Abstract:
An imaging spectrometer provides substantially simultaneous areal spectroscopy and image generation to provide improved analysis of possible skin cancer.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application 60/891,146, filed Feb. 22, 2007, the disclosure of which is incorporated herein by reference. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to medical imaging equipment and in particular to an imaging spectrometer for detection of skin cancer. 
         [0004]    The early detection of skin cancer largely relies on visual inspection of the skin and, in particular, on identification of skin patches with asymmetrical outlines, border irregularity, mottled colors, and diameters larger than a 6 mm (the skin cancer “ABC&#39;s”). While visual inspection is particularly well suited to cancer of the skin, the human eye&#39;s ability to distinguish among light frequencies is limited. The human eye is sensitive to three broad frequency bands whose relative intensities create the perception of many colors. Frequencies outside those bands are not perceptible, different frequencies within those bands may not be distinguishable, and single frequencies in areas of overlap of the bands can not be reliably distinguished from the effect of two different frequencies of light within each band. 
         [0005]    The visually observable characteristics of skin cancer cells provide reason to believe that spectral information could provide improved cancer detection. U.S. Pat. No. 4,515,165, issued May 7, 1985, describes a medical imaging system in which cancerous tissue is illuminated with different frequencies of light to provide for multi-spectral imaging of tissue. U.S. Pat. No. 5,782,770 issued Jul. 21, 1998 describes an alternative approach in which polychromatic light illuminates the tissue and a scanning slit spectrograph is used to analyze the reflected light. The spectrograph provides “hyperspectral” imaging providing spectral readings at multiple frequencies with a spectral resolution of less than 10 nm of wavelength. 
         [0006]    In both of these systems the imaging process is delayed by either the need to switch between colors or to scan a slit over an area of the imaged object. This delay creates the potential for misregistration of the image and spectrum and possible distortion of the image or spectrum if there is any movement during the acquisition process. To the extent that the switching of colors or slit scanning process require moving mechanical components, the ability to manufacture a rugged, portable and practical field device, may be adversely affected. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an imaging system that may acquire high resolution spectral and image data in one step, avoiding image registration or image distortion problems, or the need for moving components. The invention employs an optical system that remaps light from normally contiguous elements of an object onto a planar detector in a way that provides interstitial space on the detector between the light from each object element. A dispersion element then generates a spectrum extending into the interstitial space so that the detector simultaneously captures imaging and spectral information. Eliminating the need for moving parts allows perfectly registered and skew-less image and spectral analysis of the skin and allows the practical construction of a rugged handheld device. 
         [0008]    Specifically then, the present invention provides an instrument for detection of skin cancer having a multi-spectral illumination source for illuminating a region of skin and a solid-state image sensor providing multi-spectral sensitivity for imaging that region. An optical system receives light from the illuminated region of skin to optically remap regions of an object of the region onto an image sensor as discontiguous regions, and a dispersion element positioned between the image sensor and the optical system projects spectra of the discontiguous regions onto the image sensor outside of the discontiguous regions. An image processor receives the spectra and analyzes the spectra to identify cancerous features. 
         [0009]    Thus, it is one aspect of at least one embodiment of the invention to provide a spectrometer suitable for clinical use where mechanical scanning elements, which may be cumbersome and unreliable, are eliminated in favor of a fixed optical system. 
         [0010]    The light may be remapped from contiguous regions of the skin and that light of the discontiguous regions may fully characterizes the light from the contiguous region. 
         [0011]    It is thus an aspect of at least one embodiment of the invention to provide a system that samples all tissue within the region so that possibly small areas of cancer are not missed. 
         [0012]    The identification of cancerous features may provide a matching of spectral characteristics of the spectra to spectral characteristics of known skin cancer types. 
         [0013]    It is thus an aspect of at least one embodiment of the invention to allow spectral identification of possible skin cancer. 
         [0014]    The identification of cancerous features may provide an image that accentuates regions having cancerous features. 
         [0015]    It is thus an aspect of some embodiments of the invention to provide an image that may be easily reviewed and evaluated by a physician providing spectral data as an overlay. 
         [0016]    The dispersion element may be a prism. 
         [0017]    It is therefore a feature of at least one embodiment of the invention to provide a system that produces an unambiguous mapping of spectra on the image detector surface without the repeating spectrum orders that may be produced by an optical grating. 
         [0018]    The optical system may be a micro lens array. 
         [0019]    It is thus one feature of one embodiment of the invention to provide a simple optical system for providing the needed remapping. 
         [0020]    The optical system may be a set of light guides. 
         [0021]    It is a feature of at least one embodiment of the invention to provide a flexible optical system that may provide an arbitrary remapping of light for optimal spectral detection. 
         [0022]    The multi-spectral illumination source may use light emitting diodes and the system may normalize the acquired spectra against a spectrum of the light emitting diodes before analyzing the spectra to identify cancerous features. 
         [0023]    It is thus a feature of at least one embodiment of the invention to provide a system that may work with cool and long-lived light emitting diodes that nevertheless have a variable light spectrum. 
         [0024]    The image processor may receive the spectra to reconstruct the spectra into an image, and the image processor may process at least one of the spectra and the image according to information derived from the other one of the spectra and image. 
         [0025]    It is thus an aspect of at least one embodiment of the invention to allow analysis of the spectra and the image to each be informed by the analysis of the other. 
         [0026]    The image may be processed to identify likely cancerous tissue and likely non-cancerous tissue to allow comparison of spectra in the cancerous tissue and non-cancerous tissue for the identification of cancerous tissue. 
         [0027]    It is thus a feature of at least one embodiment of the invention to allow the image to be used to identify baseline non-cancerous tissue for the improved spectral detection of cancerous tissue. 
         [0028]    The spectra may be used to identify a boundary of spectrally different tissue in the image for evaluation of spatial features of the boundary in the image. 
         [0029]    It is thus another feature of at least one embodiment of the invention to allow improved spatial identification of the outline of a patch of differentiated tissue on skin through the use of spectral analysis. 
         [0030]    The instrument may be hand held and may further include a focus guide holding the instrument unit at a fixed distance from the skin when one edge of the focus guide is placed against the skin. 
         [0031]    It is thus another feature of the invention to provide a system that may rapidly acquire image and spectral data in a handheld implementation for convenient use by a physician or at home. 
         [0032]    The optical system may simultaneously measure an areal image of the region of the skin composed of multiple contiguous elements and the spectra of the multiple contiguous elements. 
         [0033]    Thus it is an aspect of at least one embodiment of the invention to provide for a system that provides for perfect registration between an image and its spectral measurement for improved analysis of both. 
         [0034]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]      FIG. 1  is a perspective view of an instrument, according to one embodiment of the present invention, showing a focus guide pressed against the skin and, in phantom, the path of light received from the patient&#39;s skin through portions of the optical system; 
           [0036]      FIG. 2  is a bottom plan view of the instrument of  FIG. 1  showing an illuminating ring of light emitting diodes within the focus guide and surrounding an objective lens; 
           [0037]      FIG. 3  is a spectrum measured by the instrument of  FIG. 1  superimposed on the spectrum of the light from the light emitting diodes such as may be used to normalize the former; 
           [0038]      FIG. 4  is a flow chart showing the various optical planes generated in the present invention including an object plane on the skin, a pupil plane used for the spectral analysis and which may be mapped to a spectrum plane, and a second image plane on an image detector; 
           [0039]      FIG. 5  is a schematic representation of the optical elements of the instrument of  FIG. 1  showing a micro lens array used as an optical remapper and a prism used as the dispersion element; 
           [0040]      FIG. 6  is a block diagram of a processor associated with the unit of  FIG. 1  receiving image and spectrum data to provide analysis of a skin tissue; 
           [0041]      FIG. 7  is a figure similar to that of  FIG. 5  showing an alternate optical remapper employing light guides; and 
           [0042]      FIG. 8  is a spectrum plane similar to the spectrum plane of  FIG. 4  but as created by the optical system of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0043]    Referring now to  FIG. 1 , an imaging spectrometer  10  of the present invention may provide for a housing  12  that may be manipulated by hand to view an object area  24  of the skin of a patient  14 . 
         [0044]    A tubular focus guide  16  may extend from the bottom of the housing  12  so that an optical assembly  18  within the housing may be precisely located with respect to the object area  24  at a desired focal distance. 
         [0045]    Referring also to  FIG. 2 , the tubular focus guide  16  may surround an objective lens  20  of the optical assembly  18 , which is preferably a fixed focus lens, whose imaging characteristics match the length of the tubular focus guide  16 . A series of different fixed focal length lenses or a variable focus lens and auto focusing mechanism may also be used in an alternative embodiment. 
         [0046]    A set of light emitting diodes  22  may be arranged in a ring about the objective lens  20  inside the tubular focus guide  16  to provide even illumination of the object area  24  on skin of the patient within the tubular focus guide  16 . Alternatively standard incandescent type or fluorescent bulbs may be used. The focus guide  16  may be transparent for ease of positioning on the imaging spectrometer  10  over a desired object area  24  or may be opaque to block interference from external illumination sources with location of the focus guide over the desired object area  24  being done by viewing of an electronic image to be described. 
         [0047]    The imaging spectrometer  10  may be wholly contained within the housing  12  or may transmit image data via a cord  44  or a wireless transmitter (not shown) to remote image processing circuitry  43 , as will be described. All or part of the analysis to be described may be done remotely, for example, by transmitting data directly to the physician&#39;s office for processing or review. 
         [0048]    Referring now to  FIG. 3 , the light emitting diodes  22  or other illumination source may produce light having a non-uniform spectrum  25  being a function of the construction of white light emitting diodes (made up of three colored light emitting diodes) as well as variations in the manufacturing process. This spectrum  25  may be measured and used for a normalization as will be described below. 
         [0049]    Referring now to  FIGS. 4 and 5 , the objective lens  20  may be aligned and focused on the object area  24  of the skin in the area of a suspicious feature  26 . The object area  24  is then illuminated by the light emitting diodes  22  and the reflected light is collected by the objective lens  20  and passed to a micro-lens array  27 . The micro-lens array is a set of lenses, either standard or anamorphic lenses, arranged contiguously over an area to capture substantially all the light passing through the area and provide multiple focal points, one focal point associated with each lens. 
         [0050]    The micro-lens array  27  thereby effectively divides the object area  24  formed of contiguous object elements  28  and remaps the light from the continuous object elements  28  to corresponding light points  30  in a pupil plane  32 . Each light point  30  generally has a smaller spatial extent than the object elements  28  provided the object elements  28  are sufficiently small to present an essentially constant field with no or little spatial variation. This constant field of each object element  28  is transformed by each lens of the micro-lens array  27  to a single, intensity value at the light point  30 . Thus, light points  30  provide spatially compressed versions of object elements  28  retaining the same spectral content and total energy of the object elements  28 . In this embodiment, substantially all of the light from the object area  24  is captured and remapped to the pupil plane  32 . 
         [0051]    The pupil plane  32  is aligned with a first object plane of a spectrometer  34  which receives the light points  30  and disperses them according to frequency onto an image (spectrum) plane  36  aligned with the face of an image detector  38 , for example, a charge coupled device (CCD) detector, which maybe a broadband monochromatic sensor. The dispersion in this case may be done by means of a prism  40  eliminating the multiple orders of spectrum produced by a grating; however, the invention contemplates that a grating may also be used by proper design of the spectrometer  34 . 
         [0052]    The dispersion axis of the prism  40  is oriented at an angle with respect to rows and columns of lenses of the micro-lens array  27  so that the light from each of the light points  30  in the pupil plane  32  are spread into a separate spectra  42  extending in interleaved fashion without interference with each other on the surface of the image detector  38 . 
         [0053]    Spectral data  45 , representing intensity values acquired from the image detector  38  at defined locations corresponding to particular frequencies of light in the spectra  42 , are received by image processing circuitry  43 . The pixel size of the image detector  38  is sufficiently small to allow multipoint measurements (e.g., 16-100 measurements) of each spectrum  42  permitting multiple different frequency bands for each object element  28  of the object area  24  to be resolved and detected. This spectral data  45  provides intensity and location information that may be used to fully characterize the spectra  42 . 
         [0054]    The spectra  42  each provide one row of a data cube  47  where position along the row provides intensity values as a function of wavelength or frequency. The location of the row in the data cube  47 , in perpendicular coordinates x and y, correspond to the x and y location of the object elements  28  in the object area  24  forming the light points  30 . Thus, one data acquisition may produce a data cube  47  of spectral and spatial data. 
         [0055]    Additional discussion of the operation of a spectrometer suitable for use in the present invention for spectrometer  34 , is described in detail in U.S. patent application 2006/0072109, naming the inventor of this application, filed Apr. 6, 2006 and hereby incorporated by reference. 
         [0056]    Referring still to  FIGS. 4 and 5 , spectral data  45  of the spectrum plane  36  is provided to image processing circuitry  43 , as will be described in additional detail below. The image processing circuitry  43  may be incorporated into the housing  12  of the imaging spectrometer  10  or in a remote computer connected to the imaging spectrometer  10  by means of a cable  47  or wireless systems such as Bluetooth transmitters and the like. 
         [0057]    Referring to  FIGS. 4 and 6 , the imaging spectrometer  10  processes the spectral data  45  to generate an electronic image  44  by integrating the energy in each spectra  42  over frequency and mapping that total energy, according to the known geometry of the optical assembly  18 , back to corresponding image elements  46  having the same geometrical relationship as the object element  28 . No image data is lost in this process (in terms of feature resolution) provided that the object elements  28  are less than half the size of any feature desired to be resolved. Appropriate selection of the micro-lens array  27  may ensure this condition. 
         [0058]    The electronic image  44  generated by the image processing circuitry  43  may be a monochromatic image or may be a color image, the later generated by partitioning each spectrum  42  into bands approximating the three primary additive colors. Alternatively, the electronic image may be constructed from other portions of the spectra  42  to create false color images or band limited images. Similarly, weighting may be applied to the colors of the spectra to provide color-weighted images. 
         [0059]    It will be understood from the above description that the imaging spectrometer  10  may thus receive light in each frame of data, from the entire object area  24  to produce at once both spectral data  45  and an electronic image  44  for all the contiguous object elements  28  in the object area  24  ensuring accurate registration between the electronic image  44  and the spectral data  45  with reduced distortion caused by motion of the imaging spectrometer  10 . 
         [0060]    Referring now to  FIG. 6 , the image processing circuitry  43  may further process the collected multipoint spectral data  45 , as indicated by process block  50 , and multipoint electronic image  44 , as indicated by process block  52 . This processing may be by means of an electronic computer (not shown) executing a stored program to process digital values representing the spectral data  45  and the electronic image  44 . 
         [0061]    First, as indicated by process block  54 , each spectrum  42  of the spectral data  45  may be normalized to the spectrum  25  of the known incident light of the light emitting diodes  22  or other light source. This normalization may simply divide each point of the spectral data  45  for each spectrum  42  by the corresponding point of the spectrum  25  to compensate for changes in intensity of the illumination at different frequencies. 
         [0062]    At process block  56 , the corrected spectral data  45  may be analyzed to extract spectral features. In the simplest case, predefined and stored reference spectrum frequency bands and normalized intensities for those bands, for known healthy and cancerous tissues, may be compared to corresponding bands and intensities of the tissue imaged by the present invention. Both the reference and actually measured spectral values may be normalized to have comparable total light energy, the total light energy being the integral of the spectra between two predetermined frequencies. This approach allows different spectral bands to be isolated or given greater weight in the analysis process. Alternatively, a library of normalized reference spectra for known healthy and cancerous tissues may be correlated with the measured spectra to identify a closest match. 
         [0063]    The spectral feature extraction process may be augmented by the step of taking an initial scan of the patient&#39;s skin near the region of the suspicious feature  26  but believed to be cancer free, or using regions in the data image away from the suspicious feature that are believed to be cancer free to provide a patient reference spectrum that may be used to compensate (for example, by looking at only spectral differences between these two spectra) for variations in underlying tissue pigmentation among individuals and lighting differences between frames. In this latter case, the reference spectral values may also be difference values. 
         [0064]    As an alternative to identifying separate cancer free areas, the image processor  43  may normalize all spectra to an average of an entire frame, and in this way correct for variations due to illumination and skin color, discolorations in the skin and externally scattered light under the assumption that the image area is large enough to contain a healthy skin sample. 
         [0065]    The present invention contemplates that the spectral feature extraction may be informed by the electronic image  44  and, in particular, by an image feature extraction performed at process block  58  in which the area of the suspicious feature  26  is demarcated using standard image processing techniques such as morphological analysis. In this case, the electronic image  44  may be used to identify regions of likely healthy tissue and regions of suspicious tissue so that the spectral features of these two regions may be compared as described above automatically, or with operator oversight. In this case, the image guides the spectral analysis. The image may also be used to provide areas over which the spectra will be averaged to increase statistical reliability or to provide weighting of the significance of spectral data  45  from different regions. 
         [0066]    Alternatively, the image feature extraction of process block  52  may be used independently to assess a demarcated area of the skin for cancer using the image based tests of asymmetry, irregular border, color and diameter (the ABC&#39;s of skin cancer detection). This demarcation process, in turn, may be informed by the spectral analysis of process block  56 , for example, by defining the boundary of the suspicious feature  26  by spectral features that may not be readily apparent to the human eye. 
         [0067]    Information from the image feature extractor of process block  58  and the spectral feature extraction of process block  56  may be weighted and combined by a rules engine  60  operating using a set of expert rules, templates or statistically derived algorithms to identify whether cancer is likely based on both spectral and image measurements. 
         [0068]    In addition, the information from the spectral feature extraction of process block  56  and the image feature extractor of process block  58  may be provided to an image generator  62  which may display on a display  64  an image  66  showing a conventional image of the object area  24  as would be visible to a human observer, and a false color image  68  superimposed on that image either identifying particular spectral features or identifying regions, per the rules engine  60 , where cancerous tissue may be likely. One or more quantitative values  74  may also be displayed indicating for example skin area, confidence values and the like. 
         [0069]    Referring now to  FIG. 7 , in an alternative embodiment of the present invention, the objective lens  20  may be followed by a fiber optic remapper  71 . The fiber optic remapper  71  may have a front face  70  consisting of the bundled ends of optical fibers arranged over a rectangular area at the image plane of the objective lens  20  to receive light from the objective lens for predetermined object elements  28 . The optical fibers are then routed separately to a rear face  72  where the ends of the fibers have been rearranged into a single continuous line. The rear face  72  of the fiber optic remapper  71  sits at the image plane of the spectrometer  34  such as that described above. In this case, and referring  FIG. 8 , the spectra  42  may extend an arbitrary distance, providing potentially greater spectral resolution. Other methods of “slicing” the image, including the use of mirror arrays and the like, are also envisioned. 
         [0070]    In yet a further embodiment, (not shown) the remapping may simply sample small areas of the object plane by blocking other areas. This is not a preferred embodiment, but can provide many of other benefits of the invention. 
         [0071]    While the preferred embodiment reconstructs image data from the spectral data, the invention also contemplates that the image could be generated using a beam splitter and second image detector  38 . 
         [0072]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.