Abstract:
This invention relates generally to a system for differentiating material characteristics using a Computed Tomography imaging system. More particularly, it relates to a detector module for use with a hybrid-scintillator/photo sensor and direct conversion imaging system comprised of an x-ray transparent top layer; an electrode on said top; a block of direct conversion material, the electrode providing a common bias to the direct conversion material; a substrate material electrically connected with the direct conversion material; a signal processing chip electrically connected with the substrate; and a connector element electrically connected to the substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to a system for differentiating material characteristics using an imaging system. More particularly, it relates to a detector module for use with a hybrid-scintillator/photo sensor and direct conversion imaging system.  
         [0002]     In at least one known computed tomography (CT) imaging system configuration having single and/or multi slice scintillator/photodiode rays, an x-ray source projects a fan-shaped, or cone-shaped, beam which is collimated to lie within an x-y-z volume of a Cartesian coordinate system. That x-y-z volume is generally referred to as an “imaging volume” and usually includes a set of x-y planes generally referred to as the “imaging planes.” An array of radiation detectors, wherein each radiation detector includes at least one detector element, is disposed within the CT system so as to receive this beam. An object, such as a patient, is disposed within the imaging plane such that the x-ray beam passes through the object. As the x-ray beam passes through the object being imaged, the x-ray beam becomes attenuated before impinging upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is responsive to the attenuation of the x-ray beam by the object. In turn, each detector element produces a separate electrical signal responsive to the beam attenuation at the detector element location. These electrical signals are referred to as x-ray “attenuation measurements”.  
         [0003]     In addition, the x-ray source and the detector array may be rotated by means of a gantry situated within the imaging volume and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, or, “projection data”, from the detector ray at one gantry angle is referred to as a “view.” A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and the detector array. In an axial scan, the projection data is processed so as to construct an image that corresponds to two-dimensional slices taken through the object.  
         [0004]     One method of reconstructing an image from a set of projection data is referred to as the “filtered back-projection technique.” This process converts the attenuation measurements from a scan into discrete integers, ranging from −1,024 to +3,072, called “CT numbers” or “Hounsfield units” (HU). These HUs are used to control the brightness of a corresponding pixel on a cathode ray tube or a computer screen display in a manner responsive to the attenuation measurements. For example, an attenuation measurement for air may convert into an integer value of −1,000 HUs (corresponding to a dark pixel) and an attenuation measurement for very dense bone matter may convert into an integer value of +3,000 (corresponding to a bright pixel), whereas an attenuation measurement for water may convert into an integer value of 0 HUs (corresponding to a grey pixel). This integer conversion or “scoring” allows a physician or a technician to determine the density of matter based upon the intensity of the computer display and thus locate anatomical landmarks and identify areas of concern.  
         [0005]     Typically, radiation detector rays that are used in imaging systems, such as the T imaging described herein, include single and/or multi-slice scintillator/photodiode etectors. A scintillation detector is constructed of scintillation material, such as cadmium tungstate (CDW04) or rare earth ceramics and operates by receiving x-ray photons emitted by an x-ray source and by converting these x-ray photons into a digital signal that is proportional to the attenuated x-ray energy received. These digital signals are then processed and turned into image data.  
         [0006]     One goal of CT imaging is to utilize multi-energy scanning techniques to differentiate tissues and/or materials having varying atomic number and densities, such as calcium and/or iodine. Historically, this has been accomplished using an imaging system having a scintillation detector either by taking single slice images with a single slice CT imaging system having two different x-ray beam filters, or by taking single slice images with a single slice CT imaging system having two different x-ray tube kVp&#39;s that exactly overlap spatially, but at a slightly later time, and then processing these two images to separate materials having varying atomic numbers and densities, using suitable known methods such as image subtraction.  
         [0007]     For example, using a single slice CT imaging system, a first single slice image would be obtained. The x-ray kVp or the filter at the x-ray tube would then be changed and a second single slice image would be obtained at the same patient location. As mentioned above, the two slices of image data would then be processed to separate the materials of varying atomic numbers and densities within the obtained sliced plane.  
         [0008]     Unfortunately, this is an expensive, time consuming and involved process and although a CT imaging system having a direct conversion (DC) detector could conceivably be utilized as the CT imaging system for performing the above-mentioned process, the DC detector would not be able to count the x-rays fast enough to support CT flux rates and/or scan times. Thus, if used in the current mode, the obtained information would suffer from a very high amount of non-linearities that would be very difficult or even impossible to correct in order to achieve artifact free scanning.  
         [0009]     The above discussed and other features and advantages of the embodiments will be appreciated and understood by those skilled in the art from the following detailed description and drawings.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The above discussed and other drawbacks and deficiencies are overcome or alleviated by a hybrid scintillation/direct conversion computed tomography (CT) imaging system comprising: a gantry, wherein the gantry defines a patient cavity and includes an x-ray source and a radiation detection apparatus, wherein the radiation detection apparatus includes a first radiation detector and a second radiation detector and wherein the x-ray source and the radiation detection apparatus are rotatingly associated with the gantry so as to be separated by the patient cavity; a patient support structure movingly associated with the gantry so as to allow communication with the patient cavity; and a processing device, wherein the processing device is communicating with the radiation detection apparatus.  
         [0011]     In order to accomplish the above mentioned, the radiation detector described herein provides for a compact, modular radiation detector that is able to move fully or partially in and out of the x-ray beam in front of an existing CT scintillator. The radiation detector is comprised of a plurality of detector modules. Each detector module is comprised of top composite layer that is radiographically transparent. Immediately underneath the composite layer is a high voltage electrode. The high voltage electrode provides the common bias to a direct conversion material. The present invention also provides for a scintillator/photodiode assembly immediately underneath the composite layer. The scintillator/photodiode assembly is connected to either a flex circuit, a multi-layer ceramic substrate or silicon substrate for signal routing to a data processing unit. Preferably, a silicon substrate is used as it has the highest run density of the known alternatives. The signals are then routed again through the substrate material to an end block connector. The connector is then connected to subsequent signal processing electronics. The foregoing and other features and advantages of the present invention will be apparent from the description that follows. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a perspective view of a CT imaging system and a patient disposed for imaging.  
         [0013]      FIG. 2  is a block schematic diagram of a hybrid scintillation/direct conversion CT imaging system in accordance with the present invention.  
         [0014]      FIG. 3  is a perspective view of a detector array in accordance with the present invention.  
         [0015]      FIG. 4  is an elevational view of a detector module of the present invention expanded along its y-axis for clarity.  
         [0016]      FIG. 5  is an elevational view of the second embodiment of the present invention  
         [0017]      FIG. 6  is an elevational view of the true aspect ratio of the detector modules of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring now to the drawings in detail, wherein like numbered elements correspond to like elements throughout.  FIGS. 1 and 2  show pictorial and block schematic views, respectively, of a computed tomography (CT) imaging system  1 . The hybrid scintillation/direct conversion CT imaging system  1  includes a gantry  2  having an x-ray source  4 , a radiation detection apparatus  6 , a patient support structure  8 , and a patient cavity  10 , wherein x-ray source  4  and radiation detection apparatus  6  are disposed so as to be separated by the patient cavity  10 . X-ray source  4  and radiation detection apparatus  6  are rotatingly disposed relative to the gantry  2  and the patient support structure  8 , so as to allow x-ray source  4  and radiation detection apparatus  6  to rotate around the patient support structure  8  when the patient support structure  8  is disposed within patient cavity  10 . X-ray projection data is obtained by rotating the x-ray source  4  and the radiation detection apparatus  6  around the patient  40  during a scan.  
         [0019]     In more detail, x-ray source  4  includes a first x-ray emitter  12  and radiation detector apparatus  6  which includes a first radiation detector  16  and second radiation detector  18 . First x-ray emitter  12  is opposingly disposed relative to first radiation detector  16  and second radiation detector  18  such that first x-ray emitter  12  is separated from first radiation detector  16  and second radiation detector  18  by patient cavity  10 . X-ray source  4  and radiation detection apparatus  6  are communicated with a control mechanism  20  associated with CT imaging system  1 . Control mechanism  20  controls the rotation and operation of x-ray source  4  and/or radiation detection apparatus  6 . Control mechanism  20  includes an x-ray controller  22  in electronic communication with the x-ray source  4 , a gantry motor controller  24 , and a data acquisition system (DAS)  26 . The DAS  26  is communicatively attached to radiation detection apparatus  6 , wherein an x-ray controller  22  provides power and timing signals to x-rays source  4 . Gantry motor controller  24  controls the rotational speed and angular position of x-ray source  4  and radiation detector apparatus  6 . DAS  26  receives the electrical signal data produced by radiation detection apparatus  6  and converts this data into digital signals for subsequent processing. CT imaging system  1  also includes an image reconstruction device  28 , a data storage device  30  and processing device, or computer  32 , wherein processing device  32  is electrically connected with image reconstruction device  28 , gantry motor controller  24 , x-ray controller  22 , data storage device  30 , an input device  34  and an output device, or display  36 . Moreover, CT imaging system  1  also includes a table controller connected with processing device  32  and patient support structure so as to control the patient support structure  8  relative to patient cavity  10 .  
         [0020]     In general, first radiation detector  16  is disposed so as to be adjacent to second radiation detector  18 . The patient  40  is disposed upon patient support structure  8  and patient support structure  8  is disposed within patient cavity  10 . Imaging system  1  is then operated so as to cause the radiation source  4  to emit and project an x-ray beam  42  toward the radiation detection apparatus  6  so as to pass through patient  40  and thereby create an attenuated x-ray beam  42  that is responsive to patient  40 . X-ray beam  42  is preferably collimated by a collimator so as to lie within an X-Y-Z volume of a Cartesian coordinate system referred to as the “imaging volume”.  
         [0021]     After passing through and becoming attenuated by patient  40 , radiation detection apparatus  6  receives the attenuated x-ray beam  42 , thus causing first radiation detector  16  to generated first detector data and second radiation detector  18  to generate second detector data, wherein first detector data and second detector data are generated in a manner responsive to attenuated x-ray beam  42 . First detector data and second detector data are then processed so as to generate image data. This is accomplished by communicating first detector data and second detector data to DAS  26 , which then converts first detector data and second detector data into digital signals responsive to patient  40 , first radiation detector  16  and second radiation detector  18 . First detector data, second detector data and the digital signals are then communicated to image reconstruction device  28  which generates image data by performing high speed reconstruction. This information is then communicated to a processing device  32 , which stores the image data in the data storage device  30  and displays the image data as an output via an output device  36 .  
         [0022]     When hybrid scintillation direct conversion imaging system  1  is used in a helical mode, adjacent slice planes though patient  40  may be viewed by multiple detector cells in the Z direction at slightly later points in time, if the rotational pitch of the radiation detection apparatus  6  has the appropriate width. Similarly, if two different types of radiation detectors were placed adjacent to each other in the Z direction, then adjacent slice planes through patient  40  could be viewed by multiple radiation detectors of a different type in the Z direction at slightly later points in time, again allowing for an appropriate rotational pitch of the radiation detection apparatus  6 .  
         [0023]     Therefore, first radiation detector  16  may be a scintillator detector/photo-sensor detector so as to allow for the collection of traditional information for creating anatomical detail for CT slices and second radiation detector  18  may be a direct conversion detector configured in x-ray counting and energy discrimination mode so as to count the attenuated x-rays and so as to measure the attenuated x-ray energy. This information on the number and energy of attenuated x-rays may provide tissue differentiation data that provides information about the elemental composition and/or density of various tissue materials, so as to allow for the discrimination of iodine, calcium and/or other materials as well. The information obtained from first radiation detector  16  and second radiation detector  18  may be super-positioned to create a single image having identically positioned and over lapping information of anatomically detail and/or tissue discrimination. Second radiation detector  18  may be of a single slice design and/or a multiple slice design. A multiple slice design may provide information on a variety of different tissue materials, whereas the single slice design may be integrated across multiple slices for improved statistics on an individual basis.  
         [0024]     Beneficially, when a second radiation detector  18  is used in the x-ray counting and energy discrimination mode, very little x-ray dose is added to the CT exam because of the low level of x-rays required for energy discrimination. This implementation may be accomplished quickly because DC detectors currently operate in the x-ray counting and energy discrimination mode in existing BMD and GAMA camera systems. This embodiment allows for obtaining superposition of anatomical detail and tissue characterization information, as well as the measurement of scatter. The DC detector may be used to pre-measure patient  40  and adjust a CT system mA within a slice and/or from slice to slice in the area of the scintillator detector. Furthermore, the energy discrimination (ED) information obtained may be used to implement smart CT correction schemes, such as beam hardening and/or smart detector correction schemes. Also, in accordance with the second embodiment multiple types of operational modes may be implemented. The x-ray beam kVP may be varied in the Z direction if an x-ray attenuation material was provided at the x-ray tube and/or at the radiation detection apparatus  6  so as to enhance the collection of multi energy x-ray information in the area of the DC detector. The information obtained via the DC detector may then be processed so as to separate different densities of materials within a patient slice plane in a simple operational mode. Additionally, some multiple multi-energy CT slices taken at the same time as standard CT slices may be obtained for a different anatomical area and later superpositioned so as to separate multiple types of materials.  
         [0025]     This embodiment allows the first x-ray beam  42  and the beam filtration to be tailored for desired system performance and also allows for the use of beams having different energies. It should be noted that each row detector can differentiate between multiple types of materials. As such, with multiple slices information can be obtained for larger amounts of different materials. The differential operation loads allow for detection of different materials using each row detector. They also allow multiple detector rows to be integrated for statistical quantification, analysis and a correction, such as for improving signal to noise ratio.  
         [0026]     The present invention also provides for a second radiation detector  18  comprised of a plurality of CT detector modules  100 . The CT detector module of the present invention provides a rigid structure which can placed in an array to form the second radiation detector  18 . This second radiation detector, in one embodiment, would be comprised of approximately 57 detector modules  100  in each row. Each module has 16 cells in the X direction and any number in the Z direction based on the number of simultaneous slices of information to be collected per CT system rotation. In another embodiment, it would be comprised of two rows of detectors butted up against one another approximately 57 detector modules  100 . This second radiation detector  18  can be placed adjacent to or partially or fully in front of the first radiation detector  16  such that the first radiation detector  16  and the second radiation detector  18  collect data simultaneously.  
         [0027]     As shown in  FIG. 4 , the CT detector module  100  of the present invention is comprised of a top graphite layer  190 , a bottom graphite layer  180 , a high voltage electrode  110 , a support  120 , a direct conversion material or scintillator/photodiode  150 , at least one ASICS chip  170 , a substrate  160  and a connector  140 .  
         [0028]     More specifically, the invention provides for the top and bottom graphite layers  180 ,  190  to serve as structural “sandwich” type layers. At least the top graphite layer  190  is x-ray translucent and readily admits x-rays to the direct conversion or scintillator/photodiode. Immediately beneath the top graphite layer  190  is a high voltage electrode  110 , which is only used in the case of the direct conversion detector.  
         [0029]     The high voltage electrode  110  is placed over the direct conversion material. In general, a charging voltage is applied between the substrate  160  and the top electrode  110  to create an electric field across the direct conversion material  150 . The detected x-ray radiation irradiates the area of direct conversion material  150 . The received x-rays create electron-hole pairs which, under the influence of the applied electric field, migrate across the direct conversion material  150 . This migration results in a measurable accumulation of charge. The amount of charge crossing the direct conversion material  150  varies proportionally with the radiation exposure. The stored charge is read-out and processed to create an image. One manner of depositing the high voltage electrode  110  on the graphite top cover  190  is to use a sputtering technique, which is known in the art. Another possibility is to employ a vapor deposition technique, also known in the art. Neither technique is intended to be a limitation of the present invention.  
         [0030]     One potential choice for the direct conversion material  150  is Cadmium Zinc Telluride, or CZT. However, many other materials could also be used. The direct conversion material may also be replaced with a scintillator/photodiode assembly. In either case, the direct conversion or scintillator/photodiode assembly is connected to either a silicon substrate, a flex circuit, or a multi-layer ceramic substrate  160  for signal run routing. Preferably, a silicon substrate  160  is used because it offers the highest run densities of the alternative substrate options.  
         [0031]     The signal runs are then routed to either one or more Application-Specific Integrated Circuits (ASICs)  170 . The ASICs  170  perform some initial signal processing. The signals are then routed again to the substrate  160  to an end block connector  140 . As before, the substrate  160  could consist of silicon substrate, a flex circuit or a multilayer ceramic substrate, although the silicon substrate is preferred.  
         [0032]     The connections between both the direct conversion material  150  and the substrate  160  and the ASICS  170  and the substrate  160  could be accomplished by several known methods. Some of these methods include bump bonding and conductive epoxy dots. However, other methods are well known in the art and could be used with the same results.  
         [0033]     The end block connector  140  generally exits through a support  120  at the first end of the detector module  101  and the support at the second end of the detector module  102  is the direct conversion material  150 . In one embodiment of the detector module, this support  120  is the only support for the detector module  100 . In this case, the detector modules  100  could be used back to back in the Z-direction, or butted up against each other. This embodiment could obviously provide faster imaging, although possibly at the expense of image resolution.  
         [0034]     In another possible embodiment of the detector module  200 , a second support  230  is used at the second end of the detector module  202 . In light of the above, this particular embodiment would not be able to provide a continuous imaging area were it butted to another detector module  200  because of the support  220 . This second embodiment of the detector module is shown in more detail in  FIG. 5  and also includes top and bottom graphite layers  280 ,  290 , a high voltage electrode  210  (For use with the direct conversion detector only), a layer of direct conversion material  250 , a substrate  260 , at least one ASICS chip  270 , a first end support  220  at the first end of the detector module  201  and an end block connector  240 . The second embodiment further includes a second end support  230  at the second end  202  of the detector module interposed between the top and bottom graphite covers  280 ,  290 .  
         [0035]     This compact modular structure of the present detector module  100  permits construction of a rigid array which can then be moved mechanically in and out of the x-ray beam of an existing CT imaging system  1  either fully or partially overlapping the existing scintillator/photodiode detector array in the Z-axis direction. The ability to move the detector array allows the use of the CT imaging system  1  in normal density-detection mode, in atomic differentiation mode and in both modes simultaneously. The present detector also permits the gathering of both density and atomic differentiation image information without increasing the x-ray fan beam width in the Z-axis direction, which could result in undesirable cone beam image artifacts.  
         [0036]     While the invention has been described with reference to an exemplary embodiment, it is to be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will included all embodiments falling within the scope of the appended claims.  
         [heading-0037]     Parts List:  
         [none]    
       
           1 . Computed Tomography imaging system  
           2 . Gantry  
           4 . X-ray source  
           6 . Radiation detection apparatus  
           8 . Patient support structure  
           10 . Patient cavity  
           12 . X-ray emitter  
           16 . First radiation detector  
           18 . Second radiation detector  
           20 . Control mechanism  
           22 . X-ray controller  
           24 . Gantry motor controller  
           26 . Data acquisition system (DAS)  
           28 . Image reconstruction device  
           30 . Data storage device  
           32 . Computer  
           34 . Input device  
           36 . Display  
           38 . Table controller  
           40 . Patient  
           42 . X-ray beam  
           100 . CT detector module  
           101  first end of  100   
           102  second end of  100   
           110 . High voltage electrode  
           120 . Support  
           140 . Connector  
           150 . Direct conversion material  
           160 . Substrate  
           170 . Application-Specific Integrated Circuits (ASICs)  
           180 . Bottom graphite layer  
           190 . Top graphite layer  
           200 . CT detector module  
           201 . First end of  200   
           202 . Second end of  200   
           210 . High voltage electrode  
           220 . First end support  
           230 . Second end support  
           240 . End block connector  
           250 . Direct conversion material  
           260 . Substrate  
           270 . ASICs  
           280 . Bottom graphite layer  
           290 . Top graphite layer