltπmethylammomum bromide (dodecyl-), cetyltπmethylammomum bromide (hexadecyl-), mynstyltnmethylammomum bromide (tetradecyl-), alkyldimethyl- benzylammomum chloride (where alkyl is C 12 , C or C 16) ), benzyldimethyldodecyl- ammonium bromide/chloride, benzyldimethyl hexadecylammonium bromide/chloride, benzyldimethyl tetradecylammomum bromide/chloπde, cetyldimethylethylammonium bromide/chloride, or cetylpyridinium bromide/chloride It has also been found that the gas and gaseous precursor filled vesicles used in the present invention may be controlled according to size, solubility and heat stability by choosing from among the various additional or auxiliary stabilizing matenals descπbed herein These matenals can affect these parameters of the vesicles, especially vesicles formulated from lipids, not only by their physical interaction with the membranes, but also by their ability to modify the viscosity and surface tension of the surface of the gas and gaseous precursor filled vesicle Accordingly, the gas and gaseous precursor filled vesicles used in the present invention may be favorably modified and further stabilized, for example, by the addition of one or more of a wide variety of (a) viscosity modifiers, includmg, for example, carbohydrates and their phosphorylated and sulfonated denvatives, polyethers, preferably with molecular weight ranges between 400 and 100,000, and di- and tnhydroxy alkanes and their polymers, preferably with molecular weight ranges between 200 and 50,000, (b) emulsifying and/or solubi zing agents including, for example, acacia, cholesterol, diethanolamine, glyceryl monostearate, lanolin alcohols, lecithin, mono- and di-glyceπdes, mono-ethanolamine, oleic acid, oleyl alcohol, poloxamer, for example, poloxamer 188, poloxamer 184, and poloxamer 181, polyoxyethylene 50 stearate, polyoxyl 35 castor oil, polyoxyl 10 oleyl ether, polyoxyl 20 cetostearyl ether, polyoxyl 40 stearate, polysorbate 20, polysorbate 40, polysorbate 60, polysorbate 80, propylene glycol diacetate, propylene glycol monostearate, sodium lauryl sulfate, sodium stearate, sorbitan mono-laurate, sorbitan mono-oleate, sorbitan mono-palmitate, sorbitan monostearate, steaπc acid, trolamine, and emulsifying wax, (c) suspending and/or viscosity-increasing agents, mcludmg, for example, acacia, agar, alginic acid, aluminum mono-stearate, bentonite, magma, carbomer 934P, carboxymethylcellulose, calcium and sodium and sodium 12, caπageenan, cellulose, dextran, gelatin, guar gum, locust bean gum, veegum, hydroxyethyl cellulose, hydroxypropyl methylcellulose, magnesium-aluminum-sihcate, Zeolites®, methylcellulose, pectin, polyethylene oxide, povidone, propylene glycol algmate, silicon dioxide, sodium alginate, tragacanth, xanthan gum, α-d-gluconolactone, glycerol and mannitol, (d) synthetic suspending agents, such as polyethylene glycol (PEG), polyvinylpyπolidone (PVP), polyvmyl alcohol (PVA), polypropylene glycol (PPG), and polysorbate, and (e) tonicity raising agents which stabilize and add tonicity, including, for example, sorbitol, mannitol, trehalose, sucrose, propylene glycol and glycerol As discussed above, die compositions of the present invention, mcludmg gas and/or gaseous precursor filled vesicles, are useful as contrast agents for diagnostic imaging, includmg, for example, ultrasound (US) imaging, computed tomography (CT) imaging, including CT angiography (CTA) imaging, magnetic resonance (MR) imagmg, magnetic resonance angiography (MRA), nuclear medicine, optical imaging and elastography In carrying out the magnetic resonance imagmg method of the present invention, the contrast agent can be used alone, or in combination with other diagnostic, therapeutic or other agents Such other agents include excipients such as flavoring or coloring matenals The magnetic resonance imaging techniques which are employed are conventional and are described, for example, in D M Kean and M A Smith, Magnetic Resonance Imaging Principles and Applications, (William and Wilkins, Baltimore 1986) Contemplated MRI techmques include, but are not limited to, nuclear magnetic resonance (NMR) and electronic spin resonance (ESR) The prefeπed imagmg modality is NMR Exemplary paramagnetic contrast agents suitable for use in the present compositions include, for example, stable free radicals, such as, for example, stable mtroxides, as well as compounds compnsmg transition, lanthanide and actinide elements, which may, if desired, be in the form of a salt or may be covalently or non-covalently bound to complexmg agents, mcludmg lipophi c denvatives thereof, or to proteinaceous macromolecules Preferable transition, lanthanide and actinide elements include, for example, Gd(m), Mn(H), Cu(D), Cr(ffl), Fe(H), Fe(m), Co(12), Er(II), Nι(ff), Eu(Hι) and Dy(m) More preferably, the elements may be Gd(HI), Mn(II), Cu(ff), Fe(H), Fe(IH), Eu(m) and Dy(m), especially Mn(H) and Gd(UI) The foregomg elements may, if desired, be in the form of a salt, includmg inorganic salts, such as a manganese salt, for example, manganese chloride, manganese carbonate, manganese acetate, and organic salts, such as manganese gluconate and manganese hydroxylapatote Other exemplary salts include salts of iron, for example, iron sulfides and ferric salts such as ferric chloride These elements may also, if desired, be bound, for example, through covalent or noncovalent association, to complexmg agents, including lipophihc derivatives thereof, or to proteinaceous macromolecules Preferable complexmg agents include, for example, diethylenetnaminepentaacetic acid (DTPA), ethylene-diaminetetraacetic acid (EDTA), l,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA), l,4,7,10-tetraazacyclododecane-N,N',N"-tπacetιc acid (DOTA), 3,6,9-tnaza-12-oxa-3,6,9- tπcarboxymethylene-lO-carboxy-13-phenyl-tπdecanoιc acid (B-19036), hydroxybenzylethylenediamine diacetic acid (HBED), N,N'-bιs(pyπdoxyl-5- phosphate)ethylene diarnme, N,N'-dιacetate (DPDP), 1 ,4,7-trιazacyclononane-N,N',N"- tπacetic acid (NOTA), 1,4,8,1 l-tetraazacyclotetradecane-N,N',N",N'"-tetraacetιc acid (TETA), kryptands (macrocyc c complexes), and desferπoxam e More preferably, the complexmg agents are EDTA, DTPA, DOTA, DO3A and kryptands, most preferably DTPA Preferable lipophihc complexes include alkylated derivatives of the complexmg agents EDTA, DOTA, for example, N,N'-bιs-(carboxydecylamιdomethyl-N-2,3- dιhydroxypropyl)-ethylenedιamιne-N,N'-dιacetate (EDTA-DDP), N,N'-bιs-(carboxy- octadecylamιdo-methyl-N-2,3-dιhydroxypropyl)-ethylenedιamιne-N,N'-dιacetate (EDTA- ODP), N,N , -Bιs(carboxy-laurylamιdomethyl-N-2,3-dιhydroxypropyl)ethylenedιamme- N,N'-dιacetate (EDTA-LDP), and the like, includmg those described in U S Patent No 5,312,617, the disclosures of which are hereby incoφorated herein by reference, in their entirety Preferable proteinaceous macromolecules mclude, for example, albumin, collagen, polyarginine, polylysine, polyhishdine, γ-globuhn and β-globulin, with albumin, polyarginine, polylysine, and polyhistidine bemg more prefeπed Suitable complexes therefore include Mn(II)-DTPA, Mn(II)-EDTA, ' Mn(n)-DOTA, Mn(JT)-DO3A, Mn(II)-kryptands, Gd(m)-DTPA, Gd(JTI)-DOTA, Gd(lTJ)- DO3A, Gd(m)-kryptands, Cr(m)-EDTA, Cu(D)-EDTA, or lron-desfeπioxamme, especially Mn(D)-DTPA or Gd(IU)-DTPA Nitroxides are paramagnetic contrast agents which mcrease both Tl and T2 relaxation rates on MRI by virtue of the presence of an unpaired electron the nitroxide molecule As known to one of ordinary skill m the art, the paramagnetic effectiveness of a given compound as an MRI contrast agent may be related, at least in part, to the number of unpaired electrons in the paramagnetic nucleus or molecule, and specifically, to the square of the number of unpaired electrons For example, gadolinium has seven unpaired electrons whereas a nitroxide molecule has one unpaired electron Thus, gadolinium is generally a much stronger MRI contrast agent than a nitroxide However, the effective correlation time, another important parameter for assessing the effectiveness of contrast agents, confers potential increased relaxivity to the nitroxides When the tumbling rate is slowed, for example, by attaching the paramagnetic contrast agent to a large molecule, it will tumble more slowly and thereby more effectively transfer energy to hasten relaxation of the water protons In gadolinium, however, the electron spin relaxation time is rapid and will limit the extent to which slow rotational correlation times can mcrease relaxivity For nitroxides, however, the electron spin correlation times are more favorable and tremendous increases m relaxivity may be attained by slowing the rotational correlation time of these molecules The gas filled vesicles of the present invention are ideal for attaining the goals of slowed rotational correlation times and resultant improvement relaxivity Although not intendmg to be bound by any particular theory of operation, it is contemplated that smce the nitroxides may be designed to coat the perimeters of the vesicles, for example, by making alkyl deπvatives thereof, the resulting correlation times can be optimized Moreover, the resulting contrast medium of the present mvention may be viewed as a magnetic sphere, a geometric configuration which maximizes relaxivity If desired, the nitroxides may be alkylated or otherwise deπvatized, such as the nitroxides 2,2,5, 5-tetramethyl-l-pyrrolιdιnyloxy, free radical, and 2,2,6,6-tetramethyl- 1 -pipendmyloxy, free radical (TMPO) Exemplary supeφaramagnetic contrast agents suitable for use in the compositions of the present invention mclude metal oxides and sulfides which expeπence a magnetic domain, feπo- or fernmagnetoc compounds, such as pure iron, magnetic iron oxide, such as magnetite, γ-Fe 2 O,, Fe 3 O 4 , manganese fernte, cobalt fernte and nickel fe ite Paramagnetic gases can also be employed m the present compositions, such as oxygen 17 gas ( 17 O 2 ) In addition, hypeφolanzed xenon, neon, or helium gas may also be employed MR whole body imagmg may then be employed to rapidly screen the body, for example, for thrombosis, and ultrasound may be applied, if desired, to aid m thrombolysis The contrast agents, such as the paramagnetic and supeφaramagnetic contrast agents descnbed above, may be employed as a component within the hpid, protem, polymer, and/or vesicle compositions In the case of vesicle compositions, the aforementioned contrast agents may be entrapped within the internal void thereof, administered as a solution with the vesicles, incoφorated with any additional stabilizing materials, or coated onto the surface or membrane of the vesicle If desired, the paramagnetic or supeφaramagnetic agents may be delivered as alkylated or other derivatives incoφorated into the compositions, especially the hpidic walls of the vesicles In particular, the nitroxides 2,2,5, 5-tetramethyl-l -pyrrohdinyloxy, free radical and 2,2,6,6-tetramethyl-l-pιperιdιnyloxy, free radical, can form adducts with long chain fatty acids at the positions of the ring which are not occupied by the methyl groups via a variety of linkages, including, for example, an acetyloxy linkage Such adducts are very amenable to incoφoration into the hpid and/or vesicle compositions of the present mvention Mixtures of any one or more of the paramagnetic agents and/or supeφaramagnetic agents in the present compositions may be used The paramagnetic and supeφaramagnetic agents may also be coadministered separately, if desired The hpid, protein, polymer, and/or vesicle compositions of the present invention, and especially the vesicle compositions, may serve not only as effective earners of the supeφaramagnetic agents descnbed above, but also may improve the effect of d e susceptibility contrast agents Supeφaramagnetic contrast agents include metal oxides, particularly iron oxides but including manganese oxides, and as iron oxides, contammg varying amounts of manganese, cobalt and nickel which experience a magnetic domain These agents are nano or microparticles and have very high bulk susceptibilities and transverse relaxation rates The larger particles, for example, particles having diameters of about 100 nm, have much higher R2 relaxivities as compared to Rl relaxivities The smaller particles, for example, particles havmg diameters of about 10 to about 15 nm, have somewhat lower R2 relaxivities, but much more balanced Rl and R2 values Much smaller particles, for example, monocrystalline iron oxide particles havmg diameters of about 3 to about 5 nm, have lower R2 relaxivities, but probably the most balanced Rl and R2 relaxation rates Ferntin can also be formulated to encapsulate a core of very high relaxation rate supeφaramagnetic iron It has been discovered that the hpid and/or vesicle compositions, especially vesicle compositions, mcludmg gas filled vesicles, can increase the efficacy and safety of these conventional iron oxide based MRI contrast agents The iron oxides may simply be incoφorated into the pid and/or vesicle compositions Preferably, in the case of vesicles formulated from lipids, the iron oxides may be incoφorated into the walls of the vesicles, for example, by bemg adsorbed onto the surfaces of the vesicles, or entrapped within the interior of the vesicles as described in U S Patent 5,088,499, the disclosures of which are hereby incoφorated herein by reference m their entirety Without bemg bound to any particular theory or theories of operation, it is believed that the compositions of the present invention, and especially the vesicle compositions, increase the efficacy of the supeφaramagnetic contrast agents by several mechanisms First, it is believed that the vesicles function to increase the apparent magnetic concentration of the iron oxide particles Also, it is believed that the vesicles increase the apparent rotational correlation time of the MRI contrast agents, including paramagnetic and supeφaramagnetic agents, so that relaxation rates are increased In addition, the vesicles appear to increase the apparent magnetic domain of the contrast medium accordmg to the manner descπbed hereinafter Certain of the vesicles of the present invention, and especially vesicles formulated from lipids, may be visualized as flexible spherical domains of differing susceptibility from the suspending medium, including, for example, the aqueous suspension of the contrast medium or blood or other body fluids, for example, in the case of intravascular injection or injection mto other body locations In the case of ferπtes or iron oxide particles, it should be noted that the contrast provided by these agents is dependent on particle size This phenomenon is very common and is often refeπed to as the "secular" relaxation of the water molecules Described in more physical terms, this relaxation mechanism is dependent upon the effective size of the molecular complex which a paramagnetic atom, or paramagnetic molecule, or molecules, may reside One physical explanation may be descnbed in the following Solomon-Bloembergen equations which define the paramagnetic contributions as a function of the T, and T 2 relaxation times of a spm A nucleus with gyromagnetic ratio g perturbed by a paramagnetic ion 1/T.M = (2/15) S(S + 1) γ 2 g 2 β 2 /r 6 [3τ c /(l + ω, 2 τ c 2 ) + lτj{\ + ω s 2 τ c 2 )] + (2/3) S(S+ 1) A 2 /h 2 [τ e /(l + ω,2τ e 2 )] and 1/T-M = (1/15) S(S + 1) γ 2 g 2 β 2 /r 6 [4τ c + 3τc/(l +ω, 2 τ c 2 ) + 13-VO + w s 2 τ c 2 )] + (1/3) S(S+ 1)A 2 /h 2 [τ e /(I + ω s 2τ e 2 )] where S is the electron spin quantum number, g is the electronic g factor, β is the Bohr magneton, ωl and ω s (657 w-) is the Larmor angular precession frequencies for the nuclear spins and electron spins, r is the ion-nucleus distance, A is the hyperfme coupling constant, τ c and τ. are the correlation times for the dipolar and scalar interactions, respectively, and h is Planck's constant See, e g , Solomon, I Phys Rev Vol 99, p 559 (1955) and Bloembergen, N J Chem Phys Vol. 27, pp 572, 595 (1957) A few large particles may have a much greater effect than a larger number of much smaller particles, pnmanly due to a larger correlation time If one were to make the iron oxide particles very large however, increased toxicity may result, and the lungs may be embohzed or the complement cascade system may be activated Furthermore, it is believed that the total size of the particle is not as important as the diameter of the particle at its edge or outer surface The domain of magnetization or susceptibility effect falls off exponentially from the surface of the particle Generally speaking, in the case of dipolar (through space) relaxation mechanisms, this exponential fall off exhibits an r 6 dependence for a paramagnetic dipole-dipole mteraction Inteφreted literally, a water molecule that is 4 angstroms away from a paramagnetic surface will be influenced 64 times less than a water molecule that is 2 angstroms away from the same paramagnetic surface The ideal situation m terms of maximizing the contrast effect would be to make the iron oxide particles hollow, flexible and as large as possible It has not been possible to achieve this heretofore and it is believed that the benefits have been unrecogmzed heretofore also By coating the inner or outer surfaces of the vesicles with the contrast agents, even though the individual contrast agents, for example, iron oxide nanoparticles or paramagnetic ions, are relatively small structures, the effectiveness of the contrast agents may be greatly enhanced In so doing, the contrast agents may function as an effectively much larger sphere wherein the effective domain of magnetization is determmed by the diameter of the vesicle and is maximal at the surface of the vesicle These agents afford the advantage of flexibility, namely, compliance While rigid vesicles might lodge in the lungs or other organs and cause toxic reactions, these flexible vesicles slide through the capillaries much more easily In contrast to the flexible vesicles described above, it may be desirable, in certain circumstances, to formulate vesicles from substantially impermeable materials such as polymer materials, mcludmg, for example, polymethyl methacrylate This would generally result m the formation of vesicles which may be substantially impermeable and relatively inelastic and bnttle In embodiments involving diagnostic imagmg, for example, ultrasound, contrast media which comprise such bnttle vesicles would generally not provide the desirable reflectivity that the flexible vesicles may provide However, by increasing the power output on ultrasound, the brittle vesicles can be made to rupture, thereby causing acoustic emissions which can be detected by an ultrasound transducer Nuclear Medicine Imagmg (NMI) may also be used m connection with the diagnostic and therapeutic method aspects of the present invention For example, NMI may be used to detect radioactive gases, such as Xe 133 , which may be incoφorated m the present compositions m addition to, or instead of, the gases discussed above Such radioactive gases may be entrapped within vesicles for use m detecting, for example, thrombosis Preferably, bifunctional chelate deπvatives are incoφorated in the walls of vesicles, and the resulting vesicles may be employed in both NMI and ultrasound In this case, high energy, high quality nuclear medicine imagmg isotopes, such as technetium 99ra or indium 1 " can be incoφorated in the walls of vesicles Whole body gamma scannmg cameras can then be employed to rapidly localize regions of vesicle uptake in vivo If desired, ultrasound may also be used to confirm the presence, for example, of a clot within the blood vessels, smce ultrasound generally provides improved resolution as compared to nuclear medicine techmques NMI may also be used to screen the entire body of the patient to detect areas of vascular thrombosis, and ultrasound can be applied to these areas locally to promote rupture of the vesicles and treat the clot As noted above, the present compositions may also be employed in connection with computed tomography (CT) imaging CT suffers from various drawbacks, and is generally less effective as compared to the diagnostic techniques discussed above Nevertheless, if a high enough concentration of the present contrast media, and especially gas filled vesicle compositions, is delivered to the region of interest, for example, a blood clot, the clot can be detected on the CT images by virtue of a decrease in the overall density of the clot ln general, a concentration of about 1/10 of 1% of gas filled vesicles or higher (on a volume basis), may be needed to be delivered to the region of interest, including the aforementioned blood clot, to be detected by CT Also, for optical imagmg, optically active gases, such as argon or neon, may be incoφorated in the present compositions In addition, optically active matenals, for example, fluorescent materials, mcludmg poφhyrm deπvatives, may also be used Elastography is an imaging technique which generally employs much lower frequency sound, for example, about 60 KHz, as compared to ultrasound which can involve over frequencies of over 1 MHZ In elastography, the sound energy is generally applied to the tissue and the elasticity of the tissue may then be determined The hpid based vesicles descπbed herein are preferably highly elastic, and they may increase the local elasticity of tissue to which they are directed This increased elasticity may then be detected with elastography If desired, elastography can be used in conjunction with other imagmg techmques, such as MRI and ultrasound A wide vaπety of methods are available for the preparation of hpid, protem, polymer, and/or vesicle compositions, such as micelles and/or hposomes Included among these methods are, for example, shakmg, drying, gas-mstallation, spray drying, and the like Suitable methods for preparing vesicle compositions from lipids are descnbed, for example, in Unger et al , U S Patent No 5,469,854, the disclosures of which are hereby incoφorated herein by reference, in their entirety As noted above, the vesicles are preferably prepared from lipids which remain m the gel state With particular reference to the preparation of micelle compositions, the following discussion is provided Micelles may be prepared using any one of a vanety of conventional micellar preparatory methods which will be apparent to those skilled in the art These methods typically involve suspension of one or more hpid compounds in an organic solvent, evaporation of the solvent, resuspension in an aqueous medium, so cation and centrifugation The foregoing methods, as well as others, are discussed, for example, in Canfield et al , Methods in Enzymology, Vol 189, pp 418-422 (1990), El- Gorab et al, Biochem Biophys Acta, \o\ 306, pp 58-66 (1973), Colloidal Surfactant, Shinoda, K , Nakagana, Tamamushi and Isejura, Academic Press, NY (1963) (especially "The Formation of Micelles", Shinoda, Chapter 1, pp 1-88), Catalysis m Micellar and Macromolecular Systems, Fendler and Fendler, Academic Press, NY (1975) The disclosures of each of the foregoing publications are incoφorated by reference herein, in their entirety As noted above, the vesicle composition may comprise hposomes A wide variety of methods are available in connection with the preparation of liposome compositions Accordingly, the hposomes may be prepared using any one of a variety of conventional hposomal preparatory techmques which will be apparent to those skilled in the art These techniques include, for example, solvent dialysis, French press, extrusion (with or without freeze-thaw), reverse phase evaporation, simple freeze-thaw, so cation, chelate dialysis, homogenization, solvent infusion, microemulsification, spontaneous formation, solvent vapoπzatoon, solvent dialysis, French pressure cell technique, controlled detergent dialysis, and others, each involving the preparation of die vesicles in various fashions See, e g , Madden et al , Chemistry and Physics of Lipids, 1990 53, 37- 46, the disclosures of which are hereby incoφorated herein by reference m their entirety Suitable freeze-thaw techniques are descπbed, for example, in International Application Serial No PCT/US89/05040, filed November 8, 1989, the disclosures of which are incoφorated herem by reference in their entirety Methods which involve freeze-thaw techmques are preferred in connection with the preparation of hposomes Preparation of the hposomes may be earned out m a solution, such as an aqueous saline solution, aqueous phosphate buffer solution, or sterile water The hposomes may also be prepared by vanous processes which mvolve shakmg or vortexmg This may be achieved, for example, by the use of a mechanical shakmg device, such as a Wig-L-Bug™ (Crescent Dental, Lyons, IL), a Mixomat (Degussa AG, Frankfurt, Germany), a Capmix (Espe Fabπk Pharmazeutischer Praeparate GMBH & Co , Seefeld, Oberay Germany), a Silamat Plus (Vivadent, Liechtenstein), or a Vibros (Quayle Dental, Sussex, England) Conventional microemulsification equipment, such as a Microfluidizer™ (Microfluidics, Wobum, MA) may also be used Spray drying may be also employed to prepare the gas-filled vesicles Utilizing this procedure, the lipids may be pre-mixed in an aqueous environment and then spray dried to produce gas-filled vesicles The vesicles may be stored under a headspace of a desired gas Many hposomal preparatory techniques which may be adapted for use the preparation of vesicle compositions are discussed, for example, in U S Patent No 4,728,578, U K Patent Apphcation GB 2193095 A, U S Patent No 4,728,575, U S Patent No 4,737,323, International Application Serial No PCT/US 85/01 161, Mayer et al Biochimica et Biophysica Ada, Vol 858, pp 161 -168 (1986), Hope et al , Biochimica et Bιophysιca Acta, \o\ 812, pp 55-65 (1985), U S Patent No 4,533,254, Mayhew et al , Methods in Enzymology, Vol 149, pp 64-77 (1987), Mayhew et al , Biochimica et Biophysica Acta, Vol 755, pp 169-74 (1984), Cheng et al, Investigative Radiology, Vol 22, pp 47-55 (1987), International Application Senal No PCT/US89/05040, U S Patent No 4,162,282, U S Patent No 4,310,505, U S Patent No 4,921,706, and Liposome Technology, Gregonadis, G , ed , Vol I, pp 29-31, 51-67 and 79-108 (CRC Press Inc , Boca Raton, FL 1984), the disclosures of each of which are hereby incoφorated by reference herem, m their entirety Lipid compositions compnsmg a gas can be prepared by agitating an aqueous solution contammg, if desired, a stabilizing matenal, in the presence of a gas The term "agitating," as used herem, means any shaking motion of an aqueous solution such that gas may be introduced from the local ambient environment mto the aqueous solution This agitation is preferably conducted at a temperature below the gel to liquid crystalline phase transition temperature of the pid The shakmg involved in the agitation of the solutions is preferably of sufficient force to result in the formation of a hpid composition, mcludmg vesicle compositions, and particularly vesicle compositions compnsmg gas filled vesicles The shakmg may be by swirling, such as by vortexing, side-to-side, or up and down motion Different types of motion may be combmed Also, the shakmg may occur by shakmg the container holding the aqueous pid solution, or by shakmg the aqueous solution within the container without shakmg the container itself The shaking may occur manually or by machine Mechanical shakers that may be used include, for example, a shaker table such as a VWR Scientific (Cerπtos, CA) shaker table, as well as any of the shakmg devices descπbed hereinbefore, with the Capmix (Espe Fabπk Pharmazeutischer Praeparate GMBH & Co , Seefeld, Oberay Germany) being preferred It has been found that certain modes of shaking or vortexing can be used to make vesicles within a prefeπed size range Shakmg is prefeπed, and it is prefeπed that the shaking be caπied out using the Espe Capmix mechanical shaker In accordance with this prefeπed method, it is preferred that a reciprocating motion be utilized to generate the pid compositions, and particularly vesicle compositions It is even more prefeπed that the motion be reciprocating in the form of an arc It is contemplated that the rate of reciprocation, as well as the arc thereof, is particularly important m connection with the formation of vesicles Preferably, the number of reciprocations or full cycle oscillations may be from about 1000 to about 20,000 per mmute More preferably, the number of reciprocations or oscillations may be from about 2500 to about 8000 per minute, with from about 3300 to about 5000 reciprocations or oscillations per mmute bemg even more prefeπed Of course, the number of oscillations may be dependent upon the mass of the contents bemg agitated Generally speaking, a larger mass may require fewer oscillations Another means for producmg shakmg mcludes the action of gas emitted under high velocity or pressure It will also be understood that preferably, with a larger volume of aqueous solution, the total amount of force may be coπespondmgly increased Vigorous shaking is defined as at least about 60 shaking motions per minute, and is prefeπed Vortexing at about 60 to about 300 revolutions per mmute is more preferred Vortexing at about 300 to about 1800 revolutions per mmute is even more prefeπed In addition to the simple shakmg methods described above, more elaborate methods can also be employed Such elaborate methods mclude, for example, liquid crystalline shakmg gas instillation processes and vacuum drying gas instillation processes, such as those descnbed m copend g U S application Serial No 08/076,250, filed June 11, 1993, the disclosures of which are incoφorated herem by reference, in their entirety Although any of a number of varymg techmques can be used, the vesicle compositions employed m the present mvention are preferably prepared usmg a shakmg technique Preferably, the shakmg technique involves agitation with a mechanical shakmg apparatus, such as an Espe Capmix (Seefeld, Oberay Germany), using, for example, the techniques disclosed in copendmg U S apphcation Serial No 160,232, filed November 30, 1993, the disclosures of which are hereby incoφorated herein by reference in their entirety The size of gas filled vesicles can be adjusted, if desired, by a variety of procedures, including, for example, microemulsification, vortexing, extrusion, filtration, somcation, homogenization, repeated freezing and thawing cycles, extrusion under pressure through pores of defined size, and similar methods Gas filled vesicles prepared in accordance with the methods described herein can range in size from less than about 1 μm to greater than about 100 μm In addition, after extrusion and sterilization procedures, which are discussed m detail below, agitation or shaking may provide vesicle compositions which can contain substantially no or minimal residual anhydrous hpid phase in the remainder of the solution (Bangham, A D , Standish, M M, & Watkins, J C , Mol Biol Vol 13, pp 238-252 (1965) If desired, the vesicles may be used as they are formed, without any attempt at further modification of the size thereof For intravascular use, the vesicles preferably have diameters of less than about 30 μm, and more preferably, less than about 12 μm For targeted intravascular use includmg, for example, bmding to certain tissue, such as cancerous tissue, the vesicles may be significantly smaller, for example, less than about 100 nm in diameter For enteric or gastrointestinal use, the vesicles may be significantly larger, for example, up to a millimeter m size Preferably, the vesicles may be sized to have diameters of from about 2 μm to about 100 μm The gas filled vesicles may be sized by a simple process of extrusion through filters wherein the filter pore sizes control the size distnbution of the resulting gas filled vesicles By usmg two or more cascaded or stacked set of filters, for example, a 10 μm filter followed by an 8 μm filter, the gas filled vesicles can be selected to have a very naπow size distnbution around 7 to 9 μm After filtration, these gas filled vesicles can remain stable for over 24 hours The sizing or filtration step may be accomplished by the use, for example, of a filter assembly when the composition is removed from a sterile vial prior to use, or more preferably, the filter assembly may be incoφorated into a syringe dunng use The method of sizing the vesicles will then comprise usmg a syringe compnsmg a barrel, at least one filter, and a needle, and may be caπied out by a step of extracting which comprises extruding the vesicles from the baπel through the filter fitted to the syringe between the baπel and the needle, thereby sizing the vesicles before they are admmistered to a patient The step of extracting may also comprise drawing the vesicles into the syrmge, where the filter may function m the same way to size the vesicles upon entrance into the synnge Another alternative is to fill such a syringe with vesicles which have already been sized by some other means, in which case the filter may function to ensure that only vesicles withm the desired size range, or of the desired maximum size, are subsequently administered by extrusion from the syringe In certain prefeπed embodiments, the vesicle compositions may be heat sterilized or filter sterilized and extruded through a filter prior to shakmg Generally speaking, vesicle compositions compnsmg a gas may be heat sterilized, and vesicle compositions compnsmg gaseous precursors may be filter sterilized Once gas filled vesicles are formed, they may be filtered for sizing as descnbed above Performing these steps pnor to the formation of gas and gaseous precursor filled vesicles provide sterile gas filled vesicles ready for administration to a patient For example, a mixing vessel such as a vial or syrmge may be filled with a filtered pid and/or vesicle composition, and the composition may be sterilized within the mixing vessel, for example, by autoclaving Gas may be instilled mto the composition to form gas filled vesicles by shaking the stenle vessel Preferably, the sterile vessel is equipped with a filter positioned such that the gas filled vesicles pass through the filter before contacting a patient The step of extruding the solution of pid compound through a filter decreases the amount of unhydrated material by breaking up any dried materials and exposing a greater surface area for hydration Preferably, the filter has a pore size of about 0 1 to about 5 μm, more preferably, about 0 1 to about 4 μm, even more preferably, about 0 1 to about 2 μm, and still more preferably, about 1 μm Unhydrated compound, which is generally undesirable, appears as amoφhous clumps of non-uniform size The sterilization step provides a composition that may be readily admmistered to a patient for diagnostic imagmg mcludmg, for example, ultrasound, MRI or CT In certain preferred embodiments, sterilization may be accomplished by heat stenhzation, preferably, by autoclaving the solution at a temperature of at least about 100 ° C, and more preferably, by autoclaving at about 100 ° C to about 130 ° C, even more preferably, about 110°C to about 130°C, still more preferably, about 120°C to about 130 °C, and even more preferably, about 130 °C Preferably, heating occurs for at least about 1 mmute, more preferably, about 1 to about 30 minutes, even more preferably, about 10 to about 20 minutes, and still more preferably, about 15 minutes If desired, the extrusion and heating steps, as outiined above, may be reversed, or only one of the two steps can be used Other modes of sterilization may be used, including, for example, exposure to gamma radiation In addition to the aforementioned embodiments, gaseous precursors contained m vesicles can be formulated which, upon activation, for example, by exposure to elevated temperature, varymg pH, or light, may undergo a phase transition from, for example, a liquid, including a liquid entrapped in a vesicle, to a gas, expanding to create the gas filled vesicles described herein This technique is descπbed in detail m copending patent applications Serial Nos 08/160,232, filed November 30, 1993 and 08/159,687, filed November 30, 1993, the disclosures of which are incoφorated herein by reference, in their entirety The prefeπed method of activating the gaseous precursor is by exposure to elevated temperature Activation or transition temperature, and like terms, refer to the boiling point of the gaseous precursor and is the temperature at which the liquid to gaseous phase transition of the gaseous precursor takes place Useful gaseous precursors are those materials which have boiling points in the range of about - 100 ° C to about 70 ° C The activation temperature is particular to each gaseous precursor An activation temperature of about 37°C, or about human body temperature, is preferred for gaseous precursors m the context of the present invention Thus, in prefeπed form, a liquid gaseous precursor is activated to become a gas at about 37 °C or below The gaseous precursor may be in liquid or gaseous phase for use m the methods of the present mvention The methods of prepaπng the gaseous precursor filled vesicles may be earned out below the boiling pomt of the gaseous precursor such that a liquid is incoφorated, for example, mto a vesicle In addition, the methods may be conducted at the boiling pomt of the gaseous precursor, such that a gas is incoφorated, for example, into a vesicle For gaseous precursors having low temperature boiling points, liquid precursors may be emulsified using a microfluidizer device chilled to a low temperature The boiling pomts may also be depressed using solvents in liquid media to utilize a precursor m liquid form Further, the methods may be performed where the temperature is mcreased throughout die process, whereby the process starts with a gaseous precursor as a liquid and ends with a gas The gaseous precursor may be selected so as to form the gas in situ in the targeted tissue or fluid, in vivo upon entering the patient or animal, pπor to use, during storage, or during manufacture The methods of producing the temperature-activated gaseous precursor filled vesicles may be carried out at a temperature below the boiling pomt of the gaseous precursor In this embodiment, the gaseous precursor may be entrapped within a vesicle such that the phase transition does not occur dunng manufacture Instead, the gaseous precursor filled vesicles are manufactured in the liquid phase of the gaseous precursor Activation of the phase transition may take place at any time as the temperature is allowed to exceed the boiling point of the precursor Also, knowing the amount of liquid m a droplet of liquid gaseous precursor, the size of the vesicles upon attaining the gaseous state may be determined .Alternatively, the gaseous precursors may be utilized to create stable gas filled vesicles which are pre-formed prior to use In this embodiment, the gaseous precursor may be added to a container housmg a pid composition at a temperature below the liquid-gaseous phase transition temperature of the respective gaseous precursor As the temperature is mcreased, and an emulsion is formed between the gaseous precursor and liquid solution, the gaseous precursor undergoes transition from the liquid to the gaseous state As a result of this heating and gas formation, the gas displaces the air m the head space above the liquid mixture so as to form gas filled vesicles which may entrap the gas of the gaseous precursor, ambient gas (e g air), or coentrap gas state gaseous precursor and ambient air This phase transition can be used for optimal mixing and formation of the contrast agent For example, the gaseous precursor, perfluorobutane, can be entrapped m the pid vesicles and as the temperature is raised beyond the boilmg point of perfluorobutane (4°C), perfluorobutane gas is entrapped in the vesicles Accordmgly, the gaseous precursors may be selected to form gas filled vesicles in vivo or may be designed to produce the gas filled vesicles in situ, during the manufacturing process, on storage, or at some time prior to use As a further embodiment of this mvention, by pre-forming the gaseous precursor in the liquid state into an aqueous emulsion, the maximum size of the vesicle may be estimated by using the ideal gas law, once the transition to the gaseous state is effectuated For the puφose of making gas filled vesicles from gaseous precursors, the gas phase may be assumed to form instantaneously and substantially no gas in the newly formed vesicle has been depleted due to diffusion into the liquid, which is generally aqueous m nature Hence, from a known liquid volume in the emulsion, one may predict an upper limit to the size of the gas filled vesicle ln embodiments of the present invention, a mixture of a hpid compound and a gaseous precursor, contammg liquid droplets of defined size, may be formulated such that upon reaching a specific temperature, for example, the boiling point of the gaseous precursor, the droplets may expand into gas filled vesicles of defined size The defined size may represent an upper limit to the actual size because the ideal gas law generally cannot account for such factors as gas diffusion into solution, loss of gas to the atmosphere, and the effects of increased pressure The ideal gas law, which can be used for calculating the increase in the volume of the gas bubbles upon transitioning from liquid to gaseous states, is as follows PV = nRT where P is pressure in atmospheres (arm), V is volume in liters (L), n is moles of gas, T is temperature in degrees Kelvin (K); and R is the ideal gas constant (22 4 L-atm/K-mole) With knowledge of volume, density, and temperature of the liquid in the mixture of liquids, the amount, for example, m moles, and volume of liquid precursor may be calculated which, when converted to a gas, may expand into a vesicle of known volume The calculated volume may reflect an upper limit to the size of the gas filled vesicle, assuming instantaneous expansion into a gas filled vesicle and negligible diffusion of the gas over the time of the expansion Thus, for stabilization of the precursor in the liquid state m a mixture wherein the precursor droplet is spherical, the volume of the precursor droplet may be determined by the equation Volume (spherical vesicle) = 4/3 πr 3 where r is the radius of the sphere Thus, once the volume is predicted, and knowing the density of the liquid at the desired temperature, the amount of liquid gaseous precursor in the droplet may be determmed In more descriptive terms, the following can be applied V gas = 4/3 π(r_> by the ideal gas law, PV=nRT substituting reveals, or, (A) n = 4/3 [πr g8 . 3 ] P/RT amount n = 4/3 [πr gas 3 P RT] « MW n Converting back to a liquid volume (B) V lιq = [4/3 [πr^ 3 ] P/RT].MW n D] where D is the density of the precursor Solving for the diameter of me liquid droplet, (C) diameter/2 = [3/4π [4/3.[πr gas 3 ] P RT] MW n D] ,/3 which reduces to Diameter = 2[[r gas 3 ] P/RT [MW n /D]] ,/3 As a further means of prepanng vesicles of the desired size for use m the methods of the present invention, and with a knowledge of the volume and especially the radius of the liquid droplets, one can use appropriately sized filters to size the gaseous precursor droplets to the appropriate diameter sphere A representative gaseous precursor may be used to form a vesicle of defined size, for example, 10 μm diameter In this example, the vesicle may be formed in the bloodstream of a human bemg, thus the typical temperature would be 37°C or 310 K At a pressure of 1 atmosphere and usmg the equation in (A), 7 54 x 10 17 moles of gaseous precursor may be required to fill the volume of a 10 μm diameter vesicle Usmg the above calculated amount of gaseous precursor and 1 -fluorobutane, which possesses a molecular weight of 76 11 , a boilmg point of 32 5 ° C and a density of 0 7789 g/mL at 20 °C, further calculations predict that 5 74 x 10 15 grams of this precursor may be required for a 10 μm vesicle Extrapolating further, and with the knowledge of the density, equation (B) further predicts that 8 47 x 10 16 mL of liquid precursor may be necessary to form a vesicle with an upper limit of 10 μm Finally, using equation (C), a mixture, for example, an emulsion containing droplets with a radius of 0 0272 μm or a coπesponding diameter of 0 0544 μm, may be formed to make a gaseous precursor filled vesicle with an upper limit of a 10 μm vesicle An emulsion of this particular size could be easily achieved by the use of an appropnately sized filter In addition, as seen by the size of the filter necessary to form gaseous precursor droplets of defined size, the size of the filter may also suffice to remove any possible bacterial contaminants and, hence, can be used as a sterile filtration as well This embodiment for preparing gas filled vesicles may be applied to all gaseous precursors activated by temperature In fact, depression of the freezing point of the solvent system allows the use of gaseous precursors which may undergo liquid-to-gas phase transitions at temperatures below 0°C The solvent system can be selected to provide a medium for suspension of the gaseous precursor For example, 20% propylene glycol irascible in buffered saline exhibits a freezing point depression well below the freezing point of water alone By mcreasmg the amount of propylene glycol or addmg materials such as sodium chlonde, the freezing point can be depressed even further The selection of appropnate solvent systems may be determined by physical methods as well When substances, solid or liquid, herein refeπed to as solutes, are dissolved in a solvent, such as water based buffers, the freezing point may be lowered by an amount that is dependent upon the composition of the solution Thus, as defined by Wall, one can express the freezing pomt depression of the solvent by the following equation Inx. = In (l-x b ) = ΔH fu ./R(l/T 0 -l/T) where x a is the mole fraction of the solvent, x b is the mole fraction of the solute, AH fus is the heat of fusion of the solvent, and T 0 is the normal freezing point of the solvent The normal freezmg point of the solvent can be obtained by solving the equation If x b is small relative to x a , then the above equation may be rewritten as follows x b = ΔH fu ,R[T-T/T 0 T] * ΔH fus ΔT/RT 0 2 The above equation assumes the change in temperature ΔT is small compared to T 2 This equation can be simplified further by expressing the concentration of the solute in terms of molality, m (moles of solute per thousand grams of solvent) Thus, the equation can be rewritten as follows X b = m/[m + 1000/m l * πvMa/1000 where Ma is the molecular weight of the solvent Thus, substituting for the fraction x b ΔT = [ Mard V1000ΔH f m or ΔT = K f m, where K f *= Mardl /1000ΔH fu . K f is the molal freezmg point and is equal to 1 86 degrees per unit of molal concentration for water at one atmosphere pressure The above equation may be used to accurately determine the molal freezing point of solutions of gaseous-precursor filled vesicles Accordingly, the above equation can be applied to estimate freezing point depressions and to determine the appropriate concentrations of liquid or solid solute necessary to depress the solvent freezing temperature to an appropnate value Methods of prepaπng the temperature activated gaseous precursor filled vesicles include (a) vortexing and or shaking an aqueous mixture of gaseous precursor and additional materials as desired, mcludmg, for example, stabilizing matenals, thickening agents and/or dispersing agents Optional vanations of this method include autoclaving before vortexing or shakmg, heating an aqueous mixture of gaseous precursor, venting the vessel contammg the mixture/suspension, shaking or permitting the gaseous precursor filled vesicle to form spontaneously and coolmg down the suspension of gaseous precursor filled vesicles, and extruding an aqueous suspension of gaseous precursor through a filter of about 0 22 μm Alternatively, filtermg may be performed dunng in vivo admmistration of the vesicles such that a filter of about 0 22 μm is employed, (b) microemulsification, whereby an aqueous mixture of gaseous precursor is emulsified by agitation and heated to form, for example, vesicles prior to administration to a patient, (c) heating a gaseous precursor in a mixture, with or without agitation, whereby the less dense gaseous precursor filled vesicles may float to the top of the solution by expanding and displacing other vesicles in the vessel and venting the vessel to release air, and (d) utilizing in any of the above methods a sealed vessel to hold the aqueous suspension of gaseous precursor and maintaining the suspension at a temperature below the phase transition temperature of the gaseous precursor, followed by autoclaving to raise the temperature above the phase transition temperature, optionally with shakmg, or permitting the gaseous precursor vesicle to form spontaneously, whereby the expanded gaseous precursor m the sealed vessel increases the pressure in the vessel, and coolmg down the gas filled vesicle suspension, after which shaking may also take place Freeze drying may be useful to remove water and organic matenals pπor to the shakmg installation method Drying installation methods may be used to remove water from vesicles By pre-entrappmg the gaseous precursor in the dried vesicles (l e prior to drying) after warming, the gaseous precursor may expand to fill the vesicle Gaseous precursors can also be used to fill dπed vesicles after they have been subjected to vacuum As the dried vesicles are kept at a temperature below their gel state to liquid crystalline temperature, the drying chamber can be slowly filled with the gaseous precursor in its gaseous state For example, perfluorobutane can be used to fill dried vesicles at temperatures above 4 °C (the boilmg pomt of perfluorobutane) Prefeπed methods for prepaπng the temperature activated gaseous precursor filled vesicles compnse shakmg an aqueous solution havmg a hpid compound in the presence of a gaseous precursor at a temperature below the liquid state to gas state phase transition temperature of the gaseous precursor This is preferably conducted at a temperature below the gel state to liquid crystalline state phase transition temperature of the hpid The mixture may be then heated to a temperature above the liquid state to gas state phase transition temperature of the gaseous precursor which can cause the precursor to volatilize and expand Heating may be then discontinued, and the temperature of the mixture may be allowed to drop below the liquid state to gas state phase transition temperature of the gaseous precursor Shakmg of the mixture may take place dunng the heatmg step, or subsequently after the mixture is allowed to cool Other methods for preparing gaseous precursor filled vesicles can involve shaking an aqueous solution of, for example, a hpid and a gaseous precursor, and separating the resulting gaseous precursor filled vesicles Conventional, aqueous-filled hposomes of the prior art are routinely formed at a temperature above the phase transition temperature of the lipids used to make them, since they are more flexible and thus useful in biological systems in the liquid crystalline state See, for example, Szoka and Papahadjopoulos, Proc Natl Acad Sci 1978, 75, 4194-4198 In contrast, the vesicles made according to certain prefeπed embodiments described herein are gaseous precursor filled, which imparts greater flexibility, since gaseous precursors after gas formation are more compressible and compliant than an aqueous solution The preparatory methods may involve shaking an aqueous solution compnsmg a hpid, m the presence of a temperature activatable gaseous precursor Preferably, the shaking is of sufficient force such that a foam is formed within a short period of time, such as about 30 minutes, and preferably within about 20 minutes, and more preferably, within about 10 minutes The shakmg may involve micro-emulsifying, microfluidizing, swirling (such as by vortexing), side-to-side, or up and down motion In die case of the addition of gaseous precursor in the liquid state, somcation may be used in addition to the shaking methods set forth above Further, different types of motion may be combined Also, the shakmg may occur by shaking the container holding me aqueous hpid solution, or by shakmg the aqueous solution within the container without shakmg the container itself Further, the shakmg may occur manually or by machine Mechanical shakers that may be used mclude, for example, the mechanical shakers descnbed hereinbefore, with an Espe Capmix (Seefeld, Oberay Germany) bemg prefeπed Another means for producmg shakmg includes the action of gaseous precursor emitted under high velocity or pressure Accordmg to the methods described herein, a gas, such as air, may also be provided by the local ambient atmosphere The local ambient atmosphere can include the atmosphere within a sealed container, as well as the external environment Alternatively, for example, a gas may be mjected mto or otherwise added to the container havmg the aqueous pid solution or into the aqueous hpid solution itself to provide a gas other than air Gases that are lighter than air are generally added to a sealed container, while gases heavier than air can be added to a sealed or an unsealed container Accordingly, the present invention includes co-entrapment of air and/or other gases along with gaseous precursors Hence, the gaseous precursor filled vesicles can be used in substantially the same manner as the gas filled vesicles descπbed herem, once activated by apphcation to die tissues of a host, where such factors as temperature or pH may be used to cause generation of the gas It is preferred that the gaseous precursors undergo phase transitions from liquid to gaseous states at near the normal body temperature of the host, and are thereby activated, for example, by the in vivo temperature of the host so as to undergo transition to trie gaseous phase therein This can occur where, for example, die host tissue is human tissue having a normal temperature of about 37°C and the gaseous precursors undergo phase transitions from liquid to gaseous states near 37°C As noted above, the pid, protein, polymer and/or vesicle compositions may be steπhzed by autoclave or steπle filtration if these processes are performed before the installation step or pπor to temperature mediated conversion of die temperature sensitive gaseous precursors within the compositions Alternatively, one or more anb- bacteπcidal agents and/or preservatives may be included m the formulation of the compositions, such as sodium benzoate, quaternary ammonium salts, sodium azide, methyl paraben, propyl paraben, sorbic acid, ascorbylpalmitate, butylated hydroxyanisole, butylated hydroxytoluene, chlorobutanol, dehydroacetic acid, ethylenediamine, monothioglycerol, potassium benzoate, potassium metabisulfite, potassium sorbate, sodium bisulfite, sulfur dioxide, and organic mercuπal salts Such stenhzation, which may also be achieved by other conventional means, such as by irradiation, may be necessary where the stabilized vesicles are used for imagmg under invasive circumstances, for example, mtravascularly or mtrapeπtonealy The appropriate means of sterilization will be apparent to the artisan based on d e present disclosure Vesicle compositions which compnse vesicles formulated from protems or polymers may be prepared by vaπous processes, as will be readily apparent to those skilled in the art, once armed with the present disclosure Exemplary processes include, for example, lnterfacial polymeπzation, phase separation and coacervation, multioπfice centπfugal preparation, and solvent evaporation Suitable procedures which may be employed or modified in accordance with the present disclosure to prepare vesicles from polymers include those procedures disclosed in Garner et al , U S Patent No 4,179,546, Garner, U S Patent No 3,945,956, Cohrs et al , U S Patent No 4,108,806, Japan Kokai Tokyo Koho 62 286534, British Patent No 1 ,044,680, Kenaga et al , U S Patent No 3,293,1 14, Morehouse et al , U S Patent No 3,401,475, Walters, U S Patent No 3,479,811 , Walters et al , U S Patent No 3,488,714, Morehouse et al , U S Patent No 3,615,972, Baker et al , U S Patent No 4,549,892, Sands et al , U S Patent No 4,540,629, Sands et al , U S Patent No 4,421,562, Sands, U S Patent No 4,420,442, Mathiowitz et al , U S Patent No 4,898,734, Lencki et al , U S Patent No 4,822,534, Herbig et al , U S Patent No 3,732,172, Himmel et al , U S Patent No 3,594,326, Sommerville et al , U S Patent No 3,015,128, Deasy, Microencapsulation and Related Drug Processes, Vol 20, Chs 9 and 10, pp 195-240 (Marcel Dekker, Inc , N Y , 1984), Chang et al , Canadian J of Physiology and Pharmacology, Vol 44, pp 115-129 (1966), and Chang, Science, Vol 146, pp 524-525 (1964), the disclosures of each of which are incoφorated herein by reference m their entirety In accordance with a prefeπed synthesis protocol, the polymer vesicles may be prepared using a heat expansion process, such as, for example, the process described m Garner et al , U S Patent No 4,179,546, Gamer, U S Patent No 3,945,956, Cohrs et al , U S Patent No 4, 108,806, Bntish Patent No 1 ,044,680, and Japan Kokai Tokkyo Koho 62 286534 In general terms, the heat expansion process may be earned out by preparing vesicles of an expandable polymer or copolymer which may contain m their void (cavity) a volatile liquid (gaseous precursor) The vesicle is then heated, plasticising the vesicle and converting the volatile liquid mto a gas, causmg the vesicle to expand to up to about several times its ongmal size When the heat is removed, the thermoplastic polymer retains at least some of its expanded shape Vesicles produced by mis process tend to be of particularly low density, and are thus prefeπed The foregomg described process is well known m the art, and may be refeπed to as the heat expansion process for prepanng low density vesicles Polymers useful in the heat expansion process will be readily apparent to those skilled in the art and mclude thermoplastic polymers or copolymers, mcludmg polymers or copolymers of many of the monomers described above Preferable of me polymers and copolymers described above include die following copolymers polyvinyhdene-polyacrylonitπle, polyvinyhdene-polyacrylonitπle-polymethyl- methacrylate, and polystyrene-polyacrylonitπle A most preferred copolymer is polyvmy dene-polyacrylonitπle Volatile liquids useful in the heat expansion process will also be well known to those skilled in the art and mclude aliphatic hydrocarbons such as ethane, ethylene, propane, propene, butane, isobutane, neopentane, acetylene, hexane, heptane, chlorofluorocarbons such as CC1 3 F, CC1 2 F 2 , CC1F,, CC1F 2 -CC1 2 F 2 , chloroheptafluoro- cyclobutane, and 1,2-dιchlorohexafluorocyclobutane, tetraalkyl silanes, such as tetramethyl silane, trimethylethyl silane, trimethyhsopropyl silane, and tnmethyl n-propyl silane, as well as perfluorocarbons, includmg the perfluorocarbons described above ln general, it is important that the volatile liquid not be a solvent for the polymer or copolymer bemg utilized It is also preferred that the volatile liquid have a boiling pomt d at is below die softening point of the involved polymer or co-polymer Boiling points of various volatile liquids and softening points of various polymers and copolymers will be readily ascertainable to one skilled m the art, and suitable combinations of polymers or copolymers and volatile liquids will be easily apparent to the skilled artisan By way of guidance, and as one skilled in the art would recognize, generally as the length of the carbon chain of the volatile liquid increases, the boiling point of that liquid increases also Also, mildly preheating die vesicles in water in the presence of hydrogen peroxide pnor to definitive heating and expansion may pre-soften the vesicle to allow expansion to occur more readily For example, to produce vesicles from synthetic polymers, vinyhdene and acrylomtnle may be copolymeπzed m a medium of isobutane liquid usmg one or more of the foregomg modified or unmodified literature procedures, such that isobutane becomes entrapped witinn the vesicles When such vesicles are then heated to a temperature of from about 80°C to about 120°C, the isobutane gas expands, which in turn expands die vesicles After heat is removed, the expanded polyv ylidene and acrylomtnle copolymer vesicles remain substantially fixed m their expanded position The resulting low density vesicles are extremely stable both dry and suspended in an aqueous media Isobutane is utilized herem merely as an illustrative liquid, with the understanding diat other liquids winch undergo liquid gas transitions at temperatures useful for the syn esis of these vesicles and formation of the very low density vesicles upon heatmg can be substituted for isobutane Similarly, monomers other than viny dene and acrylonitrile may be employed in prepaπng the vesicles In certain preferred embodiments, the vesicles which are formulated from synti etic polymers and which may be employed in the methods of the present invention are commercially available from Expancel, Nobel Industries (Sundsvall, Sweden), cludmg EXPANCEL 551 DE™ microspheres The EXPANCEL 551 DE™ microspheres are composed of a copolymer of vinyhdene and acrylonitrile which have encapsulated therein isobutane liquid Such microspheres are sold as a dry composition and are approximately 50 microns in size The EXPANCEL 551 DE™ microspheres have a specific gravity of only 0 02 to 0 05, which is between one-fiftieth and one-twentieth die density of water Included among the methods described in the aforementioned patents for the preparation of protein-based vesicles are methods which involve sonicating a solution of a protem In prefeπed form, the starting material may be an aqueous solution of a heat- denaturable, water-soluble biocompatible protem The encapsulating protem is preferably heat-sensitive so d at it can be partially insolubilized by heating during somcation Suitable heat-sensitive proteins include, for example, albumin, hemoglobin, collagen, and die like Preferably, the protem is a human protein, witii human serum albumin (HSA) being more prefeπed HSA is available commercially as a sterile 5% aqueous solution, which is suitable for use in the preparation of protein-based vesicles Of course, as would be apparent to one of ordinary skill m die art, other concentrations of albumin, as well as other protems which are heat-denaturable, can be used to prepare the vesicles Generally speaking, die concentration of HSA can vary and may range from about 0 1 to about 25% by weight, and all combinations and subcombinations of ranges dierein It may be preferable, in connection witii certain methods for the preparation of protein-based vesicles, to utilize die protem in die form of a dilute aqueous solution For albumin, it may be prefeπed to utilize an aqueous solution containing from about 0 5 to about 7 5% by weight albumin, with concentrations of less than about 5% by weight being prefeπed, for example, from about 0 5 to about 3% by weight The protein-based vesicles may be prepared using equipment which is commercially available For example, in connection with a feed preparation operation as disclosed, for example, in Cemy, et al , US Patent No 4,957,656, stainless steel tanks which are commercially available from Walker Stainless Equipment Co (New Lisbon, WI), and process filters which are commercially available from Mil pore (Bedford, MA), may be utilized The somcation operation may utilize both a heat exchanger and a flow d rough sonicating vessel, m series Heat exchanger equipment of this type may be obtained from iTT Standard (Buffalo, NY) The heat exchanger maintains operating temperature for the somcation process, with temperature controls ranging from about 65 °C to about 80 °C, depending on the makeup of the media The vibration frequency of the so cation equipment may vary over a wide range, for example, from about 5 to about 40 kilohertz (kHz), with a majonty of die commercially available sonicators operating at about 10 or 20 kHz Suitable sonicating equipment include, for example, a Sonics & Materials Vibra-Cell, equipped with a flat-tipped sonicator horn, commercially available from Sonics & Matenals, Inc (Danbury, CT) The power applied to the sonicator horn can be varied over power settings scaled from 1 to 10 by the manufacturer, as with Sonics & Materials Vibra-Cell Model VL1500 An intermediate power setting, for example, from 5 to 9, can be used It is preferred that the vibrational frequency and the power supplied be sufficient to produce cavitation in d e liquid bemg somcated Feed flow rates may range from about 50 mL/min to about 1000 mL/min, and all combinations and subcombmattons of ranges therem Residence times in the somcation vessel can range from about 1 second to about 4 minutes, and gaseous fluid addition rates may range from about 10 cubic centimeters (cc) per minute to about 100 cc/m , or 5% to 25% of the feed flow rate, and all combinations and subcombinations of ranges therem It may be preferable to carry out the somcation m such a manner to produce foaming, and especially mtense foaming, of the solution Generally speaking, intense foaming and aerosolating are important for obtaining a contrast agent having enhanced concentration and stability To promote foaming, the power input to the sonicator horn may be increased, and the process may be operated under mild pressure, for example, about 1 to about 5 psi Foaming may be easily detected by the cloudy appearance of die solution, and by the foam produced Suitable methods for die preparation of protem-based vesicles may mvolve physically or chemically altering die protein or protem derivative in aqueous solution to denature or fix the material For example, protein-based vesicles may be prepared from a 5% aqueous solution of HSA by heating after formation or during formation of the contrast agent via somcation Chemical alteration may involve chemically denaturing or fixing by binding d e protein with a difunctional aldehyde, such as glutaraldehyde For example, the vesicles may be reacted with 0 25 grams of 50% aqueous glutaraldehyde per gram of protein at pH 4 5 for 6 hours The unreacted glutaraldehyde may men be washed away from the protem The present mvention is directed to methods for diagnostic imagmg which involve die administration to a patient of a contrast agent, in combmation witii a renal vasodilator The renal vasodilator may be administered to the patient before, durmg, and/or after the administration of the contrast agent A wide variety of techniques are available for the preparation of hpid and/or vesicle compositions which comprise a bioactive agent, mcludmg renal vasodilators For example, hpid and/or vesicle compositions may be prepared from a mixture of hpid compounds, bioactive agent and gas or gaseous precursor In this case, pid and/or vesicle compositions may be prepared as descπbed above in which the compositions also compnse a bioactive agent Thus, for example, micelles can be prepared in the presence of a bioactive agent In connection wnti pid and/or vesicle compositions which compnse a gas, the preparation can involve, for example, bubblmg a gas directly mto a mixture of hpid compounds and one or more additional matenals Alternatively, die hpid and/or vesicle compositions may be preformed from pid compounds and gas or gaseous precursor In the latter case, the bioactive agent may be then added to d e hpid and/or vesicle composition pnor to use For example, an aqueous mixture of hposomes and gas may be prepared to which die bioactive agent may be added and which is agitated to provide the liposome composition The liposome composition which further comprises a bioactive agent can be readily isolated smce the gas and/or bioactive agent filled liposome vesicles generally float to die top of the aqueous solution Excess bioactive agent can be recovered from the remaining aqueous solution As tiiose skdled in the art will recognize, any of d e pid and/or vesicle compositions descπbed herein may be lyophihzed for storage, and reconstituted, for example, wid an aqueous medium (such as steπle water, phosphate buffered solution, or aqueous saline solution), widi the aid of vigorous agitation To prevent agglutination or fusion of the lipids as a result of lyophilization, it may be useful to include additives which prevent such fusion or agglutination from occuπmg Additives which may be useful include sorbitol, mannitol, sodium chloride, glucose, trehalose, polyvinylpyπohdone and poly(ethylene glycol) (PEG), for example, PEG polymers having a molecular weight of from about 400 to about 10,000, with PEG polymers having molecular weights of about 1000, 3000 (such as PEG3350) and 5000 being prefeπed These and other additives are described m the literature, such as m the U S Pharmacopeia, USP XXII, NF XVII, The United States Pharmacopeia, The National Formulary, United States Pharmacopeial Convention Inc , 12601 Twinbrook Parkway, Rockville, MD 20852, the disclosures of which are hereby incoφorated herem by reference in their entirety Lyophi zed preparations generally have the advantage of greater shelf life As discussed above, die compositions of the present invention, mcludmg gas and/or gaseous precursor filled vesicles, are useful as contrast agents for diagnostic imaging, mcludmg, for example, ultrasound (US) imaging, computed tomography (CT) imagmg, mcludmg CT angiography (CTA), magnetic resonance (MR) imagmg, mcludmg magnetic resonance angiography (MRA), nuclear medicine, optical imaging and elastography In accordance with the present invention, mere are provided methods of imagmg one or more regions of a patient, and especially die renal region of a patient The present mvention also provides me ods for diagnosing die presence or absence of diseased tissue in a patient, especially die presence or absence of diseased renal tissue The med ods of die present invention involve the administration of a contrast medium d e form, for example, of a hpid and/or vesicle composition, to a patient A renal vasodilator is also admmistered to e patient The patient is scanned using diagnostic imagmg mcludmg, for example ultrasound imagmg, to obtain visible images of an internal region, preferably d e renal region of a patient The methods may also be used to image other internal regions of me patient mcludmg, for example, me vasculature In scannmg the renal region, it is possible to include the abdommal aorta The present methods can also be used m connection widi the delivery of a bioactive agent to an internal region of a patient Wid respect to the renal vasodilator which is employed in the methods of ie present invention, a wide vanety of materials are available which may be suitable for use in the methods of the present invention and which, when administered to a patient, may impart a renal vasodilative effect, that is, dilation of blood vessels in the renal region and particular, dilation of a renal artery Preferably, the material is capable of lncreasmg the overall renal blood flow The material may act directly to mcrease die renal blood flow, or may be involved in biochemical pathways whereby an increase in renal blood flow is produced More preferably, the material acts to inhibit angiotensin converting enzyme (ACE) ACE catalyzes the conversion of die relatively inactive decapeptide angiotensin I to angiotensin II, a potent endogenous vasoconstrictor, and the destruction of bradykmin, a potent vasodilator The Pharmacological Basis of Therapeutics, Hardman, Joel G And Limbird, Lee E , eds in chief, 1996, McGraw-Hill, New York, N Y , pp 743- 751 In patients with renal artery stenosis, blood flow into ie kidney is reduced The kidney with reduced blood flow produces rerun, which increases blood pressure in order to improve blood flow mto ie kidney The increased blood pressure results from die action of ren on a plasma globulin substrate in the vascular circulation Angiotensin I is produced in this interaction, and is hydrolyzed by ACEE to Angiotensin II The administration of ACE inhibitors results in a vasodilator effect, particularly m the kidney, smce die renal vessels are particularly sensitive to the vasoconstπctor effects of Angiotensin II ACE inhibitors may be grouped accordmg to eir chemical structures Sulfhydryl-containing ACE inhibitors include captopπl (l-(3-mercapto-2-methyl-l- oxopropyl)-L-prolιne), fentiapπl (2-(2-hydroxyphenyl)3-(mercapto-l-oxopropyl)-4- thiazodinecarboxyhc acid), pivalopπl (N-cyclopentyl-N-[3-[2,2-dιmethyl-l- oxopropyl)thιo]-2-methyl-l-oxopropyl]-(S)-glycme), zofenopπl, - 1 '-/ 3-(benzoylthιo)-2- methyl- I-oxopropyl]-4-phenylthιo)-hydroxy-2,2-dιmethyl-4-(2-oxo-l-[pyrrolιdιnyl)-2H-I - benzopyran-6-carbonιtrιle L-prohne) , and alacepnl ((S)-N-[I-[3-(acetylthιo)-2-methyl-l- oxopropylJL-prolylJ-L-phenylalanme) Dicarboxyl-containing ACE inhibitors mclude enalapπl (l-[N-[l-(elhoxycarbonyl)-3-phenylpropyl]-L-alanyl]L-prolιne), enalapπlat (the parent dicarboxylic acid of enalapπl), hsmopnl ((S)-l-[N 2 -(l-carboxy-3-phenylpropyl)-L- lysylJ-L prohne dihydrate), benazepπl (3-[[l-ethoxy-carbonyl)-3-phenyl- (IS)propyl]amιno]-2, 3, 4, 5-tetrahydro-2-oxo-lH-l-(3S)benzazepιne- 1 -acetic acid), quinapπl (3S-[2[R *(R *)], 3R *]]-2-[2-[[I-ethoxycarbonyl)-3-phenylpropyl]amιno]-I- oxopropyl]-I,2,3,4-tetrahydro-3-ιsoquιnohnecarboxylιc acid), moexipπl (2-[[l- ethoxycarbonyl)-3-phenylpropyl]amιno]-l-oxopropylj-l, 2, 3, 4-tetrahydro-6, 7-dιmethoxy- 3-ιsoquιnolιnecarboxylιc acid [3S-[2R*(R*)J, 3R*]]-), ramipπl (2S- flfR *(R *)], 2a, 3aβ, 6aβ]]-l-l2-[fI-(ethoxycarbonyl)-3-phenylpropylJamιno]J- oxopropyl]octahydrocyclopenta[b]pyrrole-2-carboxylιc acid), spirapπl (mixture of 7- [2- [[l-ethoxycarbonyl)-3-phenylpropyl]amιno]-l-oxopropyl]-lH-ιndole-2-carboxylιc acιd, [8S-[7[R*(R*),8R*1]-lH-ιndole-2-carboxyhc acid, with 3-[2-4-(4-fluorobenzoyl)-l- pιpendmylJethylJ-2, (1 H, 3H)-quιnazolιnedιone) , peπndopπl (2S-[1- [R * (R *)], 2a, 3aβ, 7aβ]]-l-[2-[[l-(ethoxycarbonyl)butyl]amιno]-l-oxopropyl]octahydro- lH-ιndole-2-carboxylιc acid), mdolapπl (I-[2-[[ l-(ethoxycarbonyl)-3- phenylpropyl]amιno]-l-oxopropyljoctahydro-IH-ιndole-2-carboxyhc acιd„[2S- [I[R*(R*)],2 alph , 3a beta , 7a beta]], mdalapπl ((S)-N-(2,3-dιhydro-lH-ιnden-2yl)-N- [N-{l-ethoxycarbonyl)-3-phenylpropyl]-L-alanyl]glycme, and cilazapπl (1S- [[la,9a(R*)]]-9-[[l-ethoxycarbonyl)-3-phenylpropyl]amιno]octahydro-10-oxo-6H- pyndazmofl, 2-a][l, 2] dιazepιne-1 -carboxylic acid monohydrate) Phosphorous- containing ACE inhibitors include fosinopπl ((2a,4β)4-cyclohexyl-l-[[ [2-methyl- 1-( 1- oxopropoxy)propoxyl](4-phenylbutyl)phosphmyl] acetyl] -L-prohne) Such compounds are described, for example, m The Pharmacological Basis of Therapeutics, Hardman, Joel G And Limbird, Lee E , eds in chief, 1996, McGraw-Hill, New York, N Y , pp 743-751 , and m The Merck Index, Eleventh Ed , 1989, Merck & Co , the disclosures of which are hereby incoφorated by reference m their entirety These as well as other suitable vasodilator compounds will be readily apparent to those skilled in the art once armed with d e present disclosure Pharmaceutically acceptable salts of any of the ACE inhibitors may be used, provided the delivery, efficacy and bioavailabihty of the inhibitor is not unduly compromised Prefeπed for use in d e mediod of the present mvention are captopnl and enalapπl Captopnl is preferred for oral administration and the prefeπed oral dosage from about 25 milligrams to about 50 milligrams, depending upon patient body weight, about 0 5 to about 5 hours , preferably about 1 to about 2 hours before scannmg the patient Enalapπl is prefeπed for intravenous admmistration Generally enalapπl is mjected from about 15 mmutes to about 2 hours before scarmmg the patient If desired, die pid, protein, polymer and/or vesicle compositions descπbed herem may further compnse a targeting agent to promote targeting of tissues and/or receptors in vivo mcludmg, for example, renal tissue Suitable targeting agents, meti ods for their incoφoration into hpid and/or vesicle compositions, and methods for the use of such targeted compositions, are described, for example, in copending U S application Serial No 08/640,464, filed May 1, 1996 and U S application Serial No 08/660,032, filed June 6, 1996, the disclosures of which are hereby incoφorated herein by reference, m their entirety As one skilled in the art would recognize, administration of the p d and/or vesicle compositions described herem, as well as the renal vasodilators, can be caπied out in various fashions, mcludmg parenterally, orally, or lntraperitoneally Parenteral admmistration, which is preferred, includes administration by die following routes intravenous, intramuscular, mterstitially, mtra-arterially, subcutaneous, intraocular, lntrasynovial, transepitiie al, mcludmg transdermal, pulmonary via inhalation, ophmalmic, sublmgual and buccal, topically, including ophthalmic, dermal, ocular, rectal, and nasal inhalation via insufflation Intravenous administration is preferred among me routes of parenteral admmistration Various combinations of die hpid and/or vesicle compositions, and renal vasodilators, may be used to alter properties as desired, includmg viscosity, osmolanty or palatabihty In carrying out d e imaging me ods of the present mvention, the contrast medium, including renal vasodilator, can be used alone, or m combination with additional diagnostic, d erapeutic or other agents Such other agents mclude excipients such as flavonng or colonng matenals CT imagmg techmques which are employed are conventional and are described, for example, in Computed Body Tomography, Lee, J K T , Sagel, S S , and Stanley, R J , eds , 1983, Ravens Press, New York, N Y , especially die first two chapters thereof entitled "Physical Principles and Instrumentation" , Ter-Pogossian, M M , and "Techniques", Aronberg, D J , the disclosures of which are incoφorated by reference herein m ieir entirety Magnetic resonance imagmg techmques, particularly directed to imagmg of the renal region, are descnbed m, for example, Brown et al , "Magnetic Resonance Imagmg of die Adrenal Gland and Kidney", Topics in Magnetic Resonance Imaging, Vol 7(2), pp 90-101 (1995), Krestin, "Magnetic Resonance Imaging of the Kidneys Cuπent Status", Magnetic Resonance Quarterly, Vol 10(1), pp 2-21 (March 1994), Lee, "Recent Advances in Magnetic Resonance Imagmg of Renal Masses", Canadian Association of Radiologists .Journal, Vol 42(3), pp 185-9 (June 1991 ), Lubat et al , "Magnetic Resonance Imagmg of the Kidneys and Adrenals", Vol 2(3), pp 17-36 (June 1990), Baumgartner et al , "Magnetic Resonance Imaging of the Kidneys and Adrenal Glands", Seminars in Ultrasound, CT and MR, Vol 10(1), pp 43-62 (February 1989), Choyke e/ al , "The Role of MRI in Diseases of the Kidney", Radiologic Clinics of North Amenca Vol 26(3), pp 617-31 (May 1988), and Papanicolaou et al , "Magnetic Resonance Imaging of the Kidney", Urologic Radiology, Vol 8(3), pp 139-50 (1986) With respect to ultrasound, ultrasonic imaging techniques, includmg second harmonic imaging, and gated imaging, are well known in the art, and are descπbed, for example, m Uhlendorf, "Physics of Ultrasound Contrast Imaging Scattering in the Linear Range", IEEE Transactions on Ultrasonics, Perroelectncs, and Frequency Control, Vol 14(1 ), pp 70-79 (1994) and Sudierland, et al , "Color Doppler Myocardial Imaging A New Technique for the Assessment of Myocardial Function", Journal of the Amencan Society of Echocardiography, Vol 7(5), pp 441-458 (1994), the disclosures of which are hereby incoφorated herem by reference m their entirety In the case of diagnostic applications, such as ultrasound, CT and MRI, energy, such as ultrasonic energy, may be applied to at least a portion of the patient to image the target tissue A visible image of an internal region of the patient, preferably the renal region, may be men obtained, such that the presence or absence of diseased tissue can be ascertained Ultrasound can be used for both diagnostic and d erapeutoc puφoses In diagnostic ultrasound, ultrasound waves or attain of pulses of ultrasound may be applied widi a transducer The ultrasound is generally pulsed (intermittent) ramer than continuous, although it may be continuous, if desired Thus, diagnostic ultrasound generally involves the application of a pulse of echoes, after which, dunng a listening penod, the ultrasound transducer receives reflected signals Harmonics, ultraharmonics or subharmonics may be used The second harmonic mode may be beneficially employed, m which the 2x frequency may be received, where x is the incidental frequency This may serve to decrease die signal from the background material and enhance the signal from the transducer usmg the contrast media of the present invention which may be targeted to die desired site Otiier harmonics signals, such as odd harmonics signals, for example, 3x or 5x, may be similarly received using tins metiiod Subharmonic signals, for example, x/2 and x/3, may also be received and processed so as to form an image In addition to die pulsed metiiod, continuous wave ultrasound, for example, Power Doppler, may be applied This may be particularly useful where rigid vesicles, for example, vesicles formulated from polymethyl methacrylate or cyanomethacrylate, are employed In this case, the relatively higher energy of the Power Doppler may be made to resonate d e vesicles and thereby promote their rupture This can create acoustic emissions which may be m the subharmonic or ultraharmonic range or, in some cases, m the same frequency as the applied ultrasound It is contemplated that there may be a spectrum of acoustic signatures released in this process and die transducer so employed may receive the acoustic emissions to detect, for example, the presence of a clot In addition, die process of vesicle rupture may be employed to transfer kinetic energy to the surface, for example of a clot to promote clot lysis Thus, therapeutic thrombolysis may be achieved during a combination of diagnostic and therapeutic ultrasound Spectral Doppler may also be employed In general, the levels of energy from diagnostic ultrasound are insufficient to promote the rupture of vesicles and to facilitate release and cellular uptake of bioactive agents As noted above, diagnostic ultrasound may involve e application of one or more pulses of sound Pauses between pulses permits the reflected sonic signals to be received and analyzed The limited number of pulses used in diagnostic ultrasound limits the effective energy which is delivered to the tissue that is bemg studied Higher energy ultrasound, for example, ultrasound which is generated by therapeutic ultrasound equipment, is generally capable of causing rupture of the vesicle species In general, devices for therapeutic ultrasound employ from about 10 to about 100% duty cycles, dependmg on die area of tissue to be treated witii the ultrasound Areas of the body which are generally charactenzed by larger amounts of muscle mass, for example, backs and thighs, as well as highly vasculaπzed tissues, such as cardiovascular tissue, may require a larger duty cycle, for example, up to about 100% In therapeutic ultrasound, continuous wave ultrasound is used to deliver higher energy levels For the rupture of vesicles, continuous wave ultrasound is preferred, altiiough d e sound energy may be pulsed also If pulsed sound energy is used, the sound will generally be pulsed in echo tram lengtiis of from about 8 to about 20 or more pulses at a time Preferably, me echo train lengths are about 20 pulses at a time In addition, die frequency of d e sound used may vary from about 0 025 to about 100 megahertz (MHZ) In general, frequency for therapeutic ultrasound preferably ranges between about 0 75 and about 3 MHZ, with from about 1 and about 2 MHZ being more prefeπed In addition, energy levels may vary from about 0 5 Watt (W) per square centimeter (cm 2 ) to about 5 0 W/cm 2 , with energy levels of from about 0 5 to about 2 5 W/cm 2 being preferred Energy levels for therapeutic ultrasound involving hyperthermia are generally from about 5 W/cm 2 to about 50 W/cm 2 For very small vesicles, for example, vesicles having a diameter of less man about 0 5 μm, higher frequencies of sound are generally preferred This is because smaller vesicles may be capable of absorbing sonic energy more effectively at higher frequencies of sound When very high frequencies are used, for example, greater man about 10 MHZ, the some energy may penetrate fluids and tissues to a limited depth only Thus, external application of the sonic energy may be suitable for skin and otiier superficial tissues However, it is generally necessary for deep structures to focus the ulttasomc energy so diat it is preferentially directed witiim a focal zone Alternatively, die ultrasonic energy may be applied via interstitial probes, intravascular ultrasound catiieters or endolummal catheters Such probes or catheters may be used, for example, in the esophagus for die diagnosis and/or treatment of esophageal carcinoma In addition to the therapeutic uses discussed above, the compositions described herem can be employed in connection with esophageal carcinoma or in die coronary artenes for die treatment of atherosclerosis, as well as die therapeutic uses described, for example, m U S Patent No 5,149,319, e disclosures of which are hereby incoφorated herein by reference, in tiieir entirety A tiierapeutic ultrasound device may be used which employs two frequencies of ultrasound The first frequency may be defined as x, and d e second frequency may be defined as 2x In preferred form, die device would be designed such that die focal zones of the first and second frequencies converge to a single focal zone The focal zone of die device may then be directed to the compositions, for example, vesicle compositions, witiim die tissue m die region of interest This ultrasound device may provide second harmonic tiierapy with simultaneous application of the x and 2x frequencies of ultrasound energy It is contemplated ti at, in d e case of ultrasound involvmg vesicles, this second harmonic therapy may provide improved rupturing of vesicles as compared to ultrasound energy involving a single frequency. Also, it is contemplated diat die preferred frequency range may reside within the fundamental harmonic frequencies of the vesicles. Lower energy may also be used with this device. An ultrasound device which may be employed in connection wid the aforementioned second harmonic erapy is described, for example, in Kawabata, K. et al., Ultrasonics Sonochemistry, Vol. 3, pp. 1-5 (1996), the disclosures of which are hereby incoφorated herein by reference, in their entirety. In the case of vesicle compositions formulated from lipids, die concentration of lipid required to form a desired stabilized vesicle level may vary depending, for example, upon d e type of lipid used, and may be readily determined by routine experimentation. For example, in preferred embodiments, the concentration of 1,2-dipalmitoylphosphatidylcholine (DPPC) used to form stabilized vesicles according to- the methods of d e present invention may be from about 0.1 mg/mL to about 30 mg/mL of saline solution, more preferably from about 0.5 mg/mL to about 20 mg/mL of saline solution, and even more preferably from about 1 mg/mL to about 10 mg/mL of saline solution. The concentration of distearoylphosphatidylcholine (DSPC) used in prefeπed embodiments may be from about 0.1 mg/mL to about 30 mg/mL of saline solution, more preferably from about 0.5 mg/mL to about 20 mg/mL of saline solution, and even more preferably from about 1 mg/mL to about 10 mg/mL of saline solution. The useful dosage of contrast agent and/or renal vasodilator to be administered, and the particular mode of administration, may vary depending upon d e age, weight and the particular mammal and region thereof to be scanned, and die particular contrast medium and/or renal vasodilator to be employed. Typically, die dosage may be initiated at lower levels and increased until die desired contrast enhancement or other effect is achieved. With respect to die contrast agent, die IV dose may be less than about 10 mL for a 70 Kg patient, with lower doses being prefeπed. The dosage of the renal vasodilator may vary, for example from about 0.01 to about 100 mg/Kg, or from about 0.4 mg to about 10 g or higher. In prefeπed embodiments which involve the administration of compositions that comprise DPPC, DPPE and DPPE PEG-5000, in combination with a renal vasodilator, die renal vasodilator is preferably administered to the patient at a dosage of from about 0.01 to about 30 mg/mL, preferably from about 0.01 mg/mL to about 1 mg/mL, more preferably from about 0.01 mg/mL to about 0.1 mg/mL. The compositions descπbed herem, and especially the vesicle compositions, are useful as contrast media diagnostic imaging, and may also be suitable for use in all areas where diagnostic imaging is employed However, die stabilized vesicles are particularly useful for perfusion imagmg In accordance witii the present invention, there are provided methods of imagmg a patient generally, preferably the renal region, and or of specifically diagnosing the presence of diseased tissue in a patient, especially die renal region The imagmg process of the present mvention may be carried out by admmistermg a contrast agent and a renal vasodilator to a patient, and then scanning the patient using, for example, ultrasound, computed tomography, and/or magnetic resonance imagmg, to obtain visible images of an internal region of a patient and or of any diseased tissue in that region The methods may be particularly useful providing images of the renal region, but can also be employed more broadly, such as in imaging the vasculature or the gastrointestinal region, or in other ways as will be readily apparent to tiiose skilled in d e art The patient can be any type of mammal, but most preferably is a human The present mvention also provides metiiods of diagnosing the presence of diseased tissue in a patient, especially diseased tissue in the cardiovasculature Diseased tissue includes, for example, endotiie al tissue which results from vasculature d at supports diseased tissue As a result, the localization and visualization of endotiiehal tissue to a region of a patient which under normal circumstances is not associated with endothelial tissue provides an indication of diseased tissue m the region In addition, the methods of die present invention may be used for diagnosing die presence or absence of renal artery disease, includmg, for example, renal arteπal stenosis Otiier diseases of the cardiovasculature which may be imaged and/or diagnosed witii die methods of the present invention would be readily apparent to one of ordmary skill in the art, once armed witii die present disclosure As noted above, administration of me compositions descπbed herein may be earned out in vanous fashions, such as mtravascularly, orally, rectally, and die like, usmg a vaπety of dosage foπns When the region to be scanned is die cardiovascular region, admmistration of die contrast medium is preferably earned out mtravascularly mcludmg, for example, intravenously When the region to be scanned is d e renal region, admmistration of d e contrast medium is preferably carried out orally or mtravascularly When the region to be scanned is the gastrointestinal region, administration of me contrast medium is preferably caπied out orally or rectally Administration of the renal vasodilator may also be earned out in vaπous fashions, such as mtravascularly or orally, depending on the particular renal vasodilator employed For example, nitrovasodilators, such as nitroglyceπn, may be admimstered IV, as well as transdermally The appropriate mode or modes of admmistration of die renal vasodilator would be readily apparent to one of ordinary skill in the art, once armed with the present disclosure Administration may be continuous (constant), or intermittent (e g , pulsed), as desired, for example by continuous intravenous infusion or by intermittent intravenous infusion Vaπous combinations of the hpid, protein, polymer and/or vesicle compositions may be used to modify the relaxation behavior of the medium or to alter properties such as the viscosity, osmolaπty or palatabihty (in the case of orally admmistered matenals) The echogemcity of vesicles, and especially, gas filled vesicles, and the ability to rupture vesicles at the peak resonant frequency using ultrasound, permits the controlled delivery of bioactive agents to an internal region of a patient Specifically, the vesicles may be momtored subsequent to tiieir admmistration to a patient to determine e rate at which die vesicles arπve, for example, to a desired region Furthermore, d e vesicles may be ruptured usmg ultrasound to release a bioactive agent m die region In accordance with the present invention, tiiere are also provided methods for measurmg blood flow in the renal region of a patient In accordance with this aspect of the invention, there are provided certain embodiments which may involve first applying to the region of mterest a substantial pulse of ultrasonic energy, for example, an ulttasomc energy pulse of up to about 0 5 W/cm 2 , preferably about 0 1 W/cm 2 , to provide a "square wave" of energy This square wave energy may provide substantial reflectivity and, tiierefore, substantial bnghtness m the ultrasound image As the square wave energy becomes diluted in die bloodstream, the brightness of die coπesponding ulttasound image similarly decreases Observation and analysis of d e rate diat die square wave energy becomes diluted may provide an indication of the rate of renal blood flow In anotiier embodiment of a metiiod for measurmg blood flow in the renal region of a patient, tiiere are provided herein methods which generally involve die administration to a patient of a contrast agent m such a manner as to maintain die concentration of contrast agent in die renal region at about a constant level This may be achieved by using any of a variety of methods including, for example, a constant intravenous injection of contrast agent In prefeπed embodiments of the present invention, the pid and/or vesicle compositions may be administered by syringe, mat is, by intravenous (IV) injection Accordingly, the gas and or vesicle admmistration rates provided herem generally correspond to injection rates As would be apparent to one of ordinary skill m the art, once armed witii the present disclosure, the location on die body of die patient at which die pid and/or vesicle compositions are injected may vary and depends upon a variety of factors, includmg, for example, the particular pid and/or vesicle composition employed, the contemplated application, such as diagnostic or tiierapeutic application, and die particular region of interest For example, in the case of diagnostic ultrasound of myocardial tissue, d e hpid and/or vesicle compositions may be injected intravenously (IV), for example, m the arm of a patient The IV administration of the contrast agents described herein including, for example, the vesicle compositions, may involve administration via synnge This may be achieved, for example, by an appropnate medical technician who handles the synnge or synnges manually Alternatively, admmistration may be achieved mechanically, for example, by using a constant infusion device, such as a mechanical injector which operates usmg pneumatic or hydraulic pressure Suitable mechamcal mjectors which may be used m the metiiods of the present mvention include a Syringe Pump Model 351, commercially available from Sage Instruments (a division of Orion Research Inc , Boston, MA), a MedRad™ power mjector, commercially available from Medrad, Inc (Pittsburgh, PA) or a Liebel Flarsheim, commercially available from Liebel Flarsheim Co (Cincinnati, OH) Preferably, die contrast agent is admmistered for a period of time of at least about 1 second, more preferably from about 5 seconds up to about 30 seconds or longer, depending, for example, upon the particular contrast agent employed, the volume of injection, and die like In the case of contrast agents based on vesicles, the time period of admmistration may also depend, for example, on die concentration of vesicles in the contrast agent, die size of die vesicles and d e distnbution of vesicle size m the contrast agent Scannmg the renal region with diagnostic imagmg such as, for example, ultrasound imagmg, provides a diagnostic image of the renal region By maintaining the concentration of contrast agent in the renal region substantially constant, there may desirably be provided a substantially constant level of bnghtness in this diagnostic image Subsequent admmistration to die patient of a renal vasodilator may then increase the renal blood flow which, as discussed above, may also increase the concentration of contrast agent in the renal tissue The images obtained through imaging, such as ultrasonic imaging, may be analyzed qualitatively, but preferably quantitative techniques are used The videodensitometry signal may be digitized and analyzed to determine changes in signal intensity Vaπous comparative measurements may be made For example, comparison of videodensitometry from two kidneys may be compared to determine whether one kidney exhibits a relatively lower videodensitometπc response, indicating the likely presence of renal artery hypertension in one kidney Alternatively, die videodensitometry from one or both kidneys may be compared with that of the abdominal aorta Using the videodensitometry from die aorta as a baseline, it is possible to compare the relative enhancement the kidneys in a patient tiiought to have renal hypertension with an age- adjusted standard enhancement for a normal patient, which may provide an indication of the extent of any renal stenosis Subtractive methods may be used, for example, for comparing two kidneys in d e same patient or for comparing e videodensitometry before contrast enhancement by administration of contrast agent witii the videodensitometry after contrast enhancement For the puφoses of the methods described in accordance with the present embodiment, it is understood that die concentration of vesicles is about proportional to the bnghtness of a diagnostic image, particularly an ultrasound image, which in turn is about proportional to die rate of blood flow m the renal region Also for the puφoses of the mediods described in die present embodiment, it is understood diat d e vesicles preferably follow a first order rate of elimination However, otiier order rates, such as zero, second or tiiird order rates, may be applicable also to tiiese mediods Witii these understandings, there may be obtained an indication of die percent increase in renal flow provided by die renal vasodilator by using a simple denvation -d[vesιcles]/dT = k[vesιcles] where [vesicles] = concentration of vesicles, k = rate constant as a first order process (sec '), and T = tome (seconds) With the understanding also that tiiere is preferably a correlation between vesicle concentration, videodensitometry and renal blood flow, which correlation may be defined, for example, by a correlation constant k, dien -d[vesιcles]/dT = k[vesιcles] ~ d[vιdeodensιtometry]/dT = k[vιdeodensιtometry] where [videodensitometry] = video bnghtness m videodensitometry units (VDU's) Rearranging e above equation yields die following -d[vιdeodensιtometry]/[vιdeodensιtometry] = kdT Thus, -J " d[vιdeodensιtometry]/[vιdeodensιtometry] = J kdT or ln[vιdeodensιtometry., 0 - videodensitometry,] = kT Expansion of die foregomg equation yields [videodensitometry t ] = [videodensitometry.-,-,] = e kτ With die measurement of a baselme videodensitometry at time t = 0 which, in die present embodiment, refers to me videodensity of a diagnostic image of the renal region with a constant infusion of contrast agent, and the final videodensitometry at time T= t which, in die present embodiment, refers to the videodensity of a diagnostic image of the renal region after administration of a renal vasodilator, a rate constant for the mcrease in renal flow at a given dosage of renal vasodilator may be determined With the understanding that the blood flow m the renal region is about proportional to videodensitometry, then substitution yields [flow t ] = [flow te0 ] = e- kτ Thus, the relative increase (%) m renal blood flow may be determmed by employing a constant administration of contrast agent and once armed witii die knowledge of the change brightness of renal tissue in ultrasonic imagmg after administration of a renal vasodilator Variations in correlation between flow, videodensitometry, and microbubble concentration per unit time which may occur may affect the accuracy of the determmations of increases in renal blood flow However, coπections to any non-linearity or non-proportionality may be incoφorated mto the calculations For example, coπechons for variations between vesicle concentration and image quantitation may be provided, for example, by employing die techniques descnbed in Enksen, M , "Tissue Echo Intensity and Blood Attenuation Changes - The Pitfalls of Video Densitometry", The Second Annual International Symposium on Contrast Agents in Diagnostic Ultrasound, Atiantic City, NJ (May 7, 1996) Calibration techniques may be used to improve the accuracy of the coπelation of densitometry data with blood flow For example, an internal calibration method such as mat descnbed m Mor-Avi, V Et Al, Ultrasound in Medicine and Biology 19(8), pp 619-33 (1993) for use m calculating myocardial blood flow may be adapted for use m d e renal region Inaccuracies may remain due to non-lmearity at relatively high concentrations of contrast agent .Alternatively, an external calibration method may be used to determine die relationship between videodensity and volume of contrast agent m a model system Images obtained by scanning a patient using die calibrated contrast agent may be processed to calculate blood flow rates based on the model system Witii an external calibration technique, coπecbons may be made to compensate for non-linearities in acoustic response at higher concentrations of contrast agent and differences m rheological properties between the contrast agent and the red blood cells in d e patient The following examples are merely illustrative of the present invention and should not be considered as limiting the scope of the invention in any way Examples 1 and 2 are actual examples, and Examples 3 and 4 are prophetic Examples Example 1 This example describes the preparation of a hpid vesicle composition for use in the mediods of the present invention "DPPC" refers to dipalmitoylphosphatidylchohne, "DPPE" refers to dipalmitoylphosphatidyletiianolamine, and "DPPA" refers to dipalmitolylphosphatidic acid "PEG5000" refers to poly(ethylene glycol) polymer having a molecular weight of about 5000 "DPPE-PEG5000" refers to DPPE which is covalently bound to PEG5000, wherein the DPPE and PEG5000 are present m a weight ratio of about 20 80 "PFP" refers to perfluoropropane gas To a solution of saline, propylene glycol and glycerol (8 1 1) were added DPPC, DPPE-PEG5000 and DPPA in a molar ratio of 82 8 10 The resulting mixture was heated to about 45 °C and filtered (0 22 μm) The filtered mixture was placed in a vial and allowed to cool to room temperature The vial was placed under vacuum to evacuate any gas, after which d e vial was pressunzed with PFP The vial was ti en sealed, placed on a shaker and agitated at room temperature to provide a solution of PFP-filled vesicles havmg a mean diameter of about 2 5 μm The concentration of vesicles in die solution was about 1 5 x 10 9 vesicles/mL Example 2 This example descnbes die use of vesicles prepared according to Example 1 in the imagmg of kidneys m a mammal to detect an occluded kidney An 18kg female mongrel dog was anaestiiettzed using halotiiane (C 2 HBrClF 3 ) The renal artery leadmg to the left kidney was exposed to a 2mm pneumatic occluder (Invivometric, Healdsburg, CA) and a Doppler flow probe (Transonic, Itiiaca, NY) was attached The abdomen was closed witii sutures and ultrasound imaging was earned out using an Acoustic Imaging 5200S ultrasound machine witii a 7 5 Mhz tightly curved linear aπay transducer (Acoustic Imagmg, Phoenix, AZ) transabdominally The pneumatic occluder was inflated until die flow rate was reduced from 44 ml/min to 22 ml/mm Vesicles prepared according to Example 1 were injected at a dose of 0 02cc/kg (3 0 x 10 7 vesicles/kg) through a catheter in the cephalic vein and imaging of the left (occluded) kidney was earned out This procedure was then repeated on die right (unoccluded) kidney The left renal artery was occluded during imaging of the unoccluded kidney A dose of 330 microliters (μL) of enalapnl (1 25 mg/mL)(Vasotec, Merck, Shaφ and Doh e, West Point, PA) was admmistered tiirough the cephalic catheter The enalapπl was given 30 minutes to act, and then the pneumatic occluder was remflated to reduce the flow to 22 mL/min An additional dose of 0 02 cc/kg (3 0 x 10 7 vesicles/kg) of vesicles was admmistered for each kidney and imagmg was performed Continuous fundamental ultrasound imaging was performed over a period of four mmutes after each injection of vesicles A penod often mmutes was allowed between each injection to allow for clearance of die vesicles from the renal region The images were then captured usmg a Panasonic SV 3350 SVHS VCR (Japan) and a Macmtosh Centns 660AV computer (Apple Computer, Cupertino, CA) The captured images were analyzed using Image 1.55, an image analysis software package distnbuted by the NIH (Bethesda, MD) Regions of interest were selected from die occluded kidney before die admmistration of enalapnl ("pre-enalapπl") and after die administration of enalapπl ("post- enalapnl") and from me unoccluded kidney pre-enalapπl and post- enalapnl The data is shown m the table below A decrease in die videodensitometry (mdicated by a negative number) means diat the image became darker, and an mcrease mdicates an mcrease in bπghtness The term "precontrast" indicates a videodensitometry measurement taken pnor to the admmistration of the vesicles The term "post contrast" mdicates a videodensitometry measurement taken following the admmistration of the vesicles Data is presented as the result of the mdicated subtractions For example, [precontrast - post contrast] represents the subtraction of die videodensitometry measurement following admmistration of the vesicles from die videodensitometry measurement prior to die admmistration of d e vesicles "Pre enalapπl" means before die administration of enalapril. "Post enalapril" means following the administration of enalapril. Treatment Change in Videodensitometry measurement [Precontrast - post contrast], pre 1 1.84 enalapril, Occluded Kidney [Precontrast - post contrast], post 8.59 enalapril, Occluded Kidney Occluded Kidney [pre Enalapril - post -3.25 enalapril] [Precontrast - post contrast], pre 31.74 enalapril, Unoccluded Kidney [Precontrast - post contrast], post 49.20 enalapril, Unoccluded Kidney [pre Enalapril - post enalapril], 17.46 Unoccluded Kidney As can be seen in die above data, tiiere is a measurable increase in brightness in the unoccluded kidney after the administration of enalapril In the occluded kidney, however, the measurements remain quite constant. Example 3 This example describes die use of vesicles prepared according to Example 1 in ultrasonic imaging to measure renal blood flow in a mammal. A 20 kg dog is anestiietized using Halothane. The renal artery is exposed to a 2 mm pneumatic occluder as in Example 2, and a Doppler flow probe is attached to the renal artery in the same manner as in Example 2. Vesicles prepared according to Example 1 (6 x 10 7 vesicles/cc in saline) are preheated to 37°C, then are continuously injected at a rate of 1 cc/minute, through a catiieter in die cephalic vein. Pulsed ultrasound imaging is performed on the kidney, using a Hewlett Packard ultrasound machine (Hewlett Packard, Boston, MA). Continuous second harmonic imaging is used, with ttansmittance at 2 MHZ and receiving at 5 MHZ. A pulse having an energy of 1.0 Watts/cm 2 is then applied intermittently to rupture d e vesicles. The Example is repeated 5 times, with varying time intervals (1, 2, 3, 4, and 5 seconds, respectively) allowed between each rupturing pulse Videodensitometry is performed on the subject The reappearance of die videodensity signal following a rupturing pulse, designated "B", is obtained The signal directly correlates with renal blood flow (correlation constant r=0 88-0 98) Normalized videodensitometry signals (y) is calculated using the formula y=*A(l-10 B A1 ), where A is the peak video density and directly correlates with the quantity of blood in the renal tissue, and t is d e time interval between imaging pulse and ruptunng pulse The data are plotted in Figure 1 "A" represents die peak, or maximum, videodensity, which occurs when the flow of blood had delivered to die renal ttssue die maximum quantity of vesicles, thus generating the maximum videodensity signal Following a ruptunng pulse, the vesicles burst and the signal drops As the blood flow delivers vesicles again, d e videodensity signal reappears "B" represents the reappearance of die videodensity signal The higher the signal, the more vesicles are in the region, and coπespondmgly, the higher the blood flow rate As more vesicles are caπied into die region, die signal B increases The signal is represented in the figure as a ratio of the reappearing videodensity signal B, to the peak videodensity signal, A, l e , (B/A) The higher the rate of blood flow, the higher the ratio (B/A) is for a given normalized videodensity Therefore, it is expected diat (B/A) will increase more rapidly in d e absence of occlusion The normalized videodensity values (y) are on the y axis, and die ratio of reappearance of videodensity B to peak videodensity A (B/A) is on d e x axis Each plot represents a different time interval (1 dirough 5 seconds) following die ruptunng pulse The data illustrate the ability to quantitatively measure blood flow usmg pulsed ultrasound imagmg and continuous admmistration of a renal vasodilator Example 4 This example illustrates the use of intermittent ulttasomc imagmg with a renal vasodilator to measure blood flow dirough d e kidney of a mammal havmg a stenotic renal artery The same procedure is earned out as in Example 3, except tiiat a 20 kg dog with a stenotic renal artery is used Ultrasonic imaging is performed as m Example 3 One hour is allowed for die vesicles to be carried from the renal region by blood circulation Captopnl is administered (10 mg) followed by administration of vesicles Ultrasonic imagmg is again performed as in Example 3 The disclosures of each patent, patent apphcation and publication cited or described herein are hereby incoφorated by reference herein, their entirety Various modifications, in addition to those described herein, will be apparent to those skilled m d e art from the foregomg description Such modifications are also intended to fall witiim the scope of the appended claims

Claims: Claims What is claimed is: 1 A method for providing an image of the renal region of a patient comprising (I) administenng to the patient a vesicle composition which comprises hpid, protein or polymer vesicles and a gas or gaseous precursor, in combmation with a renal vasodilator, and (n) scannmg e patient using diagnostic imagmg to obtain a visible image of the renal region 2 A method according to Claim I wherein said vesicles comprise hpid vesicles 3 A method accordmg to Claim 2 wherem said vesicle composition comprises vesicles selected from die group consisting of micelles and hposomes 4 A method accordmg to Claim 2 wherein said lipids comprise a phosphohpid 5 A metiiod accordmg to Claim 4 wherem said phosphohpids are selected from die group consisting of phosphatidylcholme, phosphatidylethanolamme and phosphatidic acid 6 A method accordmg to Claim 5 wherein said phosphatidylcholme is selected from the group consisting of dioleoylphosphatidylcholme, dimyπstoylphosphatidylcholme, dipalmitoylphosphatidylchohne and distearoylphosphatidylcholine 7 A method accordmg to Claim 6 wherein said phosphatidylcholme compπses dipalmitoylphosphatidylcholme 8 A method according to Claim 5 wherein said phosphatidyletiianol- amine is selected from the group consisting of dipalmitoylphosphatidylethanolamine, dioleoylphosphatidyletiianolamine, N-succinyldioleoylphosphatidylethanolamme and l-hexadecyl-2-palmιtoylglycerophosphoetiιanolamιne 9 A metiiod according to Claim 8 wherein said phosphatidyletiianol- amme comprises dipalmitoylphosphatidyletiianolamme 10 A method according to Claim 5 wherem said phosphatidic acid comprises dipalmitolylphosphatidic acid 1 1 A method according to Claim 3 wherein said hpid further comprises a polymer 12 A method according to Claim 11 wherein said polymer comprises a hydrophilic polymer 13 A method according to Claim 12 wherein said hydrophilic polymer comprises polyethylene glycol 14 A method accordmg to Claim 1 wherein said vesicles compnse protein vesicles 15 A method according to Claim 14 wherem said protem compπses albumin 16 A method according to Claim 1 wherem said vesicles compnse polymer vesicles 17 A method accordmg to Claim 16 wherem said polymers compnse synthetic polymers or copolymers which are prepared from monomers selected from the group consisting of acrylic acid, metiiacryhc acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, lactic acid, glycohc acid, e-caprolactone, acrolein, cyanoacrylate, cyanomethacrylate, bisphenol A, epichlorhydnn, hydroxyalkylacrylates, siloxane, dimethylsiloxane, ediylene oxide, eti ylene glycol, hydroxyalkylmethacrylates, N-substituted acrylamides, N-substituted metiiacrylamides, N-vmyl-2-pyπohdone, 2,4-pentadιene-l-ol, vinyl acetate, acrylomtnle, styrene, p-ammo-styrene, p-ammobenzylstyrene, sodium styrene sulfonate, sodium 2- sulfoxyethyl-methacrylate, vinyl pyridine, ammoethyl methacrylates and 2- methacryloyloxytrimethyl-ammomum chloride 18 A method according to Claim 16 wherein said polymers comprise synthetic polymers or copolymers selected from the group consisting of polyacryhc acid, polyethyleneimine, polymethacry c acid, polymethylmethacrylate, polycyanomethacrylate, polysdoxane, polydimethylsiloxane, polylactic acid, poly(e- caprolactone), epoxy resin, poly( ethylene oxide), poly(etiιylene glycol), polyamide, polyvmylidene-polyacrylonitrile, polyvinyhdene-polyacrylomtrile- polymethylmethacrylate and polystyrene-polyacrylonitnle 19 A metiiod according to Claim 18 wherein said polymers compnse polyvinyhdene-polyacrylonitnle copolymer 20 A method accordmg to Claim 1 wherem said composition comprises a gas and wherein said gas compnses a fluonnated gas 21 A metiiod accordmg to Claim 20 wherem said fluonnated gas is a perfluorocarbon gas 22 A method accordmg to Claim 21 wherein said perfluorocarbon gas is selected from die group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane 23 A method accordmg to Claim 20 wherem said fluonnated gas is selected from the group consisting of sulfur hexafluonde and heptafluoropropane 24 A method according to Claim 1 wherein said composition comprises a gaseous precursor and said gaseous precursor has a boilmg point of greater than about 37°C 25 A metiiod accordmg to Claim 24 wherein said gaseous precursor comprises a fluonnated gaseous precursor 26 A metiiod according to Claim 25 wherein said fluonnated gaseous precursor comprises a perfluorocarbon gaseous precursor 27 A method according to Claim 26 wherein said perfluorocarbon gaseous precursor is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane 28 A metiiod accordmg to Claim 1 wherein said diagnostic imaging comprises computed tomography imaging 29 A method according to Claim 1 wherein said diagnostic imagmg comprises ultrasound imaging 30 A method according to Claim 3 wherein said vesicles compnse unilamellar vesicles 31 A metiiod according to Claim 30 wherem said vesicles comprise one monolayer 32 A method according to Claim 31 wherein said lipids compnse phosphohpids 33 A method according to Claim 30 wherein said vesicles comprise one bilayer 34 A method accordmg to Claim 33 wherein said lipids comprise phosphohpids 35 A method according to Claim 3 wherein said vesicles are selected from d e group consisting of o golamellar and multilamellar vesicles 36 A metiiod according to Claim 35 wherein said lipids compnse phosphohpids 37 A method according to Claim 1 wherem said renal vasodilator enhances brightness m the diagnostic image 38 A metiiod according to Claim 1 wherein said renal vasodilator substantially eliminates diagnostic artifacts in die diagnostic image 39 A metiiod according to Claim 1 wherem said vesicle composition further comprises an additional bioactive agent 40 A method according to Claim 1 further comprising applying ultrasound energy sufficient to rupture said vesicles 41 A method accordmg to Claim 40 wherein said ultrasound imagmg is pulsed harmonic imagmg and said vesicle composition is admmistered by continuous intravenous infusion 42 A method according to Claim 1 wherem said renal vasodilator is an angiotensin converting enzyme inhibitor 43 A method according to Claim 42 wherem said renal vasodilator contains a sulfhydryi group 44 A method accordmg to claim 43 wherein said renal vasodilator is selected from the group consisting of captopnl, fentiapπl, pivalopπl, zofenopπl, and alacepπl 45 A method accordmg to claim 42 wherein said renal vasodilator contains two carboxyl groups 46 A metiiod according to Claim 45 wherein said renal vasodilator is selected from the group consisting of lisinopπl, benazepπl, quinapπl, moexipπl, ramipπl, spirapπl, peπndopπl, lndolapπl, pentopnl, indalapnl, cilazapπl, enalapril, and enalapnlat 47 A method according to claim 46 wherem said renal vasodilator is selected from die group consisting of benazepπl, enalapπl, and enalapπlat 48 A method accordmg to claim 42 wherein said renal vasodilator contains phosphorus 49 A method accordmg to claim 48 wherein said renal vasodilator is fosinopπl 50 A method for providing an image of the renal region of a patient compnsmg (l) administenng to the patient a composition compnsmg a hpid, protein, or polymer, and a gas or gaseous precursor, in combmation with a renal vasodilator, and (n) scannmg die patient usmg diagnostic imaging to obtain a visible image of the renal region 51 A method accordmg to Claim 50 wherein said composition comprises 52 A method accordmg to Claim 51 wherein said hpid comprises a phosphohpid 53 A method according to Claim 52 wherem said phosphohpid is selected from the group consisting of phosphatidylcholme, phosphatidylethanolamme and phosphatidic acid 54 A method according to Claim 53 wherein said phosphatidylcholme is selected from the group consisting of dioleoylphosphatidylchohne, dimyπstoylphosphatidylchohne, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine 55 A method accordmg to Claim 54 wherein said phosphatidylcholme comprises dipalmitoylphosphatidylcholine 56 A method according to Claim 53 wherem said phosphatidylethanolamme is selected from the group consisting of dipalmitoylphosphatidylethanolamine, dioleoylphosphatidylethanolamme, N-succinyldioleoylphosphatidyletiianolamine and l-hexadecyl-2-palmιtoylglycerophosphoethanolamιne 57 A method according to Claim 56 wherein said phosphatidyletiianol- amine comprises dipalmitoylphosphatidyletiianolamine 58 A method according to Claim 53 wherein said phosphatidic acid compnses dipalmitolylphosphahdic acid 59 A method accordmg to Claim 51 wherem said pid composition compnses a vesicle composition 60 A method accordmg to Claim 59 wherem said vesicle composition compnses vesicles selected from the group consisting of micelles and hposomes 61 A method accordmg to Claim 59 wherein said pid composition compnses an emulsion, suspension, or dispersion 62 A method according to Claim 51 wherein said hpid further compnses a polymer 63 A metiiod according to Claim 62 wherein said polymer comprises a hydrophilic polymer 64 A method according to Claim 63 wherein said hydrophilic polymer comprises polyethylene glycol 65 A method according to Claim 50 wherein said composition comprises a protein 66 A method according to Claim 65 wherein said protein comprises albumin 67 A method according to Claim 62 where said polymer compnses a synthetic polymer or copolymer which is prepared from monomers selected from the group consisting of acrylic acid, metiiacryhc acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, lactic acid, gly colic acid, e-caprolactone, acrolein, cyanoacrylate, cyanomethacrylate, bisphenol A, epichlorhydnn, hydroxyalkylacrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxyalkylmethacrylates, N-substituted acrylamides, N-substituted methacrylamides, N-vιnyl-2-pyπohdone, 2,4-pentadιene-l-ol, vinyl acetate, acrylomtnle, styrene, p-amino-styrene, p-ammobenzylstyrene, sodium styrene sulfonate, sodium 2- sulfoxyethyl-methacrylate, vmyl pynd e, aminoethyl methacrylates and 2- methacryloyloxytnmethyl-animonium chloride 68 A metiiod according to Claim 67 wherein said polymer compnses a synthetic polymer or copolymer selected from the group consisting of polyacryhc acid, polyethyleneimine, polymethacry c acid, polymethylmethacrylate, polycyanomethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly(e- caprolactone), epoxy resin, poly(ethylene oxide), poly(ethylene glycol), polyamide, polyvmyhdene-polyacrylonitnle, polyvinylidene-polyacrylonitnle- polymethylmethacrylate and polystyrene-polyacrylonitπle 69 A method according to Claim 68 wherem said polymer comprises polyvmyhdene-polyacrylonitπle copolymer 70 A method accordmg to Claim 50 wherein said composition comprises a gas and wherein said gas comprises a fluonnated gas 71 A method according to Claim 70 wherein said fluonnated gas is a perfluorocarbon gas 72 A method according to Claim 71 wherem said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane 73 A method accordmg to Claim 70 wherein said fluonnated gas is selected from die group consisting of sulfur hexafluonde and heptafluoropropane 74 A metiiod accordmg to Claim 50 wherein said composition compnses a gaseous precursor and said gaseous precursor has a boiling pomt of greater than about 37°C 75 A method accordmg to Claim 74 wherem said gaseous precursor comprises a fluonnated gaseous precursor 76 A metiiod accordmg to Claim 75 wherein said fluonnated gaseous precursor comprises a perfluorocarbon gaseous precursor 77 A metiiod accordmg to Claim 76 wherem said perfluorocarbon gaseous precursor is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane 78 A method according to Claim 50 wherein said diagnostic imaging comprises computed tomography imagmg 79 A metiiod according to Claim 50 wherein said diagnostic imagmg comprises ultrasound imagmg 80 A method according to Claim 60 wherem said vesicles comprise unilamellar vesicles 81 A method according to Claim 80 wherein said vesicles comprise one monolayer 82 A method accordmg to Claim 81 wherem said lipids comprise phosphohpids 83 A method according to Claim 80 wherein said vesicles compnse one bilayer 84 A method accordmg to Claim 83 wherem said lipids compnse phosphohpids 85 A method accordmg to Claim 60 wherem said vesicles are selected from the group consisting of o golamellar and multilamellar vesicles 86 A metiiod accordmg to Claim 85 wherein said lipids comprise phosphohpids 87 A metiiod accordmg to Claim 50 wherein said renal vasodilator enhances brightness in the diagnostic image 88 A metiiod according to Claim 50 wherein said renal vasodilator substantially eliminates diagnostic artifacts in the diagnostic image 89 A metiiod according to Claim 50 wherein said vesicle composition further comprises an additional bioactive agent 90 A method for substantially eliminating diagnostic artifacts in a diagnostic image of die renal region of a patient, comprising administering to die patient, in combination with a contrast agent, an agent which mduces vasodilation of a renal artery 91 A method according to Claim 90 wherein the diagnostic image compnses a computed tomography image 92 A metiiod according to Claim 90 wherem e diagnostic image comprises an ultrasound image 93 A method according to Claim 90 wherein the diagnostic artifacts compnse computed tomography artifacts 94 A method according to Claim 90 wherein the diagnostic artifacts comprise ultrasound artifacts 95 A method accordmg to Claim 90 wherem said contrast agent compnses a vesicle composition compnsmg hpid, protem, or polymer vesicles and a gas or gaseous precursor 96 A metiiod accordmg to Claim 95 wherein said vesicle composition compnses hpid vesicles 97 A method according to Claim 96 wherein said vesicles are selected from the group consisting of micelles and hposomes 98 A method accordmg to Claim 95 wherem said composition comprises a gas and wherein said gas compnses a fluonnated gas 99 A method according to Claim 98 wherein said fluonnated gas is a perfluorocarbon gas 100 A method according to Claim 99 wherein said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane 101 A method according to Claim 99 wherein said fluonnated gas is selected from the group consisting of sulfur hexafluonde and heptafluoropropane 102 A method according to Claim 95 wherein said composition comprises a gaseous precursor and said gaseous precursor has a boiling point of greater than about 37°C 103 A metiiod accordmg to Claim 102 wherein said gaseous precursor compnses a fluonnated gaseous precursor 104 A metiiod according to Claim 103 wherem said fluonnated gaseous precursor compnses a perfluorocarbon gaseous precursor 105 A metiiod accordmg to Claim 104 wherein said perfluorocarbon gaseous precursor is selected from die group consisting of perfluoropentane, perfluorohexane and perfluoroheptane 106 A metiiod according to Claim 95 wherem said contrast agent comprises a composition comprising a pid, protein, or polymer and a gas or gaseous precursor 107 A method accordmg to Claim 95 wherein said composition is a hpid composition 108 A method according to Claim 107 wherein said hpid composition is an emulsion, suspension, or dispersion 109 A method accordmg to Claim 106 wherein said composition comprises a gas and wherem said gas comprises a fluonnated gas 1 10 A metiiod according to Claim 109 wherein said fluonnated gas is a perfluorocarbon gas 111 A method according to Claim 1 10 wherem said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroed ane, perfluoropropane, perfluorobutane and perfluorocyclobutane 112 A method according to Claim 110 wherem said fluonnated gas is selected from d e group consisting of sulfur hexafluonde and heptafluoropropane 113 A metiiod accordmg to Claim 106 wherein said composition comprises a gaseous precursor and said gaseous precursor has a boiling point of greater than about 37°C 114 A method accordmg to Claim 113 wherem said gaseous precursor compnses a fluonnated gaseous precursor 115 A metiiod according to Claim 114 wherem said fluonnated gaseous precursor compnses a perfluorocarbon gaseous precursor 116 A method accordmg to Claim 115 wherein said perfluorocarbon gaseous precursor is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane 117 A metiiod according to Claim 90 wherein said agent which mduces vasodilation of a renal artery is an angiotensin converting enzyme inhibitor 118 A method according to Claim 1 17 wherem said angiotensin converting enzyme inhibitor contains a sulfhydryl group 119 A method according to claim 118 wherein said angiotensin converting enzyme inhibitor is selected from the group consisting of captopnl, fentiapπl, pivalopπl, zofenopπl, and alacepπl 120 A method according to claim 116 wherein said angiotensin converting enzyme inhibitor contains two carboxyl groups 121 A method according to Claim 120 wherem said angiotensin converting enzyme inhibitor is selected from the group consisting of hsmopnl, benazepnl, quinapπl, moexipπl, ramipril, spirapπl, penndopπl, indolapnl, pentopnl, indalapnl, cilazapπl, enalapπl, and enalapnlat 122 A metiiod accordmg to claim 121 wherein said angiotensin converting enzyme inhibitor is selected from the group consisting of benazepnl, enalapnl, and enalapnlat 123 A method according to claim 117 wherein said angiotensin converting enzyme inhibitor contains phosphorus 124 A method according to claim 123 wherem said angiotensin converting enzyme inhibitor is fosmopπl 125 A metiiod for diagnosmg the presence of renal disease m the renal region of a patient compnsmg (1) admmistenng to the patient a vesicle composition which comprises hpid, protein, or polymer vesicles and a gas or gaseous precursor, combmation with a renal vasodilator, and (u) scanning the patient using diagnostic imaging to obtam a visible image of any disease in the renal region of the patient 126 A method according to Claim 125 wherein die renal disease is renal artery hypertension 127 A method for diagnosing the presence of renal disease in e renal region of a patient comprising (l) administering to the patient a composition compπsmg a hpid, protein, or polymer and a gas or gaseous precursor, in combination with a renal vasodilator, and (n) scannmg the patient using diagnostic imaging to obtain a visible image of disease in the renal region of the patient 128 A method according to Claim 127 wherein the renal disease is renal artery hypertension 129 A contrast agent for diagnostic imaging compπsmg a vesicle composition which compnses pid, protein, or polymer vesicles and a gas or gaseous precursor, m combination with a renal vasodilator 130 A contrast agent for diagnostic imaging which comprises a composition compnsmg a hpid, protein or polymer and a gas or gaseous precursor, in combmation witii a renal vasodilator 131 A metiiod for measurmg blood flow m the renal region of a patient, compnsmg (i) admmistenng to die patient a vesicle composition which compnses pid, protem or polymer vesicles and a gas or gaseous precursor, (n) scanning the patient usmg diagnostic imagmg to obtam a visible image of d e renal region, (in) admmistermg to the patient a renal vasodilator, (iv) continuing said scannmg, and (v) determinmg the blood flow from a videodensity versus time relationship m d e images obtained m steps (n) through (iv) 132 A method according to Claim 131 wherein said vesicle composition is admmistered to the patient to maintain a substantially constant concentration of vesicle composition in the renal region 133 A metiiod according to Claim 132 wherein said admmistration comprises constant admmistration 134 A metiiod according to Claim 1 3 comprising admmistenng said vesicle composition using a constant infusion device 135 A method according to Claim 134 wherein said constant infusion device compnses a power injector 136 A method accordmg to Claim 131 wherein said diagnostic imaging is selected from die group consisting of ulttasomc imagmg and computed tomography imaging 137 A method according to Claim 136 wherein said diagnostic imagmg compnses ultrasonic imagmg 138 A metiiod according to Claim 131 wherein said vesicles comprise hpid vesicles 139 A method according to Claim 138 wherein said vesicles comprise vesicles selected from the group consisting of micelles and hposomes 140 A metiiod according to Claim 138 wherein said pid compnses a phosphohpid 141 A method accordmg to Claim 140 wherein said phosphohpid is selected from the group consisting of phosphatidylcholme, phosphatidyletiianolamine and phosphatidic acid 142 A method according to Claim 140 wherein said phosphatidyl-choline is selected from the group consisting of dioleoylphosphatidylchohne, dimynstoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholme 143 A method accordmg to Claim 142 wherein said phosphatidylcholme comprises dipalmitoylphosphatidylcholine 144 A method accordmg to Claim 141 wherein said phosphatidyl- eti anolamine is selected from the group consisting of dipalmitoylphosphatidylethanol- amine, dioleoylphosphatidyletiianolamine, N-succinyldioleoylphosphatidyletiianolamine and l-hexadecyl-2-palmιtoylglycerophosphoethanolamιne 145 A method accordmg to Claim 144 wherein said phosphatidyleti anolamine comprises dipalmitoylphosphatidylethanolamme 146 A method according to Claim 141 wherem said phosphatidic acid compnses dipalmitolylphosphatidic acid 147 A method accordmg to Claim 138 wherein said hpid further compnses a polymer 148 A method accordmg to Claim 147 wherein said polymer compnses a hydrophilic polymer 149 A metiiod accordmg to Claim 148 wherein said hydrophilic polymer compnses polyethylene glycol 150 A method accordmg to Claim 131 wherein said vesicles compnse polymer vesicles 151 A method according to Claim 150 wherein said polymer comprises a synthetic polymer or copolymer which is prepared from monomers selected from the group consisting of acrylic acid, methacryhc acid, ethyleneimme, crotonic acid, acrylamide, etiiyl acrylate, methyl methacrylate, 2-hydroxyetiιyl methacrylate, lactic acid, glycohc acid, e-caprolactone, acrolem, cyanoacrylate, cyanomethacrylate, bisphenol A, epichlorhydπn, hydroxyalkylacrylates, siloxane, dimetiiylsiloxane, ethylene oxide, ethylene glycol, hydroxyalkylmethacrylates, N-substituted acrylamides, N-substituted methacrylamides, N-vιnyl-2-pyrrohdone, 2,4-pentadιene-l-ol, vinyl acetate, acrylomtnle, styrene, p-amino-styrene, p-aminobenzylstyrene, sodium styrene sulfonate, sodium 2- sulfoxyethyl-methacrylate, vinyl pyridine, aminoethyl methacrylates and 2- methacryloyloxytπmethyl-ammonium chloride 152 A method according to Claim 151 wherein said polymer comprises a synthetic polymer or copolymer selected from e group consisting of polyacryhc acid, polyethyleneimine, polymethacrylic acid, , polycyanomethacrylate, polymethylmethacrylate, polysiloxane, poly dimethyl siloxane, polylactic acid, poly(e- caprolactone), epoxy resin, poly(etiιylene oxide), poly(etiιylene glycol), polyamide, polyvmyhdene-polyacrylonitrile, polyvmylidene-polyacrylonitπle- polymethylmethacrylate and polystyrene-polyacrylonitnle 153 A method according to Claim 152 wherein said polymer compnses polyvmyhdene-polyacrylomtπle copolymer 154 A method accordmg to Claim 131 wherem said gas compnses a fluonnated gas 155 A metiiod accordmg to Claim 154 wherein said fluonnated gas is selected from the group consisting of a perfluorocarbon, sulfur hexafluonde and heptafluoropropane 156 A method accordmg to Claim 154 wherein said fluonnated gas compnses a perfluorocarbon 157 A method according to Claim 156 wherem said perfluorocarbon gas is selected from d e group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane 158 A method according to Claim 131 wherein said gaseous precursor has a boilmg pomt of greater than about 37 ° C 159 A metiiod according to Claim 158 wherein said gaseous precursor comprises a fluonnated compound 160 A method according to Claim 159 wherein said fluonnated compound comprises a perfluorocarbon 161 A method according to Claim 160 wherein said perfluorocarbon is selected from die group consisting of perfluoropentane, perfluorohexane and perfluoroheptane 162 A metiiod according to Claim 96 wherein said imaging comprises ultrasound imaging 163 A method according to Claim 162 further compnsmg the step of applying ultrasound energy sufficient to rupture said vesicles 164 A method accordmg to Claim 162, further comprising the application of ultrasound energy sufficient to rupture said vesicles, wherein said application occurs dunng steps (u) and (iv) 165 A method accordmg to Claim 164 wherem said vesicle composition is administered by continuous intravenous infusion 166 A method for measuring blood flow in the renal region of a patient, comprising (1) admmistenng to the patient a composition which comprises a pid, protein or polymer and a gas or gaseous precursor, (n) scarmmg die patient using diagnostic imagmg to obtain a visible image of the renal region, (in) admmistenng to me pattent a renal vasodilator, (iv) conttnumg said scanning, and (v) detemumng the blood flow from a videodensity versus time relationship in the images obtained in steps (u) through (iv)

Inventors: UNGER EVAN C, UNGER, EVAN, C.
Applicants: IMARX PHARMACEUTICAL CORP, IMARX PHARMACEUTICAL CORP.
Application year: 1997
Main classification: A61K49
IPC codes: A61K9, A61K31, A61K45, A61K49