Abstract:
Described are guidewires having at least one echolucent segment, and associated apparatuses and methods. The guidewires can be combined with devices equipped with intravascular ultrasound probes and used to effectively image regions during procedures underway in the vascular environment. The echolucent segment can have one or more echogenic markers to enable detection of the segment and/or relative movement of the segment using intravascular ultrasound.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to medical devices and procedures, and in particular aspects, to vascular guidewires and combinations thereof with other vascular devices, such as catheters, that can be beneficially used in animal and human patients when conducting procedures that employ intravascular ultrasound (IVUS) for imaging. 
         [0002]    Guidewires useful for intravascular procedures can be constructed using various materials and techniques. For example, guidewires can be constructed of segments of metallic wire formed into coils or strands, or both. Wire guides may also be coated with one or more of a wide range of coatings, such as for example, Polytetrafluoroethylene (PTFE) for reduced friction, or an anticoagulation agent like Heparin to reduce blood clotting. 
         [0003]    However, metal wire guides are highly reflective to ultrasound because the characteristic acoustic impedance of metallic substances causes substantially all of the sound waves to reflect off the device rather than passing through. Therefore, when metal guidewires are inserted into a patient and imaged using ultrasound imaging devices, various artifacts are routinely observed which can obscure important imaging features. For example, when a metallic guidewire is in use, a bright dot may be observed with a large shadow behind the wire. 
         [0004]    These artifacts may be especially severe in IVUS imaging. Due to space, size, and cost (i.e. one time use) limitations, IVUS transducers may have lower overall performance as compared to conventional transcutaneous transducers. Therefore, artifacts may be more pronounced further degrading image quality. The artifacts may be especially severe if the wire is close to the IVUS transducer further reducing opportunities to obtain usable, and perhaps critically important, clinical information from the ultrasound image. 
       SUMMARY 
       [0005]    The embodiments disclosed are directed toward guide members, such as guidewires and wire guides having similar function and purpose to those discussed above that are useable in conjunction with intravascular ultrasound procedures but which reduce or eliminate visual artifacts caused by metallic or other echogenic materials. Also included are modes of construction as well as examples of techniques and descriptions of their use. 
         [0006]    Embodiments described include guidewires that are at least partially echolucent presenting reduced visual interference in an ultrasound image when the echolucent portion is present within the imageable region. Ultrasonic waves preferably resonate in a frequency range as low as 20 khz or as high as 4 Ghz, with lower or higher frequencies possible as well depending on factors such as the imaging device used and the clinical objectives to name a few. Sound waves at any frequency cause the molecules of a physical substance they pass through (a “medium”) to vibrate. The density and the speed at which sound travels in the medium dictates how easily sound energy can pass through the medium. As sound waves pass through one medium to another different medium, the energy waves can change velocity causing some of the sound energy to be reflected off the new medium and some to pass through at a new velocity. 
         [0007]    For example in a clinical setting, as sounds waves move away from an ultrasonic transducer and through a human or animal, they may encounter several substances along the way such as muscle, bone, various liquids, air or other gases, and the like. Various clinical instruments and apparatus (such as a guidewire) may also be in the path of these waves. As the sound waves travel from one medium to another, such as from human tissue, through bodily fluids, through a guidewire, perhaps through bodily fluids again, and into the same or other human tissue behind the guidewire, the sound waves change speed at the interfaces between the different media. This speed change causes some of the sound energy to be reflected back toward the transducer, while some of the sound energy continues on away from the transducer. Generally speaking, as more sound energy is reflected back to the transducer, the reflecting medium generally appears more distinctly in the resulting ultrasound image. Therefore, materials that are “echolucent” appear less distinctly because they allow sound waves to travel through them causing fewer echoes. Materials that are “echogenic” allow fewer sound waves to travel through and cause more sound energy to be reflected. For example, as disclosed below, materials reflecting about the same amount of sound energy as soft tissue and fat result in few if any echoes being returned from these materials when they are used in a medical apparatus such as a guidewire inserted in the body adjacent to soft tissue and fat. 
         [0008]    Different materials can be chosen to adjust the “echolucence” of a device because doing so changes the characteristic acoustic impedance of the device. The characteristic acoustic impedance of a material or medium is an inherent property of that particular medium and is the product of the density of the medium and the speed of sound in the medium when no sound waves are traveling in it. Measured in Rayleigh (Rayl), 1 Rayl equals 1 newton-second per cubic meter or 1 kg/s·m 2 . Therefore the materials used in the construction of a guidewire or other similar apparatus may be varied to adjust the characteristic acoustic impedance of the material, thereby changing the ratio of sound energy passing through the material to the sound energy reflected by it. This can result in a guidewire or similar apparatus creating few if any resulting echoes under ultrasonic imaging thus reducing the visibility of the device. Reduced visibility caused by using echolucent materials makes it possible to image structures behind the guidewire (that is structures opposite the guidewire from the transducer) because sound energy can reach the more distal media by passing through the guidewire both after leaving the transducer and again on the way back to it. 
         [0009]    Various materials and modes of construction can be used to vary the echolucent properties of a guidewire or other such apparatus as disclosed. In one example, echogenic markers and marker regions are included along with the echolucent regions of the guide members. These echogenic markers may, for example, be constructed from a medium having a characteristic acoustic impedance that differs widely from that of human or animal flesh. For instance, the characteristic acoustic impedance of air at about room temperature is about 415 Rayl while the characteristic acoustic impedance of certain human or animal tissue can be in the range between about 1.5 to about 1.7 MRayl (or million Rayl)—over 3800 times higher. Thus, examples of guidewires with echogenic markers include guidewires made at least partially of materials having a characteristic acoustic impedance that is approximately equal to that of human flesh where the material also encapsulates or includes one or more voids containing air. Under ultrasound imaging, the voids (i.e. “bubbles”, or “markers”) enclosed in the material are more visible in the resulting image than the surrounding material which can be substantially invisible, or at least its visibility can be substantially reduced. 
         [0010]    Also disclosed are other materials, forms, and configurations besides air bubbles that could be used as markers, this being merely one example. Metallic flakes, elongated strips, beads, and other arrangements of marking elements or groups of elements embedded within or coupled to an echolucent guide member, or guide member portion, can therefore assist clinicians in tracking and positioning the guide members during imaging while minimizing unwanted artifacts. Also disclosed are embodiments of guide members used in conjunction with two dimensional and three dimensional intravascular ultrasound imaging devices. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGS 
         [0011]      FIG. 1  is a longitudinal cross-sectional view of the distal end of one example of a guide member and other apparatuses useful in intravascular ultrasound procedures 
           [0012]      FIG. 2  is a perspective partial cut-away view of the devices from  FIG. 1  introduced within a vascular lumen and connected to imaging equipment. 
           [0013]      FIG. 3  is a longitudinal cross-sectional view of the distal end of the guide member from  FIG. 1  in use with still other apparatuses useful in intravascular ultrasound procedures. 
           [0014]      FIG. 4  is a perspective partial cut-away view of the devices from  FIGS. 1 and 3  introduced within a vascular lumen and connected to the imaging equipment of  FIG. 1 . 
           [0015]      FIG. 5A-7C  are perspective views of other embodiments of the guide member shown in  FIGS. 1-4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments, some of which are illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
         [0017]    Illustrated in  FIG. 1  at  100  is one embodiment of a guide member  133  along with other associated apparatuses useful for intravascular ultrasound procedures in an animal or human patient. Guide member  133  is shown having a segmented arrangement of individual portions or segments joined together, these portions themselves may then also include other portions or segments as well depending on the materials used, the mode of construction, and the intended use. In the embodiment shown in  FIG. 1 , a first guide body portion  130  forming the distal end of guide member  133  is coupled to a second guide body portion  103 , with the two segments joined to one another at joint  113 . First guide body portion  130  has one or more longitudinally spaced discrete echogenic markers  129  interspersed along its length which are operable to appear during ultrasonic imaging procedures. Guide member  133  may be positioned within a first lumen  117  and can extend beyond distal end  127  of an elongate carrier body  110 . Elongate carrier body  110  can serve various functions such as maintaining an association between guide member  133  and an intravascular ultrasound probe  107  positioned within a second lumen  124 . By maintaining this association, guide member  133  can aid in maneuvering intravascular ultrasound probe  107  into position within the patient&#39;s body. 
         [0018]    Intravascular ultrasound probe  107  positioned within second lumen  124  has at its distal end an ultrasonic transducer  120  which is operable to emit ultrasonic energy and detect the reflected energy for the purpose of performing ultrasonic imaging of interior spaces of a patient&#39;s anatomy such as organs, blood vessels, and the like. Ultrasonic transducer  120  is preferably positioned distal to joint  113  so that ultrasonic energy emitted from ultrasonic transducer  120  passes through carrier body  110  and first guide body portion  130  but not through second guide body portion  103 . 
         [0019]    Elongate carrier body  110  can be constructed of any material suitable for intravascular introduction and navigation through blood vessels, organs, and other structures within a patient&#39;s body. Suitable materials include, but are not limited to, polyurethane, nylon, polyethylene, and silicone. Preferably elongate carrier body  110  includes echolucent materials to reduce or substantially eliminate sound energy reflected by the echolucent portion of carrier body  110 . By including echolucent material in elongate carrier body  110  in those regions where ultrasonic energy resonates from transducer  120 , ultrasonic waves can pass substantially unimpeded through carrier body  110  allowing the tissue surrounding carrier body  110  to be imaged. 
         [0020]    One embodiment of elongate carrier body  110  is a catheter having multiple lumens extending through some or all of the length of the catheter and exiting at or near the catheter&#39;s distal end. In this embodiment, the catheter acts to keep guide member  133  and intravascular ultrasound probe  107  properly positioned relative to one another such that guide member  133  can be used to help advance elongate carrier body  110 . Elongate carrier body  110  can then be useful for properly advancing intravascular ultrasound probe  107  to its intended region within the body. Other embodiments of elongate carrier body  110  include catheters having an intravascular ultrasound probe  107  embedded in, or otherwise coupled with, the catheter itself. 
         [0021]      FIG. 1  illustrates a first guide body portion  130  that includes an echolucent material. An echolucent material includes any material having a characteristic acoustic impedance substantially similar or about equal to the characteristic acoustic impedance of the surrounding material (e.g. bone, blood, muscle, bodily fluids, or other human or animal anatomical features of interest). Thus as sound waves pass through from the surrounding media and through first guide body portion  130 , the change in speed of the high frequency sound waves is minimized resulting in fewer echoes being returned to transducer  120  from guide body portion  130 . The results include ultrasound images where guide body portion  130  may be substantially or completely invisible. This allows the surrounding tissue or other anatomical structures of interest to be imaged rather than guide body portion  130 . 
         [0022]    In one example, the echolucent material in first guide body portion  130  may have a characteristic acoustic impedance of between about 1.5 MRayl and about 2.2 MRayl, which is approximately the range of characteristic acoustic impedances for many types of human or animal tissue. Examples of such echolucent materials include Polyethylene (PE), Polymethylpentene, and Ethyl Vinil Acetate. In another example, the echolucent material in first guide body portion  130  may have a characteristic acoustic impedance of between about 1 MRayl and about 5 MRayl, although such a material may cause reduced performance making guide body portion  130  more visible during ultrasonic imaging procedures. Examples of such materials include Acrylic, Polyvinyl Chloride (PVC), Polycarbonate, Nylon, Polystyrene, Vinyl, and Acrylonitrile Butadiene Styrene (ABS). First guide body portion  130  may, for example, include a polymeric material having a density in the range of 0.5 grams/cc to 3.5 grams/cc. In another embodiment, first guide body portion  130  includes polyethylene or another polymeric material having a density in the range of about 0.8 grams/cc to about 1.1 grams/cc. 
         [0023]    In other embodiments, first guide body portion  130  may include other echogenic structures or substances to modify the number and strength of reflected sound waves. Making a first guide body portion  130  disappear entirely from the resulting image may be undesirable and may result in a substantial reduction or complete loss of positional feedback. As a result, the clinician may, in such situations, be unable to properly maneuver and position first guide body portion  130  during the ultrasonic imaging procedure. 
         [0024]    Providing positional feedback using the ultrasonic imaging system may be achieved in various ways such as through the use of echogenic markers or marker regions as discussed below. However, it may also be advantageous to optionally include an echo-opacifier into the polymeric material used to construct first guide body portion  130  to precisely control its characteristic acoustic impedance. Examples of echo-opacifiers that might be used include tungsten nanoparticles, glass or ceramic beads, or gas filled voids or other similar structures or materials included with guide body portion  130 . By varying the concentration, placement, size and other aspects of the additives or structures, the acoustic impedance and corresponding echogenicity and echolucence may be modified to control the resulting appearance of first guide body portion  130  in an ultrasound image. 
         [0025]    The echolucence may also be reduced and echogenicity increased by using materials with characteristic acoustic impedances that differ from the characteristic acoustic impedance of the human or animal tissue in the surrounding region. For example, constructing a guide body portion  130  from PVC, which has a characteristic acoustic impedance of about 3 MRayl, can provide additional visibility of first guide body portion  130  for the clinician while still allowing some of the sound waves to pass through making it still possible to image the area behind guide body portion  130 . 
         [0026]    In other embodiments, other materials or structures may also be included in first guide body portion  130  to modify its visibility with respect to other types of imaging technologies such as to make first guide body portion  130  partially or completely radio opaque. Such an embodiment may be useful where two different imaging technologies (e.g. ultrasonography and radiography) are used during the same procedure. For example, first guide body portion  130  may be constructed of Polyethylene (which has a characteristic acoustic impedance of about 1.73 MRayl) that also includes Barium sulfate or other similar radio-opacifier. The resulting guide wire may therefore be echolucent returning very few echoes to transducer  120  presenting a reduced or minimally visible ultrasound image while also being visible during X-ray imaging. 
         [0027]    Tracking the position of first guide body portion  130  may also be achieved by including one or more echogenic markers  129  spaced longitudinally along the long axis of first guide body portion  130 . Although  FIG. 1  illustrates a first guide body portion  130  having three echogenic markers  129 , the precise number of echogenic markers  129  shown in  FIG. 1  is only illustrative. In some embodiments only one echogenic marker  129  may appear while in other embodiments numerous markers, or groups of markers, may be included. (See  FIGS. 5A through 7C  and the accompanying description below for examples.) 
         [0028]    Echogenic markers  129  include individual flakes of metal of the proper size and shape affixed to or embedded within guide member  133 , metal beads or plugs embedded within guide member  133 , metal strands or fibers adhered to the external surface of guide member  133 , or metal strands or fibers embedded within the interior of guide member  133 . Various metals might be used as an echogenic material for echogenic markers  129 . For example, substances such as stainless steel or a nickel and titanium alloy like nitinol might be formed into flakes, strands, fibers, or other forms and embedded, attached, adhered, or otherwise coupled and included with guide member  133  to form echogenic markers  129 . 
         [0029]    Other embodiments of echogenic markers  129  include a first guide body portion  130  of guide member  133  where each echogenic marker  129  includes one or more echogenic structures such as one or more empty or gas filled spaces or “bubbles” at the proper positions along the length of guide member  133 . These bubbles may be of various sizes such as large and individually positioned bubbles where each individual bubble serves as an echogenic marker  129 , or individually small bubbles arranged together to form rings, lines, or other shapes serving as echogenic markers  129  (See  FIGS. 5C-7B ). The bubbles themselves may define an empty space containing a near vacuum, or be filled with a small quantity of gas, or contain any type of matter having a characteristic acoustic impedance that substantially differs from the surrounding tissue. 
         [0030]    Other embodiments of guide member  133  are envisioned as well. For example, it is envisioned that in another embodiment of guide member  133  no joint  113  exists. In this embodiment, first guide body portion  130  is a single first guide body segment formed of echolucent material and has associated with it at least one echogenic marker  129 . In this embodiment, no concern need be given to the positioning of guide member  133  relative to ultrasonic transducer  120  (discussed in greater detail below with regard to  FIGS. 2 and 4 ) because all of guide member  133  is echolucent and no second guide body portion  103  is included. One example of such a guide member  133  is a guidewire for use in an intravascular ultrasound procedure formed from an echolucent material with one or more echogenic markers associated with the guidewire, preferably at or near its distal end. One example of such a guidewire is a polyethylene guidewire having a single echogenic marker formed from a nitinol bead embedded within the guidewire near its distal end. 
         [0031]    In the embodiment illustrated in  FIG. 1 , second guide body portion  103  may be formed from an echogenic material, or other similar material. In one preferred embodiment, second guide body portion  103  includes a metal or metallic substance such as a material containing a combination of polymeric (or other nonmetallic) and metallic fibers. Examples of a guide member  133  having echogenic properties include a guidewire constructed of strands or coils of stainless steel or nitinol, or other similarly echogenic material, having a segment coupled to the distal end constructed from Polyethylene or other similar echolucent material. 
         [0032]    Shown in  FIG. 2  is an illustration of the apparatus illustrated in  FIG. 1  in use during a procedure such as an intravascular procedure to image internal areas of a human or animal patient&#39;s body. Elongate carrier body  110  is shown in  FIG. 2  introduced into a vascular vessel  200  having a vascular lumen  204  with a vascular wall  206  and a vascular blockage  214 . Guide member  133  is shown projecting from the distal end of elongate carrier body  110  beyond intravascular ultrasound probe  107 . As ultrasonic transducer  120  of intravascular ultrasound probe  107  is energized, ultrasonic energy waves  219  begin to radiate outwardly from intravascular ultrasound probe  107  through elongate carrier body  110  into and through an imageable region  212  which is external to elongate carrier body  110  and extends through the contents of vascular lumen  204 , through vascular wall  206  and perhaps beyond. Depending on the particular implementation of intravascular ultrasound probe  107  in use, the imageable region may at any time include only a partial imageable region  211  of the total imageable region  212 . Therefore, some embodiments of intravascular ultrasound probe  107  must be rotated repeatedly to obtain updated views of the entire imageable region  212 . In one embodiment of intravascular ultrasound probe  107 , updated views of the imageable region  212  are generated automatically by an ultrasonic transducer  120  having an array of one or more radiating elements configured to electronically sweep imageable region  212  without rotating ultrasonic transducer  120 . In another embodiment of intravascular ultrasound probe  107 , updated image data from imageable region  212  is generated by manually rotating ultrasonic transducer  120  such as by the clinician applying rotational torque on ultrasound probe  107 , or by the use of a rotational device such as an electric motor coupled to ultrasound probe  107 . 
         [0033]    Regardless of how much or little of imageable region  212  is scanned or imaged at any given time, in this embodiment of intravascular ultrasound probe  107 , the imageable region is a substantially two-dimensional cross-sectional slice which can include the vascular lumen  204  and its contents, vascular wall  206 , as well as any abnormalities in vascular wall  206  such as vascular blockage  214 . The cross-sectional slice is imaged at the approximate location of ultrasonic transducer  120  as indicated by the location of imageable region  212 . It should be noted that although  FIG. 2  indicates an apparent maximum extent of the imageable region  212 , no assumption should be made from the illustration as to whether such a maximum range exists, nor how far it extends. Many factors determine the sensory capabilities of an ultrasonic imaging probe in general. Among them are the unique attributes of a particular patient, the particular location within the body relative to various organs and structures, the power and frequency of the emitted energy, as well as various other operational settings of the particular embodiment of ultrasonic transducer  120  in use. Thus no particular assumption should be made as to the degree to which imageable region  212  extends beyond the walls of elongate carrier body  110 . 
         [0034]    The result of penetrating imageable region  212  with ultrasonic energy waves  219  and detecting the return echoes may include image data indicating various information such as the extent to which vascular blockage  214  extends into vascular lumen  204 , and the type of material vascular blockage  214  is composed of, to name a few examples. In order to collect this information, elongate carrier body  110  is coupled to an image data interface device  233  through a transducer link  223  such as a data cable or wireless data link that is operable to transmit data from transducer  120  to data interface device  233 . Data interface device is also coupled to image data display device  227  by a connecting member  229  such as a data cable, wireless data link, or other similar device able to transmit data to data display device  227 . Return echoes from objects within the imageable region  212  are converted to a data stream by image data interface device  233  and the data is then passed to image data display device  227  where the data is processed into image data  225  and displayed as an image of imageable area  212  for the clinician to view, review, save for the patient to view later, archive in medical records, or use for other purposes. 
         [0035]    In some embodiments, image data display device  227  is a general purpose computer capable of operating specialized software able to capture data received through connecting member  229  from image data display device  227 , process the data into one or more images, and display this image data  225 . In other embodiments, image data display device  227  is a specialty built computer designed and built for only the purpose of capturing image data from connecting member  229  and processing the data into image data  225 . In either case, image data  225  is processed into any of various visual representations such as still frames containing individual snapshots of imageable region  212 , or as a stream of image data  225  appearing on image data display device  227  as a moving image. In the case of a moving image, image data  225  is preferably refreshed with new data from imageable region  212  at a rate of greater than 15 frames per second, more preferably greater than 20 frames per second, and most preferably 30 frames per second or more. 
         [0036]    The use of guide member  133  is shown in  FIG. 2  where guide member  133  has been introduced into the body along with elongate carrier body  110  and intravascular ultrasound probe  107 . It is commonly the case that guide member  133  is introduced into the body first, followed by elongate carrier body  110 , possibly then followed by intravascular ultrasound probe  107 . It is also common for guide member  133  to be advanced some distance through the body ahead of elongate carrier body  110  before the elongate carrier body and intravascular ultrasound probe  107  are then advanced together as well. The sequence of advancing guide member  133  followed by elongate carrier body  110  is then repeated until the area of interest is reached, or until the procedure is complete for in some cases the purpose of advancing intravascular ultrasound probe  107  is to obtain image data throughout the journey. Navigation of guide member  133  throughout this process is frequently aided by other imaging techniques such as fluoroscopy, MRI imaging, and the like. Upon arriving at the area to be imaged, or possibly in some cases throughout the journey, guide member  133  extends beyond distal end of elongate carrier body  110  as shown in  FIG. 2 . Intravascular ultrasound probe  107  can then be activated (if it is not already active) causing image data  225  to begin appearing on image data display device  227 . 
         [0037]    As can be seen in  FIG. 2 , joint  113  can be positioned proximal to ultrasonic transducer  120  and is therefore proximal to imageable region  212 . By this relative positioning of guide member  133  and intravascular ultrasound probe  107 , first guide body portion  130  comprising an echolucent material is the only portion of guide member  133  within imageable region  212 . As a result, ultrasonic energy emitted by ultrasonic transducer  120  passes through first guide body portion  130  rather than being reflected by it, and therefore first guide body portion  130  does not substantially interfere with image data  225 . This result is preferable insofar as it avoids extraneous information appearing within image data  225  that may obscure more important information, make important information more difficult to discern, or otherwise interfere with image data  225 . However, if guide member  133  is positioned such that joint  113  is distal to ultrasonic transducer  120 , second guide body portion  103  will be positioned within imageable region  212 . Extraneous information will then begin to appear in image data  225  because second guide body portion  103  is composed of an echogenic material that will not allow some or all of the ultrasonic energy emitted by ultrasonic transducer  120  to pass through it thus causing interference to appear within with image data  225 . 
         [0038]    In some cases it may be preferable for the clinician to have a visual cue appearing within image data  225  indicating the location of the echolucent portion of guide member  133  within vascular lumen  204 . This potential need is facilitated by a plurality of longitudinally spaced echogenic markers  129  which may also be implemented as marker regions having groups of markings arranged in various patterns such as a helix, lines, stripes, dots and the like (examples of various embodiments of echogenic markings are shown in  FIGS. 5A through 7C ). Ultrasonic energy  219  passes through first guide body portion  130  but is reflected back to intravascular ultrasonic transducer  120  by any echogenic markers  129  in the path of emitted ultrasonic energy  219 . Because of their small size relative to guide member  133 , echogenic markers  129  appear in image data  225 , and therefore indicate the position of first guide body portion  130  without causing substantial visual interference. Echogenic markers  129  therefore aid the clinician in maneuvering guide member  133  while still maintaining visual cues within image data  225  that do not cause substantial visual interference. 
         [0039]    Depending on the type of ultrasonic transducer used, the image data collected may be a series of two dimensional cross-sectional slices captured at various points within the patient&#39;s body and then displayed. In other embodiments, the ultrasonic transducer may be capable of generating image data which includes a three dimensional or volumetric representation of the area of interest. An example of a device having these capabilities is illustrated in  FIG. 3  and shown in operation in  FIG. 4  and described below. 
         [0040]    Illustrated in  FIG. 3  at  300  is an example of use the guide member  133  illustrated in  FIG. 1  in conjunction with a forward-looking ultrasound transducer. Guide member  133  extends beyond the distal end of an elongate carrier body  304  having an internal lumen  311  and an intravascular ultrasound probe  320  at distal end  327 . Intravascular ultrasound probe  320  includes a forward-looking transducer array  325  arranged annularly around distal end  327  of elongate carrier body  304 . Guide member  133  extends past forward-looking transducer array  325  thereby allowing ultrasonic energy emanating from forward-looking transducer array  325  to pass through and around first guide body portion  130 . As in  FIGS. 1 and 2 , joint  113  appears proximally to intravascular ultrasound probe  320  such that second guide body portion  103  composed of echogenic material is proximal to forward-looking transducer array  325  while first guide body portion  130  composed primarily of echolucent material passes through distal end  327  and is distal to forward-looking transducer array  325 . Rather than an ultrasonic transducer that radiates ultrasonic energy laterally through the side walls of the elongate carrier body  304  (as in  FIGS. 1 and 2 ), an array of multiple transducers are arranged to emit ultrasonic energy forward of transducer array  325  toward the region distal to distal end  327 . 
         [0041]    One embodiment of intravascular ultrasound probe  320  contains an array of Capacitive Micromachined Ultrasonic Transducers (CMUT) arranged in a forward-looking transducer array  325  such that ultrasonic energy is directed longitudinally ahead of transducer array  325  and carrier body  304 . Ultrasonic energy is emitted and detected by elements in the array  325  which are controlled by integrated circuits  317 . Other types of transducers and transducer arrays may be used as well such as piezoelectric transducers. In this example of elongate carrier body  304 , the CMUT transducer array is positioned at the distal end of a single lumen catheter. Other configurations are also envisioned such as multi-lumen catheters, or a forward-looking transducer array positioned next to the distal end rather than annularly around it. 
         [0042]      FIG. 4  illustrates how the devices shown in  FIG. 3  could be used in an imaging procedure such as intravascular imaging of a partially blocked internal lumen in a patient such as a vascular vessel. A blood vessel  400  appears in  FIG. 4  which is similar to the vessel appearing in  FIG. 2 . Blood vessel  400  has a vascular lumen  404 , within which has been introduced elongate carrier body  304  having an intravascular ultrasound probe  320  coupled to its distal end. Intravascular ultrasound probe  320  includes a forward-looking transducer array  325 . Ultrasonic waves  419  are generated by forward-looking transducer array  325  creating a three-dimensional imageable region  411  into which is positioned guide member  133 . Parts of vascular wall  406  and vascular blockage  414  are also within imageable region  411  as shown in  FIG. 4  given their relative position to transducer array  325 . 
         [0043]    As forward-looking transducer array  325  is activated, ultrasonic energy waves  419  begin to radiate from intravascular ultrasound probe  320  beyond distal end  327  of elongate carrier body  304  and into and through imageable region  411 . Imageable region  411  is external to elongate carrier body  304  and includes the contents of vascular lumen  404 , vascular wall  406 , vascular blockage  414 , and possibly the contents of structures and tissues outside vascular vessel  400 . The behavior of each individual transducer within the forward-looking transducer array  325  is coordinated by integrated circuits  317  so that intravascular ultrasound probe  320  operates to image all of imageable area  411  as a three-dimensional region capturing properties of the structures within the region such as volumes, densities, rates of flow of fluids through vascular lumen  404 , shapes, sizes, lengths, and other properties of objects found within the region that may be determined. It should be noted that although  FIG. 4  indicates an apparent maximum extent of the imageable region  411 , no assumption should be made from the illustration as to whether such a maximum range exists, nor to what extent it reaches. Many factors determine the sensory capabilities of an ultrasonic imaging probe in general. Among them are the unique attributes of a particular patient, the particular location within the body relative to various organs and structures, as well as the power and construction of the forward-looking transducer array  325 . Thus no particular assumption should be made as to the degree to which imageable region  411  extends beyond distal end  327 . 
         [0044]    The result of penetrating imageable region  411  with ultrasonic energy and sensing the return echoes is three-dimensional image data  425  indicating various information such as the extent to which vascular blockage  414  extends into vascular lumen  404 , the type and density of the material vascular blockage  414  is made of, and various other related information. In order to collect this information, elongate carrier body  110  is coupled to an image data interface device  233  through a transducer link  223  as described above. The data interface device  233  is in turn coupled to image data display device  227  by connecting member  229 , also as described above. Return echoes from within imageable region  411  are converted to a data stream by image data interface device  233  and the data is then passed to image data display device  227 . The data is then processed into three-dimensional image data  425  and displayed as a three-dimensional image of imageable area  411 , or possibly also viewed as a collection of two-dimensional images, or “slices”, extracted from the three-dimensional image data  425 . The clinician may then view, review, or save the images or the data for the patient to view later, archive in medical records, or use for other purposes. 
         [0045]    During the use of guide member  133  shown in  FIG. 4 , the clinician may introduce elongate carrier body  304  into the body at an appropriate point by various methods depending on the procedure required. The clinician may also introduce guide member  133  through first lumen  311  at the same time, or at another time as well. The precise order of activities used to introduce guide member  133  is unimportant to the use of the system for collecting information. Elongate carrier body  304  is advanced through the body, preferably through a vascular lumen such as a blood vessel, to the area of the body to be imaged by intravascular ultrasound probe  320 . This navigation is facilitated by guide member  133  which is often introduced well ahead of elongate carrier body  320  for the purpose of guiding it through the body. The navigation of guide member  133  may also be aided through imaging performed by other means such as by fluoroscopy, MRI imaging, and the like. In many cases it may be advantageous to operate intravascular ultrasound probe  320  to obtain image data as the elongate carrier body  304  is advanced as well. 
         [0046]      FIG. 4  illustrates the operation of intravascular ultrasound probe  320  in conjunction with guide member  133 . It can be seen in  FIG. 4  (as in  FIG. 2 ) that joint  113  is preferably positioned proximally to ultrasonic transducer  320  and is therefore proximal to imageable region  411 . By this relative positioning of guide member  133  and intravascular ultrasound probe  320 , first guide body portion  130  including echolucent material is the only portion of guide member  133  within imageable region  411 . As a result, ultrasonic energy  419  emitted from forward-looking transducer array  325  passes primarily through first guide body portion  130  rather than being substantially reflected by it and therefore first guide body portion  130  does not substantially interfere with image data  425 . This result is preferable insofar as it avoids excessive extraneous information appearing within image data  425  that may otherwise obscure important information or make important information more difficult to discern. However, if guide member  133  is positioned with joint  113  distal to forward-looking transducer array  325 , extraneous information, “noise,” “shadows,” or other interference may begin to appear in image data  425  if second guide body portion  103  is composed of echogenic material that blocks the passage of ultrasonic energy  419  through guide member  133  causing it to appear in, or interfere with, resulting image data  425 . 
         [0047]    As discussed above with regard to  FIG. 2 , in some cases it may be preferable for the clinician to maintain visual cues within image data  425  indicating the location of the echolucent portion of guide member  133  within vascular lumen  404 . This may be especially useful where the clinician is operating an automated imaging system that may rely on the echogenic markers in order to automatically position intravascular ultrasound ultrasonic probe  320 . Therefore, as noted above, at least one, and possibly more than one, longitudinally spaced echogenic marker  129  is provided as part of first guide body portion  130  of guide member  133 . Echogenic markers  129  may be individual markings or marker regions having groups of markings arranged in various patterns such as a helix, lines, stripes, dots and the like (examples of various embodiments of echogenic markers and marker regions are shown in  FIGS. 5A through 7C ). Some or all of ultrasonic energy  419  can then pass through the rest of guide member  133  with small amounts being reflected back to ultrasonic transducer array  325  by echogenic markers  129  or optionally by guide member  133  as well depending on its construction. Because of their small size relative to first guide body portion  130 , echogenic markers  129  appear in image data  425  and therefore indicate the position of first guide body portion  130  but without causing substantial visual interference. Echogenic markers  129  thereby aid the clinician in maneuvering guide member  133  through vascular lumen  404  by maintaining visual cues within three-dimensional image data  425 . 
         [0048]    Various types of visual cues may be required depending on a number of factors such as whether the imageable region is two-dimensional or three-dimensional, the location to be imaged within the body, the type of structures to be imaged, and others. Illustrated in  FIGS. 5A through 7C  are various examples of guide members similar to guide member  133  having various arrangements of echogenic markers and marker regions. In each of these embodiments described in the following  FIGS. 5A through 7C , as with echogenic markers  129  shown in  FIGS. 1 through 4  above, echogenic markers have various modes of construction. For example an echogenic marker that appears as a ring (such as echogenic marker  129  in  FIG. 1  through  FIG. 4 ) may be created by adhering a narrow band of echogenic material such as metal, or other acoustically reflective material, to the exterior surface of the guide member (such as guide member  133  in  FIG. 1 ). Likewise, a similar effect may be achieved by embedding a narrow band of echogenic material within or beneath the surface of the guide member. Other embodiments are also envisioned such as a void within the guide member filled with a vacuum, or a gas such as nitrogen, air, or other gas, or a small piece of echogenic material such as a metal bead, metal flakes, or other echogenic material within the body of the guide member. These various modes of construction can be used together as well in various combinations to create the echogenic markers described. 
         [0049]    Illustrated in  FIGS. 5A ,  5 B, and  5 C, are examples of a guide member  500  having a first guide body portion  510 , joined to a second guide body portion  503  at joint  504 . A continuous longitudinally extending echogenic marker  506  appears as well. In  FIG. 5A , echogenic marker  506  appears as a single continuous unbroken ribbon or band extending along first guide body portion  510  substantially parallel to the longitudinal axis of first guide body portion  510 . As described above, echogenic marker  506  may be created by attaching echogenic material to the exterior of guide member  500 , by embedding echogenic material within the guide member  500 , or by manufacturing guide member  500  with echogenic structures or materials within first guide body portion  510 . Other techniques may be used as well for causing echogenic marker  506  to appear during imaging as well. In  FIG. 5B , echogenic marker  506  is a single continuous longitudinally extending echogenic marker defining a helical pattern. A similar echogenic marker  506  appears in  FIG. 5C  comprising a continuous longitudinally extending echogenic marker  506  here embodied as a marker pattern having a pattern of echogenic marker elements  508  separated by discontinuities in the marker material or structure. In this embodiment, each individual echogenic marker element  508  is part of a single echogenic marker  506 . Each “dot” (marker element  508 ) in  FIG. 5C  may be an individual metal flake, metal bead, gas bubble, or other echogenic material or structure as described above with respect to markers  129 . 
         [0050]    In  FIGS. 6A ,  6 B, and  6 C, guide member  600  is illustrated having a first guide body portion  610  with an echogenic marker  606  embodied as a marker pattern with one or more longitudinally spaced echogenic marker regions  605 . Marker regions  605  include one or more echogenic marker elements  608 . Similar to previously mentioned embodiments, first guide body portion  610  is joined to a second guide body portion  603  at joint  604 . As illustrated, marker elements  608  may be separated from one another by discontinuities in the echogenic material or structure. Each of the individual marker elements  608  are constructed as discussed above with respect to marker elements  508  and markers  129 . Each individual echogenic marker element  608  in  FIG. 6A  can therefore be thought of as an echogenic “dot”. In  FIGS. 6B and 6C , each individual echogenic marker element  608  is an echogenic ring, or individual dots arranged annularly in one or more ring patterns within each marker region  605 . 
         [0051]      FIGS. 7A ,  7 B, and  7 C also illustrate various arrangements of discrete echogenic markers  708  spaced along a first guide body portion  710  of a guide member  700  also having a second guide body portion  703  joined to the first guide body portion  710  at joint  704 . In  FIGS. 7A and 7B , multiple echogenic markers  708  are illustrated as individual echogenic “dots” as discussed above. In  FIG. 7B , the echogenic markers  708  are positioned in a helical marker pattern rather than in a straight line shown in  FIG. 7A . In  FIG. 7C , each echogenic marker  708  is a configured as a band disposed around at least a portion of the perimeter or circumference of guide member  700 . As with  FIG. 5C , each “dot” or ring illustrated in  FIG. 7A through 7C  indicates an individual echogenic marker  708  formed from an echogenic substance or structure as described above such as a ring of metal, solid bead of metal, a bubble of air, metal flake, or void filled with a gas, a vacuum, or other echogenic substance. 
         [0052]    The uses of the terms “a” and “an” and “the” and similar references in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0053]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. In addition, all references cited herein are indicative of the level of skill in the art and are hereby incorporated by reference in their entirety.