Abstract:
A spectroscope includes first and second beam redirectors in optical communication with first and second fibers respectively. The first and second beam redirectors are oriented to illuminate respective first and second areas. The second area is separated from the first area by a separation distance that exceeds the separation distance between the first and second beam redirectors.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application claiming the benefit of the priority date of U.S. application Ser. No. 10/456,979, filed Jun. 6, 2003, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to spectroscopy, and in particular, to spectroscopes for detecting vulnerable plaques within a wall of a blood vessel. 
       BACKGROUND 
       [0003]    Atherosclerosis is a vascular disease characterized by a modification of the walls of blood-carrying vessels. Such modifications, when they occur at discrete locations or pockets of diseased vessels, are referred to as plaques. Certain types of plaques are associated with acute events such as stroke or myocardial infarction. These plaques are referred to as “vulnerable plaques.” A vulnerable plaque typically includes a lipid-containing pool of necrotic debris separated from the blood by a thin fibrous cap. In response to elevated intraluminal pressure or vasospasm, the fibrous cap can become disrupted, exposing the contents of the plaque to the flowing blood. The resulting thrombus can lead to ischemia or to the shedding of emboli. 
         [0004]    One method of locating vulnerable plaque is to peer through the arterial wall with infrared light. To do so, one inserts a catheter through the lumen of the artery. The catheter includes a delivery fiber that sends infrared light to a delivery mirror. Infrared light reflects off the delivery mirror toward a spot on the arterial wall. Some of this infrared light penetrates the wall, scatters off structures within the arterial wall, and re-enters the lumen. This re-entrant light falls on a collection mirror, which then guides it to a collection fiber. The collection mirror and the delivery mirror are separated from each other by a gap. Because the catheter must be narrow enough to fit through blood vessels, the collection mirror and the delivery mirror are typically separated in the axial direction. 
         [0005]    To a great extent, the separation between the delivery mirror and the collection mirror controls the depth from which most of the light gathered by the collection mirror is scattered. To gather more light from scattered from deep within the wall, one increases the gap between the collection mirror and the delivery mirror. 
         [0006]    The collection mirror and the delivery mirror are mounted in a rigid housing at the distal tip of the catheter. To enable the catheter to negotiate sharp turns, it is desirable for the rigid housing to be as short as possible. This places an upper limit on the extent of the gap between the two mirrors, and hence an upper limit on the depth from which scattered light can be gathered. 
       SUMMARY 
       [0007]    The invention is based on the recognition that one can increase the effective separation distance between a collection-beam redirector and a delivery-beam redirector by controlling the directions in which those redirectors direct light. 
         [0008]    In one aspect, the invention provides an apparatus for identifying vulnerable plaque in a wall of an artery. Such an apparatus includes a catheter defining a longitudinal axis; first and second fibers parallel to the longitudinal axis; a first fixed beam redirector in optical communication with the first fiber for directing a beam from the first fiber along a first line; and a second fixed beam redirector in optical communication with the second fiber for directing a beam from the second fiber along a second line. First and second line segments parallel to the longitudinal axis extend between the first line and the second line, with the distance between the second line segment and the longitudinal axis being greater than a distance between the first line segment and the longitudinal axis. The first and second beam redirectors are oriented such that the second line segment is longer than the first line segment. 
         [0009]    In one embodiment, the first beam redirector includes a mirror. However, the first beam redirector can also be a lens system or a diffracting element. Alternatively, by bending the first fiber, the first beam redirector becomes the distal end of the first fiber. 
         [0010]    In another embodiment, the extent to which the second line segment is long than the first line segment is chosen to enhance collection of light scattered from a target located at a selected depth behind the arterial wall. 
         [0011]    In some embodiments, the first and second beam redirectors are separated along the longitudinal axis. 
         [0012]    Other embodiments include those in which the first and second beam re-directors are oriented to define a pitch angle therebetween, the pitch angle being between 0 radians and π radians; and wherein the first separation distance is slightly greater than an average fiber diameter but less than 3 millimeters. 
         [0013]    Also included among the embodiments are those in which the first and second beam re-directors are oriented to define a pitch angle therebetween, the pitch angle being between π/2 radians and the smaller of the numerical apertures of the first and second fibers; and wherein the first separation distance is slightly greater than an average fiber diameter but less than 1.5 millimeters. 
         [0014]    In addition, there exist embodiments in which the first and second beam re-directors are oriented to define a pitch angle therebetween, the pitch angle being within a 0.5 radian window having a lower bound defined by the greater of the numerical apertures of the first and second fibers; and wherein the first separation distance is within a 0.5 millimeter window having a lower bound defined by a distance slightly greater than an average fiber diameter. 
         [0015]    Yet other embodiments include those in which the first and second beam re-directors are oriented to define a pitch angle therebetween. These include embodiments in which the pitch angle is within a 0.1 radian window centered at the sum of the numerical apertures of the first and second fibers; and wherein the first separation distance is within a 0.1 millimeter interval having a lower bound that is 0.35 millimeters greater than an average fiber diameter, and those embodiments in which the pitch angle is between 0 and π/2 radians; and the first separation distance is between 0.25 millimeters and 3 millimeters, and those embodiments in which the pitch angle is between 0.12 radians and π/2 radians; and the first separation distance is between 0.25 millimeters and 1.5 millimeters, and those embodiments in which the pitch angle is between 0.25 and 0.75 radians; and the first separation distance is between 0.25 millimeters and 0.75 millimeters. 
         [0016]    Further embodiments include those in which at least one of the first and second fibers includes an optical fiber having a numerical aperture of 0.12 radians, 0.22 radians, or 0.275 radians, and a core diameter selected from the group consisting of 9 micrometers, 62.5 micrometers, 100 micrometers, and 200 micrometers. 
         [0017]    In another aspect, the invention features an apparatus for detecting light scattered from within a wall of an artery. Such an apparatus includes: a catheter defining a longitudinal axis; first and second fibers extending along the longitudinal axis; means for causing a beam emerging from the first fiber to travel along a first line in a direction away from the catheter; and means for causing a beam traveling along a second line in a direction toward the catheter to enter the second fiber. In such an apparatus, a minimum distance between the first line and the second line increases with distance from the catheter. 
         [0018]    Embodiments include those in which at least one of the means for causing a beam emerging from the first fiber to travel along a first line in a direction away from the catheter and the means for causing a beam traveling along a second line in a direction toward the catheter to enter the second fiber includes a mirror, a lens system, a mirror in optical communication with a lens system, a bent distal portion, a diffracting element, or any combinations thereof. 
         [0019]    Other embodiments include means for rotating the means for causing a beam emerging from the first fiber to travel along a first line in a direction away from the catheter and the means for causing a beam traveling along a second line in a direction toward the catheter to enter the second fiber. 
         [0020]    Among the embodiments are those in which the rate at which the minimum distance increases with distance from the catheter is selected to enhance collection of light from a particular depth behind the wall of the artery. 
         [0021]    In another aspect, the invention features an apparatus for collecting light scattered from within a wall of an artery. Such an apparatus includes: a catheter defining an axis; a first fiber extending along the axis to a first point; a second fiber extending along the axis to a second point distal to the first point; a first beam redirector in optical communication with the first fiber to redirect light emerging from the first fiber towards a first location on the wall; and a second beam redirector in optical communication with the second fiber for redirecting light emerging from the second fiber to a second location on the wall, the second location being distal to the first location; wherein the distance between the first and second locations is greater than the distance between the first and second points. 
         [0022]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
         [0023]    Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]      FIG. 1  is a schematic of a system for identifying vulnerable plaque in a patient. 
           [0025]      FIG. 2  is a cross-section of the catheter in  FIG. 1 . 
           [0026]      FIG. 3  is a view of an optical bench at the tip assembly of the catheter in  FIG. 1 . 
           [0027]      FIG. 4  is a schematic of the paths traveled by light from the delivery fiber of  FIG. 1 . 
           [0028]      FIG. 5  is a cross-section of the spatial light distribution shown in  FIG. 4 . 
           [0029]      FIGS. 6-10  are schematics of different embodiments of beam redirectors. 
           [0030]      FIG. 11  is a contour plot of mean penetration depth as a function of separation and orientation of beam redirectors. 
           [0031]      FIG. 12  is a schematic of a pair of beam redirectors for generating the contour plot of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
     System Overview 
       [0032]      FIG. 1  shows a diagnostic system  10  for identifying vulnerable plaque  12  in an arterial wall  14  of a patient. The diagnostic system features a catheter  16  to be inserted into a selected artery, e.g. a coronary artery, of the patient. A delivery fiber  18  and a collection fiber  20  extend between a distal end  22  and a proximal end  24  of the catheter  16 . 
         [0033]    As shown in  FIG. 2 , the catheter  16  includes a sheath  26  surrounding a rotatable torque cable  28 . The delivery fiber  18  extends along the center of a torque cable  28 , and the collection fiber  20  extends parallel to, but radially displaced from, the delivery fiber  18 . The rotatable torque cable  28  spins at a rate between approximately 1 revolution per second and 400 revolutions per second. 
         [0034]    At the distal end  21  of the catheter  16 , a tip assembly  30  coupled to the torque cable  28  directs light traveling axially on the delivery fiber  18  toward an illumination spot  32  on the arterial wall  14 . The tip assembly  30  also collects light from a collection spot  34  on the arterial wall  14  and directs that light into the collection fiber  20 . 
         [0035]    The tip assembly  30  is typically a rigid housing that is transparent to infra-red light. To enable the catheter  16  to negotiate turns as it traverses the vasculature, it is desirable for the tip assembly  30  to extend only a short distance in the axial direction. 
         [0036]    A multi-channel coupler  36  driven by a motor  38  engages the proximal end  24  of the torque cable  28 . When the motor  38  spins the multi-channel coupler  36 , both the coupler  36 , the torque cable  28 , and the tip assembly  30  spin together as a unit. This feature enables the diagnostic system  10  to circumferentially scan the arterial wall  14  with the illumination spot  32 . 
         [0037]    In addition to spinning the torque cable  28 , the multi-channel coupler  36  guides light from a laser  40  (or other light source such as a light-emitting diode, a super-luminescent diode, or an arc lamp) into the delivery fiber  18  and guides light emerging from the collection fiber  20  into one or more detectors (not visible in  FIG. 1 ). 
         [0038]    The detectors provide an electrical signal indicative of light intensity to an amplifier  42  connected to an analog-to-digital (“A/D”) converter  44 . The A/D converter  44  converts this signal into digital data that can be analyzed by a processor  46  to identify the presence of a vulnerable plaque  12  hidden beneath the arterial wall  14 . 
       Optical Bench 
       [0039]      FIG. 3  shows an optical bench  48  in which are seated the collection fiber  20  and the delivery fiber  18 . The optical bench  48  is seated in a recess  50  between first and second side walls  52 A-B of the distal end of a housing  54 . The housing  54  is in turn coupled to the distal end of the torque cable  28 . The recess  50  is just wide enough to enable the collection fiber  20  and the delivery fiber  18  to nestle adjacent to each other. A floor  56  extending between the first and second side walls  52 A-B and across the recess  50  supports both the collection and delivery fibers  18 ,  20 . 
         [0040]    Just distal to the end of the delivery fiber  18 , a portion of the optical bench  48  forms a frustum  58 . The frustum  58  extends transversely only half-way across the optical bench  48 , thereby enabling the collection fiber  20  to extend distally past the end of the delivery fiber  18 . 
         [0041]    The frustum  58  has an inclined surface facing the distal end of the delivery fiber  18  and a vertical surface facing the distal end of the optical bench  48 . The inclined surface forms a 135 degree angle relative to the floor  56 . However, other angles can be selected depending on the direction in which light from the delivery fiber  18  is to be directed. A reflective material coating the inclined surface forms a beam redirector, which in this case is a delivery mirror  60 . When light exits axially from the delivery fiber  18 , the delivery mirror  60  intercepts that light and redirects it radially outward to the arterial wall  14 . Examples of other beam redirectors include prisms, lenses, diffraction gratings, and combinations thereof. 
         [0042]    The collection fiber  20  extends past the end of the delivery fiber  18  until it terminates at a plane that is coplanar with the vertical face of the frustum  58 . Just beyond the distal end of the collection fiber  20 , a portion of the optical bench  48  forms an inclined surface extending transversely across the optical bench  48  and making an angle greater than 135 degrees relative to the floor  56 . A reflective material coating the inclined surface forms a collection mirror  82 . 
         [0043]    A delivery-fiber stop  86  molded into the optical bench  48  proximal to the frustum  58  facilitates placement of the delivery fiber  18  at a desired location proximal to the delivery mirror  60 . Similarly, a collection-fiber stop  88  molded into the optical bench  48  just proximal to the collection mirror  82  facilitates placement of the collection fiber  20  at a desired location proximal to the collection mirror  82 . 
       Spatial Distribution of Scattered Light 
       [0044]    Referring to  FIG. 4 , light travels radially outward from the delivery mirror  60  toward the illumination spot  32  on the arterial wall  14 . As the light does so, it encounters the blood that fills a lumen  68 . Because of scattering by particles in the blood, many photons never reach the wall  14 . This loss of energy is shown schematically by a progressive narrowing of the beam as it nears the wall  14 . The remaining photons  61  eventually reach the arterial wall  14 . Some of these photons are reflected from the wall  14 . These specularly reflected photons  62  carry little or no information about structures  64  behind the arterial wall  14  and are therefore of little value. Of those photons  63  that penetrate the wall, many others are absorbed. The remainder  66  are scattered by structures  64  behind the wall  14 . After having been scattered, a few of these remaining photons  66  again pass through the arterial wall  14  and re-enter the lumen  68 . This remnant of the light  61  originally incident on the wall, which is referred to herein as the “re-entrant light  66 ,” carries considerable information about the structures  64  behind the arterial wall  14 . It is therefore this re-entrant light  66  that is to be guided into the collection fiber  20 . 
         [0045]    As suggested by  FIG. 4 , re-entrant light  66  tends to re-enter the lumen along concentric annular regions  70 A-F that are radially separated from the specularly reflected light  62 . Each re-entrant such annular region  70 C, best seen in  FIG. 5 , is a region through which light scattered from a particular depth within the wall  14  is most likely to re-enter the lumen  68 . Light that has penetrated only superficially into the wall  14  before being scattered generally re-enters the lumen  68  through the innermost  70 D-F such annular regions. Light that has penetrated more deeply into the wall  14  before being scattered tends to re-enter the lumen  68  through one of the outer re-entrant zones  70 A-B. 
         [0046]      FIGS. 4-5  indicate that to collect deeply-scattered light, it is desirable to collect light from a collection spot  34  that lies in an annular region  70 C that is relatively far from the illumination spot  32 . One way to achieve this is to extend the separation distance between the delivery mirror  60  and the collection mirror  82 . However, doing so results in a longer tip assembly  30 . As an alternative, the delivery mirror  60  can be angled relative to the collection mirror  82 , as shown in  FIG. 6 . 
         [0047]    In  FIG. 6 , the delivery mirror  60  is oriented to direct light radially away from the catheter, thereby delivering that light to an illumination spot  32  directly under the mirror  60 . The collection mirror  82 , however, is angled to collect light from a collection spot  34  that is further from the illumination spot  32  than the separation distance between the collection mirror  82  and the delivery mirror  60 . 
         [0048]    The collection spot  34  and the illumination spot  32  can be made further apart in ways other than by orienting the collection mirror  82 . For example, in  FIG. 7 , a refracting system  83  in the optical path between the collection fiber  20  and the collection spot  34  causes the collection spot  34  to be further from the illumination spot  32  than the separation distance between the collection mirror  82  and the delivery mirror  60 . Other optical elements, such as a diffracting system, can be used in place of a refracting system  83 . The refracting system  83  can be a discrete lens, as shown in  FIG. 7 , a collection of lenses, or a lens integrally formed with the collection fiber  20 . 
         [0049]    Alternatively, either the collection spot  34 , the illumination spot  32 , or both, can be shifted relative to each other by bending the collection fiber  20  and the delivery fiber  18 , as shown in  FIG. 8 . 
         [0050]    Separation of the collection spot  34  and the illumination spot  32  can also be achieved by orienting the delivery mirror  60 , as shown in  FIG. 9 , or by orienting both the delivery mirror  60  and the collection mirror  82 , as shown in  FIG. 10 . In both  FIGS. 9 and 10 , the movement of the illumination spot  32  can be achieved using a refracting system  83 , by using a diffracting system, or by bending the fibers  18 ,  20  as described above. 
         [0051]    In all the foregoing cases, there exists a delivery-beam redirector, through which light leaves the catheter, and a collection-beam redirector, through which scattered light enters the catheter. Whether the beam redirectors are mirrors, lenses, or ends of a bent fiber, the fact remains that they will be spatially separated from each other. 
         [0052]      FIGS. 6-10  show embodiments in which there is only one collection fiber  20  and one delivery fiber  18 . However, a catheter can also have several collection fibers  20  and/or several delivery fibers  18 , each with its associated beam-redirecting element. The beam re-directing elements associated with different delivery fibers and/or collection fibers are oriented at different angles to permit collection of light from different depths. In embodiments having multiple collection and/or delivery fibers, the spacing between fibers is between 50 and 2500 micrometers. The beam re-directing elements are oriented at angles separated by one-fourth of the numerical aperture of the fiber having the smallest numerical aperture. 
         [0053]    For a particular choice of fibers, the distance between the illumination spot  32  and the collection spot  34  determines the average penetration depth of light incident on the collection mirror  82 . This distance depends on two independent variables: the distance separating the collection mirror  82  and the delivery mirror  60 ; and the angular orientation of the collection mirror  82  relative to that of the delivery mirror  60 . For the geometry shown in  FIG. 12 , the contour plot of  FIG. 11  shows the relationship between the average penetration depth of light received at the collection mirror  82 , the separation between the collection fiber  20  and the delivery fiber  18  and the angle θ as shown in  FIG. 12 . 
         [0054]    In  FIG. 12 , a delivery mirror  60  is oriented to direct an illumination beam radially away from the catheter. A collection mirror  82  is oriented at an angle θ relative to a line normal to the wall  14 . Positive values of θ are those in which the collection mirror  82  is oriented to receive light from a collection spot  34  that is closer to the illumination spot  32  than the separation between the collection-beam redirector and the delivery-beam redirector. Conversely, negative values of θ, such as that shown in  FIG. 12 , are those in which the collection mirror  82  is oriented to receive light from a collection spot  34  that is further from the illumination spot  32  than the separation between the collection mirror  82  and the delivery mirror  60 . 
         [0055]    It is apparent from  FIG. 11  that for a given separation between the collection mirror  82  and the delivery mirror  60 , one can collect light from deeper within the wall  14  by increasing the angle θ in the negative direction. This makes possible the collection of light scattered from deep inside the wall  14  without necessarily increasing the separation between the collection mirror  82  and the delivery mirror  60 . As a result, the tip assembly  30  can be made smaller without necessarily compromising the ability to detect light scattered from deep inside the wall  14 . A suitable choice for the angle θ (also referred to as the pitch angle), depends on the numerical apertures of the collection fiber and the delivery fiber. One suitable choice is that in which the angle θ is the sum of the arcsines of the numerical apertures. 
         [0056]      FIGS. 11 and 12  are discussed in the context of mirrors as collection and delivery-beam redirectors. However, it will be apparent that similar principles apply to other types of collection-beam redirectors, such as those disclosed herein. 
         [0057]    In addition, in the discussion of  FIGS. 11 and 12 , only the angle of the collection mirror  82  is changed. However, similar effects can be achieved by properly orienting the delivery mirror  60 , or by orienting both the delivery and collection mirrors  60 ,  82  together. 
         [0058]    In some embodiments, the radian angle included between the longitudinal axes of the delivery and collection fibers  18 ,  20  (hereafter referred to as the “pitch angle”) is between 0 radians and π radians. In this case, the axial separation between the delivery fiber and the collection fiber  18 ,  20  is slightly greater than the average fiber diameter but less than 3 millimeters. As used herein, “slightly greater than” means “approximately 0.1 millimeters greater than,” and “average fiber diameter” means the average of the diameters of the collection fiber  20  and the delivery fiber  18 . 
         [0059]    In other embodiments, the pitch angle is between π/2 and the smaller of the numerical apertures of the delivery fiber  18  and the collection fiber  20 . In this case, the axial separation between the delivery fiber  18  and the collection fiber  20  is slightly greater than the average fiber diameter but less than 1.5 millimeters. 
         [0060]    In other embodiments, the pitch angle is within a 0.5 radian window having a lower bound defined by the greater of the numerical apertures of the delivery fiber  18  and the collection fiber  20 . In this case, the axial separation distance between the delivery fiber  18  and the collection fiber  20  is within a 0.5 millimeter window having a lower bound defined by a distance slightly greater than the average fiber diameter. 
         [0061]    In yet other embodiments, the pitch angle is within a 0.1 radian window centered at the sum of the numerical apertures of the collection and delivery fiber  18 . In this case, the axial separation between the delivery and collection fibers  18 ,  20  is within a 0.1 millimeter interval having a lower bound that is 0.35 millimeters greater than the average fiber diameter. 
         [0062]    Additional embodiments include those in which the pitch angle is between 0 and π/2 radians and the axial separation between the delivery and collection fibers  18 ,  20  is between 0.25 millimeters and 3 millimeters; those in which the pitch angle is between 0.12 radians and π/2 radians and the axial separation between the delivery and collection fibers  18 ,  20  is between 0.25 millimeters and 1.5 millimeters, and those in which the pitch angle is between 0.25 and 0.75 radians and the axial separation between the delivery and collection fibers  18 ,  20  is between 0.25 millimeters and 0.75 millimeters. 
         [0063]    Suitable fibers for use as a delivery fiber  18  include those having a numerical aperture of 0.12 radians and core diameters of 9 micrometers, 100 micrometers, and 200 micrometers. Suitable fibers for use as a collection fiber  20  include those having a numerical aperture of 0.22 radians and core diameters of 100 micrometers or 200 micrometers. Also suitable for use as a collection fiber  20  are fibers having a numerical aperture of 0.275 radians and a core diameter of 62.5 micrometers. 
         [0064]    The surfaces of the delivery and collection mirrors  60 ,  82  can be coated with a reflective coating, such as gold, silver or aluminum. These coatings can be applied by known vapor deposition techniques. Alternatively, for certain types of plastic, a reflective coating can be electroplated onto those surfaces. Or, the plastic itself can have a reflective filler, such as gold or aluminum powder, incorporated within it. 
         [0065]    The optical bench  48  is manufactured by injection molding a plastic into a mold. In addition to being simple and inexpensive, the injection molding process makes it easy to integrate the elements of the optical bench  48  into a single monolith and to fashion structures having curved surfaces. Examples of suitable plastics include liquid crystal polymers (LCPs), polyphenylsulfone, polycarbonate, acrylonitrile butadiene-styrene (“ABS”), polyamide (“NYLON”), polyethersulfone, and polyetherimide. Alternatively, the optical bench can be manufactured by micro-machining plastic or metal, by lithographic methods, by etching, by silicon optical bench fabrication techniques, or by injection molding metal. Materials other than plastics can be used to manufacture the housing  54  and the optical bench  48 . Such materials include metals, quartz or glass, and ceramics. 
         [0066]    The floor  56  in the illustrated embodiment is integral to the housing  54 . However, the floor  56  can also be made part of the optical bench  48 . 
         [0067]    As described herein, the housing  54  and the optical bench  48  are manufactured separately and later joined. However, the housing  54  and the optical bench  48  can also be manufactured together as a single unitary structure. 
       Using the Catheter 
       [0068]    In use, the distal tip assembly  30  is inserted into a blood vessel, typically an artery, and guided to a location of interest. Light is then directed into the delivery fiber  18 . This light exits the delivery fiber  18  at its distal tip, reflects off the delivery mirror  60  in a direction away from the plane containing the delivery and collection fibers  18 ,  20 , and illuminates an illumination spot  32  on the wall of the artery. Light penetrating the arterial wall  14  is then scattered by structures within the wall. Some of this scattered light re-enters the blood vessel and impinges on the plane and onto the collection mirror  82 . The collection mirror  82  directs this light into the collection fiber  20 . 
         [0069]    Alternatively, light incident on the wall  14  can stimulate fluorescence from structures on or within the wall  14 . The portion of this fluorescent light that is incident on the collection mirror  82  is directed into the collection fiber  20 . 
       OTHER EMBODIMENTS 
       [0070]    It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.