Abstract:
A diagnostic computed tomographic imaging apparatus, comprises a patient support structure including a tabletop to support a patient in front-down, prone position. The tabletop includes an opening to permit a breast of the patient to be vertically pendant below the tabletop. A scanning mechanism is disposed below the opening and orbitable about the breast pendent through the opening to obtain perimeter data and image reconstruction data of the breast. The scanning mechanism rotates about an axis of rotation transverse to the tabletop. One of the tabletop and scanning mechanism is movable in x-y axes to center the breast about the axis of rotation.

Description:
RELATED APPLICATION 
     This application claims the priority benefit of provisional application Ser. No. 60/759,066, filed Jan. 17, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a diagnostic optical imaging apparatus that employs a near-infrared laser as a radiation source and a detector array with restricted fields of view directed to their own patches of surface of the object being scanned to simultaneously detect the intensity of light exiting from the object for the purpose of reconstructing optical cross-sectional images of that object, and particularly where the object is movable relative to the radiation source and the detector array thereby to center the object about the scanning mechanism. 
     BACKGROUND OF THE INVENTION 
     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. 
     The x-ray absorption density resolution of present photographic x-ray methods is insufficient to provide reliable early detection of malignant 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 mammograms, 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. 
     Most mammographic apparatus in use today in clinics and hospitals require breast compression techniques that are uncomfortable at best and in many cases painful to the patient. In addition, x-rays constitute ionizing radiation, which adds a further risk factor into the use of mammographic techniques as most universally employed. 
     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. 
     In recent times, the use of light and more specifically laser light to noninvasively peer inside the body to reveal the interior structure has been investigated. This technique is called optical tomography. Rapid progress over the past decade has brought optical tomography to the brink of clinical usefulness. Optical tomography has the benefits compared to mammography of no breast compression and no ionizing radiation. 
     The patient lies prone on a scanning apparatus with one breast pendent in a scanning chamber. A laser beam impinges upon the breast; light is scattered throughout the breast and detected by an array of optical detectors. The scanning apparatus acquires data from one or several slices through the breast, parallel to the chest wall. The detection mechanism then moves some small distance away from the chest wall and the data for more slices are acquired. This process continues until the entire breast has been imaged. 
     One complication, and the subject of the present invention, is that the shape of the breast varies from patient to patient and particularly changes dramatically as the scanning proceeds from the chest wall toward the nipple. It is an absolute requirement of a 3 rd  generation optical CT scanner that the center of rotation of the scanning mechanism lay within the breast. It is desirable that the center of rotation of the scanning mechanism lies near the center of the breast. But breasts do not hang vertically in the prone position. Cooper&#39;s Droop occurs when Cooper&#39;s ligaments stretch over time, causing the breasts to sag when upright. In the prone position, breasts hang somewhat towards the feet as the scanning apparatus progresses from the chest towards the nipple (see  FIG. 3A ). A breast that is well centered at the chest wall through the axis of rotation of the scanning mechanism usually will be off-center near the nipple. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a laser imaging apparatus that acquires photon intensity data by direct observation of the surface of a scanned object, particularly of a breast, reconstructs cross-sectional images of the optical properties of the breast, and accommodates a relatively wider range of breast shapes and therefore patients. 
     It is another object of the present invention to provide a laser imaging apparatus for providing reconstructed cross-sectional images of a breast from a relatively wide range of breast shapes by maintaining the center of the breast through a center of rotation of a laser scanning mechanism of the apparatus by relative motion between the patient and the scanning mechanism. 
     It is still another object of the present invention to provide a laser imaging apparatus for providing reconstructed cross-sectional images of a breast from a relatively wide range of breast shapes by controlling the aiming of the laser used in scanning so that the breast is constantly illuminated whether or not the breast is centered in the scanning chamber. 
     In summary, the present invention provides a diagnostic computed tomographic imaging apparatus, comprising a patient support structure including a tabletop to support a patient in front-down, prone position. The tabletop includes an opening to permit a breast of the patient to be vertically pendant below the tabletop. A scanning mechanism is disposed below the opening and orbitable about the breast pendent through the opening to obtain perimeter data and image reconstruction data of the breast. The scanning mechanism rotates about an axis of rotation transverse to the tabletop. One of the tabletop and scanning mechanism is movable in x-y axes to center the breast about the axis of rotation. An alternative to translating the tabletop or the scanning mechanism is to control the aiming of the laser so that the breast is illuminated at all orbital angles during scanning, independent of the position of the breast within the scanning chamber. 
     These and other objectives of the present invention will become apparent from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of an optical scanning apparatus with a detector mechanism showing a patient positioned for optical tomographic study, with one breast pendent within the scanning chamber. 
         FIG. 2  is a top view of the scanning chamber of  FIG. 1 , showing the detector mechanism, consisting of a plurality of detectors disposed around an object being scanned and a laser light source. 
         FIG. 3A  is a cross-sectional view through the detector mechanism of  FIG. 2 , showing the breast, with pronounced Cooper&#39;s droop, being scanned, the laser light source and one detector, with the detector mechanism imaging a slice near the chest wall. 
         FIG. 3B  is a cross-sectional view through the detector mechanism of  FIG. 2 , showing the breast being scanned, the laser light source and one detector, with the detector mechanism imaging a slice near the nipple and the patient and patient support having been translated with respect to the detector mechanism. 
         FIG. 4A  is an internal structure of an optical scanning apparatus with the tabletop lifted off to show the XY tabletop translation mechanisms. 
         FIG. 4B  is a magnified view of one of the XY tabletop translation mechanisms. 
         FIG. 4C  is an internal structure of an optical scanning apparatus with the tabletop lifted off to show an automatic centering mechanism. 
         FIG. 4D  is an enlarged view of the automatic centering probe of  FIG. 4C . 
         FIG. 5A  shows a tabletop LED display indicating that the patient is off-center in the scanning chamber relative to the axis of rotation of the scanning mechanism. 
         FIG. 5B  shows the tabletop LED display indicating that the patient is on-center in the scanning chamber. 
         FIG. 6  shows an optical scanning mechanism with an XY translation mechanism 
         FIG. 7  shows a laser aiming apparatus in the scanning chamber of  FIG. 2 . 
         FIG. 8  shows the geometry of the laser aiming apparatus of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an optical scanning apparatus  2 , as described in U.S. Pats. No. 5,692,511 and 6,100,520, supports a patient  4  on a patient support surface  6 , such as a tabletop. The patient&#39;s breast  8  is pendent within a scanning chamber  10 , around which orbits, revolves a detector mechanism  12 . The tabletop  6  includes an opening  13  through which the breast protrudes into the scanning chamber  10 . The detector mechanism  12  orbits typically 360° around the vertical axis of the scanning chamber  10  and increments vertically (the “elevator” motion) between orbits to image successive slice planes. This is repeated until all the slice planes of the object have been scanned. 
       FIG. 2  shows a top view of the detector mechanism  12  from  FIG. 1 . A laser source  14  impinges on the scanned object, such as the breast  8  at point  16 . A plurality of detectors  18  define an arc surrounding the scanned object. A collimator  20  defines each detector&#39;s field of view to a small area on the surface of the scanned object. Light enters the scanned object at point  16  and exits at every point on its circumference. Three exit points,  22 ,  24  and  26  are shown, corresponding to three detectors. The entire mechanism rotates around the center of orbit rotation  28  as indicated by the curved arrow  30 . 
     Every detector is preferably collimated, aiming at the center or axis of orbit rotation  28  and the laser source also points toward the center of rotation. The detectors are spaced at equal angular increments around the center of rotation. The orbit rotation is alternately 360° clockwise for one slice plane, then 360° counterclockwise for the next slice plane, as indicated by the double arrow  30 . 
       FIG. 3A  shows a vertical cross-section through the detector mechanism of  FIG. 2  and a patient&#39;s breast with pronounced Cooper&#39;s droop. The detector mechanism  12  is shown as imaging one slice, though any number of slices can be imaged simultaneously as disclosed in U.S. Pat. No. 6,100,520. The patient&#39;s breast  8  is pendent within the scanning chamber  10 . The patient is supported by the scanning apparatus&#39; tabletop  6 . A laser  32  projects a coherent light beam  34  which impinges on the patient&#39;s breast at point  36 . A detector assembly  38  is shown, imaging point  40  on the breast. Each detector assembly consists of an opaque collimator  42  with one collimating channel  44 . The collimating channels can be round, square, hexagonal, triangular or another cross-sectional shape. The collimator restricts the fields of view of each detector assembly to a small, defined area on the surface of the scanned object, the breast. At the rear of each collimating channel is a lens  46 , which focuses the light propagating clown the collimating channel onto the photodetector  48 . The lenses are shown as plano-convex, but could be biconvex or could be eliminated if the photodetector area were larger than the collimating channel&#39;s area. The photodetectors are connected to a signal processing electronics  50 , which would typically provide amplification and analog-to-digital conversion. 
     The laser  32  could be a semiconductor diode laser, a solid-state laser or any other near-infrared light source. The photodetectors  48  could be photodiodes, avalanche photodiodes, phototransistors, photomultiplier tubes, microchannel plates or some other photosensitive device that converts incoming light photons to an electrical signal. 
     Six slice planes are shown through the breast, indicated by dashed lines, though typically there will be many more, spaced at 2-4 millimeters. The scanning mechanism  12 , consisting of the scanning chamber  10 , the laser  32 , collimators  42 , detectors  48  and the signal processing electronics  50 , is incremented vertically downward between slice planes. A requirement for optical tomographic imaging is that the object being scanned, the breast, surrounds the orbital rotation axis  28  at each slice plane. In other words, the breast must contain the axis of rotation at each slice plane. This condition is true for slice planes  52 - 60  but not for slice plane  62 . At the level of slice  62 , the laser beam  34  would miss the breast entirely at certain orbital angles. This would cause the acquired data at those orbital angles to be incorrect and the resultant reconstructed image would not be a faithful representation of the optical parameters of that slice of tissue. 
       FIG. 3B  shows the same vertical cross-section through the detector mechanism  12  as  FIG. 3A , but with the elevator having moved the scanning mechanism to the level of slice plane  62 . Additionally, the patient  4  and scanning apparatus tabletop  6  have been translated to the left with respect to the scanning mechanism, such that axis of rotation  28  is now contained within or surrounded by breast at the level of the slice plane  62 . This is the essence of the present invention: permit translation between the patient and the scanning mechanism so that the breast can be repositioned to always surround or contain the orbital rotation axis  28 . An alternative way of stating the same concept is to permit translation between the patient and the scanning mechanism so that the breast can be repositioned such that the laser beam always contacts the breast at all orbital angles around the breast. It is immaterial whether the patient is translated or the scanning mechanism is translated or both. 
       FIG. 4A  shows the internal structure of an optical scanning apparatus with the tabletop  6  lifted off. A framework  66  supports the scanning mechanism and tabletop, and therefore the patient. The detector mechanism, not shown for clarity, mounts on an elevator plate  68 , which moves vertically on three Acme leadscrews  70 . The elevator plate  68  typically at the start the scan of a breast would be at its uppermost position, then descend in increments until the entire breast has been imaged, driven by an electric motor, not shown for clarity. The detector mechanism  12  mounts in the circular hole  72  of the elevator plate. 
     At the four corners of the framework  66  are XY translation mechanisms  74 , consisting of crossed linear ball guides and rails. A magnified view of one of the XY translation mechanisms is shown in  FIG. 4B . The X-axis recirculating ball guide block  76  rides on the X-axis linear rail  78 , providing linear motion in the X, cross-table axis. The Y-axis linear rail  80  is mounted to the underside of the tabletop  6 , shown exploded for clarity. The Y-axis recirculating ball guide block  82  provides linear motion in the Y, table-length axis and is mounted to the X-axis recirculating ball guide block  76 . Four such XY translation mechanisms attach the tabletop to the framework, allowing complete XY freedom of the tabletop over a distance determined by the length of the linear rails. 
     Attached to the X-axis recirculating ball guide block in the upper-left corner is a X-axis linear optical encoder  84 . Attached to the Y-axis recirculating ball guide block in the upper-left corner is a Y-axis linear optical encoder  86 . These encoders provide the XY position of the tabletop, which information is required to locate or register the reconstructed cross-sectional images in space. This is essential for presenting 3-D representations of the breast, where all the slice images are assembled into a single volumetric image. 
     It should be understood that the x-axis and y-axis translation mechanisms  74  provide the means for centering the breast with the axis of rotation of the scanning mechanism. 
       FIG. 4C  shows the internal structure of an optical scanning apparatus with the tabletop  6  lifted off. The XY translation mechanisms  74  are not shown for clarity. An automatic centering mechanism  88  is shown. Before starting a scan, the scanning mechanism, mounted on the elevator plate  68 , is “homed” to its uppermost position via the ACME leadscrews  70 . Attached to the elevator plate is a centering cone  90  which accepts a centering probe  92 . The centering probe  92  is attached to the underside of the tabletop  6 . If the tabletop has been moved off its centered position (the tabletop hole  13  no longer being concentric to the scanning mechanism axis of rotation  28 ), the centering probe will contact the side of the centering cone and be forced to the center (bottom) of the centering cone. The centering cone is in the shape of a funnel cup, open at the top and narrows to a point at the bottom. 
     The centering probe assembly  92  is shown in  FIG. 4D . The end of the probe is a roller ball  94 , a large ball bearing, which will touch the surface of the centering cone and roll along that surface. The roller ball  94  is mounted in a polymer cup  96 , machined from a low-friction plastic such as TEFLON (trademark) or RULON (trademark). The probe end is mounted on a large machine screw  98 , which is threaded into a housing  100  and locked in place by a jam nut  102 . The machine screw  98  and housing  100  permit adjustment of the probe length. This assembly attaches to the underside of the tabletop via a mounting flange  104 . 
     The tabletop XY positioning is preferably a manual system, rather than motorized. An electromagnetic brake holds the tabletop in place once positioned. This brake (not shown) consists of an electromagnet, commonly used to secure emergency exit doors, attached to the framework, which bears on an iron plate attached to the tabletop. The operator releases the brake via a switch and moves the tabletop by hand to the desired position. The optical scanning apparatus acquires and displays the perimeter of the breast at each slice, as described in U.S. Pat. Nos. 6,044,288 and 6,029,077. The operator uses this perimeter information to recenter the breast in the scanning chamber. 
       FIGS. 5A and 5B  illustrate an LED display  103  mounted in the tabletop that the operator uses to position the tabletop. The LED display  103  comprises two lines of individual LEDs intersecting perpendicularly into a cross, having x and y axes. The general purpose computer that controls the optical scanning apparatus will calculate the breast perimeter as described in the patents above, and illuminate the LEDs accordingly. The illuminated LEDs  105  shows the extent of off-centeredness of the breast relative to the axis of rotation.  FIG. 5A  indicates that the patient position is low and to the right. The operator will move the tabletop and the control computer will continuously update the LED display. The operator will move the tabletop to the left and upward until the LED pattern is as shown in  FIG. 5B , where the LEDS are evenly lit about the center, indicating the breast is now centered. 
     The XY translation mechanisms are preferably a recirculating ball guide block riding on a linear rail. This combination provides a very low coefficient of friction, on the order of 1%. Thus the force to move the tabletop, even with a 400 lb. patient, will be on the order of 5 lbs. Many other linear positioning mechanisms could be employed, such as crossed-roller slides, polymer bushing slides, linear ball bearing slides (not recirculating) and linear tracks with rollers or cam followers. 
     The tabletop XY position sensors are preferably linear optical encoders, although linear potentiometers, linear variable displacement transducers (LVDTs) or rotary encoders or potentiometers attached to a windlass mechanism may also be used. 
     The automatic centering mechanism preferably uses a roller-ball probe entering a conical centering pocket. The centering pocket could be pyramidal, with any number of faces, or hemispherical, or virtually any other shape that is open at the top and comes to a point at the bottom. 
     The operator feedback for positioning preferably is an LED display in the tabletop. Any form of 2-dimensional display may also be employed to indicate the current and desired table position. 
     The embodiment of  FIGS. 4A-4D  discloses moving the patient on the tabletop with respect to a stationary scanning mechanism. Alternatively, the scanning mechanism could be moved with respect to a stationary tabletop and patient.  FIG. 6  illustrates an optical scanning apparatus mounted on an XY translation mechanism. 
     In  FIG. 6 , collimator  106 , shown with a conical mirror to fold the optical path 90°, collimates the light from the breast to detectors  108 , mounted on a horizontal shelf  110 . Additional disclosure of the conical mirror-collimator-detector structure is shown in WO 2004/096010. The shelf  110  is affixed to a cylindrical barrel  112  which forms the scanning chamber  10 . The cylindrical barrel  112  is mounted to an elevator plate  114  via a ball bearing, which is not shown, permitting orbital rotation around its Z-axis. The scanning mechanism  12  is rotated via chain  116  and sprocket  118  by an orbit stepping motor, hidden under the elevator plate  114 . The elevator plate  114  is mounted on three ACME leadscrews  120 . Chain  122 , driven by elevator stepping motor  124  via sprocket  126  will cause the elevator plate  114  to “crawl” up and down the ACME screws  120  via sprockets  128  (one sprocket is hidden behind the cylindrical barrel  112 ). With the combination of the two stepping motors, a precise helical scanning motion can be achieved. Helical scanning means that the source and detector continuously revolve around the subject while the subject is translated slowly through the plane of the source and detectors. Thus the trajectory of the detector is a helix, which is a screw thread shape. 
     The three ACME screws  120  are mounted on an L-shaped bracket  130  which translates in the X direction via linear bearings  132  sliding on ground rods  134 . The linear bearings  132  are shown as recirculating ball bearings, though many other types of linear bearings would suffice. X-axis stepping motor  136  effects the X translation via a gear pinion, not shown for clarity, meshed with a gear rack  138 . Rotation of the X-axis stepping motor  136  will cause the scanning mechanism to translate in the X direction. 
     The ground rods  134  are mounted on floating blocks  140 , as is the X-axis stepping motor  136  via Z-bracket  142 . The floating blocks  140  translate in the Y direction via linear bearings  144  sliding on ground rods  146 . Y-axis stepping motor  148  effects the Y translation via a gear pinion, not shown for clarity, meshed with a gear rack  150 . Rotation of the Y-axis stepping motor  148  will cause the scanning mechanism to translate in the Y direction. The Y-axis ground rods  146  are mounted on fixed blocks  152  which are mounted to a baseplate  154 . The baseplate  154  also mounts the Y-axis stepping motor  148  via Z-bracket  156 . The baseplate  154  is mounted to the floor of the scanner or is itself the floor of the scanner. 
     The optical scanning apparatus acquires and displays the perimeter of the breast at each slice, as described in U.S. Pat. Nos. 6,044,288 and 6,029,077. From this perimeter determination, the center of the breast can be determined at each slice and the controlling computer can translate the scanning mechanism in order to keep the breast centered in the scanning chamber. 
     The use of stepping motors for the X and Y movements advantageously obviates the need for any encoders, since the step count is exactly proportional to the position. Limit switches, not shown, are used to center the X and Y movements. 
     The XY translation mechanism preferably uses stepping motors for actuation and linear recirculating ball bearings sliding on ground rods for guides. DC motors, brush or brushless, rotational or linear, as actuators may also be used. Many forms of linear guide could be used as alternatives to the ground rods and ball bearings, such as the recirculating ball guides of  FIG. 4B . 
     It should be understood that the XY translation mechanism described above provides the means for means for centering the breast with the axis of rotation of the scanning mechanism. 
     An alternative to translating either the patient or the entire scanning mechanism is to control the aiming of the laser so that the breast is constantly illuminated, independent of its position in the scanning chamber.  FIGS. 7 and 8  show a laser aiming mechanism in a scanning apparatus. 
     In  FIG. 7 , the source laser, not shown, is delivered by optical fiber  158  to an SMA connector  160 . The diverging output of the SMA connector  160  is collimated into a parallel beam by lens  162 . The parallel beam is directed to a turning mirror  164  which is mounted on the shaft of a galvanometer  166 . The galvanometer  166  is a DC motor designed to move quickly over a small angular range, typically ±30° or less. They are commercially available from GSI Lumonics, Cambridge Technology, Nutfield Technology and others. They are designed to scan light beams in milliseconds to very high accuracy and stability over angular ranges of typically ±30°. The movement of the turning mirror  164  will deflect the laser beam  168  and control its landing spot on the breast  170 . In this figure, the center of rotation of the scanning mechanism is  172  and the center of this slice of the breast is  174 . The scanning mechanism consists of the laser launch mechanism  158 - 166 , a multiplicity of collimators  176  and optical detectors  178 . As the scanning mechanism rotates, the center of the breast  170  appears to describe a circle  180 . The galvanometer  166  is controlled such that the laser beam  168  always points toward the center  174  of the breast  170 . 
     The optical scanning apparatus acquires and displays the perimeter of the breast at each slice, as described in U.S. Pat. Nos. 6,044,288 and 6,029,077. From this perimeter determination, the center of the breast r can be determined.  FIG. 8  shows the same view as  FIG. 7 , with the detector components removed for clarity. The center of the breast  170  is at a distance r and angle θ from the center of rotation  172 . The distance from the turning mirror  164  to the center of rotation  108  is R. The angle of deflection φ of laser beam  168  is given by:
 
φ=tan −1 ( r *sin(θ)/( R−r *cos(θ))  Equation 1
 
     The angle of deflection of the turning mirror  164  will be ½ the angle φ. Equation 1 can be implemented in many ways: Computation by some general purpose computer or as a 2-dimensional lookup table, loaded in advance by a general purpose computer, indexed by r and θ. 
     It should be understood that the turning mirror  164  along with the perimeter data of the breast provides the means for aiming the radiation source toward the center of the breast during scanning. 
     While this invention has been described as having a 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.