Abstract:
A laser imaging apparatus comprises a platform for supporting a female patient in frontdown, prone position, including an opening permitting a breast of the patient to be vertically pendant below the surface of the platform; scanning mechanism disposed below the platform to scan the breast without breast compression. The scanning mechanism includes a source of coherent near infrared light pulses operably directed to the breast; photodetectors operably disposed to detect the light pulses after passing through the breast; circuit for deriving voltages proportional to the intensity of the received pulses; and computer programmed for storing and displaying images of tissue in the breast derived from the voltages.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to diagnostic medical imaging apparatus and more particularly to a mammography machine which employs a near-infrared pulsed laser as a radiation source.  
           [0002]    Cancer of the breast is a major cause of death among the American female population. Effective treatment of this disease is most readily accomplished following early detection of malignant tumors. Major efforts are presently underway to provide mass screening of the population for symptoms of breast tumors. Such screening efforts will require sophisticated, automated equipment to reliably accomplish the detection process.  
           [0003]    The X-ray absorption density resolution of present photographic X-ray methods is insufficient to provide reliable early detection of malignant breast tumors. Research has indicated that the probability of metastasis increases sharply for breast tumors over 1 cm in size. Tumors of this size rarely produce sufficient contrast in a mammogram to be detectable. To produce detectable contrast in photographic mammogram 2-3 cm dimensions are required. Calcium deposits used for inferential detection of tumors in conventional mammography also appear to be associated with tumors of large size. For these reasons, photographic mammography has been relatively ineffective in the detection of this condition.  
           [0004]    Most mammographic apparatus in use today in clinics and hospitals require breast compression techniques which are uncomfortable at best and in many cases painful to the patient. In addition, X-rays constitute ionizing radiation which injects a further risk factor into the use of mammographic techniques as almost universally currently employed.  
           [0005]    Ultrasound has also been suggested as in U.S. Pat. No. 4,075,883, which requires that the breast be immersed in a fluid-filled scanning chamber. U.S. Pat. No. 3,973,126 also requires that the breast be immersed in a fluid-filled chamber for an X-ray scanning technique.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to provide an imaging apparatus using light and/or near infrared coupled with ultrafast laser, thus avoiding the drawbacks of prior art X-ray equipment.  
           [0007]    It is another object of the present invention to provide a mammography apparatus wherein the patient lies in a prone face down position to the place the woman&#39;s breast in the scanning chamber in such a way as to gather the maximum amount of tissue away from the chest wall, thereby to provide maximum exposed area without breast compression.  
           [0008]    It is still another object of the present invention to provide a laser imaging apparatus that uses avalanche photodiode coupled with a low leakage precision integrator for a sensitive detection system.  
           [0009]    It is another object of the present invention to provide a laser imaging apparatus with multiplexing technique to allow for efficient gathering of scanned data.  
           [0010]    It is yet another object of the present invention to provide a laser imaging apparatus that uses femtosecond pulse width, near infrared laser pulse.  
           [0011]    Mammography apparatus of the present invention includes a non-ionizing radiation source in the form of very short pulses of near-infrared wave-length from a solid state laser pumped by a gas laser. The patient lies face down on a horizontal platform with one breast extending through an opening in the platform to hang freely inside a scanning chamber. An optical system converts the laser pulses into a horizontal fanned shaped beam which passes through the breast tissue. The breast is scanned a full 360 degrees starting at that portion of the breast which is closest to the body of the patient and is then stepped vertically downwardly and the scan is repeated at each vertical step until a complete scan of the entire breast has been completed. These light pulses are detected after passing through the breast tissue, converted into electrical signals and then recorded and/or displayed to provide an image of normal and abnormal breast tissues.  
           [0012]    These and other objects of the present invention will become apparent from the following detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a perspective view of the of the present invention, showing the patient supporting platform and operator&#39;s console;  
         [0014]    [0014]FIG. 2 is a side view partially in section of the patient support platform of FIG. 1 showing a patient positioned for mammographic study, with one of her breasts positioned within a scanning chamber;  
         [0015]    [0015]FIG. 3A is a side view partially in section of the scanning chamber;  
         [0016]    [0016]FIG. 3B is a schematic view of the scanning chamber of FIG. 3A,  
         [0017]    [0017]FIG. 4 is a top plan view of the scanning chamber which surrounds the breast of the patient;  
         [0018]    [0018]FIG. 5 is a partial perspective on the uppermost portion of the scanning chamber of FIG. 4;  
         [0019]    [0019]FIG. 6 is an enlarged view of the bearing support for the rotatable plate which carries portions of the scanning apparatus;  
         [0020]    [0020]FIG. 7 is a schematic perspective view of an array of photodiode detectors used in the present invention;  
         [0021]    [0021]FIGS. 8A and 8B are electrical schematic diagrams of the detector circuit used in the present invention;  
         [0022]    [0022]FIG. 9 is a functional block diagram of the electrical system used in the present invention;  
         [0023]    [0023]FIG. 10 is a functional block diagram of the detector electronics and multiplexer shown in FIG. 9;  
         [0024]    [0024]FIG. 11 is a schematic top plan view of the of the rotating plate carrying the rotating polygon mirror, showing a fan of laser beams generated by the rotating mirror at one of 4000 positions of the rotating plate;  
         [0025]    [0025]FIG. 12 is a flow chart of data acquisition used in the present invention;  
         [0026]    [0026]FIG. 13 is a flow chart of data reconstruction used in the present invention;  
         [0027]    [0027]FIG. 14 is an example of an image of a female breast using the present invention;  
         [0028]    [0028]FIG. 15 is an electrical schematic diagram of a clamp and time-gate switch circuit;  
         [0029]    [0029]FIG. 16 is an electrical schematic of a laser pulse pick-off circuit used in the present invention;  
         [0030]    [0030]FIG. 17A is a functional block diagram of a clamp control circuit for providing output to the clamp and time-gate switch circuit of FIG. 15;  
         [0031]    [0031]FIG. 17B is a typical response curve of a photodetector, showing the leading edge of the curve at which measurement is taken during the data acquisition phase;  
         [0032]    [0032]FIG. 18A is a representation of laser pulse train;  
         [0033]    [0033]FIG. 18B is a representation of the response of the avalanche photodiode detector to the pulse train of FIG. 18A;  
         [0034]    [0034]FIG. 18C is a similar to FIG. 18B, showing the selection of a comparator threshold level;  
         [0035]    [0035]FIG. 18D is a representation of a pulse train based on the comparator threshold level of FIG. 18C;  
         [0036]    [0036]FIG. 19 is a representation of the response of the avalanche photodiode detector to a laser pulse train traversing an air shot;  
         [0037]    [0037]FIG. 20 is a representation of the response of the avalanche photodiode detector to a laser pulse train exiting a medium, such as breast tissue;  
         [0038]    [0038]FIG. 21 is a schematic diagram of distances used in calculating time-of-arrival for the laser pulses;  
         [0039]    [0039]FIG. 22 is perspective view of another embodiment of a support structure for the orbital plate used in the present invention;  
         [0040]    [0040]FIG. 23 is a perspective view with portions broken away of the drive mechanism for lowering or raising the support plate shown in FIG. 22;  
         [0041]    [0041]FIG. 24 is a cross-section view through the support plate of FIG. 22 with the orbital plate installed in place;  
         [0042]    [0042]FIG. 25 is a perspective view with portions broken away of the orbital plate used in the support structure of FIG. 22, showing the arrangement of optics used in the present invention;  
         [0043]    [0043]FIG. 26A is schematic diagram of photons traversing a tissue, illustrating the paths taken by ballistic, snake-like or diffuse photons through the tissue;  
         [0044]    [0044]FIG. 26B is typical response curve of an avalanche photodetector, showing the portions generated by the respective ballistic, snake-like and diffuse photons after exiting the tissue;  
         [0045]    [0045]FIG. 27A is a schematic illustration of the arrival times of the laser beams at the detectors in free space; and  
         [0046]    [0046]FIG. 27B is a schematic illustration of the arrival times of the laser beams at the detectors when traversing through a tissue.  
         [0047]    [0047]FIG. 28 is a schematic diagram showing an oscillating mirror driven by a galvanometer to sweep a laser beam across a scan circle. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]    Referring first to FIGS. 1 and 2, an apparatus R in accordance with the present invention comprises an operator&#39;s console indicated at  10  which may include monitors  12  and  14 . A patient&#39;s support platform  16  overlies an enclosure  18  which houses the electronics and optics of the present invention. The platform  16  includes an opening  20  which permits one of the patient&#39;s breasts  15  to be positioned through the opening and be pendant within a scanning chamber  22 . A laser beam generated from an Argon ion pump laser  21  and a Ti:Sapphire laser is used to scan the patient&#39;s breast within the scanning chamber  22 .  
         [0049]    A detailed description of the scanning mechanism within the scanning chamber  22  will now be described. Referring to FIGS. 3A, 4,  5  and  6 , an open top, box member  24  is arranged immediately below the opening  20  in the platform  16  and houses the scanning chamber  22  which has its vertical axis aligned with the center of the opening  20 . An annular plate  26  is supported for rotation within the chamber  22  on bearings  28  and  30  (FIG. 6) which permit it to be rotated step-by-step or indexed around the interior of the scanning chamber  22 . The indexing drive for creating this rotation is indicated at  32  in FIG. 4.  
         [0050]    A ring gear  33  secured to the periphery of the annular or orbital plate  26  cooperates with the drive  32  to rotatably index the orbital plate  26 , as best shown in FIG. 4.  
         [0051]    The entire scanning chamber  22  may be moved vertically downwardly from the upmost position shown in FIG. 3 by means of elongated threaded drive rods  34  that are operably secured to the box member  24  at anchors  36  and nuts  37 . Drive motors  39  are operably connected to the threaded rods  34  by conventional means such as by belt/pulley arrangements  41 , as best shown in FIG. 3. Rotation of the threaded rods  34  is effective to lower or raise the scanning chamber  22 . The drive motors  39  are securely fixed to the box member  24  by standard means, such as brackets, and are controlled by motor  43 .  
         [0052]    Turning now to the optics of the apparatus R, the annular plate  26  carries on its upper surface a polygonal multifaceted mirror  38 , as best shown in FIGS. 3, 4, and  5 . The mirror  38  is rotatable on its own vertical axis. A ring  45  of photo-detector arrays  40  is supported on the upper surface of the scanning chamber  22  and surrounds the path traveled by the mirror  38  as it moves in an orbital path generated by revolutions of the plate  26 . The arrays  40  are fixed and stationary with respect to the scanning chamber  22 . The ring  45  is preferably concentric with the orbital path of the mirror  38 .  
         [0053]    The stepping motors  39  are used to rotate the screws  34  in order to move the scanning chamber  22  vertically downwardly through successive increments or slices following each complete orbital movement of the polygonal mirror  38  in order to successively expose portions of the breast of the patient to the pulsed laser radiation until the entire breast has been irradiated.  
         [0054]    The lasers  23  and  21  which supply the radiation for scanning the breast may be positioned within the enclosure  18 , as best shown in FIG. 2. The coherent pulsed light from the solid-state laser is directed from the laser to the polygonal multifaceted mirror  38  by means of a series of mirrors and prisms. The rotating polygon mirror  38  advantageously preserves the laser beam intensity by not diverging the beam and maintaining a controlled alignment between the projected laser beam and the respective detector  62 . A mirror  46  directs an incoming laser beam  44  to a mirror  48 , which then directs the beam to a stack of wedge prisms  50 , which turns the beam at an angle and directs it through an opening  52  in the orbital plate  26 . Two additional mirrors  54  and  56  mounted on the plate  26  then redirect the beam to the rotating polygonal mirror  38 , which generates a fan  55  of beams for each orbital position of the mirror  38 , as best shown in FIGS. 4 and 5. A shelf  35  is supported from the plate  26  and supports the wedge prisms  50 . The shelf  35  rotates with plate  26  such that the wedge prisms  50  are always oriented in the same way with respect to the plate  26  as it rotates.  
         [0055]    Referring to FIG. 3B, the speed of rotation of the multi-faceted mirror  38  used to produce the fan of laser beams  55  is controlled by system electronics  55  and is maintained at a constant speed. A hollow slip-ring assembly  53  is used to bring the electronic signals to the polygon drive motor controller  55 . While the polygon mirror  38  is rotating inside its housing, the entire mirror assembly is rotated in an orbit inside the ring  45  of detector arrays  40 . The orbital speed of the polygon mirror assembly (not the speed of rotation of the mirror itself) is controlled by the drive motor  32  and its motor controller. The orbital position of the polygon mirror assembly is determined through use of a home detector  57  and rotary encoder on the drive motor  32 . The home encoder provides a fixed reference point that is used in conjunction with the rotary encoder to determine the location of the polygon assembly  38 . Thus, for each place in the orbit of the polygon assembly  38 , the detectors  62  in the detector ring that are being swept by the fan of laser beams  55  is determined.  
         [0056]    Femtosecond wide pulses (approximately 106 fs wide) of near infra-red radiation with a wavelength in the 800 to 900 nanometer (nm) wavelength range are produced by the Ti:Sapphire mode locked laser  23 . The average laser power is in the 750 milliwatt (mw) range with a repetition rate of approximately 76.5 megahertz (MHz). The power contained in each laser pulse is approximately 9.9 nanojoules (nj) and the peak pulse power is in the  67  kilowatts (kw) range. The Ti:Sapphire laser  23  is pumped by a 7 watt Argon ion laser  21  using all spectral lines.  
         [0057]    By rotating the polygonal mirror  38  at very high speed, for example in the order of 6000 RPM, the fan-shaped beam  55  is generated and the width of the fan is such that approximately 25% of the photodiode detector arrays  40  are thus illuminated at each rotational indexed position of the plate  26 . Preferably, the mirror  38  is indexed at 4000 positions around a 360 degree circle. This scanning pattern is then repeated at successive vertically lower positions or slices of the plate as the scanning chamber is indexed downwardly by the drive motors  39 .  
         [0058]    The laser beam detector arrays  40  are positioned in the ring  45  on a top surface of the scanning chamber  22  and around the pendulant breast, as best shown in FIGS. 3, 4 and  5 . Each array  40  comprises a number of avalanche photodiodes  62 , as best shown in FIG. 7. The number of photodiodes  62  dictates the number of laser fan beam projections that can be detected as the fan  55  of laser beams sweeps across the breast.  
         [0059]    The detector  62  of each array  40  are disposed on a substrate  64 . The arrays  40  are positioned as chords of a circle around the orbital plate  26 , as best shown in FIG. 4. Each array  40  has  25  individual avalanche photodiode detectors  62 . There are  24  detector arrays  40  to form the ring of laser beam detectors, providing 600 avalanche photodiode detectors.  
         [0060]    Each of the photodiodes  62  is connected to a detector circuit  69 , as best shown in FIG. 8A. The avalanche photodiodes  62  are reversed biased to provide amplification of the detected signal. Each reversed biased detector  62  is used as a current source with the amount of current provided being a function of the number of photons  66  of laser light that impinge on each detector  62 . The number of photons reaching each detector  62  spans a wide dynamic range from no attenuation when the photons are not blocked by the breast tissue to significant attenuation when the photons pass through and eventually emerge from the breast. A current limiting series resistor  68  is used to control the amount of current that can flow through the detector  62  and thus prevents excessive current flow from occurring when the laser beam is unattenuated that otherwise could destroy the detector  62 . A suitable size decoupling capacitor  70  is used to store charge to provide the energy required when the detector  62  responds to a fast rising pulse of photon intensity.  
         [0061]    The current provided by each detector  62  in each array  40  is switched into or off to either an operational amplifier circuit  72  or an electronic integrator  73 , as best shown in FIGS. 8A and 8B. The operational amplifier circuit  72  is used as a current-to-voltage converter to produce a direct current voltage at output  74  proportional to the input current provided by each detector  62 . Thus, a DC voltage can be produced to represent the intensity of the laser beam impinging on the individual detector  62 .  
         [0062]    A fast Schottkey diode  76  provides the switching for each detector  62 . The Schottkey diode  76  is switched into or out of conduction by a clamp circuit, as will be described below, connected at  77 .  
         [0063]    The detector circuit  69  and several control circuits required to control the output of each detector  62  are referred to as detector electronics  82 , as best shown in FIG. 9. The output of detector electronics  82  is fed to a multiplexer  84 , the output of which is then fed to an analog/digital converter  86 . The output of the converter  86  is then fed to a computer  88 . The data acquired from the detector electronics  82  are used by the computer  88  to produce an image of the scanned breast by a reconstruction algorithm, to be described below, derived from computed tomography theory. The digitized slice data is converted to an image by the computer  88  using a reconstruction algorithm, which is then displayed in a monitor  90  in monochrome or pseudo-color. The raw slice data and image data can be stored on a hard drive  92  or any other storage medium, using a floppy drive  94 , a tape drive  96  or a CD-ROM drive  98 .  
         [0064]    Referring to FIG. 10, the detector electronics  82  comprises detector circuit  69  controlled by a clamp and time-gate switch circuit  102 , which is then controlled by a clamp control circuit  104 . The clamp control circuit  104  is synchronized by the computer  88  and a pulse pick-off circuit  106  to the output pulses of the mode-locked Ti:Sapphire laser  23 . Only the leading edge component of the detector response curve for the respective detectors stimulated by the laser fan beam  55  that passes through the breast are sampled by the electronic integrator  72  or an operational amplifier within the detector circuit  69 , as will be described below. This technique allows selection of only certain photons and is essential to the proper operation of the apparatus R.  
         [0065]    There are two clamp and time-gate switch circuits  102  for each detector array  40 , each detector  62  being contained in the detector circuit  100 .  
         [0066]    A multiplexer circuit  108  is provided for each detector array  40 . Each detector array has  25  photodiode detectors  62 . The output of each multiplexer circuit  108  is fed to a multiplexer circuit  110 . Each multiplexer circuit  108  is used to select the detector outputs that are appropriate for the orbital position of the rotating polygon mirror  38 . The detector outputs from the multiplexer circuit  110  are converted to a 12-bit digital word by the analog to digital converter  86 . The digital value of each detector output voltage is stored for each orbital position of the rotating mirror  38 . A buffer circuit  112  is interposed between the multiplexer circuits  108  and  110 .  
         [0067]    Referring to FIG. 11, data is acquired at each vertical or slice position of the scanning chamber  22  at 4000 locations of the polygon mirror  38  on its orbit around the breast as the orbit plate  26  is rotated to each of the 4000 locations, generally indicated by the arrow  114 . A circle is thus traced by the orbit of the polygon mirror  38 . The circle of detector arrays  40  remains fixed in place while the mirror  38  rotates on its own axis, generally indicated by the arrow  116  and is orbited around the patient&#39;s breast. The mirror  38  is shown in one of its 4000 locations in FIG. 11. At each of the 4000 locations, the rotation of the polygon mirror  38  sweeps the laser beam across a field of view  118 , which includes a scan diameter  120  within which the breast must be placed. The field of view  118  encompasses one quarter or  150  of the detectors  62 . In practice over-scanning to include  152  or more detectors for each orbit position is used for proper data acquisition.  
         [0068]    The computer  88  synchronizes the rotation of,the polygon mirror  38 , the selection of specific detectors  62  by the multiplexer circuits  108  and  110 , and analog-to-digital converter  86  conversion cycle to measure the laser beam intensity as each detector  62  is illuminated. Through this process, at each of the 4000 locations in one orbit of the mirror  38 , the output of at least  150  selected detectors  62  is measured, converted to digital format, and stored as part of the digitized slice data. The digitized slice data also contain encoding information relative to which of the 4000 locations in which of the detectors  62  is being measured.  
         [0069]    Since there are only 600 detectors  62  and data is collected from 4000 locations at each vertical or slice position of the scanning chamber  22 , a technique is required to select which of the 600 detectors outputs is sampled. The multiplexer circuits  108  and  110  are used to select which of the individual detector  62  in each of the detector arrays  40  are used as part of the  150  or more detectors for each of the 4000 locations.  
         [0070]    For example, referring to FIG. 11, for the locations shown for mirror  38 ,  150  detectors might be selected for measurement. The ratio between the 4000 locations of the mirror  38  and the 600 detectors is 6.67. Because of this ratio, for 7 successive locations of the mirror  38 , the same 150 detectors 62 might be selected for measurement. For the next 7 locations of the mirror  38 ,  2  through  151  of the detectors  62  might be selected. The step incrementing of which detectors  62  are sampled by the analog/digital converter  86  is controlled by a data acquisition algorithm, which will be described below, and the computer  88 . The exact relationship between the locations of the rotating mirror  38  and the specific detector  62  is determined by the mechanical relationship between the polygon mirror mounting location and the fixed ring of the detector arrays  40  and the individual numbering system adopted for the program.  
         [0071]    The data acquired for each vertical position of the rotating mirror  38  is referred to as slice data. This data is used to produce an image (FIG. 14) of the scanned breast by a reconstruction algorithm derived from computer tomography theory, as will be described below.  
         [0072]    Referring to FIG. 12, the acquisition algorithm used in the present invention to collect the data for each slice will now be described.  
         [0073]    The technologist performing the scan places the patient prone on the scanning table  16  with one breast pendulant through the opening  20  in the scanning chamber  22 , as best shown in FIG. 2.  
         [0074]    When the technologist starts the scan, several preset parameters are entered into the program. The speed of rotation and the number of facets on the mirror  38  are two basic values. The number of mirror facets is a physical parameter that cannot be easily changed unless the polygon mirror assembly is changed. The option to change the speed of rotation at step  122  is available in the event that some future events make this change desirable and a speed change can easily be accomplished. The available rotation speeds are 6000, 8000, 10000 and 12000 revolutions per minute (RPM).  
         [0075]    The apparatus R employs a 12-faceted mirror  38  and a mirror rotation speed of 6000 RPM, or  100  revolutions per second (RPS). The time for one facet to move the impinging laser beam through one beam fan  55  can be calculated as follows:  
         [0076]    Speed of Rotation: 100 rev/sec. 
         1 rev={fraction (1/100)} rev/sec.=0.01 sec/rev 
         [0077]    Time for 1 fan: 
         0.1 sec/12 facets=8.33×10 −4  sec (833 μsecs) 
         [0078]    The option to change the polygon mirror  38  to another number of facets is facilitated by the ability to preset the time for one fan at step  124 .  
         [0079]    Because there is a difference between the mechanical position of the swept laser beam  55  and the electronic position, another parameter, FACET DELAY, is presetable at step  126 . This parameter is established during initial scanner set up and can range in value from 0 to 833 μsecs.  
         [0080]    The fan of laser beams sweeps across an arc (slightly more than 90°) of the detectors  62 . With 600 detectors in the detector ring, 90° represents one quarter of the detector  62 , or 150 detectors.  
         [0081]    Because of the adjacent facets on the polygon mirror  38  do not form a sharp corner at the line of intersection but instead are jointed by radius, a number greater than the number of detectors  62  employed is actually used. The time the fan of laser beams sweeps across any one detector (herein called the facet dwell) is calculated as follows: 
         833 μsecs/150 detectors=5.6 μsecs/detector. 
         [0082]    The actual facet dwell is determined during initial scanner set up and is entered at step  128 .  
         [0083]    Ideally, all detectors  62  will be operational. However, in the practical situation, certain detectors  62  may be defective. This condition, within limits can be tolerated as long as the specific location of defective individual detectors is known. The defective detectors are identified during a quality control scan. The defective detectors are then ignored at step  130 .  
         [0084]    The reconstruction algorithm, which will be described below, requires an overscan of the ideal 90° fan of detectors  62 . The amount of overscan is determined during initial scanner set up and is entered at step  132 .  
         [0085]    The individual gain of detectors  62  can vary and this variation is particularly adjusted for any reconstruction algorithm. However, an over all gain value is determined during initial scanner set up and this value is entered at step  134 .  
         [0086]    The technologist is able to enter certain information concerning the specific patient, such as name, etc., as well as selecting necessary specific locations where a scan will be performed. This allows rescanning a specific location without having to rescan the entire breast. This step is generally indicated at  136 .  
         [0087]    After these parameters and data are entered, the technologist is asked at step  138  if the entered information is correct. If YES is entered, the scan commences.  
         [0088]    The first step in the scan is to return the scanning chamber  22  which carries the rotating mirror  38  and the ring of detector arrays  40  to the home position which is the extreme up position, as best shown in FIG. 3A. The motor controller that powers the motors  39  are switched to the up position and remains in this mode until home limit switches are activated. This step is generally indicated at steps  140  and  142 .  
         [0089]    After the home position has been reached, the computer checks to determine if the laser is ON, at step  144 . The laser is restarted at step  146  if the laser is not ON. The rotation of the polygon mirror  38  is initiated at step  148  and the mirror will continue to rotate at the preset speed set at step  122 .  
         [0090]    The program continues and presets the multiplex circuits  108  and  110  to select the detectors  62  that will be used as part of the initial data acquisition fan at step  150 . Since data is acquired at 4,000 individual locations in the orbit of the polygon mirror  38  and there are only  600  detectors, the set of detectors selected for data acquisition during each respective fan has been determined for this scan geometry. The table below illustrates this concept, where the actual identification number for each detector has been simplified for illustration purposes. 
         Index=4,000 orbit positions/ 600  detectors=6.67 fans/index 
         [0091]    This means that for every position or index of the rotating mirror  38  on its orbit around patient&#39;s breast, 7 fans of laser beams are generated, each fan being picked up by the same 150 detectors.  
         [0092]    In the table below, the detectors  62  that are disposed in the ring of detector arrays  40  are designated as 1, 2, 3, . . . n . . . 600. 
                                                     FAN NUMBER   FIRST DETECTOR   LAST DETECTOR                                1   525   75       2   525   75       3   525   75       4   525   75       5   525   75       6   525   75       7   525   75       8   526   76       9   526   76       10   526   76       11   526   76       12   526   76       13   526   76       14   526   76       15   527   77       16   527   77       17   527   77       18   527   77       19   527   77       20   527   77       21   527   77       —   —   —       3990   523   73       3991   523   73       3992   523   73       3993   523   73       3994   523   73       3995   523   73       3996   523   73       3997   524   74       3998   524   74       3999   524   74       4000   524   74                  
 
         [0093]    At each index or orbit location of the rotating mirror  38 , the total number of detector  62  in the fan is  150 . For example, for fan number 1, the number of detectors is (600−525)+75=150. For fan number 3999, the number of detectors is (600−496)+46=150.  
         [0094]    After the multiplex sequence is programmed, orbiting of the fan beam commences at step  152 , but data acquisition does not commence until the orbit flag signal is detected at step  154 . The orbit flag signal identifies the mechanical position in orbit that data acquisition via the multiplex sequence of detectors being sampled commences. The states for the orbit flag are 0 (continue orbiting) or 1 (initiate data acquisition sequence). Step  156  continues until the orbit flag equals 1.  
         [0095]    Preset facet period and the facet delay period are then waited out at steps  158  and  160 , after which the first detector  62  in the fan is selected to be sampled at step  162 . However, prior to actual sampling, the Ignore Detector Table is examined at step  164 . If the respective detector is accepted for sampling, then sampling proceeds. If the respective detector is defective, the detector address is incremented to the next detector in the multiplex sequence at step  168 .  
         [0096]    Sampling proceeds for the wait facet dwell at step  170 . The data is written into the respective location in the data file at step  172 . The number of detectors sampled in this cycle is examined at step.  174  to determine if the last detector in the fan has been sampled. If the last detector has been sampled, then the data file for the particular slice is closed at step  176  and the program moves to the next slice location. If the last detector has not been detected, then the detector count is incremented at  168  and the next fan of data is acquired. At step  178 , the program moves to the next slice location after the last detector is detected at  174 .  
         [0097]    After the slice data file is closed, the scanning chamber  22 , including the polygon mirror  38  and the ring of detector arrays  40 , are moved downward to the next slice location. The computer  88  monitors the downward motion. The status of the next slice location is monitored at step  180 . When the next slice location is reached, it is determined if the slice location is the end of scan location at step  182 . The computer  88  monitors the slice location and checks to determine if the last valid slice data file has been acquired. If the end slice location is detected, then it is the end of the breast scan. If the end slice location is not detected, then the next slice data file acquisition commences at step  150 . The cycle then repeats until data for the end slice have been acquired.  
         [0098]    Referring to FIG. 13, a reconstruction algorithm used in the present invention is disclosed. The raw data file is acquired during data acquisition process disclosed in FIG. 12. Raw data file is input at step  184  to generate detector fans at step  186 . To correct for gain and offset variations for the respective detectors, polynomial linearization correction is applied using information obtained from a previous phantom scan at step  188 . The linearization file is indicated at  190 .  
         [0099]    Because there is a potential offset between the electronic and mechanical centering, the centering correction is made at step  192  for individual detectors and the detector array. Center information is obtained from a prior phantom scan generally indicated at  194 .  
         [0100]    The sensitivity of individual avalanche photodiodes  62  varies and this variation must be accounted for through a detectors sensitivity correction at step  196 . Sensitivity adjustments are preformed using data acquired during prior phantom scans generally indicated at  198 .  
         [0101]    A cosine correction is made because of the fall-off of each detector fan at step  200 . Other corrections for gain control and mismatches will also be applied here. Each detector fan is convolved with a filter kernel at step  202  to process the file for back projection.  
         [0102]    The back projection step  204  projects the fan data into the image matrices with the 1/r 2  weighting applied to the data.  
         [0103]    After the data has been projected into the matrices, correction for any systematic artifacts and reconstructed density is made at step  206 . The correction factors are acquired in previous phantom scans at step  208 .  
         [0104]    Upon completion of the reconstruction steps, a file is created for the reconstructed image at step  210  and is stored for display either immediately or at a later time.  
         [0105]    An example of an image generated from a slice data of a breast is disclosed in FIG. 14. The outer band  212  is noise. The breast tissue  214  is shown surrounding a prosthesis  216  for an augmented breast.  
         [0106]    The clamp and time-gate switch circuit  102  will now be described in detail.  
         [0107]    Referring to FIG. 15, the circuit  102  comprises a clamp circuit  194  and a time-gate switch  196 . The clamp circuit  194  is provided to protect the operational amplifier  72  (or integrator) from being subjected to a voltage above the safe design parameters of the device. In response to stimulation by the femtosecond laser pulse, generally indicated at  66 , the reverse biased avalanche photodiode  62  produces a positive going pulse of current, generally indicated at  198 . The magnitude of the pulse  198  potentially could exceed the design limits of the operational amplifier  72  used to produce a voltage in response to the current pulse. To advantageously prevent this from occurring, diode  200  is reversed biased to approximately +0.8 VDC by the +5 VDC supply voltage  202  and two resistors  204  and  206 . When the pulse amplitude produced by the detector  62  increases above the biased voltage by one diode drop (approximately 0.7 VDC), diode  200  is forward biased and shunts away any further increase in signal amplitude. The shunt effect effectively clamps the signal level seen at the anode of the diode  76  to a level within design limits of the operational amplifier  72 .  
         [0108]    The time-gate switch  196  is driven by differential emitter-coupled logic (ECL) signals applied to inputs  208  and  210 , as best shown in FIG. 15. When transistor  220  is switched on, the voltage developed at the junction of the resistors  204  and  206  changes from a positive level to a negative level. The negative level voltage forward biases diode  200  and in turn reverse biases diode  76 . When the diode  76  is reversed biased, any current being provided by the detector  62  cannot reach the operational amplifier  72 . The diodes and transistors used in this circuit configuration are advantageously selected for their ability to switch at very high speeds. The effect of the circuit  196  is to switch off current provided to the operational amplifier  72  at a very high speed.  
         [0109]    The laser pulse pick-off circuit  106  will now be described in detail.  
         [0110]    Referring to FIG. 16, the occurrence of a laser pulse is detected by an increase in the current flowing in a reversed biased avalanche photodiode  222 . A femtosecond laser pulse train is disclosed in FIG. 20A. The response curve of the avalanche photodiode  222  and the delay in the peak produced by the detector  222  is shown in FIG. 20B. A representation of the point of the rising edge of the avalanche photodiode pulse used as reference point for high speed signal level comparator is shown in FIG. 20C. A resistor  224  provides current limiting to prevent damaging the detector  222  with the high current produced in response to a laser pulse  66 . A capacitor  226  is a decoupling capacitor that provides the energy that is dissipated across a resistor  228 . The current flowing through the resistor  228  produces a voltage across the resistor. The voltage is direct coupled to a comparator circuit  230 . A resistor  232  is used to adjust the threshold at which the output of the comparator  230  will switch. The output of the comparator  230  is connected to a buffer  234  and provides an ECL output signal. The ECL signal is synchronized with the occurrence of each laser pulse. The output of the circuit  106  is shown in FIG. 20D.  
         [0111]    Referring to FIGS. 17A and 17B, the clamp control circuit  104  will now be described in detail. The laser pulse pick-off circuit  106  is used to produce additional signal in synchronization with each laser pulse. The signal is used to start a time-to-amplitude converter  236 . The time-to-amplitude conversion is stopped at the appropriate time by a signal from another laser pick-off circuit  106 . The detectors  222  for the two laser pulse pick-off circuits  106  are positioned at an appropriate distance near the detector array  40 . The time of arrival t 2  through the path containing a tissue is measured during the scout scan phase and converted to a digital word with an appropriate digital value to control the address in memory where the time value is stored. During the data acquisition portion of the data acquisition sequence the memory address control  241  is used to select a value from a look-up table  250 . The look-up table  250  provides a value to an add/subtract circuit  243 . At the appropriate time, the digital time value t 2  is read from memory  240  and is modified by the value provided by the look-up table  250 . The net effect is to use the value t 2  read from memory, subtract or add a value to it to produce a new digital word A which is provided to a comparator  246 . The other input to the comparator  246  is the digital time value produced by the analog to digital converter  236 , represented by the word B. When the condition A=B is met, the comparator  246  provides a digital output to a digital/analog fine delay circuit  248 . The A=B condition starts the measurement interval for the leading edge of the detector response curve, as best shown in FIG. 17B. The analog fine delay determines the length of time during which the leading edge of detector response curve is measured. At the end of the analog delay interval, a digital signal is produced that halts the measurement interval. The look up table  250  produces a signal that controls the fine delay. The data acquisition sequence continues for the previously discussed 5.3 μsec. interval. The above sequence continues as the fan beam sweeps across the breast.  
         [0112]    An output buffer  252  produces an ECL output signal as a time-gate control signal. The output of the buffer  252  is fed to the circuit  102  at  208  and  210 , as best shown in FIG. 15.  
         [0113]    By using the time-of-flight approach, the timing of the data acquisition is automatically synchronized to the laser pulses beaming into the breast at each of the fan locations. Other approaches such as laser gating of a Kerr optical shutter or variable optical delay lines would not be practical given the number of measurement to be made in 1 second.  
         [0114]    The laser  23  produces pulses of-near infrared energy at a relatively fixed repetition rate. The laser pulses propagate at the speed of light in air, a constant. The time required for a pulse to travel a set distance is calculated as: 
         Time=Distance/Speed of Light 
         [0115]    Thus, for known distance, the time required for the pulse of energy to traverse the distance is easily calculated.  
         [0116]    The response of the photodiode detectors to the laser pulse is disclosed in FIG. 19. Note the delay in response of the detector to the laser stimulation.  
         [0117]    The response of the photodiodes to a pulse train exiting a medium is disclosed in FIG. 20. Note the propagation delay due to the relative refractive index of the tissue.  
         [0118]    The ratio of the speed of light traveling in air compared to the speed of light in a medium is referred to as the relative refractive index and is calculated as: 
         Relative Refractive Index=Speed of Light in Air/Speed of Light in Medium 
         [0119]    The time-of-flight measurement criteria must consider the speed of light in air, the speed of light in the complex medium of human tissue, and the thickness of the medium.  
         [0120]    The pulse pick-off circuit  106  is placed in a position to intercept a portion of the photons produced by the Ti:Sapphire laser  23 . The pulse pick-off circuit  106  produces a regular train of pulses based on the comparator threshold level, as best shown in FIG. 18D.  
         [0121]    The distances between the individual components in the path of the laser beam are known and fixed, as best shown in FIG. 21. Thus, the time required for an individual pulse to travel the fixed distance between individual components, for the most part mirrors used to position the laser beam, is easily determined. Also, the arrival time of an individual pulse at a selected location can be accurately predicted. The arrival time of an air shot, i.e. nothing between the polygon mirror  38  and the detectors  62 , therefore, is also known, as best shown in FIG. 21.  
         [0122]    The time required to travel the path length in air is calculated as: 
         Time in air =Path Length in air /Speed of Light in air   
         [0123]    The arrival time when the medium is air and the arrival time when the medium is human tissue can be measured. The difference between the two arrival times and the path length in human tissue can be used to calculate the relative speed of light in human tissue as shown below: 
         Speed of Light in human tissue =Path Length in human tissue /ΔTime 
         where ΔTime=Time in human tissue −Time in air   
         [0124]    The determination of the speed of light in human tissue allows time-gating of that portion of the avalanche photodiode response pulse desired to be measured and used for image reconstruction.  
         [0125]    The first few pulses of laser energy photons that have traversed through human tissue are detected as the scout phase of the data acquisition. The time difference between the expected arrival of the photons, as determined by a previously run calibration, and the actual arrival time of the photons is determined. For example, 
         Measured Arrival Time−Expected Arrival Time=ΔTime 
         t 2 −t 1 =ΔTime 
         [0126]    ΔTime is used to determine when the measurement of the detector response curve will commence on the pulses that occur after the scout phase. A look-up table or similar method is used to select when the detector measurement will commence, i.e. slightly before t 1 +ΔTime, at ΔTime, or ΔTime+t 3 , where t 3  is determined as a system calibration value.  
         [0127]    The second phase of the data acquisition is the control of length of time the leading edge of the detector response curve is measured, and the number of laser pulses used for each measurement. The starting point and the ending point of the measurement interval directly affect the contrast resolution of the resulting reconstructed image. Because of the physical variability of the optical and mechanical characteristics of the device, the beginning and ending points of the measurement interval are determined during calibration of the device. A method is provided for fine adjustment of the width of the measurement interval.  
         [0128]    A second scan, the data acquisition scan is performed. During this scan, the time-gating control factor is used to control the ECL circuit  104  that activates the time-gate switch  196  and circuit  102 . Thus, for each projection of the laser beam, only a selected portion of the respective avalanche photodiode response pulse is sampled and used as data for image reconstruction.  
         [0129]    Another embodiment of a support structure  254  for supporting the orbital plate  26  and the polygon mirror  38  is disclosed in FIG. 22. The support structure  254  includes four fixed threaded rods  256  disposed transversely through respective corners of a square or rectangular plate  258 . Each threaded rod  256  is held in position by a pair of threaded rod support brackets  260  which are attached to vertical side members  262  of a “U”-shaped assembly  264 , as best shown in FIG. 23. The “U”-shaped assembly  264  advantageously maintains the separation between the respective threaded rod support brackets  260  and the vertical alignment of the threaded rods  256 . Each threaded rod  256  has a sprocket  266  or a pulley with a threaded hole in the center. The pitch of the threaded rod and the sprocket thread is the same, such that rotation of the sprocket  266  causes it to move up or down the threaded rod  256 . The individual sprockets  266  are mated with a continuous drive chain  268  or belt.  
         [0130]    The continuous drive chain  268  is also mated with a sprocket  270  (or pulley) driven by a motor  272 . Rotation of the output shaft  274  of the drive motor  272  rotates the sprocket  270  and drives the chain  268  in the direction of rotation. The continuous chain motion advantageously synchronously rotates the individual sprocket  266  on each threaded rod  256 . Depending on the pitch of the thread and the direction of rotation, all five sprockets  266  and  270  will be driven upwardly or downwardly.  
         [0131]    The plate  258  is disposed on top of the top surface of each of the four sprockets  266 . A mounting plate  276  for the drive motor  272  is attached to the underside of the plate  258 , as best shown in FIG. 22. This configuration provides for a constant position of the drive motor  272  relative to the moving plate  258 , thus maintaining alignment of the entire drive system.  
         [0132]    The support structure  254  provides several advantages. If the chain  268  breaks, the upward or downward drive is advantageously removed from all four drive sprockets  266 . Also, the four fixed threaded rods  256  act as linear bearings for the upward and downward motion, thus eliminating the need for auxiliary vertical positioning bearings. Further, the support structure  254  provides the least amount of overall height for compactness.  
         [0133]    The plate  258  has an opening  278 . The edge of the opening  278  has an inwardly projecting flange or step  280  adapted to receive and support the outer race  282  of a bearing assembly  284 . An orbital plate  286  is pressed-fit into the opening defined by the outer race  288  of the bearing assembly  284 ; as best shown in FIG. 24. A retainer ring  290  secures the orbital plate  286  to the inner race  288 . A retainer ring  292  secures the outer race  282  to the plate  258 , as best shown in FIG. 24.  
         [0134]    The orbital plate  286  is provided with outside tooth ring gear  294  that engages with a spur gear  296  driven by an orbit drive motor  298 . The drive motor  298  is secured by conventional means to the under side of the carrier plate  258 . Rotation of the output shaft  300  of the orbit drive motor  298  produces the opposite rotation direction of the carrier plate  286 . The speed of rotation of the carrier plate  286  is a function of the ratio of the number of teeth on the ring gear  294  and number of teeth on the spur gear  296  and the speed of rotation of the orbit drive motor  298 .  
         [0135]    It will be understood that supporting the orbital plate  286  with the bearing assembly  284  advantageously provides the simplest method of maintaining concentricity between the orbital plate  286  and the detector arrays  40  mounted on the plate  258 . Further, the required amount of vertical space is minimal.  
         [0136]    The optical arrangement associated with the orbital plate  286  is disclosed in FIG. 25. A mounting pan  302  is secured to the underside of the orbital plate  286  and rotates therewith. The mounting pan  302  has a central opening  304  through which the laser beam  306  enters within the pan  302 . Turning mirrors  308  and  310  disposed within the pan  302  are adapted to turn the vertical laser beam  306  to a horizontal beam after being reflected from the mirror  308  and then to a vertical beam after being reflected from the mirror  310  and exiting through an opening  312  in the orbital plate  286 . A turning mirror  314  changes the vertical laser beam to a horizontal beam and directs it to the rotating polygon mirror  38  from which a fan beam  316  is generated. A turning mirror  318  turns the horizontal incoming laser beam vertically into the pan  302  through the opening  304 .  
         [0137]    It will be understood that the turning mirrors  308 ,  310  and  314  are fixed relative to the orbital plate  286  and thereby turns with the orbital plate  286  such that the laser beam is always oriented in the right direction when it hits the rotating polygon mirror  38 .  
         [0138]    Photons traveling through the tissue follow essentially three paths. When a beam of photons is directed into the tissue, the photons&#39; forward direction is changed--the beam is said to be scattered by the atoms and molecules in the tissue. Referring to FIG. 26A, the first photons entering the tissue  320  essentially undergo a straight forward scattering and exit the tissue with the least amount of time required to traverse the tissue. These photons are referred to as ballistic or early arriving photons  322 . Since these photons travel in essentially straight line through the tissue, the difference in the absorption of theses photons provides the best spatial resolution, i.e. true representation of the area of change in absorption in the path of these photons. The signal produced by the ballistic photons  322  is on the leading edge of the detector response curve, as best shown in FIG. 26B.  
         [0139]    The photons that exit the tissue after the ballistic photons have followed a longer path in traversing through the tissue and this path is less straight than that followed by the early arriving-ballistic photons. These late arriving photons are called snake-like photons  324 , as best shown in FIG. 26A. These photons can be thought of as signal degradation resulting in reduced spatial resolution, and the signal they produce appears later on the detector response curve than the ballistic photon component, as best shown in FIG. 26B.  
         [0140]    The photons that exit later than the snake-like photons have followed a diffuse path and exit the tissue at many points. These photons are referred to as diffuse photons  326  and make up the final components of the detector response curve, as best shown in FIG. 26B. These photons severely degrade the spatial resolution data and are considered noise.  
         [0141]    If the entire detector response from all photons (ballistic, snake-like and diffuse) are used, the ability to detect small differences within a tissue is severely compromised. Thus, only that part of the detector response curve produced by the ballistic photons is sampled for data acquisition, as best shown in FIG. 26B. The technique used to select the early portion of the photon arrival response curve shown in FIG. 26B is called time-gating, implemented by circuits  102  and  104  (FIGS. 15 and 17). Since the distance from the rotating mirror  38  to each photodetector  62  is known, any change in the time required for the photons to reach the detectors is a representation of the time required to traverse a portion of the path, i.e. through the tissue. Referring to FIG. 27A, the arrival time for each laser pulse impinging each detector in the ring  45  is determined from the known distances and the speed of light. A look-up table is generated from this free space time-of-flight data. The arrows in FIGS. 27A and 27B represent the arrival time of each laser pulse. When a tissue  328  is inserted within the scan diameter  120 , the arrival time for each laser beam passing through the tissue is delayed, the amount of delay being dependent on the length of the path traversed through the tissue, as best shown in FIG. 27B, where it is assumed, for sake of simplicity, that the speed of the laser pulse traversing through the tissue is constant. The arrival time for each laser beam traversing through the tissue is determined by observing when a response is generated at the individual detectors. The respective time-of-flight through the tissue can be determined by subtracting the free path (no tissue present) time-of-flight from the time required to traverse the path with the tissue present. The added time-of-flight is stored in the look-up table  250  and is then further increased by a delay in the range of 0-40 picoseconds, preferably 15-20 picoseconds to modulate the time at which the detector response curve is measured on succeeding laser pulses, such that the measurement is limited to that part of the detector response curve attributable to the-ballistic photons. The fine delay of 0-40 picoseconds is provided by the circuit block  248 . The resulting current produced at the detectors by the ballistic photons, after being converted to voltage, is then used to generate an image of the tissue using standard computed tomography techniques.  
         [0142]    While the present invention has been described for a structure where the detector arrays  40  are fixed in place in a circle around the tissue and the mirror  38  or source of laser beam is orbited within the circle in order to make a 360 degree scan around the tissue, it is also within the scope of the present invention to provide a set number of detectors that move synchronously with the mirror  38  or a source of laser beam around the tissue being scanned. In this respect, the detectors, formed into an arc or other geometric configuration to catch the fan beam  55 , would be disposed on the orbital plate  26 . The mirror  38  and the arc of detectors are then orbited through the 4000 locations in a circle around the tissue.  
         [0143]    The function of the rotating mirror  38 , which is to sweep the laser beam across the breast, may also be accomplished by an oscillating mirror  332  driven by a galvanometer  334 , as best shown in FIG. 28. The galvanometer mechanism produces an oscillating motion to the mirror  332 . For example, the galvanometer turns in one direction from its resting point to a certain number of degrees, say 10°, of rotation and then reverses direction and rotates an equal number of degrees in the opposite direction. The rotation and direction reversal continue as long as the drive signal is provided to the galvanometer.  
         [0144]    A laser beam  336  directed onto the mirror  332  attached to the galvanometer  334  will be swept back and forth across the breast within the scan circle  120 . Because for the mirror the angle of incidence equals the angle of reflection, 20° of galvanometer total rotation (in this case +10° to −10° of rotation) causes the laser beam to sweep through an angle that is two times of the galvanometer rotation angle. By selecting the proper location of the galvanometer and mirror relative to the scan circle center, a 90° sweep  338  across the scan circle diameter is easily obtained, as best shown in FIG. 28.  
         [0145]    The galvanometer/mirror combination is advantageously less expensive than the multi-faceted mirror. Slight modification of the data acquisition sequence would be required to accommodate the back and forth sweeping of the detector arrays  40  by the laser beam.  
         [0146]    It should be understood to the person skilled in the art that by sweeping the laser beam itself across the breast instead of using a lens system to diverge the laser beam into a fan, the laser power output is significantly decreased to maintain the same power level reaching each detector.  
         [0147]    While this invention has been described as having preferred design, it is understood that it is capable of further modification, uses and/or adaptations of the invention following in general the principle of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains, and as may be applied to the essential features set forth, and fall within the scope of the invention or the limits of the appended claims.