Abstract:
A CT detector cell is constructed to have diagonally oriented perimeter walls. With such a construction, the resulting CT detector comprised of such detector cells has improved spatial coverage (spatial density). The number of detector channels is also not increased despite the increase in spatial coverage. Moreover, the detector cells can be constructed without much variance from conventional fabrication techniques.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to CT detector design and, more particularly, to a CT detector with non-rectangular detector cells. 
     In conventional multi-row CT detectors, a two dimensional array of detector cells extend in both the x and z directions. Moreover, in conventional detectors, each cell of the array is constructed to have a rectangular-shaped active area. This active area is generally perpendicular to a plane of x-ray source rotation and, in the context of energy integrating scintillators, converts x-rays to light. The light emitted by each scintillator is sensed by a respective photodiode and converted to an electrical signal. The amplitude of the electrical signal is generally representative of the energy (number of x-rays x energy level of x-rays) detected by the photodiode. The outputs of the photodiodes are then processed by a data acquisition system for image processing. 
     As described above, each of the detector cells of the 2D array has a generally rectangular or square face, and is contiguous in both the x and z directions. As such, there is no overlapping in either of the x or z directions. This lack of overlapping places an upper limit on the spatial frequency of the region-of-interest, i.e., anatomy of interest, which can be resolved artifact free. A number of approaches have been developed to overcome the upper sampling limitations of conventional 2D detector arrays. 
     In one proposed solution, miniaturization efforts have led to a reduction in the size of the individual detector cells or pixels. Because the output of each detector cell corresponds to a pixel in a reconstructed image; conventionally, detector cells are also referred to as pixels. Segmenting the detector active area into smaller cells increases the Nyquist frequency but with the added expense of data channels and system bandwidth. Moreover, system DQE is degraded due to reduced quantum efficiency and increased electronic noise which results in a degradation in image quality. 
     In another proposed technique, focal spot deflection by deflecting the x-ray focal spot in the x and/or z direction at 2× or 4× the normal sampling rate has been found to provide additional sets of views. The different sets of views are acquired from slightly different perspectives which results in unique samples that provide overlapping views of the region-of-interest without subpixellation. A drawback of this approach is that a data acquisition system channel capable of very high sampling rates is required. Moreover, such a technique requires an x-ray source and associated hardware dedicated to rapid beam deflection. Ultimately, it has been found that focal spot deflection yields images with increased noise and reduced dose efficiency. 
     Another proposed approach to increasing sampling density of a CT detector involves the staggering of pixels. Specifically, it is has been proposed that sampling density may be improved by offsetting, in the z direction, every other channel or column of detector cells in the x direction. In one proposed approached, the offset is equal to one-half of a detector width. This proposed CT detector design as well as a more conventional CT detector design are illustrated in  FIGS. 1-2 . 
     As shown in  FIG. 1 , a conventional CT detector  2  is defined by a 2D array of detector cells  3  that are rectangular in their active area shape. As shown and described above, the array extends in both the x and z directions. In the CT detector design illustrated in  FIG. 2 , every other channel  4  (column) of detector cells  3  is offset. This provides an intermediate sample location between rows  5  increasing the number cells, decreasing cell size, or increasing the data acquisition system sampling rate. However, such a staggered design is difficult to fabricate since all the rows are not aligned. 
     Therefore, it would be desirable to design a CT detector that provides increased sampling density that is practical to fabricate yet does not over-burden the data acquisition system or necessitate an impractical number of data acquisition channels. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention is directed to a CT detector constructed to overcome the aforementioned drawbacks. The CT detector is comprised of detector cells having diagonally oriented perimeter walls. With such a construction, the CT detector has improved spatial coverage (sampling density). The number of detector channels is also not increased despite the increase in spatial coverage. Moreover, the detector cells can be constructed with a conventional cutting technique. 
     Therefore, in accordance with one aspect, the invention includes a detector cell having a generally planar active surface and a set of perimeter walls defining the generally planar active surface. The cell is constructed such that an angle of intersection formed between a pair of perimeter walls is acute. 
     According to another aspect of the invention, a detector for radiographic imaging is disclosed. The assembly comprises a detector array having a plurality of detector cells and is arranged along an x direction and a z direction perpendicular to the x direction. At least one detector cell has one edge in an x-z plane. 
     In accordance with another aspect, the invention is embodied in a CT system. The CT system includes a gantry that rotates about a plane of rotation, and an x-ray source disposed in the gantry and designed to project an x-ray beam. The system further has an x-ray detector situated parallel to the plane of gantry rotation and disposed in the gantry. The x-ray detector is configured to convert radiation projected by the x-ray source and attenuated by a subject to be imaged into a form that may be processed to reconstruct an image of the subject. The x-ray detector includes an array of detector cells, wherein each detector cell has a rhombus-shaped active area. 
     Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a plan view of a conventional rectangular CT detector matrix comprised of square-shaped detector cells. 
         FIG. 2  is a plan view of CT detector matrix with staggered detector channels. 
         FIG. 3  is a pictorial view of a CT imaging system. 
         FIG. 4  is a block schematic diagram of the system illustrated in  FIG. 1 . 
         FIG. 5  is a plan view of a CT detector matrix with detector cells having diagonal edges according to one aspect of the invention. 
         FIG. 6  is a plan view of a single exemplary detector cell in accordance with one aspect of the present invention. 
         FIG. 7  is a graph illustrating a z-axis comparison of a conventional CT detector matrix and the CT detector matrix of  FIG. 5 . 
         FIG. 8  is a plan view of a CT detector matrix with diamond-shaped detector cells according to another aspect of the invention. 
         FIG. 9  is a graph illustrating a comparison in z-axis profile between a conventional rectangular-shaped detector cell and a diamond-shaped detector cell. 
         FIG. 10  is a pictorial view of a CT system for use with a non-invasive package inspection system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1 and 2 , an exemplary computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. One skilled in the art will appreciate that the present invention is applicable with other configured CT scanners, such as those generally referred to as first generation, second generation, fourth generation, fifth generation, sixth generation, etc. scanners. Further, the present invention will be described to a CT detector cell geometry that is applicable with energy integrating cells as well as photon counting and/or energy discriminating cells. 
     Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of the gantry  12 . Detector array  18  is formed by a plurality of detectors  20  which together sense the projected x-rays that pass through a medical patient  22 . Each detector  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center or plane of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to an x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detectors  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  and gantry  12 . Particularly, table  46  moves portions of patient  22  through a gantry opening  48 . 
     As alluded to above, the present invention is directed to a CT detector comprised of individual detector cells or pixels. These cells are defined by an active surface or area and convert x-rays into a form that may be processed for image reconstruction. In this regard, the cells may, through a scintillator-photodiode combination, convert x-rays to light, detect the light, and provide an electrical signal to a data acquisition system for image reconstruction. The present invention, however, is not limited to scintillator-photodiode constructions. That is, as will be illustrated below, the present invention is also applicable with direct conversion detector cells that directly convert x-rays to electrical signal. 
     Additionally, the invention is applicable with conventional energy integrating cells as well as photon counting/energy discriminating cells. In a conventional integrating cell, the output of the scintillator or other x-ray conversion component is the product of the energy of the x-rays received and the number of x-rays received. Thus, there is no separation of the number of x-rays received from the energy level of the individual x-rays. Thus, it is possible, with energy integrating detector cells, for one cell to provide an output equal to that of another cell despite the one cell receiving more x-rays than the another cell. This equality in outputs is a result of the energy level of the x-rays received by the “another” cell being greater than the x-rays received by the “one” cell. 
     To provide photon count and/or energy discriminating information, CT detectors are increasingly being formed of energy discriminating and/or photon counting cells. These ED/PC detectors are capable of providing photon count and energy level information. Despite the differences between conventional energy integrating detectors and ED/PC detectors, there remains a need to improve spatial coverage/sampling density in both cases. Therefore, the present invention is applicable with both general types of detectors and, in fact, is not limited to a particular type of detector. Additionally, this invention is not limited to detectors for CT systems. 
     To achieve a CT detector with improved spatial coverage, detector cells with diagonal edges or perimeter walls is proposed. An exemplary construction is illustrated in  FIG. 5 . As shown, a CT detector  20  is defined by an array or matrix  50  of detector cells  52 . As shown, each detector cell  52  has a non-rectangular shape. This non-rectangularity increases the spatial coverage of the detector in the z direction. Despite the non-rectangularity in the geometry of each detector cell, as illustrated, the detector cells in each column (channel) are uniformly aligned with one another. This eases the fabrication process relative to the staggered-channel approach illustrated in  FIG. 2 . 
     As shown in  FIG. 5 , most of the detector cells are similarly shaped. However, because of the non-rectangularity of the detector cells, irregular shaped sections of the matrix must be accounted for. This is achieved by specially-shaped cells  53  that are constructed to “fill” the matrix. A skilled artisan will appreciate that each “specially-shaped” cell  53  may include multiple cells to fill the matrix. 
     Referring now to  FIG. 6 , a single exemplary detector cell  52  according to one aspect of the invention is shown. The detector cell  52  has an active area  54  that is generally parallel to the plane of x-ray projection (not shown) during data acquisition. In the exemplary illustration, the active area  54  is defined by four perimeter walls or edges  56 . As shown, the exemplary cell has the shape of a rhombus. In this regard, the angle, α 1 , formed by the intersection of edges  56 ( a ) and  56 ( b ) is acute. Likewise, the angle, α 2 , between edges  56 ( c ) and  56 ( d ), is acute. Conversely, the angle, β 1 , at the intersection of edges  56 ( a ) and  56 ( c ) and the angle, β 2 , formed at the intersection of  56 ( b ) and  56 ( d ) are each obtuse. In short, edges  56 ( b ) and  56 ( c ) are not perpendicular to the plane of gantry rotation as in conventional rectangular shaped cells; however, channel edges  56 ( a ) and  56 ( d ) are perpendicular to the plane of gantry rotation. In this regard, the diagonal edges  56 ( b ) and  56 ( c ) extend in the x-z plane whereas edges  56 ( a ) and  56 ( d ) extend only in the z direction. 
     The geometry of the detector cell can be more generally described as follows. As shown, the z boundaries of the detector cell are formed by straight diagonal edges. Thus, with the cell pitch in the z direction referenced “a” and the cell pitch in the x direction referenced “b”, the diagonal boundary makes an angle α with the x axis such that:
 
tan (α)= a /(2 b )   (Eqn. 1)
 
     For a=b, alpha is approximately 26.5 degrees. However, one skilled in the art will appreciate that the present invention is not limited to the case where a=b. For example, in one preferred embodiment, b=a√{square root over (3)}/2. In this case, which was found to be particularly optimal for sampling density, alpha is 30 degrees. With an alpha of 30 degrees, a hexagonal lattice detector matrix or array would result. Other values for alpha are of course contemplated. 
     As a result of edges  56 ( b ) and  56 ( c ) being in the x-z plane, the sampling density of the overall detector is improved, as illustrated in  FIG. 7 . Specifically, as illustrated, the z axis profile of a conventional rectangular detector cell is enveloped by the collective profiles of the diagonally edged cells illustrated in  FIGS. 5-6 . 
     Not only does the present invention provide a detector cell geometry with improved spatial coverage, it does so without requiring significant variants to conventional detector fabrication techniques. Specifically, the detector cell illustrated in  FIG. 6  can be fabricated using two cuts in a cutting process. That is, after making a straight cut, i.e., edges  56 ( a ) and  56 ( d ), the wafer or bulk of x-ray converting material need only be rotated acutely a fixed degree of rotation followed by a second cut. Thus, instead of making four ninety degree cuts, a detector according to one embodiment of the present invention can be formed with two ninety degree cuts and two acute (less than ninety degree) diagonal cuts. This can be done without requiring a significant change to a typical cutting setup. 
     Referring now to  FIG. 8 , a CT detector  20  having an array  50  of detector cells  52  shaped according to another embodiment of the present invention is shown. In this embodiment, each of the detector cells  52  is diamond-shaped. Thus, four diagonal edges rather than two, as in the cell shown in  FIG. 6 , define each cell. One advantage of the cell geometry illustrated in  FIG. 8  is that there is substantial sample overlap in the x and z directions. Moreover, the z axis profile is narrower than that of conventional rectangular detector cells. One skilled in the art will appreciate that fabrication of the diamond-shaped detector cell can be carried out with a conventional wire-saw process. 
     Referring to  FIG. 9 , the axial profile of a diamond-shaped cell relative to a rectangular-shaped cell is illustrated. As shown, notwithstanding the more narrow profile, the sampling coverage of the diamond-shaped cell is equal to that of a conventional rectangular-shaped cell. 
     The present invention may be incorporated in medical scanners, such as that shown in  FIGS. 3-4 , or non-medical scanners. Referring now to  FIG. 10 , package/baggage inspection system  100  incorporating the present invention includes a rotatable gantry  102  having an opening  104  therein through which packages or pieces of baggage may pass. The rotatable gantry  102  houses a high frequency electromagnetic energy source  106  as well as a detector assembly  108  having detector cells similar to those described herein. A conveyor system  110  is also provided and includes a conveyor belt  112  supported by structure  114  to automatically and continuously pass packages or baggage pieces  116  through opening  104  to be scanned. Objects  116  are fed through opening  104  by conveyor belt  112 , imaging data is then acquired, and the conveyor belt  112  removes the packages  116  from opening  104  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  116  for explosives, knives, guns, contraband, etc. 
     As noted above, the present invention is not limited to a particular type of detector cell. In this regard, it is contemplated that the invention can be applied to energy integrating, photon counting, or energy discriminating constructions. Thus, the invention is applicable with scintillators or direct conversion x-ray conversion material, charge collectors, such as photodiodes, charge-storage devices, charge collection anodes or cathodes, as well as, anti-scatter, collimator, and reflector grids. 
     As described herein and appreciable by one skilled in the art, the present invention provides a detector cell geometry that enables overlapping samples in the z and/or x directions without requiring additional data acquisition system channels. Moreover, the active area of each cell is equivalent to those of conventional detector cells. The detector cells can be fabricated with slight modification of a conventional wire-saw process; thus, fabrication costs are comparable to conventional detector cells. Moreover, since the diagonal and diamond-shaped cells described herein can be fabricated using wire-saw cuts of the same pitch, only a single wire-saw setup is required. Additionally, the detector cell is applicable with x direction flying-focal-spot deflection techniques, e.g., x-direction wobble, for improved sampling in the x direction. Further, for the embodiment illustrated in  FIG. 6 , the channel edges of each cell are aligned with the channel edges of each other detector cell in the channel. Thus, a conventional 1D scatter grid may be used. Also, one skilled in the art will appreciate that the present invention is applicable with CZT photon counting detectors. In such a case, the scintillator is not diced in a manner described above. The charge collection electrodes are formed with overlapping rows. 
     Therefore, the invention includes a detector cell having a generally planar active surface and a set of perimeter walls defining the generally planar active surface. The cell is constructed such that an angle of intersection formed between a pair of perimeter walls is acute. 
     A detector assembly is also disclosed. The assembly comprises a detector array having a plurality of detectors and is arranged along an x direction and a z direction perpendicular to the x direction. At least one detector of the plurality of detectors has one edge in an x-z plane. 
     The invention is also embodied in a CT system. The CT system includes a gantry that rotates about a plane of rotation, and an x-ray source disposed in the gantry and designed to project an x-ray beam. The system further has an x-ray detector situated parallel to the plane of gantry rotation and disposed in the gantry. The x-ray detector is configured to convert radiation projected by the x-ray source and attenuated by a subject to be imaged into a form that may be processed to reconstruct an image of the subject. The x-ray detector includes an array of detector cells, wherein each detector cell has a rhombus-shaped active area. 
     The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.