Abstract:
A catheter has a proximal end and a distal end and comprises an outer tube having a proximal end, an inner sheath slidingly received within the outer tube and extending distally from the outer tube, and a rotatable shaft extending from the proximal end of the outer tube to within the inner sheath. The rotatable shaft is axially fixed with respect to the outer tube and axially moveable within and with respect to the inner sheath. The rotatable shaft includes a proximal substantially rigid section and a distal flexible section. The catheter further includes a working element carried on the distal flexible section of the rotatable shaft.

Description:
PRIORITY CLAIM 
       [0001]    The present application claims the benefit of copending U.S. Provisional Patent Application Ser. No. 61/008,120, filed Dec. 17, 2007, which application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Intravascular ultrasound (IVUS) catheters provide a means of imaging internal structures in the body. Coronary IVUS catheters are used in the small coronary arteries of the heart to visualize coronary artery disease (CAD). There are other non-IVUS invasive imaging systems and catheters, such as optical coherence tomography OCT and Optical Frequency Domain Imaging (OFDI), that also provide a means of imaging internal structures within the human body. IVUS catheters (see  FIG. 1 ) use transducers  46  to create pressure waves, other imaging systems use different imaging energy sources. Both OCT and OFDI use laser light as the imaging source. 
         [0003]    There are two type of coronary IVUS catheters: mechanically scanned and electronically scanned. Mechanically scanned IVUS catheters mechanically spin the ultrasonic beam to sweep across a region of interest in the body. There are two generally accepted ways to do this. One is to mechanically rotate the transducer ( FIG. 1 ) that generates the ultrasound beam. The other is to mechanically rotate a reflective surface or mirror that directs the beam from a stationary transducer into the desired swept pattern. 
         [0004]    The preferred method is to rotate the transducer for the following reasons: Mirror based systems have additional imaging artifacts in their images as the beam is swept past mechanical supporting structures. Mirror based systems are generally larger then rotating transducer systems. 
         [0005]    Electronically scanned catheters utilize a transducer array that electronically steers the ultrasound beam. In order to maximize the size of the transducer array, electronically scanning IVUS catheters locate the transducer array on the outside of the sheath. Mechanically scanning IVUS catheters locate the transducer on the inside of a sheath. 
         [0006]    Mechanically scanning IVUS catheters  FIG. 1  have two key advantages over electronically scanning IVUS catheters. Mechanically scanning IVUS catheter transducers  46  can operate at higher frequency then electronically scanning transducers. Therefore they have higher resolution. Mechanically scanning imaging catheters operate within an ultrasonically transparent sheath  28 . The sheath prevents rotating components  33  and  34  from coming into contact with the patient&#39;s tissue and causing trauma. In addition, the sheath provides a lumen  49  that facilitates the movement of the imaging element along a portion of the distal length of the imaging catheter. Therefore, with a sheathed mechanical scanning IVUS catheter a volume of image data can be acquired over a region of interest without physically moving the catheter sheath  27  and  28  within the body. 
         [0007]    Mechanically scanning IVUS imaging catheters contain drive cables  33  to “spin” the transducer  46  within the sheath  26  and  28 . Drive cables are currently assembled by winding multiple strands of metal wire on a mandrel to create a long spring containing a repeating series of concentric rings, or windings, of the wire. Two, or more, springs are wound for each drive cable sized one to fit over the other. Adjacent springs are wound in the opposite direction of each other so that the grooves between the windings do not line up and lock together. During assembly, the inner spring is inserted into the outer spring still on its winding mandrel and then released so that it expands into the outer spring. In this way, the drive cable is extremely flexible in order to navigate small tortuous distal coronary anatomy while still providing some degree of torsional rigidity between the proximal driving end and the distal end containing the transducer. 
         [0008]    Proximal housing  25  contains engagement pins  38  (X2) that mechanically mate to the imaging system catheter interface port. Within proximal housing  25  is a connector  30  which couples in mechanical energy to the drive cable  33  and electrical energy into the transmission line  47  within the drive cable. Connector  30  is fixedly connected to drive shaft  31 , such that when rotated by the imaging system, drive shaft  31  is similarly rotated. Internal drive shaft  31  has a smooth bearing surface  37  which provides the running surface for rotational bearing  36  and snap ring  35 . Snap ring  35  is fixedly held in place by the groove in proximal housing  25 . A fluid seal  39  prevents fluids from the lumen  49 , which runs the length of the catheter, from getting into the connector  30 . The distal end of internal drive shaft  31  is connected via solder, brazing, welding or gluing bond joints  32  to the drive cable  33 , such that when drive shaft  31  is rotated, drive cable  33  is similarly rotated. 
         [0009]    Connector  30  within proximal housing  25  contains an electrical interface to couple in rotating electrical energy into the transmission line  47  that is disposed within drive cable  33  and runs its entire length. Transmission line  47  couples transmit energy from the system via connector  30 , through the drive cable  33 , and to the transducer  46  located within the distal housing  34 . The electrical excitation energy causes transducer  46  to generate a pressure wave into the lumen  49  which is filled with saline via flushing port  40 . The ultrasonic energy is coupled via the saline into the ultrasonically transparent portion of the sheath  28  and into the body. Objects in the body having acoustic impedance variations reflect back a portion of the ultrasonic pressure wave which is received by the transducer  46  after passing through sheath  28  and the saline filled lumen  49 . Transducer  46  converts the received pressure signals into electrical signals which are coupled via transmission line  47  back to connector  30  and into the imaging systems&#39; receiver. The system converts a series of scan lines acquired in the polar (R, θ) coordinate system, (similar to a beam from a lighthouse) into a slice or frame of image data by converting the polar scan lines into the Cartesian (X,Y) coordinate system for display on a X-Y scanning monitor, thus completing one rotation of the connector  30 , drive shaft  31 , drive cable  33 , and distal housing  34 . 
         [0010]    In order to move, or translate, the rotating transducer  46  along the distal portion of the length of the lumen  49 , a telescopic section is added at the proximal end of the catheter. The telescopic section contains inner proximal tubular element  26 , outer distal tubular element  50 , and anchor housing  29 . The distal end of inner proximal tubular element  26  contains an end stop  51  to prevent the inner proximal tubular element  26  from disengaging from the outer distal tubular member  50  when the telescope is fully extended. Fluid seal  41 , inside anchor housing  29  prevents fluids from lumen  49  from leaking out via the space between inner proximal tubular element  26  and outer distal tubular element  50 . Groove  52  in anchor housing  29  provides a connection point for motorized (controlled) movement of the distal outer tubular element relative to the proximal housing  25 . 
         [0011]    Due to the flexible nature of the drive cable  33 , the telescope  26 ,  29 , and  50 , and sheath  27  and  28  must provide a running surface to support drive cable  33  when it is rotating. It is also important to note that drive cable  33  is of a fixed length, so that when the outer distal tubular element  50  is translated relative to the inner proximal tubular member  26 , the transducer  46  is translated relative to the distal sheath  28 . In this way, the transducer  46  is moved along the length of the sheath  28  to acquire a volume of image data. 
         [0012]    Current telescopic devices on IVUS catheters have several shortcomings. The current telescopes design is based on a proximal inner tubular member  26  that has an inside diameter sized to accommodate the drive cable  33  and provide sufficient clearance for flushing fluid. Flushing fluid is injected into flushing port  40  to fill lumen  49 , in order to couple ultrasound energy from transducer  46 , through the fluid to sheath  28  and thereby into the patient&#39;s body. The wall thickness and therefore the outer diameter of proximal inner tubular element  26  is sized in order to provide adequate structural integrity to support the forces occurring during the movement of the telescopic section in order to reduce the likelihood of any kinking or collapsing of the inner lumen onto the spinning drive cable  33 . If the inner lumen of inner proximal tubular element compresses and catches drive cable  33  while it is spinning, the electrical connections of transmission line  47  will be severed and the imaging catheter will no longer function. A competing requirement to keep the wall thickness as thin as possible exists in order to reduce the gap between the outside diameter of drive cable  33  and the inside diameter of outer distal tubular member  50 , which will be further elaborated in the description of the current design shortcoming below. 
         [0013]    The outer distal telescopic tubular member  50  is attached at its proximal end to anchor housing  29  which contains fluid seal  41 . Fluid seal  41 , applies pressure to inner proximal tubular member  26 . For this reason, inner proximal tubular member  26  must have a smooth outer surface along its entire length in order to form a fluid seal. The distal end of the outer distal tubular member is bonded via glue  43 , to strain relief  44  and proximal shaft  27 , which is part of the catheter sheath  28 . The inside diameter of the outer distal tubular member  50 , is sized to accommodate the outside diameter of end stop  51  which must be larger than the outside diameter of the inner proximal tubular member  27 . Therefore, the inside diameter of the outer distal tubular member  50  is larger then the outside diameter of the inner proximal tubular member  26 . This creates a significant gap between the outside diameter of drive cable  33  compared to the inside diameter of the outer proximal tubular member  50 . 
         [0014]    This gap is a major deficiency of the current design. When the telescope is fully extended, the transducer is in its most proximal location within sheath  28 . Since the lumen  49  is filled with saline, the distal housing  34  and drive cable  33 , must displace this fluid as the telescope is retracted and the transducer  46  is advanced into the sheath  28 . This creates a backward force on drive cable  33 . Due to the gap between drive cable  33  and the outer proximal tubular member  50  and the flexible nature of drive cable  33 , drive cable  33  is compressed into an “S” curve as shown in  FIG. 2 . This “S” curve pulls the location of transducer  46  inward, thereby scanning the incorrect region of the anatomy and often leads to the drive cable  33  folding back over onto itself. When the drive cable  33  folds back over onto itself, the electrical connections of transmission line  47  are severed and the imaging catheter is rendered inoperative. Approximately 1% of all IVUS catheters used are returned as defected units as a result of this failure mechanism. 
         [0015]    Another short coming of existing telescope designs is that the telescope is not straight, which makes it difficult to extend and retract the telescope. This occurs because the telescope is made of polymers for cost reasons, and the inner proximal tubular member&#39;s  26  wall thickness is kept thin to keep the outside diameter of the outer telescope member as small as possible. The resultant thin wall polymer is then coiled into its packaging and during sterilization and normal shelf aging, the polymer takes a set in the coiled (non-straight) position. 
         [0016]    Another short coming of existing telescopes is the fluid seal  41  and inner proximal tubular member  26  outer running surface. The fluid seal must prevent saline from escaping during catheter flushing operations. This fluid seal is subject to pressures up 150 PSI. Current telescopic sections are made from polymers that do not have a smooth running surface for the fluid seal to slide against during telescopic action. As a result, the pressure on the fluid seal is increased to insure the seal holds against the flushing pressure. This in turn increases the friction that must be overcome when the telescope is extended or retracted. As a result, the existing telescope design is difficult to extend and retract. Another failure mechanism occurs when the user forces the outer distal tubular member  50  downward onto the inner proximal tubular member  26  which is not straight and the inner tubular member is kinked. This results in either a failed electrical connection or a mechanical defect in the drive cable which manifests itself in a non-uniformed rotation of the transducer and the associated image artifact. 
         [0017]    Another short coming of existing telescope design is the cost. The current design contains three separate tubular members which need to be individually bonded to form the telescope and this adds unnecessary cost to the assembly. The three components are the inner proximal tubular member  26 , the outer distal tubular member  50  and the proximal shaft  27  of the catheter which is bonded to the distal end of the outer proximal tubular telescopic member. 
         [0018]    A short coming of the existing drive cable  33  design is that it is flexible its entire length. This results in several shortcomings. The drive cable  33  can fail by folding back on itself as described above. The drive cable can fold back in the above S shape, while not failing, it pulls back the transducer  46  proximally so that it is pointing at a more proximal region of the artery then it should. This results in errors in length measurements on the system which is not aware of the fold hack condition. The flexible drive cable lacks torsional stiffness which can results in erratic rotational velocity of the imaging element. Erratic rotational velocity of the imaging element produces distortions in the image. 
       SUMMARY 
       [0019]    According to one embodiment, a catheter has a proximal end and a distal end and comprises an outer tube having a proximal end, an inner sheath slidingly received within the outer tube and extending distally from the outer tube, and a rotatable shaft extending from the proximal end of the outer tube to within the inner sheath. The rotatable shaft is axially fixed with respect to the outer tube and is axially moveable within and with respect to the inner sheath. The rotatable shaft includes a proximal substantially rigid section and a distal flexible section. The catheter further includes a working element carried on the distal flexible section of the rotatable shaft. 
         [0020]    The proximal rigid section of the rotatable shaft is comprised of a biocompatible material such as, for example, stainless steel. 
         [0021]    The outer tube may have a distal end sealingly engaged with the inner sheath. The rigid section of the rotatable shaft and the flexible section of the rotatable shaft may be joined at a joint substantially adjacent the distal end of the outer tube, 
         [0022]    In another embodiment, the inner sheath has a proximal portion and the catheter further includes a substantially rigid cover extending over the proximal portion of the inner sheath. 
         [0023]    The substantially rigid cover is preferably formed of a biocompatible material such as, for example, stainless steel. 
         [0024]    According to a further aspect of the invention, the catheter may further include a seal that provides a fluid seal between the inner sheath and the outer tube. The outer tube has a distal end and the seal may be at the distal end of the outer tube. 
         [0025]    The working element may be an optical element or a transducer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The features of the present invention which are believed to be novel are set forth with particularity herein. The invention, together with further features and advantages thereof, may best be understood by making reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify identical elements, and wherein: 
           [0027]      FIG. 1  is a sectional side view of a prior art intravascular catheter of the type which may be improved upon by the present invention; 
           [0028]      FIG. 2  is another sectional side view of the prior art intravascular catheter of  FIG. 1 ; 
           [0029]      FIG. 3  is a sectional view of an intravascular catheter embodying the present invention shown in a partially extended configuration; 
           [0030]      FIG. 4  is a sectional view of the intravascular catheter of  FIG. 3  shown in a fully extended configuration; 
           [0031]      FIG. 5  is a sectional view of the intravascular catheter of  FIG. 3  shown in a fully retracted configuration; and 
           [0032]      FIG. 6  is a sectional view of another intravascular catheter embodying the present invention that includes a hypo-tube fluid seal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    As will be seen from the foregoing, the embodiments of the present invention shown in  FIGS. 3-6  include improvements to both the drive cable  7 ,  8  and  9  and the telescope. These improvements make it possible, without limitation, to eliminate failure mechanisms associated with collapsing of the telescope lumen onto the drive cable, to eliminate drive cable failures resulting from drive cable fold back in the telescoping section, to reduce friction within the telescope to improve its operation and to eliminate unnecessary components in the telescope section. 
         [0034]    The drive cable  100 , according to these embodiments of the present invention, contains both a proximal rigid section  7  and a distal flexible section  9 . The rigid section is constructed of a stainless steel or other suitable material hypo tube that is welded or in some other way bonded to the flexible drive cable  9  that is similar to those used in current mechanically scanning imaging catheter designs. The rigid section  7  is welded or in some other fashion bonded  16  to the drive shaft  14  which is fixedly connected to connector  6 . The length of the rigid section  7  of the drive cable  100  is such that the bond joint  8  to the flexible section of the drive cable  9  is distal to, at or near the distal end of the outer proximal telescoping tubular member  2 . This bond location is intended to insure that a substantial portion of the flexible section  9  of the drive cable  7 ,  8  and  9 , does not enter into the telescoping section of the catheter. 
         [0035]    In operation according to various aspects of the present invention, the proximal housing  1  contains engagement pins  17  that mechanically mate to the imaging system catheter interface port. Within proximal housing  1  is a connector  6  which couples in mechanical energy to the drive cable  100  and electrical energy into the transmission line  23  within the drive cable. Connector  6  is fixedly connected to drive shaft  14 , such that when rotated by the imaging system, drive shaft  14  is similarly rotated. Internal drive shaft  14  has a smooth bearing surface  13  which provides the running surface for rotational bearing  12  and snap ring  11 . Snap ring  11  is fixedly held in place by the groove in proximal housing  1 . A fluid seal  15  prevents fluids from the lumen  24 , which runs the length of the catheter, from getting into the connector  6 . The distal end of drive shaft  14  is connected via solder, brazing, welding or gluing bond joints  16  to the drive shaft  100 , such that when drive shaft  14  is rotated, drive cable  100  is similarly rotated. The drive cable  100  carries at its distal end a working element. Here the working element is an ultrasonic transducer  22 . As may be appreciated, the working element could alternatively be an optical mirror or an optical lens, depending on the intended use of the catheter. If the working element is an optical element such as a mirror or lens, the transmission line  23  would then be replaced by an optical fiber, for example. 
         [0036]    Connector  6  within proximal housing  1  contains an electrical interface to couple in rotating electrical energy into the transmission line  23  that is disposed within drive cable  100  and runs its entire length. Transmission line  23  couples transmit energy from the system via connector  6 , through the drive cable  100 , and to the transducer  22  located within the distal housing  10 . The electrical excitation energy causes transducer  22  to generate a pressure wave into the lumen  24  which is filled with saline via flushing port  25 . The ultrasonic energy is coupled via the saline into the ultrasonically transparent portion of the sheath  4  and into the body. Objects in the body having acoustic impedance variations reflect back a portion of the ultrasonic pressure wave which is received by the transducer  22  after passing through sheath  4  and the saline filled lumen  24 . Transducer  22  converts the received pressure signals into electrical signals which are coupled via transmission line  23  back to connector  6  and into the imaging systems&#39; receiver. The system converts a series of scan lines acquired in the polar (R, θ) coordinate system, (similar to a beam from a lighthouse) into a slice or frame of image data by converting the polar scan lines into the Cartesian (X,Y) coordinate system for display on a X-Y scanning monitor, thus completing one rotation of the connector  6 , drive shaft  14 , drive cable  7 ,  8  and  9 , and distal housing  10 . 
         [0037]    In order to move, or translate, the rotating transducer  22  along the distal portion of the length of the lumen  24 , a telescopic section is added at the proximal end of the catheter. The telescopic section contains outer proximal tubular member  2 , end cap  5 , and the proximal sheath  3  which slides into the outer proximal tubular member  2 . The proximal end of proximal sheath  3  contains an end stop  18  to prevent the proximal sheath  3  from disengaging the outer proximal shaft  2  when the telescope is fully extended. Fluid seal  19 , is located inside end cap  5  and prevents fluids from lumen  24  from leaking out via the space between outer proximal tubular element  2  and the proximal sheath  3 . Strain relief  21  contains groove  20  which provides the connection point for motorized (controlled) movement of the telescoping section. As is well known, the sheath  4 , including proximal sheath  3  is formed of a biocompatible flexible material. 
         [0038]    Due to the rigid nature of the rigid section  7  of the drive cable  100 , a running surface is not required along the entire length of this section of the drive cable as is required in the prior art. Therefore the end stop  18 , bearing  12  and lock washer  11  provide the required running surfaces along this segment of the drive cable. As can be seen in  FIG. 4 , when the telescope is fully extended the two running surfaces for the rigid segment  7  of the drive cable  100  are at either ends of rigid segment  7 . When the telescope is fully retracted, the two running surfaces are at the proximal end of the rigid section  7  of drive cable  100 . It is important to note that drive cable  100  is of a fixed length, so that when the proximal sheath  3  is translated relative to the outer proximal tubular member  2 , the transducer  22  is translated relative to the distal sheath  4 . In this way, the transducer  2  is moved along a portion of the length of the sheath  4  to acquire a volume of image data. 
         [0039]    By virtue of the present invention, as described in the context of the illustrated embodiments, the outer proximal tubular member  2  wall thickness is no longer constrained by the need to slide into another segment of the telescope. Therefore the wall thickness can be increased to insure that the inner lumen can not collapse onto the spinning drive cable  100  and cause either image distortions or catheter failure. In addition, the thick wall of the outer proximal tubular member  2  will insure that the telescoping section remains straight at all times. This will improve the sliding action of the telescope. Converting the proximal most section of the telescope from an inner member to an outer member eliminates the need for an outer distal tubular member thereby reducing the part count and number of bond joint required to assembly the telescope section. 
         [0040]    The rigid section  7  of drive cable  100 , prevents the occurrence of the “S” curve in the drive cable when the drive cable is advance. This prevents the possibility of drive cable fold back and subsequent failure of the electrical connection. This improvement is expected to reduce field failure rates of mechanically scanning invasive imaging catheters significantly. 
         [0041]    Optionally, a stainless steel hypo tube  45  or other suitable rigid material can be placed over the proximal sheath  3 . This provides a smooth running surface for fluid seal  19 . 
         [0042]    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.