Abstract:
A method of detecting ionizing radiation is provided. The method includes pixelating a semiconductor substrate such that each pixel comprises a central region and a region of variable response, substantially blocking the ionizing radiation from reaching the region of variable response, and receiving the ionizing radiation with the central region.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to imaging systems using pixilated detectors, and more particularly to pixilated semiconductor detectors in imaging systems. 
   Imaging devices, such as gamma cameras and computed tomography (CT) imaging systems, are used in the medical field to detect radioactive emission events emanating from a subject, such as a patient and to detect transmission x-rays attenuated by the subject, respectively. An output, typically in the form of an image that graphically illustrates the distribution of the sources of the emissions within the object and/or the distribution of attenuation of the object is formed from these detections. An imaging device may have one or more detectors that detect the number of emissions, for example, gamma rays in the range of 140 keV, and may have one or more detectors to detect x-rays that have passed through the object. Each of the detected emissions and x-rays is typically referred to as a “count,” but may also be counted together as a ‘signal current’ and the detector determines the number of counts received at different spatial positions. The imager then uses the count tallies to determine the distribution of the gamma sources and x-ray attenuator, typically in the form of a graphical image having different colors or shadings that represent the processed count tallies. 
   A pixilated semiconductor detector, for example, a detector fabricated from cadmium zinc telluride (CZT), may provide an economical method of detecting the gamma rays and x-rays. However, a low energy tail on the energy spectrum resulting from the CZT interaction with the radiation may interfere with the ability to distinguish direct gamma rays and x-rays from scattered gamma rays and x-rays. The tail may result from a different response of the semiconductor material in the regions between the pixels compared to the response from within the pixels. 
   Another problem that may be associated with using a pixilated semiconductor detector is a loss of potential detector spatial resolution due to a gap between a detector collimator and the active detector surface. The gap is a result of known mounting technology that makes collimator exchange easier. The divergence of the gamma and x-ray photons in the gap may contribute to a degradation of a spatial resolution realizable from the detector. At least some known imaging devices use a variety of interchangeable collimators for respective different applications. Each collimator may differ in length and bore of the holes, and the weight of the collimators necessitates special handling equipment and procedures. This further increases the likelihood of a degradation of spatial resolution of the detector. 
   Furthermore, due to the fine tolerances needed to achieve accurate resolution of detector images, producing collimators having holes that are substantially aligned with each detector pixel is difficult, thus affecting image resolution. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one embodiment, a method of detecting ionizing radiation is provided. The method includes pixelating a semiconductor substrate such that each pixel comprises a central region and a region of variable response, substantially blocking the ionizing radiation from reaching the region of variable response, and receiving the ionizing radiation with the central region. 
   In another embodiment, an imaging system that includes a semiconductor detector is provided. The imaging system includes a pixilated semiconductor substrate that is responsive to ionizing radiation, the substrate including a first surface in a direction of a source of ionizing radiation, and a collimating mask covering the substrate surface, the collimating mask including a plurality of mask openings exposing a central region of a pixel of the semiconductor detector substrate to the ionizing radiation, the collimating mask including mask septa that facilitate substantially blocking the incident ionizing radiation from a region of variable response associated with the pixel. 
   In yet another embodiment, a collimating mask for a pixilated radiation detector is provided. The collimating mask includes a mask portion formed generally in a grid arrangement wherein the grid is configured to expose a central region of a pixel defined in a detector substrate of the detector, and to overlay a region surrounding the central region. 
   In still another embodiment, a detector assembly for an imaging system is provided. The detector assembly includes a radiation detector having a pixilated semiconductor substrate that includes a pixel electrode coupled to a first surface of the substrate wherein the pixel electrode defines a pixel region of the substrate, a cathode covering a second surface of the substrate, a dielectric layer covering the cathode, a collimating mask that includes a mask portion that has openings therethrough surrounded by a mask septa wherein the mask portion is configured to expose a central region of the pixel, and to overlay a region surrounding the central region. The detector assembly also includes a collimator removably couplable to the radiation detector wherein the collimator has apertures therethrough, and the apertures are configured to substantially align with the collimating mask openings. The collimator is further configured to receive another collimator such that apertures of each collimator substantially align with respect to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a graph that illustrates an exemplary energy spectrum of a single pixel of a pixilated CZT detector exposed to substantially unscattered 140 keV gamma rays; 
       FIG. 2  is a cross-sectional elevation view of an exemplary imaging device detector having a plurality of pixilated semiconductor detector elements according to an embodiment of the present invention; 
       FIG. 3  is a perspective view of the imaging device detector shown in  FIG. 2 ; 
       FIG. 4  is a schematic side elevation view of a portion of the imaging device detector shown in  FIG. 2 ; and 
       FIG. 5  is a schematic illustration of an exemplary array of imaging device detectors configured to couple to an arcuate base according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a graph  50  that illustrates an exemplary energy spectrum of a single pixel of a pixilated CZT detector exposed to substantially unscattered 140 keV gamma rays. Graph  50  includes an x-axis graduated in units of keV and a y-axis representative of an amount of total counts or count rate observed at each keV level. An energy spectrum peak  52  centered about 140 keV represents the gamma rays that have been absorbed substantially within a central region portion of the single pixel. The distribution of signal amplitudes of these events is approximately Gaussian. However, a significant number of gamma rays are also detected in the portion of the energy response spectrum that tails toward the lower energies. This tail effect is caused, in part, by Compton scattering, by gamma ray absorption events that do not confine all charge creation to within a single pixel and by non-ideal charge collection. Because the illustrated response function represents the distribution of measured signals from only a single pixel, charge that is lost from the pixel and shared with adjacent pixels is not included in the response function. As a result, gamma ray absorption events in which the charge collection is incomplete due to charge sharing with other pixels are lost from the peak region and contribute to the low energy tailing. 
     FIG. 2  is a cross-sectional elevation view of an exemplary imaging device detector  100  in accordance with one embodiment of the present invention and includes a plurality of pixilated semiconductor detector elements  102  that may be used in connection with, for example, localizing a radiation interaction event in the detector. In the exemplary embodiment, detector  100  includes a detector substrate  104  and a collimator  106 . Detector  100  may be formed of a radiation responsive semiconductor material, for example, cadmium zinc telluride (CZT) crystals. Detector elements  102  may be formed of the substrate  104  by pixelating a corresponding plurality of pixel electrodes  108  coupled to a first surface  110  of detector substrate  104  (shown as a lower surface). A cross-sectional size and shape of pixel electrodes  108  and a spacing between each of the pixel electrodes  108  facilitates determining a location and size of each pixilated detector element  102 . Specifically, each pixilated detector element  102  is located proximate a second surface  112  (shown as an upper surface) of detector substrate  104  in substantial alignment with a longitudinal axis  114  of a corresponding pixel electrode  108 . Each pixilated detector element  102  includes a central region  116 , bounded by useful limits  118 , defining an operating portion, and a region of variable response  119 . Within central region  116 , pixilated detector element  102  has a substantially uniform and repeatable response characteristic to radiation incident on second surface  112 . Detector substrate  104  in areas outside central region  116  has a response characteristic to radiation incident on second surface  112  that may be variable. An intrinsic spatial resolution of detector  100  may be defined by the size of and the spacing between each pixilated detector element  102 . Because, pixilated detector elements  102  may be non-homogeneous in response and because central region  116  has a substantially uniform and repeatable response characteristic, collimator  106  may be formed to allow gamma and x-ray photons to interact with central region  116  and to block gamma and x-ray photons from reaching region of variable response  119 . 
   Collimator  106  includes septa  120  that define apertures  122  through the collimator. A degree of collimation may be defined by a length  124  of collimator  106 , a diameter  126  of apertures  122 , a thickness  128  of apertures  122 , and an absorption coefficient of the material collimator  106  from which collimator  106  is fabricated. A surface  130  of collimator  106  that is proximate second surface  112  defines a gap  132  between detector substrate  104  and collimator  106 . A collimating mask  134  may abut and/or be coupled to second surface  112  and cover region of variable response  119 . In one embodiment, collimating mask  134  is adhered to second surface  112 . In another embodiment, collimating mask  134  is deposited on second surface  112  for example by using a vapor deposition process. A thickness  136  of collimating mask  134  may be determined based on an energy level of photons that may be incident on collimating mask  134  in operation, and an absorption coefficient of the material from which collimating mask  134  is fabricated. For example, collimating mask  134  may be fabricated from a relatively high atomic number material (e.g., an atomic number of about seventy-two or greater) that can absorb radiation of the type intended to be employed in imaging device detector  100 , such as, for examples, lead and tungsten or alloys or conglomerates thereof. 
   In operation, photons, for example emission gammas and transmission x-rays, from a source  140  are directed towards second surface  112 . A first portion  144  of the photons may arrive at an incident surface  142  of collimator  106  substantially parallel with septa  120  and in alignment with apertures  122 , and pass through collimator  106  without substantial interaction with collimator  106 . A second portion  146  of the photons may arrive at incident surface  142  of collimator  106  substantially parallel and in alignment with septa  120  and may interact with collimator  106  by absorption or scattering. A third portion  148  of the photons may arrive at incident surface  142  of collimator  106  at an angle  150  with respect to a longitudinal axis  152  of aperture  122 . If angle  150  is greater than an angle determined by length  124  and diameter  126 , a photon entering aperture  122  will interact with collimator  106  before exiting aperture  122 . If angle  150  is less than the angle determined by length  124  and diameter  126 , the photon may exit aperture  122  so as to interact with collimating mask  134  covering region of variable response  119 . Accordingly, collimating mask  134  facilitates preventing photons, that would otherwise interact with region of variable response  119 , from doing so. 
   Second surface  112  may be substantially covered by a single cathode electrode  154 . First surface  110  has a rectangular array of small, for example between about one millimeters squared (mm 2 ) and about ten mm 2 , generally square pixel electrodes  108  configured as anodes. A voltage difference applied between pixel electrodes  108  and cathode  154  during operation generates an electric field (detector field) in substrate  104 . The detector field may be, for example, about one kilovolts per centimeter to three kilovolts per centimeter. Although pixel electrodes  108  are described in the exemplary embodiment as being generally square, this shape should not be understood to be limiting, in that other shapes of pixel electrodes  108  are contemplated. 
   When a photon is incident on substrate  104 , it generally loses all its energy in substrate  104  by ionization and leaves pairs of mobile electrons  156  and holes  158  in a small localized region of substrate  104 . As a result of the detector field, holes  158  drift toward cathode  154  and electrons  156  drift toward pixel electrodes  108 , thereby inducing charges on pixel electrodes  108  and cathode  154 . The induced charges on pixel electrodes  108  are detected and identify the time at which a photon was detected, how much energy the detected photon deposited in the substrate  104  and where in the substrate  104  the photon interaction occurred. 
   To facilitate optimum detection of gamma and x-ray photons, central region  116  should be in substantial alignment with apertures  122 , collimating mask  134  should be in substantial alignment with septa  120 , and the relative dimensions of gap  132 , length  124 , diameter  126  and thickness  128  should be determined such that photons arriving at incident surface  142  are absorbed in collimator  106 , collimating mask  134 , or central region  116 . 
     FIG. 3  is a perspective view of imaging device detector  100  (shown in  FIG. 2 ). Imaging device detector  100  includes detector substrate  104  with high voltage cathode  154  covering substantially the entire second surface  112 . A dielectric layer  302  is positioned over high voltage cathode  154  to insulate the high voltage applied to high voltage cathode  154  during operation from electrical and/or other components of imaging device detector  100 . In the exemplary embodiment, dielectric layer  302  comprises Kapton™ film of about 0.01 mm to about 0.5 mm in thickness. In an alternative embodiment, other thin film dielectrics may be used, such as, but not limited to, Mylar™. Collimating mask  134  is applied to dielectric layer  302 . Collimating mask  134  includes a plurality of mask openings  304  separated by mask septa  306 . Mask openings  304  are shown, generally, as square openings, but may be fabricated as other shapes, such as hexagonal and round to meet specific requirements. Collimating mask thickness  136  may be selected to substantially reduce incident photons from interacting with region of variable response  119  and may be, for example, about one mm to six mm. If collimating mask  134  is fabricated from lead or tungsten, a typical collimating mask thickness  136  may be three mm to five mm. Thickness  136  may also be selected such that collimating mask  136  acts as a general purpose collimator, allowing imaging without using collimator  106  for certain scans, and allowing use with collimator  106  for relatively higher resolution scans. In various exemplary embodiments, a width  308  of mask septa  306  may be, for example, about 0.1 mm to about 0.5 mm. 
   In another embodiment, dielectric layer  302  may be removed, in which case, collimating mask  134  is held at a high voltage cathode voltage and insulated from surrounding low voltage components by, for example, an airgap, a dielectric coating, a dielectric layer, and/or dielectric components, such as paint, tape, and plastic parts. 
   Collimator  106  includes the plurality of apertures  122  separated by collimator septa  120 . In the exemplary embodiment, a pitch  310  of mask openings  304  and septa  306  of collimating mask  134  is substantially equal to a pitch (not shown) of central region  116  and region of variable response  119  (both shown in  FIG. 2 ). As used herein, pitch refers to a distance between identical features of a recurring pattern, for example, a distance between adjacent apertures  122  measured from a center of a first aperture  122  to a center of an adjacent aperture  122 . In various exemplary embodiments, a pitch of apertures  122  and collimator septa  120  is about equal to the pitch of collimating mask  134 . Due to a close coupling of collimating mask  134  to detector substrate  104 , the pitch of collimator  106  may deviate from the pitch of collimating mask  134  without substantially affecting the performance of imaging device detector  100 . 
     FIG. 4  is a schematic side elevation of a portion of imaging device detector  100  (shown in  FIG. 2 ).  FIG. 4  illustrates alignment tolerances between collimator  106  and collimating mask  134 . Collimating mask septa  306  are fabricated such that they are positioned adjacent region of variable response  119  and may be fabricated to a width  402  that is wider than a width  404  of region of variable response  119  and collimating mask septa  306 . Accordingly, this accommodates when collimator septa  306  may be offset a distance  403 , due to lateral misalignment of collimator  106  with respect to detector substrate  104 , a variation in collimator pitch from that of collimating mask  134 , or other reason. The amount of offset, variation, or tolerance provided is determined by width  402 , width  404 , width  128 , and gap  132 . To improve the spatial resolution of imaging device detector  100 , one or more additional stacking collimators (not shown) may be stacked on incident surface  142  of collimator  106  to effectively increase length  124 . Accordingly, system  100  including collimating mask  134  may be supplied with a factory-determined sensitivity map. Using the various exemplary embodiments described herein, collimator  106  and other additional collimators may be added without substantially changing the sensitivity map. 
     FIG. 5  is a schematic illustration of an exemplary array  500  of imaging device detectors  100  configured to couple to an arcuate base. Each imaging device detector  100  may be staggered or vertically incremented relative to each adjacent imaging device detector  100  to accommodate various mounting configuration requirements. Alternately, imaging device detectors  100  may be aligned with each adjacent imaging device detector  100  to accommodate mounting in a flat panel configuration. 
   The above-described imaging device detectors provide a cost-effective and reliable means for examining a patient. More specifically, the imaging system includes a collimating mask that is closely coupled to the surface of a planar pixilated semiconductor detector to facilitate reducing photon interaction in a region of variable response of the detector pixels. Coupling a collimating mask directly to the surface of the detector also facilitates improving the semiconductor (e.g., CZT) detector response energy spectrum, for example, reducing the characteristic tail, increasing detector imaging efficiency or the ability to tradeoff detector efficiency for higher spatial resolution, reducing response to scatter relative to direct photons (e.g., gammas and x-rays), reducing collimator handling, facilitating and simplifying collimator exchange, permitting “turn key” operation, and allowing for pre-calibration of the detector system at the factory before delivery to a customer. 
   Exemplary embodiments of pixilated photon detector methods and apparatus are described above in detail. The pixilated photon detector components illustrated are not limited to the specific embodiments described herein, but rather, components of each pixilated photon detector may be utilized independently and separately from other components described herein. For example, the pixilated photon detector components described above may also be used in combination with different imaging systems. A technical effect of the various embodiments of the systems and methods described herein include at least one of improving the semiconductor detector response energy spectrum by reducing the characteristic tail of the response and permitting simpler and easier exchange of collimators. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.