Patent Publication Number: US-2003223539-A1

Title: Method and apparatus for acquiring and storing multiple offset corrections for amorphous silicon flat panel detector

Description:
BACKGROUND OF INVENTION  
       [0001] Certain embodiments of the present invention generally relate to x-ray systems utilizing a solid state multiple element x-ray detector for producing an image; and more particularly, to techniques and apparatus for acquiring and storing offset image correction data for more than one mode of operation.  
       [0002] Solid state x-ray detectors comprising a two dimensional array of detector elements arranged in rows and columns are known in the art. A scintillator, such as cesium iodide (CsI), is deposited over the detector elements. The CsI absorbs x-rays and converts the x-rays to light. Each detector element comprises a photodiode and a field effect transistor (FET). The photodiode detects light, converts the light to a charge representative of an amount of radiation incident on the detector element and stores the charge. The FET operates as a switch to enable and disable read out of the charge stored on the photodiode. Each detector element is connected to both a row select line and a column signal line. The row select lines and column signal lines are used to activate the FET and read the level of stored charge in the photodiode. The detector may be designed with a split in each signal line at the midpoint, effectively splitting the reading of the detector into two separate operations. After an exposure, the detector is read on a row by row basis. With a detector that has split data lines, two rows may be read at the same time utilizing two sets of read out electronics. The data is then digitized for further image processing, storage, and display.  
       [0003] The signal of each detector element (or pixel) may include an offset which is independent of x-ray exposure. This offset has several sources including leakage current in the photodiodes and charge retention in the FET switches. At low signal levels, such as those used in fluoroscopic imaging, the magnitude of the offset may be larger than the x-ray signal. Furthermore, the offset is not uniform, but varies from pixel to pixel. This pixel-dependent offset is subtracted from the x-ray exposed image to produce a corrected image before viewing.  
       [0004] The offset may be isolated from the x-ray induced signal by acquiring a dark image, or an image when the detector is not exposed to x-rays. In order for the signals in the dark image to match the offset signals in the x-ray image, the dark image is acquired using the same mode of operation used to acquire the x-ray image. Because there is noise associated with the offset signals, a single dark image subtracted from an x-ray image may introduce additional noise into the corrected image. To reduce the amount of noise, several dark images may be averaged together to obtain a low-noise offset image. Additionally, the offset signals may drift with time, temperature, and other external factors. Therefore, the offset image must be updated periodically. The offset image for the mode of operation currently in use is typically updated shortly before or after an x-ray image is acquired when the x-ray signal is not present.  
       [0005] During fluoroscopy, it is often advantageous to switch between modes of operation. For example, in one mode of operation, the system may utilize only a portion of the detector, such as the center, if interested in anatomy that does not require the entire field of view. In another mode of operation, the entire field of view may be imaged with a lower resolution (larger pixel size). However, current x-ray systems store only one offset correction applicable for one mode of operation. Thus, every time the mode of operation is switched, the x-ray system must stop acquiring x-ray images in order to acquire the dark images used to create the new offset correction image. During this time, the x-ray system is no longer acquiring and displaying patient data, and thus the radiologist may need to halt the procedure until the x-ray system has completed acquiring the correction data and is ready to acquire patient data again.  
       [0006] Thus, a need exists in the industry for an x-ray system designed to switch between multiple modes of operation without interrupting the acquisition of patient data, to address the problems noted above and previously experienced.  
       SUMMARY OF INVENTION  
       [0007] In accordance with at least one embodiment, an x-ray system is provided to acquire successive images. The x-ray system includes an x-ray source to generate x-rays which are detected by a detector. The detector comprises detector elements which store levels of charge and are arranged in rows and columns. An image processor is used to sense levels of charge stored by the detector elements. First and second offset image memories are included in the image processor. The first offset image memory stores offset image data based on levels of charge for a first mode of operation and a second offset image memory stores offset image data based on levels of charge for a second mode of operation.  
       [0008] In accordance with at least one embodiment, a method for acquiring successive x-ray images using multiple modes of operation is provided. A first mode of operation comprising identifying detector elements of an x-ray detector is selected. The detector elements are used to create an image. A first offset image corresponding to the first mode of operation is selected from a plurality of stored offset images, where the plurality of stored offset images corresponds to a plurality of modes of operation. The x-ray detector is exposed to a radiation source and the detector elements store a level of charge representative of the level of radiation detected. A first image representative of the levels of charge stored by the detector elements is acquired. The first offset image is then utilized to process the first image.  
       [0009] In accordance with at least one embodiment, a method for acquiring and storing multiple offset images for an x-ray system is provided. A first mode of operation identifying detector elements included in an x-ray detector and used to create an image is defined. A first dark image representative of levels of charge stored by the detector elements is acquired when the x-ray detector is not exposed to radiation and stored in a first memory. A second mode of operation is defined which is different from the first mode of operation. A second dark image representative of the levels of charge stored by the detector elements is acquired when the x-ray detector is not exposed to radiation and stored in a second memory. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0010] The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the present invention is not limited to the arrangements and instrumentality shown in the attached drawings.  
     [0011]FIG. 1 illustrates a block diagram of an x-ray system in accordance with an embodiment of the present invention.  
     [0012]FIG. 2 illustrates the circuitry of an exemplary portion of the photodetector array which is formed by a matrix of detector elements in accordance with an embodiment of the present invention.  
     [0013]FIG. 3 illustrates a block diagram of an offset correction system utilizing two offset image memories and multiple recursive filters in accordance with an embodiment of the present invention.  
     [0014]FIG. 4 illustrates a block diagram of an offset correction system utilizing two offset image memories and a single recursive filter in accordance with an embodiment of the present invention.  
     [0015]FIG. 5 illustrates a flow chart of the steps used to acquire, store and update the offset image, and to correct an incoming x-ray image using the stored offset image in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
     [0016]FIG. 1 illustrates a block diagram of an x-ray system  14 . The x-ray system  14  includes an x-ray tube  15  which, when excited by a power supply  16 , emits an x-ray beam  17 . As illustrated, the x-ray beam  17  is directed toward a patient  18  lying on an x-ray transmissive table  20 . The portion of the beam  17  which is transmitted through the table  20  and the patient  18  impinges upon an x-ray detector  22 . The x-ray detector  22  comprises a scintillator  24  that converts the x-ray photons to lower energy photons in the visible spectrum. Contiguous with the scintillator  24  is a photodetector array  26  which converts the light photons into an electrical signal. A detector controller  27  contains electronics for operating the detector array  26  to acquire an image and to read out the signal from each photodetector element.  
     [0017] The output signal from the photodetector array  26  is coupled to an image processor  28  that includes circuitry for processing and enhancing the x-ray image signal. The image processor  28  includes at least two memories  29  and  31  for storing offset correction data. The memories  29  and  31  store a minimum of two offset images. The image processor  28  further includes one or more recursive filters as illustrated and discussed in relation to FIG. 3 and FIG. 4. The processed image is displayed on a video monitor  32  and may be archived in an image storage device  30 . The image processor  28  may additionally produce a brightness control signal which is applied to an exposure control circuit  34  to regulate the power supply  16  and thereby the x-ray exposure. The overall operation of the x-ray system  14  is governed by a system controller  36  that receives commands from an x-ray technician via an operator interface panel  38 .  
     [0018]FIG. 2 illustrates the circuitry of the photodetector array  26 , which is formed by a matrix of detector elements  40 . The detector elements  40  are arranged on an amorphous silicon wafer in a conventional two-dimensional array of m columns and n rows, where m and n are integers. For example, a typical high resolution x-ray detector is a square array of 1,000 to 4,000 rows and columns of elements. Each detector element  40  includes a photodiode  42  and a thin film transistor (TFT)  44 . The photodiodes  42  are fabricated from a large wafer area in order that the photodiode  42  will intercept a sizeable portion of the light produced by the scintillator  24 . Each photodiode  42  also has an associated capacitance that allows it to store the electrical charge resulting from the photon excitation.  
     [0019] The cathode of the photodiodes  42  in each column of the array  26  is connected by the source-drain conduction path of the associated TFT  44  to a common column signal line ( 48   −1  through  48   −m ) for the column. For example the photodiodes  42  in column  1  are coupled to the first signal line  48   −1 . The anodes of the diodes in each row are connected in common to a source of a negative bias voltage (−V). The gate electrodes of the TFTs  44  in each row are connected to a common row select line ( 46   −1  through  46   −n ), such as line  46   −1  for row  1 . The row select lines ( 46   −1  through  46   −n ) and the column signal lines ( 48   −1  through  48   −m ) are coupled to the detector controller  27  and the column signal lines ( 48   −1  through  48   −m ) also are connected to the image processor  28 .  
     [0020] In order to acquire an x-ray image using the detector  22  illustrated in FIG. 1, the x-ray system  14  performs the following sequence of operations. Initially, the detector controller  27  connects all the column signal lines ( 48   −1  through  48   −m ) to ground and applies a positive voltage (V on ) to all the row select lines ( 46   −1  through  46   −n ). The positive voltage applied to the row select lines ( 46   −1  through  46   −n ) turns on the TFT  44  in each detector element  40 , placing a positive charge on the reverse biased photodiodes  42 . Once the photodiodes  42  have been fully charged, the detector controller  27  applies a negative voltage (−V off ), which is more negative than the negative supply voltage (−V), to the row select lines ( 46   −1  through  46   −n ). This negative biasing of the row select lines ( 46   −1  through  46   −n ) turns off the TFT  44  in each detector element  40 .  
     [0021] The system x-ray tube  15  then generates an x-ray beam  17  and exposes the detector  22  to a pulse of x-ray photons. The x-ray photons are converted to lower energy photons by the scintillator  24 . When these lower energy photons strike the photodiodes  42  in the detector  26 , the electron-hole pairs are liberated and stored in the capacitance of the photodiode. The amount of charge stored in the given photodiode  42  depends upon the amount of lower energy photons which strikes it, which in turn depends upon the intensity of the x-ray energy that strikes the region of the scintillator  24  adjacent to the photodiode  42 . Therefore, the amount of charge stored in the photodiode  42  in each detector element  40  is a function of the x-ray intensity striking the corresponding region of the x-ray detector  22 .  
     [0022] After the termination of the x-ray exposure, the residual charge in each photodiode  42  is sensed. If a dark image, rather than an x-ray image, is to be acquired, the detector  22  is not exposed to a pulse of x-ray photons before the residual charge in each photodiode  42  is sensed. To sense the charge, the column signal line ( 48   −1  through  48   −m ) for each detector array column is simultaneously connected to separate sensing circuits in the image processor  28 . Any of several types of sensing circuits may be incorporated into the image processor  28 . For example, the sensing circuit may measure the voltage across the photodiode  42 , and therefore the amount of charge stored in the photodiode  42 . Alternatively, the sensing circuit may connect the associated column signal line ( 48   −1  through  48   −m ) to a lower potential than the cathode of the photodiode  42  and measure the amount of charge that flows to or from the photodiode  42 .  
     [0023] The photodiode charges may be sensed a row at a time by the detector controller  27  sequentially applying the positive voltage (V on ) to each of the row select lines ( 46   −1  through  46   −n ). When a row select line ( 46   −1  through  46   −n ) is positively biased, the detector array TFTs  44  connected to that row select line ( 46   −1  through  46   −n ) are turned on thereby coupling the associated photodiodes  42  in the selected row to their column signal lines ( 48   −1  through  48   −m ).  
     [0024] In order to decrease the amount of time required to read out the signal from each detector element  40  in the photodetector array  26 , the rows of detector elements  40  can be divided into two groups and each group simultaneously read out by separate signal sensing circuits. For example, if the detector  22  is split into two halves, the detector elements  40  in the top half of the photodetector array  26  may be read out simultaneously with the detector elements  40  in the bottom half of the photodetector array  26 .  
     [0025]FIG. 3 illustrates a block diagram of an offset correction system  60  utilizing two offset image memories  70  and  72  and recursive filters  74  and  76 . The recursive filters  74  and  76  process offset image correction data for different modes of operation. One mode of operation may acquire data from a region of interest, or a portion of the detector, such as a 1024×1024 sized matrix of pixels arranged symmetrically around the split in the detector  22 . In this mode, every row within the selected portion of the detector  22  is read out individually. Another mode of operation may acquire image data with lower resolution and utilize “binning”, wherein multiple pixels are combined to create one pixel value. Binning may be used when acquiring data from the entire field of view of the detector  22  or from a region of interest. For example, a high resolution image may not be required, or a higher frame rate than the frame rate available during high resolution imaging may be desired. Therefore, adjacent rows are read out at the same time, and a small number of neighboring pixels, such as four pixels, are combined to create a matrix with a lower resolution. Additional modes of operation may be utilized, such as selecting a region of interest other than the center of the detector  22 , imaging using a low dose or a high dose of x-ray requiring the use of different gain settings, or changing the sequence or timing in which the detector elements  40  are read. For each additional mode of operation, an additional offset image memory  70  and  72  may be included.  
     [0026] An x-ray detector  22  produces incoming images  62  at a given frame rate. By way of example only, for fluoroscopy, a typical frame rate may be  30  images per second. The system controller  36  determines if detector  22  was exposed to x-rays. If the detector  22  was not exposed to x-rays, switch  68  is placed in a position indicating “detector not exposed to x-rays”. In this configuration incoming images  62 , which are dark images or images not exposed to x-ray beam  17 , are used to create or update an offset image stored in offset image memory  70  or  72 .  
     [0027] The system controller  36  identifies the mode of operation, which may be changed by an operator through the operator interface  38 . By way of example only, MODE  1  may be a reduced region of interest, such as a 1024×1024 matrix of pixels in the center of the detector  22 , and MODE  2  may utilize binning and the entire field of view of the detector  22 . The system controller  36  communicates to the image processor  28  which sets switch  64  according to the mode of operation. When switch  64  is set to MODE  1  and switch  68  is set to “detector not exposed to x-rays”, incoming image  62  will be processed by recursive filter  74  and stored in offset image memory  70 . When switch  64  is set to MODE  2  and switch  68  is set to “detector not exposed to x-rays”, incoming image  62  will be processed by recursive filter  76  and stored in offset image memory  72 . Because the operation of the illustrated offset image memories  70  and  72  and recursive filters  74  and  76  are the same, only offset image memory  70  and recursive filter  74  will be discussed. Optionally, recursive filters  74  and  76  may utilize one or more components in common.  
     [0028] At startup and at other times as necessary, the system controller  36  may operate the detector  22  in each mode of operation automatically. When MODE  1  is selected, switch  84  is put in a “first image” position. In this configuration, the incoming dark image replaces the contents of the offset image memory  70 . Once the first dark image is stored, switch  84  is switched back to its original position, as illustrated in FIG. 3. Subsequently, one or more additional dark images are acquired. As each image is acquired, it is combined with the contents of the offset image memory  70  as discussed below. The process of acquiring initial and subsequent dark images in order to create an offset image is repeated for each mode of operation. Therefore, the acquisition of new and/or updated offset images may be transparent to the operator.  
     [0029] The recursive filter  74  acts as a temporal filter on the sequence of incoming successive images  62  to produce an offset image stored in the offset image memory  70 . As each incoming image  62  is acquired, it is combined with the contents of the offset image memory  70  using the recursive filter  74 . The action of this filter  74  can be described by the Equation 1 below: 
       a   i =(1−1 /n )( a   i−1 )+(1 /n ) p   Equation 1 
     [0030] In Equation 1, p represents an incoming pixel value, (a i−1 ) is the present pixel value in the offset image memory, and a i  is the output of the filter. It should be understood that the filter acts on each pixel, or combined pixels, if binning is used, and combines the input value of the incoming image  62 , with the corresponding value in the offset image memory  70 . The output of the filter, a for each pixel position, constitutes a new, reduced-noise offset image. This image is used to overwrite the previous contents, (a i−1 ), of the offset image memory  70 .  
     [0031] As shown in FIG. 3, the recursive filter  74  comprises multipliers  78  and  80  and adder  82 . The multiplier  78  multiplies the pixel values in the incoming image  62  by a multiplier of 1/n. The multiplier  80  multiplies pixel values stored in the offset image memory  70  by a multiplier of (1−1/n). The results of both multipliers  78  and  80  are input to the adder  82  to generate an offset image which is stored in the offset image memory  70 .  
     [0032] The value of n controls the amount of noise reduction and the speed of updating the offset image memory. Smaller values of n will produce faster updating but less noise smoothing, whereas larger values of n will produce slower updating and more smoothing. The recursive filter  74  is not limited to the components and calculations illustrated, and may achieve the noise reduction and automatic update by other suitable circuitry and/or software.  
     [0033] When the detector  22  is exposed to x-rays, the switch  68  is placed in a position indicating “detector exposed to x-rays”. With switch  68  in this position, the updating action of the recursive filters  74  and  76  is halted. The system controller sets switch  66 , which determines whether offset image memory  70  will be utilized for MODE  1  or offset image memory  72  will be utilized for MODE  2 . The offset image stored in the offset image memory  70  or  72  is subtracted from the incoming image  62  using the subtractor  86 . The subtraction removes the offset signals from the x-ray image  62  and produces a corrected image  88 . Therefore, by utilizing the offset images stored in the offset image memories  70  and  72 , successive incoming images  62  may be processed and displayed on the monitor  32  without having to halt the acquisition of patient data when switching between modes of operation.  
     [0034]FIG. 4 illustrates a block diagram of an offset correction system  92  utilizing two offset image memories  70  and  72  and a single recursive filter  94 . The recursive filter  94  processes offset image correction data for MODE I and MODE  2  to be stored in offset image memories  70  and  72 , respectively. Similar to FIG. 3, an additional offset image memory  70  and  72  may be included for each additional mode of operation.  
     [0035] The x-ray detector  22  produces incoming images  62  at a given frame rate. If the detector  22  was not exposed to x-rays, switch  68  is set to “detector not exposed to x-rays”. The system controller  36  identifies the mode of operation and sets switches  96  and  98  accordingly. A first dark image is acquired and stored as needed in offset image memories  70  and  72  as previously discussed. The offset correction system  92  operates similar to offset correction system  60 , except that the single recursive filter  94  is used to generate the offset images.  
     [0036] When the detector  22  is exposed to x-rays, the switch  68  is placed in the position indicating “detector exposed to x-rays”. The system controller sets switch  98 , which determines whether offset image memory  70  will be utilized for MODE  1  or offset image memory  72  will be utilized for MODE  2 . The offset image stored in offset image memory  70  or  72  is then subtracted from the incoming image  62  using the subtractor  86  to produced corrected image  88 .  
     [0037]FIG. 5 illustrates a flow chart of the steps which may be used to acquire, store and update the offset image, and to correct an incoming x-ray image using the offset image. At step  100 , the detector controller  27  initiates the acquisition of an image as previously discussed. The x-ray tube  15  may or may not be exposing the detector  22  to x-ray.  
     [0038] At step  102 , the system controller  36  determines what mode of operation, and thus which offset image memory  70  and  72  in the image processor memories  29  and  31  will be used. For example, the system controller  36  may determine whether the center portion of the detector  22  is being imaged, or whether binning is utilized to image the entire detector  22 . The mode of operation may be changed by the operator through the operator interface  38  before or during the diagnostic procedure as previously discussed. Because the x-ray system  14  utilizes more than one offset image memory  70  and  72  to store offset images for more than one mode, the mode of operation may be changed during a patient procedure without halting the acquisition of patient data. Once the mode of operation is determined, the system controller  36  sets switch  64  (FIG. 3) or switches  96  and  98  (FIG. 4) to the appropriate setting. For the following discussion, switches  64 ,  96  and  98  are set to MODE  1 , as illustrated in FIG. 3 and  4 .  
     [0039] At step  104 , the system controller  36  determines whether the detector  22  was exposed to x-rays during the image acquisition. If no, the incoming image  62  is a dark image. Switch  68  is set to “detector not exposed to x-rays” and flow passes to step  106 .  
     [0040] At step  106 , the image processor  28  determines whether an initial dark image should be acquired. An initial dark image may be acquired when the x-ray system  14  is started, or when a predefined parameter has been met, such as a predefined length of time passing since the offset image was updated. If an initial image is to be acquired, flow passes to step  108 , where the switch  84  is set to “first image”. After an initial dark image is acquired, switch  84  is returned to the position illustrated in FIG. 3 and  4 . Flow then returns to step  100  to process the next incoming image  62 . Alternatively, the system controller  36  may prevent subsequent x-ray exposure of detector  22  until a predetermined number of dark images have been acquired and processed for the mode of operation currently selected. If an initial image is not to be acquired at step  106 , flow passes to step  110 . The dark image is then processed by recursive filter  74 ,  94  and the updated offset image is stored in offset image memory  70  as discussed previously. Once again, the system controller  36  may prevent subsequent x-ray exposure of detector  22  until the predetermined number of dark images have been acquired and processed for the mode of operation currently selected.  
     [0041] If the system controller  36  determines at step  104  that the detector  22  has been exposed to x-rays, switch  68  is set to “detector exposed to x-rays” and flow passes to step  112 . At step  112 , the system controller  36  sets the switch  66  (FIG. 3) to the appropriate setting. Continuing the example above, switch  66  is set to MODE  1 . The image processor  28  then subtracts the offset image stored in the offset image memory  70  from the incoming image  62  with subtractor  86 . The result is the corrected image  88 , which may be displayed on the monitor  32  and/or stored in the image storage  30 . Flow then returns to step  100 , where the next incoming image  62  is acquired.  
     [0042] As discussed above, by utilizing multiple offset image memories  70  and  72  to store offset image correction data, there is no need to halt the acquisition of patient data when the mode of operation is switched during a procedure. Multiple modes of operation may be utilized to acquire successive x-ray images during a single procedure without interruption. Therefore, the acquisition of patient data need not be halted to acquire additional offset correction data when switching between modes of operation. It should be understood that although two modes of operation and two corresponding offset image memories were discussed, the system and method may utilize more than two modes of operation and corresponding offset image memories and achieve the benefits described herein.  
     [0043] While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.