Abstract:
An imaging system comprises a catheter having a lumen, a rotatable imaging probe within the catheter lumen including a distal transducer and first and second conductors coupled to the transducer, and a coupler that couples the rotatable first and second conductors to non-rotatable third and fourth conductors, respectively. The coupler includes a rotary capacitive coupler.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims the benefit of copending U.S. Provisional Patent Application Ser. No. 61/127,943, filed May 15, 2008, which application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to rotary couplers. The present invention more specifically relates to a capacitively coupled rotary coupler for use in a minimally invasive imaging catheter and system. 
         [0003]    Intravascular catheters such as intravascular ultrasonic (IVUS) catheters enable imaging of internal structures in the body. In particular, coronary IVUS catheters are used in small arteries of the heart to visualize coronary artery disease. An IVUS catheter will, in general, employ at least one high frequency (20 MHz-45 MHz) ultrasonic transducer that creates pressure waves for visualization. At least one transducer is typically housed within a surrounding sheath or catheter member and mechanically rotated for 360° visualization. 
         [0004]    The highest frequencies at which commercially available coronary imaging catheters operate are 40 MHz and 45 MHz. These high frequency probes have an axial resolution of approximately 200 microns. An axial resolution of 200 microns is insufficient to resolve structures with size features smaller than 200 microns. For example, thin-cap fibroatheromas having a thin fibrous cap of 65 microns or less in thickness cannot currently be resolved. The concern regarding thin-cap fibroatheromas is that they are prone to rupture. Plaque rupture can lead to thrombus formation and critical blockages in the coronary artery. The ability to reliably identify thin-cap fibroatheromas can aid interventional cardiologists to develop and evaluate clinical treatment strategies in order to reduce post percutaneous coronary intervention morbidity rates. Therefore, IVUS catheters and systems having improved axial resolution capable of more clearly visualizing micron sized features such as vulnerable plaques are needed in the art. The ability for such systems to operate at high transducer frequencies will be important in that effort. 
         [0005]    One of the challenges of these minimally invasive imaging systems is coupling the stationary ultrasound transceiver (transmitter/receiver) to the mechanically rotating transducer. Rotary inductive couplers are used in commercially available IVUS systems. However, rotary inductive couplers are non-ideal for very high frequency (30 MHz-300 MHz) operation because of their relatively high series inductance. At such high frequencies, series inductance will result in an insertion loss into a transmission line of the IVUS catheter. Furthermore, the insertion loss increases with increasing ultrasound imaging frequency which degrades system performance. Rotating inductive couplers also exhibit electrical impedance that can vary with rotational position. The variation of impedance with rotational position causes output signal amplitudes to vary with angular positions and further degrades system performance. The present invention addresses these and other issues towards providing imaging catheters having improved resolution and more constant level output. 
       SUMMARY 
       [0006]    In one embodiment, an imaging system comprises a catheter having a lumen, a rotatable imaging probe within the catheter lumen including a distal transducer and first and second conductors coupled to the transducer. The system further includes a coupler that couples the rotatable first and second conductors to non-rotatable third and fourth conductors. The coupler includes a rotary capacitive coupler. 
         [0007]    The coupler may comprise a parallel plate capacitor. The coupler may comprise a first parallel plate capacitor that couples the first conductor to the third conductor and a second parallel plate capacitor that couples the second conductor to the fourth conductor or a parallel plate capacitor that couples the first conductor to the third conductor and a cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0008]    The coupler may comprise a cylindrical surface concentric capacitor. The coupler may comprise a first cylindrical surface concentric capacitor that couples the first conductor to the third conductor and a second cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0009]    The coupler may comprise a conical surface concentric capacitor. The coupler comprises a conical surface concentric capacitor that couples the first conductor to the third conductor and a parallel plate capacitor that couples the second conductor to the fourth conductor, a conical surface concentric capacitor that couples the first conductor to the third conductor and a cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor, or a first conical surface concentric capacitor that couples the first conductor to the third conductor and a second conical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0010]    The coupler may be within the catheter or outside of the catheter. 
         [0011]    In another embodiment, an imaging system comprises a catheter having a lumen and a distal rotatable imaging probe within the catheter lumen including a first transducer, first and second conductors coupled to the first transducer, a second transducer, and third and fourth conductors coupled to the second transducer. The system further includes a rotary capacitive coupler that couples the rotatable first and second conductors to non-rotatable fifth and sixth conductors, respectively, and a rotary inductive coupler that couples the rotatable third and fourth conductors to non-rotatable seventh and eighth conductors, respectively. 
         [0012]    The coupler may comprise a parallel plate capacitor. The coupler may comprise a first parallel plate capacitor that couples the first conductor to the third conductor and a second parallel plate capacitor that couples the second conductor to the fourth conductor or a parallel plate capacitor that couples the first conductor to the third conductor and a cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0013]    The coupler may comprise a cylindrical surface concentric capacitor. The coupler may comprise a first cylindrical surface concentric capacitor that couples the first conductor to the third conductor and a second cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0014]    The coupler may comprise a conical surface concentric capacitor. The coupler comprises a conical surface concentric capacitor that couples the first conductor to the third conductor and a parallel plate capacitor that couples the second conductor to the fourth conductor, a conical surface concentric capacitor that couples the first conductor to the third conductor and a cylindrical surface concentric capacitor that couples the second conductor to the fourth conductor, or a first conical surface concentric capacitor that couples the first conductor to the third conductor and a second conical surface concentric capacitor that couples the second conductor to the fourth conductor. 
         [0015]    In a further embodiment, an imaging system comprises a catheter having a lumen, a rotatable imaging probe within the catheter lumen including a distal transducer, and a coupler including a rotary capacitive coupler that couples the rotatable transducer to non-rotatable first and second conductors and a rotary inductive coupler that couples the rotatable transducer to third and fourth non-rotatable conductors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further features and advantages thereof, may best be understood by making reference to the following descriptions taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify identical elements, and wherein: 
           [0017]      FIG. 1  is a high-level diagram of a catheter-based imaging system comprising a rotary coupler as part of a catheter interface module; 
           [0018]      FIG. 2  is a schematic representation of electrical signal paths of a catheter-based imaging system comprising a rotary coupler as part of a catheter interface module; 
           [0019]      FIG. 3  is a high-level diagram of a catheter-based imaging system comprising a rotary coupler as part of a catheter; 
           [0020]      FIG. 4  is a schematic representation of electrical signal paths of a catheter-based imaging system comprising a rotary coupler as part of a catheter; 
           [0021]      FIG. 5  is a side perspective view of a parallel plate capacitor; 
           [0022]      FIG. 6  is a side perspective view of a cylindrical surface concentric capacitor; 
           [0023]      FIG. 7  is a side perspective view of a conical surface concentric capacitor; 
           [0024]      FIG. 8  is a diagram of a rotary capacitive coupler located in a catheter interface module and comprised of a cylindrical surface concentric capacitor and a parallel plate capacitor; 
           [0025]      FIG. 9  is a diagram of a rotary capacitive coupler located in a catheter and comprised of a cylindrical surface concentric capacitor and a parallel plate capacitor; 
           [0026]      FIG. 10  is a diagram of a rotary capacitive coupler comprised of a conical surface concentric capacitor and a parallel plate capacitor; 
           [0027]      FIG. 11  is a diagram of a rotary capacitive coupler comprised of cylindrical surface concentric capacitors; 
           [0028]      FIG. 12  is a diagram of a rotary capacitive coupler comprised of conical surface concentric capacitors; 
           [0029]      FIG. 13  is a diagram of a rotary capacitive coupler comprised of parallel plate capacitors; and 
           [0030]      FIG. 14  is a schematic representation of electrical signal paths for a two channel system comprised of a rotary inductive coupler and a rotary capacitive coupler. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A high-level diagram of the components of a catheter-based imaging system is shown in  FIG. 1 . A catheter  1000 A is coupled mechanically and electrically to a catheter interface module  2000 A comprised of a rotary coupler  100 . An imaging engine  3000  is in electrical communication with the catheter interface module. Following the imaging engine  3000  is a display engine  4000  and a display  5000 . 
         [0032]      FIG. 2  shows an electrical schematic representation of the transmit and receive signal paths of a catheter interface module  2000 A and catheter  1000 A having a primary purpose of coupling a signal from a stationary electrical conduit to a rotating electrical conduit. Diagrams for a rotary capacitive coupler  100  and ultrasonic transducer  220  are shown. In accordance with this embodiment, the rotary capacitive coupler  100  is located outside of the catheter  1000 A and within the catheter interface module  2000 A. 
         [0033]      FIG. 3  shows a high-level diagram of the components of another catheter-based imaging system. The components  1000 B,  2000 B,  3000 ,  4000 ,  5000  of the catheter-based based imaging system in  FIG. 3  are substantially the same as the components  1000 A,  2000 A,  3000 ,  4000 ,  5000  of the catheter-based imaging system in  FIG. 1  and hence, reference characters for like elements are repeated in  FIG. 3 . However, in this embodiment, a rotary coupler  100  is located in the catheter  1000 B. 
         [0034]      FIG. 4  is an electrical schematic representation of the system signal paths of the system of  FIG. 3  and to the extent that it is the same as the electrical schematic representation in  FIG. 2 , reference characters for like elements are repeated. However, as may be noted in  FIG. 4 , the rotary capacitive coupler  100  is located in the catheter  1000 B. 
         [0035]      FIGS. 5-7  show illustrations of parallel plate and concentric capacitors which may be employed in the various embodiments described hereinafter.  FIG. 5  shows a side perspective view of a parallel plate capacitor. The capacitance of the parallel plate capacitor depends on the cross-sectional area A plate  and separation distance d plate  of two parallel plates Plate 1 ,Plate 2  and is closely approximated by 
         [0000]    
       
         
           
             C 
             = 
             
               
                 
                   ɛ 
                   0 
                 
                  
                 
                   ɛ 
                   r 
                 
                  
                 
                   A 
                   plate 
                 
               
               
                 d 
                 plate 
               
             
           
         
       
     
         [0000]    where C is the capacitance in Farads (F), A plate  is the area of each plate in square meters (m 2 ), ε r  is the relative static permittivity or dielectric constant, ε 0  is the permittivity of free space (i.e., 8.854×10 −12  F/m), and d plate  is the separation distance between the plates in meters (m). 
         [0036]      FIG. 6  shows a side perspective view of a concentric capacitor compiised of cylindrical surfaces. The capacitance per unit length of the cylindrical surface concentric capacitor depends on the radii r 1 , r 2  of the drums (or cylinders) Drum 1 , Drum 2  and is approximately 
         [0000]    
       
         
           
             
               
                 
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     
                       ? 
                     
                      
                     
                       ? 
                     
                   
                   
                     ln 
                      
                     
                       ( 
                       
                         
                           r 
                           1 
                         
                         
                           r 
                           2 
                         
                       
                       ) 
                     
                   
                 
                 . 
                 
                   
 
                 
                  
                 
                   ? 
                 
               
                
               
                 indicates text missing or illegible when filed 
               
             
              
             
                 
             
           
         
       
     
         [0000]    The reactive impedance experienced by a signal of frequency f across the capacitor is |Xc|=(2πfC −1  . 
         [0037]      FIG. 7  shows a side perspective view of a concentric capacitor comprised of conical surfaces. The capacitance of the conical surface concentric capacitor is similar to the capacitance of the cylindrical surface concentric capacitor. The cone separation distance d cone  can be varied by adjusting the relative axial position of the cones Cone 1 , Cone 2 . The ability to adjust the separation distance enables variation of the capacitance. 
         [0038]    For a given capacitance C of the capacitors of  FIGS. 5-7 , the reactive impedance |Xc| decreases as frequency f increases. Insertion loss for a rotary capacitive coupler decreases with increasing frequency and increasing capacitance. Capacitance can be increased by increasing the relative static permittivity of the capacitor gap filler material, increasing the surface area of the capacitor surfaces, or decreasing the gap between capacitor surfaces. The gap filler material can be a variety of materials including air, polyethylene, quartz, or glass. The benefit of decreased insertion loss to imaging performance is improved axial resolution of the imaging system due to use of higher transducer frequencies. 
         [0039]      FIGS. 8 and 9  illustrate separate embodiments of an IVUS system and catheter wherein a rotary capacitive coupler can either be located in a catheter interface module ( FIG. 8 ) or a catheter ( FIG. 9 ).  FIG. 8  shows a diagram of a catheter interface module  2000 A and catheter  1000 A wherein a rotary capacitive coupler  100 A is located in the catheter interface module. The rotary capacitive coupler comprises a cylindrical surface concentric capacitor  110  including concentric drums  110 A,  112 A and a parallel plate capacitor  120  including plates  120 A,  122 A, respectively. The advantage of this design is that the fixed non-rotatable drum  120 A acts as a shield to electrical noise for the parallel plate capacitor. 
         [0040]    A high frequency (&gt;40 MHz) signal travels from a send path  2  to the center conductor  212  of a catheter transmission line  210  in the catheter imaging core, through an ultrasound transducer  220 , back through a transmission line shield  214 , and finally back to the return path conductor  4 . The imaging core  200  components  212 , 214 , 220  rotate inside a catheter sheath by means of a drive motor  30 . The imaging core conductors  212 , 214  are electrically loaded by a transducer  220 . The rotary coupler  100 A comprising the cylindrical surface concentric capacitor  110  and the parallel plate capacitor  120  is used to electrically couple the fixed and rotating components. A drive shaft  32  is mechanically coupled to the rotating drum  112 A and rotating plate  122 A. 
         [0041]    When operating in a send mode the send signal along conductor  2  passes through a transmit/receive (T/R) switch  10  to the conductor  12  leading to an input transmission line  20 . The outputs of the input transmission line  20  are a send signal conductor  22  and a return signal conductor  24 . The conductors  22 , 24  are the inputs to the rotational coupler. The rotary coupler transfers (or couples) electrical signals between the send signal conductor  22  and the proximal end of the catheter transmission line  210  center conductor  202 . The rotary coupler also transfers electrical signals between the return signal conductor  24  and the proximal end of the catheter transmission line shield  204 . This is achieved with two capacitors. 
         [0042]    The send coupling capacitor  110  comprises concentric drums  110 A, 112 A. The return coupling capacitor  120  comprises parallel plates  120 A, 122 A. Regarding the two capacitors, fixed components  110 A, 120 A remain stationary while rotating components  112 A, 122 A rotate with the motor  30 , drive shaft  32 , and catheter imaging core  200 . Input transmission line center conductor  22  electrically connects to drum  110 A, and the send signal on conductor  22  is coupled to drum  112 A which is electrically connected to catheter transmission line center conductor  212 . The input transmission line shield  24  electrically connects to the fixed plate  120 A, and the signal on conductor  24  gets coupled to rotating plate  122 A which is electrically connected to catheter transmission line shield conductor  214 . A radiofrequency (RF) connector (not shown) is used to connect conductors  102 , 104  of the catheter interface module and conductors  202 , 204  of the catheter. Any RF connector can generally be used, but a subminiature RF connector such as an SMB connector is typically used. Consequently, signals on stationary input transmission line conductors  22 , 24  get coupled to the rotating catheter transmission line conductors  202 , 204 . 
         [0043]    The same rotational coupler serves to couple high frequency (&gt;40 MHz) signals generated by the transducer  220  (from ultrasound reflections) back in the reverse (or return) direction. In the return case, signals generated from the transducer  220  are sent to the receiver  8  through the imaging core conductors  202 , 204  and input transmission line input-side conductors  12 , 14 . The rotary coupler capacitively couples the signals on the imaging core proximal conductors  202 , 204  to input transmission line output-side conductors  22 , 24 . The input transmission line  20  outputs the receive signals on input-side conductors  12 , 14 . The signal on conductor  12  is sent to conductor  4  via the T/R switch  10  which would be set for the receive path. Note that the send and receive cases are never allowed to occur simultaneously. A transmit signal is sent to the transducer  220  (with the T/R switch  10  set to send) before the T/R switch  10  is set to receive. 
         [0044]    The diagram of a catheter interface module  2000 B and catheter  1000 B in  FIG. 9  is substantially the same as the diagram of the catheter interface module  2000 A and catheter  1000 A in  FIG. 8  and hence, reference characters for like elements are repeated in  FIG. 9 . A rotary capacitive coupler  100 A comprises a cylindrical surface concentric capacitor  110  having concentric drums  110 A,  112 A and a parallel plate capacitor  120  having parallel plates  120 A, 122 A and is located in the catheter. An RF connector (not shown) is used to connect conductors  22 , 24  of the catheter interface module and conductors  106 , 108  of the catheter. A subminiature RF connector such as an SMB connector is typically used. Signals on stationary input transmission line conductors  22 , 24  are coupled to the rotating catheter transmission line conductors  202 , 204 . 
         [0045]      FIGS. 10-13  show various embodiments of rotary capacitive couplers that may be employed in practicing the present invention. The diagrams of the rotary capacitive couplers, drive motor, and drive shaft in  FIGS. 10-13  are substantially the same as the diagram of the the rotary capacitive couplers, drive motor, and drive shaft in  FIG. 8  and hence, reference characters for like elements are repeated in  FIGS. 10-13 . 
         [0046]    The rotary capacitive coupler  100 B illustrated in  FIG. 10  comprises a conical surface concentric capacitor  111  having concentric conical surfaces  110 B, 112 B and a parallel plate capacitor  120  having plates  120 B, 122 B. The output-side input transmission line conductors  22 , 24  are electrically connected to the rigidly fixed conical surface  110 B and parallel plate  120 B. The rotatable conical surface  112 B is electrically connected to conductor  102  and mechanically connected to the drive shaft  32 . The rotatable parallel plate  122 B is electrically connected to conductor  104  and mechanically connected to the drive shaft  32 . 
         [0047]      FIG. 11  shows a rotary capacitive coupler  100 C comprised of two cylindrical surface concentric capacitors  113  and  115  having concentric drums  110 C,  112 C and  120 C,  122 C, respectively. The output-side input transmission line conductors  22 , 24  are electrically connected to the rigidly fixed cylindrical surfaces  110 C, 120 C. The rotatable cylindrical surface  112 C is electrically connected to conductor  102  and mechanically connected to the drive shaft  32 . The rotatable cylindrical surface  122 C is electrically connected to conductor  104  and mechanically connected to the drive shaft  32 . 
         [0048]      FIG. 12  shows a rotary capacitive coupler  100 D comprised of two conical surface concentric capacitors  117  and  119  having concentric surfaces  110 D,  112 D and  120 D,  122 D, respectively. The output-side input transmission line conductors  22 , 24  are electrically connected to the rigidly fixed conical surfaces  110 D, 120 D. The rotatable conical surface  112 D is electrically connected to conductor  102  and mechanically connected to the drive shaft  32 . The rotatable conical surface  122 D is electrically connected to conductor  104  and mechanically connected to the drive shaft  32 . 
         [0049]      FIG. 13  shows a rotary capacitive coupler  100 E comprised of two parallel plate capacitors  121  and  123  having plate pairs  110 E,  112 E and  120 E,  122 E, respectively. The output-side input transmission line conductors  22 , 24  are electrically connected to the rigidly fixed parallel plates  110 E, 120 E. The rotatable parallel plate  112 E is electrically connected to conductor  102  and mechanically connected to the drive shaft  32 . The rotatable parallel plate  122 E is electrically connected to conductor  104  and mechanically connected to the drive shaft  32 . 
         [0050]      FIG. 14  illustrates still another embodiment wherein an IVUS system comprises a rotary capacitive coupler  100  and a rotary inductive coupler  100 -HF. A catheter interface module  2000 C comprises the rotary inductive coupler  100 -HF and the rotary capacitive coupler  100  on a single rotating shaft with two sets of independent electrical connections. The catheter  1000 C comprises a high frequency (less than approximately 30 MHz) transducer  220 -HF and very high frequency (greater than approximately 30 MHz) transducer  220 . 
         [0051]    This invention overcomes drawbacks associated with rotary inductive couplers used in minimally invasive, high-frequency IVUS imaging systems and catheters. In particular, rotary capacitive couplers improve system performance by reducing insertion loss and impedance variation with angular position. The rotary capacitive couplers disclosed heretofore comprise parallel plate capacitors, cylindrical surface concentric capacitors, and conical surface concentric capacitors. A parallel plate capacitor comprises a first rigidly fixed plate and a second rotatable plate. A cylindrical surface concentric capacitor comprises a first rigidly fixed cylindrical surface and a second rotatable cylindrical surface. A conical surface concentric capacitor comprises a first rigidly fixed conical surface and a second rotatable conical surface. The exemplary rotary capacitive couplers can be combined for system performance advantages. Furthermore, a rotary inductive coupler and a rotary capacitive coupler can be used in a two channel IVUS system and catheter for high frequency and very high frequency operation. 
         [0052]    While particular embodiments of the present invention have been shown and described, modifications may be made, and it is therefore intended to cover in the appended claims, all such changes and modifications which fall within the true spirit and scope of the invention as defined by those claims.