JP2016061739A - Radiation imaging apparatus and method for adjusting the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique which is advantageous to uniformize detection performance for detecting the irradiation of radiation.SOLUTION: A radiation imaging apparatus includes a pixel array where a plurality of pixels for capturing an image by the radiation are arranged, a detection part which outputs a detection value corresponding to the irradiation of the radiation, an adjustment part which adjusts the characteristics of the detection part, so that a relation between a signal outputted from the pixel array or a processing result obtained by processing the signal and the detection result comes to a predetermined relation, and a control part which detects that the radiation is irradiated, according to the fact that the detection value exceeds a threshold.SELECTED DRAWING: Figure 7

Description

  The present invention relates to a radiation imaging apparatus and an adjustment method thereof.

  As a radiation imaging apparatus that captures an image of radiation such as X-rays, there is an apparatus that performs an imaging operation by detecting that radiation has been irradiated. In Patent Document 1, a radiographic image is detected in which a current detection unit detects a current flowing through a bias line for applying a bias voltage to two-dimensionally arranged radiation detection elements, thereby detecting the start and end of radiation irradiation. An imaging device is described.

JP 2010-268171 A

  Since the performance for detecting the irradiation of radiation (hereinafter referred to as detection performance) may vary depending on manufacturing variations of the radiation imaging apparatus, the detection performance should be adjusted before or after shipment of the radiation imaging apparatus. However, since it is difficult to make the environment for irradiating the radiation imaging apparatus constant, conventionally, it has been difficult to make the detection performance of a plurality of radiation imaging apparatuses uniform in the adjustment operation.

  An object of this invention is to provide the technique advantageous in order to equalize the detection performance which detects irradiation of a radiation.

  One aspect of the present invention relates to a radiation imaging apparatus, and the radiation imaging apparatus outputs a pixel array in which a plurality of pixels for capturing an image of radiation is arrayed and a detection value corresponding to radiation irradiation. A detection unit; an adjustment unit that adjusts characteristics of the detection unit such that a relationship between the detection value, a signal output from the pixel array or a processing result obtained by processing the signal, and the detection value; and the detection value And a control unit that detects that radiation has been irradiated in response to exceeding the threshold value.

  According to the present invention, an advantageous technique is provided for making the detection performance for detecting the irradiation of radiation uniform.

The figure which shows the structure of the radiation imaging system of one embodiment of this invention. The figure which shows the specific structural example of the radiation imaging device which comprises a radiation imaging system. The figure which shows schematically operation | movement of a radiation imaging system or a radiation imaging device. The figure which shows schematically operation | movement of a radiation imaging system or a radiation imaging device. The figure which shows operation | movement of the radiation imaging system or radiation imaging device before and after the timing at which the start of radiation irradiation is detected. The figure for demonstrating the basic principle of this invention. The figure which shows the adjustment process (calibration) by an adjustment part. The figure which shows the normal imaging operation in a radiation imaging device. The figure which shows the operation | movement at the time of the measurement of the signal conversion coefficient in a radiation imaging device. The figure which illustrates the artifact which arises due to reset. The figure for demonstrating an example of the adjustment method of a signal conversion coefficient.

  Hereinafter, a radiation imaging system of the present invention will be described by way of example with reference to the accompanying drawings.

  FIG. 1 shows a configuration of a radiation imaging system 10 according to one embodiment of the present invention. The radiation imaging system 10 can include, for example, a radiation imaging apparatus 100, a control apparatus 200, a radiation generation apparatus 310, and an exposure control apparatus 320. All or part of the configuration of the control device 200 may be incorporated in the radiation imaging apparatus 100. An apparatus constituted by all or part of the configuration of the radiation imaging apparatus 100 and all or part of the configuration of the control apparatus 200 can also be understood as a radiation imaging apparatus. In addition, a part of the configuration of the radiation imaging apparatus 100 (for example, the adjustment unit 187) may be incorporated in the control apparatus 200. The control device 200 and the exposure control device 320 may be realized as one device.

  The radiation imaging apparatus 100 includes a pixel array 110, a driving unit 120, a reading unit 130, an amplification unit (impedance conversion unit) 140, a DA converter 150, a wireless interface (I / F) 160, a control unit 170, a detection unit 180, and an adjustment. 187 and a bias power source 190 may be included. In the pixel array 110, a plurality of pixels capable of detecting radiation are arranged. The driving unit 120 drives a plurality of pixels of the pixel array 110. The reading unit 130 reads signals from a plurality of pixels of the pixel array 110.

  The DA converter 150 converts the signal (analog signal) output from the amplification unit 140 into a digital signal. The wireless I / F 160 communicates with the control device 200 (its wireless I / F 220). The wireless I / F 160 transmits, for example, a signal provided from the DA converter 150, an exposure stop command (exposure control information), a signal indicating the state of the radiation imaging apparatus 100, and the like to the control apparatus 200. Here, the exposure stop command (exposure control information) is a command or information for stopping radiation emission by the radiation generator 310. The control unit 170 controls the pixel array 110, the driving unit 120, the reading unit 130, the amplification unit 140, the DA converter 150, the wireless I / F 160, the detection unit 180, the adjustment unit 187, and the bias power source 190. For this purpose, the control unit 170 generates control signals (D-CLK, DIO, XOE, RC, SH, CLK).

  The bias power supply 190 supplies a bias voltage to the pixel array 110 via the bias line Bs (power supply line). The detection unit 180 is a circuit that outputs a detection value corresponding to radiation irradiation on the radiation imaging apparatus 100, that is, a circuit for detecting radiation irradiation. More specifically, the detection unit 180 outputs a detection value corresponding to the bias current flowing through the bias line Bs during a period during which radiation irradiation is monitored. The detection unit 180 includes a sensor 182 that outputs an electrical signal corresponding to radiation irradiation, and a processing unit 184 that processes the electrical signal output from the sensor 182 and generates a detection value.

  The adjustment unit 187 adjusts the characteristics of the detection unit 180 so that the relationship between the signal output from the pixel array 110 or the processing result obtained by processing the signal and the detection value output from the detection unit 180 is a predetermined relationship. To do. The processing result obtained by processing the signal read by the reading unit 130 is processed by the adjustment unit 187, the control unit 170, or the processing unit 210 of the control device 200, for example, by reading the signal from the pixel array 110 by the reading unit 130. Result. The adjustment by the adjustment unit 187 can typically be performed before shipment of the radiation imaging apparatus 100 or during maintenance of the radiation imaging apparatus 100.

  The control unit 170 detects that radiation has been emitted in response to the detection value output from the processing unit 184 exceeding the threshold value. The control unit 170 controls the drive unit 120 to repeat a reset operation for resetting the pixels of the pixel array 110 in units of rows until it is detected that radiation has been irradiated. When it is detected that radiation has been irradiated, the controller 170 controls the drive unit 120 to stop the reset operation. Thereby, in the pixel array 110, accumulation of electric charges according to the irradiated radiation is started. The control unit 170 determines the end of radiation irradiation based on the fact that a predetermined time has passed since the detection of radiation irradiation or the detection value output from the processing unit 184, and a signal (image) from the pixel array 110. The driving unit 120 and the reading unit 130 are controlled so as to be read out.

  The control device 200 can include, for example, a processing unit 210, a wireless I / F 220, a display unit 230, and an input unit 240 (keyboard, pointing device, etc.). The control device 200 can be configured by incorporating software (computer program) into a general-purpose computer. For example, the processing unit 210 processes signals (that is, image signals) of a plurality of pixels of the pixel array 110 read by the reading unit 130.

  The exposure control device 320 includes an exposure switch (not shown), and transmits an exposure command to the radiation generation device 310 in response to the exposure switch being turned on. The radiation generator 310 emits radiation according to the exposure command.

  FIG. 2 shows a specific configuration example of the radiation imaging apparatus 100. In the pixel array 110, a plurality of pixels P are two-dimensionally arranged so as to form a plurality of rows and a plurality of columns. The pixel P includes a conversion element CV and a switch TT. The conversion element CV converts radiation into electric charges. The conversion element CV can be composed of a scintillator that converts radiation into visible light and a photoelectric conversion element that converts visible light into electric charge. In this case, the scintillator can be shared by a plurality of conversion elements CV. The conversion element CV may be configured to convert radiation directly into charges. The conversion element CV can be composed of a MIS type or PIN type photoelectric conversion element. The switch TT can be composed of, for example, a thin film transistor (TFT). The switch TT connects the first electrode of the conversion element CV and the signal line SL according to the drive signal G (corresponding to the drive signals G1, G2, G3... Gm). The second electrode of the conversion element CV is connected to the bias line Bs. Here, a drive signal for driving the pixel P (switch TT) in the n-th row is described as Gn (n = 1 to m).

  When the driving signal Gn in the n-th row is driven to the active level by the driving unit 120, the switch TT of the pixel P in the n-th row is turned on (conductive), and the charge accumulated in the conversion element CV of the pixel P is changed. The signal is transferred to the signal line SL through the switch TT. That is, when the driving signal Gn in the nth row is driven to the active level by the driving unit 120, the signal of the pixel P in the nth row is output to the signal line SL. In this embodiment, the active level is a high level, but the active level may be a low level.

  The reading unit 130 reads a signal from the pixel P via the signal line SL. The readout unit 130 includes an integration amplifier 131, a variable amplifier 132, a sample hold circuit 133, and a buffer amplifier 134 for each column in the pixel array 110. The signal output to the signal line SL is amplified by the integrating amplifier 131 and the variable amplifier 132, sampled and held by the sample and hold circuit 133, and amplified by the buffer amplifier 134. The reading unit 130 includes a multiplexer 135, and a signal output from the buffer amplifier 134 provided for each column is selected by the multiplexer 135 and output to the amplification unit 140.

  The integrating amplifier 131 includes an operational amplifier, an integrating capacitor, and a reset switch. The signal output to the signal line SL is input to the inverting input terminal of the operational amplifier 105, the reference voltage Vref is input to the non-inverting input terminal, and the amplified signal is output from the output terminal. The integration capacitor is disposed between the inverting input terminal and the output terminal of the operational amplifier. The variable amplifier 132 amplifies the signal output from the integrating amplifier 131 at an amplification factor specified by the control unit 170. The sample hold circuit 133 can be composed of a sampling switch and a sampling capacitor.

  The reset switch of the integrating amplifier 131 is controlled to be on (conductive) / off (non-conductive) by the control signal RC. The sampling switch of the sample hold circuit 133 is controlled to be turned on / off by a control signal SH. The multiplexer 135 selects a signal read from the pixel array 110 through the plurality of signal lines SL according to the control signal CLK.

  The bias power supply 190 supplies a bias voltage to the second electrode of the conversion element CV of each pixel P of the pixel array 110 via the bias line Bs. The detection unit 180 detects a bias current flowing between the bias power source 190 and the second electrode of the pixel P constituting the pixel array 110 through the bias line Bs during the period of monitoring the radiation irradiation. Specifically, in a period during which radiation irradiation is monitored, the detection unit 180 outputs a detection value corresponding to the bias current flowing through the bias line Bs.

  The sensor 182 can be constituted by, for example, a current-voltage conversion circuit and an AD conversion circuit that converts an output voltage of the current-voltage conversion circuit into a digital signal. The current-voltage conversion circuit can be constituted by, for example, a transimpedance amplifier including an operational amplifier and a transimpedance or a shunt resistor, but is not limited to this example.

  The drive unit 120 generates a drive signal G that controls the switches TT of the pixels P of the pixel array 110 in units of rows in accordance with control signals (D-CLK, DIO, and XOE) supplied from the control unit 170. The driving unit 120 includes a shift register, and the control signal D-CLK is a clock signal supplied as a shift clock to the shift register. The control signal DIO is a shift pulse supplied to the shift register, and the control signal XOE is an output enable signal for the shift register.

  3 and 4 schematically show the operation of the radiation imaging system 10 or the radiation imaging apparatus 100. When the radiation imaging apparatus 100 is activated in step S310, the radiation imaging apparatus 100 enters a standby state. In the standby state, the plurality of pixels P of the pixel array 110 are reset in a predetermined order in units of rows (step S314). In this example, a plurality of pixels P are reset in units of rows from the first row to the m-th row in the order of row numbers, and thereafter, the same processing is repeated by returning to the first row.

  In step S312, the control unit 170 determines whether radiation irradiation has been started based on the detection value output from the detection unit 180. Specifically, when the detection value exceeds a predetermined threshold, the control unit 170 determines that radiation irradiation has started, and proceeds to step S316 (this means that the resetting of the pixel P is stopped). To do). On the other hand, until the detected value exceeds a predetermined threshold, the resetting of the pixel P is switched in step S314. That is, in steps S312 and S314, the pixel P of the pixel array 110 is reset in units of rows until it is determined that the radiation has been emitted while monitoring whether the radiation has been started. The period until it is determined in step S312 that radiation irradiation has started is the “reset” period in FIG.

  In step S316, the control unit 170 determines the end of radiation irradiation, and if it is determined that the radiation irradiation has ended, the control unit 170 proceeds to step S320. On the other hand, until it is determined that radiation irradiation has ended, the control unit 170 controls the drive unit 120 so as to continue accumulating charges according to the irradiated radiation in step S318. Here, it may be determined that the irradiation of the radiation has ended when a predetermined time has elapsed since the detection of the irradiation of the radiation, or the irradiation of the radiation has ended when the detection value falls below the reference value. You may judge. Alternatively, it may be determined that the irradiation of radiation has been completed by another method. In step S <b> 320, the control unit 170 controls the reading unit 130 to read a signal from the pixel array 110.

  The period until it is determined in step S316 that the radiation irradiation has ended is the “accumulation” period in FIG. Further, the period after it is determined in step S316 that the irradiation of radiation has been completed is a period of “reading a signal from the reading unit” in FIG.

  FIG. 5 shows in detail a part of FIG. 4 (before and after the timing when the start of radiation irradiation is detected). In this example, it is determined that radiation irradiation has started immediately after the reset of the first Gn row is completed. In FIG. 5, “bias current” is a signal detected by the sensor 182, that is, a bias current flowing through the bias line Bs. The bias current flowing through the bias line Bs can include three components (first, second, and third components). The first component is a component proportional to the electric charge accumulated in the conversion element CV, and includes information indicating the amount of irradiated radiation. In FIG. 5, the first component is described as “radiation component”. The second component is a component called external noise. The external noise is, for example, a component of about 50 to 60 Hz generated by an electromagnetic field generated from a commercial power source, or a component of several Hz to several kHz generated when pressure or an impact is applied to the housing. External noise is generated regardless of whether the switch TT is on (conducting) or off (non-conducting). In FIG. 5, the second component is described as “external noise component”. The third component is switching noise generated by switching on / off the switch TT of the pixel P. In FIG. 5, the third component is not described.

  In FIG. 5, “sampling” indicates sampling of the output of the sensor 182 by the processing unit 184. As information for determining the start of radiation irradiation, the output (sample value) of the sensor 182 sampled by the processing unit 184 may be used. However, when the external noise component cannot be ignored, it is desirable to remove the external noise component.

  The processing unit 184 performs a calculation for removing the external noise component and generates a detection value. For example, the processing unit 184 samples the output of the sensor 182 in a state where the switch TT of the pixel P is turned on. Let S be the sampling value. The processing unit 184 samples the output of the sensor 182 in a state where the switch TT of the pixel P is turned off. The sampling value is N. The processing unit 184 calculates a difference between S and N, and outputs this difference as a detection value. As a result, the external noise component can be removed. However, since the external noise component varies with time, it is desirable to use S and N sampled at the closest possible time. That is, assuming that S sampled at the yth time is S (y), N sampled at the yth time is N (y), and a sample value from which the external noise component is removed is X (y), the processing unit 184 is as follows. Can perform simple operations.

X (y) = S (y)-{N (y) + N (y-1)} / 2 (1)
The method of removing the external noise component as described above can be called CDS (correlated double sampling). The calculation using CDS is not limited to the equation (1). For example, either N (y) or N (y-1) may be used for the calculation of X (y), or non-adjacent sample values such as S (y-1) and N (y-2) May be used.

In addition, when switching noise due to ON / OFF of the switch TT cannot be ignored, it is desirable to determine the start of radiation irradiation based on a sampling value from which the switching noise has been removed. For example, the processing unit 184 samples switching noise in advance, and outputs a value obtained by subtracting the switching noise value from the sampling value at the time of determination as a detection value. In addition, when the amount of switching noise is different for each row, it is known that the amount of switching noise in the same row is highly reproducible. Therefore, a method of removing switching noise by subtracting the sampling value one frame before the same row can be considered. When the number of rows of the pixel array 110 is Y, S before one frame is S (y−Y), and N before one frame is N (y−Y). That is, the processing unit 184 can perform the following calculation: X (y) = [S (y) − {N (y) + N (y−1)} / 2]
− [S (y−Y) − {N (y−Y) + N (y−1−Y)} / 2] (2)
The method of removing switching noise as described above can be referred to as frame correction. The calculation to which the frame correction is applied is not limited to the expression (2). For example, S and N before k frames may be used (k> 1). The calculation may be performed using only S or N.

  As a method for sampling S, a method is preferable in which sampling is performed a plurality of times during a period in which the switch TT is turned on, and the result obtained by adding (averaging) a plurality of sampling values obtained thereby is S. According to this method, high frequency noise can be removed. Similarly, the sampling of N can be performed by sampling a plurality of times during the period when the switch TT is off, and adding (averaging) a plurality of sampling values obtained thereby can be set to N. Such processing can be called addition processing.

  In application of addition processing, depending on the response speed of the sensor 182 (current-voltage conversion circuit), the delay from when the switch TT is switched on / off until the bias current changes may not be negligible. In such a case, it is preferable to provide a delay from the time when the switch TT is switched from the off state to the on state until the time when the sampling of S is started. Further, in the transition time when the switch TT is switched from the OFF state to the ON state, a signal due to radiation irradiation does not sufficiently appear in the output of the sensor 182. Therefore, it is desirable not to perform sampling at such a transition time, or to discard values sampled at such a transition time. The same applies to N sampling.

  In one example, the processing unit 184 may be configured to perform processing in the order of addition processing, CDS, and frame correction, and to determine the start of radiation irradiation based on the integrated value of the sample values subjected to these processing.

  The basic principle of the present invention will be described with reference to FIG. Since the detection performance for detecting that radiation has been irradiated may vary depending on manufacturing variations of the radiation imaging apparatus 100, the detection performance should be adjusted before or after the shipment of the radiation imaging apparatus 100. However, since it is difficult to make the environment for irradiating radiation to the radiation imaging apparatus 100 constant, conventionally, it has been difficult to make the detection performance of the plurality of radiation imaging apparatuses 100 uniform in the adjustment operation.

  FIG. 6 shows a relationship between a signal read from the pixel array 110 after the radiation irradiation or a processing result obtained by processing the signal and a detection value output from the detection unit 180 when the radiation is irradiated. It is shown schematically. Hereinafter, a signal read from the pixel array 110 by the reading unit 130 after irradiation of radiation or a processing result obtained by processing the signal will be referred to as a pixel signal. The pixel signal and the detected value have a proportional relationship, and the proportional relationship is specified by the slope of the straight line (proportional constant). This inclination is called a signal conversion coefficient γ. The signal conversion coefficient γ is changed by adjusting the characteristics of the detection unit 180 by the adjustment unit 187. The characteristic of the detection unit 180 can be adjusted, for example, by changing the characteristic of the sensor 182 (for example, when the sensor 182 is formed of a transimpedance amplifier, the gain of the transimpedance amplifier). Alternatively, the characteristics of the detection unit 180 can be changed by changing the content of processing in the processing unit 184. The change in the processing content in the processing unit 184 can be, for example, a change in all or part of the above-described addition processing, CDS, and frame correction.

  The signal conversion coefficient γ does not depend on the radiation irradiation condition, the thickness of the subject, the distance between the radiation generator 310 and the radiation imaging apparatus 100, the radiation irradiation range, and the like. Therefore, it is easy to keep the signal conversion coefficient γ of the plurality of radiation imaging apparatuses 100 within an allowable range. That is, it is easy to make the radiation detection performance uniform in the plurality of radiation imaging apparatuses 100 through adjustment of the signal conversion coefficient γ.

  In the radiation imaging apparatus 100, the control unit 170 determines that radiation has been emitted when the detection value output from the detection unit 180 exceeds the threshold T. The threshold T can be set so as not to erroneously detect radiation irradiation due to the influence of noise. The minimum value of the pixel signal exceeding the threshold T can be referred to as a detection limit Smin. The relationship between the signal conversion coefficient γ, the threshold value T, and the detection limit Smin is given as follows.

Smin = T / γ (3)
In FIG. 6, Sa is the target detection limit Smin, and Sb is the current detection limit Smin. In other words, γa is the target signal conversion coefficient γ, and γb is the current signal conversion coefficient γ. The signal conversion coefficient γ can be adjusted by changing the characteristics of the sensor 182 (for example, the gain of the transimpedance amplifier when the sensor 182 is formed of a transimpedance amplifier) by the adjustment unit 187. Alternatively, the signal conversion coefficient γ can be adjusted by changing the contents of processing in the processing unit 184 by the adjustment unit 187, for example, by changing all or part of the above-described addition processing, CDS, and frame correction.

  An adjustment process (calibration) by the adjustment unit 187 will be described with reference to FIG. First, in step S712 (measurement process), the adjustment unit 187 measures the signal conversion coefficient γ. Next, in step S714 (determination step), the adjustment unit 187 determines whether the signal conversion coefficient γ measured in step S712 is within the allowable range of the target signal conversion coefficient γa. Then, the adjustment unit 187 ends the adjustment process when the signal conversion coefficient γ measured in step S712 is within the allowable range of the signal conversion coefficient γa, and proceeds to step S716 (adjustment process) when it is not within the allowable range. Adjust with.

  The measurement of the signal conversion coefficient γ in step S712 in FIG. 7 will be described with reference to FIGS. 8, 9, and 10. First, as a comparative example, a normal imaging operation in the radiation imaging apparatus 100 will be described with reference to FIG. The operation shown in FIG. 8 is the same as the operation shown in FIG. 4, but the detection value output from the detection unit 180 is shown in FIG. 8. In a normal imaging operation, a threshold value for determining that radiation irradiation has started is set to T1. The detection value output from the detection unit 180 gradually increases due to the irradiation of radiation. The control unit 170 determines that radiation irradiation has started when the detected value exceeds the threshold value T1, and controls the drive unit 120 to stop the reset operation. Thereby, in the pixel array 110, accumulation of electric charges according to the irradiated radiation is started.

  Next, the operation at the time of measuring the signal conversion coefficient γ in the radiation imaging apparatus 100 will be described with reference to FIG. In addition, although it seems that the irradiation time of radiation differs in FIG. 8, FIG. 9, this does not intend that the irradiation time of radiation differs at the time of normal imaging operation and the time of measurement of the signal conversion coefficient γ, This is because the relationship between the drive signal Gb and the radiation irradiation time is clearly shown in FIG.

  When measuring the signal conversion coefficient γ, the threshold value T1 is changed to the threshold value (adjustment threshold value) T2. T2 is a value larger than T1, and T2 can be set to a value of about 10 to 20 times T1, for example. Changing the T threshold value T1 to T2 larger than that means delaying the timing at which it is determined that radiation irradiation has started. In the example shown in FIG. 9, the irradiation of radiation is stopped when the drive signal Gb is driven to the active level and the pixels P in the corresponding row are reset. Further, since the detection value output from the detection unit 180 changes with a delay with respect to the actual irradiation of radiation, it is determined that the detection value has exceeded the threshold value T2 when the drive signal Gn is driven to the active level, Reset operation is stopped.

  FIG. 10A illustrates an average value for each row of signals read by the reading unit 130 when the driving shown in FIG. 8 (driving at the time of normal imaging) is performed. For example, Ia is an average value of signals of a plurality of pixels constituting the third row to which the drive signal G3 is applied. FIG. 10B illustrates an average value for each row of the signal read by the reading unit 130 when the driving shown in FIG. 9 (driving when measuring the signal conversion coefficient) is performed. . For example, Ib is an average value of signals of a plurality of pixels constituting the b-th row to which the drive signal Gb is applied.

  There is a corresponding time delay after the detection value is calculated by the detection unit 180 after the radiation is irradiated and until the control unit 170 detects the start of the radiation irradiation accordingly. Even during this time delay, the conversion elements of the pixels P in the row where the drive signal G is driven to the active level are reset. As a result, an artifact as exemplified in FIGS. 10A and 10B is generated. The artifact is larger at the time of measuring the signal conversion coefficient γ in which the threshold is set to T2 larger than T1. In the example shown in FIGS. 9 and 10B, the pixels P in the b-th row to the n-th row are reset even after the radiation irradiation is stopped, so the average value of the signals of the pixels in the row is 0. It has become.

  Since the signal conversion coefficient γ has a correlation with the size of the artifact determined by the state of the pixel P when the detected value reaches the threshold value T2, the signal conversion coefficient γ is obtained by quantifying the size of the artifact. Can do. Here, in order to quantify the size of the artifact when the detected value reaches the threshold T2, the artifact amount I (x) of the x-th row is, for example, the value of the row that was not reset during radiation irradiation. It can be defined as the difference between the signal value of the pixel and the signal value of the pixel in the xth row. Further, the signal conversion coefficient γ can be defined as in the following equation. If the signal value can be read from the pixel P in the x-th row when the detected value reaches the threshold value T2, the artifact amount I (x) is not reset during the irradiation with the signal value. This is an amount that can be obtained based on the signal values of the pixels in the row.

γ = T2 / (α × ΣI (x)) (4)
ΣI (x) is an integrated value of I (x) in the time until the detection value output from the detection unit 180 reaches the threshold T2 (in the example of FIG. 10B, from the first row to the n-th row). Integrated value of I (x)). α is a constant indicating the transfer efficiency of the switch TT and can be obtained in advance by simulation or actual measurement. α × ΣI (x) corresponds to the amount of charge lost due to reset at the time of radiation irradiation, and has a correlation with the amount of charge generated in the conversion element CV at the time of radiation irradiation. That is, α × ΣI (x) can be understood as the horizontal axis (pixel signal (evaluation value)) in FIG.

  Further, the calculation of the signal conversion coefficient γ is not limited to the above example. The signal conversion coefficient γ is output from the detection unit 180 and the value of the signal read by the reading unit 130 from the pixel P irradiated with radiation or the result of processing (for example, calculating the average value, calculating the sum). It is only a coefficient indicating the relationship with the detected value. Therefore, the signal conversion coefficient γ can be calculated based on the value of the signal read by the reading unit 130 from the pixel P irradiated with radiation or the processing result obtained by processing the signal and the detection value output from the detection unit 180.

  The radiation imaging apparatus 100 according to the present embodiment cannot read a signal from the pixel P by the reading unit 130 or another circuit during radiation irradiation. Therefore, the signal conversion coefficient γ is calculated using the artifact amount that can be obtained from the signal read by the reading unit 130 after the end of radiation irradiation. However, in a configuration in which a signal can be read from the pixel P at the time of radiation irradiation, the signal conversion coefficient γ can be calculated based on the signal.

  Next, the adjustment of the signal conversion coefficient γ in step S716 in FIG. 7 will be described together with the processes in steps S712 and S714 with reference to FIG. Here, it is assumed that the sensor 182 includes a current-voltage conversion circuit and an AD conversion circuit that converts an output voltage of the current-voltage conversion circuit into a digital signal. FIG. 11 shows a waveform obtained by converting the drive signal Gn in the nth row, the drive signal Gn + 1 in the (n + 1) th row, and the bias current flowing through the bias line Bs into a voltage by the current-voltage conversion circuit. In this example, the switch TT of the pixel P is turned on when the drive signal becomes high level, and turned off when the drive signal becomes low level. The time when the switch TT of the pixel P is turned on is TH, the time when the switch TT is turned off is TL, and the sampling period in which the processing unit 184 samples the output of the sensor 182 (the output of the AD conversion circuit) is shown as TS. In one example, TH = TL = 16 μsec and TS = 1 μsec.

  In one row reset, 32 bias current signals are output from sensor 182. In the y-th reset, it is assumed that the drive signal Gn switches to the high level at time t0. The change of the driving signal Gn to the high level appears as a change in the input to the AD conversion circuit at time t1. The 16 bias current signals supplied to the processing unit 184 from time t1 to time t5 are set as effective values S (y) in the y-th reset. Similarly, the 16 bias current signals supplied to the processing unit 184 from time t5 to time t9 correspond to the case where the switch TT is in a non-conduction state. The processing unit 184 calculates the noise value N (y for the y-th initialization operation) from the total value of the middle 8 sample values output from time t7 to time t8 among the 16 sample values from time t5 to time t9. ).

  The processing unit 184 calculates γ using the bias current signal of n samples after the delay time TD from the time t0. An example is shown in FIG. In one example, when the adjustment unit 187 first executes step S712, the adjustment unit 187 measures γ with the initial value of the delay time TD set to t3 and the number of samples set to n. If it is determined in step S714 that γ is not within the allowable range, the adjustment unit 187 changes the sampling condition (specifically, the delay time TD and / or the number of samples n) in step S716, S712 and S714 are executed again. Changing the delay time TD means changing the sampling timing.

  Such processing is repeated while changing the delay time TD or the number of samples n in step S716 until it is determined in step S714 that γ is within the allowable range. Here, in one example, the delay time TD can be changed by 1 μs from an initial value t3 to t3 + 1 μs, t3-1 μs, t3-2 μs,. Similarly, in one example, the sample number n can be changed from the initial value n to n + 1, n + 2... Or n−1, n−2,.

  The above embodiment is merely an exemplary embodiment of the present invention, and various modifications are possible. For example, in the reset operation, the switches of the pixels in the even rows are sequentially turned on, and then the switches of the pixels in the odd rows are sequentially turned on. In the read operation, the switches of the pixels are sequentially turned on from the first row to the last row. Also good. Alternatively, in the reset operation, the pixel switches are sequentially turned on from the first row to the last row, and in the read operation, the switches of the even-numbered pixels are turned on in turn and then the switches of the odd-numbered pixels are turned on in turn. Also good. In both the reset operation and the read operation, the switches of the pixels in the even rows may be sequentially turned on, and then the switches of the pixels in the odd rows may be sequentially turned on.

  In the reset operation, not only the switches of the pixels in one row at a time but also the switches of the pixels in a plurality of rows may be simultaneously turned on. For example, after resetting all even-numbered pixels while simultaneously conducting a plurality of even-numbered pixels, all odd-numbered pixels may be reset while simultaneously conducting a plurality of odd-numbered pixels. The reset operation does not need to be performed in ascending order or descending order of the row numbers, and rows that are continuously reset may be rows that are not adjacent to each other.

  The functions in the above embodiments can also be realized by a computer executing a program. Also, means for supplying a program to a computer, for example, a computer-readable recording medium such as a CD-ROM recording such a program, or a transmission medium such as the Internet for transmitting such a program is also applied as an embodiment of the present invention. Can do. The above program can also be applied as an embodiment of the present invention. The above program, recording medium, transmission medium, and program product are included in the scope of the present invention.

110: Pixel array, P: Pixel, 180: Detection unit, 187: Adjustment unit, 170: Control unit

Claims (10)

A pixel array in which a plurality of pixels for capturing an image of radiation is arranged;
A detection unit that outputs a detection value according to radiation irradiation;
An adjustment unit that adjusts the characteristics of the detection unit such that a relationship between a signal output from the pixel array or a processing result obtained by processing the signal and the detection value is a predetermined relationship;
A control unit for detecting that radiation has been irradiated in response to the detection value exceeding a threshold;
A radiation imaging apparatus comprising: A power source for supplying a voltage to the pixel array via a power line;
The detection unit outputs a value corresponding to a current flowing through the power line as the detection value;
The radiation imaging apparatus according to claim 1. Each pixel of the pixel array includes a conversion element that converts radiation into an electrical signal, and a switch that connects the conversion element and a signal line according to a drive signal,
The detection unit includes a sensor that outputs an electrical signal according to radiation irradiation, and a processing unit that processes the electrical signal output from the sensor and generates the detection value.
The processing unit obtains the detection value by correlated double sampling of a signal output from the sensor while the switch is on and a signal output from the sensor while the switch is off. ,
The radiation imaging apparatus according to claim 2. The processing unit samples one or more signals output from the sensor while the switch is on, and one or more signals output from the sensor when the switch is off. Sample
The adjustment unit is configured to adjust a sampling condition by the processing unit,
The radiation imaging apparatus according to claim 3. The sampling condition includes a condition for at least one of sampling timing and number.
The radiation imaging apparatus according to claim 4. The adjustment unit uses a signal output from the pixel array in response to radiation applied in a state where an adjustment threshold is set instead of the threshold, or a processing result obtained by processing the signal and the adjustment threshold. Adjusting the characteristics of the detection unit based on
The radiation imaging apparatus according to claim 1, wherein:   In a state where the adjustment threshold is set, a reset operation for resetting the plurality of pixels in a predetermined order is repeated until the detection value exceeds the adjustment threshold, and the detection value exceeds the adjustment threshold. The radiation imaging apparatus according to claim 6, wherein the reset operation is stopped in response to the signal, and a signal is read from the pixel array after the radiation irradiation is completed. In the signal output from the pixel array, artifacts are generated due to pixels that are reset after radiation irradiation is started,
The adjustment unit adjusts the characteristics of the detection unit while evaluating the relationship based on the artifact and the adjustment threshold.
The radiation imaging apparatus according to claim 7. A pixel array in which a plurality of pixels for capturing an image of radiation is arranged, a detection unit that outputs a detection value corresponding to the irradiation of radiation, and irradiation of radiation in response to the detection value exceeding a threshold value An adjustment method for adjusting a radiation imaging apparatus, comprising:
A measurement step for obtaining a relationship between a signal output from the pixel array or a processing result obtained by processing the signal and the detection value;
An adjustment step of adjusting the characteristics of the detection unit so that the relationship becomes a predetermined relationship when the relationship deviates from an allowable range;
A method for adjusting a radiation imaging apparatus, comprising: The measurement step includes
Setting the threshold value of the radiation imaging apparatus to an adjustment threshold value greater than the threshold value;
Irradiating the radiation imaging apparatus with the adjustment threshold set, and storing charges in the pixel array;
Obtaining the relationship based on a signal output from the pixel array and the adjustment threshold,
The radiation imaging apparatus in which the adjustment threshold value is set repeats a reset operation for resetting the plurality of pixels in a predetermined order until the detection value exceeds the adjustment threshold value, and the detection value satisfies the adjustment threshold value. Stops the reset operation in response to exceeding, operates to read out signals from the pixel array after the end of radiation irradiation,
In the signal output from the pixel array, artifacts are generated due to pixels that are reset after radiation irradiation is started,
In the step of obtaining the relationship, the relationship is obtained based on the artifact.
The method of adjusting a radiation imaging apparatus according to claim 9.