Abstract:
A system for reading data is described. The system includes a detector that includes a first semiconductor device and a second semiconductor device. The second semiconductor device is configured to remove a need to scrub the detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to imaging systems and more particularly to systems and methods for reading data from a detector. 
         [0002]    A solid state x-ray detector includes of an array of pixels having a plurality of switches and photodiodes over which Cesium Iodide (CsI) is deposited. The CsI absorbs x-rays and converts the x-rays to light, which is detected by the photodiodes. Each photodiode, due to its construction, acts as a capacitor and stores charge. Initialization of the detector takes place prior to an x-ray exposure, when each photodiode is charged to an initial voltage. The detector is then exposed to the x-rays, which are absorbed by the CsI. The light that is emitted in proportion to a portion of the x-rays partially discharges each photodiode. After the exposure, a voltage on each photodiode is restored to the initial voltage. An amount of charge used to restore the initial voltage on the photodiode is measured, which becomes a measure of an x-ray dose integrated by a pixel during a length of the exposure. 
         [0003]    The detector is read or alternatively scrubbed on a row-by-row basis, as controlled by the switches associated with each photodiode. Reading is performed whenever an image acquired by the detector includes exposure data or alternatively offset data. Scrubbing is similar to reading except that data acquired from scrubbing is not interesting, and is therefore discarded. Scrubbing is performed to maintain proper bias on the photodiodes during idle periods, perhaps to reduce a plurality of effects of lag, which is incomplete charge restoration of the photodiodes, or alternatively to maintain a plurality of thresholds of the switches. The thresholds may shift if the switches are kept in an “off” state for long periods, among other reasons. Scrubbing restores charge on each photodiode and the charge need not be measured. If the charge is measured, data acquired from scrubbing can be discarded. 
         [0004]    Scrubbing is performed to keep the detector ready for use largely due to the less than ideal characteristics of amorphous silicon used to fabricate the detector. 
         [0005]    However, scrubbing is undesirable for several reasons. Scrubbing represents non-productive overhead, uses power to perform, the dissipation of which is undesirable especially in low power applications. Scrubbing may create an access time latency before the commencement of the exposure to allow completion of the scrub prior to the start of the exposure. The time used to scrub takes away from the detector&#39;s availability. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect, a system for reading data is described. The system includes a detector that includes a first semiconductor device and a second semiconductor device. The second semiconductor device is configured to remove a need to scrub the detector. 
         [0007]    In another aspect, an imaging system for reading data is described. The imaging system includes an energy source configured to generate energy that passes through a subject, and a detector configured to receive a portion of the energy. The detector includes a first semiconductor device and a second semiconductor device and the second semiconductor device is configured to remove a need to scrub the detector. 
         [0008]    In yet another aspect, a method for reading data is described. The method includes removing a need to scrub a first detector by coupling a first semiconductor device to a second semiconductor device. 
         [0009]    In still another aspect, a method for removing image artifacts is described. The method includes coupling a first semiconductor device to a second semiconductor device and a photodiode, and during an activation of the second semiconductor device, applying a potential to the second semiconductor device that is more negative than a potential applied to an anode of the photodiode. The method further includes during the activation of the second semiconductor device, applying a potential to the second semiconductor device similar to a potential of a data line attached to the first semiconductor device subsequently after applying the potential to the second semiconductor device that is more negative than the potential applied to the anode of the photodiode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of an embodiment of an imaging system. 
           [0011]      FIG. 2  is a block diagram of another embodiment of an imaging system. 
           [0012]      FIG. 3  is a block diagram of an embodiment of a detector of the imaging system of  FIG. 2 . 
           [0013]      FIG. 4  is a circuit diagram of an embodiment of a photo detector array of the detector. 
           [0014]      FIG. 5  is a timing diagram illustrating an embodiment of a method of reading data from the photo detector array of  FIG. 4 . 
           [0015]      FIG. 6  is a circuit diagram of another embodiment of a photo detector array of the detector. 
           [0016]      FIG. 7  is a timing diagram illustrating an embodiment of a method of reading data from the photo detector array of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  is a block diagram of an embodiment of an imaging system  14 . Imaging system  14  includes source  15 , such as an x-ray tube or a gamma ray source, which, when excited by a power supply  16 , emits a beam  17 , such as an x-ray beam or a gamma ray beam. The beam  17  is directed toward a subject  18 , such as a patient or a phantom, lying on a transmissive table  20 . A portion of the beam  17  which is transmitted through the transmissive table  20  and the subject  18  impinges upon a detector  22 , such as an x-ray or a gamma ray detector. Detector  22  includes a scintillator layer  24  that converts a plurality of higher energy photons of the portion of beam  17  to lower energy photons. The lower energy photons have energies lower than the higher energy photons. Contiguous with the scintillator layer  24  is a photo detector array  26 , which converts the lower energy photons into a plurality of electrical signals. A detector controller  27  includes electronics that operates photo detector array  26  to acquire an image, such as an x-ray image or a gamma ray image, and to read an electrical signal from each photo detector element of photo detector array  26 . Examples of the x-ray image include a radiographic image, an image of a chest of subject  18 , an angiographic image, a cardio graphic image, and a mammographic image. 
         [0018]    The electrical signals from the photo detector array  26  are coupled to an image processor  28  that includes circuitry that processes and enhances, such as amplifies or filters, the electrical signals to generate the image. As an example, image processor  28  might apply image reconstruction, such as filtered backprojection (FBP) or maximum intensity projection (MIP) to generate a computed tomography (CT) image or other algorithms to generate diagnostic images from the electrical signals. As used herein, the term controller is not limited to just those integrated circuits referred to in the art as a controller, but broadly refers to a processor, a computer, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, a Field Programmable Gate Array, and any other programmable circuit. Moreover, as used herein, the term processor is not limited to just those integrated circuits referred to in the art as a processor, but broadly refers to a controller, a computer, a microcontroller, a microcomputer, a programmable logic controller, an application specific integrated circuit, a Field Programmable Gate Array, and any other programmable circuit. 
         [0019]    The image is displayed on a monitor  32  and may be archived in an image storage device  30 . Examples of image storage device  30  include a computer-readable medium, such as a hard drive, a volatile memory, a non-volatile memory, a floppy disk, a compact disc—read only memory (CD-ROM), a magneto-optical disk (MOD), and a digital versatile disc (DVD). Examples of monitor  32  include a cathode ray tube (CRT) and a liquid crystal display (LCD). The image processor  28  additionally produces a brightness control signal which is applied to an exposure control circuit  34  to regulate the power supply  16  and to reduce or alternatively increase an exposure of the portion of beam  17  on detector  22 . The overall operation of imaging system  14  is governed by a system controller  36  that receives commands from a technician or a person via an operator interface panel  38 , such as a mouse or a keyboard. 
         [0020]      FIG. 2  is a block diagram of another embodiment of an imaging system  50 . Imaging system  50  includes imaging system  14 . Moreover, imaging system  50  includes a source  52  and a detector  54 . Source  52  can be an x-ray source or a gamma ray source. Detector  54  includes a scintillator layer  56  and a photo detector array  58 . Source  52  is located at an angle, such as ranging from and including 1 degrees to 179 degrees, with respect to source  15 . Moreover, detector  54  forms an angle, such as ranging from and including 1 degrees to 179 degrees, with respect to detector  22 . Source  52  and detector  54  are used to acquire to a view of subject  18  different than a view acquired by source  15  and detector  22 . 
         [0021]    Source  52  generates a beam  60 , such as an x-ray beam or a gamma ray beam, directed toward subject  18  when source  52  is excited by power supply  16 . Upon passing through subject  18 , a portion of beam  60  is transmitted towards detector  54 . Scintillator layer  56  converts a plurality of higher energy photons of the portion of beam  60  to a plurality of lower energy photons having energies lower than the higher energy photons. The lower energy photons of the portion of beam  60  lie within a spectrum that can be detected by the photo detector array  58 . Photo detector array  58  converts the lower energy photons of the portion of beam  60  into a plurality of electrical signals. Image processor  28  reads the electrical signals from detector  54 , and processes and enhances the electrical signals to generate an image, such as an x-ray image or a gamma ray image. For example, image processor  28  applies image reconstruction, such as FBP or MIP or other image processing algorithms, to the electrical signals received from photo detector array to generate the image. The image generated from the electrical signals output by detector  54  is stored in image storage device  30 . Image processor  28  produces a brightness control signal that is applied to exposure control circuit  34  to regulate power supply  16  and an exposure of the portion of beam  60  incident on detector  54 . 
         [0022]      FIG. 3  is a block diagram of an embodiment of a detector  70 , which is an example of any of detectors  22  and  54 . Detector  70  includes a scintillator layer  72 , fabricated from a scintillator, such as cesium iodide (CsI). Detector  70  includes a photo detector array  74  and a substrate  76 . Photo detector array  74  is an example of one of photo detector array  26  and photo detector array  58 . Scintillator layer  72  is an example of one of scintillator layer  24  and scintillator layer  56 . 
         [0023]    Scintillator layer  72  absorbs the portion of one of the beams  17  and  60  to convert the higher energy photons of the portion of the one of beams  17  and  60  into the lower energy photons of the portion of the one of beams  17  and  60 . Photo detector array  74  receives the lower energy photons of the portion of the one of beams  17  and  60  to generate a plurality of electrical signals. Substrate  76  supports photo detector array  74  and scintillator layer  72 . 
         [0024]      FIG. 4  is a circuit diagram of an embodiment of a photo detector array  80 , which is an example of photo detector array  74 . Photo detector array  80  includes a plurality of scan lines  82  and  84 , and a plurality of data lines  86  and  88 . Photo detector array  80  is formed by a matrix of pixels or detector elements  90 . Detector elements  90  are arranged on substrate  76 . Each detector element  90  includes a photodiode  92  made of a material, such as silicon. Examples of silicon include amorphous silicon and crystalline silicon. Moreover, each detector element  90  includes a thin film field effect transistor (FET)  94 . The photodiode  92  is fabricated over a large portion of detector element  90  in order that the photodiode  92  will intercept a sizeable portion of the light produced by scintillator layer  72 . Each photodiode  92  has a capacitance that allows the photodiode  92  to store an electrical charge, which is then partially or alternatively wholly discharged due to an excitation by the lower energy photons of the portion of one of beams  17  and  60 . 
         [0025]    The cathode of each photodiode  92  in each detector element  90  of each column of the photo detector array  80  is connected via a source-drain conduction path of the FET  94  to one of data lines  86  and  88 . Data lines  86  and  88  are connected to a plurality of sensing circuits  96  and sensing circuits  96  maintain data lines  86  and  88  at a constant potential at all times. The sensing circuits  96  are included in the image processor  28 . The anode of each photodiode  92  is connected to a common electrode  98 . A gate electrode of FET  94  in each row is connected to one of scan lines  82  and  84 . Each scan line  82  and  84  runs the full dimension of detector  70 . Scan lines  82  and  84  are coupled to the detector controller  27 . In another embodiment, photo detector array  80  is formed of any integer, m, of scan lines and any integer, n, of data lines. 
         [0026]      FIG. 5  is a timing diagram  110  illustrating an embodiment of a method of reading data from a detector. To acquire an image, such as an x-ray image or a gamma ray image, by using detector  70 , initially, detector  70  is scrubbed  112 . Scrubbing  112  may be performed to maintain a known potential or a known voltage on the photodiodes  92  during idle periods, to reduce a plurality of effects of image retention or lag, and/or to protect a plurality of operating characteristics of the FETs  94 . Sensing circuits  96  restore charge of the photodiodes  92  of detector elements  90  of detector  70  during scrubbing  112 . One of scan lines  82  and  84  being scrubbed  112  operates detector elements  90  connected to a corresponding one of data lines  86  and  88 . For example, detector element  90  connected to data line  86  is operated to scrub  112  scan line  82 . During scrub  112 , a high negative voltage, such as −a volts, is applied to the common electrode  98  by a power source (not shown), where a is a positive real number other than zero. The sensing circuits  96  apply a low negative voltage, such as −b volts, to data lines  86  and  88  that is lower than the high negative voltage, where b is a positive real number. The number b is lower than the number a. 
         [0027]    Moreover, during scrub  112 , detector controller  27  switches scan lines  82  and  84  from a voltage, such as −c volts, more negative than the high negative voltage of common electrode  98  to a positive voltage, causing the FETs  94  attached to scan lines  82  and  84  to begin to conduct, where c is a positive real number. The number c is higher than the number a. Detector controller  27  includes a plurality of drive circuits to drive or provide power to scan lines  82  and  84 . During scrub  112 , the photodiode  92  continues to store charge until a voltage across the photodiode  92  is equal to a voltage difference between a corresponding one of data lines  86  and  88  and common electrode  98  and until the photodiode  92  is charged to the known voltage, after which the FETs  94  are switched off. For example, the photodiode  92  continues to store charge until a voltage across the photodiode  92  is equal to a voltage difference between data line  88  and common electrode  98 . To end scrub  112 , the FETs  94  are switched off by detector controller  27 , which reapplies, to scan lines  82  and  84 , a potential, such as −c or −d volts, that is more negative than the high negative voltage of common electrode  98 , where d is a positive real number. The number d is higher than the number a. Image data used to produce an image, such as an x-ray or a gamma ray image, is not acquired during scrub  112 . 
         [0028]    Before system controller  28  controls power supply  16  to activate one of sources  15  and  52 , system controller  28  makes an exposure request  114 . Upon receiving exposure request  114 , detector controller  27  determines whether scrub  112  has ended. Upon determining that scrub  112  has not ended, detector controller  27  does not grant  116  exposure request  114  received from system controller  28 . On the other hand, upon determining that scrub  112  has ended, detector controller  27  grants  116  exposure request  114  received from system controller  28 . A scrub latency  118 , which is a time difference between a start of exposure request  114  and a start of grant  116  of exposure request  114  is developed. Detector  70  is exposed  120  to the portion of one of beams  17  and  60 , which is controlled by an amount of power supplied to a corresponding one of sources  15  and  52 . Detector  70  is exposed  120  during grant  116  of exposure request  114 . 
         [0029]    When the lower energy photons of the portion of one of beams  17  and  60  strike photodiode  92 , the photodiode  92  conducts and a capacitance of photodiode  92  is partially discharged. An amount of charge removed from the capacitance of photodiode  92  depends upon an amount of the lower energy photons of the portion of one of beams  17  and  60 , and the amount depends upon an intensity and duration of the portion of one of beams  17  and  60  that strikes scintillator layer  72  during exposure  120 . 
         [0030]    Upon termination of the exposure  120  of detector  70  to the portion of one of beams  17  and  60 , a charge in each photodiode  92  is restored to the known voltage before the exposure. The exposure  120  of detector  70  to the portion of one of beams  17  and  60  terminates when system controller  28  controls power supply  16  to discontinue supplying power to one of sources  15  and  52 . 
         [0031]    Upon termination of the exposure  120  of detector  70  to the portion of one of beams  17  and  60 , detector controller  27  reads  122  detector  70  by sequentially applying a positive voltage to scan lines  82  and  84 . The image data is acquired by sensing circuits  96  during read  122 . When one of scan lines  82  and  84  is positively biased, the FETs  94  connected to the one of scan lines  82  and  84  are turned on to couple the corresponding photodiodes  92  in the selected row to a corresponding one of data lines  86  and  88 . For example, when scan line  82  is positively biased, the FET  94  connected to the scan line  82  is turned on to couple the photodiode  92  to data line  86 . An amount of charge used to restore the voltage difference between the one of data lines  86  and  88  and common electrode  98  to the known voltage is measured by the sensing circuits  96 . 
         [0032]    Detector  70  is scrubbed  112  after detector  70  is read  122 . Sensing circuits  96  restore charge, if necessary, to photodiode  92  during scrub  112  in order to restore the potential across the photodiode  92 . If sensing circuits  96  measure the charge used to restore the voltage across photodiode  92  during scrub  112 , the measurement is discarded. 
         [0033]      FIG. 6  is a circuit diagram of an embodiment of a photo detector array  130 , which is another example of photo detector array  74 . Photo detector array  130  includes scan lines  82  and  84 , data lines  86  and  88 , FETs  94 , and photodiodes  92 . Moreover, photo detector array  130  includes a plurality of FETs  132 , such as an N-type metal-oxide semiconductor FET. A source electrode of FET  94  is coupled to a source electrode of FET  132 . Moreover, a drain electrode of FET  132  is coupled to a maintenance potential electrode  134  and a gate electrode of FET  132  is coupled to a maintenance control electrode  136 . Moreover, the source electrode of FET  132  is coupled to the cathode of photodiode  92 . 
         [0034]    Photodiodes  92 , FET  94 , and FET  132  form a pixel  138 . Maintenance control electrode  136  is electrically connected to a power supply  140 , such as a voltage source. Moreover, maintenance potential electrode  134  is electrically coupled to a power supply  142 , such as a voltage source. 
         [0035]    It is noted that in another embodiment, another type of device, such as a bipolar junction transistor (BJT), can be used, with a selection of supply voltages  140  and  142 , instead of FET  132 . For example, a base of the BJT is coupled to maintenance control electrode  136 , an emitter of the BJT is coupled to maintenance potential electrode  134 , and a collector of the BJT is coupled to the cathode of photodiode  92  and the source electrode of FET  94 . Examples of the BJT include an NPN BJT and a PNP BJT. Moreover, other types of devices, such as a P-type MOSFET, a Junction FETs (JFETs), metal-semiconductor FET (MESFETs), or a diode, can be used instead of FET  132 . In another embodiment, photo detector array  130  includes any number of pixels  138 . 
         [0036]      FIG. 7  is a timing diagram  150  illustrating an embodiment of a method of reading data from photo detector array  130 . Power supply  140  applies a voltage  152  to maintenance control electrode  136  to maintain a bias across photodiode  92 . A magnitude of voltage  152  applied to FETs  132  to activate or turn on FETs  132  is more positive, such as e volts, than the low negative voltage, such as −b volts, applied by the sensing circuits  96  to data lines  86  and  88 , and the more positive voltage  152  is applied to maintain bias across photodiode  92 , where e is a positive real number. The number e is higher than  0 , which is greater than −b. The higher the voltage e applied to maintenance control electrode  136  of FETs  132 , the “harder” FETs  132  will turn “on”, i.e. with lower impedance. A voltage applied to maintenance potential electrode  134  from power supply  142  is similar to a voltage applied to at least one of data lines  86  and  88  to maintain bias across photodiode  92 . For example, the voltage applied to maintenance potential electrode  134  ranges from and including a voltage applied to data line  86  to a voltage applied to common electrode  98 . As another example, when a voltage applied to data line  86  is −f volts and a voltage applied to common electrode  98  is −g volts, the voltage applied to maintenance potential electrode  134  ranges from and including −f volts to −g volts, where f and g are real and positive numbers and f is less than g. As another example, the voltage applied to maintenance potential electrode  134  ranges from and including a voltage applied to data line  88  to a voltage applied to common electrode  98 . As yet another example, when a voltage applied to data line  88  is −f volts and a voltage applied to common electrode  98  is −g volts, the voltage applied to maintenance potential electrode  134  ranges from and including −f volts to −g volts. Photodiode  92  charges when the bias is maintained across photodiode  92 . 
         [0037]    Moreover, the bias across photodiode  92  is maintained at a time at which sensing circuits  96  and detector controller  27  are deactivated, turned off, or not powered by a power supply (not shown) within detector  70 . Additionally, the bias across photodiode  92  is maintained at a time during which detector  70  is at least one of not exposed  120 , not read  122 , and not integrating offset data. Sensing circuits  96  are controlled by system controller  36  to prevent sensing circuits  96  from reading  122  from detector  70  until a certain time. The prevention until the certain time integrates offset data. 
         [0038]    When exposure request  114  is received from system controller  28 , detector controller  27  is not scrubbing  112  and does not need to wait to finish scrubbing  112  before granting  116  exposure request  114 . When exposure request  114  is received from system controller  28 , power supply  140  reduces voltage supplied to gate electrodes of FETs  132  to −c or alternatively −d volts to deactivate or turn off FETs  132 . By turning off FETs  132 , the bias is discontinued to be maintained across photodiode  92  and photodiode  92  discontinues to be charged by maintenance potential electrode  134 . When the bias is discontinued to be maintained across photodiode  92 , detector controller  27  grants  116  exposure request  114  received from system controller  28 . At an end of exposure request  114 , detector  70  is read  122  by sensing circuits  96 . Detector  70  is read  122  by sensing circuits  96  and scrub  112  is not performed by using FETs  132 . A non-scrub latency  154 , which is a time difference between a start of exposure request  114  and a start of grant  116  of grant  116  of exposure request  114  is less than scrub latency  118 . 
         [0039]    In one embodiment, the voltage applied to maintenance potential electrode  134  is equal to a value at one of data lines  86  and  88 . In another embodiment, when the bias is maintained across photodiode  92  of detector  54 , detector  22  is exposed to the portion of beam  17 . After exposure of detector  22  to the portion of beam  17 , the bias is discontinued to be maintained across photodiode  92  of detector  54 . Moreover, when the bias is discontinued to be maintained across photodiode  92  of detector  54 , the detector  54  is exposed to the portion of beam  60 . After exposure of detector  54  to the portion of beam  60 , detectors  22  and  54  are read  122  by sensing circuits  96 . It is noted that exposing detector  22  during maintenance of the bias across photodiode  92  of detector  54  reduces, such as eliminates, the effect of scatter, generated by the portion of beam  17 , on detector  54 . 
         [0040]    It is further noted that offset data is read by sensing circuits  96  from detector  70  in a similar, such as the same, manner in which the image data is read from detector  70  during read  122  except that there is no exposure  120  made. Image Processor  28  subtracts the offset data from the image data to account for any non-zero signal contribution outside of exposure signal, such as diode leakage and any differences between a potential of maintenance potential electrode  134  and a potential of any one of data lines  86  and  88 .Moreover, during activation or turning on of FET  132 , artifacts are removed by applying to maintenance potential electrode  134 , the voltage that is more negative, such as −h volts, than a voltage, such as −i volts, applied to common electrode  98 , where h and i are positive real numbers. The number h is higher or greater than the number i. Additionally, during the activation of FET  132 , artifacts are removed by applying to FET  132  a voltage that is similar to a potential of data line  86  attached to transistor FET  94  subsequently after applying to maintenance potential electrode  134 , the voltage that is more negative, such as −h volts, than a voltage, such as −i volts, applied to common electrode. 
         [0041]    Technical effects of the herein described systems and methods for reading data include reducing a time and power utilized to scrub  112  detector  70 . Moreover, removing a need to scrub  112  detector  70  also results in non-scrub latency  154  being less than the scrub latency  118 . Power is limited when detector  70  is portable and is powered by a battery. The power utilized by sensing circuits  96  to scrub  112  detector  70  is saved when detector  70  is not scrubbed. 
         [0042]    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.