IMPROVED METHODS FOR DIAGNOSTIC IMAGING INVOLVING THE USE OF A CONTRAST AGENT AND A CORONARY VASODILATOR

Abstract: Improved method for providing an image of an internal region of a patient, especially the cardiovascular region. Embodiments of the invention involve the administration to the patient of a contrast agent which comprises a vesicle composition comprising lipid or polymer vesicles and a gas or gaseous precursor, in combinationwith a coronary vasodilator. The patient is scanned using diagnostic imaging, such as ultrasound, to obtain a visible image of the region. The methods are particularly useful for diagnosing the presence of diseased tissue in the cardiovascular region of a patient, as well as for measuring blood flow in the cardiovascular region of a patient.

Description: IMPROVED METHODS FOR DIAGNOSTIC IMAGING INVOLVING THE USE OF A CONTRAST AGENT AND A CORONARY VASODILATOR Field of the Invention The present invention relates to improved methods for diagnostic imaging. More particularly, the present invention relates to improved methods for diagnostic imaging which involve the administration to a patient of a contrast agent and a coronary vasodilator. Background of the Invention Ultrasound is a valuable diagnostic imaging technique for studying various areas of the body including, for example, the vasculature, such as tissue micro vasculature. Ultrasound provides certain advantages over other diagnostic techniques. For example, diagnostic techniques involving nuclear medicine and X-rays generally results in exposure of the patient to ionizing electron radiation. Such radiation can cause damage to subcellular material, including deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and proteins. 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 of the tissue. The reflection of sound waves off of 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 various 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. Ultrasound imaging techniques often involve the use of contrast agents. 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 characterized 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, unilamellar, oligolamellar and/or multilamellar vesicles, such as liposomes, micelles and the like. As discussed more fully hereinafter, the effectiveness of bubbles as contrast agents depends upon various factors, including, for example, the size and/or elasticity of the bubble. The effectiveness of bubbles as contrast agents depends upon various 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 comprise bubbles having a diameter of greater than about 10 μm can be dangerous since micro vessels 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 preferred. The volume weighted mean diameter of the bubbles should be about 7 to about 20 μm. The use of echocardiography for the diagnosis of coronary artery disease has generally been limited to indirect methods which involve the detection and quantitation of abnormalities in wall motion. Echocardiography has also been used in connection with methods for detecting pathologies of the heart to identify cardiac masses, thrombi, vegetative lesions (endocarditis), myxomas, and other lesions. See, e.g. , M.L. Marcus et al. , Cardiac Imaging: A Companion to Braunwald's Heart Disease, W.B. Saunders Co., Harcourt Brace Jovanovich, Inc. , Philadelphia, PA, pp. 536-556 (1991). The viability of currently available ultrasound contrast agents and methods involving their use is highly dependent on a variety of factors, including the particular region being imaged. For example, difficulty is encountered in obtaining useful diagnostic images of heart tissue and the surrounding vasculature due, at least in part, to the large volume of blood which flows through the chambers of the heart relative to the volume of blood which flows in the blood vessels of the heart tissue itself. The high volume of blood flowing through the chambers of the heart can result in insufficient contrast in ultrasound images of the heart region, especially the heart tissue. The high volume of blood flowing through the chambers of the heart can also produce diagnostic artifacts including, for example, shadowing or darkening, in ultrasound images of the heart. Diagnostic artifacts can be highly undesirable since they may hamper or even prevent visualization of a region of interest. Thus, 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 various areas of the body. In CT, the radiodensity (electron density) of matter is measured and is expressed in terms of Hounsefield Units (HU). Hounsefield 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 similarity in 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 iodine compounds, for example, have been developed for this purpose. For the gastrointestinal tract, barium sulfate is used extensively to increase the radiodensity 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 imaging (MRI) is another diagnostic imaging 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 TI (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 in 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 TI, T2, or the proton density. Generally speaking, MRI requires the use of contrast agents. If MRI is performed without employing 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 primarily on paramagnetic contrast agents for MRI. Paramagnetic contrast agents involve materials which contain unpaired electrons. The unpaired electrons act as small magnets within the main magnetic field to increase the rate of longitudinal (TI) and transverse (T2) relaxation. Paramagnetic contrast agents typically comprise 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 ligands. 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 arises out of intrinsic peristaltic motions and motions from respiration or cardiovascular action. In 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. , Magnetic Resonance Imaging, 6: 124 (1988). Other contrast agents may be less sensitive to variations in pulse sequence and may provide more consistent contrast. However, high concentrations of particulates, such as ferrites, can cause magnetic susceptibility artifacts which are particularly evident, for example, in the colon where the absorption of intestinal fluid occurs and the superparamagnetic material may be concentrated. Toxicity is another problem which is generally associated with currently available contrast agents for MRI. For example, ferrites often cause symptoms of nausea after oral administration, as well as flatulence and a transient rise in serum iron. The gadolinium ion, which is complexed in Gd-DTPA, is highly toxic in free form. The various environments of the gastrointestinal tract, including increased acidity (lower pH) in the stomach and increased alkalinity (higher pH) in 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 in connection with thallium 201 ( 201 TI) in an effort to improve the visualization of viable myocardial tissue in nuclear medicine. Vasodilators can improve visualization by increasing blood flow to the myocardium which enables the 201 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 (TI 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. Accordingly, new and/or better diagnostic imaging methods which permit the regulation of the flow of blood through the chambers of the heart and/or the heart tissue, and/or new and/or better diagnostic imaging methods which permit the quantification of blood flow in the heart region are needed. The present invention is directed to these, as well as other, important ends. Summary of the Invention The present invention is directed, in part, to improved methods for diagnostic imaging. Specifically, in one embodiment, there is provided an improved method for providing an image of the cardiovascular region of a patient. The method comprises administering to the patient a vesicle composition comprising lipid or polymer vesicles and a gas or gaseous precursor, in combination with a coronary vasodilator. The patient is scanned using diagnostic imaging to obtain a visible image of the cardiovascular region. Another embodiment of the invention also relates to a method for providing an image of the cardiovascular region of a patient. The method comprises administering to the patient a lipid composition comprising a lipid and a gas or gaseous precursor, in combination with a coronary vasodilator. The patient is scanned using diagnostic imaging to obtain a visible image of the cardiovascular region. Still another embodiment of the present invention relates to a method for diagnosing the presence of diseased tissue in the cardiovascular region of a patient. The method comprises administering to the patient a vesicle composition comprising lipid or polymer vesicles and a gas or gaseous precursor, in combination with a coronary vasodilator. The patient is scanned using diagnostic imaging to obtain a visible image of any diseased tissue in me cardiovascular region of a patient. Yet another embodiment of the invention relates to a method for diagnosing the presence of diseased tissue in the cardiovascular region of a patient. The method comprises administering to the patient a lipid composition comprising a lipid and a gas or gaseous precursor, in combination with a coronary vasodilator. The patient is scanned using diagnostic imaging to obtain a visible image of any diseased tissue in the cardiovascular region of the patient. In another embodiment of the invention, there is provided a method for measuring blood flow in the cardiovascular region of a patient. The method comprises (i) administering to the patient a vesicle composition which comprises lipid or polymer vesicles and a gas or gaseous precursor, (ii) scanning the patient with diagnostic imaging to obtain a visible image of the cardiovascular region, (iii) administering to the patient a coronary vasodilator, (iv) continuing said scanning, and (v) determining the blood flow from a videodensity versus time relationship in the images obtained in steps (ii) through (iv). These and other aspects of the invention will become more apparent from the present specification and claims. 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 synthetic or naturally-occurring compound which is generally amphipathic and biocompatible. The lipids typically comprise a hydrophilic component and a hydrophobic component. Exemplary lipids include, for example, fatty acids, neutral fats, phosphatides, glycolipids, surface-active agents (surfactants), aliphatic alcohols, waxes, terpenes and steroids. "Lipid composition" refers to a composition which comprises a lipid compound, typically in an aqueous medium. Exemplary lipid compositions include suspensions, emulsions and vesicle compositions. The lipid compositions described herein may also comprise a bioactive agent. "Vesicle" refers to a spherical 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 described herein, or polymeric materials, including natural, synthetic and semi- synthetic polymers. Preferred vesicles are those which comprise walls or membranes formulated from lipids. In these 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 concentric. Lipids may be used to form a unilamellar vesicle (comprised of one monolayer or bilayer), an oligolamellar vesicle (comprised of about two or about three monolayers or bilayers) or a multilamellar vesicle (comprised of more than about three monolayers or bilayers). Similarly, the vesicles prepared from polymers may comprise one or more concentric walls or membranes. The walls or membranes of vesicles prepared from polymers 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, liposomes, micelles, bubbles, microbubbles, microspheres, lipid- and/or polymer-coated bubbles, microbubbles and/or microspheres, microballoons, microcapsules, aerogels, clathrate bound vesicles, 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 material, including, for example, a targeting ligand, coronary vasodilator and/or bioactive agent, as desired. "Liposome" refers to a generally spherical cluster or aggregate of amphipathic compounds, including lipid compounds, typically in the form of one or more concentric layers, for example, bilayers. They may also be referred to herein as lipid vesicles. The liposomes may be formulated, for example, from ionic lipids and/or non-ionic 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 H2 phase configuration. In other preferred embodiments, the micelles may comprise a bilayer configuration. "Aerogel" refers to generally spherical 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 resorcinol and formaldehyde), as well as natural materials, such as polysaccharides. "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 hydroxy apatite. 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 in which there is encapsulated a gaseous precursor. The vesicles may be minimally, partially or substantially completely filled with the gas and/or gaseous precursor. In certain preferred embodiments, the vesicles may be substantially or 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. Preferably, greater than about 60% of the internal void of the substantially filled vesicles consists of a gas, 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, with greater than about 90% being still more preferred. In particularly preferred embodiments, greater than about 95% of the internal void of the vesicles consists of a gas, with about 100% being especially preferred. Although not considered a preferred embodiment of the present invention, the vesicles may also contain, if desired, no or substantially no gas or gaseous precursor. "Vesicle composition" refers to a composition, typically in an aqueous medium, which comprises vesicles. The vesicle compositions described herein may also comprise a bioactive agent. "Emulsion" refers to a lipoidal mixture of two or more liquids and is generally in the form of a colloid. The lipids may be heterogeneously dispersed throughout the emulsion. Alternatively, the lipids may be aggregated in the form of, for example, clusters or layers, including mono- or bilayers. "Suspension" refers to a mixture of finely divided liquid or solid particles floating in a liquid which 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 portion(s) of the lipids generally face inwardly in association with a 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 in the hexagonal phase structure. "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 include, for example, areas being imaged with diagnostic imaging and/or areas being treated with a bioactive agent. The phrase "cardiovascular region of a patient" refers to the region of the patient defined by the heart (myocardium) and the vasculature leading directly to and from the heart. The phrase "vasculature," as used herein, denotes the blood vessels (including arteries, veins and the like) in the body or in an organ or part of the body. The phrase "circulatory system" refers to the cardiovascular region and the entire vasculature. The phrase "ventricular region of a patient" refers to the region of the heart defined by the right and left ventricles. The phrase "atrial region of a patient" refers to the region of the heart defined by the right and left atria. "Bioactive agent" refers to a substance which may be used in connection with an application that is therapeutic or diagnostic in nature, such as in methods for diagnosing the presence or absence of a disease in a patient and/or in methods for the treatment of disease in a patient. As used herein, "bioactive agent" refers also to substances which are capable of exerting a biological effect in vitro and/or in vivo. The bioactive agents may be neutral or positively or negatively charged. Examples of suitable bioactive agents include diagnostic agents, pharmaceuticals and/or drugs, including coronary vasodilators, synthetic organic molecules, proteins, peptides, vitamins, steroids, steroid analogs, and genetic material, including nucleosides, nucleotides and polynucleotides. Preferably, the bioactive agent comprises a pharmaceutical and/or drug. "Diagnostic agent" refers to any agent which may be used in connection with methods for imaging an internal region of a patient, including the cardiovascular region, and/or in methods for diagnosing the presence or absence of a disease in a patient, especially diseases of the heart including, for example, myocardial ischemia and myocardial infarction. Exemplary diagnostic agents include, for example, contrast agents for use in connection with ultrasound, magnetic resonance imaging or computed tomography of a patient including, for example, the lipid and/or vesicle compositions described herein. "Polymer", as used herein, 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, trimers and oligomers. The polymer may be synthetic, naturally-occurring or semisynthetic. In preferred form, the term "polymer" refers to molecules which comprise 10 or more repeating units. "Thickening agent" refers to any of a variety of generally hydrophilic materials which, when incoφorated in the lipid and/or vesicle compositions described herein, may act as viscosity modifying agents, emulsifying and/or solubilizing 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. "Genetic material" refers generally to nucleotides and polynucleotides, including deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The genetic material 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 combination of the two. The DNA and RNA may optionally comprise unnatural nucleotides and may be single or double stranded. "Genetic material" 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. "Pharmaceutical" 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 in a patient. Therapeutically useful peptides, polypeptides and polynucleotides may be included within the meaning of the term pharmaceutical or drug. Preferred pharmaceuticals and/or drugs are coronary vasodilators. "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 interest. Diagnostic artifacts may be manifested as undesired darkening and/or shadowing in the diagnostic image. Diagnostic artifacts in the cardiovascular region are believed to arise from an undesirably high volume of ventricular and/or atrial blood flow relative to myocardial blood flow. "Ultrasound artifact, " "computed tomography artifact" and "MRI artifact" refer respectively to diagnostic artifacts associated with ultrasound, computed tomography and MRI. "Ventricular blood flow" refers to the flow of blood through the ventricles of the heart in one or more cardiac cycles. "Atrial blood flow" refers to the flow of blood through the atria of the heart in one or more cardiac cycles. "Myocardial blood flow" refers to the flow of blood in the vasculature in the cardiovascular region, including the blood vessels in the heart, in one or more cardiac cycles. "Cardiac cycle" refers to a complete contractile period of the heart, and includes both the diastole and systole periods. "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, myocardial tissue. Generally speaking, videodensitometric analysis involves the use of a computer system which is capable of digitizing a full region of analog video data into 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 interest. Thus, in connection with diagnostic imaging of the cardiovascular region, the term "brightness" refers to the level of contrast of a diagnostic image of the cardiovascular region, including the heart 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 prior art and which may be obtained with the methods of the present invention. Enhanced diagnostic images may be manifested by an increase in brightness in the diagnostic image, substantial elimination of diagnostic artifacts in the diagnostic image, and/or the like. Thus, in connection with diagnostic imaging of the cardiovascular region, including the heart tissue and the vasculature associated therewith, an enhanced diagnostic image may be manifested, for example, by increased brightness in the diagnostic image of the cardiovascular region and/or a substantial elimination in the occurrence of diagnostic artifacts in the diagnostic image of the cardiovascular region. "Increased brightness" refers to an increase in the brightness of a diagnostic image which may be obtained with the methods of the present invention. Preferably, the increase in brightness provided by the methods of the present invention are at least discernible 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 brightness of at least about 10 VDUs (gray levels). More preferably, in accordance with the embodiment described herein, the methods of the present invention provide an increased brightness 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 150 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 occurrence 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 invention 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 invention 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 prior art diagnostic method, with the elimination of at least about 100% being still more preferred. The terms "administered" and "administration" refer generally to the administration to a patient of a biocompatible material, including, for example, the lipid and/or vesicle compositions described herein. "Biocompatible" refers to materials which are generally not injurious to biological functions and which will not result in any degree of unacceptable toxicity, including allergenic responses and disease states. "In combination with" refers to the co-administration of a bioactive agent, such as a coronary vasodilator, with a lipid and/or vesicle composition. The term "co-administration" means that the bioactive agent may be administered before, during, or after the administration of the lipid and/or vesicle composition. In embodiments in which the bioactive agent is included in the lipid and/or vesicle composition, the bioactive agent may be combined with the lipid and/or 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 lipids (in the case of vesicle compositions which comprise vesicles formulated from lipids) which are contained within the vesicle layer(s) or wall(s). It is contemplated that the bioactive agent may be located on the surface of a vesicle. In this case, the bioactive agent may interact chemically with the surface of the vesicle and remain substantially adhered thereto. Such interaction may take the form of, for example, electrostatic interactions, hydrogen bonding, van der Waal's forces or covalent bonding. Also, the bioactive agent may interact with the surface of the vesicle 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. "Coronary vasodilator" refers to a bioactive agent which, when administered to a patient, causes dilation of the vasculature in the cardiovascular region. 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 cardiovascular 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 composition comprising a lipid 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 coronary vasodilator is administered to the patient. It is contemplated that the coronary vasodilator provides unique advantages in connection with methods for diagnostic imaging, especially ultrasound, relative to methods for diagnostic imaging which have been available heretofore. In this connection, coronary vasodilators are capable of dilating blood vessels in the cardiovascular region of a patient. This, in turn, may produce an alteration in the patient's blood flow in the cardiovascular region. It has been surprisingly and unexpectedly found that this alteration in the flow of blood in the cardiovascular region may produce profound changes in the quality of diagnostic images of the cardiovascular region. The present invention is directed, at least in part, to exploiting this surprising and unexpected discovery to provide improved methods for diagnostic imaging of the cardiovascular region. The present invention provides a simple and effective means for obtaining improved images of the cardiovascular region of a patient. Diagnostic imaging, such as ultrasound imaging of the cardiovascular region, may involve the use of a contrast agent, for example, a contrast agent comprising gas filled vesicles, which is administered intravenously. After injection, the contrast agent may be carried in the bloodstream to the cardiovascular 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 chambers of the heart, including the ventricular and atria regions of the heart, as well as through the vasculature of the heart itself. Energy, for example, ultrasound, may be applied, and a diagnostic image of the cardiovascular region may be generated. It has been observed that the volume of blood which flows through the ventricular and atrial regions of the heart, in any given period of time, is generally significantly greater than the volume of blood which flows, in the same period of time, in the vasculature of the heart (the myocardial blood flow). Thus, in the case of ultrasound imaging of the cardiovascular region involving, for example, a contrast agent comprising gas filled vesicles, an undesirably high degree of backscatter may occur from contrast agent which is contained in the blood flowing through the chambers of the heart relative to the backscatter that occurs from contrast agent which is contained in the blood flowing through the vasculature of the heart. The backscatter from the blood in the heart chambers may interfere with the backscatter from the blood in the vasculature, resulting in poor contrast and/or visualization of the heart tissue. The backscatter from the blood in the heart chambers may also result in the formation of diagnostic artifacts in the diagnostic image. As noted above, diagnostic artifacts can be highly undesirable since they may hamper or even prevent visualization of a region of interest. The inventors have surprisingly and unexpectedly found that, in connection with methods for the diagnostic imaging of the cardiovascular region which involve the administration to a patient of a contrast agent comprising lipids and a gas or gaseous precursor, and especially a contrast agent comprising polymer or lipid vesicles and a gas or gaseous precursor, the additional administration of a coronary vasodilator can desirably enhance the resulting diagnostic image. While the inventors do not wish to be bound by any theory or theories of operation, it is believed that the enhanced diagnostic images may be due to an alteration in coronary blood flow caused by the coronary 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 coronary vasodilator may increase myocardial blood flow and, therefore, the concentration of contrast agent in the myocardial tissue. This increase in contrast agent in the myocardial tissue may provide increased brightness in diagnostic images of the cardiovascular region which may result in improved tissue visualization. It is believed also that the administration of a coronary vasodilator may increase the flow of blood (and concentration of contrast agent) in the myocardial tissue relative to the flow of blood (and concentration of contrast agent) in one or more of the chambers of the heart. This may result in a reduction or substantial elimination of diagnostic artifacts in images of the cardiovascular region. The degree of increase in myocardial blood flow which may be provided using the methods of the present invention may vary and depends, for example, on the particular lipid and/or vesicle composition administered to the patient, the particular coronary vasodilator administered to the patient, the respective dosages of the lipid/vesicle composition and coronary vasodilator administered to the patient, and the like. Generally speaking, any increase in myocardial blood flow which is obtained using the methods of the present invention may provide enhanced diagnostic images. In accordance with preferred embodiments of the present invention, enhanced diagnostic images may be obtained by increasing myocardial blood flow by greater than about 10%, for example, about 20, 30, 40 or 50%. Preferably, enhanced diagnostic images may be obtained by increasing myocardial blood flow by greater than about 50%, for example, about 60, 70, 80, 90 or 100%, with increased myocardial blood flow of greater than about 100%, for example, about 110, 120, 130, 140 or 150%, being more preferred. Even more preferably, enhanced diagnostic images may be obtained by increasing myocardial blood flow by greater than about 150% , for example, about 160, 170, 180 or 190%, with an increase of about 200% being still more preferred. In certain particularly preferred embodiments, enhanced diagnostic images may be obtained by increasing myocardial 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 coronary vasodilator. Thus, in preferred form, the methods of the present invention involve the administration to a patient of a contrast agent in combination with a coronary vasodilator. Generally speaking, the coronary 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 in the art, once armed with the present disclosure, the order of administration of the contrast agent and coronary vasodilator, as well as the manner in which they are administered may be varied, 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 myocardial tissue, and the rate of blood flow in the cardiovascular 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 in the cardiovascular region of the patient a substantially constant flow or concentration of contrast agent. Administration of a coronary vasodilator may provide an increased flow of blood in the myocardial region of the patient which, in turn, may provide increased brightness in the diagnostic image of the myocardial tissue. It is believed that this increase in brightness may be related directly to the increase in myocardial blood flow. This increased contrast may provide information concerning myocardial reserve, which is typically defined as the difference between resting and maximal coronary blood flow. See, e.g. , M.L. Marcus et al. , Cardiac Imaging: A Companion to Braunwald's Heart Disease, W.B. Saunders Co. , Harcourt Brace Jovanovich, Inc. , Philadelphia, PA, pp. 536-556 (1991). Coronary reserve can be altered by disease processes, nonhemodynamic conditions, hemodynamic alterations and drug effects. With particular reference to diseases, both noncardiac and cardiac diseases can influence coronary flow reserve. Noncardiac diseases exert their influence on coronary flow reserve primarily by affecting hemodynamics or the oxygen-carrying or oxygen-delivering capacity of the blood. Classic examples include febrile illnesses associated with tachycardia, changes in arterial pressure and myocardial contractility, diseases associated with severe hypoxia such as emphysema, and anemia secondary to chronic blood loss. Cardiac diseases that typically influence coronary reserve include pathologic problems of the conduit coronary vessels, such as congenital coronary anomalies, or coronary obstructive disease. Diffuse diseases of the coronary vasculature also can decrease coronary reserve. These may include collagen vascular diseases, syndrome X, or the diffuse atherosclerosis that frequently occurs in cardiac transplant patients. Pathologic types of cardiac hypertrophy, including volume induced and pressure-induced hypertrophy of the right or left ventricles, may profoundly decrease flow reserve in humans. It is not known whether the hypertrophy associated with various types of athletic conditioning or thyrotoxicosis is associated with a decrease in coronary flow reserve. Also, large coronary collaterals may influence coronary flow reserve by increasing baseline flow in the vessels supplying the collaterals, allowing only limited perfusion of collateral-dependent myocardium or permitting coronary steal to develop. In embodiments which employ a constant administration or infusion of contrast agent, the rate of response of the patient to the coronary vasodilator may also provide information concerning the presence or absence of disease, including coronary artery disease, in the patient. Specifically, the time required for the patient to respond to the coronary vasodilator as evidenced, for example, by the rate of increase in myocardial blood flow, may provide information concerning the integrity of the patient's blood vessels. Thus, a patient having substantially no coronary heart disease, as measured, for example, by a substantial lack of coronary occlusions, may respond substantially rapidly to the coronary vasodilator, as measured by an increase in the myocardial blood flow. Conversely, a patient having one or more occlusions in the coronary blood vessels may respond to the coronary vasodilator at a rate which is slower than the rate at which a patient having no or fewer occlusions responds. In embodiments of the present invention, the contrast agents may comprise a lipid composition comprising a lipid and a gas or gaseous precursor. In connection with lipid compositions, and especially lipid 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 lipid 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 of Lipid Bilayers (CRC Press, Boca Raton, FL 1990), at p. 139 for main chain melting transitions of saturated diacyl-sn-glycero-3-phosphocholines. The gel state to liquid crystalline state phase transition temperatures of various lipids will be readily apparent to those skilled in the art and are described, for example, in Gregoriadis, 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-Glycero-3-PhosphochoIines: Main Chain Melting Transition Temperatures Main Phase Number of Carbons in Transition Acyl Chains Temperature (° C) 1,2-02:0) -1.0 1,2-03:0) 13.7 1,2-(14:0) 23.5 1,2-05:0) 34.5 1,2-(16:0) 41.4 1,2-07:0) 48.2 1,2-(18:0) 55.1 1,2-(19:0) 61.8 l,2-(20:0) 64.5 1,2-(21:0) 71.1 l,2-(22:0) 74.0 TABLE 1 Saturated Diacyl-sn-Glycero-3-Phosphocholines : Main Chain Melting Transition Temperatures Main Phase Number of Carbons in Transition Acyl Chains Temperature (° C) l,2-(23:0) 79.5 l ,2-(24:0) 80.1 See, for example, Derek Marsh, CRC Handbook of Lipid 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 lipid compositions at least a minor amount, for example, about 1 to about 10 mole percent, based on the total amount of lipid employed, of a negatively charged lipid. Suitable negatively charged lipids include, for example, phosphatidylserine, phosphatidic acid, and fatty acids. Without intending to be bound by any theory or theories 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 by 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 touching 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 single, larger vesicle. A continuation of this process of consolidation will, of course, lead to significant degradation of the vesicles. The lipid 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 lipid and/or vesicle compositions. With particular reference to vesicle compositions, for example, micelles and/or liposomes, any of the materials or combinations thereof which are known to those skilled in the art as suitable for their preparation may be used. The lipids used may be of natural, synthetic or semi- synthetic origin. 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 lipid compositions include, for example, fatty acids; lysolipids; phosphocholines; phosphatidylcholine with both saturated and unsaturated lipids, including dioleoylphosphatidylcholine; dimyristoylphosphatidylcholine; dipentadecanoylphosphatidylcholine; dilauroyl- phosphatidylcholine; dipalmitoylphosphatidylcholine (DPPC); distearoylphosphatidyl- choline (DSPC); and diarachidonylphosphatidylcholine (DAPC); phosphatidylethanol- amines, such as dioleoylphosphatidylethanolamine, dipalmitoylphosphatidylethanol- amine (DPPE) and distearoylphosphatidylethanolamine (DSPE); phosphatidylserine; phosphatidylglycerols, including distearoylphosphatidylglycerol (DSPG); phosphatidylinositol; sphingolipids, such as sphingomyelin; glycolipids, such as ganglioside GM1 and GM2; glucolipids; sulfatides; glycosphingolipids; phosphatidic acids, such as dipalmitoylphosphatidic acid (DPP A) and distearoylphosphatidic acid (DSP A); palmitic acid; stearic acid; arachidonic acid; oleic acid; lipids bearing biocompatible polymers, such as chitin, hyaluronic acid, polyvinylpyrrolidone or polyethylene glycol (PEG), the latter being also referred to herein as "pegylated lipids", with preferred lipids bearing polymers including DPPE-PEG, which refers to the lipid DPPE having a PEG polymer attached thereto, including, 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-, oligo- or polysaccharides; cholesterol, cholesterol sulfate and cholesterol hemisuccinate; tocopherol hemisuccinate; lipids witii ether and ester-linked fatty acids; polymerized lipids (a wide variety of which are well known in the art); diacetyl phosphate; dicetyl phosphate; stearylamine; cardiolipin; phospholipids with short chain fatty acids of about 6 to about 8 carbons in length; synthetic phospholipids 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 liposomes including 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 ricinoleate, ethoxylated soybean sterols, ethoxylated castor oil, polyoxyethylene-polyoxypropylene polymers and polyoxyethylene fatty acid stearates; sterol aliphatic acid esters, including 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 glucuronide, ergosterol glucuronide, cholesterol gluconate, lanosterol gluconate and ergosterol gluconate; esters of sugar acids and alcohols including lauryl glucuronide, stearoyl glucuronide, myristoyl glucuronide, lauryl gluconate, myristoyl gluconate and stearoyl gluconate; esters of sugars and aliphatic acids, including sucrose laurate, fructose laurate, sucrose palmitate, sucrose stearate, glucuronic acid, gluconic acid and polyuronic acid; saponins, including sarsasapogenin, smilagenin, hederagenin, oleanolic acid and digitoxigenin; glycerols, including glycerol dilaurate, glycerol trilaurate, glycerol dipalmitate, glycerol and glycerol esters, such as glycerol tripalmitate, glycerol distearate, glycerol tristearate, glycerol dimyristate and glycerol trimyristate; long chain alcohols, including n-decyl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol and n-octadecyl alcohol; 6-(5-cholesten-30-yloxy)-l- thio-3-D-galactopyranoside; digalactosyldiglyceride; 6-(5-cholesten-3/3-yloxy)hexyl-6- amino-6-deoxy-l-thio-j3-D-galactopyranoside; 6-(5-cholesten-30-yloxy)hexyl-6-.amino-6- deoxyl-1-thio-α-D-mannopyranoside; 12-(((7'-diethylaminocoumarin-3-yl)carbonyl)- methylamino)-octadecanoic acid; N-[12-(((7'-diethylaminocoumarin-3-yl)carbonyl)- methylamino)-octadecanoyl]-2-aminopalmitic acid; cholesteryl)4'-trimethyl- ammonio)butanoate; N-succinyldioleoylphosphatidylethanol-amine; 1 ,2-dioleoyl-sn- glycerol; l,2-dipalmitoyl-sn-3-succinylglycerol; l ,3-dipalmitoyl-2-succinylglycerol; 1- hexadecyl-2-palmitoyl-glycerophosphoethanolamine and palmitoylhomocysteine, and/or combinations thereof. If desired, a cationic lipid may be used, such as, for example, N-[l-(2,3- dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTMA), 1 ,2-dioleoyloxy- 3-(trimethylammonio)propane (DOTAP); and l,2-dioleoyl-3-(4'-trimethylammonio)- butanoyl-sn-glycerol (DOTB). If a cationic lipid is employed in the lipid compositions, the molar ratio of cationic lipid to non-cationic lipid may be, for example, from about 1:1000 to about 1:100. Preferably, the molar ratio of cationic lipid to non-cationic lipid may be from about 1 :2 to about 1 : 10, with a ratio of from about 1 : 1 to about 1 :2.5 being preferred. Even more preferably, the molar ratio of cationic lipid to non- cationic lipid may be about 1: 1. In the case of lipid compositions which contain both cationic and non- cationic lipids, a wide variety of lipids may be employed as the non-cationic lipid. Preferably, this non-cationic lipid comprises one or more of DPPC, DPPE and dioleoylphosphatidylethanolamine. In lieu of the cationic lipids listed above, lipids bearing cationic polymers, such as polylysine or polyarginine, as well as alkyl phosphonates, alkyl phosphinates and alkyl phosphites, may also be used in the lipid compositions. In preferred embodiments, the lipid compositions comprise phospholipids, particularly one or more of DPPC, DPPE, DPPA, DSPC, DSPE, and DAPC (20 carbons), with DPPC, DPPE and/or DPPA being especially preferred. Saturated and unsaturated fatty acids may also be employed in the lipid 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, myristic, palmitic and stearic acids. Suitable unsaturated fatty acids that may be used include, for example, lauroleic, physeteric, myristoleic, linoleic, linolenic, palmitoleic, petroselinic and oleic acids. Examples of branched fatty acids that may be used include, for example, isolauric, isomyristic, isopalmitic and isostearic acids. In addition to lipid compositions and/or vesicle compositions formulated from lipids, the methods of the present invention may also involve vesicles formulated from polymers which may be of natural, semi-synthetic (modified natural) or synthetic origin, with semi-synthetic and synthetic polymers being preferred. As used herein, the term polymer denotes a compound comprised of two or more repeating monomeric units, and preferably 10 or more repeating monomeric 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 occurring polysaccharides. Such polysaccharides include, for example, arabinans, fructans, fucans, galactans, galacturonans, glucans, mannans, xylans (such as, for example, inulin), levan, fucoidan, carrageenan, galatocarolose, pectic acid, pectins, including amylose, pullulan, glycogen, amylopectin, cellulose, dextran, dextrin, dextrose, polydextrose, pustulan, chitin, agarose, keratan, chondroitan, dermatan, hyaluronic acid, alginic acid, xanthan gum, starch and various 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, lyxose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, erythrulose, ribulose, xylulose, psicose, fructose, sorbose, tagatose, mannitol, sorbitol, lactose, sucrose, trehalose, maltose, cellobiose, glucuronic acid, gluconic acid, glucaric acid, galacturonic acid, mannuronic acid, glucosamine, galactosamine, and neuraminic 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 invention include polyethylenes (such as, for example, polyethylene glycol, polyoxyethylene, and polyethylene terephthlate), polypropylenes (such as, for example, polypropylene glycol), polyurethanes (such as, for example, poly vinyl alcohol (PVA), polyvinylchloride and polyvinylpyrrolidone), polyamides including nylon, polystyrene, polylactic acids, fluorinated hydrocarbons, fluorinated carbons (such as, for example, polytetrafiuoroethylene), and polymethylmethacrylate, and derivatives thereof. Preferred are biocompatible synthetic polymers or copolymers prepared from monomers, such as acrylic acid, methacrylic acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate (HEMA), lactic acid, glycolic acid, e-caprolactone, acrolein, cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkyl-acrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxyalkyl-methacrylates, N- substituted acrylamides, N-substituted methacrylamides, N-vinyl-2-pyrrolidone, 2,4- pentadiene-1-ol, vinyl acetate, acrylonitrile, styrene, p-amino-styrene, p-amino-benzyl- styrene, sodium styrene sulfonate, sodium 2-sulfoxyethylmethacrylate, vinyl pyridine, aminoethyl methacrylates, 2-methacryloyloxy-trimethylammonium chloride, and polyvinylidene, as well polyfunctional crosslinking monomers such as N,N'- methylenebisacrylamide, ethylene glycol dimethacrylates, 2,2'-(p-phenylenedioxy)- diethyl dimethacrylate, divinylbenzene, triallylamine and methylenebis-(4-phenyl- isocyanate), including combinations thereof. Preferable polymers include polyacrylic acid, polyethyleneimine, poly methacrylic acid, polymethylmethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly (e-caprolactone), epoxy resin, poly(ethylene oxide), poly (ethylene glycol), and polyamide (nylon) polymers. Preferable copolymers include polyvinylidene-polyacrylonitrile, polyvinylidene-polyacrylonitrile-polymethyl- methacrylate, polystyrene-polyacrylonitrile and poly d,l lactide co-glycolide polymers. A preferred copolymer is polyvinylidene-polyacrylonitrile. Other suitable biocompatible monomers and polymers will be readily apparent to those skilled in the art, once armed with the present disclosure. Vesicles derived from 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 lipid and/or vesicle compositions employed in the present methods may also comprise a gas, such as an inert gas. The gas provides the lipid and/or vesicle compositions with enhanced reflectivity, particularly in connection with vesicle compositions in which the gas is entrapped within the vesicles. This may increase the effectiveness of the vesicle compositions as contrast agents. Preferred gases are gases which are inert and which are biocompatible, that is, gases which are not injurious to biological function. Preferred gases include those selected from the group consisting of air, noble gases, such as helium, rubidium hyperpolarized xenon, hyperpolarized argon, hyperpolarized helium, neon, argon, xenon, carbon dioxide, nitrogen, fluorine, oxygen, sulfur-based gases, such as sulfur hexafluoride and sulfur tetrafluoride, fluorinated gases, including, for example, partially fluorinated gases or completely fluorinated gases. Exemplary fluorinated gases include the fluorocarbon gases, such as the perfluorocarbon gases, and mixtures thereof. Paramagnetic gases, such as 17 O 2 , may also be used in the lipid and/or vesicle compositions. In preferred embodiments, the gas utilized in the compositions described herein comprises a fluorinated gas. Such fluorinated gases include materials 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 fluorinated fluorocarbons) being more preferred. Preferably, the perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluoropentane, perfluorocyclobutane and mixtures thereof. More preferably, the perfluorocarbon gas is perfluoropropane or perfluorobutane, with perfluoropropane being 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 preferred 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, perfluorotripropylamine and perfluorotributylamine. It may also be desirable to incoφorate 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 lipid and/or vesicle compositions described herein are agents which are sensitive to pH. These agents include 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, including 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 in the art, and its preparation is described, for example, in Thanassi, Biochemistry, Vol. 9, no. 3, pp. 525-532 (1970); Fitzpatrick 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 incoφorated herein by reference, in their entirety. In addition to, or instead of being sensitive to changes in pH, the gaseous precursor materials may also comprise 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 including, for example, halothane, enflurane, isoflurane, desflurane and sevoflurane. As the artisan will appreciate, a particular perfluorocarbon may exist in the liquid state when the lipid 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 lipid 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 preferred 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 being 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 the 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 indicates that as pressure increases, the effective boiling point increases also. Conversely, as pressure decreases, the effective boiling point decreases. A wide variety of materials may be used as temperature-sensitive gaseous precursors in the compositions described herein. It is only required that the material be capable of undergoing a phase transition to the gas phase upon passing through the appropriate temperature. Suitable gaseous precursors include, for example, hexafluoroacetone, isopropyl acetylene, allene, tetrafluoroallene, boron trifluoride, 1,2-butadiene, 2,3-butadiene, 1,3-butadiene, l,2,3-trichloro-2-fluoro-l ,3-butadiene, 2-methy 1-1 , 3 -butadiene, hexafluoro- 1,3 -butadiene, butadiyne, 1-fluorobutane, 2-methylbutane, perfluorobutane, 1-butene, 2-butene, 2-methyl-l-butene, 3-methyl-l- 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, crotononitrile, cyclobutane, methylcyclobutane, octafluorocyclobutane, perfluorocyclo- butene, 3-chlorocyclopentene, perfluorocyclopentane, octafluorocyclopentene, cyclopropane, perfluorocyclopropane, 1,2-dimethyl-cyclopropane, 1, 1-dimethyl- cyclopropane, 1,2-dimethylcyclopropane, ethylcyclopropane, methylcyclopropane, diacetylene, 3-ethyl-3-methyl diaziridine, 1,1 , 1-trifluorodiazoethane, dimethyl amine, hexafluorodimethylamine, dimethylethylamine, bis(dimethylphosphine)amine, perfluorohexane, perfluoroheptane, perfluorooctane, 2,3-dimethyl-2-norbornane, perfluorodimethylamine, dimethyloxonium chloride, l,3-dioxolane-2-one, 4-methyl- 1,1,1,2-tetrafluoroethane, 1, 1,1-trifluoroethane, 1, 1,2,2-tetrafluoroethane, 1 , 1,2- trichloro- 1 , 2 , 2-trifluoroethane , 1,1 -dichloroethane , 1 , 1 -dichloro- 1 ,2,2 , 2-tetra- fluoroethane, 1 ,2-difluoroethane, l-chloro-l, l,2,2,2-pentafluoroethane, 2-chloro-l , l- difluoroe thane, l, l-dichloro-2-fluoroethane, l-chloro-l , l ,2,2-tetrafluoroethane, 2- chloro-l, l-difluoroethane, chloroethane, chloropentafluoroethane, dichlorotrifluoro- ethane, fluoroethane, perfluoroethane, nitropentafluoroethane, nitrosopentafluoroethane, perfluoroethylamine, ethyl vinyl ether, 1,1 -dichloroethane, 1,1 -dichloro- 1,2- difluoroethane, 1,2-difluoroethane, methane, trifluoromethanesulfonyl chloride, trifluoromethanesulfonylfluoride, bromodifluoronitrosomethane, bromofluoro- methane, bromochlorofluoromethane, bromotrifluoromethane, chlorodifluoronitro- methane, chlorodinitromethane, chlorofluoromethane, chlorotrifluoromethane, chlorodifluoromethane, dibromodifluoromethane, dichlorodifluoromethane, dichloro- fluoromethane, difluoromethane, difluoroiodomethane, disilanomethane, fluoromethane, iodomethane, iodotrifluoromethane, nitrotrifluoromethane, nitrosotrifluoromethane, tetrafluoromethane, trichlorofluoromethane, trifluoromethane, 2-methylbutane, methyl ether, methyl isopropyl ether, methyllactate, methylnitrite, methylsulfide, methyl vinyl ether, neopentane, nitrous oxide, 1,2,3-nonadecanetricarboxylic acid 2-hydroxy- trimethyl ester, l-nonene-3-yne, 1,4-pentadiene, n-pentane, perfluoropentane, 4-amino- 4-methylpentan-2-one, 1-pentene, 2-pentene (cis and trans), 3-bromopent-l-ene, perfluoropent-1-ene, tetrachlorophthalic acid, 2,3,6-trimethyl-piperidine, propane, 1,1, 1,2, 2, 3 -hexafluoropropane, 1,2-epoxypropane, 2,2-difluoropropane, 2- aminopropane, 2-chloropropane, heptafluoro-1-nitropropane, heptafluoro-1- nitrosopropane, 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-trifluoropropyne, 3-fluorostyrene, sulfur (di)- decafluoride (S 2 F 10 ), 2,4-diaminotoluene, trifluoroacetonitrile, trifluoromethyl peroxide, trifluoromethyl sulfide, tungsten hexafluoride, vinyl acetylene and vinyl ether. Perfluorocarbons are both preferred gases and preferred gaseous precursors for use in connection with the compositions employed in the methods of the present invention. Included among such perfluorocarbons are saturated perfluorocarbons, unsaturated perfluorocarbons, and cyclic perfluorocarbons. The saturated perfluorocarbons, which are usually preferred, have the formula C n F 2n+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 2jl , 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 containing from about 5 to about 12 carbons are liquids at room temperature. In either case, both liquid and/or gaseous perfluorocarbons may be incoφorated in the compositions of the present invention. In addition to the perfluorocarbons, it may be desirable to utilize stable fluorocarbons which are not completely fluorinated. Such fluorocarbons include 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 lipid 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 incoφorated 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 incoφorated in the lipid and/or vesicle compositions irrespective 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 lipid compositions in which the lipids are aggregated randomly, as well as in vesicle compositions, including vesicle compositions which are formulated from lipids, such as micelles and liposomes. 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 comprises a gas or gaseous precursor and one or more lipids. This promotes the formation of stabilized vesicles within which the gas or gas precursor is encapsulated. In addition, a gas may be bubbled directly into an aqueous mixture of lipid 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 herein by reference, in their entirety. Suitable methods for incoφorating the gas or gas precursor in cationic lipid compositions are disclosed also in U.S. Patent No. 4,865,836, the disclosures of which are hereby incoφorated herein by reference. Other methods would be apparent to one skilled in the art based on the present disclosure. Preferably, the gas may be instilled in the lipid and/or vesicle compositions after or during the addition of the stabilizing material and/or during formation of vesicles. In preferred embodiments, the gaseous substances and/or gaseous precursor materials are incoφorated in vesicle compositions, with micelles and liposomes being preferred. As discussed in detail below, vesicles in which a gas or gaseous precursor or both are encapsulated are advantageous in that they provide improved reflectivity in vivo. As discussed more fully hereinafter, it is preferred that the lipid 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 lipid 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 diffusability in aqueous media. Exemplary highly stable gases include perfluorocarbons since they are generally less diffusible and relatively insoluble in aqueous media. Accordingly, their use may promote the formation of highly stable vesicles. In certain embodiments, it may be desirable to use a fluorinated compound, especially a perfluorocarbon compound, which may be in the liquid state at the temperature of use of the lipid and/or vesicle compositions, including, for example, the in vivo temperature of the human body, to assist or enhance the stability of the lipid and/or vesicle compositions, and especially, the gas filled vesicles. Suitable fluorinated compounds include, for example, fluorinated surfactants, such as fluorinated 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), perfluorodecalin, perfluorododecalin, perfluorooctyliodide, perfluorotripropylamine, and perfluorotributylamine. 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 derived from a gaseous precursor, which may also be a perfluorocarbon, for example, perfluoropentane. In the latter case, the lipid 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 theories of operation, it is believed that, in the case of vesicle compositions, the liquid fluorinated compound may be situated at the interface between the gas and the membrane or wall surface of the vesicle. There may be thus formed a further stabilizing layer of liquid fluorinated compound on the internal surface of the stabilizing compound, for example, a biocompatible lipid 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 invention, 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 fluorinated compound, such as a perfluorocarbon, when combined with a gas or gaseous precursor ordinarily used to make the lipid and/or vesicle compositions described herein, 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. Perfluorinated surfactants, such as ZONYL ® fluorinated surfactants, may be used to stabilize the lipid and/or vesicle compositions, and to act, for example, as a coating for vesicles. Preferred perfluorinated surfactants are the partially fluorinated phosphocholine surfactants. In these preferred fluorinated surfactants, the dual alkyl compounds may be fluorinated at the terminal alkyl chains and the proximal carbons may be hydrogenated. These fluorinated phosphocholine surfactants may be used for making the targeted lipid 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 being even more preferred. In connection with particular uses, for example, intravascular use, including magnetic resonance imaging of the vasculature, it may be preferred that the vesicles be no larger that about 30 μm in diameter, with smaller vesicles being preferred, for example, vesicles of no larger than about 12 μ in diameter. In certain preferred 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 preferred, and vesicles having a mean diameter of about 3 μm or less being even more preferred. 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 traveling 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 including, 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 herein 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 0.6778ξ 1.2 fluorobutane 2-methyl 72.15 27.8 0.6201 2.6 butane (isopentane) 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 preferred for use as the gas or gaseous precursors, as well as additional stabilizing components. As noted above, it is preferred 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 interface 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 in 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 lipid and/or polymeric compounds discussed above, the compositions described herein may comprise one or more stabilizing materials. Exemplary of such stabilizing materials are, for example, biocompatible polymers. The stabilizing materials 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 sulfur hexafluoride, improved vesicle compositions may be obtained when one or more stabilizing materials are utilized in the formation of the gas and gaseous precursor filled vesicles. These compounds may help improve the stability and the integrity of the vesicles with regard to their size, shape and/or other attributes. The terms "stable" or "stabilized", as used herein, means that the vesicles may be substantially resistant to degradation, including, for example, loss of vesicle structure or encapsulated gas or gaseous precursor, for a useful period of time. 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 preferred form, 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 described herein, including 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 described herein may be attributable, at least in part, to the materials from which the vesicles are made, including, for example, the lipids, and/or polymers described 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 characteristics are described more fully hereinafter. The materials from which the vesicles are constructed are preferably biocompatible lipid and/or polymer materials, and of these, the biocompatible lipids are preferred. In addition, because of the ease of formulation, including the capability of preparing vesicles immediately prior to administration, these vesicles may be conveniently made on site. The biocompatible polymers useful as stabilizing materials for preparing the gas and gaseous precursor filled vesicles may be of natural, semi-synthetic (modified natural) or synthetic origin. As used herein, the term polymer denotes a compound comprised of two or more repeating monomeric units, and preferably 10 or more repeating monomeric 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 occurring polysaccharides. Such polysaccharides include, for example, arabinans, fructans, fucans, galactans, galacturonans, glucans, mannans, xylans (such as, for example, inulin), levan, fucoidan, carrageenan, galatocarolose, pectic acid, pectins, including amylose, pullulan, glycogen, amylopectin, cellulose, dextran, dextrin, dextrose, polydextrose, pustulan, chitin, agarose, keratan, chondroitan, dermatan, hyaluronic acid, alginic acid, xanthan gum, starch and various 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, lyxose, allose, altrose, glucose, mannose, gulose, idose, galactose, talose, erythrulose, ribulose, xylulose, psicose, fructose, sorbose, tagatose, mannitol, sorbitol, lactose, sucrose, trehalose, maltose, cellobiose, glucuronic acid, gluconic acid, glucaric acid, galacturonic acid, mannuronic acid, glucosamine, galactosamine, and neuraminic acid, and naturally occurring derivatives thereof. Exemplary semi-synthetic polymers include carboxymethylcellulose, hydroxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, and methoxycellulose. Exemplary synthetic polymers suitable for use in the present invention include polyethylenes (such as, for example, polyethylene glycol, polyoxyethylene, and polyethylene terephthlate), polypropylenes (such as, for example, polypropylene glycol), polyurethanes (such as, for example, polyvinyl alcohol (PVA), poly viny .chloride and polyvinylpyrrolidone), polyamides including nylon, polystyrene, polylactic acids, fluorinated hydrocarbons, fluorinated carbons (such as, for example, polytetrafluoroethylene), and polymethylmethacrylate, and derivatives thereof. Methods for the preparation of vesicles which employ polymers as stabilizing compounds 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 incoφorated herein by reference, in their entirety. Particularly preferred embodiments of the present invention involve vesicles which comprise three components: (1) a neutral lipid, for example, a nonionic or zwitterionic lipid, (2) a negatively charged lipid, and (3) a lipid bearing a stabilizing material, for example, a hydrophilic polymer. Preferably, the amount of the negatively charged lipid will be greater than about 1 mole percent of the total lipid present, and the amount of lipid bearing a hydrophilic polymer will be greater than about 1 mole percent of the total lipid present. Exemplary and preferred negatively charged lipids include phosphatidic acids. The lipid bearing a hydrophilic polymer will desirably be a lipid 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, polyvinyl alcohol, and polyvinylpyrrolidone and copolymers thereof, with PEG polymers being preferred. 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 being more preferred. The PEG or other polymer may be bound to the lipid, for example, DPPE, through a covalent bond, such as an amide, carbamate or amine linkage. In addition, the PEG or other polymer may be linked to a targeting ligand, or other phospholipids, with a covalent bond including, for example, amide, ester, ether, thioester, thioamide or disulfide bonds. Where the hydrophilic polymer is PEG, a lipid bearing such a polymer will be said to be "pegylated. " In preferred form, the lipid bearing 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 lipid is distearoylphosphatidylethanolamine-polyethylene glycol 5000 (DSPE-PEG5000). In certain preferred embodiments of the present invention, the lipid 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 comprise 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 material bound to the lipid membrane or skin of the vesicle by the DPPE moiety, with the PEG moiety free to surround the vesicle membrane or skin, and thereby form a physical barrier 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, in the given ratios. It is also theorized 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 during which the stabilized vesicles may function as diagnostic imaging contrast media. The vesicle compositions may be prepared from other materials, in addition to the materials described above, provided that the vesicles so prepared meet the stability and other criteria set forth herein. These materials may be basic and fundamental, and form the primary 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 in addition to that afforded by the basic stabilizing material. However, it is not always possible to determine whether a given material is a basic or an auxiliary agent, since the functioning of the material 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 combination of a biocompatible lipid and water or saline when shaken will often give a cloudy solution subsequent to autoclaving for sterilization. Such a cloudy solution may function as a contrast agent, but is aesthetically objectionable and may imply instability in the form of undissolved or undispersed lipid 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 lipid 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 skin of the vesicle, thus providing additional stabilization. As examples of such further basic or auxiliary stabilizing materials, there are conventional surfactants which may be used; see D'Arrigo U.S. Patents Nos. 4,684,479 and 5,215,680. Additional auxiliary and basic stabilizing materials include such agents as peanut oil, canola oil, olive oil, safflower oil, corn oil, or any other oil commonly known to be ingestible which is suitable for use as a stabilizing compound in accordance with the teachings herein. Various auxiliary and basic stabilizing materials are disclosed, for example, in U.S. application Serial No. 08/444,574, filed May 19, 1995, the disclosures of which are incoφorated herein by reference, in their entirety. In addition, compounds used to make mixed micelle systems may be suitable for use as basic or auxiliary stabilizing materials, and these include, for example, lauryltrimethylammonium bromide (dodecyl-), cetyltrimethylammonium bromide (hexadecyl-), myristyltrimethylammonium bromide (tetradecyl-), alkyldimethylbenzylammonium chloride (where alkyl is C 12 , C 14 or C 16 ,), benzyldimethyldodecylammonium bromide/chloride, benzyldimethyl hexadecylammonium bromide/chloride, benzyldimethyl tetradecylammonium bromide/chloride, 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 materials described herein. These materials 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, including, for example, carbohydrates and their phosphorylated and sulfonated derivatives; polyethers, preferably with molecular weight ranges between 400 and 100,000; and di- and trihydroxy alkanes and their polymers, preferably with molecular weight ranges between 200 and 50,000; (b) emulsifying and/or solubilizing agents including, for example, acacia, cholesterol, diethanolamine, glyceryl monostearate, lanolin alcohols, lecithin, mono- and di- glycerides, 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, stearic acid, trolamine, and emulsifying wax; (c) suspending and/or viscosity-increasing agents, including, for example, acacia, agar, alginic acid, aluminum mono-stearate, bentonite, magma, carbomer 934P, carboxymethylcellulose, calcium and sodium and sodium 12, carrageenan, cellulose, dextran, gelatin, guar gum, locust bean gum, veegum, hydroxyethyl cellulose, hydroxypropyl methylcellulose, magnesium-aluminum-silicate, Zeolites ® , methylcellulose, pectin, polyethylene oxide, povidone, propylene glycol alginate, silicon dioxide, sodium alginate, tragacanth, xanthan gum, α-d-gluconolactone, glycerol and mannitol; (d) synthetic suspending agents, such as polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyvinyl 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 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. In 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. Exemplary paramagnetic contrast agents suitable for use in the present compositions include, for example, stable free radicals, such as, for example, stable nitroxides, as well as compounds comprising 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 complexing agents, including lipophilic derivatives thereof, or to proteinaceous macromolecules. Preferable transition, lanthanide and actinide elements include, for example, Gd(III), Mn(II), Cu(II), Cr(III), Fe(II), Fe(III), Co(II), Er(II), Ni(II), Eu(III) and Dy(III). More preferably, the elements may be Gd(III), Mn(II), Cu(II), Fe(II), Fe(III), Eu(III) and Dy(III), especially Mn(II) and Gd(III). The foregoing elements may, if desired, be in the form of a salt, including inorganic salts, such as a manganese salt, for example, manganese chloride, manganese carbonate, manganese acetate, and organic salts, such as manganese gluconate and manganese hydroxylapatite. 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 complexing agents, including lipophilic derivatives thereof, or to proteinaceous macromolecules. Preferable complexing agents include, for example, diethylenetriaminepentaacetic 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"-triacetic acid (DOTA) , 3 , 6, 9-triaza- 12-oxa-3 ,6 , 9-tricarboxymethylene- 10-carboxy- 13-pheny 1- tridecanoic acid (B-19036), hydroxybenzylethylenediamine diacetic acid (HBED), N,N'-bis(pyridoxyl-5-phosphate)ethylene diamine, N,N' -diacetate (DPDP), 1,4,7- triazacyclononane-N,N',N"-triacetic acid (NOTA), 1,4,8,11-tetraazacy clotetradecane- N,N',N",N'"-tetraacetic acid (TETA), kryptands (macrocyclic complexes), and desferrioxamine. More preferably, the complexing agents are EDTA, DTPA, DOTA, DO3A and kryptands, most preferably DTPA. Preferable lipophilic complexes include alkylated derivatives of the complexing agents EDTA, DOTA, for example, N,N'-bis- (carboxydecylamidomethyl-N-2,3-dihydroxypropyl)-ethylenediamine-N,N'-diacetate (EDTA-DDP); N,N'-bis-(carboxy-octadecylamido-methyl-N-2,3-dihydroxypropyl)- ethy lenediamine-N , N ' -diacetate (EDTA : ODP) ; N , N ' -Bis(carboxy-laury lamidomethy 1- N-2,3-dihydroxypropyl)ethylenedi.amine-N,N'-diacetate (EDTA-LDP); and the like, including 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 include, for example, albumin, collagen, polyarginine, polylysine, polyhistidine, γ-globulin and 0-globulin, with albumin, polyarginine, polylysine, and polyhistidine being more preferred. Suitable complexes therefore include Mn(II)-DTPA, Mn(II)-EDTA, Mn(II)-DOTA, Mn(II)-DO3A, Mn(II)-kryptands, Gd(III)-DTPA, Gd(III)-DOTA, Gd(III)-DO3A, Gd(III)-kryptands, Cr(III)-EDTA, Cu(II)-EDTA, or iron- desferrioxamine, especially Mn(II)-DTPA or Gd(III)-DTPA. Nitroxides are paramagnetic contrast agents which increase both TI and T2 relaxation rates on MRI by virtue of the presence of an unpaired electron in the nitroxide molecule. As known to one of ordinary skill in 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 increase relaxivity. For nitroxides, however, the electron spin correlation times are more favorable and tremendous increases in 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 in relaxivity. Although not intending to be bound by any particular theory of operation, it is contemplated that since the nitroxides may be designed to coat the perimeters of the vesicles, for example, by making alkyl derivatives thereof, the resulting correlation times can be optimized. Moreover, the resulting contrast medium of the present invention may be viewed as a magnetic sphere, a geometric configuration which maximizes relaxivity. If desired, the nitroxides may be alkylated or otherwise derivatized, such as the nitroxides 2,2,5, 5-tetramethyl-l-pyrrolidinyloxy, free radical, and 2,2,6,6- tetramethyl-1-piperidinyloxy, free radical (TMPO). Exemplary supeφaramagnetic contrast agents suitable for use in the compositions of the present invention include metal oxides and sulfides which experience a magnetic domain, ferro- or ferrimagnetic compounds, such as pure iron, magnetic iron oxide, such as magnetite, γ-Fe 2 O 3 , Fe 3 O 4 , manganese ferrite, cobalt ferrite and nickel ferrite. Paramagnetic gases can also be employed in the present compositions, such as oxygen 17 gas ( 17 O 2 ). In addition, hypeφolarized xenon, neon, or helium gas may also be employed. MR whole body imaging may then be employed to rapidly screen the body, for example, for thrombosis, and ultrasound may be applied, if desired, to aid in thrombolysis. The contrast agents, such as the paramagnetic and supeφaramagnetic contrast agents described above, may be employed as a component within the lipid 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 lipidic walls of the vesicles. In particular, the nitroxides 2,2,5,5- tetramethyl-1-pyrrolidinyloxy, free radical and 2,2,6,6-tetramethyl-l-piρeridinyloxy, 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 lipid and/or vesicle compositions of the present invention. 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 lipid and/or vesicle compositions of the present invention, and especially the vesicle compositions, may serve not only as effective carriers of the supeφaramagnetic agents described above, but also may improve the effect of the susceptibility contrast agents. Supeφaramagnetic contrast agents include metal oxides, particularly iron oxides but including manganese oxides, and as iron oxides, containing 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 RI relaxivities. The smaller particles, for example, particles having diameters of about 10 to about 15 nm, have somewhat lower R2 relaxivities, but much more balanced RI and R2 values. Much smaller particles, for example, monocrystalline iron oxide particles having diameters of about 3 to about 5 nm, have lower R2 relaxivities, but probably the most balanced RI and R2 relaxation rates. Ferritin can also be formulated to encapsulate a core of very high relaxation rate supeφaramagnetic iron. It has been discovered that the lipid and/or vesicle compositions, especially vesicle compositions, including 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 lipid 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 being 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 in their entirety. Without being 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 according to the manner described 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 into other body locations. In the case of ferrites 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 referred 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 in which a paramagnetic atom, or paramagnetic molecule, or molecules, may reside. One physical explanation may be described in the following Solomon-Bloembergen equations which define the paramagnetic contributions as a function of the T, and T 2 relaxation times of a spin V2 nucleus with gyromagnetic ratio g perturbed by a paramagnetic ion: 1/T.M = (2/15) S(S + 1) γ 2 g 2 5 2 /r 6 [3* /(l + ψ ) + 7τ c /(l + ω s 2 τ c 2 )] + (2/3) S(S + 1) A 2 /If [r (\ + ^ 2r e 2 )l and 1/T 2 M = (1/15) S(S + 1) γ 2 g 2 /3 2 /r 6 [4 τ + 3τc/(l +φ h *) + 13τ c /(l + w s 2 τ c 2 )] + (1/3) S(S+ 1)A 2 /If [ /(l + φ lj 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 e 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, primarily 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 embolized 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 interaction. 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 in 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 unrecognized 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 determined 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 polymeric materials including, for example, polymethyl methacrylate. This would generally result in the formation of vesicles which may be substantially impermeable and relatively inelastic and brittle. In embodiments involving diagnostic imaging, for example, ultrasound, contrast media which comprise such brittle vesicles would generally not provide the desirable reflectivity that the flexible vesicles may provide. However, by increasing the power output on ultrasound, the brittle microspheres can be made to rupture, thereby causing acoustic emissions which can be detected by an ultrasound transducer. Nuclear Medicine Imaging (NMI) may also be used in 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 in the present compositions in addition to, or instead of, the gases discussed above. Such radioactive gases may be entrapped within vesicles for use in detecting, for example, thrombosis. Preferably, bifunctional chelate derivatives 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 imaging isotopes, such as technetium 99m or indium 1 " can be incoφorated in the walls of vesicles. Whole body gamma scanning 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, since ultrasound generally provides improved resolution as compared to nuclear medicine techniques. 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. For optical imaging, optically active gases, such as argon or neon, may be incoφorated in the present compositions. In addition, optically active materials, for example, fluorescent materials, including poφhyrin derivatives, 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 lipid based vesicles described 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 imaging techniques, such as MRI and ultrasound. A wide variety of methods are available for the preparation of lipid and/or vesicle compositions, such as micelles and/or liposomes. Included among these methods are, for example, shaking, drying, gas-installation, spray drying, and the like. Suitable methods for preparing vesicle compositions from lipids are described, for example, in Unger et al. , U.S. Patent No. 5,469,854, the disclosures of which are incoφorated herein by reference. As noted above, the vesicles are preferably prepared from lipids which remain in 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 variety of conventional micellar preparatory methods which will be apparent to those skilled in the art. These methods typically involve suspension of one or more lipid compounds in an organic solvent, evaporation of the solvent, resuspension in an aqueous medium, sonication 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. Ada, Vol. 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 in 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 liposomes. A wide variety of methods are available in connection with the preparation of liposome compositions. Accordingly, the liposomes may be prepared using any one of a variety of conventional liposomal preparatory techniques 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, sonication, chelate dialysis, homogenization, solvent infusion, microemulsification, spontaneous formation, solvent vaporization, solvent dialysis, French pressure cell technique, controlled detergent dialysis, and others, each involving the preparation of the 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 in their entirety. Suitable freeze-thaw techniques are described, for example, in International Application Serial No. PCT/US89/05040, filed November 8, 1989, the disclosures of which are incoφorated herein by reference in their entirety. Methods which involve freeze-thaw techniques are preferred in connection with the preparation of liposomes. Preparation of the liposomes may be carried out in a solution, such as an aqueous saline solution, aqueous phosphate buffer solution, or sterile water. The liposomes may also be prepared by various processes which involve shaking or vortexing. This may be achieved, for example, by the use of a mechanical shaking device, such as a Wig-L-Bug™ (Crescent Dental, Lyons, IL), a Mixomat (Degussa AG, Frankfurt, Germany), a Capmix (Espe Fabrik 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, Woburn, 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 liposomal preparatory techniques which may be adapted for use in the preparation of vesicle compositions are discussed, for example, in U.S. Patent No. 4,728,578; U.K. Patent Application GB 2193095 A; U.S. Patent No. 4,728,575; U.S. Patent No. 4,737,323; International Application Serial No. PCT/US85/01161; Mayer et al., Biochimica et Biophysica Acta, Vol. 858, pp. 161-168 (1986); Hope et al., Biochimica et Biophysica Acta, Vol. 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 Serial 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, Gregoriadis, 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 herein, in their entirety. Lipid compositions comprising a gas can be prepared by agitating an aqueous solution containing, if desired, a stabilizing material, in the presence of a gas. The term "agitating," as used herein, means any shaking motion of an aqueous solution such that gas may be introduced from the local ambient environment into the aqueous solution. This agitation is preferably conducted at a temperature below the gel to liquid crystalline phase transition temperature of the lipid. The shaking involved in the agitation of the solutions is preferably of sufficient force to result in the formation of a lipid composition, including vesicle compositions, and particularly vesicle compositions comprising gas filled vesicles. The shaking may be by swirling, such as by vortexing, side-to-side, or up and down motion. Different types of motion may be combined. Also, the shaking may occur by shaking the container holding the aqueous lipid solution, or by shaking the aqueous solution within the container without shaking 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 (Cerritos, CA) shaker table, as well as any of the shaking devices described hereinbefore, with the Capmix (Espe Fabrik 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 preferred size range. Shaking is preferred, and it is preferred that the shaking be carried out using the Espe Capmix mechanical shaker. In accordance with this preferred method, it is preferred that a reciprocating motion be utilized to generate the lipid compositions, and particularly vesicle compositions. It is even more preferred 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 in 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 minute. 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 minute being even more preferred. Of course, the number of oscillations may be dependent upon the mass of the contents being agitated. Generally speaking, a larger mass may require fewer oscillations. Another means for producing shaking includes 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 correspondingly increased. Vigorous shaking is defined as at least about 60 shaking motions per minute, and is preferred. Vortexing at about 60 to about 300 revolutions per minute is more preferred. Vortexing at about 300 to about 1800 revolutions per minute is even more preferred. In addition to the simple shaking methods described above, more elaborate methods can also be employed. Such elaborate methods include, for example, liquid crystalline shaking gas instillation processes and vacuum drying gas instillation processes, such as those described in copending U.S. application Serial No. 08/076,250, filed June 11, 1993, the disclosures of which are incoφorated herein by reference, in their entirety. Although any of a number of varying techniques can be used, the vesicle compositions employed in the present invention are preferably prepared using a shaking technique. Preferably, the shaking technique involves agitation with a mechanical shaking apparatus, such as an Espe Capmix (Seefeld, Oberay Germany), using, for example, the techniques disclosed in copending U.S. application 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, sonication, 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 μ . In addition, after extrusion and sterilization procedures, which are discussed in detail below, agitation or shaking may provide vesicle compositions which can contain substantially no or minimal residual anhydrous lipid phase in the remainder of the solution. (Bangham, A.D. , Standish, M.M, & Watkins, J.C. , J. 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 including, for example, binding 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 in 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 distribution of the resulting gas filled vesicles. By using two or more cascaded or stacked set of filters, for example, a 10 μm filter followed by an 8 μ filter, the gas filled vesicles can be selected to have a very narrow size distribution 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 during use. The method of sizing the vesicles will then comprise using a syringe comprising a barrel, at least one filter, and a needle; and may be carried out by a step of extracting which comprises extruding the vesicles from the barrel through the filter fitted to the syringe between the barrel and the needle, thereby sizing the vesicles before they are administered to a patient. The step of extracting may also comprise drawing the vesicles into the syringe, where the filter may function in the same way to size the vesicles upon entrance into the syringe. 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 within the desired size range, or of the desired maximum size, are subsequently administered by extrusion from the syringe. In certain preferred embodiments, the vesicle compositions may be heat sterilized or filter sterilized and extruded through a filter prior to shaking. Generally speaking, vesicle compositions comprising a gas may be heat sterilized, and vesicle compositions comprising gaseous precursors may be filter sterilized. Once gas filled vesicles are formed, they may be filtered for sizing as described above. Performing these steps prior 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 syringe may be filled with a filtered lipid and/or vesicle composition, and the composition may be sterilized within the mixing vessel, for example, by autoclaving. Gas may be instilled into the composition to form gas filled vesicles by shaking the sterile 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 lipid 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 administered to a patient for diagnostic imaging including, for example, ultrasound or CT. In certain preferred embodiments, sterilization may be accomplished by heat sterilization, 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 minute, 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 outlined 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 in vesicles can be formulated which, upon activation, for example, by exposure to elevated temperature, varying 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 described in detail in 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 preferred 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 in the context of the present invention. Thus, in preferred 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 in the methods of the present invention. The methods of preparing the gaseous precursor filled vesicles may be carried out below the boiling point of the gaseous precursor such that a liquid is incoφorated, for example, into a vesicle. In addition, the methods may be conducted at the boiling point 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 points may also be depressed using solvents in liquid media to utilize a precursor in liquid form. Further, the methods may be performed where the temperature is increased throughout the 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, prior 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 point of the gaseous precursor. In this embodiment, the gaseous precursor may be entrapped within a vesicle such that the phase transition does not occur during 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 in 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 housing a lipid composition at a temperature below the liquid-gaseous phase transition temperature of the respective gaseous precursor. As the temperature is increased, 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 in 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 in the lipid vesicles and as the temperature is raised beyond the boiling point of perfluorobutane (4°C), perfluorobutane gas is entrapped in the vesicles. Accordingly, 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 invention, 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 in nature. Hence, from a known liquid volume in the emulsion, one may predict an upper limit to the size of the gas filled vesicle. In embodiments of the present invention, a mixture of a lipid compound and a gaseous precursor, containing 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, in 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 in 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 7iτ 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 determined. In more descriptive terms, the following can be applied: V eas = 4/3 τr(r sas ) by the ideal gas law, PV=nRT substituting reveals, V gas = nRT/P gas or, (A) . n n = = A 4/IX3 T [IπTr, gas 3 ] P/RT amount n = 4/3 [τrr gas 3 P/RT] « MW n Converting back to a liquid volume (B) V liq = [4/3 [τrr gas 3 ] P/RT] » MW n /D] where D is the density of the precursor. Solving for the diameter of the liquid droplet, (C) diameter/2 = [3/4τr [4/3«[τπr gas 3 ] P/RT] MW„/D] 1/3 which reduces to Diameter = 2[[r gas 3 ] P/RT [MW n /D]]" 3 . As a further means of preparing vesicles of the desired size for use in 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 being, thus the typical temperature would be 37°C or 310 K. At a pressure of 1 atmosphere and using 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. Using the above calculated amount of gaseous precursor and 1-fluorobutane, which possesses a molecular weight of 76.11 , a boiling 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 corresponding 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 appropriately sized filter. In addition, as seen by die 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 miscible in buffered saline exhibits a freezing point depression well below the freezing point of water alone. By increasing the amount of propylene glycol or adding materials such as sodium chloride, the freezing point can be depressed even further. The selection of appropriate solvent systems may be determined by physical methods as well. When substances, solid or liquid, herein referred 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 point depression of the solvent by the following equation: Inx a = In (l-x„) = ΔH U! /R(1/T 0 -1/T) where x a is the mole fraction of the solvent; x b is the mole fraction of the solute; ΔH fus is the heat of fusion of the solvent; and T 0 is the normal freezing point of the solvent. The normal freezing 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 fus /R[T-T 0 /T 0 T] * ΔH fus ΔT/RT„ 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 = mMa/1000 where Ma is the molecular weight of the solvent. Thus, substituting for the fraction x b : ΔT = rM a RT o 2 /1000ΔH fu -]m or ΔT = K f m, where K f = M-RT. lOOOΔH ft .. K f is the molal freezing 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 appropriate value. Methods of preparing the temperature activated gaseous precursor filled vesicles include: (a) vortexing and/or shaking an aqueous mixture of gaseous precursor and additional materials as desired, including, for example, stabilizing materials, thickening agents and/or dispersing agents. Optional variations of this method include autoclaving before vortexing or shaking; heating an aqueous mixture of gaseous precursor; venting the vessel containing the mixture/suspension; shaking or permitting the gaseous precursor filled vesicle to form spontaneously and cooling 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, filtering may be performed during in vivo administration 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 shaking, or permitting the gaseous precursor vesicle to form spontaneously, whereby the expanded gaseous precursor in the sealed vessel increases the pressure in the vessel, and cooling down die gas filled vesicle suspension, after which shaking may also take place. Freeze drying may be useful to remove water and organic materials prior to the shaking installation method. Drying installation methods may be used to remove water from vesicles. By pre-entrapping the gaseous precursor in the dried vesicles (i.e. prior to drying) after warming, the gaseous precursor may expand to fill the vesicle. Gaseous precursors can also be used to fill dried 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 boiling point of perfluorobutane). Preferred methods for preparing the temperature activated gaseous precursor filled vesicles comprise shaking an aqueous solution having a lipid 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 lipid. 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. Shaking of the mixture may take place during the heating 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 lipid and a gaseous precursor, and separating the resulting gaseous precursor filled vesicles. Conventional, aqueous-filled liposomes 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 preferred 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 comprising a lipid, in 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 shaking may involve micro- emulsifying, microfluidizing, swirling (such as by vortexing), side-to-side, or up and down motion. In the case of the addition of gaseous precursor in the liquid state, sonication may be used in addition to the shaking methods set forth above. Further, different types of motion may be combined. Also, the shaking may occur by shaking the container holding the aqueous lipid solution, or by shaking the aqueous solution within die container without shaking the container itself. Further, the shaking may occur manually or by machine. Mechanical shakers that may be used include, for example, the mechanical shakers described hereinbefore, with an Espe Capmix (Seefeld, Oberay Germany) being preferred. Another means for producing shaking includes the action of gaseous precursor emitted under high velocity or pressure. According 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 injected into or otherwise added to the container having the aqueous lipid solution or into the aqueous lipid 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 described herein, once activated by application to the 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 the gaseous phase therein. This can occur where, for example, the 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 lipid and/or vesicle compositions may be sterilized by autoclave or sterile filtration if these processes are performed before the installation step or prior to temperature mediated conversion of the temperature sensitive gaseous precursors within the compositions. Alternatively, one or more anti-bactericidal agents and/or preservatives may be included in the formulation of the compositions, such as sodium benzoate, quaternary ammonium salts, sodium azide, methyl paraben, propyl paraben, sorbic acid, ascorbylpalmitate, butylated hydroxyanisole, butylated hydroxy toluene, chlorobutanol, dehydroacetic acid, ethylenediamine, monothioglycerol, potassium benzoate, potassium metabisulfite, potassium sorbate, sodium bisulfite, sulfur dioxide, and organic mercurial salts. Such sterilization, which may also be achieved by other conventional means, such as by irradiation, may be necessary where the stabilized vesicles are used for imaging under invasive circumstances, for example, intravascularly or intraperitonealy. The appropriate means of sterilization will be apparent to the artisan based on the present disclosure. Vesicle compositions which comprise vesicles formulated from polymers may be prepared by various processes, as will be readily apparent to those skilled in the art, once armed with the present disclosure. Exemplary processes include, for example, interfacial polymerization, phase separation and coacervation, multiorifice centrifugal 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, 114, 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 in their entirety. In accordance with a preferred synthesis protocol, the vesicles may be prepared using a heat expansion process, such as, for example, the process described 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, British Patent No. 1 ,044,680, and Japan Kokai Tokkyo Koho 62 286534. In general terms, the heat expansion process may be carried out by preparing vesicles of an expandable polymer or copolymer which may contain in their void (cavity) a volatile liquid (gaseous precursor). The vesicle is then heated, plasticising the vesicle and converting the volatile liquid into a gas, causing the vesicle to expand to up to about several times its original size. When the heat is removed, the thermoplastic polymer retains at least some of its expanded shape. Vesicles produced by this process tend to be of particularly low density, and are thus preferred. The foregoing described process is well known in the art, and may be referred to as the heat expansion process for preparing low density vesicles. Polymers useful in the heat expansion process will be readily apparent to those skilled in the art and include thermoplastic polymers or copolymers, including polymers or copolymers of many of the monomers described above. Preferable of the polymers and copolymers described above include the following copolymers: polyvinylidene-polyacrylonitrile, polyvinylidene-polyacrylonitrile-polymethyl- methacrylate, and polystyrene-polyacrylonitrile. A most preferred copolymer is poly vinylidene-polyacry lonitrile . Volatile liquids useful in the heat expansion process will also be well known to those skilled in die art and include: 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 3 , CC1F 2 -CC1 2 F 2 , chloroheptafluoro- cyclobutane, and 1,2-dichlorohexafluorocyclobutane; tetraalkyl silanes, such as tetramethyl silane, trimethylethyl silane, trimethylisopropyl silane, and trimethyl n- propyl silane; as well as perfluorocarbons, including the perfluorocarbons described above. In general, it is important that the volatile liquid not be a solvent for the polymer or copolymer being utilized. It is also preferred that the volatile liquid have a boiling point that is below the 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 in 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 the vesicles in water in the presence of hydrogen peroxide prior 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, vinylidene and acrylonitrile may be copolymerized in a medium of isobutane liquid using one or more of the foregoing modified or unmodified literature procedures, such that isobutane becomes entrapped within 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 the vesicles. After heat is removed, the expanded polyvinylidene and acrylonitrile copolymer vesicles remain substantially fixed in their expanded position. The resulting low density vesicles are extremely stable both dry and suspended in an aqueous media. Isobutane is utilized herein merely as an illustrative liquid, with the understanding that other liquids which undergo liquid/gas transitions at temperatures useful for the synthesis of these vesicles and formation of the very low density vesicles upon heating can be substituted for isobutane. Similarly, monomers other than vinylidene and acrylonitrile may be employed in preparing the vesicles. In certain preferred embodiments, the vesicles which are formulated from synthetic polymers and which may be employed in the methods of the present invention are commercially available from Expancel, Nobel Industries (Sundsvall, Sweden), including EXPANCEL 551 DE™ microspheres. The EXPANCEL 551 DE™ microspheres are composed of a copolymer of vinylidene and acrylonitrile which have encapsulated tiierein 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 the density of water. The present invention is directed to methods for diagnostic imaging which involve the administration to a patient of a contrast agent, in combination with a coronary vasodilator. The coronary vasodilator may be administered to the patient before, during, and/or after the administration of the contrast agent. A wide variety of techniques are available for the preparation of lipid and/or vesicle compositions which comprise a bioactive agent, including coronary vasodilators. For example, lipid and/or vesicle compositions may be prepared from a mixture of lipid compounds, bioactive agent and gas or gaseous precursor. In this case, lipid and/or vesicle compositions may be prepared as described above in which the compositions also comprise bioactive agent. Thus, for example, micelles can be prepared in the presence of a bioactive agent. In connection with lipid and/or vesicle compositions which comprise a gas, the preparation can involve, for example, bubbling a gas directly into a mixture of lipid compounds and one or more additional materials. Alternatively, the lipid and/or vesicle compositions may be preformed from lipid compounds and gas or gaseous precursor. In the latter case, the bioactive agent may be then added to the lipid and/or vesicle composition prior to use. For example, an aqueous mixture of liposomes and gas may be prepared to which the 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 since the gas and/or bioactive agent filled liposome vesicles generally float to the top of the aqueous solution. Excess bioactive agent can be recovered from the remaining aqueous solution. As those skilled in the art will recognize, any of the lipid and/or vesicle compositions described herein may be lyophilized for storage, and reconstituted, for example, with an aqueous medium (such as sterile water, phosphate buffered solution, or aqueous saline solution), with 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 occurring. Additives which may be useful include sorbitol, mannitol, sodium chloride, glucose, trehalose, polyvinylpyrrolidone 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 preferred. These and other additives are described in the literature, such as in 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 herein by reference in their entirety. Lyophilized preparations generally have the advantage of greater shelf life. As discussed above, the compositions of the present invention, including gas and/or gaseous precursor filled vesicles, are useful as contrast agents for diagnostic imaging, including, for example, ultrasound (US) imaging, computed tomography (CT) imaging, including CT angiography (CTA), magnetic resonance (MR) imaging, including magnetic resonance angiography (MRA), nuclear medicine, optical imaging and elastography. In accordance with the present invention, there are provided methods of imaging one or more regions of a patient, and especially the cardiovascular region of a patient. The present invention also provides methods for diagnosing the presence or absence of diseased tissue in a patient, especially the presence or absence of diseased tissue in the cardiovascular region of a patient. The methods of the present invention involve the administration of a contrast medium in the form, for example, of a lipid and/or vesicle composition, to a patient. A coronary vasodilator is also administered to the patient. The patient is scanned using diagnostic imaging including, for example ultrasound imaging, to obtain visible images of an internal region, preferably the cardiovascular region of a patient. The methods may also be used to image the gastrointestinal region or the lymphatic system, and more broadly to image other internal regions of the patient including, for example, the vasculature. The phrase "gastrointestinal region" or "gastrointestinal tract, " as used herein, includes the region of a patient defined by the esophagus, stomach, small and large intestines and rectum. The present methods can also be used in connection with the delivery of a bioactive agent to an internal region of a patient. With respect to the coronary vasodilator which is employed in the methods of the present invention, the following discussion is provided. A wide variety 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 coronary vasodilative effect, that is, dilation of blood vessels in the cardiovascular region of a patient. Preferably, the material which is employed as a coronary vasodilator is capable of increasing the overall myocardial blood flow, as well as increasing the myocardial blood flow relative to the ventricular and/or atrial blood flow. The coronary vasodilators which may be employed in the present methods include materials which are classified as arterial and/or venous vasodilators, as well as preload and preload/afterload reducers. Included among the coronary vasodilators which may be used in the methods of the present invention are coronary nitro vasodilators, including nitroglycerin, isosorbide dinitrate, isosorbide tetranitrate and sodium nitroprusside; phosphodiesterase inhibitors, including amrinone, milrinone and vesnarinone; direct coronary vasodilators, including hydralazine and nicorandil; adrenergic receptor antagonists, including prazosin and other quinazoline derivatives, phentolamine, labetalol, carvedilol and bucindolol; calcium (Ca 2+ ) channel blocking agents, including nifedipine and amlodipine; sympathomimetics, such as dobutamine; cyclic nucleotide phosphodiesterase isoenzyme inhibitors, including vinpocetine, milrinone, amrinone, pimobendan, cilostamide, enoximone, peroximone, vesnarinone, rolipram, zaprinast and dipyridamole; adenosine; and alkaloids, including papeverine. Preferred among these coronary vasodilators are nitroprusside, nitroglycerin, isosorbide dinitrate, isosorbide tetranitrate, nifedipine and dipyridamole, with dipyridamole being more preferred. Other coronary vasodilators which would be suitable for use in the methods of the present invention, including those exemplified above, would be apparent to one of ordinary skill in the art once armed with the present disclosure. If desired, the lipid and/or vesicle compositions described herein may further comprise a targeting agent to promote targeting of tissues and/or receptors in vivo including, for example, myocardial tissue. Suitable targeting agents, methods for their incoφoration into lipid 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, in their entirety. As one skilled in the art would recognize, administration of the lipid and/or vesicle compositions described herein, as well as the coronary vasodilators, can be carried out in various fashions, including parenterally, orally, or intraperitoneal ly. Parenteral administration, which is preferred, includes administration by the following routes: intravenous; intramuscular; interstitially; intra-arterially; subcutaneous; intraocular; intrasynovial; transepithelial, including transdermal; pulmonary via inhalation; ophthalmic; sublingual and buccal; topically, including ophthalmic; dermal; ocular; rectal; and nasal inhalation via insufflation. Intravenous administration is preferred among the routes of parenteral administration. Various combinations of the lipid and/or vesicle compositions, and coronary vasodilators, may be used to alter properties as desired, including viscosity, osmolarity or palatability. In carrying out the imaging methods of the present invention, the contrast medium, including coronary vasodilator, can be used alone, or in combination with additional diagnostic, therapeutic or other agents. Such other agents include excipients such as flavoring or coloring materials. CT imaging techniques 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 the 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 in 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 cardiovascular region, may be then obtained, such that the presence or absence of diseased tissue can be ascertained. With respect to ultrasound, ultrasonic imaging techniques, including second harmonic imaging, and gated imaging, are well known in the art, and are described, for example, in Uhlendorf, "Physics of Ultrasound Contrast Imaging: Scattering in the Linear Range", IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 14(1), pp. 70-79 (1994) and Sutherland, et al. , "Color Doppler Myocardial Imaging: A New Technique for the Assessment of Myocardial Function", Journal of the American Society of Echocardiography , Vol. 7(5), pp. 441-458 (1994), the disclosures of which are hereby incoφorated herein by reference in their entirety. Ultrasound can be used for both diagnostic and therapeutic puφoses. In diagnostic ultrasound, ultrasound waves or a train of pulses of ultrasound may be applied with a transducer. The ultrasound is generally pulsed rather than continuous, although it may be continuous, if desired. Thus, diagnostic ultrasound generally involves the application of a pulse of echoes, after which, during a listening period, the ultrasound transducer receives reflected signals. Harmonics, ultraharmonics or subharmonics may be used. The second harmonic mode may be beneficially employed, in which the 2x frequency may be received, where x is the incidental frequency. This may serve to decrease the signal from the background material and enhance the signal from the transducer using the contrast media of the present invention which may be targeted to the desired site. Other harmonics signals, such as odd harmonics signals, for example, 3x or 5x, may be similarly received using this method. Subharmonic signals, for example, x/2 and x/3, may also be received and processed so as to form an image. In addition to the pulsed method, 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 the vesicles and thereby promote their rupture. This can create acoustic emissions which may be in the subharmonic or ultraharmonic range or, in some cases, in the same frequency as the applied ultrasound. It is contemplated that there may be a spectrum of acoustic signatures released in this process and the transducer so employed may receive the acoustic emissions to detect, for example, the presence of a clot. In addition, the 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 the 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 being 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, depending on the area of tissue to be treated with the ultrasound. Areas of the body which are generally characterized by larger amounts of muscle mass, for example, backs and thighs, as well as highly vascularized 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, although the sound energy may be pulsed also. If pulsed sound energy is used, the sound will generally be pulsed in echo train lengths of from about 8 to about 20 or more pulses at a time. Preferably, the echo train lengths are about 20 pulses at a time. In addition, the frequency of the 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 preferred. In addition, energy levels may vary from about 0.5 Watt (W) per square centimeter (cm 2 ) to about 5.0 W/crrf , 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 than 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 than about 10 MHZ, the sonic energy may penetrate fluids and tissues to a limited depth only. Thus, external application of the sonic energy may be suitable for skin and other superficial tissues. However, it is generally necessary for deep structures to focus the ultrasonic energy so that it is preferentially directed within a focal zone. Alternatively, the ultrasonic energy may be applied via interstitial probes, intravascular ultrasound catheters or endoluminal catheters. Such probes or catheters may be used, for example, in the esophagus for the diagnosis and/or treatment of esophageal carcinoma. In addition to the therapeutic uses discussed above, the compositions described herein can be employed in connection with esophageal carcinoma or in the coronary arteries for the treatment of atherosclerosis, as well as the therapeutic uses described, for example, in U.S. Patent No. 5,149,319, the disclosures of which are hereby incoφorated herein by reference, in tiieir entirety. A therapeutic ultrasound device may be used which employs two frequencies of ultrasound. The first frequency may be defined as x, and the second frequency may be defined as 2x. In preferred form, the device would be designed such that the focal zones of the first and second frequencies converge to a single focal zone. The focal zone of the device may then be directed to the compositions, for example, vesicle compositions, within the tissue in the region of interest. This ultrasound device may provide second harmonic therapy with simultaneous application of the x and 2x frequencies of ultrasound energy. It is contemplated that, in the case of ultrasound involving vesicles, this second harmonic therapy may provide improved rupturing of vesicles as compared to ultrasound energy involving a single frequency. Also, it is contemplated that the 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 with the aforementioned second harmonic therapy 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, the concentration of lipid required to form a desired stabili.zed vesicle level may vary depending, for example, upon the 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 the 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 preferred 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 coronary vasodilator to be administered, and the particular mode of administration, may vary depending upon the age, weight and the particular mammal and region thereof to be scanned, and the particular contrast medium and/or coronary vasodilator to be employed. Typically, the dosage may be initiated at lower levels and increased until the desired contrast enhancement is achieved. With respect to the contrast agent, the IV dose may be less than about 10 mL for a 70 Kg patient, with lower doses being preferred. The dosage of the coronary 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 preferred embodiments which involve the administration of compositions that comprise DPPC, DPPE and DPPE PEG-5000, in combination with a coronary vasodilator, the coronary 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 described herein, and especially the vesicle compositions, are useful as contrast media in diagnostic imaging, and may also be suitable for use in all areas where diagnostic imaging is employed. However, the stabilized vesicles are particularly useful for perfusion imaging. In accordance with the present invention, there are provided methods of imaging a patient generally, preferably the cardiovascular region, and/or in specifically diagnosing the presence of diseased tissue in a patient, especially the cardiovascular region. The imaging process of the present invention may be carried out by administering a contrast agent and a coronary vasodilator to a patient, and then scanning the patient using, for example, ultrasound, computed tomography, and/or magnetic resonance imaging, 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 in providing images of the cardiovascular 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 those skilled in the art. The patient can be any type of mammal, but most preferably is a human. The present invention also provides methods of diagnosing the presence of diseased tissue in a patient, especially diseased tissue in the region of the cardiovasculature. Diseased tissue includes, for example, endothelial tissue which results from vasculature that supports diseased tissue. As a result, the localization and visualization of endothelial tissue to a region of a patient which under normal circumstances is not associated with endothelial tissue provides an indication of diseased tissue in the region. In addition, the methods of the present invention may be used for diagnosing the presence or absence of coronary artery disease, including, for example, coronary artery disease associated with atherosclerotic disease, hypertrophic cardiomyopathy, dilated cardiomyopathy, and infiltrative or restrictive cardiomyopathy. Other diseases of the cardiovasculature which may be imaged and/or diagnosed with the methods of die present invention would be readily apparent to one of ordinary skill in the art, once armed with the present disclosure. As noted above, administration of the compositions described herein may be carried out in various fashions, such as intravascularly, orally, rectally, and the like, using a variety of dosage forms. When the region to be scanned is the cardiovascular region, administration of the contrast medium is preferably carried out intravascularly including, for example, intravenously. When the region to be scanned is the gastrointestinal region, administration of the contrast medium is preferably carried out orally or rectally. Administration of the coronary vasodilator may also be carried out in various fashions, such as intravascularly or orally, depending on the particular coronary vasodilator employed. For example, nitro vasodilators, such as nitroglycerin, may be administered IV, as well as transdermally. The appropriate mode or modes of administration of the coronary vasodilator would be readily apparent to one of ordinary skill in the art, once armed with the present disclosure. Various combinations of the lipid and/or vesicle compositions may be used to modify the relaxation behavior of the medium or to alter properties such as the viscosity, osmolarity or palatability (in the case of orally administered materials). The present invention may be performed with ultrasound, computed tomography or MRI according to conventional methods known by skilled artisans. Ultrasound is unlike nuclear medicine and X-rays since it does not expose the patient to the harmful effects of ionizing radiation. Moreover, unlike magnetic resonance imaging, ultrasound is relatively inexpensive and can be conducted as a portable examination. In using the ultrasound technique, sound is transmitted into a patient or animal via a transducer. When the sound waves propagate through the body, they encounter interfaces from tissues and fluids. Depending on the acoustic properties of the tissues and fluids in the body, the ultrasound sound waves are partially or wholly reflected or absorbed. When sound waves are reflected by an interface they are detected by the receiver in the transducer and processed to form an image. The acoustic properties of the tissues and fluids within the body determine the contrast which appears in the resultant image. Computed tomography imaging principles and techniques 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. , Ch. 1 , pp. 1-7 (Raven Press, NY 1983). In carrying out the magnetic resonance imaging method of the present invention, the contrast agent and coronary vasodilator can be used alone, or in combination with other diagnostic, therapeutic or other agents. Such other agents include excipients such as flavoring or coloring materials. 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 techniques include, but are not limited to, nuclear magnetic resonance (NMR) and electronic spin resonance (ESR). The preferred imaging modality is NMR. The echogenicity of vesicles, and especially, gas filled vesicles, and die 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 monitored subsequent to their administration to a patient to determine the rate at which the vesicles arrive, for example, to a desired region. Furthermore, the vesicles may be ruptured using ultrasound to release a bioactive agent in the region. In accordance with the present invention, there are also provided methods for measuring blood flow in the cardiovascular 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 interest a substantial pulse of ultrasonic energy, for example, an ultrasonic energy pulse of greater than about 0.5 W/cm 2 , to provide a "square wave" of energy. This square wave energy may provide substantial reflectivity and, therefore, substantial brightness in the ultrasound image. As the square wave energy becomes diluted in the bloodstream, the brightness of the corresponding ultrasound image similarly decreases. Observation and analysis of the rate that the square wave energy becomes diluted may provide an indication of the rate of blood flow in the myocardium. In another embodiment of a method for measuring blood flow in the cardiovascular region of a patient, there are provided herein methods which generally involve the administration to a patient of a contrast agent in such a manner as to maintain the concentration of contrast agent in the cardiovascular 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 preferred embodiments of the present invention, the lipid and/or vesicle compositions may be administered by syringe, that is, by intravenous (IV) injection. Accordingly, the gas and/or vesicle administration rates provided herein generally correspond to injection rates. As would be apparent to one of ordinary skill in the art, once armed with the present disclosure, the location on the body of the patient at which the lipid and/or vesicle compositions are injected may vary and depends upon a variety of factors, including, for example, the particular lipid and/or vesicle composition employed, the contemplated application, such as diagnostic or therapeutic application, and the particular region of interest. For example, in the case of diagnostic ultrasound of myocardial tissue, the lipid and/or vesicle compositions may be injected intravenously (IV), for example, in the arm of a patient. The IV administration of the contrast agents described herein including, for example, the vesicle compositions, may involve administration via syringe. This may be achieved, for example, by an appropriate medical technician who handles the syringe or syringes manually. Alternatively, administration may be achieved mechanically, for example, by using a constant infusion device, such as a mechanical injector which operates using pneumatic or hydraulic pressure. Suitable mechanical injectors which may be used in the methods of the present invention include a Syringe Pump Model 351 , commercially available from Sage Instruments (a division of Orion Research Inc., Boston, MA), a MedRad™ power injector, commercially available from Medrad, Inc. (Pittsburgh, PA) or a Liebel Flarsheim, commercially available from Liebel Flarsheim Co. (Cincinnati, OH). Preferably, the contrast agent is administered 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 the like. In the case of contrast agents based on vesicles, the time period of administration may also depend, for example, on the concentration of vesicles in the contrast agent, the size of the vesicles and the distribution of vesicle size in the contrast agent. Scanning the cardiovascular region with diagnostic imaging such as, for example, ultrasound imaging, provides a diagnostic image of the cardiovascular region. By maintaining the concentration of contrast agent in the cardiovascular region substantially constant, there may desirably be provided a substantially constant level of brightness in this diagnostic image. Subsequent administration to the patient of a coronary vasodilator may then increase the myocardial blood flow which, as discussed above, may also increase the concentration of contrast agent in the myocardial tissue. For the puφoses of the methods described in accordance with the present embodiment, it is understood that the concentration of vesicles is about proportional to the brightness of a diagnostic image, particularly an ultrasound image, which in turn is about proportional to the rate of blood flow in the cardiovascular region. Also for the puφoses of the methods described in the present embodiment, it is understood that the vesicles preferably follow a first order rate of elimination. However, other order rates, such as zero, second or third order rates, may be applicable also to these med ods. With these understandings, there may be obtained an indication of the percent increase in myocardial flow provided by the coronary vasodilator by using a simple derivation: -d[vesicles]/dT = k[vesicles] where [vesicles] = concentration of vesicles; k = rate constant as a first order process (sec '); and T = time (seconds). With me understanding also that there is preferably a correlation between vesicle concentration, videodensitometry and myocardial blood flow, which correlation may be defined, for example, by a correlation constant k, then: -d[ vesicles ]/dT = kfvesicles] = d[videodensitometry]/dT = k[ videodensitometry] where [videodensitometry] = video brightness in videodensitometry units (VDU's). Rearranging the above equation yields the following: -d[videodensitometry]/[videodensitometry] = kdT Thus, - d[videodensitometry]/[videodensitometry] = j kdT or ln[videodensitometry t=0 - videodensitometryj = kT Expansion of the foregoing equation yields: [videodensitometryj = [videodensitometry [=0 ] = e kT With the measurement of a baseline videodensitometry at time t = 0 which, in the present embodiment, refers to the videodensity of a diagnostic image of the cardiovascular region with a constant infusion of contrast agent, and the final videodensitometry at time T= t which, in the present embodiment, refers to the videodensity of a diagnostic image of the cardiovascular region after administration of a coronary vasodilator, a rate constant for the increase in myocardial flow at a given dosage of coronary vasodilator may be determined. With the understanding that the blood flow in the cardiovascular region is about proportional to videodensitometry, then substitution yields: [flowj = [flow 1=0 ] = e kT Thus, the relative increase (%) in myocardial blood flow may be determined by employing a constant administration of contrast agent and once armed with the knowledge of the change in brightness of myocardial tissue in ultrasonic imaging after administration of a coronary vasodilator. Variations in correlation between flow, videodensitometry, and microbubble concentration per unit time which may occur may affect the accuracy of the determinations of increases in myocardial blood flow. However, corrections to any non-linearity or non-proportionality may be incoφorated into the calculations. For example, corrections for variations between vesicle concentration and image quantitation may be provided, for example, by employing the techniques described in Eriksen, M. , "Tissue Echo Intensity and Blood Attenuation Changes - The Pitfalls of Video Densitometry", The Second Annual International Symposium on Contrast Agents in Diagnostic Ultrasound, Atlantic City, NJ (May 7, 1996). The invention is further described in the following ex.amples. Example 1 is an actual example, while Example 2 is a prophetic example. These examples are for illustrative puφoses only, and are not to be construed as limiting the appended claims. Example 1 This example describes the preparation of a gas filled vesicle composition for use in the methods of the present invention. "DPPC" refers to dipalmitoylphosphatidylcholine; "DPPE" refers to dipalmitoylphosphatidylethanol- amine; 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 in 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 the vial was pressurized with PFP. The vial was men sealed, placed on a shaker and agitated at room temperature to provide a solution of PFP- filled vesicles having a mean diameter of about 2.5 μm. The concentration of vesicles in the solution was about 1.5 x 10 9 vesicles/mL. Example 2 This example includes a description of methods for imaging the cardiovascular region of a patient in accordance with methods within the scope of the present invention. A 20 Kg pig is anesthetized with isoflurane and a Swan Ganz catheter is inserted to monitor hemodynamics. Dose range studies are conducted using the gas filled vesicles prepared in Example 1 to determine the minimum dose required to enhance the myocardium echo, which is defined as greater than 10 videodensitometry units as determined by repeated videodensity measurements. After the dose range studies are performed, dipyridamole is administered intravenously at a dose of 0.56 mg/Kg over a period of 4 minutes. After a 5 minute latency period, dose range studies are repeated. A comparison of the dose range studies performed before administration (control) and after admimstration (test) of dipyridamole indicates that a significantly smaller dosage of the contrast agent (gas filled vesicle composition) is required to obtain a medically useful diagnostic image of the cardiovascular region when a vasodilator is also administered to the patient. In addition, diagnostic artifacts are substantially eliminated in the studies involving the use of dipyridamole. The disclosures of each patent, patent application and publication cited or described in this document are hereby incoφorated by reference herein, in their entirety. Various modifications of the invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

Claims: What is claimed is: CLAIMS 1. A method for providing an image of the cardiovascular region of a patient comprising (i) administering to the patient a vesicle composition which comprises lipid or polymer vesicles and a gas or gaseous precursor, in combination with a coronary vasodilator, and (ii) scanning the patient using diagnostic imaging to obtain a visible image of the cardiovascular region. 2. A method according to Claim 1 wherein said vesicles comprise lipid vesicles. 3. A method according to Claim 2 wherein said vesicle composition comprises vesicles selected from the group consisting of micelles and liposomes. 4. A method according to Claim 2 wherein said vesicles comprise unilamellar vesicles. 5. A method according to Claim 4 wherein said vesicles comprise one monolayer. 6. A method according to Claim 5 wherein said lipid comprises a phospholipid. 7. A method according to Claim 4 wherein said vesicles comprise one bilayer. 8. A method according to Claim 7 wherein said lipid comprises a phospholipid. 9. A method according to Claim 2 wherein said vesicles are selected from the group consisting of oligolamellar and multilamellar vesicles. 10. A method according to Claim 2 wherein said lipid comprises a phospholipid. 11. A method according to Claim 10 wherein said phospholipid is selected from the group consisting of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. 12. A method according to Claim 11 wherein said phosphatidylcholine is selected from the group consisting of dioleoylphosphatidylcholine, dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoyl- phosphatidylcholine . 13. A method according to Claim 12 wherein said phosphatidylcholine comprises dipalmitoylphosphatidylcholine. 14. A method according to Claim 11 wherein said phosphatidyl- ethanolamine is selected from the group consisting of dipalmitoylphosphatidylethanol- amine, dioleoylphosphatidylethanolamine, N-succinyldioleoylphosphatidylethanolamine and l-hexadecyl-2-palmitoylglycerophosphoethanolamine. 15. A method according to Claim 14 wherein said phosphatidylethanolamine comprises dipalmitoylphosphatidylethanolamine. 16. A method according to Claim 11 wherein said phosphatidic acid comprises dipalmitoly -phosphatidic acid. 17. A method according to Claim 2 wherein said lipid further comprises a polymer. 18. A method according to Claim 17 wherein said polymer comprises a hydrophilic polymer. 19. A method according to Claim 18 wherein said hydrophilic polymer comprises polyethylene glycol. 20. A method according to Claim 1 wherein said vesicles comprise polymer vesicles. 21. A method according to Claim 20 wherein said polymer comprises a synthetic polymer or copolymer which is prepared from monomers selected from the group consisting of acrylic acid, methacrylic acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, lactic acid, glycolic acid, e-caprolactone, acrolein, cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkylacrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxy alky lmethacrylates, N-substituted acrylamides, N-substituted methacrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-l-ol, vinyl acetate, acrylonitrile, styrene, p-amino-styrene, p-aminobenzylstyrene, sodium styrene sulfonate, sodium 2-sulfoxyethyl-methacrylate, vinyl pyridine, aminoethyl methacrylates and 2-me acryloyloxytrimethyl-ammonium chloride. 22. A method according to Claim 20 wherein said polymer comprises a synthetic polymer or copolymer selected from the group consisting of polyacrylic acid, polyethyleneimine, polymethacrylic acid, polymethylmethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly (e-caprolactone), epoxy resin, poly(ethylene oxide), poly(ethylene glycol), polyamide, polyvinylidene-polyacrylonitrile, polyvinylidene-polyacrylonitrile-polymethylmethacrylate and polystyrene- polyacrylonitrile. 23. A method according to Claim 22 wherein said polymer comprises polyvinylidene-polyacrylonitrile copolymer. 24. A method according to Claim 1 wherein said gas comprises a fluorinated gas. 25. A method according to Claim 24 wherein said fluorinated gas is selected from the group consisting of a perfluorocarbon, sulfur hexafluoride and heptafluoropropane . 26. A method according to Claim 25 wherein said fluorinated gas comprises a perfluorocarbon. 27. A method according to Claim 26 wherein said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane. 28. A method according to Claim 1 wherein said gaseous precursor has a boiling point of greater than about 37 °C. 29. A method according to Claim 28 wherein said gaseous precursor comprises a fluorinated compound. 30. A method according to Claim 29 wherein said fluorinated compound comprises a perfluorocarbon. 31. A method according to Claim 30 wherein said perfluorocarbon is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane . 32. A method according to Claim 1 wherein said diagnostic imaging is selected from the group consisting of ultrasound imaging and computed tomography imaging. 33. A method according to Claim 32 wherein said diagnostic imaging comprises ultrasound imaging. 34. A method according to Claim 1 wherein said coronary vasodilator is selected from the group consisting of arterial vasodilators, venous vasodilators, preload reducers and preload/afterload reducers. 35. A method according to Claim 34 wherein said coronary vasodilator is selected from the group consisting of nitroprusside, nitroglycerin, isosorbide dinitrate, isosorbide tetranitrate, nifedipine and dipyridamole. 36. A method according to Claim 35 wherein said coronary vasodilator is dipyridamole. 37. A method according to Claim 1 wherein said coronary vasodilator enhances brightness in the diagnostic image. 38. A method according to Claim 1 wherein said coronary vasodilator substantially eliminates diagnostic artifacts in the diagnostic image. 39. A method according to Claim 1 wherein said vesicle composition further comprises an additional bioactive agent. 40. A method for providing an image of the cardiovascular region of a patient comprising (i) administering to the patient a lipid composition which comprises a lipid and a gas or gaseous precursor, in combination with a coronary vasodilator, and (ii) scanning the patient using diagnostic imaging to obtain a visible image of the cardiovascular region. 41. A method according to Claim 40 wherein said lipid comprises a phospholipid. 42. A method according to Claim 41 wherein said phospholipid is selected from the group consisting of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. 43. A method according to Claim 40 wherein said lipid composition comprises a vesicle composition. 44. A method according to Claim 43 wherein said vesicle composition comprises vesicles selected from the group consisting of micelles and liposomes. 45. A method according to Claim 40 wherein said diagnostic imaging is selected from the group consisting of ultrasound imaging and computed tomography imaging. 46. A method according to Claim 45 wherein said diagnostic imaging comprises ultrasound imaging. 47. A method for diagnosing the presence of diseased tissue in the cardiovascular region of a patient comprising (i) administering to the patient a vesicle composition which comprises lipid or polymer vesicles and a gas or gaseous precursor, in combination with a coronary vasodilator, and (ii) scanning the patient using diagnostic imaging to obtain a visible image of any diseased tissue in the cardiovascular region of the patient. 48. A method according to Claim 47 wherein said vesicles comprise lipid vesicles. 49. A method according to Claim 48 wherein said vesicle composition comprises vesicles selected from the group consisting of micelles and liposomes. 50. A method according to Claim 48 wherein said lipid comprises a phospholipid. 51. A method according to Claim 50 wherein said phospholipid is selected from the group consisting of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. 52. A method according to Claim 51 wherein said phosphatidyl- choline is selected from the group consisting of dioleoylphosphatidylcholine, dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine. 53. A method according to Claim 52 wherein said phosphatidylcholine comprises dipalmitoylphosphatidylcholine. 54. A method according to Claim 51 wherein said phosphatidylethanolamine is selected from the group consisting of dipalmitoylphosphatidyl- ethanolamine, dioleoylphosphatidylethanolamine, N-succinyldioleoylphosphatidyl- ethanolamine and 1 -hexadecyl-2-palmitoylglycerophosphoethanolamine . 55. A method according to Claim 54 wherein said phosphatidyl- ethanolamine comprises dipalmitoylphosphatidylethanolamine. 56. A method according to Claim 51 wherein said phosphatidic acid comprises dipalmitolylphosphatidic acid. 57. A method according to Claim 48 wherein said lipid further comprises a polymer. 58. A method according to Claim 57 wherein said polymer comprises a hydrophilic polymer. 59. A method according to Claim 58 wherein said hydrophilic polymer comprises polyethylene glycol. 60. A method according to Claim 47 wherein said vesicles comprise polymer vesicles. 61. A method according to Claim 60 wherein said polymer comprises a synthetic polymer or copolymer which is prepared from monomers selected from the group consisting of acrylic acid, methacrylic acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, lactic acid, glycolic acid, e-caprolactone, acrolein, cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkylacrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxy alky lmethacrylates, N-substituted acrylamides, N-substituted methacrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-l-ol, vinyl acetate, acrylonitrile, styrene, p-amino-styrene, p-aminobenzylstyrene, sodium styrene sulfonate, sodium 2-sulfoxyethyl-methacrylate, vinyl pyridine, aminoethyl methacrylates and 2-methacryloyloxytrimethyl-∑unmonium chloride. 62. A method according to Claim 60 wherein said polymer comprises a synthetic polymer or copolymer selected from the group consisting of polyacrylic acid, polyethyleneimine, polymethacrylic acid, polymethylmethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly (e-caprolactone), epoxy resin, poly(ethylene oxide), poly(ethylene glycol), polyamide, polyvinylidene-polyacrylonitrile, polyvinylidene-polyacrylonitrile-polymethylmethacrylate and polystyrene- polyacrylonitrile. 63. A method according to Claim 62 wherein said polymer comprises polyvinylidene-polyacrylonitrile copolymer. 64. A method according to Claim 47 wherein said gas comprises a fluorinated gas. 65. A method according to Claim 64 wherein said fluorinated gas is selected from the group consisting of a perfluorocarbon, sulfur hexafluoride and heptafluoropropane . 66. A method according to Claim 65 wherein said fluorinated gas comprises a perfluorocarbon. 67. A method according to Claim 66 wherein said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane. 68. A method according to Claim 47 wherein said gaseous precursor has a boiling point of greater than about 37°C. 69. A method according to Claim 68 wherein said gaseous precursor comprises a fluorinated compound. 70. A method according to Claim 69 wherein said fluorinated compound comprises a perfluorocarbon. 71. A method according to Claim 70 wherein said perfluorocarbon is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane. 72. A method according to Claim 47 wherein said diagnostic imaging is selected from the group consisting of ultrasound imaging and computed tomography imaging. 73. A method according to Claim 72 wherein said diagnostic imaging comprises ultrasound imaging. 74. A method according to Claim 47 wherein said coronary vasodilator is selected from the group consisting of arterial vasodilators, venous vasodilators, preload reducers and preload/afterload reducers. 75. A method according to Claim 74 wherein said coronary vasodilator is selected from the group consisting of nitroprusside, nitroglycerin, isosorbide dinitrate, isosorbide tetranitrate, nifedipine and dipyridamole. 76. A method according to Claim 75 wherein said coronary vasodilator is dipyridamole. 77. A method according to Claim 47 wherein said coronary vasodilator increases brightening in the diagnostic image. 78. A method according to Claim 47 wherein said coronary vasodilator substantially eliminates diagnostic artifacts in the image. 79. A method for diagnosing the presence of diseased tissue in the cardiovascular region of a patient comprising (i) administering to the patient a lipid composition which comprises a lipid and a gas or gaseous precursor, in combination with a coronary vasodilator, and (ii) scanning the patient using diagnostic imaging to obtain a visible image of any diseased tissue in the cardiovascular region of the patient. 80. A method according to Claim 79 wherein said lipid comprises a phospholipid. 81. A method according to Claim 80 wherein said phospholipid is selected from the group consisting of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. 82. A method according to Claim 79 wherein said lipid composition comprises a vesicle composition. 83. A method according to Claim 80 wherein said vesicle composition comprises vesicles selected from the group consisting of micelles and liposomes. 84. A method according to Claim 79 wherein said diagnostic imaging is selected from the group consisting of ultrasound imaging and computed tomography imaging. 85. A method according to Claim 84 wherein said diagnostic imaging comprises ultrasound imaging. 86. A method for measuring blood flow in the cardiovascular region of a patient comprising (i) administering to the patient a vesicle composition which comprises lipid or polymer vesicles and a gas or gaseous precursor, (ii) scanning the patient with diagnostic imaging to obtain a visible image of the cardiovascular region, (iii) administering to the patient a coronary vasodilator, (iv) continuing said scanning, and (v) determining the blood flow from a videodensity versus time relationship in the images obtained in steps (ii) through (iv). 87. A method according to Claim 86 wherein said vesicle composition is administered to the patient to maintain a substantially constant concentration of vesicle composition in the cardiovascular region. 88. A method according to Claim 87 wherein said administration comprises constant administration. 89. A method according to Claim 88 comprising administering said vesicle composition using a constant infusion device. 90. A method according to Claim 89 wherein said constant infusion device comprises a power injector. 91. A method according to Claim 86 wherein said diagnostic imaging is selected from the group consisting of ultrasound imaging and computed tomography imaging. 92. A method according to Claim 91 wherein said diagnostic imaging comprises ultrasonic imaging. 93. A method according to Claim 86 wherein said vesicles comprise lipid vesicles. 94. A method according to Claim 93 wherein said vesicle composition comprises vesicles selected from the group consisting of micelles and liposomes. 95. A method according to Claim 93 wherein said lipid comprises a phospholipid. 96. A method according to Claim 95 wherein said phospholipid is selected from the group consisting of phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid. 97. A method according to Claim 96 wherein said phosphatidylcholine is selected from the group consisting of dioleoylphosphatidylcholine, dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine. 98. A method according to Claim 97 wherein said phosphatidylcholine comprises dipalmitoylphosphatidylcholine. 99. A method according to Claim 96 wherein said phosphatidyl- ethanolamine is selected from the group consisting of dipalmitoylphosphatidylethanol- amine , dioleoylphosphatidy lethanolamine , N-succiny Idioleoy Iphosphatidylethanolamme and 1 -hexadecyl-2-palmitoylglycerophosphoedιanolamine . 100. A method according to Claim 99 wherein said phosphatidylethanolamine comprises dipalmitoylphosphatidylethanolamine. 101. A method according to Claim 96 wherein said phosphatidic acid comprises dipalmitolylphosphatidic acid. 102. A method according to Claim 96 wherein said lipid further comprises a polymer. 103. A method according to Claim 102 wherein said polymer comprises a hydrophilic polymer. 104. A method according to Claim 103 wherein said hydrophilic polymer comprises polyethylene glycol. 105. A method according to Claim 86 wherein said vesicles comprise polymer vesicles. 106. A method according to Claim 105 wherein said polymer comprises a synthetic polymer or copolymer which is prepared from monomers selected from the group consisting of acrylic acid, methacrylic acid, ethyleneimine, crotonic acid, acrylamide, ethyl acrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, lactic acid, glycolic acid, e-caprolactone, acrolein, cyanoacrylate, bisphenol A, epichlorhydrin, hydroxyalkylacrylates, siloxane, dimethylsiloxane, ethylene oxide, ethylene glycol, hydroxyalkylmethacrylates, N-substituted acrylamides, N-substituted me acrylamides, N-vinyl-2-pyrrolidone, 2,4-pentadiene-l-ol, vinyl acetate, acrylonitrile, styrene, p-amino-styrene, p-aminobenzylstyrene, sodium styrene sulfonate, sodium 2-sulfoxyethyl-methacrylate, vinyl pyridine, aminoethyl methacrylates and 2-memacryloyloxytrimethyl-ammonium chloride. 107. A method according to Claim 105 wherein said polymer comprises a synthetic polymer or copolymer selected from the group consisting of polyacrylic acid, polyethyleneimine, polymethacrylic acid, polymethylmethacrylate, polysiloxane, polydimethylsiloxane, polylactic acid, poly (e-caprolactone), epoxy resin, poly (ethylene oxide), poly (ethylene glycol), polyamide, polyvinylidene- polyacrylonitrile, polyvinylidene-polyacrylonitrile-polymethylmethacrylate and polystyrene-polyacrylonitrile . 108. A method according to Claim 107 wherein said polymer comprises polyvinylidene-polyacrylonitrile copolymer. 109. A method according to Claim 86 wherein said gas comprises a fluorinated gas. 110. A method according to Claim 109 wherein said fluorinated gas is selected from the group consisting of a perfluorocarbon, sulfur hexafluoride and heptafluoropropane . 111. A method according to Claim 110 wherein said fluorinated gas comprises a perfluorocarbon. 112. A method according to Claim 111 wherein said perfluorocarbon gas is selected from the group consisting of perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane and perfluorocyclobutane. 113. A method according to Claim 86 wherein said gaseous precursor has a boiling point of greater than about 37 °C. 114. A method according to Claim 113 wherein said gaseous precursor comprises a fluorinated compound. 115. A method according to Claim 114 wherein said fluorinated compound comprises a perfluorocarbon. 116. A method according to Claim 115 wherein said perfluorocarbon is selected from the group consisting of perfluoropentane, perfluorohexane and perfluoroheptane . 117. A method according to Claim 86 wherein said coronary vasodilator is selected from the group consisting of arterial vasodilators, venous vasodilators, preload reducers and preload/afterload reducers. 118. A method according to Claim 117 wherein said coronary vasodilator is selected from the group consisting of nitroprusside, nitroglycerin, isosorbide dinitrate, isosorbide tetranitrate, nifedipine and dipyridamole. 119. A method according to Claim 118 wherein said coronary vasodilator is dipyridamole. 120. A method according to Claim 1 wherein said lipid comprises a surfactant. 121. A method according to Claim 120 wherein said gas or gaseous precursor is a perfluorocarbon. 122. A method according to Claim 120 wherein said gas or gaseous precursor is selected from the group consisting of nitrogen, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane and perfluoroheptane. 123. A method according to Claim 122 wherein said gas or gaseous precursor is a combination of nitrogen and perfluorohexane. 124. A method according to Claim 122 wherein said gas or gaseous precursor is a combination of nitrogen and perfluoropropane. 125. A method according to Claim 120 wherein said vesicle composition has been rehydrated from a lyophilized composition. 126. A method according to Claim 40 wherein said lipid comprises a surfactant. 127. A method according to Claim 126 wherein said gas or gaseous precursor is a perfluorocarbon. 128. A method according to Claim 126 wherein said gas or gaseous precursor is selected from the group consisting of nitrogen, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane and perfluoroheptane. 129. A method according to Claim 128 wherein said gas or gaseous precursor is a combination of nitrogen and perfluorohexane. 130. A method according to Claim 128 wherein said gas or gaseous precursor is a combination of nitrogen and perfluoropropane. 131. A method according to Claim 126 wherein said lipid composition has been rehydrated from a lyophilized composition. 132. A method according to Claim 47 wherein said lipid comprises a surfactant. 133. A method according to Claim 132 wherein said gas or gaseous precursor is a perfluorocarbon. 134. A method according to Claim 132 wherein said gas or gaseous precursor is selected from the group consisting of nitrogen, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane and perfluoroheptane. 135. A method according to Claim 134 wherein said gas or gaseous precursor is a combination of nitrogen and perfluorohexane. 136. A method according to Claim 134 wherein said gas or gaseous precursor is a combination of nitrogen and perfluoropropane. 137. A method according to Claim 132 wherein said vesicle composition has been rehydrated from a lyophilized composition. 138. A method according to Claim 79 wherein said lipid comprises a surfactant. 139. A method according to Claim 138 wherein said gas or gaseous precursor is a perfluorocarbon. 140. A method according to Claim 138 wherein said gas or gaseous precursor is selected from the group consisting of nitrogen, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane and perfluoroheptane. 141. A method according to Claim 140 wherein said gas or gaseous precursor is a combination of nitrogen and perfluorohexane. 142. A method according to Claim 140 wherein said gas or gaseous precursor is a combination of nitrogen and perfluoropropane. 143. A method according to Claim 138 wherein said lipid composition has been rehydrated from a lyophilized composition. 144. A memod according to Claim 86 wherein said lipid comprises a surfactant. 145. A method according to Claim 144 wherein said gas or gaseous precursor is a perfluorocarbon. 146. A method according to Claim 144 wherein said gas or gaseous precursor is selected from the group consisting of nitrogen, perfluoromethane, perfluoroethane, perfluoropropane, perfluorobutane, perfluorocyclobutane, perfluoropentane, perfluorohexane and perfluoroheptane. 147. A method according to Claim 146 wherein said gas or gaseous precursor is a combination of nitrogen and perfluorohexane. 148. A method according to Claim 146 wherein said gas or gaseous precursor is a combination of nitrogen and perfluoropropane. 149. A method according to Claim 144 wherein said vesicle composition has been rehydrated from a lyophilized composition.

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