Patent Publication Number: US-2011071400-A1

Title: Systems and methods for making and using intravascular ultrasound imaging systems with sealed imaging cores

Description:
TECHNICAL FIELD 
     The present invention is directed to the area of intravascular ultrasound imaging systems and methods of making and using the systems. The present invention is also directed to ultrasound imaging systems having motors disposed within sealed imaging cores, as well as methods for making and using the motors, sealed imaging cores, and intravascular ultrasound systems. 
     BACKGROUND 
     Intravascular ultrasound (“IVUS”) imaging systems have proven diagnostic capabilities for a variety of diseases and disorders. For example, IVUS imaging systems have been used as an imaging modality for diagnosing blocked blood vessels and providing information to aid medical practitioners in selecting and placing stents and other devices to restore or increase blood flow. IVUS imaging systems have been used to diagnose atheromatous plaque build-up at particular locations within blood vessels. IVUS imaging systems can be used to determine the existence of an intravascular obstruction or stenosis, as well as the nature and degree of the obstruction or stenosis. IVUS imaging systems can be used to visualize segments of a vascular system that may be difficult to visualize using other intravascular imaging techniques, such as angiography, due to, for example, movement (e.g., a beating heart) or obstruction by one or more structures (e.g., one or more blood vessels not desired to be imaged). IVUS imaging systems can be used to monitor or assess ongoing intravascular treatments, such as angiography and stent placement in real (or almost real) time. Moreover, IVUS imaging systems can be used to monitor one or more heart chambers. 
     IVUS imaging systems have been developed to provide a diagnostic tool for visualizing a variety is diseases or disorders. An IVUS imaging system can include a control module (with a pulse generator, an image processor, and a monitor), a catheter, and one or more transducers disposed in the catheter. The transducer-containing catheter can be positioned in a lumen or cavity within, or in proximity to, a region to be imaged, such as a blood vessel wall or patient tissue in proximity to a blood vessel wall. The pulse generator in the control module generates electrical pulses that are delivered to the one or more transducers and transformed to acoustic pulses that are transmitted through patient tissue. Reflected pulses of the transmitted acoustic pulses are absorbed by the one or more transducers and transformed to electric pulses. The transformed electric pulses are delivered to the image processor and converted to an image displayable on the monitor. 
     BRIEF SUMMARY 
     In one embodiment, a catheter assembly for an intravascular ultrasound system includes a catheter having a length, a distal end, and a proximal end. The distal end is configured and arranged for insertion into patient vasculature. A sealed imaging core is disposed in the distal end of the catheter. The sealed imaging core is configured and arranged to provide a watertight environment within the sealed imaging core. The sealed imaging core has a proximal end, a distal end, and a length that is substantially less than the length of the catheter. The sealed imaging core includes a motor, at least one fixed transducer, a tilted mirror, and at least one sonolucent fluid. The motor includes a magnet and at least two magnetic field windings. The magnet is configured and arranged to rotate upon generation of a magnetic field by the at least two magnetic field windings. The at least one fixed transducer is configured and arranged for transforming applied electrical signals to acoustic signals, transmitting the acoustic signals, receiving corresponding echo signals, and transforming the received echo signals to electrical signals. The tilted mirror is coupled to the magnet such that rotation of the magnet causes a corresponding rotation of the tilted mirror. The tilted mirror is configured and arranged to redirect acoustic signals transmitted from the at least one fixed transducers to patient tissue. The at least one sonolucent fluid is disposed in the sealed imaging core and fills open space within the sealed imaging core. At least one transducer conductor is electrically coupled to the at least one fixed transducer within the sealed imaging core. The at least one transducer conductor extends from the at least one fixed transducer to a location that is external to the sealed imaging core. The at least one stator conductor is electrically coupled to the magnetic field windings within the sealed imaging core. The at least one stator conductor extends from the magnetic field windings to a location that is external to the sealed imaging core. 
     In another embodiment, a catheter assembly for an intravascular ultrasound system includes a catheter having a length, a distal end, and a proximal end. The distal end is configured and arranged for insertion into patient vasculature via a guidewire. An imaging core is disposed in the distal end of the catheter. The imaging core is configured and arranged to provide a watertight environment within the imaging core. The imaging core has a proximal end, a distal end, and a length that is substantially less than the length of the catheter. The imaging core includes a motor, at least one fixed transducer, and a signal redirection unit. The motor includes a magnet and at least two magnetic field windings. The magnet is configured and arranged to rotate upon generation of a magnetic field by the at least two magnetic field windings. The magnet has an inner surface at an inner diameter and an outer surface at an outer diameter. The inner diameter is defined by an aperture along a longitudinal axis of the at least one transducer. The magnet aperture is configured and arranged to allow passage of the guidewire. The at least two magnetic field windings are disposed around at least a portion of both the inner surface and the outer surface of the magnet. The at least one fixed transducer has an aperture defined along a longitudinal axis of the at least one transducer. The at least one fixed transducer aperture is configured and arranged to allow passage of the guidewire. The at least one fixed transducer is configured and arranged for transforming applied electrical signals to acoustic signals, transmitting the acoustic signals, receiving corresponding echo signals, and transforming the received echo signals to electrical signals. The signal redirection unit is coupled to the magnet such that rotation of the magnet causes a corresponding rotation of at least a portion of the signal redirection unit. The signal redirection unit includes a tilted mirror configured and arranged to redirect acoustic signals transmitted from the at least one fixed transducers to patient tissue. At least one transducer conductor is electrically coupled to the at least one transducer and in electrical communication with the proximal end of the catheter. At least one stator conductor is electrically coupled to the magnetic field windings and in electrical communication with the proximal end of the catheter. 
     In yet another embodiment, a method for imaging a patient using an intravascular ultrasound imaging system includes inserting a catheter into patient vasculature. The catheter includes a sealed, watertight imaging core coupled to the guidewire. The imaging core is electrically coupled to a control module by at least one transducer conductor. The imaging core has at least one fixed transducer and a magnet that rotates by application of a magnetic field generated from at least two magnetic field windings. The magnet has an inner surface at an inner diameter and an outer surface at an outer diameter. The at least two magnetic field windings are disposed around at least a portion of both the inner surface and the outer surface of the magnet. The transducer emits acoustic signals directed at a tilted mirror configured and arranged to rotate with the magnet and redirect the acoustic signals to patient tissue. At least one electrical signal is transmitted from the control module to the at least one transducer. A magnetic field is generated to cause the magnet to rotate. At least one acoustic signal is transmitted from the at least one transducer to the tilted mirror. At least one echo signal received from a tissue-boundary between adjacent imaged patient tissue is redirected to the at least one transducer by the tilted mirror. At least one transformed echo signal is transmitted from the at least one transducer to the control module for processing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. 
       For a better understanding of the present invention, reference will be made to the following Detailed Description, which is to be read in association with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of one embodiment of an intravascular ultrasound imaging system, according to the invention; 
         FIG. 2  is a schematic side view of one embodiment of a catheter of an intravascular ultrasound imaging system, according to the invention; 
         FIG. 3  is a schematic perspective view of one embodiment of a distal end of the catheter shown in  FIG. 2  with an imaging core disposed in a lumen defined in the catheter, according to the invention; 
         FIG. 4  is a schematic longitudinal cross-sectional view of one embodiment of a catheter assembly including a sealed imaging core disposed in a catheter and a guidewire extending through the sealed imaging core, according to the invention; 
         FIG. 5A  is a schematic transverse cross-sectional view of one embodiment of the guidewire of  FIG. 4  extending through a transducer disposed in the imaging core of  FIG. 4  such that a blind spot is formed by the guidewire during imaging, according to the invention; 
         FIG. 5B  is a schematic transverse cross-sectional view of one embodiment of the guidewire, transducer, and blind spot of  FIG. 5A , the blind spot rotated ninety degrees from the location shown in  FIG. 5A , according to the invention; 
         FIG. 6A  is a schematic perspective view of one embodiment of a magnetic field winding suitable for use with the imaging core of  FIG. 4 , according to the invention; 
         FIG. 6B  is a schematic end view of one embodiment of the magnet of  FIG. 4  disposed in the magnetic field winding of  FIG. 4 , according to the invention; 
         FIG. 7  is a schematic longitudinal cross-sectional view of one embodiment of a sealed imaging core disposed in a lumen of a catheter, the imaging core including a fixed transducer and a rotatable mirror, according to the invention; 
         FIG. 8  is a schematic longitudinal cross-sectional view of another embodiment of a sealed imaging core disposed in a lumen of a catheter, the imaging core including a fixed transducer and a rotatable mirror, according to the invention; and 
         FIG. 9  is a schematic longitudinal cross-sectional view of one embodiment of a sealed imaging core disposed in a lumen of a catheter, the imaging core including a rotatable transducer, according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to the area of intravascular ultrasound imaging systems and methods of making and using the systems. The present invention is also directed to ultrasound imaging systems having motors disposed within sealed imaging cores, as well as methods for making and using the motors, sealed imaging cores, and intravascular ultrasound systems. 
     Suitable intravascular ultrasound (“IVUS”) imaging systems include, but are not limited to, one or more transducers disposed on a distal end of a catheter configured and arranged for percutaneous insertion into a patient. Examples of IVUS imaging systems with catheters are found in, for example, U.S. Pat. Nos. 7,306,561; and 6,945,938; as well as U.S. Patent Application Publication Nos. 20060253028; 20070016054; 20070038111; 20060173350; and 20060100522, all of which are incorporated by reference. 
       FIG. 1  illustrates schematically one embodiment of an IVUS imaging system  100 . The IVUS imaging system  100  includes a catheter  102  that is coupleable to a control module  104 . The control module  104  may include, for example, a processor  106 , a pulse generator  108 , a drive unit  110 , and one or more displays  112 . In at least some embodiments, the pulse generator  108  forms electric pulses that may be input to one or more transducers ( 312  in  FIG. 3 ) disposed in the catheter  102 . In at least some embodiments, signals from the drive unit  110  may be used to control a motor (see e.g.,  416  in  FIG. 4 ) driving an imaging core ( 306  in  FIG. 3 ) disposed in the catheter  102 . In at least some embodiments, electric pulses transmitted from the one or more transducers ( 312  in  FIG. 3 ) may be input to the processor  106  for processing. In at least some embodiments, the processed electric pulses from the one or more transducers ( 312  in  FIG. 3 ) may be displayed as one or more images on the one or more displays  112 . In at least some embodiments, the processor  106  may also be used to control the functioning of one or more of the other components of the control module  104 . For example, the processor  106  may be used to control at least one of the frequency or duration of the electrical pulses transmitted from the pulse generator  108 , the rotation rate of the imaging core ( 306  in  FIG. 3 ) by the motor, the velocity or length of the pullback of the imaging core ( 306  in  FIG. 3 ) by the motor, or one or more properties of one or more images formed on the one or more displays  112 . 
       FIG. 2  is a schematic side view of one embodiment of the catheter  102  of the IVUS imaging system ( 100  in  FIG. 1 ). The catheter  102  includes an elongated member  202  and a hub  204 . The elongated member  202  includes a proximal end  206  and a distal end  208 . In  FIG. 2 , the proximal end  206  of the elongated member  202  is coupled to the catheter hub  204  and the distal end  208  of the elongated member is configured and arranged for percutaneous insertion into a patient. In at least some embodiments, the catheter  102  defines at least one flush port, such as flush port  210 . In at least some embodiments, the flush port  210  is defined in the hub  204 . In at least some embodiments, the hub  204  is configured and arranged to couple to the control module ( 104  in  FIG. 1 ). In some embodiments, the elongated member  202  and the hub  204  are formed as a unitary body. In other embodiments, the elongated member  202  and the catheter hub  204  are formed separately and subsequently assembled together. 
       FIG. 3  is a schematic perspective view of one embodiment of the distal end  208  of the elongated member  202  of the catheter  102 . The elongated member  202  includes a sheath  302  and a lumen  304 . An imaging core  306  is disposed in the lumen  304 . The imaging core  306  includes an imaging device  308  coupled to a distal end of a rotatable driveshaft  310 . 
     The sheath  302  may be formed from any flexible, biocompatible material suitable for insertion into a patient. Examples of suitable materials include, for example, polyethylene, polyurethane, plastic, spiral-cut stainless steel, nitinol hypotube, and the like or combinations thereof. 
     One or more transducers  312  may be mounted to the imaging device  308  and employed to transmit and receive acoustic pulses. In a preferred embodiment (as shown in  FIG. 3 ), an array of transducers  312  are mounted to the imaging device  308 . In other embodiments, a single transducer may be employed. In yet other embodiments, multiple transducers in an irregular-array may be employed. Any number of transducers  312  can be used. For example, there can be two, three, four, five, six, seven, eight, nine, ten, twelve, fifteen, sixteen, twenty, twenty-five, fifty, one hundred, five hundred, one thousand, or more transducers. As will be recognized, other numbers of transducers may also be used. 
     The one or more transducers  312  may be formed from one or more known materials capable of transforming applied electrical pulses to pressure distortions on the surface of the one or more transducers  312 , and vice versa. Examples of suitable materials include piezoelectric ceramic materials, piezocomposite materials, piezoelectric plastics, barium titanates, lead zirconate titanates, lead metaniobates, polyvinylidenefluorides, and the like. 
     The pressure distortions on the surface of the one or more transducers  312  form acoustic pulses of a frequency based on the resonant frequencies of the one or more transducers  312 . The resonant frequencies of the one or more transducers  312  may be affected by the size, shape, and material used to form the one or more transducers  312 . The one or more transducers  312  may be formed in any shape suitable for positioning within the catheter  102  and for propagating acoustic pulses of a desired frequency in one or more selected directions. For example, transducers may be disc-shaped, block-shaped, rectangular-shaped, oval-shaped, and the like. The one or more transducers may be formed in the desired shape by any process including, for example, dicing, dice and fill, machining, microfabrication, and the like. 
     As an example, each of the one or more transducers  312  may include a layer of piezoelectric material sandwiched between a conductive acoustic lens and a conductive backing material formed from an acoustically absorbent material (e.g., an epoxy substrate with tungsten particles). During operation, the piezoelectric layer may be electrically excited by both the backing material and the acoustic lens to cause the emission of acoustic pulses. 
     In at least some embodiments, the one or more transducers  312  can be used to form a radial cross-sectional image of a surrounding space. Thus, for example, when the one or more transducers  312  are disposed in the catheter  102  and inserted into a blood vessel of a patient, the one more transducers  312  may be used to form an image of the walls of the blood vessel and tissue surrounding the blood vessel. 
     In at least some embodiments, the imaging core  306  may be rotated about a longitudinal axis of the catheter  102 . As the imaging core  306  rotates, the one or more transducers  312  emit acoustic pulses in different radial directions. When an emitted acoustic pulse with sufficient energy encounters one or more medium boundaries, such as one or more tissue boundaries, a portion of the emitted acoustic pulse is reflected back to the emitting transducer as an echo pulse. Each echo pulse that reaches a transducer with sufficient energy to be detected is transformed to an electrical signal in the receiving transducer. The one or more transformed electrical signals are transmitted to the control module ( 104  in  FIG. 1 ) where the processor  106  processes the electrical-signal characteristics to form a displayable image of the imaged region based, at least in part, on a collection of information from each of the acoustic pulses transmitted and the echo pulses received. In at least some embodiments, the rotation of the imaging core  306  is driven by the motor (see e.g.,  416  in  FIG. 4 ). 
     As the one or more transducers  312  rotate about the longitudinal axis of the catheter  102  emitting acoustic pulses, a plurality of images are formed that collectively form a radial cross-sectional image of a portion of the region surrounding the one or more transducers  312 , such as the walls of a blood vessel of interest and the tissue surrounding the blood vessel. In at least some embodiments, the radial cross-sectional image can be displayed on one or more displays  112 . 
     In at least some embodiments, the imaging core  306  may also move longitudinally along the blood vessel within which the catheter  102  is inserted so that a plurality of cross-sectional images may be formed along a longitudinal length of the blood vessel. In at least some embodiments, during an imaging procedure the one or more transducers  312  may be retracted (i.e., pulled back) along the longitudinal length of the catheter  102 . In at least some embodiments, the catheter  102  includes at least one telescoping section that can be retracted during pullback of the one or more transducers  312 . In at least some embodiments, the motor (see e.g.,  416  in  FIG. 4 ) drives the pullback of the imaging core  306  within the catheter  102 . In at least some embodiments, the motor pullback distance of the imaging core is at least 5 cm. In at least some embodiments, the motor pullback distance of the imaging core is at least 10 cm. In at least some embodiments, the motor pullback distance of the imaging core is at least 15 cm. In at least some embodiments, the motor pullback distance of the imaging core is at least 20 cm. In at least some embodiments, the motor pullback distance of the imaging core is at least 25 cm. 
     The quality of an image produced at different depths from the one or more transducers  312  may be affected by one or more factors including, for example, bandwidth, transducer focus, beam pattern, as well as the frequency of the acoustic pulse. The frequency of the acoustic pulse output from the one or more transducers  312  may also affect the penetration depth of the acoustic pulse output from the one or more transducers  312 . In general, as the frequency of an acoustic pulse is lowered, the depth of the penetration of the acoustic pulse within patient tissue increases. In at least some embodiments, the IVUS imaging system  100  operates within a frequency range of 5 MHz to 60 MHz. 
     In at least some embodiments, one or more transducer conductors  314  electrically couple the transducers  312  to the control module  104  (See  FIG. 1 ). In at least some embodiments, the one or more transducer conductors  314  extend along a longitudinal length of the rotatable driveshaft  310 . 
     In at least some embodiments, the catheter  102 , with one or more transducers  312  mounted to the distal end  208  of the imaging core  308 , may be inserted percutaneously into a patient via an accessible blood vessel, such as the femoral artery, at a site remote from the target imaging location. The catheter  102  may then be advanced through the blood vessels of the patient to the target imaging location, such as a portion of a selected blood vessel. 
     A catheter assembly includes a motor that is at least partially disposed in the imaging core. The motor includes a rotor and a stator. The rotor is a rotatable magnet and the stator includes a plurality of magnetic field windings configured and arranged to rotate the magnet by a generated magnetic field. Examples of IVUS imaging systems with motors that use rotatable magnets driven by magnetic field windings include, for example, U.S. patent application Ser. Nos. 12/415,724; 12/415,768; and 12/415,791, all of which are incorporated by reference. 
     In at least some embodiments, the magnetic field windings (“windings”) are also disposed in the imaging core. In alternate embodiments, the windings are disposed external to the catheter. In at least some embodiments, the windings are disposed external to a patient during an imaging procedure. In at least some embodiments, the imaging core is configured and arranged for coupling to a guidewire. In at least some embodiments, the imaging core is configured and arranged for insertion into the lumen of the catheter. 
     In at least some embodiments, the imaging core is configured and arranged such that rotation of the magnet causes a corresponding rotation of the one or more transducers configured and arranged to transmit energy to patient tissue and receive corresponding echo signals. In alternate embodiments, the one or more transducers do not rotate. Instead, the imaging core is configured and arranged such that rotation of the magnet causes a corresponding rotation of a tilted mirror configured and arranged to redirect energy between the one or more fixed transducers and patient tissue. Exemplary embodiments of imaging cores with a rotating mirror and fixed transducer are described below, with reference to  FIGS. 4 ,  7  and  8 . An exemplary embodiment of an imaging core with a rotating transducer is described above, with reference to  FIG. 3 . Additionally, other exemplary embodiments of imaging cores with rotating transducers are described below, with reference to  FIGS. 9 and 10 . In at least some embodiments, the motor is configured and arranged for rotating both the one or more transducers and the mirror. 
     Air in the imaging core may disrupt signal propagation between the one or more transducers and patient tissue. Thus, it is typically desirable to remove air from the imaging core prior to an imaging procedure. In some cases, open space within the imaging core is filled with a sonolucent fluid, such as a saline fluid, with impedance that matches (or nearly matches) patient tissue or fluids at or around a target imaging location. Air bubbles, however, may develop over time. Accordingly, some conventional IVUS systems may require one or more saline flushes be performed before, or even during, an imaging procedure to maintain an environment within a catheter that is conducive to signal propagation between the one or more transducers and patient tissue. 
     In order to reduce or prevent the formation of air bubbles in the imaging core, the catheter assembly includes a sealed and preferably watertight imaging core (“imaging core”). In at least some embodiments, the catheter assemblies utilize imaging cores that do not need to be flushed during an imaging procedure. In at least some embodiments, the imaging cores, once filled with a fluid and sealed, do not need to be flushed prior to an imaging procedure. The magnet is disposed in the imaging core. In at least some embodiments, the windings are also disposed in the imaging core. In at least some embodiments, when a mirror is used to redirect acoustic or echo signals, the mirror is disposed in the imaging core. An exemplary embodiment of an imaging core configured and arranged for being used in conjunction with a guidewire is described below, with reference to  FIG. 4 . Alternate embodiments of imaging cores for use without guidewires are described below, with reference to FIGS.  3  and  7 - 10 . 
       FIG. 4  is a schematic longitudinal cross-sectional view of one embodiment of a catheter assembly  400  that includes a catheter  402  and a guidewire  408 . An imaging core  404  is disposed within a body  406  of the catheter  402 . In at least some embodiments, the imaging core  404  is disposed in proximity to a distal end of the catheter  402 . In at least some embodiments, the imaging core  404  has a length that is substantially less than a length of the catheter  402 . 
     The imaging core  404  is configured and arranged to be guided to a target imaging location by the guidewire  408 . The catheter  402  can be configured and arranged to receive the guidewire  408 , for example, via a guidewire lumen  410  defined along at least a portion of the catheter  402 . In at least some embodiments, the guidewire lumen  410  extends from a proximal guidewire port  412  to a distal tip of the imaging core  404 . In at least some embodiments, the guidewire lumen  410  defines a distal guidewire port  414  through which the guidewire  408  can extend. In at least some embodiments, the guidewire lumen  410  extends along at least a portion of the imaging core  404 . In at least some embodiments, the guidewire lumen  410  extends along an entire length of the imaging core  404 . In at least some embodiments, the guidewire lumen  410  extends along substantially entirely the length of the catheter  402 . In at least some embodiments, the proximal guidewire port  412  is defined in a portion of the catheter  402  that is proximal to the imaging core  404 . In at least some embodiments, the proximal guidewire port  412  is defined in a portion of the catheter  402  that is in proximity to a proximal end of the catheter  402 . In at least some embodiments, the distal guidewire port  414  is defined in a portion of the imaging core  404 . In at least some embodiments, the distal guidewire port  414  is defined in a portion of the catheter  402  that is distal to the imaging core  404 . 
     The imaging core  404  includes a motor  416  and one or more transducers  418 . The motor  416  includes a rotatable magnet  420  and windings  422 . One or more transducer conductors  424  extend from the one or more transducers  418  along at least a portion of the catheter  402 . In at least some embodiments, the one or more transducer conductors  424  electrically couple the one or more transducers  418  to the control module ( 104  in  FIG. 1 ). In at least some embodiments, the one or more transducer conductors  424  extend along at least a portion of the catheter  402  as shielded electrical cables, such as a coaxial cable, or a twisted pair cable, or the like. In at least some embodiments, the one or more transducer conductors  424  may be attached to contacts on the distal end of the catheter  402  that, in turn, are connected to control module contacts. 
     One or more stator conductors  426  extend from the windings  422  along at least a portion of the catheter  402 . In at least some embodiments, the one or more stator conductors  426  electrically couple the windings  422  to the control module ( 104  in  FIG. 1 ). In at least some embodiments, the one or more stator conductors  426  may be attached to contacts on the distal end of the catheter  402  that, in turn, are connected to control module contacts. In at least some embodiments, at least a portion of the distal ends of the one or more transducer conductors  424  and the one or more stator conductors  426  are disposed in the imaging core  404  while at least a portion of the of the proximal ends of the one or more transducer conductors  424  and the one or more stator conductors  426  are disposed external to the imaging core  404 . In at least some embodiments, the one or more transducer conductors  424  and the one or more stator conductors  426  extend through one or more sealed portions of the imaging core  404 . 
     In at least some embodiments, a sonolucent sheath  428  radially surrounds the imaging core  404 . In at least some embodiments, the sheath  428  is formed from one or more materials with an acoustic impedance that is within 20 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sheath  428  is formed from one or more materials with an acoustic impedance that is within 15 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sheath  428  is formed from one or more materials with an acoustic impedance that is within 10 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sheath  428  is formed from one or more materials with an acoustic impedance that is within 5 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. 
     In at least some embodiments, the sheath  428  has a reduced level of attenuation of the ultrasound beam by virtue of a sufficiently thin wall. For example, a thin rigid tube made from a material such as polyimide, has reduced attenuation when the tube wall thickness is less than one wavelength of an ultrasound signal transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  418 . In at least some embodiments, the sheath  428  has a wall thickness that is no more than, or less than one-half of the wavelength of an ultrasound signal transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  418 . In at least some embodiments, the sheath  428  has a wall thickness that is no more than, or less than one-quarter of the wavelength of an ultrasound signal transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  418 . 
     Without wishing to be held to any particular values, in one example ultrasound signals transmitting from the one or more transducers  418  have a frequency of 40 MHz, and the speed of sound in surrounding tissue is 1,500 m/sec, so the ultrasound wavelength is about 0.04 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.04 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.02 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.01 mm. 
     In at least some embodiments, the sheath  428  is configured and arranged to provide cushion to the imaging core  404  when the imaging core  404  contacts patient tissue during an imaging procedure, thereby reducing the likelihood that the tissue contact will cause a non-uniform rotation of the magnet  420  (which may adversely affect images generated by the imaging procedure). In at least some embodiments, the imaging core  404  includes a tapered proximal end  430 , a tapered distal end  432 , or both. The one or more tapered ends  430  and  432  may facilitate axial movement of the imaging core  404  through patient vasculature. 
     In at least some embodiments, a sonolucent fluid is disposed in the imaging core  404  to displace open space within the imaging core  404 , and also to lubricate interfaces between moving and non-moving components of the imaging core  404  during an imaging procedure. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 20 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 15 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 10 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 5 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid also provides lubrication for the interface between the rotating portions of the imaging core  404  and the sheath  428 . 
     It may be detrimental to the patient for the sonolucent fluid to leak into patient tissues or fluids (e.g., following a breach of the imaging core  404 ). Thus, it may be an advantage to avoid using excessive amounts of sonolucent fluid. In at least some embodiments, the volume of the sonolucent fluid is no greater than 300 nano-liters. In at least some embodiments, the volume of the sonolucent fluid is no greater than 250 nano-liters. In at least some embodiments, the volume of the sonolucent fluid is no greater than 200 nano-liters. 
     A magnetic field generated by the windings  422  causes the magnet  420  to rotate along a longitudinal axis of the magnet  420 . In at least some embodiments, the longitudinal axis of the magnet  420  is parallel to the longitudinal axis of the imaging core  404 . In at least some embodiments, an applied current creates the magnetic field in the windings  422 . In some embodiments, the rotation of the magnet  420  causes a corresponding rotation of the one or more transducers  418 . In other embodiments, the one or more transducers  418  are fixed and the rotation of the magnet  420  causes a corresponding rotation of a tilted mirror configured and arranged to redirect energy beams (e.g., acoustic signals and echo signals) to and from the one or more fixed transducers  418 . 
     In  FIG. 4 , the imaging core  404  includes one or more fixed transducers  418  and a signal redirection unit  434  coupled to the magnet  420 . In at least some embodiments, the signal redirection unit  434  is coupled to the magnet  420  such that rotation of the magnet  420  causes a corresponding rotation of the signal redirection unit  434 . The signal redirection unit  434  includes a tilted mirror  436 . In at least some embodiments, the signal redirection unit  434  includes a mount  438  configured and arranged to couple the mirror  436  to the magnet  420 . In at least some embodiments, the signal redirection unit  434  includes one or more sonolucent materials  440  disposed over a reflective surface  442  of the mirror  436 . 
     In at least some embodiments, the one or more sonolucent materials  440  have an acoustic impedance that is within 20 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the one or more sonolucent materials  440  have an acoustic impedance that is within 15 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the one or more sonolucent materials  440  have an acoustic impedance that is within 10 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. In at least some embodiments, the one or more sonolucent materials  440  have an acoustic impedance that is within 5 percent of an acoustic impedance of patient tissue or fluid at or near a target imaging site within the patient. 
     In at least some embodiments, the one or more sonolucent materials  440  are formed by a molding process. In at least some embodiments, the one or more sonolucent materials  440 , the mirror  436 , and the mount  438  are configured and arranged such that the signal redirection unit  434  has an even weight distribution around an axis of rotation of the signal redirection unit  434 . In at least some embodiments, the axis of rotation of the signal redirection unit  434  is parallel to the longitudinal axis of the magnet  420 . In at least some embodiments, the signal redirection unit  434  is substantially cylindrically shaped. 
     Any impedance-matching material suitable for insertion into a patient may be used to form one or more of the sheath  428 , the sonolucent fluid disposed in the imaging core  404 , or the one or more sonolucent materials  440  of the signal redirection unit  434 . Examples of suitable impedance-matching materials may include one or more rubbers (e.g., polyurethanes, room temperature vulcanization rubber, or the like or combinations thereof), one or more plastics (e.g., polyethylene, ethyl vinyl acetate, or the like or combinations thereof), one or more liquids (e.g., furfuryl alcohol, butylene glycol, polyethylene glycol, ethylnapthalene, castor oil, linseed oil, paraffin oil, silicon oil, water, salt water, sea water, or the like or combinations thereof), or the like or combinations thereof. 
     In at least some embodiments, the tilted mirror  436  is angled to redirect acoustic signals emitted from the one or more fixed transducers  418  to a direction that is roughly perpendicular to the longitudinal axis of the magnet  420 . In at least some embodiments, the tilted mirror  436  is angled to redirect acoustic signals emitted from the one or more fixed transducers  418  to a direction that is not perpendicular to the longitudinal axis of the magnet  420 . In at least some embodiments, the tilted mirror  436  is angled to redirect acoustic signals emitted from the one or more fixed transducers  418  to one or more of a plurality of angles that are within a 120 degree range with respect to a transverse axis of the magnet  420 . In at least some embodiments, the tilted mirror  436  is angled to redirect acoustic signals emitted from the one or more fixed transducers  418  to one or more of a plurality of angles that are within a 90 degree range with respect to the transverse axis of the magnet  420 . 
     In at least some embodiments, the tilted mirror  436  has a concave shape that is configured and arranged to focus the ultrasound beam emitted from transducer  418  to a non-diverging beam within the surrounding patient tissues. In at least some embodiments, the mount  438  is formed from one or more materials that attenuate ultrasound energy that may transmit through tilted mirror  436 , thus reducing, or even preventing, reflections from distal parts, such as the magnet  420 . In at least some embodiments, one or more materials may be disposed between the tilted mirror  436  and the mount  438  that attenuate ultrasound energy that may transmit through tilted mirror  436 , thus reducing, or even preventing, reflections from distal parts, such as the magnet  420 . 
     In at least some embodiments, the one or more transducers  418  are disposed at a proximal end of the imaging core  404  and emit acoustic signals distally along the longitudinal axis of the imaging core  404 , towards the reflective surface  442  of the tilted mirror  436 . It may be an advantage to dispose the one or more transducers  418  at a proximal end of the imaging core  404  so that the one or more transducer conductors  424  do not extend along rotating portions of the imaging core  404  (e.g., the magnet  420 , the signal redirection unit  434 , or the like). 
     In at least some embodiments, the magnet  420  is a permanent magnet. The magnet  420  may be formed from many different magnetic materials suitable for implantation including, for example, neodymium-iron-boron, or the like. One example of a suitable neodymium-iron-boron magnet is available through Hitachi Metals America Ltd, San Jose, Calif. 
     In at least some embodiments, the magnet  420  is cylindrical. In at least some embodiments, the magnet  420  has a magnetization M of no less than 1.4 T. In at least some embodiments, the magnet  420  has a magnetization M of no less than 1.5 T. In at least some embodiments, the magnet  420  has a magnetization M of no less than 1.6 T. In at least some embodiments, the magnet  420  has a magnetization vector that is perpendicular to the longitudinal axis of the magnet  420 . 
     In at least some embodiments, the one or more transducers  418  are ring-shaped. In at least some embodiments, the one or more transducers  418  are C-shaped. In at least some embodiments, the one or more transducers  418  are fixedly coupled to an inner surface of the imaging core  404 . 
     When, as shown in  FIG. 4 , the one or more transducers  418  are ring-shaped and the guidewire  408  extends through the one or more transducers  418 , the guidewire  408  may obstruct some of the acoustic signals from reaching patient tissue, and may also obstruct some of the echo signals from patient tissues from reaching the one or more transducers  418  (i.e., a blind spot may be formed).  FIG. 5A  is a schematic transverse cross-sectional view of one embodiment of the guidewire  408  extending through the one or more transducers  418 . A blind spot  502  is formed over a portion of the one or more transducers  418  because the guidewire  408  obstructs the path of signals transmitting between the one or more transducers  418  and patient tissue. The positioning of the blind spot  502  rotates with the rotation of the magnet ( 420  in  FIG. 4 ). Arrow  504  shows the direction of the reflected acoustic signals when emitted from the one or more transducers  418  and reflected from the mirror ( 436  in  FIG. 4 ). As shown in  FIG. 5A , when the mirror ( 436  in  FIG. 4 ) is tilted such that acoustic signals emitted from the one or more transducers  418  are reflected upward, the blind spot  502  is positioned beneath the guidewire  408 . 
     Similarly, as shown in  FIG. 5B , when the magnet ( 420  in  FIG. 4 ) is rotated clockwise ninety degrees, the mirror ( 436  in  FIG. 4 ) is also rotated clockwise ninety degrees. Accordingly, arrow  506  shows the direction of reflected acoustic signals being to the right. As a result, the blind spot  502  is rotated clockwise ninety degrees to the left of the guidewire  408 . 
     In at least some embodiments, the imaging core  404  has a diameter that is no greater than 1.3 millimeters. In at least some embodiments, the imaging core  404  has a diameter that is no greater than 1.2 millimeters. In at least some embodiments, the imaging core  404  has a diameter that is no greater than 1.1 millimeters. In at least some embodiments, the imaging core  404  has a diameter that is no greater than one millimeter. In at least some embodiments, the imaging core  404  has a diameter that is no greater than 0.9 millimeters. In at least some embodiments, the imaging core  404  has a diameter that is no greater than 0.8 millimeters. 
     In at least some embodiments, the guidewire  408  has a diameter that is no greater than 0.5 millimeters. In at least some embodiments, the guidewire  408  has a diameter that is no greater than 0.4 millimeters. In at least some embodiments, the guidewire  408  has a diameter that is no greater than 0.3 millimeters. In at least some embodiments, the guidewire  408  has a diameter that is no greater than 0.2 millimeters. 
     In at least some embodiments, the one or more transducers  418  have an outer diameter no greater than 1.2 millimeters. In at least some embodiments, the one or more transducers  418  have a diameter no greater than 1.1 millimeters. In at least some embodiments, the one or more transducers  418  have a diameter no greater than one millimeter. In at least some embodiments, the one or more transducers  418  have a diameter no greater than 0.9 millimeters. In at least some embodiments, the one or more transducers  418  have a diameter no greater than 0.8 millimeters. 
     In at least some embodiments, the one or more transducers  418  have an inner diameter no greater than 0.5 millimeters. In at least some embodiments the one or more transducers  418  have an inner diameter no greater than 0.4 millimeters. In at least some embodiments, the one or more transducers  418  have an inner diameter no greater than 0.3 millimeters. In at least some embodiments, the one or more transducers  418  have an inner diameter no greater than 0.2 millimeters. 
     In preferred embodiments, the windings  422  are formed from rigid or semi-rigid materials using multiple-phase winding geometries. It will be understood that there are many different multiple-phase winding geometries and current configurations that may be employed to form a rotating magnetic field. For example, the windings  422  may include, for example, a two-phase winding, a three-phase winding, a four-phase winding, a five-phase winding, or more multiple-phase winding geometries. It will be understood that a motor may include many other multiple-phase winding geometries. In a two-phase winding geometry, for example, the currents in the two windings are out of phase by 90°. For a three-phase winding, there are three lines of sinusoidal current that are out of phase by zero, 120°, and 240°, with the three current lines also spaced by 120°, resulting in a uniformly rotating magnetic field that can drive a cylindrical rotor magnet magnetized perpendicular to the current lines. 
     In at least some embodiments, the windings  422  utilize a three-phase winding geometry.  FIG. 6A  is a schematic perspective view of one embodiment of the windings  422  configured using a three-phase winding geometry for forming a rotating magnetic field around the magnet ( 420  in  FIG. 4 ). The windings  422  are disposed on three arms  604 - 606  to form an inner section  608 , a radial section  610 , and an outer section  612 . The windings  422  also include a proximal end  614  and a distal end  616 . In at least some embodiments, the windings  422  are configured and arranged to be disposed around both an inner surface and an outer surface of the magnet ( 420  in  FIG. 4 ), the inner surface at an inner diameter  620  and the outer surface at an outer diameter  622  of the magnet ( 420  in  FIG. 4 ). 
     In at least some embodiments, the inner section  608  of the windings  422  extend along a length of the guidewire lumen ( 410  in  FIG. 4 ) along the inner surface at the inner diameter  620  of the magnet ( 420  in  FIG. 4 ) to a distal end of the magnet ( 420  in  FIG. 4 ). In at least some embodiments, at least a portion of the inner section  608  of the windings  422  is integrated into the guidewire lumen  410 . The radial section  610  extends radially outward from the inner section  608 . In at least some embodiments, the radial section  610  is disposed in proximity to the distal end  616  of the windings  422 . In at least some embodiments, the radial section  610  extends along a distal end of the magnet ( 420  in  FIG. 4 ). In alternate embodiments, the radial section  610  is disposed in proximity to the proximal end  614  of the windings  422  and extends along a proximal end of the magnet ( 420  in  FIG. 4 ). 
     The outer section  612  of the windings  422  extends from the radial section  610  of the windings  422 . In at least some embodiments, the outer section  612  extends along the outer surface at the outer diameter  622  of the magnet ( 420  in  FIG. 4 ). It may be an advantage to have both the inner section  608  of the windings  422  extending along the inner surface of the magnet ( 420  in  FIG. 4 ) and the outer section  612  of the windings  422  extending along the outer surface of the magnet ( 420  in  FIG. 4 ) because the surface area of the magnet ( 420  in  FIG. 4 ) over which the windings  422  are disposed is increased. Thus, the current flowing along the windings  422  can be used to provide additional torque for rotating the magnet ( 420  in  FIG. 4 ) as compared to windings that extend over just one of the diameters of the magnet ( 420  in  FIG. 4 ). 
     Although other geometries may also form a rotating magnetic field, the three-phase geometry  602  may have the advantages of allowing for a more compact motor construction than other geometries. In at least some embodiments, the windings are constructed by forming slits in a solid tube, and are equivalent to single-turn coils. By using a single-turn geometry, there are no winding cross-overs, which may result in thinner windings. One property of a three-phase winding geometry  602  is that only two of the three windings disposed on the arms  604 - 606  need to be driven, while the third winding is a common return that mathematically is equal to the third phase of current. In at least some embodiments, the arms  604 - 606  may be supported by a substrate to increase mechanical stability. In at least some embodiments, the arms  604 - 606  are constructed from a solid tube formed from one or more metals or metal alloys (e.g., beryllium-copper, tungsten-copper, or the like), leaving most of the material in tact, and removing only material needed to prevent electrical shorting between the arms  604 - 606 . 
     In at least some embodiments, the arms  604 - 606  each have thicknesses of no more than 0.004 inches (0.1 mm). In at least some embodiments, the arms  604 - 606  each have thicknesses of no more than 0.003 inches (0.08 mm). In at least some embodiments, the arms  604 - 606  each have thicknesses of no more than 0.002 inches (0.05 mm). In at least some embodiments, the arms  604 - 606  are formed from a cylindrical material with a plurality of slits are defined along at least a portion of a longitudinal length of each of the arms  604 - 606 . In at least some embodiments, at least one of the slits is back filled with one or more electrically insulating materials, such as an epoxy. 
       FIG. 6B  is a schematic distal end view of one embodiment of the magnet  420  disposed in the winding  422 . In  FIG. 6B , a plurality of current paths are shown along the radial section  610  of the arms  604 - 606  from the inner surface of the magnet  420  to the outer surface of the magnet  420 , the inner surface being at the inner diameter  620  and the outer surface being at the outer diameter  622 . 
     In alternate embodiments, the imaging core is configured and arranged for insertion into a catheter, but that is not configured and arranged for being guided by a guidewire. In at least some embodiments, the imaging core is disposed in a catheter defining a lumen into which the imaging core can be disposed and at least partially pulled back there through during an imaging procedure, as discussed above with reference to  FIG. 3 . 
     In at least some embodiments, the imaging core includes a fixed transducer and a rotating mirror disposed in a lumen of a catheter.  FIG. 7  is a schematic longitudinal cross-sectional view of another embodiment of a distal end of a catheter  702 . The catheter  702  includes a sheath  704  and a lumen  706 . A sealed and preferably watertight imaging core  708  is disposed in the lumen  706  at the distal end of the catheter  702 . In at least some embodiments, a sonolucent fluid is disposed in the imaging core to displace air pockets within the imaging core  708 . 
     In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 20 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 15 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 10 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid has an acoustic impedance that is within 5 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid also provides lubrication for the interface between the rotating portions of the imaging core  404  and the sheath  428 . 
     The imaging core  708  includes a rotatable driveshaft  710  with a motor  712  and a mirror  714  coupled to the driveshaft  710  and configured and arranged to rotate with the driveshaft  710 . In at least some embodiments, the mirror  714  is part of a signal redirection unit  734 . The imaging core  708  also includes one or more transducers  716  defining an aperture  718  extending along a longitudinal axis of the one or more transducers  716 . In at least some embodiments, the one or more transducers  716  are positioned between the motor  712  and the mirror  714 . In at least some embodiments, the one or more transducers  716  are configured and arranged to remain stationary while the driveshaft  710  rotates. In at least some embodiments, the driveshaft  710  extends through the aperture  718  defined in the one or more transducers  716 . In at least some embodiments, the aperture  718  is formed from a material, or includes a coating, or both, such as polytetrafluoroethylene coated polyimide tubing, that reduces drag between the rotatable driveshaft  710  and the stationary (relative to the driveshaft  710 ) aperture  718  of the one or more transducers  716 . 
     One or more motor conductors  720  electrically couple the motor  712  to the control module ( 104  in  FIG. 1 ). In at least some embodiments, the one or more of the motor conductors  720  may extend along at least a portion of the longitudinal length of the catheter  702  as braided electrical cables, or as shielded, twisted pair cable, or the like. One or more transducer conductors  722  electrically couple the one or more transducers  716  to the control module ( 104  in  FIG. 1 ). In at least some embodiments, the one or more of the catheter conductors  722  may extend along at least a portion of the longitudinal length of the catheter  702  as shielded electrical cables, such as a coaxial cable, or a twisted pair cable, or the like. 
     The magnet  724  is coupled to the driveshaft  710  and is configured and arranged to rotate the driveshaft  710  during operation. In at least some embodiments, the magnet  724  is rigidly coupled to the driveshaft  710 . In at least some embodiments, the magnet  724  is coupled to the driveshaft  710  by an adhesive. 
     In at least some embodiments, the imaging core  708  includes a proximal end cap  736 . In at least some embodiments, the proximal end cap  736  provides structure to the proximal portion of the imaging core  708 . In at least some embodiments, the proximal end cap  736  is rigid enough to withstand lateral forces (i.e., off-axis forces) typically encountered during normal operation within patient vasculature such that the operation of the motor  712  is not interrupted. In at least some embodiments, a proximal end of the driveshaft  710  contacts the proximal end cap  736 . In at least some embodiments, the proximal end cap  736  defines a drag-reducing element  738  for reducing drag caused by the rotating driveshaft  710  contacting the proximal end cap  736 . The drag-reducing element  738  can be any suitable device for reducing drag including, for example, one or more bushings, one or more bearings, or the like or combinations thereof. In at least some embodiments, the drag-reducing element  738  facilitates uniformity of rotation of the driveshaft  710 . 
     In at least some embodiments, the imaging core  708  is sealed by an inner sheath  740 . In at least some embodiments, the inner sheath  740  is rigid. In at least some embodiments, the inner sheath  740  is rigid enough to withstand lateral forces (i.e., off-axis forces) typically encountered during normal operation within patient vasculature such that the mirror  714  does not contact the inner sheath  740 . In at least some embodiments, the inner sheath  740  is filled with a sonolucent fluid. In at least some embodiments, the one or more motor conductors  720  and the one or more catheter conductors  722  extend through the inner sheath  740 . 
     In at least some embodiments, the sonolucent fluid within the sealed and watertight imaging core  708  has an acoustic impedance that is within 20 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid within the sealed and watertight imaging core  708  has an acoustic impedance that is within 15 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid within the sealed and watertight imaging core  708  has an acoustic impedance that is within 10 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. In at least some embodiments, the sonolucent fluid within the sealed and watertight imaging core  708  has an impedance that is within 5 percent of an acoustic impedance of patient tissue or fluid at the ultrasound imaging frequency, at or near a target imaging site within the patient. 
     In at least some embodiments, the wall thickness of the sheath  720  is less than the wavelength of an ultrasound beam transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  716 . In some embodiments, the wall thickness of the sheath  720  is no more than, or less than one-half of the wavelength of an ultrasound beam transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  716 . In some embodiments, the wall thickness of the sheath  720  is no more than, or less than one-quarter of the wavelength of an ultrasound beam transmitting in patient fluid or tissue at the imaging frequency of the one or more transducers  716 . 
     Without wishing to be held to any particular values, in one example ultrasound signals transmitting from the one or more transducers  418  have a frequency of 40 MHz, and the speed of sound in surrounding tissue is 1,500 m/sec, so the ultrasound wavelength is about 0.04 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.04 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.02 mm. In at least some embodiments, when the ultrasound wavelength is about 0.04 mm, the sheath  428  is constructed from polyimide having a wall thickness of no more than 0.01 mm. 
     In at least some embodiments, the rotatable mirror is positioned proximal to the one or more fixed transducers.  FIG. 8  is a schematic longitudinal cross-sectional view of another embodiment of a distal end of a catheter  802 . The catheter  802  defines a lumen  804  within which a sealed and preferably watertight imaging core  806  is disposed. The imaging core  806  is sealed by a sheath  840 . The imaging core  806  includes one or more fixed transducers  808 , a motor  810 , and a rotating mirror  812  proximal to the one or more transducers  808 . The one or more transducers  808  electrically couple to the control module ( 104  in  FIG. 1 ) via one or more transducer conductors  814 . In at least some embodiments, the one or more transducer conductors  814  extend through the sheath  840 . 
     The motor  810  includes a rotatable magnet  816  and windings  818 . The windings  818  are provided with power to generate a magnetic field from the control module ( 104  in  FIG. 1 ) via one or more stator conductors  820 . In at least some embodiments, the motor  810  is disposed in a housing  822  with a distal end cap  824 . In at least some embodiments, the one or more stator conductors  820  extend through the sheath  840 . 
     The mirror  812  includes a magnet  826  and a tilted reflective surface  828 . In at least some embodiments, the mirror  812  is configured and arranged to rotate with the magnet  816 . In at least some embodiments, the mirror  812  is not mechanically coupled to the end cap  824 . The magnet  816  is magnetically coupled to the mirror  812  through the end cap  824 . 
     The end cap  824  can be formed from a rigid or semi-rigid material (e.g., one or more metals, alloys, plastics, composites, or the like). In at least some embodiments, the end cap  824  is coated with a slick material (e.g., polytetrafluoroethylene, or the like) to reduce friction between the end cap  824  and the rotating magnet  816  and mirror  812 . In at least some embodiments, at least one of the magnet  816  or the mirror  812  has a tapered end contacting the end cap  824  to reduce friction during rotation. 
     In at least some embodiments, the imaging core  806  includes a support hub  830  disposed at a distal end of the imaging core  806 . In at least some embodiments, the windings  818  are supported on one end by the support hub  830  and on the opposite end by the end cap  824 . In at least some embodiments, the motor  810  includes a motor shaft  832  providing a longitudinal axis about which the magnet  816  rotates. In at least some embodiments, the motor shaft  832  is coupled on one end by the support hub  830  and on the opposite end by the end cap  824 . In at least some embodiments, the one or more transducers  808  are coupled to a transducer shaft  834  extending distally from the end cap  824 . In at least some embodiments, the mirror  812  defines an aperture through which the transducer shaft  834  extends. In at least some embodiments, the one or more transducer conductors  814  are at least partially disposed in the transducer shaft  834 . In at least some embodiments, the one or more transducer conductors  814  are at least partially disposed in the motor shaft  832 . In alternate embodiments, the one or more transducer conductors  814  extend around an outer surface of one or more of the motor  810  or the mirror  812 . 
     In at least some embodiments, the imaging core includes one or more rotatable transducers.  FIG. 9  is a schematic longitudinal cross-sectional view of one embodiment of a distal end of a catheter  902 . The catheter  902  includes a sheath  904  and a lumen  906 . A sealed and preferably watertight imaging core  908  is disposed in the sheath  904 . The sheath  904  is disposed in the lumen  906  at the distal end of the catheter  902 . The imaging core  908  includes a rotatable driveshaft  910  with one or more transducers  912  coupled to a distal end of the driveshaft  910  and a transformer  914  coupled to a proximal end of the driveshaft  910 . The imaging core  908  also includes a motor  916  coupled to the driveshaft  910 . One or more imaging core conductors  918  electrically couple the one or more transducers  912  to the transformer  914 . In at least some embodiments, the one or more imaging core conductors  918  extend within the driveshaft  910 . One or more catheter conductors  920  electrically couple the transformer  914  to the control module ( 104  in  FIG. 1 ). In at least some embodiments, the one or more of the catheter conductors  920  may extend along at least a portion of a length of the catheter  902  as shielded electrical cables, such as a coaxial cable, or a twisted pair cable, or the like. In at least some embodiments, the one or more catheter conductors  920  extend through the sheath  904 . 
     The transformer  914  is disposed on the imaging core  908 . In at least some embodiments, the transformer  914  includes a rotating component  922  coupled to the driveshaft  910  and a stationary component  924  disposed spaced apart from the rotating component  914 . In some embodiments, the stationary part  924  is proximal to, and immediately adjacent to, the rotating component  922 . The rotating component  922  is electrically coupled to the one or more transducers  912  via the one or more imaging core conductors  918  disposed in the imaging core  908 . The stationary component  916  is electrically coupled to the control module ( 104  in  FIG. 1 ) via one or more conductors  920  disposed in the lumen  906 . Current is inductively passed between the rotating component  922  and the stationary component  924  (e.g., a rotor and a stator, or a rotating pancake coil and a stationary pancake coil, or the like). 
     In at least some embodiments, the transformer  914  is positioned at a proximal end of the imaging core  908 . In at least some embodiments, the components  922  and  924  of the transformer  914  are disposed in a ferrite form. In at least some embodiments, the components  922  and  924  are smaller in size than components conventionally positioned at the proximal end of the catheter. 
     The motor  916  includes a magnet  926  and windings  928 . In at least some embodiments, the magnet  926  is a permanent magnet with a longitudinal axis, indicated by a two-headed arrow  930 , which is coaxial with the longitudinal axis of the imaging core  908  and the driveshaft  910 . 
     In at least some embodiments, the magnet  926  is coupled to the driveshaft  910  and is configured and arranged to rotate the driveshaft  910  during operation. In at least some embodiments, the magnet  926  defines an aperture  934  along the longitudinal axis  930  of the magnet  926 . In at least some embodiments, the driveshaft  910  and the one or more imaging core conductors  918  extend through the aperture  934 . In at least some other embodiments, the drive shaft  910  is discontinuous and, for example, couples to the magnet  926  at opposing ends of the magnet  926 . In which case, the one or more imaging core conductors  918  still extend through the aperture  934 . In at least some embodiments, the magnet  926  is coupled to the driveshaft  910  by an adhesive. Alternatively, in some embodiments the driveshaft  910  and the magnet  926  can be machined from a single block of magnetic material with the aperture  934  drilled down a length of the driveshaft  910  for receiving the imaging core conductors  918 . The windings  928  are provided with power from the control module ( 104  in  FIG. 1 ) via one or more motor conductors  936 . In at least some embodiments, the one or more motor conductors  936  extend through the sheath  904 . 
     The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.