US20040171938A1 - Diagnostic tomographic laser imaging apparatus - Google Patents

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
• 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.[0001]
• 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.[0002]
• 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.[0003]
• 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.[0004]
• 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.[0005]
OBJECTS AND SUMMARY OF THE INVENTION
• 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.[0006]
• 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'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.[0007]
• 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.[0008]
• 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.[0009]
• It is yet another object of the present invention to provide a laser imaging apparatus that uses femtosecond pulse width, near infrared laser pulse.[0010]
• 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.[0011]
• These and other objects of the present invention will become apparent from the following detailed description.[0012]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a perspective view of the of the present invention, showing the patient supporting platform and operator's console;[0013]
• 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;[0014]
• FIG. 3A is a side view partially in section of the scanning chamber;[0015]
• FIG. 3B is a schematic view of the scanning chamber of FIG. 3A,[0016]
• FIG. 4 is a top plan view of the scanning chamber which surrounds the breast of the patient;[0017]
• FIG. 5 is a partial perspective on the uppermost portion of the scanning chamber of FIG. 4;[0018]
• FIG. 6 is an enlarged view of the bearing support for the rotatable plate which carries portions of the scanning apparatus;[0019]
• FIG. 7 is a schematic perspective view of an array of photodiode detectors used in the present invention;[0020]
• FIGS. 8A and 8B are electrical schematic diagrams of the detector circuit used in the present invention;[0021]
• FIG. 9 is a functional block diagram of the electrical system used in the present invention;[0022]
• FIG. 10 is a functional block diagram of the detector electronics and multiplexer shown in FIG. 9;[0023]
• 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;[0024]
• FIG. 12 is a flow chart of data acquisition used in the present invention;[0025]
• FIG. 13 is a flow chart of data reconstruction used in the present invention;[0026]
• FIG. 14 is an example of an image of a female breast using the present invention;[0027]
• FIG. 15 is an electrical schematic diagram of a clamp and time-gate switch circuit;[0028]
• FIG. 16 is an electrical schematic of a laser pulse pick-off circuit used in the present invention;[0029]
• 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;[0030]
• 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;[0031]
• FIG. 18A is a representation of laser pulse train;[0032]
• FIG. 18B is a representation of the response of the avalanche photodiode detector to the pulse train of FIG. 18A;[0033]
• FIG. 18C is a similar to FIG. 18B, showing the selection of a comparator threshold level;[0034]
• FIG. 18D is a representation of a pulse train based on the comparator threshold level of FIG. 18C;[0035]
• FIG. 19 is a representation of the response of the avalanche photodiode detector to a laser pulse train traversing an air shot;[0036]
• 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;[0037]
• FIG. 21 is a schematic diagram of distances used in calculating time-of-arrival for the laser pulses;[0038]
• FIG. 22 is perspective view of another embodiment of a support structure for the orbital plate used in the present invention;[0039]
• 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;[0040]
• FIG. 24 is a cross-section view through the support plate of FIG. 22 with the orbital plate installed in place;[0041]
• 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;[0042]
• FIG. 26A is schematic diagram of photons traversing a tissue, illustrating the paths taken by ballistic, snake-like or diffuse photons through the tissue;[0043]
• 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;[0044]
• FIG. 27A is a schematic illustration of the arrival times of the laser beams at the detectors in free space; and[0045]
• FIG. 27B is a schematic illustration of the arrival times of the laser beams at the detectors when traversing through a tissue.[0046]
• FIG. 28 is a schematic diagram showing an oscillating mirror driven by a galvanometer to sweep a laser beam across a scan circle.[0047]
DETAILED DESCRIPTION OF THE INVENTION
• Referring first to FIGS. 1 and 2, an apparatus R in accordance with the present invention comprises an operator's console indicated at[0048]10 which may includemonitors12 and14. A patient'ssupport platform16 overlies anenclosure18 which houses the electronics and optics of the present invention. Theplatform16 includes anopening20 which permits one of the patient'sbreasts15 to be positioned through the opening and be pendant within ascanning chamber22. A laser beam generated from an Argonion pump laser21 and a Ti:Sapphire laser is used to scan the patient's breast within thescanning chamber22.
• A detailed description of the scanning mechanism within the[0049]scanning chamber22 will now be described. Referring to FIGS. 3A, 4,5 and6, an open top,box member24 is arranged immediately below theopening20 in theplatform16 and houses thescanning chamber22 which has its vertical axis aligned with the center of theopening20. Anannular plate26 is supported for rotation within thechamber22 onbearings28 and30 (FIG. 6) which permit it to be rotated step-by-step or indexed around the interior of thescanning chamber22. The indexing drive for creating this rotation is indicated at32 in FIG. 4.
• A[0050]ring gear33 secured to the periphery of the annular ororbital plate26 cooperates with thedrive32 to rotatably index theorbital plate26, as best shown in FIG. 4.
• The[0051]entire scanning chamber22 may be moved vertically downwardly from the upmost position shown in FIG. 3 by means of elongated threadeddrive rods34 that are operably secured to thebox member24 atanchors36 and nuts37. Drivemotors39 are operably connected to the threadedrods34 by conventional means such as by belt/pulley arrangements41, as best shown in FIG. 3. Rotation of the threadedrods34 is effective to lower or raise thescanning chamber22. Thedrive motors39 are securely fixed to thebox member24 by standard means, such as brackets, and are controlled bymotor43.
• Turning now to the optics of the apparatus R, the[0052]annular plate26 carries on its upper surface a polygonalmultifaceted mirror38, as best shown in FIGS. 3, 4, and5. Themirror38 is rotatable on its own vertical axis. A ring45 of photo-detector arrays40 is supported on the upper surface of thescanning chamber22 and surrounds the path traveled by themirror38 as it moves in an orbital path generated by revolutions of theplate26. Thearrays40 are fixed and stationary with respect to thescanning chamber22. The ring45 is preferably concentric with the orbital path of themirror38.
• The stepping[0053]motors39 are used to rotate thescrews34 in order to move thescanning chamber22 vertically downwardly through successive increments or slices following each complete orbital movement of thepolygonal mirror38 in order to successively expose portions of the breast of the patient to the pulsed laser radiation until the entire breast has been irradiated.
• The[0054]lasers23 and21 which supply the radiation for scanning the breast may be positioned within theenclosure18, as best shown in FIG. 2. The coherent pulsed light from the solid-state laser is directed from the laser to the polygonalmultifaceted mirror38 by means of a series of mirrors and prisms. Therotating polygon mirror38 advantageously preserves the laser beam intensity by not diverging the beam and maintaining a controlled alignment between the projected laser beam and therespective detector62. Amirror46 directs anincoming laser beam44 to amirror48, which then directs the beam to a stack ofwedge prisms50, which turns the beam at an angle and directs it through anopening52 in theorbital plate26. Twoadditional mirrors54 and56 mounted on theplate26 then redirect the beam to the rotatingpolygonal mirror38, which generates afan55 of beams for each orbital position of themirror38, as best shown in FIGS. 4 and 5. Ashelf35 is supported from theplate26 and supports thewedge prisms50. Theshelf35 rotates withplate26 such that thewedge prisms50 are always oriented in the same way with respect to theplate26 as it rotates.
• Referring to FIG. 3B, the speed of rotation of the[0055]multi-faceted mirror38 used to produce the fan oflaser beams55 is controlled bysystem electronics55 and is maintained at a constant speed. A hollow slip-ring assembly53 is used to bring the electronic signals to the polygondrive motor controller55. While thepolygon mirror38 is rotating inside its housing, the entire mirror assembly is rotated in an orbit inside the ring45 ofdetector arrays40. The orbital speed of the polygon mirror assembly (not the speed of rotation of the mirror itself) is controlled by thedrive motor32 and its motor controller. The orbital position of the polygon mirror assembly is determined through use of ahome detector57 and rotary encoder on thedrive motor32. The home encoder provides a fixed reference point that is used in conjunction with the rotary encoder to determine the location of thepolygon assembly38. Thus, for each place in the orbit of thepolygon assembly38, thedetectors62 in the detector ring that are being swept by the fan oflaser beams55 is determined.
• 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[0056]laser23. 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 the67 kilowatts (kw) range. The Ti:Sapphire laser23 is pumped by a 7 wattArgon ion laser21 using all spectral lines.
• By rotating the[0057]polygonal mirror38 at very high speed, for example in the order of 6000 RPM, the fan-shapedbeam55 is generated and the width of the fan is such that approximately 25% of thephotodiode detector arrays40 are thus illuminated at each rotational indexed position of theplate26. Preferably, themirror38 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 thedrive motors39.
• The laser[0058]beam detector arrays40 are positioned in the ring45 on a top surface of thescanning chamber22 and around the pendulant breast, as best shown in FIGS. 3, 4 and5. Eacharray40 comprises a number ofavalanche photodiodes62, as best shown in FIG. 7. The number ofphotodiodes62 dictates the number of laser fan beam projections that can be detected as thefan55 of laser beams sweeps across the breast.
• The[0059]detector62 of eacharray40 are disposed on asubstrate64. Thearrays40 are positioned as chords of a circle around theorbital plate26, as best shown in FIG. 4. Eacharray40 has25 individualavalanche photodiode detectors62. There are24detector arrays40 to form the ring of laser beam detectors, providing 600 avalanche photodiode detectors.
• Each of the[0060]photodiodes62 is connected to adetector circuit69, as best shown in FIG. 8A. The avalanche photodiodes62 are reversed biased to provide amplification of the detected signal. Each reversedbiased detector62 is used as a current source with the amount of current provided being a function of the number ofphotons66 of laser light that impinge on eachdetector62. The number of photons reaching eachdetector62 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 limitingseries resistor68 is used to control the amount of current that can flow through thedetector62 and thus prevents excessive current flow from occurring when the laser beam is unattenuated that otherwise could destroy thedetector62. A suitablesize decoupling capacitor70 is used to store charge to provide the energy required when thedetector62 responds to a fast rising pulse of photon intensity.
• The current provided by each[0061]detector62 in eacharray40 is switched into or off to either anoperational amplifier circuit72 or anelectronic integrator73, as best shown in FIGS. 8A and 8B. Theoperational amplifier circuit72 is used as a current-to-voltage converter to produce a direct current voltage atoutput74 proportional to the input current provided by eachdetector62. Thus, a DC voltage can be produced to represent the intensity of the laser beam impinging on theindividual detector62.
• A[0062]fast Schottkey diode76 provides the switching for eachdetector62. TheSchottkey diode76 is switched into or out of conduction by a clamp circuit, as will be described below, connected at77.
• The[0063]detector circuit69 and several control circuits required to control the output of eachdetector62 are referred to asdetector electronics82, as best shown in FIG. 9. The output ofdetector electronics82 is fed to amultiplexer84, the output of which is then fed to an analog/digital converter86. The output of theconverter86 is then fed to acomputer88. The data acquired from thedetector electronics82 are used by thecomputer88 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 thecomputer88 using a reconstruction algorithm, which is then displayed in amonitor90 in monochrome or pseudo-color. The raw slice data and image data can be stored on ahard drive92 or any other storage medium, using afloppy drive94, atape drive96 or a CD-ROM drive98.
• Referring to FIG. 10, the[0064]detector electronics82 comprisesdetector circuit69 controlled by a clamp andtime-gate switch circuit102, which is then controlled by aclamp control circuit104. Theclamp control circuit104 is synchronized by thecomputer88 and a pulse pick-off circuit106 to the output pulses of the mode-locked Ti:Sapphire laser23. Only the leading edge component of the detector response curve for the respective detectors stimulated by thelaser fan beam55 that passes through the breast are sampled by theelectronic integrator72 or an operational amplifier within thedetector circuit69, as will be described below. This technique allows selection of only certain photons and is essential to the proper operation of the apparatus R.
• There are two clamp and[0065]time-gate switch circuits102 for eachdetector array40, eachdetector62 being contained in the detector circuit100.
• A[0066]multiplexer circuit108 is provided for eachdetector array40. Each detector array has25photodiode detectors62. The output of eachmultiplexer circuit108 is fed to amultiplexer circuit110. Eachmultiplexer circuit108 is used to select the detector outputs that are appropriate for the orbital position of therotating polygon mirror38. The detector outputs from themultiplexer circuit110 are converted to a 12-bit digital word by the analog todigital converter86. The digital value of each detector output voltage is stored for each orbital position of therotating mirror38. Abuffer circuit112 is interposed between themultiplexer circuits108 and110.
• Referring to FIG. 11, data is acquired at each vertical or slice position of the[0067]scanning chamber22 at 4000 locations of thepolygon mirror38 on its orbit around the breast as theorbit plate26 is rotated to each of the 4000 locations, generally indicated by thearrow114. A circle is thus traced by the orbit of thepolygon mirror38. The circle ofdetector arrays40 remains fixed in place while themirror38 rotates on its own axis, generally indicated by thearrow116 and is orbited around the patient's breast. Themirror38 is shown in one of its 4000 locations in FIG. 11. At each of the 4000 locations, the rotation of thepolygon mirror38 sweeps the laser beam across a field of view118, which includes ascan diameter120 within which the breast must be placed. The field of view118 encompasses one quarter or150 of thedetectors62. In practice over-scanning to include152 or more detectors for each orbit position is used for proper data acquisition.
• The[0068]computer88 synchronizes the rotation of,thepolygon mirror38, the selection ofspecific detectors62 by themultiplexer circuits108 and110, and analog-to-digital converter86 conversion cycle to measure the laser beam intensity as eachdetector62 is illuminated. Through this process, at each of the 4000 locations in one orbit of themirror38, the output of at least150 selecteddetectors62 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 thedetectors62 is being measured.
• Since there are only 600[0069]detectors62 and data is collected from 4000 locations at each vertical or slice position of thescanning chamber22, a technique is required to select which of the 600 detectors outputs is sampled. Themultiplexer circuits108 and110 are used to select which of theindividual detector62 in each of thedetector arrays40 are used as part of the150 or more detectors for each of the 4000 locations.
• For example, referring to FIG. 11, for the locations shown for[0070]mirror38,150 detectors might be selected for measurement. The ratio between the 4000 locations of themirror38 and the 600 detectors is 6.67. Because of this ratio, for 7 successive locations of themirror38, the same 150detectors 62 might be selected for measurement. For the next 7 locations of themirror38,2 through151 of thedetectors62 might be selected. The step incrementing of whichdetectors62 are sampled by the analog/digital converter86 is controlled by a data acquisition algorithm, which will be described below, and thecomputer88. The exact relationship between the locations of therotating mirror38 and thespecific detector62 is determined by the mechanical relationship between the polygon mirror mounting location and the fixed ring of thedetector arrays40 and the individual numbering system adopted for the program.
• The data acquired for each vertical position of the[0071]rotating mirror38 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.
• Referring to FIG. 12, the acquisition algorithm used in the present invention to collect the data for each slice will now be described.[0072]
• The technologist performing the scan places the patient prone on the scanning table[0073]16 with one breast pendulant through theopening20 in thescanning chamber22, as best shown in FIG. 2.
• 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[0074]mirror38 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 atstep122 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).
• The apparatus R employs a 12-[0075]faceted mirror38 and a mirror rotation speed of 6000 RPM, or100 revolutions per second (RPS). The time for one facet to move the impinging laser beam through onebeam fan55 can be calculated as follows:
• Speed of Rotation: 100 rev/sec.[0076]
• 1 rev={fraction (1/100)} rev/sec.=0.01 sec/rev
• Time for 1 fan:[0077]
• 0.1 sec/12 facets=8.33×10⁻⁴sec (833 μsecs)
• The option to change the[0078]polygon mirror38 to another number of facets is facilitated by the ability to preset the time for one fan atstep124.
• Because there is a difference between the mechanical position of the swept[0079]laser beam55 and the electronic position, another parameter, FACET DELAY, is presetable atstep126. This parameter is established during initial scanner set up and can range in value from 0 to 833 μsecs.
• The fan of laser beams sweeps across an arc (slightly more than 90°) of the[0080]detectors62. With 600 detectors in the detector ring, 90° represents one quarter of thedetector62, or 150 detectors.
• Because of the adjacent facets on the[0081]polygon mirror38 do not form a sharp corner at the line of intersection but instead are jointed by radius, a number greater than the number ofdetectors62 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.
• The actual facet dwell is determined during initial scanner set up and is entered at[0082]step128.
• Ideally, all[0083]detectors62 will be operational. However, in the practical situation,certain detectors62 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 atstep130.
• The reconstruction algorithm, which will be described below, requires an overscan of the ideal 90° fan of[0084]detectors62. The amount of overscan is determined during initial scanner set up and is entered atstep132.
• The individual gain of[0085]detectors62 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 atstep134.
• 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[0086]136.
• After these parameters and data are entered, the technologist is asked at[0087]step138 if the entered information is correct. If YES is entered, the scan commences.
• The first step in the scan is to return the[0088]scanning chamber22 which carries the rotatingmirror38 and the ring ofdetector arrays40 to the home position which is the extreme up position, as best shown in FIG. 3A. The motor controller that powers themotors39 are switched to the up position and remains in this mode until home limit switches are activated. This step is generally indicated atsteps140 and142.
• After the home position has been reached, the computer checks to determine if the laser is ON, at[0089]step144. The laser is restarted at step146 if the laser is not ON. The rotation of thepolygon mirror38 is initiated atstep148 and the mirror will continue to rotate at the preset speed set atstep122.
• The program continues and presets the[0090]multiplex circuits108 and110 to select thedetectors62 that will be used as part of the initial data acquisition fan atstep150. Since data is acquired at 4,000 individual locations in the orbit of thepolygon mirror38 and there are only600 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
• This means that for every position or index of the[0091]rotating mirror38 on its orbit around patient's breast, 7 fans of laser beams are generated, each fan being picked up by the same 150 detectors.
• In the table below, the[0092]detectors62 that are disposed in the ring ofdetector arrays40 are designated as 1, 2, 3, . . . n . . . 600.

  FAN NUMBER	FIRSTDETECTOR	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

• At each index or orbit location of the[0093]rotating mirror38, the total number ofdetector62 in the fan is150. For example, forfan number 1, the number of detectors is (600−525)+75=150. For fan number 3999, the number of detectors is (600−496)+46=150.
• After the multiplex sequence is programmed, orbiting of the fan beam commences at[0094]step152, but data acquisition does not commence until the orbit flag signal is detected atstep154. 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). Step156 continues until the orbit flag equals 1.
• Preset facet period and the facet delay period are then waited out at[0095]steps158 and160, after which thefirst detector62 in the fan is selected to be sampled atstep162. However, prior to actual sampling, the Ignore Detector Table is examined at step164. 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 atstep168.
• Sampling proceeds for the wait facet dwell at[0096]step170. The data is written into the respective location in the data file atstep172. 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 atstep176 and the program moves to the next slice location. If the last detector has not been detected, then the detector count is incremented at168 and the next fan of data is acquired. Atstep178, the program moves to the next slice location after the last detector is detected at174.
• After the slice data file is closed, the[0097]scanning chamber22, including thepolygon mirror38 and the ring ofdetector arrays40, are moved downward to the next slice location. Thecomputer88 monitors the downward motion. The status of the next slice location is monitored atstep180. When the next slice location is reached, it is determined if the slice location is the end of scan location atstep182. Thecomputer88 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 atstep150. The cycle then repeats until data for the end slice have been acquired.
• 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[0098]step184 to generate detector fans atstep186. To correct for gain and offset variations for the respective detectors, polynomial linearization correction is applied using information obtained from a previous phantom scan atstep188. The linearization file is indicated at190.
• Because there is a potential offset between the electronic and mechanical centering, the centering correction is made at[0099]step192 for individual detectors and the detector array. Center information is obtained from a prior phantom scan generally indicated at194.
• The sensitivity of[0100]individual avalanche photodiodes62 varies and this variation must be accounted for through a detectors sensitivity correction atstep196. Sensitivity adjustments are preformed using data acquired during prior phantom scans generally indicated at198.
• A cosine correction is made because of the fall-off of each detector fan at[0101]step200. Other corrections for gain control and mismatches will also be applied here. Each detector fan is convolved with a filter kernel atstep202 to process the file for back projection.
• The[0102]back projection step204 projects the fan data into the image matrices with the 1/r²weighting applied to the data.
• After the data has been projected into the matrices, correction for any systematic artifacts and reconstructed density is made at[0103]step206. The correction factors are acquired in previous phantom scans atstep208.
• Upon completion of the reconstruction steps, a file is created for the reconstructed image at[0104]step210 and is stored for display either immediately or at a later time.
• An example of an image generated from a slice data of a breast is disclosed in FIG. 14. The[0105]outer band212 is noise. Thebreast tissue214 is shown surrounding aprosthesis216 for an augmented breast.
• The clamp and[0106]time-gate switch circuit102 will now be described in detail.
• Referring to FIG. 15, the[0107]circuit102 comprises aclamp circuit194 and atime-gate switch196. Theclamp circuit194 is provided to protect the operational amplifier72 (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 at66, the reversebiased avalanche photodiode62 produces a positive going pulse of current, generally indicated at198. The magnitude of thepulse198 potentially could exceed the design limits of theoperational amplifier72 used to produce a voltage in response to the current pulse. To advantageously prevent this from occurring,diode200 is reversed biased to approximately +0.8 VDC by the +5VDC supply voltage202 and tworesistors204 and206. When the pulse amplitude produced by thedetector62 increases above the biased voltage by one diode drop (approximately 0.7 VDC),diode200 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 thediode76 to a level within design limits of theoperational amplifier72.
• The[0108]time-gate switch196 is driven by differential emitter-coupled logic (ECL) signals applied toinputs208 and210, as best shown in FIG. 15. Whentransistor220 is switched on, the voltage developed at the junction of theresistors204 and206 changes from a positive level to a negative level. The negative level voltageforward biases diode200 and in turnreverse biases diode76. When thediode76 is reversed biased, any current being provided by thedetector62 cannot reach theoperational amplifier72. The diodes and transistors used in this circuit configuration are advantageously selected for their ability to switch at very high speeds. The effect of thecircuit196 is to switch off current provided to theoperational amplifier72 at a very high speed.
• The laser pulse pick-[0109]off circuit106 will now be described in detail.
• Referring to FIG. 16, the occurrence of a laser pulse is detected by an increase in the current flowing in a reversed[0110]biased avalanche photodiode222. A femtosecond laser pulse train is disclosed in FIG. 20A. The response curve of theavalanche photodiode222 and the delay in the peak produced by thedetector222 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. Aresistor224 provides current limiting to prevent damaging thedetector222 with the high current produced in response to alaser pulse66. A capacitor226 is a decoupling capacitor that provides the energy that is dissipated across aresistor228. The current flowing through theresistor228 produces a voltage across the resistor. The voltage is direct coupled to acomparator circuit230. Aresistor232 is used to adjust the threshold at which the output of thecomparator230 will switch. The output of thecomparator230 is connected to abuffer234 and provides an ECL output signal. The ECL signal is synchronized with the occurrence of each laser pulse. The output of thecircuit106 is shown in FIG. 20D.
• Referring to FIGS. 17A and 17B, the[0111]clamp control circuit104 will now be described in detail. The laser pulse pick-off circuit106 is used to produce additional signal in synchronization with each laser pulse. The signal is used to start a time-to-amplitude converter236. The time-to-amplitude conversion is stopped at the appropriate time by a signal from another laser pick-off circuit106. Thedetectors222 for the two laser pulse pick-offcircuits106 are positioned at an appropriate distance near thedetector array40. The time of arrival t₂through 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 thememory address control241 is used to select a value from a look-up table250. The look-up table250 provides a value to an add/subtractcircuit243. At the appropriate time, the digital time value t₂is read frommemory240 and is modified by the value provided by the look-up table250. The net effect is to use the value t₂read from memory, subtract or add a value to it to produce a new digital word A which is provided to acomparator246. The other input to thecomparator246 is the digital time value produced by the analog todigital converter236, represented by the word B. When the condition A=B is met, thecomparator246 provides a digital output to a digital/analogfine delay circuit248. 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 table250 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.
• An[0112]output buffer252 produces an ECL output signal as a time-gate control signal. The output of thebuffer252 is fed to thecircuit102 at208 and210, as best shown in FIG. 15.
• 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.[0113]
• The[0114]laser23 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
• Thus, for known distance, the time required for the pulse of energy to traverse the distance is easily calculated.[0115]
• 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.[0116]
• 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.[0117]
• 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:[0118]
• Relative Refractive Index=Speed of Light in Air/Speed of Light in Medium
• 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.[0119]
• The pulse pick-[0120]off circuit106 is placed in a position to intercept a portion of the photons produced by the Ti:Sapphire laser23. The pulse pick-off circuit106 produces a regular train of pulses based on the comparator threshold level, as best shown in FIG. 18D.
• 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[0121]polygon mirror38 and thedetectors62, therefore, is also known, as best shown in FIG. 21.
• The time required to travel the path length in air is calculated as:[0122]
• Time_{in air}=Path Length_{in air}/Speed of Light_{in air}
• 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:[0123]
• Speed of Light_{in human tissue}=Path Length_{in human tissue}/ΔTime
• where ΔTime=Time_{in human tissue}−Time_{in air}
• 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.[0124]
• 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,[0125]
• Measured Arrival Time−Expected Arrival Time=ΔTime
• t₂−t₁=ΔTime
• 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[0126]₁+ΔTime, at ΔTime, or ΔTime+t₃, where t₃is determined as a system calibration value.
• 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.[0127]
• A second scan, the data acquisition scan is performed. During this scan, the time-gating control factor is used to control the[0128]ECL circuit104 that activates thetime-gate switch196 andcircuit102. 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.
• Another embodiment of a[0129]support structure254 for supporting theorbital plate26 and thepolygon mirror38 is disclosed in FIG. 22. Thesupport structure254 includes four fixed threadedrods256 disposed transversely through respective corners of a square orrectangular plate258. Each threadedrod256 is held in position by a pair of threadedrod support brackets260 which are attached tovertical side members262 of a “U”-shapedassembly264, as best shown in FIG. 23. The “U”-shapedassembly264 advantageously maintains the separation between the respective threadedrod support brackets260 and the vertical alignment of the threadedrods256. Each threadedrod256 has asprocket266 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 thesprocket266 causes it to move up or down the threadedrod256. Theindividual sprockets266 are mated with acontinuous drive chain268 or belt.
• The[0130]continuous drive chain268 is also mated with a sprocket270 (or pulley) driven by amotor272. Rotation of theoutput shaft274 of thedrive motor272 rotates thesprocket270 and drives thechain268 in the direction of rotation. The continuous chain motion advantageously synchronously rotates theindividual sprocket266 on each threadedrod256. Depending on the pitch of the thread and the direction of rotation, all fivesprockets266 and270 will be driven upwardly or downwardly.
• The[0131]plate258 is disposed on top of the top surface of each of the foursprockets266. A mountingplate276 for thedrive motor272 is attached to the underside of theplate258, as best shown in FIG. 22. This configuration provides for a constant position of thedrive motor272 relative to the movingplate258, thus maintaining alignment of the entire drive system.
• The[0132]support structure254 provides several advantages. If thechain268 breaks, the upward or downward drive is advantageously removed from all fourdrive sprockets266. Also, the four fixed threadedrods256 act as linear bearings for the upward and downward motion, thus eliminating the need for auxiliary vertical positioning bearings. Further, thesupport structure254 provides the least amount of overall height for compactness.
• The[0133]plate258 has anopening278. The edge of theopening278 has an inwardly projecting flange or step280 adapted to receive and support theouter race282 of a bearingassembly284. Anorbital plate286 is pressed-fit into the opening defined by theouter race288 of the bearingassembly284; as best shown in FIG. 24. Aretainer ring290 secures theorbital plate286 to theinner race288. Aretainer ring292 secures theouter race282 to theplate258, as best shown in FIG. 24.
• The[0134]orbital plate286 is provided with outsidetooth ring gear294 that engages with aspur gear296 driven by anorbit drive motor298. Thedrive motor298 is secured by conventional means to the under side of thecarrier plate258. Rotation of theoutput shaft300 of theorbit drive motor298 produces the opposite rotation direction of thecarrier plate286. The speed of rotation of thecarrier plate286 is a function of the ratio of the number of teeth on thering gear294 and number of teeth on thespur gear296 and the speed of rotation of theorbit drive motor298.
• It will be understood that supporting the[0135]orbital plate286 with the bearingassembly284 advantageously provides the simplest method of maintaining concentricity between theorbital plate286 and thedetector arrays40 mounted on theplate258. Further, the required amount of vertical space is minimal.
• The optical arrangement associated with the[0136]orbital plate286 is disclosed in FIG. 25. A mountingpan302 is secured to the underside of theorbital plate286 and rotates therewith. The mountingpan302 has acentral opening304 through which thelaser beam306 enters within thepan302. Turning mirrors308 and310 disposed within thepan302 are adapted to turn thevertical laser beam306 to a horizontal beam after being reflected from themirror308 and then to a vertical beam after being reflected from themirror310 and exiting through anopening312 in theorbital plate286. Aturning mirror314 changes the vertical laser beam to a horizontal beam and directs it to therotating polygon mirror38 from which afan beam316 is generated. Aturning mirror318 turns the horizontal incoming laser beam vertically into thepan302 through theopening304.
• It will be understood that the turning mirrors[0137]308,310 and314 are fixed relative to theorbital plate286 and thereby turns with theorbital plate286 such that the laser beam is always oriented in the right direction when it hits therotating polygon mirror38.
• Photons traveling through the tissue follow essentially three paths. When a beam of photons is directed into the tissue, the photons' 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[0138]tissue320 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 arrivingphotons322. 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 theballistic photons322 is on the leading edge of the detector response curve, as best shown in FIG. 26B.
• 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-[0139]like photons324, 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.
• 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[0140]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.
• 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[0141]circuits102 and104 (FIGS. 15 and 17). Since the distance from the rotatingmirror38 to eachphotodetector62 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 ring45 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 atissue328 is inserted within thescan diameter120, 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 table250 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 thecircuit block248. 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.
• While the present invention has been described for a structure where the[0142]detector arrays40 are fixed in place in a circle around the tissue and themirror38 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 themirror38 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 thefan beam55, would be disposed on theorbital plate26. Themirror38 and the arc of detectors are then orbited through the 4000 locations in a circle around the tissue.
• The function of the[0143]rotating mirror38, which is to sweep the laser beam across the breast, may also be accomplished by anoscillating mirror332 driven by agalvanometer334, as best shown in FIG. 28. The galvanometer mechanism produces an oscillating motion to themirror332. 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.
• A[0144]laser beam336 directed onto themirror332 attached to thegalvanometer334 will be swept back and forth across the breast within thescan circle120. 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°sweep338 across the scan circle diameter is easily obtained, as best shown in FIG. 28.
• 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[0145]detector arrays40 by the laser beam.
• 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.[0146]
• 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.[0147]

Claims (23)

I claim:

1. A laser imaging apparatus, comprising:

a) platform for supporting a female patient in front-down, prone position and having an opening permitting a breast of the patient to be vertically pendant below the surface of the platform;

b) scanning mechanism including a multi-faceted mirror adjacent the underside of said platform, said mirror being-rotated about its own axis and orbited around the pendent breast;

c) a source of coherent near infrared narrow light pulses operably directed to said multi-faceted mirror;

d) optical reflectors directing said light pulses onto the facets of said mirror from a point spaced from said platform for reflection in a series of horizontal fan shaped beams through a breast pendent below said platform;

e) photodetectors operably disposed to detect the light pulses after passing through the breast;

f) circuit for deriving voltages proportional to the intensity of the received pulses; and,

g) computer programmed for storing and displaying images of tissue in the breast derived from the voltages.

2. An apparatus as inclaim 1, and further comprising:

a) a platform carrying said orbiting mirror and said photodetectors, said platform being adapted for stepping said orbiting mirror and photodetectors vertically downwardly in small increments following each orbit.

3. An apparatus as defined byclaim 1, in which said light pulses are of the order of 100 femtoseconds in width and a wavelength of the order of 850 nanoseconds.

4. An apparatus as defined byclaim 1, in which said source of light pulses includes a titanium sapphire laser and means including an argon ion laser to pump said titanium sapphire laser.

5. An apparatus as defined byclaim 1, in which said detecting means includes a stationary ring of reverse biased avalanche diodes surrounding the orbital path of said mirror.

6. A method of providing imaging of humanoid breast tissue including:

a) supporting a patient in face-down, prone position on a horizontal surface with a breast vertically pendent through an opening in said surface;

b) directing a succession of narrow coherent near infrared light pulses through said breast in a horizontal pattern from a plurality of positions completely surrounding said breast;

c) repeating said last step in a plurality of closely vertically spaced horizontal planes until the entire breast has been scanned;

d) detecting said pulses after passage through said breast tissue and;

e) deriving images of the tissue of said breast from said detected pulses by computed tomography reconstruction.

7. A scanning chamber for use in scanning human tissue, comprising:

a) a frame;

b) photodetectors disposed in a ring on said frame;

c) a rotatable plate supported on said frame, said plate including an opening for permitting the human tissue to be suspended in said opening;

d) said plate having an axis of rotation substantially coincident with center of said ring;

e) a rotatable multi-faceted mirror disposed on said plate such that said mirror makes a complete orbit around said opening when said plate is rotated; and

f) said mirror being positioned such that a laser pulse reflected therefrom is directed across said opening and through the tissue and impinge on said photodetectors during a complete orbit of said mirror.

8. A scanning chamber, as inclaim 7, wherein:

a) said frame is movable vertically.

9. A scanning chamber as inclaim 7, wherein:

a) drive screws are operably associated with said frame such that rotations of said screws are effective to lower or raise said plate.

10. A scanning chamber as inclaim 7, wherein:

a) said frame includes a bottom shelf with a central opening; and

b) wedge prism disposed over said central opening adapted to direct a laser pulse passing through said wedge prism to said mirror.

11. A scanning chamber as inclaim 7, wherein:

a) bearing assembly adapted to rotatably support said plate from said frame.

12. A scanning chamber as inclaim 11, wherein:

a) said bearing assembly includes an outer race secured to an opening on said frame; and

b) an inner race secured to said plate.

13. A scanning chamber as inclaim 7, wherein:

a) said plate includes a ring gear; and

b) drive motor operably connected to said ring gear for rotating said plate.

14. A scanning chamber as inclaim 7, wherein:

a) said frame includes a bottom shelf with a central opening; and

b) turning mirrors disposed on said shelf adapted to direct a vertical laser beam passing through said central opening to said rotating mirror.

15. A laser imaging apparatus, comprising:

a) a scanning chamber including a source of laser beam for passing through a tissue and at least one photodetector adapted to respond to the laser beam exiting the tissue;

b) an operational amplifier circuit connected to said at least photodetector for converting the current output of said at least detector to voltage; and

c) a clamp circuit for protecting said operational amplifier from overvoltage.

16. A laser imaging apparatus, as inclaim 15 wherein:

a) a time-gate switch circuit for sampling only a portion of the amplitude of the response curve of said at least one detector.

17. A laser imaging apparatus as inclaim 15, wherein:

a) said at least one detector is reverse biased avalanche photodiode.

18. A laser imaging apparatus as inclaim 15, wherein:

a) said laser source comprises femtosecond pulse width, near infrared laser pulses.

19. A laser imaging apparatus, comprising:

a) a scanning chamber including a source of femtosecond pulse width near infrared laser beam for passing through a tissue and a plurality of photodetectors arranged around the tissue and adapted to respond to the laser beam exiting the tissue;

b) a moving mirror for directing said source of laser beam to each of said photodetectors;

c) an analog to digital converter operably connected to said photodetectors for converting output of said photodetectors to digital form; and

d) a computer programmed to generate an image of the scanned tissue from the output of said converter.

20. A laser imaging apparatus as inclaim 19, wherein:

a) said photodetectors are reverse biased avalanche photodiodes.

21. A method for scanning a tissue, comprising the steps of:

a) providing a series of laser pulses through the tissue;

b) providing a photodetector disposed to receive the laser pulses after traversing the tissue;

c) determining the free flight arrival time of the laser pulses at the detector when the tissue is not present;

d) determining the delayed arrival time of the laser pulses when the tissue is present in the path of the laser pulses;

e) determining the added time-of-flight of the laser pulses due to the presence of the tissue in the path;

f) adding an increment of time to the added time-of-flight; and

g) sampling the response of the photodetector after a delay of the added time-of-flight and the increment of time, from the free flight arrival time.

22. A method as inclaim 21, wherein:

a) selecting the increment of time from the range 0-40 picoseconds.

23. A scanning chamber for use in scanning human tissue, comprising:

a) a frame;

b) a rotatable plate supported on said frame, said plate including an opening for permitting the human tissue to be suspended in said opening;

c) photodetectors disposed in on said plate;

d) said plate having an axis of rotation substantially coincident with center of said ring;

e) an oscillating mirror disposed on said plate such that said mirror makes a complete orbit around said opening when said plate is rotated; and

f) said mirror being positioned such that a laser pulse reflected therefrom is directed across said opening and through the tissue and impinge on said photodetectors during a complete orbit of said mirror.

Images