Abstract:
Steering electrodes are used to improve the uniformity and efficiency of solid-state semiconductor x-ray detectors. The steering electrodes are insulated from the semiconductor material so as to prevent surface current flows that degrade the signal to noise ratio of the detected signal. A simple fabrication technique employing photolithographic techniques may be employed

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to x-ray detectors, and in particular, to a solid-state semiconductor detector such as cadmium zinc telluride (CZT) used for quantitative x-ray imaging.  
         [0002]     Measurements of the x-ray absorption by an object, for example, at two different x-ray energies, can reveal information about the composition of that object as decomposed into two selected basis materials. In the medical area, the selected basis materials are frequently bone and soft tissue. The ability to distinguish bone from soft tissue allows x-ray images to yield quantitative information about in vivo bone density for the diagnosis of osteoporosis and other bone disease.  
         [0003]     Alternatively, the selection of other basis materials allows dual energy x-ray measurements to be used for the analysis of body composition by distinguishing between fat and non-fat tissue, or for baggage scanning by distinguishing between explosive and non-explosive materials.  
         [0004]     High resistivity solid-state semiconductors such as cadmium zinc telluride (CZT) may be used to detect x-rays passing through a measured object in a single or dual energy x-ray system. In a planar-contact CZT detector, a voltage is imposed between an anode and cathode positioned on opposite faces of a CZT crystal. X-rays pass through the anode into the crystal to release electrons that are attracted to the cathode. The number of released electrons is proportional to the photon energy allowing high and low energy x-ray photons to be distinguished by pulse height.  
         [0005]     Detectors of this design can exhibit a variation in the energy measurement dependent on the location of the x-ray radiation interaction within the crystal. This measurement variation results in a reduction in accuracy as well as poor energy resolution. The prior art has addressed this measurement variation by reducing the anode size and attaching “steering electrodes” to the crystal surrounding the anode. The steering electrodes are operated at an intermediate voltage between the voltages of the anode and cathode to shape the electrical field within the detector improving charge collection.  
       SUMMARY OF THE INVENTION  
       [0006]     The finite resistance of the CZT crystal allows a surface current to flow between the steering electrode and the anode which presents a practical limit to the steering grid voltage above which energy measurement degradation begins to occur once again. The surface current is also believed to introduce noise into the detected signal.  
         [0007]     Accordingly, the present invention places a thin layer of highly insulating material between the steering electrode and the CZT crystal preventing surface current flow. Manufacturing is simplified by placing the steering electrode on an insulating support holding the CZT crystal and placing the insulating layer on the steering electrode as attached to the insulating support. This approach also simplifies routing the electrodes for multi-electrode arrays.  
         [0008]     The steering electrode&#39;s close proximity to the CZT crystal allows the electric field from the steering electrode to penetrate the CZT crystal, providing the necessary electron steering effect, while avoiding noise currents between the steering electrode and anode, and making it possible to increase the steering voltage significantly over what could otherwise be obtained using an electrode directly on or in the crystal.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0009]      FIG. 1  is a side elevational cross section through an x-ray detector of the present invention having an insulating support and a CZT crystal and showing the associated cathode, anode, and steering electrodes, the latter being insulated from the CZT crystal;  
         [0010]      FIG. 2  is a top planar view of the insulating support showing the placement of the steering electrodes in a grid pattern and showing the location of the crystals and their anodes in a staggered parallelogram configuration for improved sampling in a scanning x-ray machine; and  
         [0011]      FIG. 3  is a figure similar to that of  FIG. 2  showing an alternative staggered configuration of electrodes using rectangular detector elements. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring now to  FIG. 1 , a solid-state, dual energy x-ray detector system  10  may include a monolithic CZT crystal  12  having a front surface  14  normally facing a source of x-ray photons  16  and a rear surface  20  on the opposite side of the CZT crystal from the front surface.  
         [0013]     A cathode  22  is applied to the front surface  14  of the CZT crystal  12 , and an anode  24  is applied to the rear surface  20  of the CZT crystal  12  to provide a biasing electrical field between them. Generally, the cathode  22  will cover the entire front surface  14  but the anode will cover only a small area centered on the rear surface  20 . Both the cathode  22  and anode  24  may be applied directly to the CZT crystal  12 , for example, by sputtering, and are preferably formed of a conductive metal such as platinum. The front surface  14  of the CZT crystal  12  may also be protected by a light opaque, x-ray transparent material such as aluminized Mylar.  
         [0014]     The CZT crystal  12  with attached cathode  22  and anode  24  may be supported at the rear surface  20  by an insulating support  26  abutting the rear surface  20 . The gap between the insulating support  26  and rear face  20  is exaggerated in  FIG. 1  to indicate how elements of insulation and electrodes are attached to the surfaces prior to assembly. The insulating support  26  holds on its front surface, facing the CZT crystal, an anode contact  28 , flanked by steering electrodes  30  also held on the front surface of the insulating support.  
         [0015]     When the CZT crystal  12  is placed against the insulating support  26 , the anode contact  28  will align with and electrically connect to the anode  24 . This connection may be enhanced through the use of a conductive epoxy or the like. In this configuration, the steering electrodes  30  will be proximate to the CZT crystal  12  but separated from the CZT crystal by an insulating layer  32  attached to the steering electrodes  30 . Although a solid insulating layer  32  is shown, air insulation may also be used instead or in addition with the spacing of up to 0.2 millimeters. In the preferred embodiment, the dielectric constant of the insulating layer should be well matched to that of the solid-state material. In either case, direct electrical flow between the steering electrode  30  and the anode  24  may be avoided. On the other hand, it has been determined that the electrical field produced by the steering electrodes  30  will penetrate the CZT crystal and help steer electrons  36  generated by the interaction of the CZT crystal  12  and the x-ray photons  16  to the anode  24 .  
         [0016]     Placement of the steering electrodes  30  on the insulating support  26 , rather than directly on the CZT crystal  12 , greatly simplifies experimentation with different electrode configurations and allows a variety of electrode patterns to be used with single sets of CZT crystal  12  to achieve different product configurations.  
         [0017]     The steering electrodes  30  on one side of the insulating support  26  may communicate via plate-through holes  38  with traces  40  on the opposite side of the insulating support  26 , the latter which may conduct a steering voltage to the steering electrodes  30 . Likewise, a plate-through hole  42  may allow communication between anode contact  28  and a grounding trace  44  providing grounding for the anode  24 . An additional plate-through hole  45 , displaced from the CZT crystal  12 , allows a convenient attachment point of a high voltage lead  46  from the plate-through hole  45  to the cathode  22  to apply a biasing voltage to that cathode  22 . The plate-through hole  45  also communicates with a trace  47  also on the opposite side of the insulating support  26  conducting a biasing voltage to the cathode  22 .  
         [0018]     These traces  40 ,  44 ,  47  may cross underneath different pixel regions  15  of the x-ray detector system  10 , as will be described below, and by being thus removed from the CZT crystal  12  by the thickness of the insulating support  26  and possibly ground planes of that support, may have reduced effect on the electric fields and hence the operation of those different pixel regions  15 . The insulating support  26  together with its electrodes  30 ,  28  and plate-through holes  38 ,  42  and  45  and traces,  44 ,  40  and  47  may be readily fabricated on a ceramic material such as alumina using standard photolithography techniques and sputtering of metallic layers, or by using standard printed circuit board techniques in which a metal-clad insulating material is etched to produce the necessary traces and treated to produce the plate through holes.  
         [0019]     In operation of the x-ray detector system  10 , the cathode  22  will be biased on the order of one thousand volts with respect to the ground of anode contact  28  to accelerate electrons  36  to the anode  24 . The steering electrode  30  may be placed at a lower voltage, for example, one hundred volts to provide the necessary steering action. A power supply  31  will be connected to provide the necessary bias voltages. Outputs from the anode  24  may be connected to an amplifier to provide a signal to a processing computer to produce a quantitative image of the x-ray photons segregated by energy according to techniques well known in the art.  
         [0020]     Referring now also to  FIG. 2 , generally the x-ray detector system  10  may provide for multiple detector elements on a single CZT crystal  12 . In this case, multiple anodes  24  will be placed on the insulating support  26 , each surrounded by steering electrodes  30 .  
         [0021]     The steering electrodes  30  surrounding each anode  24  (and equal area anode contact  28 ) describe by their perimeter a pixel region  15  associated with each anode contact  28 . The pixel regions  15  describe areas which may independently detect x-ray photons  16  to produce a quantitative detection value that will be mapped to individual pixels in a resultant image.  
         [0022]     In the embodiment shown in  FIG. 2 , the pixel regions  15  are generally parallelograms tiling in rows and slanted columns. In this embodiment, each parallelogram pixel region  15  has a first base  52  generally perpendicular to a scan direction  54  in which the x-ray detector system  10  will be scanned to collect information over an area of the patient. Sidewalls  56  of the parallelogram and the pixel regions  15  are angled such that the centers of the pixel regions  15  defined approximately by the center of the anode contact  28  for a first row of pixel regions  15 , follow paths  60  that interleave with paths  62  followed by centers of the pixel regions  15  of a second row of pixel regions  15 . In this way, larger pixel regions  15  may provide higher spatial resolution sampling to improve the resultant image.  
         [0023]     Referring now to  FIG. 3 , in an alternative embodiment, the pixel regions  15  may be rectangular with the pixel regions  15  of a first row staggered with respect to the second row to provide interleaved paths  60  and  62  as before. The rectangular pixel regions  15  of  FIG. 3  provide the advantage of a more compact detection region limiting the effective size of a convolution kernel (a function of the project width of the pixel regions  15  on a line perpendicular to the scan direction  54 ) that can make a resultant image less distinct.  
         [0024]     Referring still to  FIG. 3 , a convenient form factor for the x-ray detector system  10  has two rows each having eight pixel regions  15 . Multiple detector systems  10  of this or similar form factors may be ganged edgewise to provide arbitrary continuations of the rows. For an x-ray detector system  10  having rectangular pixel regions  15 , pixel regions  15   a  and  15   b  at a first and second row of a right edge of the x-ray detector system  10  may be cut at an angle with respect to the scan direction  54  to equally reduce the area of the pixel regions  15   a  and  15   b . Similarly reduced pixel regions  15   c  and  15   d  at a first and second row of a left edge of a next x-ray detector system  10 ′ may be placed in close proximity to their counterpart pixel regions  15   b  and  15   a . The area of each pixel region  15   a - 15   d  is reduced by half the width of the joint gap between x-ray detector system  10  and  10 ′, which then preserves the regular lateral of the other pixel regions  15 . In another embodiment, the area of each pixel region  15   a - 15   d  is reduced to slightly less than half to accommodate the joint gap between x-ray detector system  10  and  10 ′. This provides two virtual pixel regions, the first being a combination of the signals from pixel regions  15   a  and  15   d , and the second being a combination of the pixel regions  15   b  and  15   c . The slightly reduced detection area of these detectors virtual pixel regions may be corrected mathematically by a weighting factor applied by the computer receiving the signals.  
         [0025]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.