fig1 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 fig2 , 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 collection spot 34 on the arterial wall 14 and directs that light into the collection fiber 20 . 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 . 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 fig1 ). 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 . fig3 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 . 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 . 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 . 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 to fig4 , 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 . as suggested by fig4 , 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 fig5 , 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 . fig4 - 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 fig6 . in fig6 , 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 . 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 fig7 , 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 fig7 , a collection of lenses , or a lens integrally formed with the collection fiber 20 . 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 fig8 . separation of the collection spot 34 and the illumination spot 32 can also be achieved by orienting the delivery mirror 60 , as shown in fig9 , or by orienting both the delivery mirror 60 and the collection mirror 82 , as shown in fig1 . in both fig9 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 . 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 . fig6 - 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 . 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 fig1 , the contour plot of fig1 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 fig1 . in fig1 , 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 fig1 , 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 . it is apparent from fig1 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 . fig1 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 . in addition , in the discussion of fig1 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 . 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 . 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 . 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 . 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 . 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 . suitable fibers for use as a delivery fiber 18 include those having a numerical aperture of 0 . 12 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 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 and a core diameter of 62 . 5 micrometers . 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 . 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 . 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 . 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 .
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 .