Abstract:
A guidewire with optical sensing capabilities is based on a multiplexed optical coherence domain reflectometer (OCDR), which allows it to sense location, thickness, and structure of the arterial walls or other intra-cavity regions as it travels through the body during minimally invasive medical procedures. This information will be used both to direct the guidewire through the body by detecting vascular junctions and to evaluate the nearby tissue. The guidewire contains multiple optical fibers which couple light from the proximal to distal end. Light from the fibers at the distal end of the guidewire is directed onto interior cavity walls via small diameter optics such as gradient index lenses and mirrored corner cubes. Both forward viewing and side viewing fibers can be included. The light reflected or scattered from the cavity walls is then collected by the fibers, which are multiplexed at the proximal end to the sample arm of an optical low coherence reflectometer. The guidewire can also be used in nonmedical applications.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to guidewires for medical procedures and more particularly to guidewires with optical sensing capabilities. 
     Optical coherence domain reflectometry (OCDR) is a technique developed by Youngquist et al. in 1987 (Youngquist, R. C. et al., “Optical Coherence-Domain Reflectometry: A New Optical Evaluation Technique,” 1987, Optics Letters 12(3):158-160). Danielson et al. (Danielson, B. L. et al., “Guided-Wave Reflectometry with Micrometer Resolution,” 1987, Applied Physics 26(14): 2836-2842) also describe an optical reflectometer which uses a scanning Michelson interferometer in conjunction with a broadband illuminating source and cross-correlation detection. OCDR was first applied to the diagnosis of biological tissue by Clivaz et al. in January 1992 (Clivaz, X. et al., “High-Resolution Reflectometry in Biological Tissues,” 1992, Optics Letters 17(1):4-6). A similar technique, optical coherence tomography (OCT), has been developed and used for imaging with catheters by Swanson et al. in 1994 (Swanson, E. A. et al., U.S. Pat. Nos. 5,321,501 and 5,459,570). Tearney et al. (Tearney, G. J. et al., “Scanning Single-Mode Fiber Optic Catheter-Endoscope for Optical Coherence Tomograph,” 1996, Optics Letters 21(7):543-545) also describe an OCT system in which a beam is scanned in a circumferential pattern to produce an image of internal organs. U.S. Pat. No. 5,570,182 to Nathel et al. describes method and apparatus for detection of dental caries and periodontal disease using OCT. However, as OCT systems rely on mechanical scanning arms, miniaturizing them enough to operate on a guidewire would be very difficult. 
     Polarization effects in an OCDR system for birefringence characterization have been described by Hee et al. (Hee, M. R. et al., “Polarization-sensitive low-coherence reflectometer for birefringence characterization and ranging,” J. Opt. Soc. Am. B, Vol. 9, No. 6, June 1992, 903-908) and in an OCT system by Everett et al. (Everett, M. J. et al., “Birefringence characterization of biological tissue by use of optical coherence tomography,” Optics Letters, Vol. 23, No. 3, Feb. 1, 1998, 228-230). 
     In a prior art OCDR scanning system  10 , shown in FIG. 1, light from a low coherence source  12  is input into a 2×2 fiber optic coupler  14 , where the light is split and directed into sample arm  16  and reference arm  18 . An optical fiber  20  is connected to the sample arm  16  and extends into a device  22 , which scans an object  24 . Reference arm  18  provides a variable optical delay. Light input into reference arm  18  is reflected back by reference mirror  26 . A piezoelectric modulator  28  may be included in reference arm  18  with a fixed mirror  26 , or modulator  28  may be eliminated by scanning mirror  26  in the Z-direction. The reflected reference beam from reference arm  18  and a reflected sample beam from sample arm  16  pass back through coupler  14  to detector  30  (including processing electronics), which processes the signals by techniques that are well known in the art to produce backscatter profile (or “image”) on display  32 . 
     SUMMARY OF THE INVENTION 
     This invention is a guidewire with sensing capabilities based on a multiplexed optical coherence domain reflectometer (OCDR), which allow it to sense location, thickness, and structure of the arterial walls or other intra-cavity regions as it travels through the body during minimally invasive medical procedures. This information will be used both to direct the guidewire through the body by detecting vascular junctions and to evaluate the nearby tissue. The guidewire contains multiple optical fibers which couple light from the proximal to distal end. Light from the fibers at the distal end of the guidewire is directed onto interior cavity walls via small diameter optics, such as gradient index lenses and mirrored corner cubes. The light reflected or scattered from the cavity walls is then collected by the fibers which are multiplexed at the proximal end to the sample arm of an optical low coherence reflectometer. The resulting data, collected sequentially from the multiple fibers, provides information about branching of arteries necessary for guiding the guidewire through the arterial system. It also can be used to locate small structural abnormalities in the arterial or cavity wall (such as aneurysms or arteriovenous malformations) that are currently not resolvable by existing techniques. The guidewire can also be used in nonmedical applications. 
     By multiplexing between a number of sensor fibers with an optical switch, the OCDR system of the invention has multiple sequentially accessed sensor points consisting of the tip of each multiplexed fiber. These sensor points measure the scattering of light as a function of distance from the fiber tip, thus determining both the distance between the fiber tip and the nearest tissue and any structure in that tissue. For guiding purposes, a number of these fibers are embedded in the guidewire with their tips at the distal end of the guidewire. Miniature collimating and reflection optics deflect the light from the fiber tips toward the vascular walls, thus sensing any branching of the vasculature or abnormalities in the walls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art OCDR scanning system. 
     FIG. 2A is a schematic diagram of an OCDR guidewire optical sensing system with multiplexed sample arm. 
     FIG. 2B is a schematic diagram of an OCDR guidewire optical sensing system with a multiplexed sample arm and optical circulator. 
     FIG. 2C is a schematic diagram of an OCDR guidewire optical sensing system with multiplexed sample arm using polarized light. 
     FIGS. 3A, B are side and top views of a rotating helix reference mirror. 
     FIGS. 4A, B are sectional and side views of an OCDR optical sensing guidewire. 
     FIGS. 5A, B are displays generated by the guidewire sensing system. 
     FIG. 6 shows a guidewire with OCDR scanning fibers at various positions along its length. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention uses a multiplexed optical coherence domain reflectometer in a guidewire as an aid in guiding the wire during minimally invasive medical procedures or in nonmedical in situ probes. 
     The guidewire optical sensing system  40  is illustrated in FIG.  2 A. The device is based on an optical coherence domain reflectometer (OCDR) which has been multiplexed. Except for the multiplexed feature, the system is similar to the prior art system  10  of FIG.  1 . Output from a low coherence light source  12  is split at the 2×2 fiber optic coupler  14  and directed through a multiplexed sample arm  42  toward the sample  24  and through a reference arm  18  to reference mirror  26 . Reflections from the mirror  26  and backscattered light from the sample  24  are recombined at the coupler  14  and propagated to the detector  30  (and light source  12 ). Constructive interference creates a signal at the detector  30  when the sample and reference reflections have traveled approximately the same optical group delay. The shorter the coherence length of the source, the more closely the sample and reference arm group delays must be matched for constructive interference to occur. By imposing a changing optical delay in the reference arm  18  with a known velocity, either by scanning mirror  26  in the Z-direction or with a piezomodulator  28  (with fixed mirror  26 ), the amplitudes and longitudinal positions of reflections from the sample  24  can be measured with high precision. The sample arm  42  contains a multiplexer  44  for switching between several (e.g., 8) fibers,  20 - 1  . . .  20 - 8 , allowing sequential spatially distinct regions to be diagnosed consecutively using the same basic OCDR system. The fibers can be placed anywhere in the guidewire. Some fibers can be forward viewing and some can be side viewing. 
     An alternate embodiment, guidewire optical sensing system  50 , is shown in FIG.  2 B. Guidewire sensing system  50  is similar to guidewire sensing system  40  of FIG. 2A, except that an optical circulator  52  is added to the system and detector  30  is replaced by balanced detector unit  54 . Balanced detector unit  54  includes a pair of detectors  56 ,  58  with associated processing electronics, and produces a backscatter profile on display  32 . 
     OCDR/OCT systems are based on white light Michelson interferometers, in which light from a source is split via a beamsplitter into two arms, a reference arm and a sample arm. Light is then reflected back to the beamsplitter in both arms. The light returning to the beamsplitter is then split, half returning to the source and the rest going to a detector. The light returning to the source is wasted and can cause the source to lase, reducing the bandwidth of the source. 
     The optical circulator  52  has three ports, as shown in FIG.  2 B. The first port is connected to the output of source  12  and the second port is connected to coupler  14 . Thus light from source  12  passes through optical circulator  52  to coupler  14  and into reference arm  18  and multiplexed sample arm  42 , as before. In system  40  of FIG. 2A, the light returning to coupler  14  from reference and sample arms  18 ,  42  would be split, with some going to detector  30 , where useful information is obtained, and some going back to source  12 . In system  50  of FIG. 2B, some of the light passing back through coupler  14  goes to detector unit  54  and some goes back to the second port of optical circulator  52 . But light returning to the second port of optical circulator  52  cannot pass back through the first port to source  12 . Instead the light passes through the third port to detector unit  54 . 
     Thus putting an optical circulator  52  in the source arm between source  12  and coupler  14  allows the light that would have returned to the source  12  to be sent to another detector. Detector unit  54  contains a pair of balanced detectors  56 ,  58 . Detector  58  receives the light which passes directly from coupler  14  while detector  56  receives the light which passes back through optical circulator  52 . Thus detector unit  54  can utilize all the reflected light. In the balanced detection scheme, the signal on the second detector is subtracted from the first. The signal caused by heterodyning between light in the reference and sample arms is 180 degrees out of phase on the two detectors. 
     The use of optical circulator  52  provides three benefits: (1) it protects source  12  from optical back reflections which can cause it to lase; (2) it allows detector unit  54  to collect twice as much light, enhancing system sensitivity; (3) balanced detection is achieved by subtracting the signal on one detector from the other which eliminates source or ring noise as fluctuations in source intensity appear equally on both detectors and thus cancel when the two signals are subtracted. 
     Another embodiment, guidewire optical sensing system  49 , is shown in FIG.  2 C. Guidewire sensing system  49  is similar to guidewire sensing system  40  of FIG. 2A, except that the polarization of the light through the system is controlled by polarization maintaining (PM) fibers and optics. Mismatches between the polarization states of the light returning from the reference and sample arms  42 ,  18  in system  40  causes reduction in the coherent interference between light from the two arms and thus losses of signal. Control of the polarization state of the light in the system can both eliminate losses in signal due to depolarization of the light and provide the additional capability of measurement of the birefringence of the sample  24 . In this embodiment, linearly polarized light is introduced into the system either through use of a linearly polarized broadband light source  12  or by placing linear polarizer  51  directly after an unpolarized source  12 . The linear polarization of the light is then maintained through the use of PM fibers and a PM fiber optic coupler  14  where the linear polarization is one of the two modes of the PM fiber and PM coupler  14 . The polarization state of the light returning from the reference arm  18  is modified by either a waveplate or faraday rotator  53  so as to be equally split between the two modes (orthogonal polarizations) of the PM fiber. A polarization beam splitter  55  in the detector arm splits the two polarizations and directs them to two separate detectors  57 ,  59  of detector unit  54 . In one embodiment, the optical fibers  20 - 1  . . .  20 - 8  in the sample arm  42  are not polarization maintaining. In this case, the polarization beam splitter  55  ensures that the polarization state of the light from the reference and sample arms  42 ,  18  is matched on each detector  57 ,  59 , thus eliminating the losses due to depolarization of the light. The light returning from the sample arm  42  is then measured by summing the signals from the two detectors  57 ,  59 . In another embodiment, the optical fibers  20 - 1  . . .  20 - 8  in the sample arm  42  are polarization maintaining. The fibers  20 - 1  . . .  20 - 8  can be oriented such that the light leaving the fibers is linearly polarized at an angle approximately 45° relative to the fast axis of birefringence of the sample  24 . Alternatively a quarter waveplate  85  (shown in FIG. 4B) can be placed at the distal end of each fiber  2 - 1  . . .  20 - 8  to cause the light entering the sample to be circularly polarized. In either case, the total light in all polarization states returning from the sample  24  is once again determined by summing the signal from the two detectors  57 ,  59 . In addition, detector unit  54  includes means for ratioing the output signals from detectors  57 ,  59 ; the birefringence of the sample  24  is determined based on the arc tangent of the ratio of the signals from the two detectors  57 ,  59 . 
     As previously described, a variable optical delay can be produced in reference arm  18  by scanning reference mirror  26  back and forth in the Z-direction (see FIGS. 1,  2 A-B). However, there are two key issues in varying the axial length of the reference arm: linearity of the axial scan and duty cycle. 
     A rotating helix reference mirror  60 , shown in FIGS. 3A, B, can be used to smoothly vary the path length in the reference arm of the OCDR system. Mirror  60  is formed of a disk  62  with a radius R which varies from R 1  to R 2  over its entire circumference. Lateral edge surface  64  of disk  62  is a highly reflective mirror so that a collimated light beam  66  incident thereon at normal incidence will be reflected back. Collimated light beam  66  is formed by collimating the diverging light from optical fiber  70  which forms the reference arm of the OCDR system. Lens  68  is used to collimate the output of fiber  70 . When the beam is reflected back by surface  64 , lens  68  focuses the light back into fiber  70 . 
     When mirror  60  is positioned so that beam  66  is incident on point  72 , at which the radius R=R 2 , the longest radius, the path length ΔZ between lens  68  and surface  64  is the shortest. As mirror  60  is rotated about shaft  74 , which fits into central opening  76  and is turned by motor  78 , the path length ΔZ increases as R decreases. As mirror  60  completes an entire 360 degree revolution, R=R 1 , the shortest radius, is reached and ΔZ has increased by ΔR=R 2 −R 1 . Beam  66  then returns to point  72  and starts a new cycle. In each cycle, the path length ΔZ changes by AR, or the optical path length change in the reference arm AL changes by 2ΔZ=2ΔR=2(R 2 −R 1 ). Disk  62  can typically be about 2 inches in diameter and 0.2 inches thick, with a ΔR of about 0.2 inches. Thus the optical path length will be varied by about half an inch on each cycle. 
     As shown in FIGS. 4A, B, the guidewire  80  consists of a plurality (e.g., seven) of optical fibers, including peripheral fibers  82  and a central fiber  84  embedded in a guidewire body  86  of x-ray blocking material. The number of fibers in the guidewire is dependent on the limit of the guidewire diameter and requirements for flexibility, the fiber optic diameter, and the desired speed of acquisition. Either single or multiple mode optical fibers can be used. Single mode fibers are preferable for maximizing the longitudinal resolution. However, multimedia fibers can be made smaller, thus minimizing the guidewire&#39;s size and maximizing its flexibility. Average sizes for single mode fibers are on the order of 100 μm diameter. Thus seven fibers, as shown in FIG. 4A, would lead to a guidewire approximately 400 microns in diameter (including 50 microns of x-ray absorbing material around the fibers). Miniature optics  88  (GRIN lenses  90  and mirrored corner cubes  92 , for instance) can be used for collimating and directing the light emerging from the fiber tips of the peripheral fibers onto the adjacent arterial or cavity wall. The optical elements  88  extend through guidewire body  86  to allow light to be transmitted to and received from the surrounding area. The x-ray absorber provides conventional radiographs of the guidewire position while the central optical fiber  84  scans ahead and the peripheral optical fibers  82  scan to the sides of the device. Preferably, the tip  94  of this device is somewhat elongated to allow easier navigation of bends in the circulatory pathways. The central fiber extends into tip  94  and looks directly ahead. 
     Miniature optics  88  can be eliminated and just the bare fiber tip can be used, as with central fiber  84 ; different combinations of optical elements, e.g. GRIN lens  90  without corner cube  92  or corner cube  92  without GRIN lens  90 , can also be used. Thus with different optical arrangements, foreward and/or side viewing can be obtained. 
     The scan data can be displayed, as shown in FIGS. 5A, B, as a radial pie slice  96  for each sideways looking fiber and a separate plot  102  for the forward looking fiber. As seen in FIG. 5A, each segment  96  is a scan obtained by one of the side viewing fibers, which have been multiplexed to produce a 360 degree view. The boundaries  98  represent the artery walls. Since there are only a discrete number of fibers and sectors  96 , there are some discontinuities in the boundaries  98 . However, boundary  100  is clearly much farther away and represents a junction with a secondary artery. 
     A guidewire device  110  as shown in FIG. 6 may have a plurality of peripheral fibers, e.g. fibers  112 - 1 ,  112 - 2 , embedded in guidewire body  114  with individual fibers extending to different lengths along the body  114 . Each fiber may terminate in optical elements  116 , e.g. GRIN lens  118  and corner cube  120 , for side viewing, or some of the fibers can be forward viewing, as central fiber  122  which extends ahead of fiber  112 - 1 . Thus features found a different locations along the length of the catheter can be viewed without moving the guidewire. 
     Applications for the invention include any method or procedure where accurate guidewire positioning is beneficial, including angioplasty, stroke treatment, aneurysm, arteriovenous malformations, ophthalmic surgery, laparoscopic surgery, arthroscopic surgery, treatment of colorectal disorders, sinus disorders, ear surgery, pneumothoracic surgery, spinal surgery, bladder surgery, esophageal surgery, uteral disorders, and essentially any treatment that requires accurate information about tissue structures while using a guidewire inside a body cavity. In addition to medical applications, the invention can be used for nonmedical instruments which are to be used to inspect and probe in situ locations. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.