Abstract:
A spectroscope includes an optical fiber extending through a catheter, and in communication with an optical system. The optical system has a finite focal length.

Description:
RELATED APPLICATIONS 
     Pursuant to 35 USC 120, this application is a divisional application of, and claims the benefit of the priority date of U.S. patent application Ser. No. 10/289,741, filed Nov. 7, 2002, the contents of which are herein incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The invention relates to spectroscopy, and in particular, to spectroscopes for detecting vulnerable plaques within a wall of a blood vessel. 
     BACKGROUND 
     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. 
     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 for illuminating a spot on the arterial wall with infrared light. Various particles in the blood, as well as the arterial wall itself, scatter or reflect much of this light. A small portion of the light, however, penetrates the arterial wall, scatters off structures deep within the wall. Some of this deeply-scattered light re-enters the lumen. This re-entrant light be collected by a collection fiber within the catheter and subjected to spectroscopic analysis. 
     Light scattered only by the blood or reflected off the vessel walls surface contains no information about structures within the wall. To the extent that such light enters the collection fiber, it represents unwanted noise. Hence, the collection fiber preferably rejects such light and directs only re-entrant light into the collection fiber. 
     SUMMARY 
     The invention is based on the recognition that one can use the differing spatial distributions of specularly reflected light and re-entrant light to preferentially guide re-entrant light into the collection fiber. 
     In one aspect, the invention includes a spectroscope having an optical fiber extending through a catheter. An obstruction is placed so as to occlude a portion of a field-of-view of the optical fiber. 
     A variety of obstructions are within the scope of the invention. For example, in some embodiments, the obstruction includes a ledge extending across a chord of fiber core so as to occlude a region bounded by the chord and by a boundary of the core. In other embodiments, the obstruction includes a tab extending at least part way across the core. In yet other embodiments, the obstruction includes a disk disposed to occlude a circular portion of the core. 
     The obstruction need not be adjacent to the fiber. For example, in spectroscopes that include a mirror in optical communication with the optical fiber, the obstruction can be a non-reflective region of the mirror shaped to obstruct a portion of the fibers field-of-view. Or, for spectroscopes that include a perforated mask enclosing the fiber, the obstruction can be walls forming an aperture in optical communication with the fiber, the aperture being shaped to obstruct a portion of the fibers field-of-view. For spectroscopes that include a transparent sheath surrounding the fiber, the obstruction can be an opaque band in optical communication with the fiber and positioned to obstruct a portion of the fibers field-of-view. 
     The resulting field of view of the optical fiber depends in part on the shape of the obstruction. In some embodiments, the obstruction has a shape selected to form a field-of-view in the shape of a truncated ellipse. This includes the special case of a truncated circle, a circle being an ellipse with coincident foci. In other embodiments, the obstruction has a shape selected to form a crescent-shaped field-of-view. In yet other embodiments, the obstruction has a shape selected to form an annular field-of-view. Additional embodiments include those in which the obstruction has a diffracting edge, the geometry of which is selected to form a selected field-of-view. 
     Another aspect of the invention includes a spectroscope having an optical fiber that extends through a catheter. An optical system having a finite focal length is disposed to be in optical communication with the fiber. 
     In some embodiments, the optical system includes an optical element having a curved surface. Examples of such optical elements include mirrors and lenses. The curved surface can be a cylindrical surface, or it can be a paraboloid, an ellipsoid, a hyperboloid, or a sphere. In other embodiments, the optical system includes an optical element with a spatially varying index of refraction. 
     Another aspect of the invention includes a spectroscope having a catheter and an optical fiber extending through the catheter. In this aspect, a diffracting element configured to form a selected field-of-view is in optical communication with the fiber. 
     Embodiments of the spectroscope include those in which the diffracting element is a transmissive diffracting element and those in which the diffracting element is reflective diffracting element. Other embodiments include those in which the diffracting element is a diffraction grating, an amplitude grating, a phase grating, or a holographic grating. 
     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. 
     Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic of a system for identifying vulnerable plaque in a patient. 
         FIG. 2  is a cross-section of the catheter in  FIG. 1 . 
         FIG. 3  is a view of an optical bench at the tip assembly of the catheter in  FIG. 1 . 
         FIG. 4  is a schematic of the paths traveled by light from the illumination fiber of  FIG. 1 . 
         FIG. 5  is a cross-section of the spatial light distribution shown in  FIG. 4 . 
         FIG. 6  is a transverse cut of the collection fiber and optical bench showing a known collection-fiber stop. 
         FIG. 7  is a schematic of the field-of-view of the collection fiber of  FIG. 6  superimposed on the spatial light distribution of  FIG. 4 . 
         FIGS. 8 and 9  are longitudinal and transverse cross-sections of an optical bench having an extended collection-fiber stop. 
         FIG. 10  is a schematic of a field-of-view of resulting from the extended collection-fiber stop of  FIGS. 8 and 9 . 
         FIGS. 11 and 12  are longitudinal and transverse cross-sections of an optical bench having a tab protruding from the collection-fiber stop. 
         FIG. 13  is a schematic of a crescent-shaped field of view resulting from the tab of  FIGS. 11 and 12 . 
         FIGS. 14 and 15  are longitudinal and transverse cross-sections of an optical bench having an occluding disk supported on a post protruding from the collection-fiber stop. 
         FIG. 16  is a plan view of a mask on the collection mirror. 
         FIG. 17  is a schematic of a perforated shell enclosing the distal tip assembly. 
         FIG. 18  is a schematic of a banded sheath enclosing the distal tip assembly. 
         FIG. 19  is a schematic of a mirror having a curved surface in optical communication with the collection fiber. 
         FIGS. 20 and 21  are schematics of exemplary fields-of-view as modified by mirrors having various curved surfaces. 
         FIG. 22  is a schematic of a lens in optical communication with the collection fiber. 
         FIG. 23  is a schematic of a transmissive diffracting element in optical communication with the collection fiber. 
         FIG. 24  is a schematic of a reflective diffractive element integrated onto the surface of the collection mirror. 
     
    
    
     DETAILED DESCRIPTION 
     System Overview 
       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 . 
     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. 
     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 field-of-view  34  on the arterial wall  14  and directs that light into the collection fiber  20 . 
     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 . 
     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 ). 
     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 
       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 . 
     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 . 
     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 . 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 re-director, 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 re-directors include prisms and diffraction gratings. 
     Spatial Distribution of Light 
     Referring to  FIG. 4 , as the light travels radially outward from the delivery mirror  60 , it encounters the blood that fills a lumen  68 . As a result of scattering by particles in the blood, a large number of 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 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 . 
     As suggested by  FIG. 4 , re-entrant light  66  tends to re-enter the lumen along an annular re-entrant zone that is radially separated from the specularly reflected light  62 .  FIG. 5 , which illustrates the spatial distribution of light from the viewpoint of the catheter  16 , shows such a re-entrant zone  70  surrounding an illumination spot  32 . Photons received from within the re-entrant zone  70  are predominantly those that have been scattered from within the arterial wall  14 . The re-entrant zone  70  has an inner circumference  74  and an outer circumference  76 . Between the inner circumference  74  and the illumination spot  32  lies a specular zone  78 . Photons received from the specular zone  78  are predominantly those that have undergone specular reflection. Proceeding radially outward beyond the outer circumference  76 , one comes to a dark zone  80 , where the number of photons of either type is so small as to be immeasurable. 
     Modifying the Field-of-View 
     To collect as many photons of re-entrant light  66  as possible, the field-of-view  32  should overlap the re-entrant zone  70  to the greatest extent possible. To the extent that the field-of-view  32  extends outside the re-entrant zone  70 , it should extend into the dark zone  80  and away from the specular zone  78 . 
     Modifying the Field-of-View with an Obstruction 
     Referring back to  FIG. 3 , 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 a 135 degree angle relative to the floor  56 . A reflective material coating the inclined surface forms a collection mirror  82 . This collection mirror  82  reflects light incident from the arterial wall  14  into the distal end of the collection fiber  20 . The collection mirror  82  and the collection fiber  20  together form a collection subsystem  84  that collects light from a field-of-view  32 . 
     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 . 
     Referring now to  FIG. 6 , the collection fiber has an optically transmissive core  90  surrounded by a protective cladding  92 . The collection-fiber stop  88  extends upward from the floor  56  to provide an abutment surface for the collection fiber  20 . A portion of the cladding  92  rests on the abutment surface. The core  90  does not rest on the abutment surface and therefore remains unobstructed. 
     A distal tip assembly  94  configured as shown in  FIG. 6  results in a field-of-view  32  shaped like an ellipse  96  with its major axis  98  extending along the radial direction, as shown in  FIG. 7 . The extent to which the ellipse  96  overlaps the re-entrant zone  70  is one measure of how effective the collection subsystem  84  is at guiding re-entrant light into the collection fiber  20 . 
     The extent of the overlap between the ellipse  96  and the re-entrant zone  70  depends on the eccentricity of the ellipse and its position relative to the re-entrant zone  70 . The eccentricity of the ellipse  96  is governed by the angular orientation of the collection mirror  82 . Its position relative to the re-entrant zone  70  is controlled by varying the position and angle of the delivery mirror  60  relative to the collection mirror  82 . 
     To avoid collecting photons from the specular zone  78 , the ellipse  96  is positioned to be tangent to the inner circumference  74  of the re-entrant zone  70 , with its minor axis  100  located radially outward from the point of tangency. 
       FIGS. 8 and 9  show an extended collection-fiber stop  102  forms an abutment surface that extends part-way across the core  90  of the collection fiber. The occluded portion of the core  90  is bounded by a chord extending across the core  90  and by an arc that forms part of the boundary between the core  90  and the cladding  92 . The resulting modified field-of-view is a truncated ellipse  106  having a base  108 , as shown in  FIG. 10 . A dotted line  110  outlines a portion  112  of the ellipse truncated by the extended collection-fiber stop  102 . To avoid collecting photons from the specular zone  78 , the truncated ellipse  106  is positioned such that the base  108  of the truncated ellipse  106  is tangent to the inner circumference  74  of the re-entrant zone  70 . 
     The overlap between the truncated ellipse  106  and the re-entrant zone  70  in FIG.  10  is greater than the overlap between the full ellipse  96  and the re-entrant zone  70  in  FIG. 7 . The extent to which these overlaps differ represents an increase in the number of photons gathered from the re-entrant zone  70 . 
     Other beam-shaping structures can be used to prevent light from illuminating the entire core  90  and to thereby shape the field-of-view  32 . In  FIGS. 11 and 12 , for example, a tab  118  having a curved distal tip  119  protrudes vertically upward from the collection-fiber stop  88  and obstructs part of the core  90 . When placed in front of a collection fiber  20  having a suitably high numerical aperture, this results in a crescent shaped field-of-view  120  as shown in  FIG. 13 . 
     Another example, shown in  FIGS. 14 and 15 , is an occulting disk  122  mounted on a post  124  that protrudes from the collection-fiber stop  88 . The post  124  supports the occulting disk  122  so that its center coincides with the center of the core  90 . The diameter of the occulting disk  122  is slightly smaller than the diameter of the core  90 . The difference between the diameter of the occulting disk  122  and that of the core  90  is selected to provide an annular field-of-view that closely matches the size and shape of the re-entrant zone  70 . 
     Structures that effectively block light from entering a portion of the collection fiber need not be adjacent to the collection fiber  20 , as shown in  FIGS. 11-12  and in  FIGS. 14-15 . In fact, such structures can be placed anywhere along the optical path between the collection fiber  20  and the arterial wall  14 . For example, a beam-shaping structure that effectively obstructs a portion of the core  90  can be a mask  125  formed directly on the collection mirror  82 , as shown in  FIG. 16 . Another example of such a structure is a perforated shell  127  rotationally coupled to the torque cable  28 , as shown in  FIG. 17 . The perforated shell  127  has a delivery aperture  129  to permit light from the delivery fiber  18  to pass through the shell  127  unimpeded, and a collection aperture  131  shaped to block a portion of the light incident on the collection mirror  82 . In another example, shown in  FIG. 18 , an opaque band  133  on a transparent distal tip  135  of the stationary sheath  26  is positioned to obscure a portion of the collection mirror  82 . The band  131  extends circumferentially around the sheath so that the collection mirror  82  is obscured as the torque cable  28  rotates the collection mirror  82 . 
     Any of the foregoing beam-shaping structures can have an edge that is modified to diffract light incident thereon in a manner that causes the field-of-view to have a pre-selected geometry. Such an edge can be formed by providing protrusions or indentations having a dimension on the order of the wavelength of light to be observed. 
     Modifying the Field-of-View with an Optical System 
     The beam-shaping function of the foregoing obstructions can also be achieved by providing an optical system in optical communication with the collection mirror  82 . Such an optical system can include a collection mirror  82  with a curved surface, a lens assembly, or both. For example, in  FIG. 19 , the collection mirror  82  has a cylindrical surface rather than a planar surface. The resulting field-of-view for the configuration shown in  FIG. 19  is an ellipse  126  having an aspect ratio closer to unity, as shown in  FIG. 20 . Other curved surfaces can result in fields-of-view or alternatively an ellipse  128  in which it is the minor axis of an ellipse  128 , rather than the major axis, that extends radially, as shown in  FIG. 21 . 
     Curved surfaces other than a cylindrical surface can also be used to shape the field-of-view to more closely approximate the shape of the re-entrant zone  70 . For example, the curved surface can be a conic surface, such as a paraboloid, a hyperboloid, or an ellipsoid. Alternatively, the surface can be a spherical surface. 
     Optical elements other than reflecting surfaces can also be used to shape the field-of-view. For example, in  FIG. 22 , a lens assembly  130  disposed in optical communication with the collection fiber  20  provides control over the shape of the field-of-view. 
     The lens assembly  130  can include one or more discrete lenses. One or more lenses in the lens assembly can have a suitably curved surface. Another lens suitable for use in a lens assembly is a GRIN (graduated index of refraction) lens having a spatially varying index of refraction. In addition, the lens assembly  130  need not be composed of discrete lenses but can instead include a lens that is integral with the distal end of the collection fiber  20 . Such a lens  132 , an example of which is shown in  FIG. 22 , can be made by shaping the distal end of the collection fiber  20  so that it has the desired optical characteristics. 
     The beam-shaping function provided by the foregoing examples of optical systems can also be provided by a diffracting element  134  placed along the optical path, as shown in  FIG. 23 . Examples of diffracting elements  134  include diffraction gratings, amplitude gratings, spatial light modulators, and holographic gratings. The diffracting element can be a transmissive or reflective. A transmissive diffracting element  134  can be placed anywhere along the optical path traversed by the collection beam, either integrated onto the distal end of the collection fiber  20 , as shown in  FIG. 23 , or mounted separately on the optical path, either between the collection fiber  20  and the mirror  82  as shown in  FIG. 23 , or between the collection mirror  82  and the arterial wall. A reflective diffracting element  134  can be integrated directly onto the surface of the collection mirror  82  as shown in  FIG. 24 . 
     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. 
     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. 
     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 . 
     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 
     In use, the distal tip assembly  94  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 . 
     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 
     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.