Abstract:
Rotating element catheters and catheter assemblies employ clutch assemblies for preventing rotational energy from being transmitted from a motor drive unit to the catheter element under defined circumstances. The catheter assembly includes an elongate member in which there is disposed a rotatable catheter drive cable. The catheter drive cable may have an operative element, e.g., an ultrasonic transducer or an artherectomy blade, distally mounted thereon for providing diagnostic or therapeutic functions to the physician. To control the rotation of the catheter drive shaft, the clutch assembly is configured such that the catheter drive shaft is operated in a drive mode (i.e., it is allowed to rotate) and in a release mode (i.e., it is prevented from rotating). The clutch assembly includes a driver member comprising a first cylindrical body, and a driven member comprising a second cylindrical body in a concentric relationship with the first cylindrical body. The first cylindrical body can be a rigid cylindrical member, and the second cylindrical body can be a compliant tube interference fit about the cylindrical member. Or the first cylindrical body can be a receptacle with cutouts, and the second cylindrical body can be a cylindrical member disposed within the receptacle, with the surface of the cylindrical member being exposed through the cutouts. A spring clamp can then be interference fitted over the receptacle and exposed arcuate surfaces of the cylindrical member.

Description:
RELATED APPLICATION 
     This application is related to application Ser. No. 09/548,860, application Ser. No. 09/548,690, and application Ser. No. 09/548,692, all filed concurrently herewith and all expressly incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to catheters, and more particularly to catheters having rotatable operative elements. 
     BACKGROUND 
     Currently, there exist rotating element catheters, which can be used by physicians to provide a diagnostic or therapeutic effect within the body tissue of a patient, e.g., ultrasonic imaging or artherectomy. A typical rotating element catheter includes a flexible drive cable that extends the length of the catheter body, terminating proximally in a motor drive unit. An operative element, e.g., an ultrasonic transducer or artherectomy blade, is distally mounted to the drive cable. Operation of the drive unit rotates the drive cable, which, in turn, rotates the operative element at high speeds to produce the desired diagnostic or therapeutic effect. Due to the nature of placing indiscriminately rotating elements inside a patient, there is always a risk that the rotating element could inadvertently damage tissue if the catheter is defective or mishandled. 
     For example, some ultrasonic imaging catheters can provide two-dimensional 360° images along the length of a blood vessel by rotating an ultrasonic transducer at high speeds, while linearly moving the ultrasonic transducer in the distal direction relative to the catheter member. If the distal end of the catheter member is kinked, or otherwise formed into a tight curve, there exists the possibility, however so slight, that the rotating ultrasonic transducer could perforate through the catheter member and damage the surrounding tissue. This is caused, in part, by the fact that the drive unit is designed to maintain the speed of the transducer at a set level, accordingly increasing or decreasing the torque that is applied to the drive cable. In doing so, the drive unit does not discriminate between normal frictional loads, i.e., frictional loads caused by normal friction between the drive cable and catheter member, and abnormal friction loads, i.e., frictional loads caused by an abnormal circumstance, e.g., the boring of the transducer through the wall of the catheter member. 
     As a precaution, these types of ultrasonic imaging catheters are designed, such that the drive shaft fails if the torque required to rotate the ultrasonic transducer becomes too great. This design contemplates providing a circumferential space between the drive cable and the catheter member along a portion of the catheter, allowing the drive cable to wind or ball up within the space when the torque applied to the drive cable exceeds a critical magnitude. Presumably, such an excess in force will occur if the rotating ultrasonic transducer begins to perforate the catheter member, resulting in a failed drive cable, and preventing the ultrasonic transducer from further boring through the catheter member. 
     Typically, however, the drive shaft fails, not because the ultrasonic transducer is boring through the catheter member, but rather because the drive cable is subjected to excessive frictional forces. Such forces are often a result of having to route the catheter through the tortuous vasculature of a patient, forcing the drive cable to rotate through many curves. Any mishandling of the catheter while operating the motor drive unit, e.g., overtightening the touhy-borst valve through which the catheter is introduced into the patient, exacerbates this situation. Because the drive unit is designed to maintain the rotation of the ultrasonic transducer at a uniform speed, the motor drive unit increases the torque that is applied to the drive cable to compensate for any increase in frictional force, thereby risking failure of the drive cable. In fact, of all the failed ultrasonic imaging catheters returned to the assignee of this application, approximately seventy percent fail as a result of this phenomenon. 
     There thus remains a need to prevent premature failure of a drive cable within a catheter, while minimizing the potential risk of inadvertently damaging tissue by the rotating operative element distally mounted on the drive cable. 
     SUMMARY OF THE INVENTION 
     The present inventions are broadly directed to rotating element catheters and catheter assemblies that employ concentric members to prevent rotational energy from being transmitted from a motor drive unit to the catheter element under defined circumstances. 
     In accordance with a first aspect of the present inventions, a catheter assembly includes an elongate member in which there is disposed a rotatable catheter drive shaft, e.g., a flexible drive cable. The catheter drive shaft may have an operative element, e.g., an ultrasonic transducer or an artherectomy blade, distally mounted thereon for providing diagnostic or therapeutic functions to the physician. In the case of ultrasonic imaging, the elongate member can take the form of a telescoping guide sheath slidably disposed about an imaging core (i.e., the catheter drive shaft and ultrasonic transducer) to provide the physician with two-dimensional 360° ultrasonic images of surrounding body tissue. 
     To control the rotation of the catheter drive shaft, the catheter assembly includes a driver member comprising a first cylindrical body, and a driven member comprising a second cylindrical body in a concentric relationship with the first cylindrical member. The driven member is rotatably coupled (either directly or indirectly) to the proximal end of the catheter drive shaft. The first cylindrical body cooperates with the second cylindrical body, e.g., frictionally or magnetically, such that the driven and driver members are rotatably engaged with each other before the applied torque exceeds a critical magnitude, and rotatably disengaged with each other after the applied torque exceeds the critical magnitude. The driven and driver members are preferably located entirely within the catheter, e.g., in a proximal hub configured to interface with a motor drive unit, but a portion of the entirety of the driven and driver members can be located elsewhere, e.g., in the motor drive unit. 
     In accordance with a second aspect of the present inventions, one end of a compliant tube composed of a material, such as rubber or silicone, is interference fitted over a rigid cylindrical member. One of a driven member and a driver member includes the compliant tube, and the other of the driven member and driver member includes the rigid cylindrical member. In the preferred embodiment, the member that includes the compliant tube comprises another rigid cylindrical member over which the other end of the compliant tube is affixed by suitable means, e.g., bonding. 
     In accordance with a third aspect of the present inventions, a cylindrical member is disposed within the cylindrical cavity of a receptacle, wherein the receptacle includes one or more cutouts to expose one or more arcuate surfaces of the cylindrical member. One of a driven member and a driver member includes the cylindrical member, and the other of the driven member and driver member includes the receptacle. A spring clamp is interference fit around the receptacle and the exposed arcuate surfaces of the cylindrical member. 
    
    
     Other and further objects, features, aspects, and advantages of the present invention will become better understood with the following detailed description of the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate both the design and utility of preferred embodiments of the present invention, in which: 
     FIG. 1 is a schematic view of an ultrasonic imaging system constructed in accordance with the present inventions; 
     FIG. 2 is a longitudinal section of a first preferred embodiment of an automatic clutch assembly employed in the system of FIG. 1; 
     FIG. 3 is a side view of the clutch assembly of FIG. 2; 
     FIG. 4 is a side view of a second preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 5 is a diagram showing the magnitude of a torque applied to a catheter drive shaft within the imaging system over a time period in response to a varying frictional load of the catheter drive shaft; 
     FIG. 6 is a side view of a third preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 7 is a side view of a fourth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 8 is a partially cut-away side view of a fifth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 9 is a partially cut-away side view of a sixth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 10 is a cross-sectional view taken along the line  10 — 10  of FIG. 9; 
     FIG. 11 is a partially cut-away side view of a seventh preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 12 is a cross-sectional view taken along the line  12 — 12  of FIG. 11; 
     FIG. 13 is a partially cut-away side view of an eighth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 14 is a cross-sectional view taken along the line  14 — 14  of FIG. 13; 
     FIG. 15 is a side view of a ninth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 16 is a side view of a tenth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 17 is a cross-sectional view taken along the line  17 — 17  of FIG. 16; 
     FIG. 18 is a side view of an eleventh preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 19 is a cross-sectional view taken along the line  19 — 19  of FIG. 18; 
     FIG. 20 is a side view of a twelfth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 21 is a cross-sectional view taken along the line  21 — 21  of FIG. 20; 
     FIG. 22 is a cross-sectional view taken along the line  22 — 22  of FIG. 20; 
     FIG. 23 is a side view of a thirteenth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 24 is a cross-sectional view taken along the line  24 — 24  of FIG. 23; 
     FIG. 25 is a cross-sectional view taken along the line  25 — 25  of FIG. 23; 
     FIG. 26 is a side view of a fourteenth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 27 is a cross-sectional view taken along the line  27 — 27  of FIG. 26; 
     FIG. 28 is a cross-sectional view taken along the line  28 — 28  of FIG. 26; 
     FIG. 29 is a side view of a fifteenth preferred embodiment of an automatic clutch assembly employed in the imaging system of FIG. 1; 
     FIG. 30 is a cross-sectional view taken along the line  30 — 30  of FIG. 29; and 
     FIG. 31 is a cross-sectional view taken along the line  31 — 31  of FIG.  29 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an exemplary ultrasound imaging catheter system  100 , constructed in accordance with the present invention, is provided for ultrasonically imaging a patient&#39;s internal body tissue  194 , e.g., the wall of an artery. The catheter system  100  generally includes a flexible ultrasonic imaging catheter  102 , which houses an ultrasonic imaging core  108 , a motor drive unit  104  (MDU) for providing a source of rotational energy to the imaging core  108 , and an ultrasonic signal processing unit  106  operatively connected to the imaging core  108  for providing an ultrasonic image of the targeted tissue to a physician. 
     The catheter  102  includes an elongate telescoping catheter body  110 , which facilitates the rotational and longitudinal translation of the imaging core  108 . In particular, the catheter body  110  includes an outer guide sheath  112  with an imaging lumen  114 . The imaging core  108  is disposed within the imaging lumen  114 , allowing the imaging core  108  to be rotationally and longitudinally translated with respect to the guide sheath  112 . 
     The imaging core  108  comprises a flexible catheter drive shaft  110 , i.e., a drive cable, with an ultrasonic transducer  118  distally mounted thereon. As is well known in the art, the transducer  118  is composed of a layer of piezoelectrical material, with acoustic matching and backing layers suitably formed on the opposite sides thereof (not individually shown). The drive cable  116  is preferably designed, such that it possesses a high torsional stiffness and a low bending stiffness. For example, the drive cable  116  can be made of two counterwound layers of multifilar coils that are fabricated using techniques disclosed in Crowley et al., U.S. Pat. No. 4,951,677, the disclosure of which is fully and expressly incorporated herein by reference. Thus, the transducer  118  rotates about a longitudinal axis in response to the application of a torque on the proximal end of the drive cable  116 . The imaging core  108  further includes signal wires  114  (shown in FIG.  2 ), which are suitably connected to the transducer  118  by suitable means, e.g., welding. The signal wires  114  are routed through the drive cable  116  from the transducer  118 , extending out the proximal end of the drive cable  116 . 
     The outer guide sheath  112  can be generally divided into three sections: an acoustic window  120 , a main section  122 , and an telescoping section  124 . The acoustic window  120  houses the transducer  118 , and when filled with a suitable imaging solution, allows ultrasonic energy UE to be transmitted between the transducer  118  and the surrounding body tissue. The proximal end of the acoustic window  120  is suitably bonded to the distal end of the main section  122 , which extends almost the entire length of the guide sheath  112 . The main section  122  is characterized by a relatively stiff structure, which not only facilitates advancement of the catheter body  110  through the tortuous vasculature of the patient, but also facilitates advancement of the imaging core  108  through the imaging lumen  114 . The distal end of the telescoping section  124  is suitably bonded to the proximal end of the main section  122 , and includes a semi-rigid tube  125  through which a smaller diameter semi-rigid tube  126  is slidably disposed. The semi-rigid tube  126  extends proximally from the telescoping section  124  and serves to provide rigidity to the drive cable  116  outside of the guide sheath  112 . 
     In this regard, the semi-rigid tube  126  includes a lumen  128  through which the proximal end of the drive cable  116  extends. Although the drive cable  116  rotates relative to the semi-rigid tube  126 , as will be described in further detail below, the drive cable  116  and semi-rigid tube  126  are longitudinally affixed with respect to each other. Thus, relative translation of the semi-rigid tube  126  in the distal direction necessarily translates the imaging core  108  in the distal direction with respect to the guide sheath  112 . Similarly, relative translation of the semi-rigid tube  126  in the proximal direction necessarily translates the imaging core  108  in the proximal direction with respect to the guide sheath  112 . To facilitate the telescoping action of the catheter  102 , the telescoping section  124  includes an anchor housing  130  for connection to a rigid pullback arm  190  of the MDU  104 , as will be described in further detail below. 
     The catheter  102  further includes a proximal hub  132 , which mates with a hub  186  of the MDU  104 . The catheter hub  132  provides the necessary mechanical interface between the imaging core  108  and the MDU  104 , as well as the electrical interface between the imaging core  108  and the signal processing unit  106 . In the illustrated embodiment, the catheter hub  132  is configured as a male adapter, with the MDU hub  186  being configured as a female adapter. 
     Referring specifically to FIG. 2, the catheter hub  132  includes a rigid housing  134  composed of a suitable material, e.g., plastic, and is molded in a shape that facilitates firm seating of the catheter hub  132  within the MDU hub  186 . The housing  134  further includes a pair of spring clamps (not shown), which interact with the MDU hub  186  to removably affix the catheter hub  132  therein. 
     The proximal end of the housing  134  includes a transverse wall  136  from which opposing distally and proximally extending cylindrical walls  138  and  140  extend. The cylindrical walls  138  and  140  respectively include cavities  142  and  144 , which are in communication with each other through the transverse wall  136 . The semi-rigid tube  126  is permanently fixed within the distal cylindrical wall cavity  142  using adhesive  146 . In this regard, the semi-rigid tube  126  is affixed to and extends through the adhesive  146 , across the transverse wall  136 , and into the proximal cylindrical wall cavity  144 . A flexible rubber grommet  148  is suitably mounted to the distal end of the housing  134 , around the distal cylindrical wall  138  and abutting the distal face of the transverse wall  136 . The grommet  148  receives and provides stress relief for the drive cable  116  and semi-rigid tube  126 . 
     The catheter hub  132  further includes an automatic clutch assembly  200 , which is firmly and rotatably seated within a cavity  152  of an inner cylindrical wall  150  formed within the housing  134 . The cylindrical wall  150  is an axial alignment with the distal and proximal cylindrical walls  138  and  140 , and thus, the clutch assembly  200  is in axial alignment with the drive cable  116 . The clutch assembly  200  is configured to advantageously operate the drive cable  116  in either a drive mode or a release mode. Specifically, when a torque T is applied to the proximal end of the drive cable  116 , the clutch assembly  200  provides a means for permitting rotation of the drive cable  116  before the applied torque T exceeds a critical magnitude (drive mode), and provides a means for preventing rotation of the drive cable  116  after the applied torque T exceeds the critical magnitude (release mode). 
     To this end, the clutch assembly  200  comprises a driven member  202  and a driver member  204 , which, as will be described in further detail below, interact with each other to provide the aforementioned clutching function. The driven member  202  comprises a generally cylindrical rigid member  208 , which is composed of a suitable rigid material, e.g., stainless steel. The cylindrical member  208  includes an elongate shaft  210  with a proximally facing boss  212 . The boss  212  and the shaft  210  can be molded as an integral unit, or can alternatively be affixed to each other using suitable means, e.g., welding. 
     The driven member  202  is rotatably coupled to the drive cable  118 . Specifically, the driven member  202  is held in axial alignment with the drive cable  118  by a bushing  154 , which is composed of a suitably rigid bearing material, e.g., bronze. The bushing  154  is suitably bonded within the cavity  152  of the cylindrical wall  150 , with the boss  212  of the driven member  202  being rotatably disposed with the bushing  154 . Likewise, a seal  156  is suitably bonded within the cavity  144  of the cylindrical wall  140 , with the shaft  210  being rotatably disposed within the seal  156 . The driven member  202  is rotatably engaged with the drive cable  116  by suitably mounting the distal end of the shaft  210  to the proximal end of the drive cable  116 , e.g., by welding. It is noted that a portion of the shaft  210  is hollow, which allows the signal wires  114  from the drive cable  116  to extend therethrough. 
     The driver member  204  comprises a generally cylindrical rigid member  214 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  214  includes a proximally facing receptacle  216  with a cavity  218  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104  (shown in FIG.  1 ), when the catheter hub  132  is mated with the MDU hub  186 . To facilitate proper and firm engagement with the motor drive shaft  184 , the receptacle  216  and motor drive shaft  184  are keyed, such that the receptacle  216  rotatably engages the motor drive shaft  184  when inserted into the cavity  218 . The driver member  204  is held in longitudinal abeyance by a rigid arcuate member  158 , which is mounted through the housing  134  and engages an annular recess  160  formed in the cylindrical member  214 . 
     The driver member  204  further includes a coil spring  206 , which integrally rotates with and is affixed to the cylindrical member  214 . As will be discussed in further detail below, the coil spring  206  interacts with the cylindrical rigid member  208  of the driven member  202  in a manner that actuates the clutching action between the driven member  202  and the driver member  204 . 
     The catheter hub  132  further includes an inductive coupler  162 , which is firmly seated within the housing  134  in an axial relationship with the clutch assembly  200 . The inductive coupler  162  provides the means for inductively coupling the electrical energy from the signal wires  114 , which rotate by virtue of their association with the rotating drive cable  116 , and a stationary platform, i.e., the signal processing unit  106 . To this end, the inductive coupler  162  includes a disk-shaped magnetic rotor  164  and a disk-shaped magnetic stator  166 , which are located adjacent each other in a coaxial manner. The shaft  210  of the driven member  202  extends entirely through the inductive coupler  162 , where it is rotatably engaged with the rotor  164 . Thus, the rotor  164  of the inductive coupler  162  integrally rotates with the driven member  202 . The signal wires  114  extend from a transverse hole (not shown) made in the shaft  210 , and are suitably connected to the rotor  164 . Lead-in signal wires  168  are mounted between the stator  166  and an electrical jack  170  mounted on the housing  134 . In this manner, electrical signals can be transmitted between the electrical jack  170  and the signal wires  114  within the drive cable  116  when the imaging core  108  is rotating. 
     The catheter hub  132  further includes an infusion port  172  formed from the housing  134 , which is in fluid communication with the cavity  144  of the distal cylindrical wall  140 . Because the lumen  128  of the semi-rigid tube  126  (shown in FIG. 1) is in fluid communication with the cavity  144 , the infusion port  172  is in fluid communication with the imaging lumen  114  of the guide sheath  112 . Thus, the acoustic window  120  can be filled with a suitable imaging fluid, e.g., a saline solution, introduced through the infusion port  172 . 
     Referring back to FIG. 1, the MDU  104  provides the means for rotationally and longitudinally translating the imaging core  108  with respect to the guide sheath  112 . In particular, the MDU  104  comprises a casing  180  in which there is firmly affixed a motor  182  and the aforementioned motor drive shaft  184  (motor and shaft shown in phantom). As briefly discussed above, the MDU hub  186  mates with the proximal catheter hub  132 , with the distal end of the motor drive shaft  184  being rotatably engaged with the driver member  204  of the clutch assembly  200 . The casing  180  is mounted to a carriage  188  and is in a sliding relationship therewith. A drive train (not shown) is coupled between the casing  180  and the motor  182 , and is configured to longitudinally translate the casing  180  with respect to the carriage  188  in a controlled manner when engaged with the motor  182 . 
     Further details regarding the use of a single motor to actuate both rotation of a drive shaft and longitudinal translation of a drive unit casing are disclosed in U.S. Pat. No. 6,004,271, the disclosure of which is fully and expressly incorporated herein by reference. Alternatively, separate and distinct motors can be used to respectively actuate rotation of the motor drive shaft  184  and longitudinal movement of the casing  180 . Further details regarding the use of two motors to respectively actuate rotation of a drive shaft and longitudinal translation of a drive unit casing are disclosed in U.S. Pat. No. 6,013,030, the disclosure of which is fully and expressly incorporated herein by reference. 
     The MDU  104  further includes a rigid pull back arm  190 , one end of which is mounted to the anchor housing  130  of the guide sheath  112 , and the other end of which is mounted to the carriage  188 . In this manner, when the MDU  104  is operated, the rotating imaging core  108  longitudinally translates in relation to the guide sheath  112 , since the imaging core  108  is longitudinally engaged with the casing  180  via the catheter hub  132 , and the guide sheath  112  is fixed in place by the pullback arm  190 . 
     The MDU  104  includes feedback circuitry with an encoder (not shown), which senses the loss of rotational speed in the presence of an increased friction force between the imaging core  108  and the catheter body. In response, the feedback circuitry increases the current delivered to motor  182 , maintaining the motor drive shaft  184  at the set speed. This increased current translates to an increased torque T applied to the proximal end of the drive cable  116 . 
     The signal processing unit  106  generally comprises a controller, data interpretation unit, monitor, keyboard, etc. (not individually shown). The signal processing unit  106  is electrically coupled to the transducer  118  of the imaging core  108  through the MDU  104 . Specifically, a power/data cable  192  transmits input/output data between the MDU  104  and signal processing unit  106 , while providing DC electrical power to the MDU  104 . Upon mating of the catheter hub  132  with the MDU hub  186 , the MDU  104  is, in turn, electrically coupled to the imaging core  108  via signal wires  114  connected to the electrical jack  170  (shown in FIG.  2 ). 
     During operation, the signal processing unit  106  transmits electrical signals to the transducer  118  via the afore-described electrical path. In response, the transducer  118  is electrically excited, emitting ultrasonic energy UE through the acoustic window  120  into the surrounding body tissue. The ultrasonic energy UE is reflected from the surrounding body tissue, back through the acoustic window  120 , and into the transducer  118 . The ultrasonic excited transducer  118 , in turn, emits electrical signals, which are transmitted back to the signal processing unit  106  via the electrical path. By virtue of the fact that the transducer  118  is being simultaneously rotated and longitudinally translated during this process, the received electrical signals represent a multitude of 360° data slices, which are constructed by the signal processing unit  106  into a two-dimensional image of the body tissue. 
     As stated above, the MDU  104  attempts to maintain the speed of the motor drive shaft  184  at a set speed, by increasing or decreasing the torque applied to the motor drive shaft  184  in response to a variable frictional load. The clutch assembly  200 , however, provides a check on the MDU  104 . In the presence of normal frictional loads, the clutch assembly  200  automatically engages the motor drive shaft  184  with the drive cable  116 , in which case, the drive cable  116  rotates with the motor drive shaft  184  (drive mode). In the presence of abnormal frictional loads, however, the clutch assembly  200  automatically disengages the motor drive shaft  184  from the drive cable  116 , in which case the drive cable  116  does not rotate with the motor drive shaft  184  (release mode). 
     Referring to FIG. 3, the motor drive shaft  184  (shown partially in phantom) is shown applying the torque T to the proximal end of the drive cable  116  (via the clutch assembly  200 ) in a clockwise direction. As noted above, the current magnitude of the applied torque T at any given time depends on the frictional load. Taking the current magnitude of the applied torque T into account, the clutch assembly  200  allows the drive cable  116  to be alternately operated between the drive mode and the release mode. To this end, the driven member  202  and the driver member  204  are conditionally affixed to each other. That is, the driven member  202  is rotatably engaged with the driver member  204  before the current magnitude of the applied torque T exceeds the critical magnitude, and is rotatably disengaged with the driver member  204  after the current magnitude of the applied torque T exceeds the critical magnitude. 
     In particular, the coil spring  206  is affixed to the cylindrical member  214  of the driver member  204  by bending the proximal end of the coil spring  206  into engagement with a hole  220  formed in the cylindrical member  214 . Alternatively, the coil spring  206  can be affixed to the cylindrical member  214  by bending the distal end of the coil spring  206  into engagement with the hole  220 , as shown in the automatic clutch assembly  300  depicted in FIG.  4 . 
     Referring back to FIG. 3, the coil spring  206  provides the means for effecting the aforementioned clutching action. Specifically, the body of the coil spring  206 , which, in the illustrated embodiment, is represented by seven and one-half coils  222 , is interference fitted over the boss  212 , such that a frictionally engaging relationship is formed therebetween. In this regard, the normal inner diameter (the inner diameter in the absence of an external force) of the coil spring  206  is slightly less than the outer diameter of the boss  212 . Preferably, the outer surface of the driven member  202  is polished to a substantially uniform diameter to provide a substantially uniform contact between the coil spring  206  and boss  212 . 
     The coil spring  206  is preferably wound in a direction, such that it tends to “unwind” in the presence of the applied torque T. That is, the interference fit between the coil spring  206  and the boss  212  decreases as the applied torque T increases. Thus, if the proximal end of the coil spring  206  is affixed to the cylindrical member  214  of the driver member  204  (as shown in FIG.  2 ), the coil spring  206  is wound in the counterclockwise direction from the proximal end. In contrast, if the distal end of the coil spring  206  is affixed to the cylindrical member  214  of the driver member  204  (as shown in FIG.  3 ), the coil spring  206  is wound in the clockwise direction from the proximal end. 
     By way of nonlimiting example, the outer and inner diameters of the coil spring  206  can be 0.160 and 0.124 inches, with the diameter of the wire being 0.018 inches. Assuming an exemplary interference fit between the coil spring  206  and driven member  202  of between 0.001 and 0.002 inches (in the absence of an applied torque), the outer diameter of the boss  212  is preferably between 0.122 and 0.123 inches. 
     The operation of the clutch assembly  200  will now be described. FIG. 5 specifically depicts the magnitude of the applied torque T (solid line) and the magnitude of a representative frictional load variance in the drive cable  116  (dashed line) over time. FIG. 5 also indicates the particular mode in which the drive cable  116  is operated, assuming that the drive cable  116  is initially operated in the drive mode. Note that the magnitude of the applied torque T tracks the magnitude of the frictional load, which results from the tendency of the MDU  104  to maintain the motor drive shaft  184  at a uniform speed. The lag between the magnitude of the applied torque T and the magnitude of the frictional load represents the time taken for the MDU  104  to adjust the magnitude of the applied torque T in response to the change in the magnitude of the frictional load. 
     As can be seen from FIG. 5, as long as the frictional load remains normal, the current magnitude of the applied torque T remains below the critical magnitude. Thus, operation of the drive cable  116  is maintained in the drive mode. Specifically, as long as the critical magnitude is not exceeded, the current magnitude of the applied torque T does not overcome the frictional force generated by the interference fit between the coil spring  206  and the boss  212  (in spite of the reduced interference fit due to the “unwinding” of the coil spring  206  in the presence of the applied torque T). Thus, the driven member  202  remains rotatably engaged with the driver member  204 . As a result, the drive cable  116  is rotatably coupled to, and integrally rotates with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the drive mode. 
     As can be seen from FIG. 5, once the frictional load becomes abnormal, the drive cable  116  is operated in the release mode. Specifically, once the critical magnitude is exceeded, the current magnitude of the applied torque T overcomes the frictional force generated by the interference fit between the coil spring  206  and the boss  212  (facilitated by the decrease in the interference fit due to the “winding” of the coil spring  206  in the presence of the applied torque T). Thus, the driven member  202  becomes rotatably disengaged with the driver member  204 . As a result, the drive cable  116  is rotatably uncoupled from, and does not integrally rotate with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the release mode. 
     As can be seen from FIG. 5, once the drive cable  116  is operated in the release mode, the current magnitude of the applied torque T drops to a level well below the critical magnitude. At this point, the current magnitude of the applied torque T tracks the magnitude of the frictional force between the rotatably disengaged coil spring  206  and boss  212 , which generally remains uniform. The substantial drop in the current magnitude of the applied torque T is due to the frictional changes in the clutch assembly  200 . Specifically, the transition from a rotatably engaged relationship to a rotatably disengaged relationship (i.e., transition from drive mode to release mode) is determined by a frictional force between the coil spring  206  and boss  212  that is based upon a stationary coefficient of friction. Once this transition is made, the frictional force between the coil spring  206  and the boss  212  is based upon a dynamic coefficient of friction, which, as is well known, is less than the stationary coefficient of friction. The reduced frictional force translates to a reduced applied torque needed to maintain the motor drive shaft  184  at a uniform set speed. 
     As long as the MDU  104  maintains rotation of the motor drive shaft  184 , once the drive cable  116  is operated in the release mode, operation of the drive cable  116  does not return to the drive mode until the frictional load of the drive cable  116  drops below the frictional force between the disengaged coil spring  206  and boss  212 . It can thus be said that the clutch assembly  200  has a built-in hysteresis, ensuring that the drive cable  116  will not be operated in the drive mode until the frictional load is well within the normal range, e.g., by retracting the catheter or loosening the touhy-borst valve. Once this occurs, operation of the drive cable  116  returns to the drive mode, and the current magnitude of the applied torque T again tracks the magnitude of the frictional load. It should be noted that the imaging core  108  can be repeatedly cycled between the drive mode and release mode without wearing out the clutch assembly  200  due to the intrinsic ability of the coil spring  206  to consistently return to its normal diameter. 
     FIG. 6 depicts an alternative embodiment of an automatic clutch assembly  400 , which is constructed in accordance with the present inventions. Like the clutch assembly  200  described above, the clutch assembly  400  includes a driven member  402  and a driver member  404  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  400  is frictionally actuated by the coil spring  206 . The clutch assembly  400  differs from the clutch assembly  200 , however, in that the driven member  402 , rather than the driver member  404 , includes the coil spring  206 . 
     Specifically, the driven member  402  is similar to the above-described driven member  202  (see FIG.  3 ), with the exception that it includes the coil spring  206 , which is affixed to the cylindrical member  208  by bending the distal end of the coil spring  206  into engagement with the boss  212  by suitable means, e.g., welding. Alternatively, the coil spring  206  can be affixed to the cylindrical member  208  by bending the proximal end of the coil spring  206  into engagement with the boss  212 , as shown in the automatic clutch assembly  500  depicted in FIG.  7 . 
     Referring back to FIG. 6, the driver member  404  includes a generally cylindrical rigid member  414 , which is constructed similarly to the above-described cylindrical member  214  (see FIG.  3 ), with the exception that the cylindrical member  414  includes a distally facing transitional shaft  420 . The body of the coil spring  206  is interference fit about the transitional shaft  420  in the same manner as that described above with respect to the coil spring  206  and boss  212  (see FIG.  3 ). Again, the coil spring  206  is preferably wound in a direction, such that it tends to “unwind” in the presence of the applied torque T. Thus, if the distal end of the coil spring  206  is affixed to the boss  212  (as shown in FIG.  6 ), the coil spring  206  is wound in the clockwise direction from the proximal end. In contrast, if the proximal end of the coil spring  206  is affixed to the boss  212  (as shown in FIG.  7 ), the coil spring  206  is wound in the counterclockwise direction from the proximal end. 
     The operation of the clutch assembly  400  is identical to that of the clutch assembly  200 , with the exception that the coil spring  206  frictionally interacts with the transitional shaft  420  of the driver member  404 , rather than the boss  212  of the driven member  202 . 
     FIG. 8 depicts another alternative embodiment of an automatic clutch assembly  600 , which is constructed in accordance with the present invention. Like the clutch assembly  200  described above, the clutch assembly  600  includes a driven member  602  and a driver member  604  that are conditionally affixed to each other. The clutch assembly  600  differs from the clutch assembly  200 , however, in that the driver member  604  resides in the MDU  104 , rather than in the catheter hub  132  (shown in FIG.  1 ). 
     Specifically, the driver member  604  comprises the motor drive shaft  184  itself. The driven member  602  includes the above-described cylindrical member  208  (see FIG.  3 ), as well as a generally cylindrical rigid member  614 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  614  includes a distally facing receptacle  620  with a cavity  622  formed therein, wherein the boss  212  (shown partially in phantom) of the cylindrical member  208  is mounted by suitable means, e.g., bonding. Like the above-described cylindrical member  214  (see FIG.  3 ), the cylindrical member  614  further includes a proximally facing receptacle  616  with a cavity  618  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104  (shown in FIG.  2 ), when the catheter hub  132  is mated with the MDU hub  186 . The receptacle  616  and motor drive shaft  184 , however, are not keyed, such that the motor drive shaft  184  freely rotates with the cavity  618  absent restraint. 
     The driven member  602  further includes the coil spring  206 , which is seated within an annular recess  626  formed within the cavity  618 , with the distal end of the coil spring  206  being suitably mounted to the receptacle  616  distally adjacent the cavity  618 . The diameter of the annular recess  626  is slightly greater than the normal outer diameter of the coil spring  206 , whereby expansion of the coil spring  206  is allowed, i.e., the coil spring  206  is allowed to “unwind.” The normal inner diameter of the coil spring  206  is slightly smaller than the outer diameter of the distal end of the motor drive shaft  184 , such that the coil spring  206  can be interference fitted over the distal end of the motor drive shaft  184 . Again, the coil spring  206  is preferably wound in a direction, such that it tends to “unwind” in the presence of the applied torque T. In the illustrated embodiment, the coil spring  206  is wound in the clockwise direction from the proximal end. As can be seen, the cavity  618  within the receptacle  616  tapers to a diameter equal to the diameter of the distal end of the motor drive shaft  184 . Thus, when the distal end of the motor drive shaft  184  is inserted into the receptacle  616 , it is guided into an interference fitted with the coil spring  206 . 
     The operation of the clutch assembly  600  is identical to that of the clutch assembly  200 , with the exception that the coil spring  206  frictionally interacts with the motor drive shaft  184  of the driver member  604 , rather than the boss  212  of the driven member  202 . 
     FIGS. 9 and 10 depict another alternative embodiment of an automatic clutch assembly  700 , which is constructed in accordance with the present inventions. Like the clutch assembly  200  described above, the clutch assembly  700  allows the drive cable  116  to be alternately operated between the drive mode and the release mode, as dictated by the magnitude of the applied torque T. To this end, the clutch assembly  700  includes a driven member  702  and a driver member  704 , which are conditionally affixed to each other. That is, the driven member  702  is rotatably engaged with the driver member  704  before the current magnitude of the applied torque T exceeds the critical magnitude, and is rotatably disengaged from the driver member  704  after the current magnitude of the applied torque T exceeds the critical magnitude. Unlike the clutch assembly  200 , however, the clutch assembly  700  utilizes a watch spring  706 , rather than the coil spring  206 , to effect the frictional clutching action. 
     Specifically, the driver member  704  comprises a generally cylindrical rigid member  714 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  714  includes a proximally facing receptacle  716  with a cavity  718  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104 , when the catheter hub  132  is mated with the MDU hub  186  (shown in FIG.  2 ). To facilitate proper and firm engagement with the motor drive shaft  184 , the receptacle  716  and motor drive shaft  184  are keyed, such that the receptacle  716  rotatably engages the motor drive shaft  184  when inserted into the cavity  718 . The cylindrical member  714  further includes a distally facing transitional shaft  720  and the watch spring  706 , which is wound around the transitional shaft  720 , with one end of the watch spring  706  being suitably bonded to the transitional shaft  720  (best shown in FIG.  10 ). 
     The driven member  702  comprises a generally cylindrical rigid member  708 , which is composed of a suitable rigid material, e.g., stainless steel. The cylindrical member  708  includes an elongate shaft  710  with a proximally facing receptacle  712  having a cavity  713  formed therein. The receptacle  712  and the shaft  710  can be molded as an integral unit, or can alternatively be affixed to each other using suitable means, e.g., welding. 
     The watch spring  706  provides the means for effecting the aforementioned clutching action. Specifically, the watch spring  706  is interference fitted within the cavity  713 , such that a frictionally engaging relationship is formed between the watch spring  706  and the receptacle  712 . In this regard, the normal outer diameter (the outer diameter in the absence of an external force) of the watch spring  706  is greater than the inner diameter of the cavity  713 . Preferably, the cavity  713  is polished to a substantially uniform diameter to provide a substantially uniform contact between the watch spring  706  and the receptacle  712 . The watch spring  706  is preferably wound in a direction, such that it tends to “wind” in the presence of the applied torque T. That is, the interference fit between the watch spring  706  and the receptacle  712  decreases as the applied torque T increases. In the illustrated embodiment, the watch spring  706  is wound in the counterclockwise direction from the inside. 
     The operation of the clutch assembly  700  is similar to that of the clutch assembly  200  described with respect to FIG.  5 . Specifically, as long as the critical magnitude is not exceeded, the current magnitude of the applied torque T does not overcome the frictional force generated by the interference fit between the watch spring  706  and the receptacle  712  (in spite of the reduced interference fit due to the “winding” of the watch spring  706  in the presence of the applied torque T). Thus, the driven member  702  remains rotatably engaged with the driver member  704 . As a result, the drive cable  116  is rotatably coupled to, and integrally rotates with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the drive mode. 
     Once the critical magnitude is exceeded, the current magnitude of the applied torque T overcomes the frictional force generated by the interference fit between the watch spring  706  and the receptacle  712  (facilitated by the decrease in the interference fit due to the “winding” of the watch spring  706  in the presence of the applied torque T). Thus, the driven member  702  becomes rotatably disengaged with the driver member  704 . As a result, the drive cable  116  is rotatably uncoupled from, and does not integrally rotate with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the release mode. 
     FIGS. 11 and 12 depict another alternative embodiment of an automatic clutch assembly  800 , which is constructed in accordance with the present inventions. Like the clutch assembly  700  described above, the clutch assembly  800  includes a driven member  802  and a driver member  804  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  800  is frictionally actuated by the watch spring  706 . The clutch assembly  800  differs from the clutch assembly  700 , however, in that the driven member  802 , rather than the driver member  804 , includes the watch spring  706 . 
     Specifically, the driven member  802  includes a generally cylindrical rigid member  808 , which is constructed similarly to the above-described cylindrical member  708  (see FIG.  9 ), with the exception that the cylindrical member  808  does not include a receptacle  712 . Thus, the cylindrical member  808  is formed solely by an elongate shaft  810 . The driven member  802  further includes the watch spring  706 , which is wound around the proximal end of the shaft  810 , with one end of the watch spring  706  being suitably bonded to the shaft  810  (best shown in FIG.  12 ). 
     The driver member  804  includes a generally cylindrical rigid member  814 , which is constructed similarly to the above-described cylindrical member  714  (see FIG.  9 ), with the exception that the cylindrical member  814  includes a distally facing receptacle  820  having a cavity  822  formed therein, rather than the transitional shaft  720 . The watch spring  706  is interference fitted within the cavity  822  in the same manner as that described above with respect to the watch spring  706  and the cavity  714  of the receptacle  712  (see FIG.  9 ). Again, the watch spring  706  is preferably wound in a direction, such that it tends to “wind” in the presence of the applied torque T. In the illustrated embodiment, the watch spring  706  is wound in the clockwise direction from the inside. 
     The operation of the clutch assembly  800  is identical to that of the clutch assembly  700 , with the exception that the watch spring  706  friction ally interact s with the receptacle  820  of the driver member  804 , rather than the receptacle  712  of the driven member  702 . 
     FIGS. 13 and 14 depict another alternative embodiment of an automatic clutch assembly  900 , which is constructed in accordance with the present invention. Like the clutch assembly  700  described above, the clutch assembly  900  includes a driven member  902  and a driver member  904  that are conditionally affixed to each other. The clutch assembly  900  differs from the clutch assembly  200 , however, in that the driver member  904  resides in the MDU  104 , rather than in the catheter hub  132  (shown in FIG.  1 ). 
     Specifically, the driver member  904  comprises the motor drive shaft  184  itself. The driver member  904  further includes the watch spring  706  , which is wound around the distal end of the drive shaft  184 , with one end of the watch spring  706  being suitably bonded to the motor drive shaft  184  (best shown in FIG.  14 ). 
     The driven member  902  includes a generally cylindrical rigid member  908 , which is constructed similarly to the above-described cylindrical member  708  (see FIG.  9 ), with the exception that the cylindrical member  908  includes a proximally facing boss  912 , rather than the receptacle  712 . The driven member  902  further includes a generally cylindrical rigid member  914 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  914  includes a distally facing receptacle  920  with a cavity  922  formed therein, wherein the boss  912  (shown partially in phantom in FIG. 13) of the cylindrical member  908  is mounted by suitable means, e.g., bonding. Like the above-described cylindrical member  714  (see FIG.  9 ), the cylindrical member  914  further includes a proximally facing receptacle  916  with a cavity  918  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104 , when the catheter hub  132  is mated with the MDU hub  186  (shown in FIG.  2 ). 
     The watch spring  706  is interference fitted within the cavity  918  in the same manner as that described above with respect to the watch spring  706  and the cavity  713  of the receptacle  712  (see FIG.  9 ). Again, the watch spring  706  is preferably wound in a direction, such that it tends to “wind” in the presence of the applied torque T. In the illustrated embodiment, the watch spring  706  is wound in the counterclockwise direction from the inside. 
     The operation of the clutch assembly  900  is identical to that of the clutch assembly  700 , with the exception that the watch spring  706  frictionally interacts with the receptacle  916  of the driven member  902 , rather than the receptacle  712  of the driven member  702 . 
     FIG. 15 depicts another alternative embodiment of an automatic clutch assembly  1000 , which is constructed in accordance with the present inventions. Like the clutch assemblies  200  and  700  described above, the clutch assembly  1000  allows the drive cable  116  to be alternately operated between the drive mode and the release mode, as dictated by the magnitude of the applied torque T. To this end, the clutch assembly  1000  includes a driven member  1002  and a driver member  1004 , which are conditionally affixed to each other. That is, the driven member  1002  is rotatably engaged with the driver member  1004  before the current magnitude of the applied torque T exceeds the critical magnitude, and is rotatably disengaged from the driver member  1004  after the current magnitude of the applied torque T exceeds the critical magnitude. Unlike the clutch assemblies  200  and  700 , however, the clutch assembly  1000  utilizes a compliant member  1006 , rather than a spring, to effect the frictional clutching action. 
     Specifically, the driver member  1004  comprises a generally cylindrical rigid member  1014 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  1014  includes a proximally facing receptacle  1016  with a cavity  1018  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104 , when the catheter hub  132  is mated with the MDU hub  186  (shown in FIG.  2 ). To facilitate proper and firm engagement with the motor drive shaft  184 , the receptacle  1016  and motor drive shaft  184  are keyed, such that the receptacle  1016  rotatably engages the motor drive shaft  184  when inserted into the cavity  1018 . The cylindrical member  1014  further includes a distally facing transitional shaft  1020  and the compliant tube  1006 , which is composed of a suitably compliant material, e.g., rubber or silicone. The proximal end of the compliant tube  1006  is disposed over and suitably bonded to the transitional shaft  1020 . 
     The driven member  1002  comprises a generally cylindrical rigid member  1008 , which is composed of a suitable rigid material, e.g., stainless steel. The cylindrical member  1008  includes an elongate shaft  1010  with a proximally facing boss  1012 . The boss  1012  and the shaft  1010  can be molded as an integral unit, or can alternatively be affixed to each other using suitable means, e.g., welding. 
     The compliant tube  1016  provides the means for effecting the aforementioned clutching action. Specifically, the distal end of the compliant tube  1006  is interference fitted over the boss  1012 , such that a frictionally engaging relationship is formed therebetween. In this regard, the normal outer diameter (the outer diameter in the absence of an external force) of the compliant tube  1006  is less than the outer diameter of the boss  1012 . Preferably, the boss  1012  is polished to a substantially uniform diameter to provide a substantially uniform contact between the compliant tube  1006  and the boss  1012 . Because the inner diameters of the proximal and distal ends of the compliant tube  1006  are the same, the outer diameter of the boss  1012  is preferably equal to the outer diameter of the transitional shaft  1020 . 
     Although the compliant tube  1006  in the illustrated embodiment is conditionally affixed to the boss  1012 , the compliant tube  1006  can alternatively be conditionally affixed to the transitional shaft  1020 . That is, the distal end of the compliant tube  1006  can be disposed over and suitably bonded to the boss  1012 , and the proximal end of the compliant tube  1006  can be interference fitted over the boss  1012 , such that a frictionally engaging relationship is formed therebetween. 
     The operation of the clutch assembly  1000  is similar to that of the clutch assembly  200  described with respect to FIG.  5 . Specifically, as long as the critical magnitude is not exceeded, the current magnitude of the applied torque T does not overcome the frictional force generated by the interference fit between the compliant tube  1006  and the boss  1012  (or the compliant tube  1006  and the transitional shaft  1020  if the compliant tube  1006  is conditionally affixed to the transitional shaft). Thus, the driven member  1002  remains rotatably engaged with the driver member  1004 . As a result, the drive cable  116  is rotatably coupled to, and integrally rotates with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the drive mode. 
     Once the critical magnitude is exceeded, the current magnitude of the applied torque T overcomes the frictional force generated by the interference fit between the compliant tube  1006  and the boss  1012  (or the compliant tube  1006  and the transitional shaft  1020  if the compliant tube  1006  is conditionally affixed to the transitional shaft). Thus, the driven member  1002  becomes rotatably disengaged with the driver member  1004 . As a result, the drive cable  116  is rotatably uncoupled from, and does not integrally rotate with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the release mode. 
     FIGS. 16 and 17 depict another alternative embodiment of an automatic clutch assembly  1100 , which is constructed in accordance with the present inventions. Like the clutch assemblies  200 ,  700 , and  1000  described above, the clutch assembly  1100  allows the drive cable  116  to be alternately operated between the drive mode and the release mode, as dictated by the magnitude of the applied torque T. To this end, the clutch assembly  1100  includes a driven member  1102  and a driver member  1104 , which are conditionally affixed to each other. That is, the driven member  1102  is rotatably engaged with the driver member  1104  before the current magnitude of the applied torque T exceeds the critical magnitude, and is rotatably disengaged from the driver member  1104  after the current magnitude of the applied torque T exceeds the critical magnitude. Unlike the clutch assemblies  200 ,  700 , and  1000 , however, the clutch assembly  1100  utilizes rigid bodies to effect the frictional clutching action. 
     Specifically, the driver member  1104  comprises a generally cylindrical rigid member  1114 , which is molded from a suitably rigid material, e.g., plastic. The cylindrical member  1114  includes a proximally facing receptacle  1116  with a cavity  1118  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104 , when the catheter hub  132  is mated with the MDU hub  186  (shown in FIG.  2 ). To facilitate proper and firm engagement with the motor drive shaft  184 , the receptacle  1116  and motor drive shaft  184  are keyed, such that the receptacle  1116  rotatably engages the motor drive shaft  184  when inserted into the cavity  1118 . The cylindrical member  1114  further includes a distally facing receptacle  1120  with a cavity  1122  formed therein. 
     The driven member  1102  comprises a generally cylindrical rigid member  1108 , which is composed of a suitable rigid material, e.g., stainless steel. The cylindrical member  1108  includes an elongate shaft  1110  with a proximally facing boss  1112 . The boss  1112  and the shaft  1110  can be molded as an integral unit, or can alternatively be affixed to each other using suitable means, e.g., welding 
     The boss  1112  is disposed within the cavity  1122  of the receptacle  1120 , with the outer diameter of the boss  1112  being slightly less than the diameter of the cavity  1122 , such that, absent any external binding force, the boss  1112  can rotate freely within the cavity  1122 . To facilitate axial alignment between the driven member  1102  and driver member  1104 , the proximal face of the boss  1112  includes a centered pin  1124  (shown in phantom in FIG.  16 ), and the receptacle  1120  includes a centered pin hole  1126  (also shown in phantom) proximally adjacent the cavity  1122 , wherein the pin  1124  and pin hole  1126  engage each other to center the boss  1112  within the cavity  1122  of the receptacle  1120 . 
     The spring clamp  1106  is interference fit about the receptacle  1120  and boss  1112  to provide a binding force between the receptacle  1120  and boss  1112 . Specifically, the longitudinal center of the receptacle  1120  includes a pair of opposing circumferential cutouts  1128  and a pair of adjacent bridge sections  1130 . Thus, the boss  1112  includes a pair of opposing arcuate surfaces  1132  that is exposed through the respective cutouts  1128 . The spring clamp  1106  is interference fit around the pair of bridge sections  1130  and the pair of exposed arcuate surfaces  1132 , such that a frictionally engaging relationship is formed among the spring clamp  1106 , receptacle  1120 , and boss  1112 . 
     Once the critical magnitude is exceeded, the current magnitude of the applied torque T overcomes the frictional force generated by the interference fit among the spring clamp  1106 , receptacle  1120 , and boss  1112 . Thus, the driven member  1102  becomes rotatably disengaged with the driver member  1104 . As a result, the drive cable  116  is rotatably uncoupled from, and does not integrally rotate with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the release mode. 
     FIGS. 18 and 19 depict another alternative embodiment of an automatic clutch assembly  1200 , which is constructed in accordance with the present inventions. Like the clutch assembly  1100  described above, the clutch assembly  1200  includes a driven member  1202  and a driver member  1204  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  1200  is frictionally actuated by the spring clamp  1106 . The clutch assembly  1200  differs from the clutch assembly  1100 , however, in that the driven member  1202  houses the driver member  1204 , rather than vice versa. 
     Specifically, the driven member  1202  includes a generally cylindrical rigid member  1208 , which is constructed similarly to the above-described cylindrical member  1108  (see FIG.  16 ), with the exception that the cylindrical member  1208  includes a proximally facing receptacle  1212  having a cavity  1213  formed therein, rather than the boss  1112 . The driver member  1204  includes a generally cylindrical rigid member  1214 , which is constructed similarly to the above-described cylindrical member  1114  (see FIG.  16 ), with the exception that the cylindrical member  1214  includes a distally facing transitional shaft  1220 , rather than the transitional shaft  1120 . 
     The transitional shaft  1220  is disposed within the cavity  1213  of the receptacle  1212 , with the outer diameter of the transitional shaft  1220  being slightly less than the diameter of the cavity  1213 , such that, absent any external binding force, the transitional shaft  1220  can rotate freely within the cavity  1213 . To facilitate axial alignment between the driven member  1202  and driver member  1204 , the distal face of the transitional shaft  1220  includes a centered pin  1224  (shown in phantom in FIG.  18 ), and the receptacle  1212  includes a centered pin hole  1226  (also shown in phantom in FIG. 18) distally adjacent the cavity  1213 , wherein the pin  1224  and pin hole  1226  engage each other to center the transitional shaft  1220  within the cavity  1213  of the receptacle  1212 . 
     The spring clamp  1106  is interference fit about the receptacle  1212  and transitional shaft  1220  to provide a binding force between the receptacle  1212  and transitional shaft  1220 . Specifically, the longitudinal center of the receptacle  1212  includes a pair of opposing circumferential cutouts  1228  and a pair of adjacent bridge sections  1230 . Thus, the transitional shaft  1220  includes a pair of opposing arcuate surfaces  1232  that is exposed through the respective cutouts  1228 . The spring clamp  1106  is interference fit around the pair of bridge sections  1230  and the pair of exposed arcuate surfaces  1232 , such that a frictionally engaging relationship is formed among the spring clamp  1106 , receptacle  1212 , and transitional shaft  1220 . 
     The operation of the clutch assembly  1200  is identical to that of the clutch assembly  1100 , with the exception that the spring clamp  1106 , the receptacle  1212  of the driven member  1202 , and transitional shaft  1220  of the driver member  1204  frictionally interact with each other, rather than the spring clamp  1106 , receptacle  1120  of the driver member  1104 , and boss  1112  of the driven member  1102 . 
     FIGS. 20-22 depict another alternative embodiment of an automatic clutch assembly  1300 , which is constructed in accordance with the present inventions. Like the clutch assemblies  200 ,  700 ,  1000 , and  1100  described above, the clutch assembly  1300  allows the drive cable  116  to be alternately operated between the drive mode and the release mode, as dictated by the magnitude of the applied torque T. To this end, the clutch assembly  1300  includes a driven member  1302  and a driver member  1304 , which are conditionally affixed to each other. That is, the driven member  1302  is rotatably engaged with the driver member  1304  before the current magnitude of the applied torque T exceeds the critical magnitude, and is rotatably disengaged from the driver member  1304  after the current magnitude of the applied torque T exceeds the critical magnitude. Unlike the clutch assemblies  200 ,  700 ,  1000 , and  1100 , however, the clutch assembly  1300  utilizes magnetic forces, rather than frictional forces, to effect the clutching action. 
     Specifically, the driver member  1304  comprises a generally cylindrical rigid member  1314 , which is molded from a ferrous material. The cylindrical member  1314  includes a proximally facing receptacle  1316  with a cavity  1318  formed therein for receiving the distal end of a rigid motor drive shaft  184  from the MDU  104 , when the catheter hub  132  is mated with the MDU hub  186  (shown in FIG.  2 ). To facilitate proper and firm engagement with the motor drive shaft  184 , the receptacle  1316  and motor drive shaft  184  are keyed, such that the receptacle  1316  rotatably engages the motor drive shaft  184  when inserted into the cavity  1318 . The cylindrical member  1314  further includes a distally facing transitional shaft  1320 . 
     The driven member  1302  comprises a generally cylindrical rigid member  1308 , which is composed of a suitable rigid material, e.g., stainless steel. The cylindrical member  1308  includes an elongate shaft  1310  with a proximally facing receptacle  1312 . having a cavity  1313  formed therein. The receptacle  1312  and the shaft  1310  can be molded as an integral unit, or can alternatively be affixed to each other using suitable means, e.g., welding. 
     The transitional shaft  1320  is disposed within the cavity  1313  of the receptacle  1312 . To facilitate axial alignment between the driven member  1302  and driver member  1304 , the distal face of the transitional shaft  1320  includes a centered pin  1324  (shown in phantom in FIG.  20 ), and the receptacle  1312  includes a centered pin hole  1326  (also shown in phantom) distally adjacent the cavity  1313 , wherein the pin  1324  and pin hole  1326  engage each other to center the transitional shaft  1320  within the cavity  1313  of the receptacle  1312 . 
     A magnetic system provides the means for effecting the aforementioned clutching action. Specifically, the transitional shaft  1320  of the cylindrical member  1314  is composed of a ferrous material, and includes four outwardly extending permanent magnets  1328 , which are circumferentially affixed about the transitional shaft  1320  by suitable means, e.g., bonding. In the illustrated embodiment, adjacent magnets  1328  are separated by 90° and substantially extend the length of the transitional shaft  1320 . As can be seen, each magnet  1328  includes a north pole N and a south pole S, with the polarities of each magnet  1328  being opposite with respect to the two adjacent magnets  1328 . 
     The receptacle  1312  of the cylindrical member  1308  is composed of a ferrous material, and includes four inwardly extending ferrous elements  1330  and four outwardly extending ferrous arcs  1332 , which are circumferentially disposed about the cavity  1313 . In the illustrated embodiment, adjacent ferrous elements  1330  are separated by 90° and substantially extend the length of the receptacle  1312 . The ferrous arcs  1332  are interlaced between the ferrous elements  1330 , and likewise, are separated by 90° and substantially extend the length of the receptacle  1312 . In the embodiment illustrated in FIG. 21, the ferrous elements  1330  and arcs  1332  are formed from the deformed inner surface of the receptacle  1312 . In an alternative embodiment illustrated in FIG. 22, the ferrous elements  1330  and arcs  1332  are formed from four curvilinear flanges. 
     The four ferrous elements  1330  are located outwardly adjacent the four magnets  1328 , respectively, such that a magnetically engaging relationship is formed between the magnets  1328  and ferrous elements  1330 . As can be seen, the transitional shaft  1320 , by virtue of its ferrous composition, advantageously provides a magnetic return (indicated by arrows) between the inward poles of adjacent magnets  1328 , and the receptacle  1312 , by virtue of its ferrous composition, provides a magnetic return (indicated by arrows) between the outward poles of adjacent magnets  1328 . The outwardly extending arcs  1332  facilitate the magnetically engaging relationship between the magnets  1328  and ferrous elements  1330 , by concentrating the magnetic force at the ferrous elements  1330 . 
     The operation of the clutch assembly  1300  is similar to that of the clutch assembly  200  described with respect to FIG.  5 . Specifically, as long as the critical magnitude is not exceeded, the current magnitude of the applied torque T does not overcome the attractive magnetic force generated between the magnets  1328  and ferrous elements  1330 . Thus, the driven member  1302  remains rotatably engaged with the driver member  1304 . As a result, the drive cable  116  is rotatably coupled to, and integrally rotates with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the drive mode. 
     Once the critical magnitude is exceeded, the current magnitude of the applied torque T overcomes the attractive magnetic force generated between the magnets  1328  and ferrous elements  1330 . Thus, the driven member  1302  becomes rotatably disengaged with the driver member  1304 . As a result, the drive cable  116  is rotatably uncoupled from, and does not integrally rotate with, the motor drive shaft  184 . That is, the drive cable  116  is operated in the release mode. 
     FIGS. 23-25 depict another alternative embodiment of an automatic clutch assembly  1400 , which is constructed in accordance with the present inventions. Like the clutch assembly  1300  described above, the clutch assembly  1400  includes a driven member  1402  and a driver member  1404  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  1400  is magnetically actuated. The clutch assembly  1400  differs from the clutch assembly  1300 , however, in that the driver member  1404  houses the driven member  1402 , rather than vice versa. Also, the driven member  1402  is magnetic and the driver member  1404  is ferrous, rather than vice versa. 
     Specifically, the driven member  1402  includes a generally cylindrical rigid member  1408 , which is constructed similarly to the above-described cylindrical member  1308  (see FIG.  20 ), with the exception that the cylindrical member  1408  includes a proximally facing boss  1412 , rather than the receptacle  1312 . The driver member  1404  includes a generally cylindrical rigid member  1414 , which is constructed similarly to the above-described cylindrical member  1314  (see FIG.  20 ), with the exception that the cylindrical member  1414  includes a distally facing receptacle  1420  with a cavity  1422  formed therein, rather than the transitional shaft  1320 . 
     The boss  1412  is disposed within the cavity  1422  of the receptacle  1420 . To facilitate axial alignment between the driven member  1402  and driver member  1404 , the distal face of the boss  1412  includes a centered pin  1424  (shown in phantom in FIG.  23 ), and the receptacle  1420  includes a centered pin hole  1426  (also shown in phantom in FIG. 23) proximally adjacent the cavity  1422 , wherein the pin  1424  and pin hole  1426  engage each other to center the boss  1412  within the cavity  1422  of the receptacle  1420 . The boss  1412  is composed of a ferrous material, and includes four outwardly extending permanent magnets  1428 , which are circumferentially affixed about the boss  1412  by suitable means, e.g., bonding. 
     The receptacle  1420  is composed of a ferrous material, and includes four inwardly extending ferrous elements  1430  and four outwardly extending ferrous arcs  1432 , which are circumferentially disposed about the cavity  1422 . In the embodiment illustrated in FIG. 24, the ferrous elements  1430  and arcs  1432  are formed from the deformed inner surface of the receptacle  1420 . In an alternative embodiment illustrated in FIG. 25, the ferrous elements  1430  and arcs  1432  are formed from four curvilinear flanges. 
     The four ferrous elements  1430  are located outwardly adjacent the four magnets  1428 , respectively, such that a magnetically engaging relationship is formed between the magnets  1428  and ferrous elements  1430 . As can be seen, the boss  1412 , by virtue of its ferrous composition, advantageously provides a magnetic return (indicated by arrows) between the inward poles of adjacent magnets  1428 , and the receptacle  1420 , by virtue of its ferrous composition, provides a magnetic return (indicated by arrows) between the outward poles of adjacent magnets  1428 . The outwardly extending arcs  1432  facilitate the magnetically engaging relationship between the magnets  1428  and ferrous elements  1430 , by concentrating the magnetic force at the ferrous elements  1430 . 
     The operation of the clutch assembly  1400  is identical to that of the clutch assembly  1300 , with the exception that the magnets  1428  of the driven member  1402  and the ferrous elements  1430  of the driver member  1404  magnetically interact with each other, rather than the magnets  1328  of the driver member  1304  and the ferrous elements  1330  of the driven member  1302 . 
     FIGS. 26-28 depict another alternative embodiment of an automatic clutch assembly  1500 , which is constructed in accordance with the present inventions. Like the clutch assembly  1300  described above, the clutch assembly  1500  includes a driven member  1502  and a driver member  1504  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  1500  is magnetically actuated. The clutch assembly  1500  differs from the clutch assembly  1300 , however, in that the driven member  1502  is magnetic and the driver member  1504  is ferrous, rather than vice versa. 
     Specifically, the driven member  1502  includes a generally cylindrical rigid member  1508 , which is constructed similarly to the above-described cylindrical member  1308  (see FIG.  20 ), and includes a proximally facing receptacle  1512  having a cavity  1513  formed therein. The driver member  1504  includes a generally cylindrical rigid member  1514 , which is constructed similarly to the above-described cylindrical member  1314  (see FIG.  20 ), and includes a transitional shaft  1520 . 
     The transitional shaft  1520  is disposed within the cavity  1513  of the receptacle  1512 . The receptacle  1512  is composed of a ferrous material, and includes four inwardly extending permanent magnets  1528 , which are circumferentially disposed around the cavity  1513 , and are affixed to the receptacle  1512  by suitable means, e.g., bonding. The transitional shaft  1520  is composed of a ferrous material, and includes four outwardly extending ferrous elements  1530  and four inwardly extending ferrous arcs  1532 , which are circumferentially disposed around the transitional shaft  1520 . In the embodiment illustrated in FIG. 27, the ferrous elements  1530  and arcs  1532  are formed from the deformed outer surface of the transitional shaft  1520 . In an alternative embodiment illustrated in FIG. 28, the ferrous elements  1530  and arcs  1532  are formed from four curvilinear flanges. 
     The four ferrous elements  1530  are located inwardly adjacent the four magnets  1528 , respectively, such that a magnetically engaging relationship is formed between the magnets  1528  and ferrous elements  1530 . As can be seen, the receptacle  1512 , by virtue of its ferrous composition, advantageously provides a magnetic return (indicated by arrows) between the outward poles of adjacent magnets  1528 , and the-transitional shaft  1520 , by virtue of its ferrous composition, provides a magnetic return (indicated by arrows) between the inward poles of adjacent magnets  1528 . The inwardly extending arcs  1532  facilitate the magnetically engaging relationship between the magnets  1528  and ferrous elements  1530 , by concentrating the magnetic force at the ferrous elements  1530 . 
     The operation of the clutch assembly  1500  is identical to that of the clutch assembly  1300 , with the exception that the magnets  1528  of the driven member  1502  and the ferrous elements  1530  of the driver member  1504  magnetically interact with each other, rather than the magnets  1328  of the driver member  1304  and the ferrous elements  1330  of the driven member  1302 . 
     FIGS. 29-31 depict another alternative embodiment of an automatic clutch assembly  1600 , which is constructed in accordance with the present inventions. Like the clutch assembly  1300  described above, the clutch assembly  1600  includes a driven member  1602  and a driver member  1604  that are conditionally affixed to each other, wherein the clutching function of the clutch assembly  1600  is magnetically actuated. The clutch assembly  1600  differs from the clutch assembly  1300 , however, in that the driver member  1604  houses the driven member  1602 , rather than vice versa. 
     Specifically, the driven member  1602  includes a generally cylindrical rigid member  1608 , which is constructed similarly to the above-described cylindrical member  1308  (see FIG.  20 ), with the exception that the cylindrical member  1608  includes a proximally facing boss  1612 , rather than a receptacle  1312 . The driver member  1604  includes a generally cylindrical rigid member  1614 , which is constructed similarly to the above-described cylindrical member  1314  (see FIG.  20 ), with the exception that the cylindrical member  1614  includes a distally facing receptacle  1620  having a cavity  1622  formed therein, rather than a transitional shaft  1320 . 
     The boss  1612  is disposed within the cavity  1622  of the receptacle  1620 . The receptacle  1620  is composed of a ferrous material, and includes four inwardly extending permanent magnets  1628 , which are circumferentially disposed around the cavity  1622 , and are affixed to the receptacle  1620  by suitable means, e.g., bonding. The boss  1612  is composed of a ferrous material, and includes four outwardly extending ferrous elements  1630  and four inwardly extending ferrous arcs  1632 , which are circumferentially disposed around the boss  1612 . In the embodiment illustrated in FIG. 30, the ferrous elements  1630  and arcs  1632  are formed from the deformed outer surface of the boss  1612 . In an alternative embodiment illustrated in FIG. 31, the ferrous elements  1630  and arcs  1632  are formed from four curvilinear flanges. 
     The four ferrous elements  1630  are located inwardly adjacent the four magnets  1628 , respectively, such that a magnetically engaging relationship is formed between the magnets  1628  and ferrous elements  1630 . As can be seen, the receptacle  1620 , by virtue of its ferrous composition, advantageously provides a magnetic return (indicated by arrows) between the outward poles of adjacent magnets  1628 , and the boss  1612 , by virtue of its ferrous composition, provides a magnetic return (indicated by arrows) between the inward poles of adjacent magnets  1628 . The inwardly extending arcs  1632  facilitate the magnetically engaging relationship between the magnets  1628  and ferrous elements  1630 , by concentrating the magnetic force at the ferrous elements  1630 . 
     The operation of the clutch assembly  1600  is identical to that of the clutch assembly  1300 , with the exception that the magnets  1628  of the driven member  1602  and the ferrous elements  1630  of the driver member  1604  magnetically interact with each other, rather than the magnets  1328  of the driver member  1304  and the ferrous elements  1330  of the driven member  1302 . 
     With regard to any of the above-described clutch assemblies, the critical magnitude of the applied torque T, i.e., the point at which the driven member and driver member are rotatably uncoupled from each other, can be selected by “tuning” these clutch assemblies, i.e., altering the materials from which the elements are composed, altering the size of or spatial relationship between the elements, etc. To ensure proper clutching action, a simple fixture with a built-in torque watch can be used to apply a measured torque to these clutch assemblies, whereby the critical magnitude of the applied torque can be determined and compared against an optimum critical magnitude. 
     While preferred embodiments have been shown and described, it will be apparent to one of ordinary skill in the art that numerous alterations may be made without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited except in accordance with the following claims.