Abstract:
The present invention relates to probe apparatuses and component combinations thereof that are used to recognize possibly abnormal living tissue using a detected early increase in microvascular blood supply and corresponding applications. In one embodiment there is disclosed an apparatus that emits broadband light obtained from a light source onto microvasculature of tissue disposed within a human body and receives interacted light that is obtained from interaction of the broadband light with the microvasculature for transmission to a receiver. Different further embodiments include combinations of optical fibers, polarizers and lenses that assist in the selection of a predetermined depth profile of interacted light. In another embodiment, a kit apparatus is described that has various probe tips and/or light transmission elements that provide for various combinations of predetermined depth profiles of interacted light. In a further embodiment, a method of making a spectral data probe for a depth range detection selectivity for detection of blood within microvasculature of tissue is described.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority to U.S. Application No. 60/801,947 entitled “Guide-To-Colonoscopy By Optical Detection Of Colonic Micro-Circulation And Applications Of Same”, which was filed on May 19, 2006, the contents of which are expressly incorporated by reference herein. This application is also a continuation-in-part and claims priority to co-pending U.S. patent application Ser. No. 11/261,452 entitled “Multi-Dimensional Elastic Light Scattering”, filed Oct. 27, 2005 with the same assignee as the present invention, the disclosure of which is incorporated in its entirety herein by reference.  
         [0002]     This application is also related to co-pending U.S. Patent Application with Attorney Docket No. 16936-58277, entitled “Method of Recognizing Abnormal Tissue Using the Detection of Early Increase in Microvascular Blood Content” that is being filed on the same day as this application, and which is also incorporated in its entirety by reference.  
         [0003]     Some references, which may include patents, patent applications and various publications, are cited and discussed in the description of this invention. The citation and/or discussion of such references is provided merely to clarify the description of the present invention and is not an admission that any such reference is “prior art” to the invention described herein. All references cited and discussed in this specification are incorporated herein by reference in their entireties and to the same extent as if each reference was individually incorporated by reference. 
     
    
     STATEMENT AS TO RIGHTS UNDER FEDERALLY-SPONSORED RESEARCH  
       [0004]     This invention was made with Government support under Grant Nos. R01CA109861 and U01CA111257 awarded by the National Institutes of Health of the United States. Accordingly, the United States Government may have certain rights in this invention pursuant to the grant. 
     
    
     FIELD OF THE INVENTION  
       [0005]     The present invention relates generally to light scattering and absorption, and in particular to probe apparatuses and component combinations thereof that are used to recognize possibly abnormal living tissue using a detected early increase in microvascular blood supply and corresponding applications including in vivo tumor imaging, screening, detecting and treatment, and, in particular, “Early Increase in microvascular Blood Supply” (EIBS) that exists in tissues that are close to, but are not themselves, the lesion or tumor and in tissues that precede the development of such lesions or tumors.  
       BACKGROUND OF THE INVENTION  
       [0006]     Optical probes are known that detect optical signals. Simple optical probes will transmit broadband or a laser light to a target with one optical fiber, and receive the light such as light that is elastically scattered from a specimen, fluorescent light, Raman scattered light, etc., with another optical fiber. The received backscattered light can be channeled to a receiver, such as a CCD array, and the spectrum of the signal is recorded therein.  
         [0007]     While such probes work sufficiently for their intended purposes, new observations in terms of the type of measurements that are required for diagnostic purposes have required further enhancements and improvements. The present invention sets forth enhancements and improvements for a variety of different probes that are useful in detecting “Early Increase in microvascular Blood Supply” (EIBS).  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention relates generally to light scattering and absorption, and in particular to probe apparatuses and component combinations thereof that are used to recognize possibly abnormal living tissue using a detected early increase in microvascular blood supply and corresponding applications  
         [0009]     In one embodiment there is disclosed an apparatus that emits broadband light obtained from a light source onto microvasculature of tissue disposed within a human body and receives interacted light that is obtained from interaction of the broadband light with the microvasculature for transmission to a receiver. Different further embodiments include combinations of optical fibers, polarizers and lenses that assist in the selection of a predetermined depth profile of interacted light.  
         [0010]     In another embodiment, a kit apparatus is described that has various probe tips and/or light transmission elements, such that each different combination preferably provides for a different predetermined depth profile of interacted light.  
         [0011]     In a further embodiment, a method of making a spectral data probe for a depth range detection selectivity for detection of blood within microvasculature of tissue is described. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:  
         [0013]     FIGS.  1 ( a ) and ( b ) show schematically according to one embodiment of the present invention a fiber-optic polarization-gated probe: (a) side view and (b) cross-section view of distal (i.e., close to tissue surface) tip.  
         [0014]      FIG. 2  shows according to one embodiment of the present invention photographically a polarization-gated probe in an accessory channel of an endoscope.  
         [0015]      FIG. 3  illustrates a probe kit according to the present invention.  
         [0016]     FIGS.  4 ( a )-( j ) illustrate various configurations of the probe according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Various embodiments of the invention are now described in detail. Referring to the drawings, like numbers indicate like components throughout the views, As used in the description herein and throughout the claims that follow, the meaning of “a”, “an”, and “the” includes plural reference unless the context clearly dictates otherwise Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. Moreover, titles or subtitles may be used in the specification for the convenience of a reader, which shall have no influence on the scope of the present invention. Additionally, some terms used in this specification are more specifically defined below.  
         [0018]     The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention, For convenience, certain terms may be highlighted, for example using italics and/or quotation marks. The use of highlighting has no influence on the scope and meaning of a term; the scope and meaning of a term is the same, in the same context, whether or not it is highlighted. It will be appreciated that same thing can be said in more than one way. Consequently, alternative language and synonyms may be used for any one or more of the terms discussed herein, not is any special significance to be placed upon whether or not a term is elaborated or discussed herein. Synonyms for certain terms are provided. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.  
         [0019]     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 pertains. In the case of conflict, the present document, including definitions will control.  
         [0020]     As used herein, “around”, “about” or “approximately” shall generally mean within 20 percent, preferably within 10 percent, and more preferably within 5 percent of a given value or range. Numerical quantities given herein are approximate, meaning that the term “around”, “about” or “approximately” can be inferred if not expressly stated.  
         [0021]     The present invention, in one aspect, relates to a probe apparatus that is used for optically examining a target for tumors or lesions using what is referred to as “Early Increase in microvascular Blood Supply” (EIBS) that exists in tissues that are close to, but are not themselves, the lesion or tumor. While the abnormal tissue can be a lesion or tumor, the abnormal tissue can also be tissue that precedes formation of a lesion or tumor, such as a precancerous adenoma, aberrant crypt foci, tissues that precede the development of dysplastic lesions that themselves do not yet exhibit dysplastic phenotype, and tissues in the vicinity of these lesions or pre-dysplastic tissues.  
         [0022]     A particular application described herein is for detection of such lesions in colonic mucosa in early colorectal cancer (“CRC”), but other applications are described as well.  
         [0023]     The target is a sample related to a living subject such as a human being or animal. The sample may be a part of the living subject such that the sample is a biological sample, wherein the biological sample may have tissue developing a cancerous disease.  
         [0024]     The neoplastic disease is a process that leads to a tumor or lesion, wherein the tumor or lesion is an abnormal living tissue (either premalignant or cancerous), such as pancreatic cancer, a colon cancer, an adenomatous polyp of the colon, a liver cancer, a lung cancer, a breast cancer, or other cancers.  
         [0025]     The measuring step is preferably performed in vivo, though it can be performed ex vivo as well. The measuring step may further comprise the step of acquiring an image of the target. The image, obtained at the time of detection, can be used to later analyze the extent of the tumor, as well as its location.  
         [0026]     In one embodiment, the probe projects a beam of light to a target that has tissues with blood circulation therein. At least one spectrum of light scattered from the target is then measured, and blood supply information related to the target is obtained from the measured at least one spectrum. The obtained blood supply information comprises data related to at least one of blood content, blood oxygenation, blood flow and blood volume.  
         [0027]     The probe can be used to obtain different optical measurements. According to one embodiment, it may be used to obtain a first set of the blood supply information from a first location of the target and then obtain a second set of the blood supply information from a second location of the target. The first set of the blood supply information at a first location of the target and the second set of the blood supply information at a second location of the target can then be compared to determine the status of the target. One can compare the data to indicate whether the tumor or lesion exists at all by comparison to previously established microvascular blood content values from patients who harbor neoplasia and from those who are neoplasia free.  
         [0028]     In one embodiment, a probe apparatus comprises a light source configured and positioned to project a beam of light to a target; and means for measuring at least one spectrum of light scattered from the target; and means for obtaining blood supply information related to the target from the measured at least one spectrum.  
         [0029]     The probe apparatus may further comprise a detector that obtains a first set of the blood supply information at a first location of the target. The same detector can be used to obtain a second set of the blood supply information at a second location of the target. Spectral data which is then obtained from the probe apparatus is analyzed and used to determine whether the tissue that has been inspected is abnormal. This analysis is fully described in the application referred to earlier as co-pending U.S. Patent Application with Attorney Docket No. 16936-58277, entitled “Method of Recognizing Abnormal Tissue Using the Detection of Early Increase in Microvascular Blood Content” that is being filed on the same day as this application, and as such is not described further herein.  
         [0030]     In one embodiment, at least one spectrum of light scattered from the target is measured by a fiber optic probe according to the present invention, wherein the fiber optic probe comprises a polarization-gated fiber optic probe configured to detect the blood content information. The light source comprises an incoherent light source (such as a xenon lamp).  
         [0031]     In one embodiment, the fiber optic probe includes a proximal end portion, an opposite, distal portion, and a body portion with a longitudinal axis defined between the proximal end portion and the distal portion. The body portion is formed with a cavity along the longitudinal axis. At least one first type of fiber is used for delivering a beam of energy to a target, wherein the at least one first type fiber is at least partially positioned within the cavity of the body portion. An optical element is positioned at the proximal end portion and configured to focus the beam of energy to the target. At least one second type fiber is used for collecting scattered energy from the target, wherein the at least one second type fiber is at least partially positioned within the cavity of the body portion.  
         [0032]     The fiber optic probe may further comprise at least one linear polarizer optically coupled to the at least one first type fiber and the at least one second type fiber and positioned proximate to the proximal end portion, and wherein the optical element is positioned at the proximal end portion and configured to focus the scattered energy from the target to the at least one linear polarizes for the at least one second type fiber to collect.  
         [0033]     The optical element comprises at least one of a ball lens, a graded refractive index lens, an aspheric lens, cylindrical lens, convex-convex lens, and plano-convex lens, although preferably just a single lens is used. Lenses other than these above-mentioned lenses can also be used. It is further noted that different lenses can be used to assist in discriminating measurements and to achieve different tissue penetration depths. Thus, for example, to achieve the shortest penetration depth, a lens can be positioned at the focal distance from the end of the light-collecting fibers with the fibers positioned symmetrically around the axis of the lens. This configuration further increases the intensity of collected light, particularly when a probe is at a distance from tissue, and provides improved stability of the signals collected by the probe in terms of different distances from tissue (if a probe is not in contact with tissue) and pressures exerted by the probe onto tissue (if a probe is in contact with tissue). Shorter penetration depth can also be achieved by using a lens with a shorter focal distance, smaller numerical aperture of the illumination and/or collection fibers, and larger distance between illumination and collection fiber. In principle, penetration depths from a few tens of microns to a few millimeters can be achieved by choosing a proper combination of these probe characteristics.  
         [0034]     The at least one first type fiber comprises an illumination fiber, wherein the illumination fiber is optically coupled to the light source.  
         [0035]     The at least one second type fiber can also be formed with one or more collection fibers, wherein the one or more collection fibers are optically coupled to an imaging spectrograph and a CCD at the distal end portion, which imaging spectrograph is used to obtain an image of the target. The body portion comprises a tubing.  
         [0036]     The following further details of the preferred embodiments that will further describe the invention.  
         [0037]     Without intent to limit the scope of the invention, exemplary instruments, apparatus, methods and their related results according to the embodiments of the present invention are given below. Note that titles or subtitles may be used in the examples for convenience of a reader, which in no way should limit the scope of the invention. Moreover, certain theories are proposed and disclosed herein; however, in no way they, whether they are right or wrong, should limit the scope of the invention so long as the invention is practiced according to the invention without regard for any particular theory or scheme of action.  
         [0038]     Polarization Gating: Polarization gating has been previously used to selectively record short-traveling photons as well as to increase contrast for photons emerging from deeper tissue. As has been shown by our group, the differential polarization signal Δ I (λ)=I∥(λ)−I ⊥ (λ) is primarily contributed by scatterers located close to the tissue surface and, therefore, particularly sensitive to the properties of the superficial tissues, e. g. epithelial. Our experiments showed that the contribution to the differential polarization signal from deeper tissue structures rapidly decreases with “optical distance” (aka. “optical depth”) to the structure and, hence, with depth (optical distance τ=L/ls with L “physical” depth and ls photon mean free path length in tissue). Because optical density of the epithelium is much smaller than that of underlying connective tissue, in the colon, differential polarization signals are primarily collected from the epithelium plus the mucosal connective tissue with the depth of penetration determined by the design of the probe. For example, if a probe&#39;s depth of penetration is about two optical distances or, put alternatively, about two mean free path lengths, it corresponds to approximately the colonic mucosal connective depth of ˜100 μm. This near-surface portion of subepithelial stoma contains a network of capillaries supplying oxygen to the epithelium. Co-polarized signal I∥, arbitrarily polarized signal I∥+I 195   and cross-polarized signal I ⊥  contain information about progressively deeper tissue, up to several millimeters below the surface for certain probe configurations.  
         [0039]     Polarization Gated Fiber-Optic Probe to Detect EIBS: In one aspect, a fiber-optic probe has been developed to accurately detect blood supply in tissue mucosa.  FIGS. 1A and 1B  illustrate the design of the probe and  FIG. 2  shows a photograph of the probe protruding from an accessory channel of a colonoscope. The probe  100  has one or more 100 μm-diameter fibers, one delivery fiber  1   10  used for delivery of linearly polarized light from a Xe-lamp (not shown) onto the tissue surface and the other two fibers  120  and  122  for collecting scattered light from the tissue. A positive lens  130  was positioned at the focal distance from the fiber tips. Several lens types were also tested, including ball, graded refractive index (GRIN), and aspherical lenses. All of the different types of lenses could be used and these provide different performance of the probe in terms of the depth of penetration. In the configuration where the lens  130  was positioned at the focal distance from the fiber tips, it focused light backscattered from a sample onto different fibers  120  and  122 , depending on the angle of backscattering. It also ensured that all collection fibers receive scattered light from the same tissue site, which coincides with the illumination spot. The lens  130  does not have to be positioned at the focal distance from the fibers  110 ,  120 ,  122 , but this configuration provides better performance in terms of 1) shorter penetration depth, in particularly for the polarization gated signal, 2) increases signal level and, thus, time required to collect the signal with sufficient signal-to-noise ratio, 3) prevents collection of specular reflection from probe and tissue surfaces, and 4) improves stability of the measurements in terms of probe displacement from tissue surface in non-contact geometry or the pressure exerted by the probe onto a sample. In the proximal end of the probe  100 , the collection fibers  120 ,  122  are coupled to an imaging spectrograph and a CCD. Two thin film polarizers  140 ,  142  were mounted on the proximal tip of the probe to polarize the incident light and enable collection of both polarization components (i.e. parallel I∥ and I ⊥  perpendicular to the incident polarization) of the backscattered light to allow for polarization gating. All components of the probe  100  were made from FDA approved materials.  
         [0040]     At least some or all of the components of the probe  100  can be selected according to their characteristics or variables such as optical parameters, relative positions, geometrical dimensions to assist with the detection of the interacted light at different tissue penetration depths. A lens at the probe tip is one of these components that assist in allowing the selecting of a desired penetration depth. For example, to achieve a shorter penetration depth, a lens can be positioned at the focal distance from the end of the fibers with the fibers positioned symmetrically around the axis of the lens. Furthermore, one can use a lens with a shorter focal distance, smaller numerical aperture of the illumination and/or collection fibers, and a larger distance between the illumination and collection fiber. For example, probes were fabricated with a GRIN lens with the penetration depth in colon tissue for polarization-gated signal ˜85 microns (˜1.7 mean free path lengths) and that for cross-polarized light ˜260 microns. A ball lens probe with penetration depths ˜23 and 275 microns was also developed. As such, it is apparent that penetration depths from a few tens of microns to a few millimeters can be achieved by choosing a proper combination of probe characteristics.  
         [0041]     Accordingly, the present invention provides for a probe kit  300 , illustrated in  FIG. 3  that contains a plurality of interchangeable probe tips  310 - 1  to  310 -n and a plurality of interchangeable optical transmission elements  320 - 1  to  320 - n , where n is an integer greater than 1. Different combinations of these allow for a variety of depth selectivity.  
         [0042]     FIGS.  4 ( a )-( j ) illustrate various configurations of the probe according to the present invention. FIGS.  4 ( a )-( e ) illustrate probe configurations that have a single depth selectivity based upon the various characteristics of the components included.  FIG. 4   a  shows an embodiment in which there is only a single delivery fiber  410   a  and a single collection fiber  420   a . There may be a polarizer  440   a .  FIG. 4   b  is similar to  FIG. 4   a , but further includes the usage of two polarizers  440   b  and  442   b .  FIGS. 4   c ,  4   d  and  4   e  illustrate versions with two collection fibers  420   c ,  422   c ;  420   d ,  422   d ; and  420   e ,  422   e , respectively. In each of these embodiments there are two or three polarizers,  440   c  and  442   c ;  440   d  and  442   d ; and,  440   e ,  442   e  and  444   e  as shown, respectively, in various configurations relative to the optical fibers.  
         [0043]     FIGS.  4 ( f )-( j ) illustrate probe configurations in which a single probe, including both the probe tip and the transmission delivery element, can have more than one depth selectivity.  
         [0044]     In  FIG. 4 ( f ) there exist pairs of collection fibers, and each pair has the same collection depth. Thus collection pair  420   f   1  and  420   f   2  have penetration depth  1 , collection pair  422   f   1  and  422   f   2  have penetration depth  2 , and collection pair  424   f   1  and  424   f   2  have penetration depth  3 . Each of the fibers in each pair is spaced the same distance from the delivery optical fiber  410   f . There are two polarizers  440   f  and  442   f  as shown.  
         [0045]     In  FIG. 4 ( g ) there is not a collection pair, but rather individual collection fibers  420   g ,  422   g ,  424   g ,  426   g  and  428   g , that each has a different spacing from the delivery optical fiber  410   g . Certain of the collection optical fibers have a different numerical aperture than the others, shown as  426   g  and  428   g . There are two polarizers  440   g  and  442   g  as shown.  
         [0046]     The  FIG. 4 ( h ) embodiment is similar to the  FIG. 4 ( f ) embodiment, except at the penetration depth  3  there is an additional fiber pair that includes collection optical fibers  426   h   1  and and  426   h   4 , each of which has a numerical aperture different that the other collection pair at penetration depth  3 . There are two polarizers  440   h  and  442   h  as shown.  
         [0047]      FIG. 4 ( i ) illustrates a probe having a delivery fiber  410   i , and three collection fibers  420   i ,  422   i  and  424   i , each spaced a different distance from the delivery fiber  410   i . There is either no polarizer or one polarizer (not labeled).  
         [0048]      FIG. 4 ( j ) illustrates a probe that is same as the probe illustrated in  FIG. 4 ( i ) except that it includes two polarizers  440 ( j ) and  442 ( j ), rather than none or one polarizer.  
         [0049]     The following discussion sets forth the light path for a three-fiber, two polarizer version of a probe, such as illustrated in  FIG. 4 ( b )-( e ). A lamp/light source emits unpolarized light. This light is coupled into a delivery fiber  410 . Unpolarized light emerges from this fiber  410  and passes through the first polarizer  420  and becomes linearly polarized. This light is diverging with angle of divergence depending on the numerical aperture (NA) of the fiber  410 . Typical NA is about 0.22, which means that the angle of divergence is ˜25 degrees. Fibers with NA&#39;s between 0.1 and 0.5 are also available. This polarized but diverging beam then passes through a lens  430 , gets collimated, and impinges upon tissue. The lens  430  is positioned at a focal distance from the fibers  410  and  440 . Two collection fibers  440  collect the light that interacts with, such as by backscattering, the tissue. The spot on tissue surface, which is formed such that the light that emerges from tissue from this spot can reach and can get collected by one of the collection fibers  440 , will be referred to as the “collection spot” for a given collection fiber  440 . If tissue surface is in the focal plane of the lens  430  (GRIN lenses typically have the focal planes coinciding with their surfaces) all illumination and collection spots coincide. One of the collection fibers  440  shares the same polarizer  420  with the delivery fiber  410  and the other fiber  440  is “behind” a second polarizer  450  with the axis of polarization orthogonal (or, to be more general, just different) to the axis of polarization of the first polarizer  420 . The light that interacts, such as by being backscattered, from tissue has both polarization components. Each of these polarizers  420  and  450 , selects light polarized in a particular way and only this light reaches the corresponding collection fiber  440 . The first fiber  440  collects light that is polarized along the same direction as the incident light. This is co-polarized light. The other fiber  440  collects the cross-polarized light. On the other (proximal) ends, the light transmitted through the collection fibers  440  is coupled into a spectrometer and a detector (not shown). The spectrometer and a detector can be a single linear array detector (one for each fiber) or an imaging spectrometer and a CCD (which is more expensive). The detector records the spectrum of light intensity from each fiber, which becomes the co-polarized (I ∥ ) and cross-polarized signals/spectra (I ⊥ ). These spectra are then transmitted to a computer or a CPU. The computer can process these spectral data. Four different spectral curves can be looked at: 1) Differential polarization (or what is called polarization-gated) signal is calculated as I ∥ −I ⊥ ; 2) The total (or arbitrarily polarized) signal is found as I ∥ +I ⊥ ; 3) Co-polarized signal I 81 ; and 4) Cross-polarized signal I ⊥ . Each of these four signals is preferentially sensitive to tissue up to its own penetration depth. In principle, one does not have to use two polarizers and measure both co- and cross-polarized signals. If shallow penetration depth is not desired, one can use just a single polarizer and only one collection fiber (co-polarized signal only), two polarizers and collect cross-polarized signal, no polarizer and collect I ∥+ I ⊥ , etc.  
         [0050]     Specific characteristic combinations for two different probes  100  are provided below.  
       EXAMPLE 1  
     Ball Lens Probe  
       [0000]    
       
          Ball lens diameter: 2 mm,  
          focal length: 1.1 mm,  
          fiber core diameter: 200 microns,  
          numerical aperture (NA): 0.22,  
          distance between illumination and detection fibers: 0.5 mm,  
          spot size on tissue surface: 0.5 mm,  
          output (incident on tissue) beam divergence: 5 deg,  
          outer diameter of the probe: 2.6 mm.  
       
     
       EXAMPLE 2  
     GRIN Lens Probe  
       [0000]    
       
          GRIN lens diameter: 1.8 mm,  
          focal length: 2.4 mm,  
          fiber core diameter: 200 um,  
          NA: 0.22,  
          distance between illumination and detection fibers: 0.7 mm,  
          spot size: 0.7 mm,  
          output beam divergence: 3 deg,  
          outer diameter of the probe: 2.5 mm.  
       
     
         [0067]     In addition to modifying characteristics of the probe tips and the optical transmission elements as mentioned above, there are other characteristics that can be modified.  
         [0068]     These modifications include altering where the end of the collection fiber is relative to the delivery fiber in order to get different angular ranges for this depth selective probe. Positioning fibers away from the focal distance is essentially equivalent to using fibers with a larger diameter that are positioned in the focal plane, which in turn is equivalent to a greater divergence of incident or collected light beams. Greater divergence results in addition of both longer and shorter light paths inside tissue. Overall, in most cases this will result in deeper penetration. It should be noted, however, that by “defocusing” the probe, the “defocusing” configuration is less efficient in terms of intensity the probe collects.  
         [0069]     Another characteristic is the distance to the tissue surface. This distance can be controlled by choosing a proper spacer between the lens and the tissue surface. (As discussed above, most GRIN lenses have their focal plane coinciding with their sides but this is not necessarily the case with other lens types. If other lenses are used, one can put a spacer to distance the probe from tissue.) If this distance is different from the focal length, penetration depth is greater.  
         [0070]     The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teachings.