Abstract:
A scanning optical head for a catheter is locally controlled by a motor at an insertion end of the catheter uses a hollow motor through which a longitudinal optical path of the catheter passes. This permits the motor to be positioned between a control base of the catheter and avoids rotating the whole fiber, and therefore makes the beam scanning stable and accurate. In addition, because there is no coupling component, it also eliminates the light reflection between additional surfaces as well as varying fiber birefringence, which becomes a cause of noise when imaging the deep structure.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to optical catheters, and, in particular, to an optical catheter with a motorized rotary optical cap for 360° azimuthal scanning of the beam without occlusion of the beam, the catheter design being scalable to outer diameter sizes smaller than 2 mm. 
     BACKGROUND OF THE INVENTION 
     Optical catheters are increasingly used for a wide variety of optical diagnostic procedures and interventions. Accessing tissues via blood vessels, or other orifices, as opposed to by open surgery, reduces damage to the body, facilitating recovery. Generally the finer the catheter, the smaller the blood vessel it can enter. The present invention is particularly suited to providing catheters having small diameters and requiring reliable rotation. 
     A reliable rotary fibre scanner is a critical part of optical coherence tomography (OCT) systems employed for medical imaging and diagnosis, particularly, to intravascular and cardiovascular applications. In particular up to 70% of heart attacks are thought to be caused by vulnerable plaque in arterial walls. OCT systems with rotary fibres may provide a useful tool to image and analyze the plaques. 
     Most legacy technologies use a rotary motor at a base end of the fibre outside the body, and rotate the whole fibre to perform the circular scan (e.g. U.S. Pat. No. 6,445,939). As the fibre have lengths around 1.5 m, and the fibers are bent and twisted, and subject to different forces in use, the rotation of the base end does not necessarily correspond to equal angular changes at the inserted end of the catheter, and neither stable nor uniform rotary actuation is provided. In application, the rotation at the inserted end of the fibre suffers from non-uniform movements, which directly results in scanning errors during the measurement (non-uniform rotational distortion). Because those errors appear to be random, it is almost impossible to calibrate and compensate them by post-processing. As scanning techniques require higher precision in terms of positioning and momentum of the inserted end, this technique becomes increasingly problematic. Diameters of typically used optical fibre are ˜150 μm, it is very difficult to make a part to rotate the laser beam emitted from optical fibre. Furthermore a coupling component which passes the light from a non-rotating fibre to a rotating fibre at the base end has been known to pose problems with losses. Time-varying fibre birefringence may also be a problem with this type of scanning technique. This is a known problem in the art, and various solutions have been proposed. For example, U.S. Pat. No. 6,891,984 teaches provision of a viscous damping fluid located within the sheath to provide drag. 
     A second technique involves placing the motor at a distal end of the catheter (e.g. “Micromotor endoscope catheter for in vivo, ultrahigh-resolution optical coherence tomography” Herz et al. Optics Letters v. 29, No. 9, Oct. 1, 2004, “In vivo three-dimensional microelectromechanical endoscope swept source optical coherence tomography” Su et al. Optics Express v. 15, No. 16, 10390, Aug. 6, 2007, and “Endoscopic optical coherence tomography system” Xiadong et al. Proc. of SPIE v. 6357, 63574B, 2006). This has disadvantages that power and control wires occlude the sensor over part of the radial scan of the device. 
     Concentric drive endoscopes are also known (“A concentric three element radial scanning optical coherence tomography endoscope” Bonnema et al. J. Biophoton., 2, No. 6-7, 353-356, 2009), but a concentric drive brings with it severe constrains on the flexibility of the endoscope, and is not suitable for passage through winding blood vessels. 
     Finally MEMS devices with limited rotation have been designed that avoid the above problems, and place a motor near the distal output of the beam, but between the beam exit and the base end (e.g. “MEMS based non-rotatory circumferential scanning optical probe for endoscopic optical coherence tomography” Xu et al. Proc. of SPIE-OSA, v. 6627, 662715-1, 2007; “New endoscope sees what lies beneath” MIT Tech, Rev., Dec. 3. 2009). However such devices introduce several other difficulties, including the diameter, the complexity of the device, and increased optical insertion loss from multiple reflections, the complexity and cost of forming the device, etc. 
     As power supplies and electromagnetic actuation are all highly problematic, there are limited possibilities for actuation of catheters. 
     Accordingly there is a need for a catheter having a motorized rotational control at the insertion end, for higher control, but avoids the problems of wires crossing the scanning beam, and provides a reliable, relatively low cost fabrication, assembly, and use. 
     SUMMARY OF THE INVENTION 
     Applicant has conceived and tested a motor drive system for a catheter that consists of a hollow motor that can surround the fibre (or other light path) that extends longitudinally through the catheter. 
     In accordance with the present invention a catheter is provided, the catheter comprising: an electromagnetic path passing longitudinally through a body of the catheter from a base end to an insertion end; and a motor encircling the longitudinal path at the control end, surrounding the longitudinal path. The motor is anchored to the catheter and has a rotor extending away from the control end, in a direction of the insertion end. The rotor is coupled to an electromagnetic path element that caps the longitudinal path, to redirect a beam between the longitudinal path and a substantially radial direction, in at least one mode of operation. 
     The path element may be coupled to the catheter via the rotor. The motor may comprise a plurality of actuable piezoelectric elements secured to the anchoring in a ring around the longitudinal path for selectively engaging the rotor, which is in the form of a cylinder concentrically mounted about the ring, or may drive the rotor in a circular or helical motion. The motor may be a squiggle motor. 
     The electromagnetic path may comprise an optical fibre; such as an optical fiber that extends through the catheter, through the motor, and at least partly into the rotor, and preferably completely through the rotor. The optical fibre may have a lensed tip, and the tip may have a higher OD than a remainder of the optical fibre. The tip may be a colimating lens or a ball lens. The optical fiber may have a protective casing. The optical fiber may be rotatably mounted to the catheter, and may be rotatably mounted to the catheter but retained against longitudinal movement with respect to the catheter, for example at the insertion end, preferably at an inner through bore of the rotor. The optical fibre may be unrestrained with respect to rotation or longitudinal movement of the catheter at the insertion end. 
     The electromagnetic path element may comprise a deflector, consisting of a reflective, diffractive, or refractive surface. The deflector may consisting of a dichroic mirror for deflecting only some modes along the electromagnetic path. The electromagnetic path element may further include one or more lenses. The deflector may be mounted rigidly to a cap that is affixed to the rotor; or to an optical fibre that provides the longitudinal path. 
     Further features of the invention will be described or will become apparent in the course of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a catheter in accordance with an embodiment of the invention, having a rotor extending concentrically away from a base of the catheter upon which an electromagnetic path element may be rotationally mounted; 
         FIGS. 2   a,b,c  are schematic illustrations of three electromagnetic path elements for coupling to the catheter of  FIG. 1 ; 
         FIG. 3  is an image of a commercially available squiggle motor; and 
         FIGS. 4   a,b  are images of a prototype catheter with and without an end cap. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows two schematic illustrations of a catheter: on top a perspective drawing, and below a cross-sectional view. The catheter comprises a covering  10  which is secured to motor  12  for driving a pin threaded shaft  14 , which is accordingly a rotor. It will be noted that an internal structure of the motor  12  is not shown in the cross-sectional drawing, but that independently controlled pairs of parallel piezoelectric actuators that selectively threadedly engage the pin thread on the shaft  14  may be used, for example as in a squiggle motor. In the illustrated embodiment, motor  12  and covering  10  are coupled by an intermediate part  15 , which provides a lip  16  for sealed connection to the covering  10  at a proximal end, and a cavity  17  at the distal end for receiving the motor  12 , the motor  12 , and covering  10  are glued to the intermediate part  15  at respective ends. The outer surface of motor  12  has a cross-section that is substantially square, with rounded edges, whereas the outer surfaces of the intermediate part  15 , and cover  10  (unflexed) are substantially cylindrical, thus a rotational base of support is provided against which the motor  12  acts. This rotational support relies on the covering  10  to have sufficient inertia, even when bent and torsioned, to resist the forces exerted on the shaft  14 . 
     The motor  12  and shaft  14 , intermediate part  15 , and covering  10  are all hollow, collectively forming throughbore concentric with a longitudinal axis  18 . In the embodiment of  FIG. 1 , leads for power supply and control of motor  12  pass from the outer surface into the throughbore at the intermediate part  15 , which throughbore the leads  19  follow to the base of the catheter (not in view). However, it will be appreciated that the leads  19  could be held to an outer surface of the covering  10 , or enter the throughbore at any other position. For example a hole can be bored or otherwise provided through the intermediate part  15 , or covering  10  at any point for this purpose. 
     The catheter shown in  FIG. 1  has a throughbore that permits definition of an optical path concentric with the longitudinal axis  18  in a number of ways. The shaft  14  is adapted to be driven by motor  12  such that the shaft  14  describes a helical path, which can be decomposed into circular motion (azimuthal direction) and translational motion (in the direction of longitudinal axis  18 ). A pitch of the pin thread of the shaft  14  is chosen for adequate motor  12  performance and the manner in which actuable elements of the motor  12  selectively engage the pin thread. Another consideration is a desired amount of translational motion. This may depend on an intended scanning process. For example, if it is desirable to image a particular region (e.g., using Optical Coherence Tomography, Raman spectroscopy, or non-linear optical techniques such as CARS, SERS, SHG, THG, etc.) and to provide a high spatial accuracy map of a lumen in which the catheter is inserted, and if the translational motion of the catheter at the base end is not as accurate as the motor&#39;s motion (e.g. if the catheter follows a torturous path), it may be preferable to stabilize the catheter at a single point, and then scan by the continued action of the motor. It may be preferable to stabilize the catheter near the insertion end, for example, by partial inflation of a balloon (or other stabilizer if blocking the lumen is not desired), as may be provided elsewhere on the catheter. In such a case, a pitch p of the pin thread, (i.e. the translational distance per 360° rotation) can be chosen to correspond with a resolution r of the desired image in the longitudinal direction. For example, if n scans are to be averaged to produce the image at each pixel, p=r/n would be a natural choice. If p is much less than the desired longitudinal resolution, scanning of different depths may be performed during respective rotations within a given longitudinal resolution. The higher p is, the slower the actuation, and/or the smaller a resolution in the azimuthal direction. The longer shaft  14  extends (d), the greater the number (d/r) of longitudinal resolution pixel provided on the map and having the higher spatial accuracy. The shaft  14  may retract until mechanically stopped by a stopper (on the threading or at the surface), or as controlled by the motor  12  to minimize the extent of the catheter that is inflexible, during insertion or retraction of the catheter. The length of the shaft  14  may be constrained by limits to flexibility of the catheter given its outer diameter, or the angular inertia which may exceed the rotational support base provided by the covering  10 . Alternatively, motion at the catheter base may be used to control the transverse direction. 
     It will be appreciated that the pin thread on the shaft  14  may be effectively used to secure an end cap for the catheter onto the shaft  14 . As the shaft  14  has an outer diameter less than that of the motor  12 , tips shown in  FIGS. 2   a,b  do not enlarge the catheter. Nonetheless a wide variety of tips can be used to rigidly secure end caps of different configurations to the shaft  14 . While the tips shown use a mirrored surface for reflecting the beam, it will be appreciated that refraction or grating-based surfaces may alternatively be used to form a light path element to redirect light between the longitudinal axis and a direction substantially perpendicular thereto. These may further include actuable members, for example, to permit selective engagement of a mirror for the end cap. 
       FIG. 2   a  schematically illustrates an embodiment of a tip  20  threadedly coupled to shaft  14  of  FIG. 1 . The tip  20  shown consists of a nut body  22 , box threaded  24  to match pin threading of the shaft  14 , with a throughbore concentric with the longitudinal axis  18 , and a mirror  24 , preferably a first surface mirror, rigidly mounted to the nut body  22  for redirecting light between a radial direction and the longitudinal direction. Mirror  24  may be provided in any number of forms, such as: glass ball cleaving, semiconductor etching, and metal plate punching, and commercially available products, such as a micro-prism made by Tower Optical Corp. (Boynton Beach, Fla.). While the mirror  24  is shown mounted along a single side of the nut body  22 , it will be appreciated that the mirror  24  could be mounted to the shaft  14  at three sides as only a single side of the tip  20  provides a window for a beam. Naturally the mirror  24  could be replaced with a refractive-based or diffraction-based optical path element. The mirror  24  may be dichroic and the substrate may be transparent, permitting the same optical path to be used at different wavelengths for guidance of the catheter (the beam continuing through the mirror  24  at certain wavelengths substantially along the longitudinal axis  18 ), and for radial scanning, at wavelengths for which the mirror  24  is reflective to a high degree. 
     The throughbore of the catheter houses an optical path, which is in the form of a fibre  25 , that is clad in a protective steel sheath  26 , although it could be otherwise. At a tip of the fibre  25 , a ball lens  28  provides for a focusing of the light from the optical fibre  25 , and for collecting light reflected from mirror  24 . In the present embodiment, the sheath  26  is rotatably mounted to the nut body  22  and shaft  14  via a washer and spacers which effectively retain the sheath  26  axially to the nut body  22 , allowing some rotation of the sheath  26 . Accordingly, at the end cap, the optical fiber  25  is rotationally free, reducing torsional tension within the fibre. Specifically, the centre washer is solidly connected with sheath  26 . When the tip  20  rotates and moves, the head cap pulls or pushes the sheath  26  forward or backward. The two spacers sandwiching the washer are used to reduce the rotating friction, permitting damped rotation of the sheath  26  and fibre  25  together. The sheath  26  is so closely fitting with the fibre  25  that it resists relative movement in terms of rotation and translation, and thus the fibre  25  is locked in motion with the tip  20 . Thus the fibre  25  is driven by the tip  20  in the present embodiment, at the insertion end of the catheter, and a constant distance is provided between the ball lens  28  and the mirror  24 . 
     The constant distance between the ball lens  28  and mirror  24  ensures a focusing of a beam through the fibre  25  at a constant radial distance from the axis  18 , which is important for stability of the rotationally scanning beam, and the interpretation of the response signal, which may be highly sensitive to phase offset, for example, if interferometric detection is used. 
     It will be appreciated that, variants of this system are possible. The sealing system may be avoided and the fibre  25 , or fibre  25  and sheath  26  together, may be designed to move with little friction within the throughbore of the catheter. The interface between the sheath  26  and nut body  22  and/or the sheath  26  and fibre  25  may be rotational and not translational (telescopically along the longitudinal axis  18 ), rotational and telescoping, or telescopic and non-rotational. Rotational may be preferred if changing torsion of the fibre  25  introduces artifacts in the signal, and the torturous path of the optical fibre lead to interaction of the covering  10  and fibre  25  that results in excessive torsional stresses. Telescoping connection may be preferred if changing a distance between the ball lens  28  and mirror  24  can be used to intentionally set a different scan depth for the sample. Control over a separation of the ball lens  28  and mirror  24  may be provided at the base of the catheter, or by an additional actuator. 
       FIG. 2   b  schematically illustrates another embodiment again having tip  20  with nut body  22 , as described in  FIG. 2   a . Herein features identified by the same reference number denote substantially equivalent elements, and their descriptions are not repeated. The tip  20  of  FIG. 2   b  further comprises a lens  31  (which could alternatively be located before the mirror  24 ) for focusing light from the fibre  25 . Fibre  25 , instead of a ball lens, has a collimating tip  30  for issuing a beam in a substantial ray, which reflects off of the mirror  24 , and is focused by lens  31 . This embodiment makes data much less sensitive to the position of the end of the fibre  25 , and provides focusing/collecting optics as close to the sample as possible. The fibre  25  need not be coupled to the shaft  14 , but may be coupled to the motor  12  along with the covering  10  to improve a rotational support of the motor  12 . Furthermore, as the shaft  14  now drives only the tip  20 , and does not grip or twist the fibre  25 , an inertial load on the motor  12  is substantially decreased. The fibre and sheath may substantially float within the casing, having no retainers for holding any part at any longitudinal location. Furthermore, given that longitudinal displacements may be much more accurate than torsional displacements, a base end of the fibre may be axially operated to change a focus, angle or other mode of operation of the catheter. 
       FIG. 2   c  schematically illustrates a third embodiment wherein the end cap of the longitudinal optical path through the catheter is provided by a snug fitting of the sheath  26  and the shaft  14 . As the shaft  14  is shown having a smallest diameter bore, it will be the only section of the throughbore of the catheter to provide gripping contact with the sheath  26 . This may be a sealed coupling having an O-ring or elastomeric plug at the shaft/sheath and sheath/fibre interfaces. In the illustrated embodiment a reflector  33  is coupled to the ball lens  28 , however the ball lens  28  could be cleaved to provide the reflective surface, or the reflector  33  could equally be affixed to one or more of the sheath  26 , shaft  14 , and fibre  25  (which could extend beyond the tip of the sheath  26 ). Driving the fiber at the insertion end of the catheter shares some disadvantages with the base-driven prior art catheters, in that an unknown resistance to the rotation may be encountered, and torsion on the fibre may affect fibre birefringence. Nonetheless a higher accuracy is provided by controlling the fibre at the insertion end, as opposed to at the base end, and various liquid media can be used to mitigate torsional resistance, especially if rotation of the fibre is limited to a few turns. 
     Other squiggle motors are known in the art that provide rotational (and no axial) motion. Such squiggle motors could alternatively be used. In such cases, it is unnecessary to secure the fibre  25  or sheath  26  to the shaft  14  as required in the embodiment of  FIG. 2   a , as the distance between the fibre  25  and tip  20  are not expected to change in operation. 
     EXAMPLES 
     A prototype device has been produced. A commercially available miniature rotary motor, squiggle motor (outer diameter 1.8 mm, as shown in  FIG. 3 , shaft outer diameter 1.1 mm, thread pitch 0.16 mm) was obtained from New Scale Technologies (Victor, N.Y.).  FIG. 3  is an image of the squiggle motor. Other squiggle motors, including some with smaller shaft OD (e.g., 1.5 mm are commercially available). A threaded shaft of the miniature motor was bored to produce a through hole along its centre axis with an inner diameter of 0.4 mm. This proved difficult, but the details are not provided here, as hollow shafts, even of these dimensions, can be provided in a variety of ways known in the art by forming techniques that are expected to be easier than boring an axial hole through a solid shaft as done in the present instance. 
     A fibre with a protective steel sheath was obtained from (SMF-130V, Prime Optical Fiber Corporation), having an OD of 0.125 mm. The fibre had a ball lens formed thereon by a fusion splicer, FSM-45PM-LDF, Fujikura). A cap as shown in  FIG. 1  was produced to mate with the threaded shaft. The tiny mirror was solidly mounted with the motor&#39;s rotating screw shaft with a 45° reflecting angle using an epoxy. Washers and spacers were produced. The catheter was assembled by placing the spacer and washers around the tip of the sheathed fibre near the ball lens, inserting the assembly through the shaft (which was in place within the motor), until the ball lens extended a short distance from the threaded shaft, and then the cap was placed over the shaft and screwed into place. 
     When driving signal was applied to the leads of the motor, which was held in secured against rotation by a mounting, the threaded screw shaft traced a helical path back and forth. Specifically, as per the specifications of the squiggle motor, a 2.5-3.8 V, (operational power 300 mW) DC power supply supplied to leads resulted in longitudinal motion of about 7 mm/s and azimuthal scanning at a rate of 2400 rpm, although various rates can be obtained depending on the applied voltage. 
     When motor&#39;s threaded shaft and mirror rotates, the light emitted from fibre becomes a rotary side-view scanning beam. This device avoids driving rotating of the fibre from the base end, and therefore makes the beam scanning stable and smooth, as is particularly important for high resolution scanning. In addition, because no coupling component is used, it also avoids the light reflection between surfaces in still-rotation convert devices at the base of the catheter, which is very important to the weak signal back scattered from deep structure.  FIGS. 4   a,b  are images of the operating prototype.  FIG. 4   a  shows a ball lens of an optical fibre exiting the threaded shaft, and in  FIG. 4   b  a cap is shown assembled over the threaded shaft, and providing a mirrored surface for reflecting light between radial and longitudinal (i.e. axial) directions. 
     Other advantages that are inherent to the structure are obvious to one skilled in the art. The embodiments are described herein illustratively and are not meant to limit the scope of the invention as claimed. Variations of the foregoing embodiments will be evident to a person of ordinary skill and are intended by the inventor to be encompassed by the following claims.