Patent Publication Number: US-2017354395-A1

Title: Imaging Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 62/100,756, filed 7 Jan. 2015, which is hereby incorporated by reference as though fully set forth herein. 
    
    
     BACKGROUND 
     The instant disclosure relates to imaging, including medical imaging. In particular, the instant disclosure relates to apparatuses, systems, and methods for creating three-dimensional volumetric images. 
     Ultrasound transducers are utilized in a variety of medical applications. In many applications, the transducer is mounted in a catheter that can be navigated through a patient&#39;s vasculature and/or body organs to a site of interest. 
     In many such catheters, in order to obtain a three-dimensional volumetric image of the tissue being imaged, the transducer is rotated around a longitudinal axis of the catheter in order to obtain a plurality of two-dimensional image slices for assembly into a three-dimensional volumetric image. The transducer, for example a phased two dimensional image array, can be rotated via a motor or a manual actuator (e.g., a finger slider), either of which necessitates a relatively complex, relatively large diameter, and expensive catheter structure. For example, a motorized continuously-rotating transducer typically requires a rotating drivewire, a rotating energized (“hot”) lead, and a rotating ground lead, as well as electrical slip rings or rotary transformers in or near the catheter handle. 
     BRIEF SUMMARY 
     Disclosed herein is an apparatus for imaging tissue that includes: an acoustic imaging element (e.g., a phased array two-dimensional imaging transducer) having an active face that emits energy along a beam path and towards a tissue to be imaged; and an acoustically transmissive oscillating energy deflector positioned within the beam path. The acoustically transmissive oscillating energy deflector can be an acoustically transparent prism or lens. A drive assembly can be coupled to and operable to oscillate the acoustically transmissive energy deflector. For example, the drive assembly can include a motor and/or piezomotor, a cyclically inflatable element, and/or can be a cyclically fluid-driven assembly. Oscillating the deflector allows the capture of multiple closely-spaced and/or overlapping two-dimensional image slices that can be assembled to create a three-dimensional volumetric image. 
     In certain embodiments, the acoustically transmissive energy deflector oscillates at a frequency of between 15 Hz and 30 Hz, allowing for the capture of between 30 and 60 volumes per second (e.g., one volume in each direction of the oscillation). It can also oscillate through a range of 70 degrees. 
     It is contemplated that the acoustic imaging element and the acoustically transmissive energy deflector can be disposed within an enclosure, such as the catheter shaft or an inflatable balloon or membrane. 
     Also disclosed herein is an apparatus for imaging tissue including: an acoustic imaging element (e.g., a phased array two-dimensional imaging transducer) having an active face that emits energy along a beam path and towards a tissue to be imaged; and an oscillating reflective acoustic mirror deflector positioned within the beam path. The reflective acoustic mirror can be secured to the apparatus via at least one elastic element biased such that, when the elastic element is in a relaxed position, the acoustic mirror forms an angle of zero degrees with the active face of the acoustic imaging element (e.g., it lays flat against the apparatus/imaging element). A drive assembly, including one or more of a cyclically inflatable element, a motor, and a piezomotor, can be coupled to and operable to oscillate the acoustic mirror. 
     In another embodiment, an apparatus for volumetrically imaging tissue includes: a shaft; an imaging element disposed within the shaft, the imaging element including an active face that emits energy along a beam path and towards a tissue to be imaged; an energy deflector (e.g., a prism, lens, or acoustic mirror) positioned within the beam path; and a drive assembly coupled to the energy deflector operable to oscillate the energy deflector. The apparatus can also include a sensor for measuring a rotational or deflected position of the energy deflector as it oscillates. For example, the drive assembly can include a stepper motor. 
     The apparatus can also include a processor to assemble a three-dimensional volumetric image of the tissue to be imaged from a plurality of two-dimensional image slices of the tissue, wherein each image slice of the plurality of two-dimensional image slices is associated with a corresponding rotational or deflected position of the energy deflector. It is contemplated that the processor can also include additional functions, such as graphical user interface (“GUI”) presentation, system control, deflection control, and the like. 
     The imaging element will emit energy along the beam path to form two-dimensional image slices at a frame rate, and the energy deflector will oscillate at an oscillation frequency. It is contemplated that the frame rate and the oscillation frequency will be integer multiples of each other, and can be identical (e.g., the integer can be 1). 
     in still another embodiment, an apparatus for imaging tissue includes: an imaging element having an active face that emits energy along a beam path and towards a tissue to be imaged; an asymmetric transmissive lens positioned within the beam path; and an enclosure within which the imaging element and the energy deflector are disposed. 
     It should be understood from the foregoing summary and the detailed description that follows that, to form a three-dimensional volumetric image, the imaging element (e.g., a phased array two-dimensional imaging transducer) can have its electronically-scanned two-dimensional image plane mechanically deflected in a deflection direction, which is out of or at an angle to the imaging element&#39;s own two-dimensional image plane. This allows a set of closely spaced and/or overlapping two-dimensional image slices to be acquired, which, when assembled, create the three-dimensional volumetric image. 
     It should also be understood from the foregoing summary and the detailed description that follows that the two-dimensional image slices may not be perfectly parallel to each other in space, for example if the deflection mechanism swings the energy deflector about a hinge or pivot axis. Thus, as used herein, the term “deflection” (and its derivatives, such as “deflect” or “deflector”) includes not just pure rotation, but also a combination of rotation and translation. The various processors described herein can also compensate for any non-parallel offset in two-dimensional image slices. 
     The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representative intracardiac echocardiography (“ICE”) catheter. 
         FIG. 2  is a close-up and partially cut-away view of the distal section of the catheter depicted in  FIG. 1 . 
         FIG. 3 a    is an axial cross-section of a first embodiment of an imaging device as disclosed herein taken along line  3 - 3  in  FIG. 2 . 
         FIG. 3 b    is an axial cross-section of a second embodiment of an imaging device as disclosed herein taken along line  3 - 3  in  FIG. 2 . 
         FIG. 3 c    is an axial cross-section of a third embodiment of an imaging device as disclosed herein taken along line  3 - 3  in  FIG. 2 . 
         FIG. 3 d    is an axial cross-section of a fourth embodiment of an imaging device as disclosed herein taken along line  3 - 3  in  FIG. 2 . 
         FIG. 4  defines an angle α for the oscillation of an energy deflector  24  according to embodiments disclosed herein. 
         FIG. 5  is a block diagram of a system to construct a three-dimensional volumetric image according to aspects of the teachings herein. 
         FIG. 6  is a graphical representation of the use of the imaging devices disclosed herein to capture a plurality of two-dimensional image slices for assembly into a three-dimensional volumetric image. 
         FIG. 7  illustrates an imaging element fitted with an asymmetric lens. 
         FIG. 8 a    is a close-up and cut-away view of the distal end of an ICE catheter including an imaging element fitted with an asymmetric lens, such as shown in  FIG. 7 . 
         FIGS. 8 b -8 d    are axial cross-sections of  FIG. 8 a    taken along lines b-b, c-c, and d-d, respectively. 
         FIG. 9  illustrates an embodiment of an ICE catheter including an oscillating acoustic mirror according to the teachings herein. 
         FIG. 10  illustrates an alternative construction of an ICE catheter including an oscillating acoustic mirror as disclosed herein. 
         FIG. 11  illustrates the use of an inflatable balloon to encapsulate the imaging elements and energy deflectors disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides three dimensional imaging apparatuses, systems, and methods. For purposes of illustration, certain exemplary embodiments will be described herein in the context of an intracardiac echocardiography (“ICE”) device, such as the ViewFlex™ Xtra ICE Catheter of St. Jude Medical, Inc. It is contemplated, however, that the apparatuses, systems, and methods described herein can be used in other contexts, including, without limitation, intravascular ultrasound (“IVUS”) devices and optical coherence tomography (“OCT”) devices. 
       FIG. 1  depicts a representative ICE catheter  10 . ICE catheter  10  generally includes a handle  12  and a shaft  14 , which has a proximal section  16  (connected to handle  12 ) and a distal section  18 . The basic construction and features of ICE catheter  10  (e.g., steerability of distal section  18 ) will be familiar to the ordinarily skilled artisan. Thus, in the interest of brevity, the features of ICE catheter  10  will only be described in detail herein to the extent necessary to understand the instant disclosure. 
     As shown in the close up and partially cut-away view of  FIG. 2 , distal section  18  of shaft  14  includes therein an imaging element  20 . Imaging element  20  includes an active face  22  that emits and receives energy along a beam path and towards a tissue to be imaged (e.g., a cardiac surface). Distal section  18  can be somewhat larger in diameter than proximal section  16  in order to accommodate imaging element  20  and the other aspects of the disclosure described below. 
     In the case of ICE catheter  10 , imaging element  20  is an acoustic element, and more particularly an ultrasound element. For example, imaging element  20  can be a multi-element (e.g., 64 element) phased or linear ultrasound two-dimensional transducer array or any other suitable ultrasound transducer (including a single-element transducer) or arrangement of multiple ultrasound transducers (each of which can, for example, be either single- or multi-element). It should be understood, however, that any suitable imaging element can be employed, including both acoustic and/or electromagnetic (e.g., optical, near-infrared) elements. In general, the ordinarily skilled artisan will appreciate how to select and configure a suitable imaging element  20  for a given application of the teachings herein. 
     Distal section  18  also includes an energy deflector  24  positioned in the beam path of imaging element  20 . Energy deflector  24  acts to deflect, steer, shape, focus, defocus, or otherwise alter the energy emitted by imaging element  20  as it passes therethrough (in the case of an acoustic prism or acoustic lens) or reflects therefrom (in the case of an acoustic mirror). The ordinarily skilled artisan will understand from the foregoing disclosure that imaging element  20  emits energy towards the tissue being imaged through (e.g., in the case of a lens or prism) or off of (e.g., in the case of an acoustic mirror) energy deflector  24 , which redirects the energy as it propagates. 
       FIGS. 3 a  through 3 d    depict, in axial cross-section, various embodiments of energy deflector  24 . In a first embodiment, depicted in  FIG. 3 a   , energy deflector  24  is a transmissive prism  24   a . In another embodiment, depicted in  FIG. 3 b   , energy deflector  24  is a transmissive lens or lensed prism  24   b . The use of a transmissive lens or lensed prism  24   b  allows the energy  26   b  to maintain a tighter pattern after passing through and being deflected by energy deflector  24  than is the case with energy  26   a  passing through ordinary transmissive prism  24   a  in  FIG. 3 a   . That is, the transmissive lens or lensed prism  24   b  results in a thinner (i.e., more collimated) imaging plane or beam width after the beam passes therethrough.  FIG. 3 c    depicts an alternative configuration of a transmissive lens or lensed prism  24   c . As used herein, the term “transmissive” means deflection element  24  has beam energy passing through its bulk in at least one direction. That passage may involve a single pass (e.g., as shown in  FIGS. 3 a  and 3 b   ) or multiple passes involving an internal reflection within deflection element  24 . In general, the person of ordinary skill in the art will be familiar with the principles relevant to the selection, configuration, and/or design of acoustic prisms and acoustic lenses, such as  24   a ,  24   b , and  24   c , given the nature and purpose of the device within which the same is installed, the corresponding configuration and purpose of imaging element  20  (e.g., the number and arrangement of transducers and/or the number and arrangement of elements within transducers), and the like. For example, certain design considerations for acoustic prisms are discussed in Li et al.,  Unidirectional acoustic transmission through a prism with near - zero refractive index,  Appl. Phys. Lett. 103, 053505 (2013), which is hereby incorporated by reference as though fully set forth herein. 
     Those of ordinary skill in the art will appreciate that imaging element  20  will not only emit energy through (or off of) deflection element  24 , but will also receive incoming acoustic energy through (or off of) deflection element  24 . Thus, it is desirable to design distal section  18  (e.g., imaging element  20 , deflection element  24 , and the like) to reduce multiple internal reflections and reverberations in or off of deflection element  24 . It is also desirable to ensure that any gaps (e.g., the varying-size gap between imaging element  20  and deflection element  24  as deflection element  24  oscillates) are filled with an acoustically-transmissive liquid or other flowable material that minimizes acoustic reflections due to acoustic impedance mismatches. For example, as discussed below, one such flowable material is saline. In other embodiments, however, a permanent gel can be used to fill the gaps as they vary. 
       FIG. 3 d    shows yet another alternative embodiment where energy deflector  24  is a non-prismatic lens  24   d.    
     It should be understood, however, that  FIGS. 3 a  through 3 d    are exemplary embodiments of acoustically transmissive energy deflectors  24  according to the teachings herein. The ordinarily skilled artisan will appreciate that other configurations of energy deflector  24  can be used without departing from the scope of the instant disclosure, depending upon the device within which energy deflector  24  is installed, the intended use therefor, the nature and configuration of imaging element  20 , and the material of which energy deflector  24  is made (although an acoustically transmissive deflection element  24  will generally be made of a material having low acoustic attenuation and an acoustic impedance not very different from surrounding saline). For example, suitable materials for the acoustic prisms described herein include, without limitation, metamaterials as described by Li et al. (cited above), phononic crystals, TPX, Upilex, and silicone rubber. 
     The ordinarily skilled artisan will also understand that shaft  14  can be filled with a medium (e.g., saline, as described above) in order to facilitate the transmission of ultrasonic energy emitted by imaging element  20  as it propagates towards, off of, and/or through energy deflector  24 . Advantageously, saline acts as an acoustic coupling material to reduce loss/reflection of acoustic energy at the transducer interface. 
     It will be understood from the description herein that, for each rotational position of energy deflector  24  relative to the longitudinal axis of shaft  14  (see the arrows in  FIGS. 3 a -3 d    and  4 ), imaging element  20  will be able to capture a corresponding two-dimensional image slice of the tissue to be imaged. It will further be understood that a plurality of two-dimensional image slices, corresponding to a plurality of rotational or deflected positions of energy deflector  24  as it rotates, oscillates, and/or deflects, can be assembled to produce a three-dimensional volumetric image of the tissue to be imaged as discussed in further detail below. 
     Rather than rotating the entirety of catheter  10  to capture various rotational orientations as is the case in some prior art devices, and rather than rotating imaging element  20  within catheter  10 , which introduces additional complexity to the construction of catheter  10 , as is the case in other extant devices, energy deflector  24  can be rotated by itself to capture a plurality of two-dimensional image slices that collectively define a three-dimensional volumetric image. More particularly, energy deflector  24  can be oscillated about the longitudinal axis of shaft  14  (e.g., on a hinge or pivot that runs parallel to the longitudinal axis of shaft  14 ); as energy deflector  24  oscillates, the energy passing therethrough (or reflected therefrom, in the case of an acoustic mirror, as described below) will impinge upon a different slice or portion of the tissue to be imaged. 
     The pivot or hinge about which energy deflector  24  rotates or oscillates may be positioned running through energy deflector  24 , on a side of energy deflector  24 , or elsewhere. For example, in some embodiments, a drive shaft (or drive wire)  28  can be attached to energy deflector  24  at one end and to a motor  30  (shown schematically in  FIG. 2 ) at its other end. Motor  30  can be engaged to oscillate or position drive shaft  28 , and therefore energy deflector  24 , back and forth. Alternatively, motor  30  can be engaged to rotate drive shaft  28 , with suitable mechanisms used to convert the rotational motion of drive shaft  28  into oscillatory motion of energy deflector  24  in distal section  18 . 
     In some embodiments, motor  30  can be a piezomotor (e.g., a rotary piezomotor), which may be situated in handle  12  and connected by drive shaft  28  to energy deflector  24 . In other embodiments, the piezomotor can he disposed within distal section  18 , which advantageously simplifies construction by reducing or eliminating the need for drive shaft  28 . 
     Motor  30  can also be a stepper motor, a reversible stepper motor, or a servo motor. 
     In still other embodiments, energy deflector  24  can be rotated by an inflatable balloon or membrane, wherein inflation of the balloon or membrane forces deflection element  24  to move via contact therewith and/or mechanical coupling thereto. Such a balloon or membrane may be oscillated in inflation-extent for rotational scanning as by a fluid or gas piston residing in handle  12 . The piston can be driven, for example by a linear piezoactuator, and can be fluidically connected to the driving or oscillating balloon or membrane via a pressurized fluid lumen in shaft  14 . An advantage of the foregoing embodiment is that it minimizes or avoids the torque of drive shaft  28 , which can cause unintended wholesale rotation of the entire distal section  18 , as opposed to just the deflection element  24  as desired. 
     In further embodiments, drive shaft  28  may include a rotatable drive wire inside a rotationally-static tube (e.g., a rotatable nitinol drive wire in a rotationally-static nitinol tube). By attaching the body of motor  30  to the containment tube and the drive shaft of motor  30  to the drive wire, it is possible to minimize unwanted torque being delivered to distal section  18 . The drive wire may have a lubricious coating (e.g., polytetrafluouroethylene (PTFE), such as TEFLON®) to minimize stick/slip events between the drive wire and the tube within which it is constrained. 
     As yet another alternative, energy deflector  24  can be driven as disclosed in U.S. application Ser. No. 12/347,116, which is hereby incorporated by reference as though fully set forth herein. The fluid (e.g., saline) driving the impeller as disclosed in the foregoing patent application can also advantageously act as described above to facilitate the transmission of ultrasonic energy emitted by imaging element  20 . The fluid could also cyclically drive a bellows- or balloon-type mechanism (e.g., as the fluid fills the balloon or bellows, a (described below) gets progressively larger; as fluid is drained therefrom, a gets progressively smaller), rather than turning an impeller. 
     According to additional aspects of the disclosure, energy deflector  24  is driven by a temperature-driven shape memory actuator. 
     According to certain aspects, energy deflector  24  oscillates through a range of about 70 degrees (e.g., ±α, as shown in  FIG. 4 , are each about 35 degrees from the “neutral” position, which is shown by a dashed line). Of course, it is within the scope of the instant teachings for ±α to be other values, recognizing that greater oscillatory ranges will yield additional two-dimensional image slices, and therefore a larger (that is, including a greater number of two-dimensional image slices) three-dimensional volumetric image. 
     The ordinarily skilled artisan will appreciate from the present disclosure that the upper limit on the oscillation frequency of energy deflector  24  will be dictated by the speed of sound in the medium to be imaged and by excessive fluid drag/cavitation associated with any saline or liquid surrounding energy deflector  24  as it moves. In certain embodiments, however, the oscillation frequency of energy deflector  24  will be between about 15 Hz and about 30 Hz. 
     Indeed, it is desirable to oscillate energy deflector  24  at the frequency at which imaging element  20  emits energy (commonly referred to as the “frame rate”), or at a rate that is an integer multiple of the frame rate, to improve the efficiency with which the three-dimensional volumetric image is assembled. For example, in one embodiment, a single 3D volume is gathered by 180 degrees of the full 360 degrees of phase of the full oscillation cycle (that is, each full cycle over the angular deflection limits can provide 2 sequential volumes). 
     As described above, each rotational position of energy deflector  24  is associated with a corresponding two-dimensional image slice of the tissue to be imaged. Thus, for example, a two-dimensional image slice can be taken at each degree step as energy deflector  24  oscillates from, e.g., −35 degrees to 35 degrees (i.e., a 180 degree of phase or half cycle of a full sine wave oscillation) relative to the position designated as “neutral” for a total of 71 two-dimensional image slices. These 71 two-dimensional image slices can be assembled into a single three-dimensional volumetric image of the tissue to be imaged. Depending on the oscillation rate of energy deflector  24 , multiple volumetric images can be created each second, which facilitates the smooth depiction of cardiac motion. 
       FIG. 5  is a block diagram of a system  50  to gather a plurality of such two-dimensional image slices and then assemble a three-dimensional volumetric image therefrom. System  50  can also be employed to monitor an ablation device. As shown in  FIG. 5 , system  50  can include an ultrasonic imaging module  52 , a navigation/localization module  54 , and an ablation module  56 , all of which can be under control of and/or executed on a processor  58 . An ECG  60  can also be provided. 
     In the embodiment depicted in  FIG. 5 , catheter  10  (and, in particular, imaging element  20  thereof) is in communication with ultrasonic imaging module  52 . A rotational sensor  62  is also in communication with ultrasonic imaging module  52 . In particular, rotational sensor  62  measures the rotational position or deflection of energy deflector  24  and provides that information to ultrasonic imaging module  52 , so that it can be associated with the corresponding two-dimensional image slice (that is, so that each two-dimensional image slice has an associated angle α and/or frame number that can be used to order or sequence the image slice when assembling the three-dimensional volumetric image). Additional rotational sensors  62  and/or position sensors (e.g., localization elements) can also be provided within distal section  18 . This allows determination of both the deflection of energy deflector  24  relative to distal section  18  and, optionally, the spatial orientation and position of distal section  18  relative to the anatomy, which will be influenced by the practitioner and the heating heart. 
     As described above, in certain aspects of the disclosure, motor  30  is a stepper or servo motor, such that the various rotational positions of energy deflector  24  are known (e.g., by a motor-integrated encoder). Alternatively, a rotary encoder (which can be mechanical, optical, magnetic, capacitive, or of any other suitable technology) can be used at distal section  18  to output the rotational position of energy deflector  24 . 
     Another suitable rotational sensor  62  is an electromagnetic coil. As described above, two such coils can be used, with a first mounted on energy deflector  24  and a second mounted on distal section  18  itself. This enables one to detect the rotational position of energy deflector  24 , for example by driving the first coil and detecting the first coil using the second coil (e.g., the coil on distal section  18  itself) via mutual induction coupling. Likewise, to determine the orientation and position of distal section  18 , an external magnetic field can be applied and the response of the second coil can be measured (as is generally known in connection with magnetic field-based localization systems, including those referenced herein). 
     In turn, ultrasonic imaging module  52 , under control of (and/or executing on) processor  58 , assembles a plurality of such image slices, according to their associated rotational positions, into a volumetric image. To aid in understanding the assembly of a plurality of image slices into a volumetric image by processor  36 ,  FIG. 6  is a schematic representation of the intersection of a plurality of image slices  34  with a tissue volume to be imaged  40 . 
     Navigation/localization module  54  is operable to detect the position, and, in some aspects, rotational orientation, of a medical device, such as catheter  10  and/or ablator  64 , within a localization field. When navigation/localization module  54  also localizes catheter  10 , the localization of catheter  10  can be used to identify the location of the tissue depicted in the three-dimensional volumetric image assembled as discussed above. 
     In some embodiments, navigation/localization module  54  is the EnSite™ Velocity™ cardiac mapping and visualization system of St. Jude Medical, Inc., which operates on the principle that, when electrical currents are passed through a resistive medium, the voltage sensed by a tracking electrode can be used to determine the position of a medical device within the body. Other similar systems that rely upon electrical fields to localize a medical device within a patient&#39;s body can also be used. Other systems, however, may be used in connection with the present teachings, including for example, the CARTO navigation and location system of Biosense Webster, Inc., the AURORA® system of Northern Digital Inc., or Sterotaxis&#39; NIOBE® Magnetic Navigation System, all of which utilize magnetic fields rather than electrical fields. The localization and mapping systems described in the following patents (all of which are hereby incorporated by reference in their entireties) can also be used: U.S. Pat. Nos. 6,990,370; 6,978,168; 6,947,785; 6,939,309; 6,728,562; 6,640,119; 5,983,126; and 5,697,377. Insofar as various navigation; localization systems (including those mentioned above) are well known, however, a detailed explanation thereof is not necessary to the instant disclosure. 
     Ablator  64  is also in communication with ablation module  56 . Ablator  64  can include a radiofrequency ablation catheter and a radiofrequency generator to drive the catheter, but other manners of ablation (e.g., ultrasound ablation, cryogenic ablation, laser ablation) are contemplated. A detailed description of ablation module  56 , however, is not necessary to the understanding of the teachings herein. Instead, it will suffice to mention that the imaging teachings herein can be applied to good advantage to observe and monitor the progress of a lesion being created by ablator  64 . 
     In another aspect of the disclosure, energy deflector  24  does not oscillate to capture the plurality of image slices  34  that are assembled into the desired three-dimensional volumetric image. Instead, as shown in  FIGS. 7 and 8   a - 8   d,  energy deflector  24  is an asymmetric transmissive lens  24   e . An asymmetric transmissive lens  24   e  is twisted or warped such that, at its ends, it captures the outermost image slices (that is, those corresponding to large values of the angle θ, as measured from a line normal to the surface of imaging element  20 ) and, toward its center, it captures the more central image slices (that is, those where the angle θ, as measured from a line normal to the surface of imaging element  20 , is close to zero). 
     As such, asymmetric transmissive lens  24   e  facilitates the capture of image slices at various rotational orientations, without mechanical oscillation, such as by sequentially activating one or more elements within imaging element  20  and “walking” the activated elements along the length of imaging element  20  (e.g., activating a moving window of 8 elements of a total of 64 elements within imaging element  20 ). 
     In certain embodiments, asymmetric lens  24   e  can capture image slices over a range of up to about 30 degrees (e.g., ±about 15 degrees from the “neutral” position normal to the surface of imaging element  20 , shown in  FIG. 8 c   ). Arrows b, c, and d in  FIG. 7  show the direction in which the image slice is taken at various points along asymmetric prism  24   e.    
     The image slices can be gathered by sequentially activating different elements subsets of elements) within imaging element  20 . Each activation can he termed an “aperture,” and, by “walking” the aperture along imaging element  20 , the plurality of two-dimensional image slices can be captured. Advantageously, this avoids any mechanical oscillation of or within distal section  18 . 
     It should also be understood, by analogy to  FIGS. 3 a -3 d    and their corresponding description, that asymmetric prism  24   e  could be replaced or supplemented with various configurations of an asymmetric lens to achieve similar results. 
     An alternative aspect of the instant teachings is illustrated in  FIGS. 9 and 10 . In this embodiment, energy deflector  24  takes the form of a reflective acoustic mirror  24   f  that reflects the acoustic energy impinging thereon (including both outgoing energy emitted from imaging element  20  and incoming acoustic energy returning from the tissue being imaged). Suitable materials for acoustic mirror  24   f  include, without limitation, stainless steel, titanium, and tungsten. 
     As shown in  FIG. 9 , acoustic mirror  24   f  is secured to distal section  18  via hinges  90 . For insertion through a patient&#39;s vasculature, acoustic mirror  24   f  can be stowed flat against the surface of imaging element  20 . Indeed, hinges  90  can he elastic elements that bias acoustic mirror  24   f  into the flat, stowed position by default. 
     In vivo, acoustic mirror  24   f  can be deployed and caused to oscillate using any of the mechanisms described above (e.g., a motor, a piezomotor, a fluid driven impeller, a piezo-driven fluidic piston, a bellows, or balloon, etc.). Once deployed, and as acoustic mirror  24   f  oscillates, it will define an angle w with the face of imaging element  20 . In certain aspects, acoustic mirror  24   f  oscillates through a total range of about 20 degrees. For example, if one assumes a “neutral” acoustic mirror  24   f  position of ψ=45 degrees (that is, the position of acoustic mirror  24   f  corresponding to the central two-dimensional image slice), then the total oscillatory range can be defined as 35≦ψ≦55. 
     Advantageously, the volumetric range imaged by a reflective oscillating acoustic mirror  24   f  is twice the oscillatory range. Thus, if acoustic mirror  24   f  oscillates through a range of about 20 degrees total, it will be able to image a three-dimensional volume spanning about 40 degrees total (e.g., a total of 41 two-dimensional image slices). This yields a two-times advantage over extant mechanical wobblers, which typically move the entire transducer within the imaging device tip. 
     It is contemplated that acoustic mirror  24   f  can be a permanent part of catheter  10 . For example, as shown in  FIG. 10 , acoustic mirror  24   f  can be enclosed within distal section  18  in much the same fashion as the prismatic and/or lensed embodiments discussed above. Alternatively, acoustic mirror  24   f  can be provided as part of an external assembly designed to clip on to or slip over an extant ICE catheter (or other medical device); in this aspect, any connections necessary to acoustic mirror  24   f  may, for example, be routed along the outside of shaft  14 . 
     It should also be understood, by analogy to  FIGS. 3 a -3 d    and their corresponding description, that acoustic mirror  24   f  could be modified (e.g., to have a curved, rather than planar, reflective surface) to achieve analogous results. 
     In still another embodiment, illustrated in  FIG. 11 , imaging element  20  and energy deflector  24  are enclosed in a balloon  100  secured to distal section  18 . Although  FIG. 11  is not drawn to scale, it should be understood that balloon  100  can be egg-shaped and have an outer diameter of about 3-4 times the diameter of catheter  10  (e.g., of shaft  14 ). The use of balloon  100  protects the heart wall from trauma (e.g., hypothetical abrasion resulting from the oscillation of energy deflector  24 ) and minimizes or eliminates vibration of the tip of catheter  10 . Balloon  100  can also be configured to drive energy deflector  24 , for example by inflating balloon  100  with pumped saline and deflating balloon  100  under saline suction. 
     The devices disclosed herein can gather three-dimensional volumes during oscillation of energy deflector  24 . It is also contemplated that the systems disclosed herein can control the oscillation of energy deflector  24  to “lock on” to a particular region of tissue, or even particular two-dimensional image slice(s). This ability to “lock on” to a target can save time (e.g., a practitioner need not manually re-aim the ICE catheter periodically) and/or resources (e.g., it may reduce or eliminate the need for a practitioner dedicated to aiming the ICE catheter). 
     Although several embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. 
     For example, imaging element  20  can include one or more capacitive micromechanical ultrasound transducers (“CMUT”). 
     As another example, the hinge or pivot about which energy deflector  24  deflects may not only rotate energy deflector  24 , but also allow for translation of energy deflector  24 . 
     As yet another example, imaging element can alternatively be coupled to a higher-power energy source, which can allow the use of imaging element for ablation as well (e.g., high intensity focused ultrasound (“HIFU”) ablation). 
     All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. 
     It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.