Abstract:
One aspect of the present invention is a detector array for a computed tomographic imaging system having a z-direction corresponding to an image slice thickness direction and that is arc-shaped in a direction transverse to the z-direction. The detector array has a plurality of detector modules configured so that the detector array has active regions of differing thicknesses.  
     This detector array embodiment provides an optimized detector array for certain imaging situations, for example, in cardiac imaging applications in which increased coverage is required only in a relatively small central portion of a field of view.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to detectors for computed tomography (CT) imaging systems, and more particularly to optimizations of such detectors for medical and other applications and to imaging systems using such optimized detectors.  
           [0002]    In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.  
           [0003]    In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane 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, i.e., projection data, from the detector array 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, or view angles, during one revolution of the x-ray source and detector.  
           [0004]    In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. In another mode of operation of the CT imaging system, a helical scan is used to obtain projection data for images.  
           [0005]    More particularly, and referring to FIGS. 1 and 2, one known computed tomograph (CT) imaging system embodiment  10  includes a gantry  12  representative of a “third generation” CT scanner. 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 gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through an object  22 , for example a medical patient. In at least one embodiment of the present invention, detector array  18  is fabricated in a multi-slice configuration. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam. As the x-ray beam passes through a patient  22 , the bean is attenuated. During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 .  
           [0006]    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 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 detector elements  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 image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 .  
           [0007]    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  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 .  
           [0008]    Multiple slice detector arrays  18  increase the rate at which a scan of a given volume can be performed by acquiring data for several parallel image slices at the same time. For example, and referring to FIGS. 3 and 4, one known prior art detector array  18  includes a plurality of detector modules  50 . Each detector module includes an array of detector elements  20 . Particularly, each x-ray detector module  50  includes a plurality of scintillators  52  positioned above and adjacent corresponding photodiodes  54 , a semiconductor device  56 , and at least one flexible electrical cable  58 . Photodiodes  54  are either individual photodiodes or a multi-dimensional photodiode array. Photodiodes  54  are optically coupled to scintillators  52  and generate electrical outputs on lines  60 , wherein the outputs are representative of light output by corresponding scintillators  52 . Each photodiode  54  produces a separate electrical output  60  that is a measurement of the beam attenuation for a specific element  20 . Photodiode output lines  60  are, for example, physically located on one side of module  50  or on a plurality of sides of module  50 . As shown in FIG. 4, photodiode outputs  60  are located at top and bottom of the photodiode array.  
           [0009]    Semiconductor device  56  includes two semiconductor switches  62  and  64 . Switches  62  and  64  each include a plurality of field effect transistors (FET) (not shown) arranged as a multidimensional array. Each FET includes an input line electrically connected to a photodiode output  60 , an output line, and a control line (not shown). FET output and control lines are electrically connected to flexible cable  58 . Particularly, one-half of photodiode output lines  60  are electrically connected to each FET input line of switch  62  with the remaining one-half of photodiode output lines  60  electrically connected to the FET input lines of switch  64 .  
           [0010]    Flexible electrical cable  58  includes a plurality of electrical wires  66  connecting its ends. FET output and control lines are electrically connected to cable  58 . Particularly, each FET output and control line is wire bonded to a wire  66  of one end of cable  58 . FET output and control lines are wire bonded to wires  66  in the same manner as photodiode outputs (not shown) are wire bonded to the FET input lines (also not shown). Cables  58  are secured to detector module  50  using mounting brackets  68  and  70 .  
           [0011]    Referring to FIG. 5, after mounting detector modules  50  into detector array  18 , unconnected cable  58  ends are coupled to DAS 32 so that an electrical path exists between photodiode  52  outputs and DAS 32, and so that FET control lines  72  are electrically connected to DAS 32 to enable semiconductor device FETs  74 . In a four-slice CT imaging system  10  using the prior art detector array  18  embodiment of FIGS. 3, 4, and  5 , each column of detector module  50  is electrically connected to four DAS 32 channels, i.e., two channels within each flexible electrical cable  58 . (In general, an N channel system would have N channels connected to each column of detector module  50 , with N/2 channels within each flexible electrical cable  58 .) One exemplary channel is represented, in part, in FIG. 5. DAS 32 is coupled across a rotating gantry  12  slip ring  76  to computer  36  and image reconstructor or processor  34 . Each detector element  20  includes a photodiode  54  that is coupled to a plurality of FETs  74 , only one of which is shown. In a four-slice CT imaging system, each channel is coupled to the output of one-fifth of FETs  74 . (Of the FETs not shown in FIG. 5, one set connects unused diode elements to ground during a scan.) Computer  36  provides a control signal instructing a controller  78  to turn on one or more FETs  74  per channel per data interval during an imaging scan, resulting in an analog signal from a corresponding one or more photodiodes  54  being applied to a preamp  82 . The output signal from preamp  82  is converted to a digital signal by analog to digital converter  84  and sent across slip ring  76  to image reconstructor  34 .  
           [0012]    For reconstruction of medical images without motion artifacts, it is desirable to rotate gantry  12  as rapidly as possible to obtain a set of views for image reconstruction. It is correspondingly desirable to sample the outputs of photodiodes  54  as rapidly as possible to obtain images with as high a resolution as possible. However, the highest sampling rate is limited by the bandwidth of data communication across slip ring  76 , among other things. In some applications, it is desirable to image as large an extent in the z-direction as possible in as little time as possible. For these applications, it has been necessary to effectively combine outputs of detector elements  20  in adjacent rows of detector array  18  transverse to the z-direction by turning on more than one FET  74  at a time. This combination allows a greater extent of a patient to be imaged in the z-direction in a shorter time, but the reconstructed images correspond to thicker slices of the imaging volume in the z-direction (i.e., lower z-axis resolution).  
           [0013]    Detector elements  20  are only  1 . 25  mm in extent in the z-direction in one known detector array  18 . Moreover, even though one known detector array  18  provides  16  rows of detector elements  20 , one known imaging system  10  using such a detector array only provides sufficient DAS 32 electronics to process four image slices at a time. Therefore, cardiac imaging applications require either that a helical scan be performed or that multiple axial scans be performed, with table  46  being stepped between the axial scans. Providing more rows of detector elements  20  in detector modules  50  of detector array  18  would reduce the time needed to acquire data for a complete image of a patient&#39;s heart, but this advantage could be gained only at the expense of a much greater number of DAS  32  channels.  
           [0014]    It would therefore be desirable to provide a multislice detector array optimized for one or more imaging applications, including medical imaging applications. It would also be desirable to provide an imaging system using such a detector array that had a reduced need for additional DAS channels and additional bandwidth.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    There is therefore provided, in one embodiment of the present invention, a detector array for a computed tomographic imaging system having a z-direction corresponding to an image slice thickness direction and that is arc-shaped in a direction transverse to the z-direction. The detector array has a plurality of detector modules configured so that the detector array has active regions of differing thicknesses.  
           [0016]    This detector array embodiment provides an optimized detector array for certain imaging situations, for example, in cardiac imaging applications in which increased coverage is required only in a relatively small central portion of a field of view. Such detector array embodiments also reduce the number of detector acquisition system (DAS) channels and the corresponding bandwidth needed to process information from the detector array, because detector elements and their associated electronics are not provided where they are not needed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a pictorial view of a prior art CT imaging system.  
         [0018]    [0018]FIG. 2 is a block schematic diagram of the prior art system illustrated in FIG. 1.  
         [0019]    [0019]FIG. 3 is a perspective drawing of a prior art multislice detector array.  
         [0020]    [0020]FIG. 4 is a perspective drawing of a prior art detector module of the detector array shown in FIG. 3.  
         [0021]    [0021]FIG. 5 is a simplified schematic diagram illustrating the concept of a DAS “channel.” 
         [0022]    [0022]FIG. 6 is a perspective drawing of one embodiment of a multislice detector array of the present invention.  
         [0023]    [0023]FIG. 7 is a perspective drawing of one representative type of detector module of the present invention useful for detector arrays of the type shown in FIG. 6.  
         [0024]    [0024]FIG. 8 is a simplified schematic representation of the “active” area of another embodiment of a multislice detector array of the present invention. The “active” area is that area covered by detector elements and facing the radiation source.  
         [0025]    Detector elements are not shown. (The schematic representation of FIG. 8 is a projection of the active area onto a two-dimensional surface. The actual detector embodiment represented has a curvature similar to that shown in FIG. 6.)  
         [0026]    [0026]FIG. 9 is simplified schematic representation of another multislice detector array of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    In one embodiment of the present invention and referring to FIGS. 6 and 7, a detector array  86  is provided in place of detector array  18  of FIGS.  1 - 4  in a CT imaging system  10 . Detector array  86  provides a first quantity of full field of view (FOV) slices for general body coverage, and a smaller FOV with a second, greater number of slices for more specialized scanning. Examples of specialized scanning include, but are not limited to, cardiac or other organ scanning, and head, neck, and limb scanning.  
         [0028]    Detector array  86  comprises a plurality of different types of detector modules. Wings  88  and  90  utilize a first type of detector module, for example, the prior art detector module  50  of FIG. 4. A central region  92  utilizes a different type of detector module  94  of the present invention. For example, while detector modules  50  and  94  each comprise a rectangular array of detector elements  20 , detector modules  94  provide a greater number of detector elements  20  in the z-direction (i.e., more detector rows) than do detector modules  50 . To accommodate detector modules  94 , rails  96  and  98  (or at least one of them) are shaped to provide a thick (i.e., large in the z-direction) “window” in the center of detector array  18 . In one embodiment, to accommodate the additional detector elements  20  of detector module  86 , multiple metal layers are used for the larger array of photodiodes  54 , for semiconductor device  56  and/or for semiconductor switches  62  and  64 . Also in one embodiment, flexible electrical cables  58  of detector modules  86  are multi-layer electrical cables.  
         [0029]    Both the relative and absolute sizes of detector modules in wings  88  and  90  and those in central region  92  can be selected to provide embodiments providing specialized coverage for imaging. More particularly, embodiments of the present invention provide high resolution, large z-extent coverage in a selected portion or portions of detector array  86  where it is most useful for a particular application, for example, in central portion  92 . Because a lesser coverage is provided elsewhere by detector array  86 , extra DAS channels do not have to be provided for other regions of detector array  86  (e.g., wings  88  and  90 .) To reduce the number of DAS 32 channels required, outputs of detector elements  20  are configurable for selective combination using FETs  74  (see FIG. 5). In one embodiment, the number of DAS 32 channels required is further reduced by utilizing different detector element  20  sizes to reduce resolution in some areas of detector array  86 , for example, or by hard-wiring outputs of multiple detector elements  20  together.  
         [0030]    In another embodiment of a detector array  100  of the present invention and referring to FIG. 8, the active area of detector  100  has a total dimension in the z-direction of 12 cm (dimension A). This thickness represents ninety-six parallel rows of detector elements  20  (not shown in FIG. 8) in a central region  102  that provides 16 cm of coverage (dimension B). Thus, detector modules (not shown in FIG. 8) in central region  102  have ninety-six detector elements in the z-direction. In one embodiment, detector modules in region  102  each have sixteen detector elements in a direction transverse to the z-direction, and fourteen modules are arranged adjacent one another in the direction transverse to the z-direction. These modules form central region  102  having the desired dimensions.  
         [0031]    Wings  104  and  106  of detector array  100  allow detector array  100  to provide a FOV of 48 cm (dimension C). In the embodiment represented in FIG. 8, wings  104  and  106  comprise detector modules (not shown) having thirty two detector rows, and thus having a 4 cm extent in the z-direction (Dimension D). In one embodiment, each of these detector modules also has sixteen detector elements in a direction transverse to the z-direction and each wing  104 ,  106  comprises twenty-two modules adjoined in the direction transverse to the z-direction. These modules form wings  104 ,  106  having the desired dimensions.  
         [0032]    In other embodiments, and referring to FIG. 9, a detector array  108  of the present invention comprises more than two sizes of detector modules, thus providing three (or more) regions  110 ,  112 ,  114  of different thicknesses in the z-direction. These additional embodiments provide FOVs optimized for other specialized types of scans. In some detector array embodiments, the thickest portion of the detector array is not necessarily in the center of the array, nor is the detector array itself necessarily symmetrical.  
         [0033]    In summary, detector array embodiments of the present invention provide detector arrays having regions of unequal thicknesses in the z-direction. The dimension, locations, and numbers of regions are different in different embodiments, depending upon the type or types of scans for which the detector array and imaging system is specialized. However, in each case, the largest width FOV is not provided across the entire z-axis thickness of the detector array. Because a full FOV is not provided across the entire thickness of the detector array, it is also not necessary to provide DAS 32 channel circuitry to receive data for a full FOV of the detector array from each slice. Thus, both DAS 32 and detector array resources are optimized. Detector array embodiments of the present invention can be utilized in place of detector arrays  18  in conventional CT imaging systems such as imaging system  10  of FIGS. 1 and 2.  
         [0034]    More particularly, sampling of analog outputs of detector elements  20  by DAS 32 proceeds at a frequency that is governed by speed and resolution requirements of imaging system  10 . Outputs of detector elements  20  can be sampled separately. If lower resolution is acceptable, detector  20  outputs can be combined in pairs, for example, or in other combinations. In addition, sets of detector  20  outputs (or sets of combined detector  20  outputs) can be combined or multiplexed so that they share a single preamplifier  82  and analog-to-digital converter  84  of DAS 32.  
         [0035]    In at least one embodiment of a detector array of the present invention, detector modules are tiled in two dimensions, one of which is the z-direction. In one embodiment, all of the tiled detector modules are the same size and have the same number of detector elements  20 . Thicker regions of the detector array have more tiled detector modules in the z-direction than thinner regions.  
         [0036]    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.