Abstract:
A CT detector module utilizes a simplified FET that effectively sums detector cells in the X direction, allowing for a doubling of scan slices in the Z direction with the same or a lesser number of DAS channels found in conventional CT detector modules.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a Continuation In Part of U.S. patent application Ser. No. 09/751,891 filed Dec. 29, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to radiation detectors of the scintillating type, and more particularly to a computer tomograph (CT) detector module having a reduced complexity interconnect and to methods for preparing and using the same.  
           [0003]    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.  
           [0004]    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 the x-ray source and detector. 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 of 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 in a cathode ray tube display.  
           [0005]    At least one known detector CT imaging system includes a plurality of detector modules, each having a scintillator array optically coupled to a semiconductor photodiode array that detects light output by the scintillator array. These known detector module assemblies require an increasing number of scintillator/diode rows in the Z direction, along with associated electronics to support a desire for increasing the number of CT slices of information gathered per CT rotation.  
           [0006]    Accordingly, it would be desirable to provide an improved CT detector module design which effectively sums detector cells in the X direction, while allowing for a doubling of scan slices in the Z direction, with the same or lesser number of DAS channels found in a conventional CT detector module.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    There is therefore provided, in one embodiment of the invention, an enhanced CT detector module utilizing a simplified FET that effectively sums detector cells in the X direction, allowing for a doubling of scan slices in the Z direction with the same or a lesser number of DAS channels found in conventional CT detector modules.  
           [0008]    This incorporates a simpler FET/decoder chip than would be required for a module with an increased number of detector rows in the Z direction or for a module with an option to electrically combine cells in the X direction while increasing detector rows sampled in the Z direction. Fewer FETs are provided although the same number as conventional modules may be used providing a simpler decoder design.  
           [0009]    Also, this invention permits summing detector cells in the X direction, which allows a doubling of scan slices in the Z direction with the same number of DAS channels but avoids using more FET switches, a more complex decoder and/or more FET decoder horizontal lines (X-direction) than products designed to accomplish this electronically would use. This invention also reduces overall FET /decoder size and cost, and improves reliability over modules designed to accomplish this electronically.  
           [0010]    In addition, this and other embodiments of the invention provide for including a simplified concept wherein some cells float (i.e. they are left open) and their collected charge will re-distribute itself among the neighboring cells. This embodiment allows cell summing in the X direction with a much simpler interconnect scheme (i.e., far fewer FET switches and a simplified decoder). In one embodiment, there is no increase in the number of FET switches/detector pixel to accomplish a doubling of the scan slices. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a pictorial view of a CT imaging system.  
         [0012]    [0012]FIG. 2 is a block schematic of the system illustrated in FIG. 1.  
         [0013]    [0013]FIG. 3 is a perspective view of one embodiment of a CT system detector array of the present invention.  
         [0014]    [0014]FIG. 4 is a perspective view of one of the detector module assemblies of the detector array shown in FIG. 3.  
         [0015]    [0015]FIG. 5 is a top view of an 8×16 cell array in accordance with one embodiment of the invention.  
         [0016]    [0016]FIG. 6 is a top view of an 8×16 cell array in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring to FIG. 1 and FIG. 2, a computed tomography (CT) imaging system  10  is shown as including a gantry  12  representative of a third generation CT scanner. Gantry  12  has an x-ray source that  14  that projects a beam of x-rays  16  toward a detector array  18  on 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. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . Detector array  18  may be fabricated in a single slice or multi-slice configuration. In a multi-slice configuration, detector array  18  has a plurality of rows of detector elements  20 , only one of which is shown in FIG. 2.  
         [0018]    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 a 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 storage device  38 .  
         [0019]    Computer  36  also receives commands and scanning parameters from an operator via console  40  that has an input device such as a keyboard and a mouse. 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 .  
         [0020]    As shown in FIGS. 3 and 4, detector array  18  includes a plurality of detector module assemblies  50  (also referred to as detector modules), each module comprising an array of detector elements  20 . Each detector module  50  includes a high-density photosensor array  52  and a multidimensional scintillator array  54  positioned above and adjacent to photosensor array  52 . Particularly, scintillator array  54  includes a plurality of scintillator elements  56 , while photosensor array  52  includes photodiodes  58 , a switch apparatus  60 , and a decoder  62 . A material such as a titanium dioxide—filled epoxy fills the small spaces between scintillator elements. Photodiodes  58  are individual photodiodes. In another embodiment, photodiodes  58  are deposited or formed on a substrate. Scintillator array  54 , as known in the art, is positioned over or adjacent photodiodes  58 . Photodiodes  58  are optically coupled to scintillator array  54  and have electrical output lines for transmitting signals representative of the light output by scintillator array  54 . Each photodiode  58  produces a separate low level analog output signal that is a measurement of beam attenuation for a specific scintillator of scintillator array  54 . Photodiode output lines (not shown in FIG. 3 or  4 ) may, for example, be physically located on one side of module  20  or on a plurality of sides of module  20 . In the embodiment illustrated in FIG. 4, photodiode outputs are located at opposing sides of the photodiode array.  
         [0021]    In one embodiment, and as shown in FIG. 3, detector array  18  includes fifty-seven detector modules  50 . Each detector module  50  includes a photosensor array  52  and scintillator array  54 , each having a detector element  20  array size of 16×16. As a result, array  18  is segmented into 16 rows and 912 columns (16×57 modules) allowing up to N=16 simultaneous slices of data to be collected along a z-axis with each rotation of gantry  12 , where the z-axis is an axis of rotation of the gantry.  
         [0022]    Switch apparatus  60  is a multidimensional semiconductor switch array. Switch apparatus  60  is coupled between photosensor array  52  and DAS  32 . Switch apparatus  60 , in one embodiment, includes two semiconductor switch arrays  64  and  66 . Switch arrays  64  and  66  each include a plurality of field effect transistors (FETS) (not shown) arranged as a multidimensional array. Each FET includes an input electrically connected to one of the respective photodiode output lines, an output, and a control (not shown) arranged as a multidimensional array.  
         [0023]    Each FET includes an input electrically connected to one of the respective photodiode output lines, an output, and a control (not shown). FET outputs and controls are connected to lines that are electrically connected to DAS  32  via a flexible electrical cable  68 . Particularly about one-half of the photodiode output lines are electrically connected to each FET input line of switch  64  with the other one-half of photodiode output lines electrically connected to FET input lines of switch  66 . Flexible electrical cable  68  is thus electrically coupled to photosensor array  52  and is attached, for example, by wire bonding.  
         [0024]    Decoder  62  controls the operation of switch apparatus  60  to enable, disable, and/or combine photodiode  58  outputs depending upon a desired number of slices and slice resolution for each slide. Decoder  62  in one embodiment, is a FET controller as known in the art. Decoder  62  includes a plurality of output and control lines coupled to switch apparatus  60  and DAS  32 . Particularly, the decoder outputs are electrically coupled to the switch apparatus control lines to enable switch apparatus  60  to transmit the proper data from the switch apparatus inputs to the switch apparatus outputs.  
         [0025]    Utilizing decoder  62 , specific FET&#39;s within switch apparatus  60  are selectively enabled, disabled, or combined so that specific photodiode  58  outputs are electrically connected to CT system DAS  32 . Decoder  62  enables switch apparatus  60  so that a selected number of rows of photosensor array  52  are connected to DAS  32 , resulting in a selected number of slices of data being electrically connected to DAS  32  for processing.  
         [0026]    As shown in FIG. 3, detector modules  50  are formed in a detector array  18  and secured in place by rails  70  and  72 . FIG. 3 shows rail  72  secured in place, while rail  70  is positioned to be secured over electrical cable  68 , over module substrate  74 , and mounting bracket  76 . Screws (not shown in FIG. 3 or  4 ) are then threaded through holes  78  and  80  and into threaded holes  82  of rail  70  to secure modules  50  in place. Flanges  84  of mounting brackets  76  are held in place by compression against rails  70  and  72  (or by bonding, in one embodiment) and prevent detector modules  50  from “rocking”. Mounting brackets  76  also clamp flexible cable  68  against substrate  74 . In one embodiment, flexible cable  68  is also adhesively bonded to substrate  74 . If desired, photosensor array can be adhesively bonded to the substrate. Flexible cable  68  is also electrically and mechanically bonded to photosensor array  52 , for example, by wire bonding.  
         [0027]    [0027]FIGS. 5 and 6 illustrate alternative embodiments of a photodiode wherein some cells float (i.e. they are left open) and their collected charge automatically re-distributes itself among the adjacent connected cells (only one half of a 16×16 diode array is shown in each of FIG. 5 and FIG. 6). Cells labeled “x” are electrically connected, and cells labeled “o” are those cells that float or are open. This allows cell summing in the X direction with a simpler interconnect scheme than is currently known (i.e. fewer FET switches and a simplified decoder compared to known system improvement designs). In one embodiment there is no increase in the number of FET switches/detector pixel. There is also the capability to increase spatial resolution with the staggered cell design through using interpolation schemes between rows or slices.  
         [0028]    In an alternative embodiment, the silicon in the open cell regions allows for a tailoring of the point response or charge collection response. This tailoring is accomplished by modifying the silicon diode sensitivity profile within each diode sensing cell. Such tailoring optionally is selected by a radiologist (or other purchaser) prior to purchase of the CT system to tailor the scan/data collection/sensitivity parameters to the purchaser&#39;s desires.  
         [0029]    In one embodiment, this concept is used with a finer cell pitch in the X direction than current cell pitches. Also a slice to slice interpolation is performed in one embodiment. In an exemplary embodiment, the slice to slice interpolation is performed with a tailored charge response and/or a tailored open cell silicon design to provide a scheme that is used at all times. This scheme may provide more data slices with fewer DAS channels and/or higher resolution than current designs. This scheme may open the requirements for reflectors, scintillator cell sizes, and other module design parameters.  
         [0030]    Referring to FIG. 5, one half of a 16×16 diode array comprising connectable cells, is shown with a Z direction. An X direction is also shown. In this illustrated embodiment of the invention a selected number of cells are combined in the X direction (the number being at least one cell less than the full number of such connectable cells). DAS is thus of constant bandwidth whereby the number of rows that can be processed are doubled.  
         [0031]    Referring to FIG. 6, one half of a 16×16 diode array, showing connectable cells, is shown with a Z direction and an X direction. In this illustrated embodiment of the invention, an alternative selected number of cells are combined in the X direction (the number being at least one cell less than the full number of such connectable cells). In an embodiment of this invention, when cells on either side of a center cell are disconnected and only a center cell is connected, the charge on the side cells diffuses and is collected by the center cell. Instead of using FETs to connect cells together, this embodiment uses disconnected cells and allows any charge on a disconnected cell to redistribute itself to cells around the disconnected cells.  
         [0032]    Additionally, the design of the photodiode can be modified to change how the charge is distributed. For example, one can tailor cells to redistribute mostly in rows or in columns. Also, the cells in a column is tailor-able to redistribute the charge in all eight adjacent cells, depending upon diffusion in p+ region in cells left unconnected. Typically, current collection in most systems is very crisp so that current cells collect charge in a crisply defined manner, with minimum cross talk to neighboring cells. In this concept rather than minimize cross talk, advantage is taken of it.  
         [0033]    One embodiment herein which may be employed to take advantage of the cross talk is to modify the doping of a silicon chip. In this concept, the Si diode cell doping profile is changed whereby the diode structure can be changed. In another modification, a bias can be applied in open pixel to drive the charge.  
         [0034]    To preferentially distribute charge, the slopes of P+ doping profile of the diode cell are made asymmetric, the side with the most p+ area, will be the side to which the charge will preferentially migrate in such an embodiment. If there is no change in the diode structure, (i.e. symmetric diode) that results in a symmetric charge redistribution, which is also an acceptable embodiment so it is the easiest way to accomplished results of this invention.  
         [0035]    If the p+ regions are moved (i.e. change their locations), a preferential redistribution occurs to cells that are closer to the disconnected regions. The concentration of dopant during the doping of a silicon chip can be changed to give higher concentration in one direction than in another direction, and this moves the charge preferentially. PIN type structure may be employed but embodiments of the invention can use other configurations (e.g. PN structures). The invention is utilized to disconnect some cells and collect the charges from adjacent cells to obtain combinations in x and z directions.  
         [0036]    A further method of enhancing the summation of X cells in an embodiment of this invention involves the application of a bias on an open pixel. One may apply a bias voltage to drive charge to adjacent pixels (e.g., to a connected channel corresponding to a middle pixel or to a channel which is connected to a DAS channel). Accordingly, biasing in the either direction can be performed.  
         [0037]    In one embodiment, the positive bias can be 0 to 10 volts for a positive bias, e.g. 2 volts, just enough to encourage the migration of charge. This will help to avoid conduction in these regions. This allows flexibility in the number of slices or X resolution with a fixed number of DAS channels.  
         [0038]    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.