Patent Publication Number: US-2020289091-A1

Title: Internal ultrasound assembly fluid seal

Description:
This application claims the benefit of U.S. Provisional Application Ser. No. 61/787,357, filed on Mar. 15, 2013, which is incorporated herein by reference in its entirety. 
     The present disclosure relates to structure and methods in medical uses of ultrasound. In particular, this disclosure relates to seals around moving parts in medical ultrasound devices. 
    
    
     BACKGROUND 
     Commonly, devices for subcutaneous medical applications require a motor to couple a moving part in a fluid environment. Fluid environments can present hazardous conditions for certain motors. One example is in the use of ultrasound for imaging, therapy or other medical uses. In such use, ultrasound energy or waves are transmitted through a medium and can reflect, scatter or otherwise attenuate when they reach a surface or border having a significant difference in acoustic impedance. For example, in ultrasound imaging of the human body, ultrasound waves may be applied externally (e.g. by placing a transducer on the skin) or internally (e.g. by placing a transducer within a vessel or organ), and travel through the body&#39;s internal fluids, which is a large proportion of water. When the waves strike a bone, organ or other body portion that provides an acoustic interface—i.e., a border of two significantly different acoustic impedances—then the waves are reflected or otherwise attenuated. A transducer (which may be the same transducer that supplied the ultrasound waves or another) receives the reflected or attenuated waves, and an image of a portion of the body can be generated. 
     A gel is placed between the skin and transducer to reduce reflection or other attenuation between the transducer and the skin. When a transducer is placed within the body, commonly it is inside a protective envelope, such as a tube, catheter or similar housing or enclosure. The material of such an envelope may be selected for its similarity in acoustic impedance to that of bodily fluids, so that there is little or no attenuation as ultrasound waves travel from that material to the fluids or tissues of the body. The inner pocket or volume of the enclosure within which the transducer is placed needs a coupling medium having an acoustic impedance similar to that of the envelope material and the body&#39;s fluids, to allow maximum transmission of the ultrasound signal. Without such a medium, e.g. if the inside of the body simply includes air or another gas, significant reflection or other attenuation will occur when the ultrasound energy from the transducer hits the boundary where the gas meets the material of the envelope. Suitable coupling media include biocompatible fluids such as saline, oils such as mineral oil, alcohols, and other fluids. 
     In ultrasound devices in which the transducer can turn, pivot or otherwise move, a seal between or around mechanisms to move the transducer may be necessary to limit or prevent the coupling medium from corroding, fouling or otherwise interfering with performance of such mechanisms. For example, in mechanisms using a motor that operates a turning or otherwise mobile shaft, with the shaft connected to the transducer or its seat or other holder, a seal may be needed between a chamber holding the transducer and coupling medium and the motor. In another example, a motor is combined with a tool to drill a bore through a clot or plaque in a vein or artery. Corrosive and/or electrolytic coupling media may be incompatible with electrical connections or other parts of a motor, drive shaft or other mechanism. Piezoelectric motors generally need dry conditions to operate, as they require a high friction contact area between a stator and a clutch. If fluid (whether generally corrosive or not) touches that contact area or interface, the friction will be substantially reduced, thereby also reducing the torque output of the motor. 
     Accordingly, to prevent fluid from contacting parts of such devices, such as motors, a seal should be included between the motor and the fluid environment, to prevent the fluid from gaining access to the motor. Examples of such a structure are disclosed below. 
     SUMMARY 
     Among other things, there are shown embodiments of apparatus for ultrasound procedures that include a housing, the housing having a chamber defined at least partially by an acoustic window for transmission of ultrasound signals. The chamber may have a uniform diameter in some embodiments. A transducer for emitting and/or receiving ultrasound signals is within the chamber, and a shaft is operatively connected to the transducer, the shaft adapted to move with respect to the wall in at least one of rotation and translation so that the transducer moves in response to movement of the shaft. A seal partially bounds the chamber, with a part of the seal fixed to the housing and the seal extending across the chamber&#39;s diameter, and the seal has an opening through which the shaft extends, with a portion of the seal around the opening engaging the outside of the shaft to create a fluid-tight connection between the seal and the shaft. 
     As exemplary embodiments, the seal can include a body having a lip portion that is elastically bent with respect to the rest of the body. The lip portion may have a rounded convex surface facing the opening, e.g. with a portion of the rounded convex surface engaging the outside of the shaft to create a fluid-tight connection between the seal and the shaft. A portion of the body can be fixed substantially perpendicular to the housing, and or the lip portion may be substantially annular. If seal has a body including a lip portion, the seal may have a first unstressed position when the shaft does not extend through the opening in which the lip portion is substantially planar with respect to the rest of the body, and a second stressed position when the shaft extends through the opening in which the lip portion is elastically bent with respect to the rest of the body. 
     The seal may be initially in the shape of a disc with the opening having a diameter smaller than an outer diameter of the shaft. When the shaft extends through the opening, the disc is elastically deformed. In such deformation, the disc can form substantially a portion of a cone in particular examples, and/or be substantially uniformly deformed. 
     In other embodiments, the seal may include an O-ring and an O-ring holder. For example, such an O-ring holder may engage the shaft with the O-ring fixed to the housing. The shaft is movable with respect to the O-ring holder, and/or the O-ring holder is movable with respect to the O-ring, in some instances. In particular examples the O-ring does not move with the shaft. 
     Embodiments of apparatus for ultrasound procedures as disclosed below may include a transducer for emitting and/or receiving ultrasound signals, with the transducer being within a chamber that is defined at least partially by a wall forming an acoustic window for transmission of ultrasound signals. The apparatus further includes a motor and a shaft operated by the motor and operatively connected to the transducer. The shaft is adapted to move with respect to the wall in at least one of rotation and translation. A seal is provided next to (e.g. abutting or adjacent) the motor and around at least part of the shaft, the seal engaging the outside of the shaft to create a fluid-tight connection between the seal and the shaft. 
     Particular examples include a housing that encloses the transducer, the motor, the shaft and the seal. The housing can feature a wall to which the seal is fixed around the entire circumference of the wall. The seal can include an opening smaller than the diameter of the shaft, through which the shaft passes, with at least a portion of the seal bending when the shaft extends through the seal. The seal, in some embodiments, includes a substantially circular line around the opening, wherein when the shaft extends through the seal the seal portion between the line and the opening bends substantially around the line. In other embodiments, seals can include an O-ring and an O-ring holder, the holder having an opening through which the shaft extends, and the O-ring fixed with respect to the wall. 
     These and other embodiments are discussed further below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a part cross-sectional view of an application end of an embodiment of an ultrasound device as disclosed herein. 
         FIG. 2  is a cross-sectional view of an embodiment of a portion of the device of  FIG. 1 . 
         FIG. 3  is an end view of a portion of the embodiment shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of an embodiment of a portion of the device of  FIG. 1 . 
         FIG. 5  is an end view of a portion of the embodiment shown in  FIG. 4 . 
         FIG. 6  is a cross-sectional view of an embodiment of a portion of the device of  FIG. 1 . 
         FIG. 7  is a part cross-sectional view of an application end of an embodiment of an ultrasound device as disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the claims is thereby intended, such alterations and further modifications in the illustrated embodiments, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates. 
     Referring now generally to the drawings, there is shown an embodiment of a device  20  for application of ultrasound internally to a patient. Although the embodiment is described herein for use in the context of ultrasound applications, device  20  and the sealing apparatuses described can be used in any number of structural applications in which a motor must be sealed from the fluid environment. As particular examples, device  20  is or is part of a device or system for imaging, such as for intravascular ultrasound (IVUS) imaging. Other embodiments can include devices for therapeutic or diagnostic uses within the body, or for ultrasound devices used outside of the body. In the embodiment indicated schematically in  FIG. 1 , device  20  is a catheter or other flexible elongated or tubular housing or member  22 , and in a particular example is sized and configured for insertion into and/or travel along the vascular system. Member  22  has an application end  23  enclosed by a wall  24 , with at least part of wall  24  defining a boundary of internal chamber  26 . Wall  24  may be continuous (e.g. monolithic or one-piece), i.e. defining some or all of catheter  22  and encircling or containing chamber  26 , or in other embodiments a portion of wall  24  surrounding chamber  26  may be different from and fixed to the rest of catheter  22 . Chamber  26  in this embodiment has a uniform diameter, with no steps, corners or sharp irregularities that may undesirably attenuate ultrasound waves. Within catheter  22  and chamber  26  in this embodiment is a transducer  28  for sending and/or receiving ultrasound signals. One or more motors  30  are connected directly or indirectly to transducer  28  so as to turn transducer  28  around a longitudinal axis of device  20 , pivot transducer  28  around an axis substantially perpendicular to that longitudinal axis, and/or provide other motions to transducer  28 . 
     Catheter  22  in the illustrated embodiment is an elongated device of plastic or other sturdy flexible material that is substantially transparent to or presenting a minimal barrier to passage of ultrasound signals. For example, when used within a blood vessel containing body tissues and blood, it is preferable for catheter  22  (or at least some or all of wall  24 ) to be constructed of a material which has acoustic impedances similar to that of body fluids such as blood. Possible materials could include, for example, a polymer material such as high density polyethylene, polymethylpentene (PMP), or acrylonitrile butadiene styrene (ABS). It has been determined that a preferred thickness of at least the portion of catheter  22  which serves as the viewing window should be at least ½ of the wavelength of the center frequency. Alternatively, the thickness can be N*½ of the wavelength, where N is a positive integer. 
     Wall  24  surrounds chamber  26 , which is at the distal end of device  20  in the illustrated embodiment, and extends proximally. Wall  24  is a monolithic part of a catheter  22  in some embodiments, and in others wall  24  is at the application end surrounding all or part of chamber  26 . Wall  24  may extend toward the control end of device  20  beyond chamber  26  in some examples. The proximal end of wall  24  and/or catheter  22  may extend outside of the patient during use, and the control end may include a handle or other operating portion (e.g. an imaging system and/or a maneuvering system (not shown)). Particular embodiments of catheter  22  or at least chamber  26  are cylindrical, and are sized for insertion into and passage through blood vessels, such as insertion into the femoral artery and passage through it toward the heart. 
     Transducer  28  is indicated schematically in the drawings. The term “transducer” should be understood to include an assembly of two or more parts as well as a single piece. For instance, transducer  28  can include a body or backing  40 , a transducer element  42  attached to one side of body  40 , and a matching layer (not shown) attached to one side of element  42 . The matching layer is attached to one side of element  42  and may be focused or non-focused. The matching layer has acoustic impedance generally between that of element  42  and the medium surrounding transducer  28  in order to minimize mismatched acoustic impedance between transducer  28  and the medium surrounding transducer  28  (e.g. mineral oil). In some embodiments, transducer  28  includes an element  42  and matching layer but no body  40 . In this embodiment, transducer  28  is pivotable and/or rotatable through action or influence of motor  30 , so that with element  42  on the side of body  40  as indicated, a generally lateral (i.e. away from the longitudinal axis) and forward ultrasound beam direction is possible depending on the motion of transducer  28 . Body  40  may be substantially opaque to or reflective of ultrasound signals, so that such signals are effectively only projected in one general direction outward from element  42 , e.g. to one side or in a limited angular range radially from body  40 . Embodiments of transducer  28  may be capable in particular examples of sending and receiving ultrasound waves in a range of frequencies which are typically used in medical ultrasound procedures, such as, for example, in the range from 2 MHz to 50 MHz. 
     Transducer  28  is electronically connected to a power source and to an imaging system (not shown). Examples of connections include conductors (e.g. wires or cables) along wall  24 , through a central lumen of a motor shaft, via slip ring connections, and/or via metallic film(s) along wall  24 . Transducer  28  may be mounted in a pivoting mechanism or otherwise linked to motor  30  or a shaft (which rotates, travels longitudinally, or otherwise moves) to permit transducer  28  to turn, pivot, or otherwise move. Embodiments of such examples are discussed and shown in Application Ser. Nos. 61/713,135; 61/713,172; 61/714,275; and 61/748,773, all of which are incorporated by reference in their entireties. 
     Motor  30  may be a rotary or linear motor and includes a shaft  44  for connecting or linking to transducer  28  or a mechanism connected to it. Multiple-motor embodiments are also considered (e.g.  FIG. 7 ), which shows an example of a rotary motor  30   a  and a linear motor  30   b  with respective shafts  44   a  and  44   b . In this example, motor  30   a  turns hollow shaft  44   a  about a longitudinal axis L of device  20 , and shaft  44   a  is linked to transducer  28  as schematically indicated so that transducer  28  likewise turns around axis L. Motor  30   b  moves shaft  44   b  forward and backward along axis L and through shaft  44   a  in this example, with shaft  44   b  engaging or linked to transducer  28  off-center. Forward motion of  44   b  tends to pivot transducer  28  clockwise about an axis into the page (perpendicular to axis L), and rearward motion of shaft  44   b  tends to pivot or allow pivoting of transducer  28  counterclockwise around an axis into the page. 
     Embodiments of piezoelectric or electromagnetic micromotors of a size and configuration suitable for containment within catheter  22  may be used. For example, a particular embodiment of a rotary motor (e.g. motor  30   a ) is a two-phase, coreless, brushless DC electromagnetic motor, which has few components, small size and minimal complexity. A piezoelectric micromotor is of a small size, such as having a diameter in the range from 0.3 mm to 4 mm in particular embodiments, and can exhibit a high torque-to-size ratio. An example of a linear motor (e.g. motor  30   b ) is an electromagnetic motor similar to a voice coil, used extensively for loudspeakers, which operate by creating a high static magnetic flux (e.g. by a permanent magnet) in the lateral direction (e.g. perpendicular to the longitudinal axis of the motor). An electrically conductive coil is placed through this flux and when current is applied to the coil a force in the axial direction is created, pulling or pushing shaft  44   b.    
     A seal  50  is provided forward of motor  30  (e.g. engaging or adjacent to the forward-most part of motor  30   a  in the illustrated embodiment) to separate chamber  26  from motor  30 . Seal  50  in the illustrated embodiment is a wall or membrane that extends across the entire diameter or width of chamber  26 , e.g. contacting wall  24  around a full circumference and forming an end of chamber  26  toward the control end of device  20 . Seal  50  may be unitary, formed with or as part of wall  24  and of the same material as wall  24 , or may be separately formed and inserted into and joined with the inside of wall  24 . For example, seal  50  may be formed concurrently with wall  24 , as by molding, or may be separately formed or prepared and fixed to or within wall  24 , as by adhesive joining or welding. Outermost portion of seal  50  (e.g. its outer diameter) is attached to wall  24 , either in the acoustic window or in the catheter behind it. The attachment may be to an inner surface of wall  24 , or to different component such as an end surface of motor  30  so that chamber  26  is isolated from the rest of device  20 . In any case, the attachment is to a stationary surface with respect to the movable shaft  44 . As indicated in the drawings, one or more shafts (e.g. shafts  44   a  and/or  44   b , associated with motors  30   a  and/or  30   b ) extend through seal  50  in order to link or connect to transducer  28 . In such embodiments, seal  50  thus provides not only a general wall bounding chamber  26 , but also inhibits or prevents flow of fluid out of chamber  26  around shaft(s)  44  extending through seal  50 . 
     A particular embodiment of seal  50  is shown in  FIGS. 2-3  which is configured to provide a seal between chamber  26  and motor  30  during rotating motion of shaft  44 . Seal  50  is in the form of a disc seal. Seal  50  is a disc of low-friction polymer or elastomer material, and in a particular embodiment of silicone. An opening  54  is in seal  50 , and in the illustrated embodiment is in the middle of seal  50  (i.e. along the central longitudinal axis of device  20 ). Opening  54  is slightly smaller than the outer diameter of shaft  44 . Opening  54  is a hole smaller than the diameter of shaft  44  in particular embodiments, and in other embodiments could be a hole combined with a slit, a slit itself, or otherwise configured. In the illustrated embodiment, shaft  44  is centrally located, i.e. along the central longitudinal axis of device  20 , and therefore opening  54  is also centrally located. It will be understood that other embodiments may have the location of shaft  44  and opening  54  off-center. 
     As seal  50  is pulled over shaft  44  (or shaft  44  is inserted through seal  50 ), opening  54  is elastically deformed, enlarging or stretching out over the circumference of shaft  44 . The elastic properties of the stretched material of seal  50  causes seal  50  to exert a compressive force or stress on shaft  44  to create a seal between seal  50  and shaft  44 . As indicated in the embodiment of  FIG. 4 , seal  50  may stretch along the length of shaft  44  while remaining attached to wall  24 , so that much or all of seal  50  becomes substantially conical and/or reduces in thickness when shaft  44  is forced through it. The low-friction nature of the material of seal  50  permits shaft  44  to turn with minimal hindrance even under some pressure from fluid in chamber  26 . 
     Seal  50  may be formed concurrently with wall  24 , or may be separately formed or prepared and fixed to or within wall  24 . In other embodiments, seal  50  is attached to other components of device  20  so that chamber  26  is isolated from the rest of device  20 , or particularly from motor  30 . For example, seal  50  can be attached to an end surface or other portion of the motor  30 , or it may be attached to a sheath extending within catheter  22 . In any case, the attachment is to a stationary surface with respect to the movable shaft  44 . Generally seal  50  is attached to a component which is stationary with respect to wall  24 . In any case, the outermost portion of seal  50  (e.g. its outer diameter) is attached either in the acoustic window or in the catheter behind it. 
     A further embodiment is shown in  FIGS. 4-5 . In that embodiment, seal  50 ′ is configured to provide a seal between chamber  26  and motor  30  during rotation of shaft  44 . Seal  50 ′ is a lipped seal made of a low-friction polymer or elastomer. Seal  50 ′ includes a body portion  52 , and an opening  54 ′ having a diameter smaller than the shaft  44 . Body portion  52  is substantially planar or disc-shaped in this embodiment, extending substantially perpendicular to the longitudinal axis and joining wall  24  so as to form part of the enclosure of chamber  26 . The thickness of body portion  52  is preferably uniform, so that the contact between body portion  52  and shaft  44  will be uniform around the circumference of shaft  44 . 
     An annular inner part  56  of body portion  52  surrounds opening  54 ′ and forms a lip. In this embodiment, the lip or annular center  56  is a part that elastically bends as shaft  44  is pressed through it. A line, score or other feature  57  may be placed in body portion  52 , for example concentric with opening  54 ′, that promotes bending or provides a particular bending location. An inward surface  58  of annular center  56  that borders or defines opening  54 ′ is rounded in the embodiment illustrated in  FIG. 2 , to ease insertion of shaft  44  through opening  54 ′ and to ensure that a portion of surface  58  will press evenly and without the potential for gaps on shaft  44  as shaft  44  turns. Surface  58  may describe part of a torus, be convex, and/or describe a semicircle or other part of a curve in cross-section, in specific examples. 
     In some embodiments, body portion  52  is substantially planar prior to inserting shaft  44  through opening  54 ′. When device  20  is being assembled, shaft  44  is pressed against body portion  52  at opening  54 ′, and annular part  56  bends away from shaft  44  (e.g. at line or feature  57 ) to allow shaft  44  to pass through opening  54 ′, without tearing body portion  52 . While the part of body portion  52  between annular part  56  and wall  24  of device  20  remains substantially perpendicular to wall  24 , as indicated in the embodiment of  FIG. 2 , annular part  56  angles toward chamber  26  so as to be substantially conical or having a concave surface (e.g. at location C) facing chamber  26 . A portion of rounded or convex surface  58  engages shaft  44  around the entire circumference of shaft  44 . 
     Body portion  52  is prepared with annular part  56  already bent to an extent as it is formed or attached in device  20 . Insertion of shaft  44  through opening  54 ′ bends annular part  56  at least slightly more (i.e. through elastic deformation) so that surface  58  applies a force to shaft  44  which creates a fluid seal at the engagement of surface  58  and shaft  44 . The force effectively seals surface  58  against the outer surface of shaft  44 . In some embodiments, fluid pressure from coupling medium in chamber  26  will tend to press annular part  56  against shaft  44 , maintaining or strengthening the seal against fluid escaping chamber  26  between annular part  56  of seal  50 ′ and shaft  44 . 
       FIG. 6  shows an embodiment of seal  50 ″ that is configured for shaft(s)  44  that move longitudinally, including reciprocating longitudinal movement. Seal  50 ″ includes an O-ring  60  and an O-ring holder or gland  62 , which together form a fluid-tight barrier between shaft  44  and wall  24 . O-ring  60  in the illustrated embodiment is a circular torus of elastomeric or other sealing material, having an outer diameter that engages or abuts against the inner diameter of wall  24 . 
     O-ring holder or gland  62 , in the particular illustrated embodiment, is a round, spool-shaped piece having a longitudinal opening  64  for shaft  44  through a body  66 . A central groove or space  68  around body  66  is between side flanges  70 . The unstressed (i.e. natural) outer diameter of ring  60  is slightly larger than an inner diameter of wall  24 , so that when ring  60  is inserted into device  20  a press or interference fit exists between ring  60  and wall  24 . The inner diameter of the torus of ring  60  is at least slightly larger than the outer diameter of shaft  44  and at least slightly smaller than the diameter of opening  64 . In that way, ring  60  is slightly oversized so that when the ring is placed in space  68  between holder  62  and wall  24 , ring  60  is compressed, and the compression force creates a fluid seal between chamber  26  and motor  30 . 
     Holder  62  is of a low-friction material, perhaps similar to the materials noted above with respect to seals  50 ,  50 ′ to permit easy movement, sliding, or rolling motion of holder  62  with respect to ring  60 . In particular embodiments, holder  62  is attached to shaft  44  so that longitudinal movement of shaft  44  moves holder  62  as well. The low-friction material of holder  62  allows ring  60  to move within space  68  (or at least rotate with respect to holder  62 ). During such movement, the inner diameter of ring  60  engages the diameter of opening  64  as the outer diameter of ring  60  engages the inner diameter of wall  24 , preserving the seal. Similarly, ring  60  is able to move longitudinally along wall  24  while preserving the seal. In that way, ring  60  follows holder  62  as holder  62  moves in unison with longitudinal movement of shaft  44 . It will be understood that the size of holder  62  and/or the size of space  68  measured along the length of shaft  44  may be related or tailored to the length of travel of shaft  44 . That is, the longer space  68  is, the longer ring  60  can move within it and the longer the amount of longitudinal travel of shaft  44  is allowed for, and conversely a shorter distance of travel by shaft  44  may need only a relatively short holder  62  and space  68 . 
     In an alternative embodiment (not shown), holder  62  is shaped in an opposite fashion with respect to axis L. In that embodiment, holder  62  is attached to wall  24  with a space and flanges that are directed inward toward axis L. In that embodiment, the unstressed (i.e. natural) outer diameter of ring  60  is slightly larger than the inner diameter of the space between the flanges of holder  62 . The inner diameter of the torus of ring  60  is at least slightly smaller than the outer diameter of shaft  44 . In that way, similar to the embodiment of  FIG. 6 , ring  60  is slightly oversized so that when the ring is placed in the space between holder  62  and shaft  44 , ring  60  is compressed, and the compression force creates a fluid seal between chamber  26  and motor  30 . In this embodiment ring  60  may move with the longitudinal movement of shaft  44 , but holder  62  will remain stationary relative to wall  24  while shaft  44  moves. Ring  60  can move around in holder  62  and still maintain the seal as was explained in the above example. 
     Specific embodiments of device  20  may have seal(s)  50 ,  50 ′ and/or  50 ″ having an outer diameter of approximately 2.5 mm, i.e. about the inner diameter of wall  24  and/or chamber  26 . An inner diameter of about 0.8 mm for seal(s)  50 ,  50 ′ and/or  50 ″ is proposed, in light of the expected outer diameter of embodiments of shaft  44 . Of course, it will be understood that size and configuration of the outer and inner diameters of seal embodiments may depend on geometry and size of shaft(s)  44 , and of wall  24  and/or chamber  26 . 
     It will be understood that features or attributes noted with respect to one or more specific embodiments may be used or incorporated into other embodiments of the structures and methods disclosed. Multiple seals  50 ,  50 ′ and/or  50 ″ may be used in particular embodiments, as where a first seal is placed at a boundary of chamber  26  and around a first shaft  44   a , and a second seal is placed between motor  30   a  that turns shaft  44   a  and motor  30   b  which operates shaft  44   b . For example, where shaft  44   a  is a hollow shaft and shaft  44   b  operates through the lumen of shaft  44   a , a first seal  50  (or other embodiment(s)) is around shaft  44   a  at the boundary of chamber  26 , as discussed above. A second seal  50 ″ (or other embodiment(s)) is between motor  30   a  and  30   b  (e.g. attached to or adjacent the rear of motor  30   a ) and sealingly fitted around shaft  44   b . In such a configuration, some liquid from chamber  26  may escape through hollow shaft  44   a , i.e. between the inner diameter of shaft  44   a  and the outer diameter of shaft  44   b . Motor  30   a  in this example can be an electromagnetic motor that is not significantly susceptible to small amounts of escaping coupling medium, which is in any case largely or entirely contained within shaft  44   a . The second seal, around shaft  44   b  and otherwise fixed to catheter  22  (e.g. wall  24 ), maintains any such escaped coupling medium away from motor  30   b.    
     While the embodiments have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only particular embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.