Abstract:
A method and apparatus for identifying tissue structures in advance of a mechanical medical instrument during a medical procedure. A mechanical tissue penetrating medical instrument ( 22 ) has a distal end for penetrating tissue in a penetrating direction. An optical wavefront analysis system ( 32 - 50 ) provides light to illuminate tissue ahead of the medical instrument and receives light returned by tissue ahead of the medical instrument. An optical fiber ( 30 ) is coupled at a proximal end to the wavefront analysis system and attached at a distal end to the medical instrument proximate the distal end of the medical instrument. The distal end of the fiber has an illumination pattern directed substantially in the penetrating direction for illuminating the tissue ahead of the medical instrument and receiving light returned therefrom. The wavefront analysis system provides information about the distance from the distal end of the medical instrument to tissue features ahead of the medical instrument.

Description:
RELATED APPLICATION 
     This application is based on provisional patent application No. 60/283,068, filed Apr. 11, 2001, and hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to medical procedures and apparatuses for performing medical procedures and, in particular, to methods and apparatuses for identifying tissues structures in advance of a tissue penetrating medical instrument. 
     BACKGROUND OF THE INVENTION 
     During invasive medical procedures employing a tissue penetrating medical instrument, such as a needle or a scalpel, there is a need to identify tissue structures in advance of the medical instrument so as to control the depth of penetration of the tip or cutting edge of the medical instrument. This may be done to ensure that the penetration is to the right depth, but usually more importantly, this is done to ensure that the medical instrument does not penetrate too deeply and thereby damage tissue structure unnecessarily. 
     During surgery important tissue is often damaged inadvertently. For example, biopsy patients often report a loss of sensation or motion control due to nerve damage sustained during surgery. It would be desirable to provide surgeons with a means to visualize better or measure the distance of the tip of a surgical device, such as a scalpel, to nerve tissue so that the surgeon can avoid penetrating tissue so deeply as to damage the nerve tissue. 
     Another current medical problem occurs during tests of certain body fluids or during localized injections, where a needle used to extract body fluids or inject medication must penetrate one body structure without penetrating a subsequent structure. For example, in performing a spinal tap it is desirable to penetrate the dural membrane containing the spinal fluid, but highly undesirable to penetrate, or even touch the spinal cord itself as that can cause sever injury and potentially permanent paralysis. Yet, this is generally carried out without the aid of any means for seeing the physical relationship of the tip of the spinal tap needle to the spinal structures or even a way of measuring the distance from the tip to structure that must not be penetrated. Similarly, in carotid injections in small laboratory animals it is often difficult to find the artery or vein in small animals, and it can be very difficult to keep from penetrating entirely through the artery or vein, once it is found. This can make drug delivery excessively time consuming and difficult to control. 
     The use of optical coherence domain reflectometry (“OCDR”) has been disclosed as a technique for examining the reflectivity of an animal structure to a limited depth therein. In OCDR a short coherence length light source is used in a scanning Michelson interferometer to determine the distance of a point from which light is scattered to a reference position. This is disclosed, for example, in Swanson et al. U.S. Pat. No. 5,459,579 entitled METHOD AND APPARTUS FOR PERFORMING OPTICAL MEASUREMENTS, hereby incorporated by reference in its entirety. In particular, Swanson et al. discloses the use of a fiber optic Michelson interferometer having a test fiber placed adjacent to the surface of an animal structure and coupled through a lens for illuminating the structure and coupling the backscattered light back into the fiber. The backscattered light is then interfered with a scanning reference reflector to measure the reflectivity profile of the structure to a limited depth therein. However, the usefulness of this technique is limited by the size and vulnerability of the lens assembly associated with the test fiber. 
     Tearney, et al., U.S. Pat. No. 6,134,003, entitled METHOD AND APPARATUS FOR PERFORMING OPTICAL MEASUREMENTS USING A FIBER OPTIC IMAGING GUIDE WIRE, CATHETER OR ENDOSCOPE, hereby incorporated by reference in its entirety, extends OCDR to optical coherence tomography (“OCT”). Tearney et al. discloses that an OCDR test fiber may be combined with a catheter or endoscope having a scanning imaging system at the distal end thereof for obtaining multiple measurements of the distance to a body structure used to create a tomograhic image of the structure. Like Swanson et al., the disclosure of Tearney et al. is limited by the use of an optical system at the distal end of the test fiber, and it is static with respect to tissue depth. Colston et al. U.S. Pat. No. 6,175,669 entitled OPTICAL COHERENCE DOMAIN REFLECTOMETRY GUIDEWIRE, which discloses a fiber optic OCDR system for guiding a guidewire through an arterial system is similarly limited. 
     A method for analyzing tissue based on the information obtained from OCT is also discussed in Song et al., “Simultaneous measurements of thickness and refractive index of microstructures in obscure specimens by optical coherence tomography,” Optik Volume 111, Issue 12, pages 541-543 (2000). Like Swanson et al. and Tearney et al., the disclosure of Song et al. is limited optics and essentially static depth measurements. 
     A technique for analyzing the contours of eye surfaces using OCT to provide autofocussing of a laser scalpel in eye surgery has been disclosed in Wei et al. U.S. Pat. No. 6,004,314 entitled OPTICAL COHERENCE TOMOGRAPHY ASSISTED SURGICAL APPARATUS. While in-line tomography, that is, obtaining OCDR data relative to various locations along the axis of propagation of the OCDR light is disclosed, this system is limited to use with a non physically invasive laser tool applied to the eye, which is especially adapted for light transmission, and is limited by the use of large, free-space optics for coupling the OCDR into the eye. 
     Winston et al. U.S. Pat. No. 6,228,076 entitled SYSTEM AND MTHOD FOR CONTROLLING TISSUE ABLATION describes the combining OCDR with an endoscope used for laser ablation to distinguish tissue. However, it is limited in applicability by the size of the endoscope and distal optics. 
     In both manual and robotic surgery, the surgeon and surgical robot must know: (a) where a cut is occurring, and (b) what is being cut. This information describes the current state of a procedure. In addition, ideally the surgeon and robot would know where a cut is about to occur and what is about to be cut. This information, referred to as feedforward, is much harder information to acquire. 
     A surgical robotic arm typically provides, from sensors mounted thereon, six position values, six velocity values, and six acceleration values. Combining this information with basic anatomy information can let the robot know where and what it is cutting and to estimate what it is cutting, but does not allow the robot actually to sense the tissue that is being cut or to sense ahead of the cut. Such advance, or feedforward, information, as well as feedback, has been supplied for surgical procedures by, magnetic resonance imaging (“MRI”) and ultrasound on a routine basis. However, neither of these techniques has sub-millimeter resolution. So, for delicate work they are not optimal. In addition, MRI is severely constrained in application by its size, its cost, and the strength of the magnets involved. Ultrasound has considerably worse spatial resolution than MRI. 
     Accordingly, there is a need for a method and system that can be used both to determine the current state of a mechanical tissue penetrating medical instrument, such as a mechanical scalpel, biopsy needle, or injection needle, to scan tissue ahead of the medical instrument while it is moving, to identify and avoid damaging tissue structures such as blood vessels and nerves. 
     SUMMARY OF THE INVENTION 
     The aforementioned need is met by the present invention, which provides a method and apparatus for identifying tissue structures in advance of a mechanical medical instrument during a medical procedure. A mechanical tissue penetrating medical instrument has a distal end for penetrating tissue in a penetrating direction. An optical wavefront analysis system provides light to illuminate tissue ahead of the medical instrument and receives light returned by tissue ahead of the medical instrument. An optical fiber is coupled at a proximal end to the wavefront analysis system and attached at a distal end to the medical instrument proximate the distal end of the medical instrument. The distal end of the fiber has an illumination pattern directed substantially in the penetrating direction for illuminating the tissue ahead of the medical instrument and receiving light returned therefrom. The wavefront analysis system provides information about the distance from the distal end of the medical instrument to tissue features ahead of the medical instrument. 
     In one embodiment, the medical instrument is a needle for extracting body fluid or injecting medication. The optical fiber is embedded in the needle longitudinally thereof. Multiple fibers may be used to provide improved information content. In another embodiment one or more optical fibers are embedded in a scalpel. Preferably, the wavefront analysis system is an OCDR system, but other means for analyzing the wavefront information could be used. 
     The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1(   a ) is an illustration of a test fiber and associated lens adjacent tissue in a fiber optic Michelson interferometer OCT system for surface scanning according to the prior art. 
         FIG. 1(   b ) is an illustration of a two-dimensional slice of an OCT image produced by the prior art system of  FIG. 1(   a ). 
         FIG. 2(   a ) is an illustration of a test fiber adjacent tissue in a fiber optic Michelson interferometer OCDR system for axial scanning according to the present invention. 
         FIG. 2(   b ) is an illustration of a one-dimensional slice of an OCDR image produced by the method and apparatus of the present invention. 
         FIG. 3  is an exemplary needle for use in a tissue boundary identification method and apparatus in accordance with the present invention. 
         FIG. 4  is a block diagram of an OCDR analysis system for use with a tissue penetrating medical instrument in accordance with the present invention. 
         FIG. 5  is a picture of the tissue structure of the spinal sheath, synovial fluid and cord of a pig along the axis of advancement and ahead of a stationary penetrating medical instrument, produced in accordance with the present invention. 
         FIG. 6  is a picture of the tissue structure of the abdomen of a mouse along the axis of advancement and ahead of a penetrating medical instrument moving at a substantially steady rate, produced in accordance with the present invention. 
         FIG. 7(   a ) shows a first embodiment of a spinal block needle equipped with a fiber in accordance with the present invention. 
         FIG. 7(   b ) shows a second embodiment of a spinal block needle equipped with a fiber in accordance with the present invention. 
         FIG. 7(   c ) shows a third embodiment of a spinal block needle equipped with a fiber in accordance with the present invention. 
         FIG. 8  shows a tissue-penetrating device equipped with a fiber tip in accordance with the present invention. 
         FIG. 9  shows a side view of a tissue-penetrating needle equipped with a transmitting fiber and a receiving fiber in accordance with an alternative embodiment of the present invention. 
         FIG. 10  is an illustration of the use of a transmitting fiber and a receiving fiber as shown in  FIG. 9  in relation to tissue, in accordance with the present invention. 
         FIG. 11  shows a scalpel equipped with a plurality of fibers in accordance with the present invention. 
         FIG. 12  shows a flow chart of a representative control system using feed back to guide a mechanical tissue penetrating medical instrument according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The method and apparatus of the present invention uses optical fibers aligned along a needle, scalpel or other mechanical cutting tool to both transmit and receive optical information directly in front of the cutting surface. OCDR is used to determine the depth of boundaries immediately in advance of a cutting surface such as a needle tip or scalpel blade. This information can be used to alert either a medical professional or a surgical robot to the presence of an upcoming boundary and possibly identify the tissue. 
     The basic concept of prior art OCT  10  is illustrated by  FIGS. 1(   a ) and  1 ( b ), which shows a typical test fiber  12  and associated distal optic  14  in relation to a tissue sample  16  to be scanned. This prior art system, which does not mechanically penetrate the tissue, can produce a two-dimensional tomographic image  17  of relatively limited depth of, for example, 1-2 millimeters. 
     In contrast, the apparatus for identifying tissue structures ahead of a mechanical tissue penetrating instrument  18  according to the present invention, employs an axial, dynamic OCDR system, as shown in  FIGS. 2(   a ) and  2 ( b ). In this case, a fiber  20  is imbedded in a mechanical tissue penetrating medical instrument such as needle  22 , and the tissue  24  is scanned in the axial direction of movement of the instrument as it penetrates the tissue. This produces a one-dimensional axial scan  26 , as shown in  FIG. 2(   b ). Since the fiber is embedded in the instrument, only the fiber core is actually required, the instrument itself providing the mechanical protection for the core. No optic is required to be associated with the end of the fiber, as it has been found surprisingly that, in this application, backscattered light captured by the fiber end face is sufficient to provide the desired one-dimensional image. Because a single mode fiber has such a small diameter, on the order of 12.5 micrometers (micron), it can be carried into the tissue by the mechanical instrument, thereby probing the depth of the tissue ahead of the instrument as it is advanced. 
     Turning to  FIG. 3 , a typical instrument with which the present invention is to be used is a cored needle  28 , such as the type of needle that is used to inject a spinal block in a surgical patient. Such a needle is typically about one millimeter in diameter. As shown, the core is filled with a single mode fiber  30 , surrounded by cladding, when the needle is inserted into an animal subject. While the needle penetrates deeper and deeper into the tissue, the light emitted from the core propagates ahead and some is scattered back into the end face of the fiber. As the needle is advanced, the fiber “sees ahead” of the needle 1-2 millimeters, which is enough to stop the needle before a critical boundary is penetrated. 
       FIG. 4  shows a block diagram of a typical OCDR system suitable for use in the present invention. OCDR is a distance-measuring tool that relies on the fact that if a sufficiently broad band source is used in an interferometric system, interference fringes can only be generated when the two optical path lengths are equal. The depth resolution of the interferometer is directly proportional to the bandwidth of the source through 
               Δ   ⁡     (   nl   )       ∝       1     c   ⁢           ⁢   Δ   ⁢           ⁢   v       .           
The optical path length to the sample can be measured through knowledge of the position of the reference mirror, which is obtained by other means.
 
     There are several ways of scanning the position of the reference mirror. By way of example, the first method involves scanning the reference mirror with a predetermined position and velocity profile. This method is simple to implement but requires compensation for the doppler shift of the moving reference mirror. Alternatively, a series of fixed reflectors at different lengths can be scanned by a rotating prism or mirror. Although it is more complex to implement, doppler compensation is not required and allowable scan rates are much higher. 
     In  FIG. 4 , the needle  28  carries the test fiber  30  into the tissue that is penetrated. Light generated by a low-coherence source  32  is coupled through a source fiber  34  and fiber coupler  36  both to the test fiber  28  and a reference fiber  38 . The reference fiber illuminates a scanning reference reflector  40 , which reflects light back into the reference fiber. Light reflected back from tissue  42  along the test fiber and from the reference  40  along the reference fiber is couple to a detector fiber  44  and produces interference at the detector  46 . The analog output of the detector is processed by signal processor  48 , whose output is analyzed by computer  50  to display an axial tomographic image of the tissue in front of the needle as the needle progresses through the tissue. 
     The operation of the present invention is shown by  FIGS. 5 and 6 , which are pictures of actual data produced by the invention. In  FIG. 5 , the needle  28  has been placed on the surface of the spinal cord of an ex vivo laboratory pig and held stationary. In the picture of  FIG. 5 , the vertical axis represents, downwardly, the distance into the spinal from the end of the penetrating medical instrument. The horizontal axis represents time. Item  52  is a reflection off the distal face of the test fiber  30 , item  54  is the spinal cord dural, and item  56  is the boundary of the spinal nerves. Since the needle has been held stationary, the reflections are substantially linear, the variations coming from random movements. In  FIG. 6 , the needle  28  has been inserted into the abdomen of an ex vivo laboratory mouse and advanced forward at a substantially constant rate so as to penetrate the structures therein. Item  58  is the reflection of the distal face of the test fiber. Item  60  is the boundary of the stomach. Item  62  is the boundary of the liver. It can be seen that as time (depth of penetration) progresses, the boundary reflections move closer in the picture to the fiber face reflection. (Item  64  is an artifact caused by rapid puncture through the liver.) 
     Alternative embodiments of spinal block needles according to the present invention are shown in  FIGS. 7(   a ),  7 ( b ) and  7 ( c ). In  FIG. 7(   a ), the needle  66  has an outer sheath  68  and a removable inner core  70 . The needle is inserted into and penetrates the tissue close to the spinal cord for injecting anesthetic. A test fiber  72  is embedded in a groove  74  in the core  70 , the distal end of  76  of the fiber being disposed proximate the distal end of the sheath  68 . The test fiber is used in accordance with the present invention to guide the needle close to the spinal cord without penetrating it. The inner core  70  is then removed, along with the fiber, and the anesthetic is injected. 
     In  FIG. 7(   b ), a needle  78  is like needle  66 , except that it is equipped with several test fibers  80 ,  82  and  84  coupled alternately to the OCDR system. This enables the measurements to be averaged to reduce measurement errors. 
     In  FIG. 7(   c ), a needle  80  is like the needle  66  of  FIG. 7(   a ), except that a test fiber  86  is embedded in a groove in the inner wall  88  of the needle. The needle may or may not be equipped with a core. 
     In all of  FIGS. 7(   a )-( c ) it is to be recognized that the fiber could be replaced with a waveguide other than a fiber, for example a semiconductor deposited into a groove etched into a wall of the needle, or a metal microtube. Such an alternative waveguide is preferably coupled at its proximal end to a fiber for connection to the OCDR system. 
       FIG. 8  illustrates either a needle or a scalpel where the tip of the glass fiber is itself a cutting tool. Thus, the mechanical tissue penetrating instrument  90  comprises an outer rigid wall  92 , which could be either tubular (in the case of a needle) or flat (in the case of a scalpel, and an inner fiber core  94  which terminates in a tip  96 . The tip is a sharp edge that not only provides narrow beam forward illumination, but acts as the cutting edge for the instrument. This device is particularly adapted for use in cosmetic surgery, where very fine, sharp cutting edges are needed. Since breakage of the edge can be immediately detected by the OCDR system the use of such a glass cutting tool is enabled with minimum risk that a broken edge will cause patient damage. 
     As indicated above, a needle or other mechanical tissue-penetrating instrument may be equipped with two fibers. Thus, in  FIG. 9 , a needle  98  is equipped with fibers  100  and  102 .  FIG. 10  shows the respective fields of view of fibers  100  and  102 . Similarly, FIG.  11  shows a scalpel  103  equipped with a plurality of fibers  105 . The return from such multiple fibers can be used in several ways: (a) to average the inputs as a means of speckle reduction; (b) to increase the light collection efficiency of the system; and (c) to aid in tissue discrimination. 
     The distance light travels from a fiber to a boundary and directly back to the fiber face—neglecting scattering—is: OPD 1 =2 nt. The distance light travels from one fiber, to a boundary and back to the other fiber is: 
               OPD   2     =     2   ⁢   n   ⁢           d   2     4     +     t   2                 
The difference is:
 
                 OPD   2     -     OPD   1       =     2   ⁢     n   ⁡     (             d   2     4     +     t   2         -   t     )               
For small motion along z we can get:
 
               Δ   ⁡     (       OPD   2     -     OPD   1       )       =     2   ⁢     n   ⁡     (             d   2     4     +       (     Δ   ⁢           ⁢   z     )     2         -     Δ   ⁢           ⁢   z       )               
Assuming the motion is small enough not to alter the location of the boundary in question, the only unknown in the above equation is the index of refraction of the intervening medium, which can be computed. In addition, the width of overlap cones of the respective fibers is a function of the scattering strength of the medium, as is the direct return signal as well.
 
     The present invention can be used as part of a control system for automatic or quasi-automatic robotic surgery. This is illustrated by  FIG. 12 . A position input is provided by a physician at  104 . an adder  106  receives an error signal from the OCDR system  108  and produces a sum signal to a surgical too control system  110 . 
     To appreciate the control capability offered by the invention, it is useful to think of the information that it provides in terms of a state vector, each element of the vector describing a state or condition of a surgical system. For example, in a robotic needle probe, an ideal state vector would include elements representing at least the following parameters: needle position, {right arrow over (r)}; angular position, {right arrow over (φ)}; needle velocity, {right arrow over (r)}; needle angular velocity, {right arrow over (φ)}; needle acceleration, {right arrow over (r)}; needle angular acceleration, {right arrow over (rφ)}; tissue type at tip location; tissue type 0.25 mm beyond tip location; issue type 0.50 mm beyond tip location; tissue type as far in front of the cutting edge of the instrument as it is possible to measure. In the event that it is impossible to identify the exact tissue type, it is still useful to identify the location of a change in tissue type. Knowledge of the position vector {right arrow over (r)} of the cutting edge, and the anatomy of the subject could be used to make a useful estimate of the location and type of a tissue boundary. 
     The present invention can be used to augment a conventional state vector to provide feedforward, as well as feedback, information as described above. This can be used either to provide valuable information to a surgeon through an appropriate interface, or in the control algorithm for a surgical robot. 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.