JP2019193247A - Imaging system, control method of the same, and control program - Google Patents

Abstract

To make it possible to perform correction easily.SOLUTION: The imaging system includes: an input voltage setting unit (101) for applying an input voltage to an Amp transistor (253); a setting output value acquisition unit (102) for acquiring a corresponding output for each pixel (211); and a calibration characteristic deriving unit (103) for deriving calibration characteristics used for calibration on the basis of reverse characteristics of input output characteristics by deriving input and output characteristics that indicate a relationship between an input voltage and an output.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an imaging system including a sensor element that generates an electrical signal based on a dose of incident radiation.

  The sensor element that outputs an electrical signal according to incident radiation, for example, the dose of X-rays, is a direct conversion type that converts X-rays directly into electrical signals, and photoelectric conversion after converting X-rays into light by a scintillator There is an indirect conversion type that converts an electric signal by an element.

  A panel for X-ray imaging is configured by providing the sensor element for each pixel and arranging the pixels on a substrate (panel) in a two-dimensional matrix. In such a panel, a thin film transistor element (TFT (Thin Film Transistor) element) is used to control each pixel. In both the direct conversion type and the indirect conversion type, an electric signal (charge) generated according to the X-ray dose is accumulated in the capacitance in each pixel.

  A device that transfers this accumulated capacitance to an amplifier outside the panel via a TFT element is called a passive pixel type. A capacitor that amplifies the accumulated capacitance by using a TFT element as an amplifying element and transmits it to an external circuit is called an active pixel type. Since the active pixel type can be amplified within the pixel, a large signal can be obtained for the same dose as compared with the passive pixel type. For this reason, even if it is a low irradiation amount, there exists an advantage that an appropriate signal is obtained.

  When detecting X-rays using sensor elements, a plurality of sensor elements are provided in a two-dimensional matrix on the panel, so it is necessary to correct variations in output voltage due to differences in sensor element characteristics. There is (calibration). In particular, in the active pixel type, an amplifying unit is provided for each pixel, and the amplifying unit has high nonlinearity and has variations in gain and offset. Therefore, correction is necessary.

  Therefore, Patent Document 1 describes a calibration method that uses a correction value obtained from a density value obtained by photoelectric scanning two density reference plates having different densities and a previously known density value. ing.

  Patent Document 2 describes a method of correcting variations between bits of an analog signal output from an image sensor by previously reading a reference surface a plurality of times and obtaining a correction value for each bit from the result. ing.

Japanese Patent Laid-Open No. 61-53868 (published March 17, 1986) JP 57-119565 A (published July 26, 1982)

  However, the conventional techniques as described above have the following problems. In Patent Document 1, photoelectric scanning is performed in advance on a density reference plate whose density value is known. That is, the density value is obtained by irradiating the density reference plate with X-rays before imaging. However, X-ray irradiation needs to be performed under strict control and cannot be performed easily.

  Patent Document 2 is the same in that it is read in advance, and has the same adverse effect as Patent Document 1.

  One embodiment of the present invention has been made in view of the above problems, and an object thereof is to realize an imaging system that can easily perform correction (calibration).

  In order to solve the above problems, an imaging system according to one embodiment of the present invention includes a pixel including a sensor element that generates an electrical signal based on a dose of incident radiation, and an amplifier transistor that amplifies the electrical signal. A plurality of imaging systems, a voltage application unit that applies an input voltage to the amplifier transistor at a predetermined interval, an acquisition unit that acquires an output corresponding to the input voltage for each pixel, and for each pixel, A calibration characteristic deriving unit for deriving an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage, and deriving a calibration characteristic used for calibration based on an inverse characteristic of the input output characteristic; It is equipped with.

  In order to solve the above problems, an imaging system control method according to an aspect of the present invention includes a sensor element that generates an electrical signal based on a dose of incident radiation, and an amplifier transistor that amplifies the electrical signal. An imaging system control method comprising a plurality of pixels including a voltage application step of applying an input voltage to the amplifier transistor at a predetermined interval, and an acquisition step of acquiring an output corresponding to the input voltage for each pixel, Calibration for deriving an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage for each pixel, and deriving a calibration characteristic to be used for calibration based on an inverse characteristic of the input output characteristic And a process characteristic deriving step.

  According to one embodiment of the present invention, calibration characteristics corresponding to a pixel can be derived without causing radiation to enter the sensor element. Therefore, it is possible to easily calibrate the output of the pixel.

It is a figure which shows the outline | summary of the imaging system which concerns on this embodiment. It is a figure which shows the circuit structure in the pixel contained in an imaging sensor main body. It is a block diagram which shows the principal part structure of a calibration apparatus. (A) is a graph which shows the relationship between the input voltage which the input voltage setting part set, and the output which the output value acquisition part for setting acquired, (b) is a graph which shows the reverse characteristic of the graph shown to (a) It is. It is a flowchart which shows the flow of the process in which a calibration setting part derives | leads-out a calibration characteristic. It is a circuit diagram for demonstrating the process which derives | leads-out a calibration characteristic. It is a figure for demonstrating the example which thins out a part and derives the reverse characteristic. It is a figure for demonstrating the example which expresses an inverse characteristic with the type | formula containing three parameters. It is a circuit diagram for giving a different reference potential for every pixel. It is a figure for demonstrating the reason that the characteristic of a pixel can be used effectively by varying a reference potential for every pixel, and (a) is a figure showing an example at the time of making a reference potential of a pixel into a specific potential. FIG. 8B is a diagram illustrating an example in which the reference potential is changed for each pixel 211. It is a figure for demonstrating the derivation | leading-out example of the input-output characteristic which concerns on other embodiment. It is a figure for demonstrating the derivation | leading-out example of the input-output characteristic which concerns on other embodiment. (A) is a figure which shows the change of the input-output characteristic when a threshold voltage fluctuates, (b) is a figure which shows the reverse characteristic. It is a figure for demonstrating the reason which is not influenced by the fluctuation | variation of a threshold voltage by using a difference value.

Embodiment 1
〔Overview〕
Hereinafter, an embodiment of the present invention will be described in detail. First, an overview of the imaging system 1 according to the present embodiment will be described with reference to FIG. FIG. 1 is a diagram showing an outline of the imaging system 1. As shown in FIG. 1, the imaging system 1 includes a calibration device 10, an imaging sensor 20, and a display device 30. In the imaging system 1, calibration characteristics are derived by the calibration device 10, and an image captured by the imaging sensor 20 is calibrated and displayed on the display device 30 using the derived calibration characteristics. The calibration characteristic is a characteristic used for calibration (correction), and is used to correct a relative difference between the pixels 211 included in the image sensor 20, such as a correction formula, a correction value, and a correction coefficient. is there.

  Although details will be described later, the calibration device 10 acquires the characteristics of the pixels 211 by applying a voltage to each pixel 211 included in the imaging sensor body 21 of the imaging sensor 20, and calibration characteristics based on the acquired characteristics. Is derived.

Thereby, the calibration apparatus 10 can derive | lead-out a calibration characteristic, without imaging in the imaging sensor 20 previously.
Therefore, unlike the prior art, it is not necessary to perform various preparations, settings, management, and the like in order to perform X-ray imaging in advance, and calibration characteristics can be easily derived.

  In the imaging system 1, the captured image can be calibrated using the calibration characteristics. Therefore, the imaging system 1 can easily perform calibration.

  The imaging sensor 20 includes an imaging sensor main body 21, a voltage generation unit 22, a row selection unit 23, and a reading unit 24.

  The imaging sensor main body 21 is a main body of an imaging sensor including a plurality of pixels 211 arranged in a two-dimensional matrix.

  The voltage generation unit 22 generates a voltage to be applied to the pixel 211 and applies the voltage generated for each column.

  The row selection unit 23 selects a row to which the voltage generated by the voltage generation unit 22 is applied. Since the voltage generation unit 22 applies each column, the voltage generated by the voltage generation unit 22 can be applied to each pixel 211 by selecting a row by the row selection unit 23.

  The reading unit 24 reads the output from the pixel 211 and transmits it to the calibration device 10. Note that the reading unit 24 includes an AFE 241 described later.

[In-pixel circuit]
Next, a circuit configuration in the pixel 211 will be described with reference to FIG. FIG. 2 is a diagram illustrating a circuit configuration in the pixel 211. As shown in FIG. 2, the imaging sensor main body 21 covers an array in which a plurality of pixels 211 are arranged in a two-dimensional matrix (an example of 512 vertical × 512 horizontal in FIG. 2) and the front of the array. It consists of a scintillator (not shown). The scintillator has an X-ray light conversion function of receiving X-rays and converting the received X-rays into light.

  As shown in FIG. 2, the in-pixel circuit 250 includes a calibration / reset switch 251, a photodiode (sensor element) 252, an Amp transistor (amplifier transistor) 253, and a reed switch 254.

  The calibration / reset switch 251 is a switch for applying a calibration setting voltage or a reset voltage to the gate electrode of the Amp transistor 253. Here, the calibration setting voltage is a voltage applied to derive calibration characteristics. The reset voltage is a voltage for resetting the charge generated by the photodiode 252.

  The output of the photodiode 252 is connected to the gate electrode of the Amp transistor 253. Thus, when the photodiode 252 receives light and radiation (electrical signal) is generated due to incidence of radiation, the voltage of the gate electrode of the Amp transistor 253 connected to the photodiode 252 changes. The Amp transistor 253 outputs a voltage change of the gate electrode as a current change between the drain and the source.

  The Amp transistor 253 is a transistor that amplifies the electric signal.

  The reed switch 254 is a switch for outputting a current between the drain and source of the Amp transistor 253 to the outside of the pixel 211, and is controlled by the reading unit 24.

  In addition, the current output from the pixel 211 is amplified and A / D converted by an AFE (analog front end) 241 of the reading unit 24 and output to the calibration device 10.

[Configuration of Calibration Device 10]
Next, the calibration apparatus 10 will be described with reference to FIG. FIG. 3 is a block diagram showing a main configuration of the calibration apparatus 10. As illustrated in FIG. 3, the calibration apparatus 10 includes a calibration setting unit 100, an imaging output value acquisition unit 110, a calibration unit 120, and an output unit 130.

  The calibration setting unit 100 sets the calibration characteristics of each pixel 211 of the image sensor 20. The calibration characteristic is a characteristic used for calibration. An appropriate value can be output by calibrating the output from the image sensor 20 using the calibration characteristics.

  More specifically, the calibration setting unit 100 includes an input voltage setting unit (voltage application unit) 101, a setting output value acquisition unit (acquisition unit) 102, a calibration characteristic derivation unit 103, and calibration characteristic derivation data 104. including.

  The input voltage setting unit 101 sets an input voltage for deriving calibration characteristics, instructs the voltage generation unit 22 to apply it to the pixel 211. More specifically, the input voltage setting unit 101 sequentially sets input voltages according to a predetermined condition and applies them to the pixels 211 each time. Information indicating the set input voltage is stored in the calibration characteristic derivation data 104. The flow of the input voltage setting process will be described later.

  The setting output value acquisition unit 102 acquires, for each pixel 211, an output when the input voltage set by the input voltage setting unit 101 is applied to the pixel 211, and stores it in the calibration characteristic derivation data 104.

  The calibration characteristic deriving unit 103 is based on the relationship between the input voltage set by the input voltage setting unit 101 and the output acquired by the setting output value acquisition unit 102, which is stored in the calibration characteristic deriving data 104. Is derived. A method of deriving an example of calibration characteristics will be described with reference to FIG. FIG. 4A is a graph showing the relationship between the input voltage set by the input voltage setting unit 101 and the output acquired by the setting output value acquisition unit 102. FIG. 4 shows an example of one pixel 211. The horizontal axis of the graph shown in FIG. 4A is input (corresponding to voltage), and the vertical axis is output (code value). Here, the output of a certain pixel 211 when the input is changed from 0 to 128 is shown. In the example shown in FIG. 4A, the output is 0 until the input exceeds 0 to 32, and then the output changes from 0 to 65536 while the input is about 70, and the input is changed to 70. Beyond that, the output remains 65536.

  The calibration characteristic deriving unit 103 calculates an input-output characteristic (input output) as shown in FIG. 4A from the input voltage set by the input voltage setting unit 101 and the output acquired by the setting output value acquisition unit 102. When the characteristic) is obtained, the inverse characteristic of the input-output characteristic is derived as the calibration characteristic. An example of the derived calibration characteristics is shown in FIG. In FIG. 4B, the horizontal axis is output (code value), and the vertical axis is input (corresponding to voltage). In this way, by using the inverse characteristic of the input-output characteristic as the calibration characteristic, the input to the pixel 211 can be derived from the value output from the pixel 211.

  As a result, even if the characteristics of each pixel 211 are different, calibration characteristics corresponding to the pixel 211 can be derived, and calibration can be performed appropriately. For example, assume that there are three pixels, a pixel 211A, a pixel 211B, and a pixel 211C, and their input-output characteristics are as follows.

  Here, when the output from the pixel 211A is 3000, the output from the pixel 211B is 3300, and the output from the pixel 211C is 2940, the output values are different for the three pixels. However, if calibration is performed using the calibration characteristics, it can be seen that the input of all three pixels was V2, that is, the same light amount (dose) was input to the three pixels. As described above, the calibration can be easily and appropriately performed by using the inverse characteristic of the input-output characteristic as the calibration characteristic.

  The imaging output value acquisition unit 110 acquires an output when imaging processing is performed in the imaging sensor 20. The acquired output is transmitted to the calibration unit 120.

  The calibration unit 120 calibrates the output transmitted from the imaging output value acquisition unit 110 using the calibration characteristics set by the calibration setting unit 100, and outputs the value after calibration to the output unit 130. Send.

  The output unit 130 transmits the value calibrated by the calibration unit 120 to, for example, the external display device 30. And in the display apparatus 30, the imaging result by the imaging sensor 20 is displayed. The display device 30 may be realized by a device external to the calibration device 10 or may be included in the calibration device 10.

[Process flow]
Next, with reference to FIGS. 5 and 6, the flow of processing in which the calibration setting unit 100 derives the calibration characteristics will be described. FIG. 5 is a flowchart showing a flow of processing in which the calibration setting unit 100 derives calibration characteristics. FIG. 6 is a circuit diagram for explaining processing for deriving calibration characteristics. Hereinafter, an example in which the input voltage is changed from V0 to V127 will be described. However, the input voltage to be applied is not limited to 128 levels from V0 to V127. Further, the input voltage to be applied may be in ascending order from V0 to V127, or may be in descending order from V127 to V0. Moreover, it is not necessary to change monotonously. The range of the input voltage to be changed may be set over a range that can change during actual imaging.

  As shown in FIG. 5, first, the input voltage setting unit 101 sets an input voltage and applies it to the pixel 211 (S101, voltage application step). In the circuit diagram, all the calibration / reset switches 251 shown in FIG. 6 are connected to provide an input voltage V0 for deriving calibration characteristics.

  Next, the setting output value acquisition unit 102 acquires an output corresponding to the input voltage for each pixel 211 (S102, acquisition step). That is, in the state where the input voltage V0 is applied, the reading unit 24 sequentially reads the outputs of all the pixels. And if the output of all the pixels 211 arrange | positioned at the imaging sensor main body 21 is acquired, the calibration setting part 100 will determine whether all the voltages which should be applied were applied (S103). When all the voltages to be applied are not applied (NO in S103), the input voltage setting unit 101 changes the input voltage (S104). That is, the input voltage is changed from V0 to V1. Then, the process returns to step S101.

  On the other hand, if all the voltages to be applied have been applied (YES in S103), that is, if the input voltage has been changed to V127, the calibration characteristic deriving unit 103 determines the calibration characteristics from the input-output characteristics. (S105, calibration characteristic deriving step).

[Embodiment 2]
Another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  In the above-described embodiment, the reverse characteristics are derived using all points obtained in the input-output characteristics (points indicating the relationship between the input voltage and the output). In the present embodiment, not all the points in the input-output characteristics are used, but the inverse characteristics are derived by thinning out a part. For example, a reverse characteristic is derived by thinning out part of a section where the relationship between the input voltage and the output is substantially linear. This will be described with reference to FIG. FIG. 7 is a diagram for explaining an example in which a reverse characteristic is derived by thinning a part. As shown in FIG. 7, the reverse characteristics are derived by thinning out part of the section (from point B to point G in FIG. 7) where the relationship between the input voltage and the output is approximately linear. For the thinned section, the input voltage is derived from points before and after the obtained output (a point corresponding to an output larger than the obtained output and a point corresponding to a smaller output). For example, in the example shown in FIG. 7, if the obtained output is 49152, the corresponding input voltage is derived by performing linear interpolation using point C and point D. If the obtained output is 16384, the corresponding input voltage is derived by performing linear interpolation using the points E and F.

  Thereby, the calculation amount used for the inverse transformation can be reduced.

[Embodiment 3]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

In the present embodiment, the calibration characteristic deriving unit 103 expresses the calibration characteristic by an expression (α × X ^β + γ) including three parameters (α, β, γ) as an inverse characteristic of the input-output characteristic. Here, X indicates the obtained output. For example, in the example shown in FIG. 8, α = 0.059839149, β = 0.54761691, and γ = 35.395547187. Thereby, the corresponding input voltage can be obtained only by substituting the output obtained for X into the above equation. The three parameters can be derived from the graph showing the inverse characteristics of the input-output characteristics using the prior art, and the description thereof is omitted.

[Embodiment 4]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  In the present embodiment, a reset voltage determination unit (not shown) included in the calibration setting unit 100 gives a different reference potential for each pixel 211. Thereby, the reference potential corresponding to the characteristic of each pixel 211 can be obtained, and the characteristic of each pixel 211 can be used effectively. This will be described in detail with reference to FIGS. FIG. 9 is a circuit diagram for applying a different reference potential for each pixel 211. FIG. 10 is a diagram for explaining the reason why the characteristics of the pixel 211 can be effectively used by making the reference potential different for each pixel 211. FIG. 10A shows the reference potential of the pixel 211 as a specific potential. (B) shows an example in which the reference potential is varied for each pixel 211.

  As shown in FIG. 9, in this embodiment, the calibration / reset voltage line 910 is shared only in the direction orthogonal to the calibration / reset control line 911. Thereby, the calibration / reset voltage line 910 can apply the same reference potential to the pixels 211 in the row direction. The calibration / reset control line 911 can control the calibration / reset switch 251 for the pixels 211 in the column direction. Thereby, a different reference potential can be applied to each pixel 211.

  FIG. 10A shows an example of input-output characteristics of three pixels 211. When there are three pixels 211 having different characteristics as shown in FIG. 10A, when the reference potential is a specific potential (for example, 70), the pixel 211 indicated by the characteristic 1002 has a portion where the characteristics change. Although it can be used effectively, in the pixel 211 indicated by the characteristic 1001 and the characteristic 1003, a portion where the characteristic changes can hardly be used. On the other hand, as shown in FIG. 10B, the reference potential is made different for each pixel 211, for example, the reference potential of the pixel 211 indicated by the characteristic 1001 is 90, and the reference potential of the pixel 211 indicated by the characteristic 1002 If the potential is 70 and the reference potential of the pixel 211 indicated by the characteristic 1003 is 48, a portion where the characteristic changes in each pixel 211 can be used effectively.

  In addition, since the reference potential differs for each pixel 211, a value corresponding to a different reference potential is output for each pixel 211 as it is. For this, if the difference between the obtained output and the output in the case of the overall reference potential at the time of measurement (for example, 70) is derived and multiplied by an appropriate gain, the data of the desired number of bits is obtained. Good.

[Embodiment 5]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  In the present embodiment, the calibration setting unit 100 determines whether the derivation of the calibration characteristic is necessary again after a lapse of a predetermined period after deriving the calibration characteristic. Is derived.

  Specifically, after deriving the calibration characteristics, after a predetermined period has elapsed, the input voltage setting unit 101 sets a specific input voltage (for example, V60), instructs the voltage generation unit 22 to apply it to the pixel 211. . Then, the setting output value acquisition unit 102 acquires an output when the input voltage is applied to the pixel 211. Then, the calibration setting unit 100 compares the output when the input voltage V60 was applied last time with the current output. If the difference exceeds a predetermined value, the calibration setting unit 100 derives the calibration characteristics again. The method for deriving the calibration characteristics is the same as that described above.

  As a result, the gain and offset of the pixel 211 change due to changes in the surrounding environment such as temperature, humidity, and operating time, and the calibration characteristics can be appropriately derived even when the characteristics change.

  Note that the determination as to whether or not to perform calibration again may be performed using a plurality of input voltages. Further, the configuration may be such that the user is notified that it is better to perform the calibration again instead of performing the calibration again automatically.

[Embodiment 6]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  In the present embodiment, the calibration setting unit 100 derives the input-output characteristics by using a predetermined number of measurement points and interpolating between the measurement points without using all the points in the input-output characteristics. Further, the calibration setting unit 100 derives the input-output characteristic using an interpolation formula that varies depending on the section of the measurement point.

Specifically, this will be described with reference to FIG. FIG. 11 is a diagram for explaining an example of deriving input-output characteristics using different interpolation formulas depending on sections. In FIG. 11, the vertical axis represents the input voltage Vg (V), and the horizontal axis represents the output current Id (μA) corresponding to the input voltage Vg. In the present embodiment, the calibration setting unit 100 determines that the input voltage Vg is an interval A (V _RESET >Vg> V _border , I _RESET > Id from the reset voltage V _RESET (point 1101) to the threshold V _border (point 1102). > I _border ) (first interval), the measurement interval is shortened and interpolation is performed by linear (primary) interpolation. Also, for the section B (Vg <V _border , Id <I _border ) (second section) smaller than the threshold V _border (point 1102), the measurement interval is increased and nonlinear (second-order or higher order polynomial, exponential function, etc.) ) Interpolate by interpolation. For example, in section A, interpolation is performed using Vg = a · Id + b (a and b are coefficients), and in section B, Vg = f _nl (Id) (f _nl () is a nonlinear function (second-order or higher polynomial, exponent) Function))). Examples of second-order or higher polynomials include Lagrange interpolation, Spline interpolation, Hermite interpolation, and the like.

  In section A where the amount of change from the reset voltage is small, the S / N ratio (signal-to-noise ratio) is small, so the derivation accuracy is improved by shortening the measurement interval and performing linear interpolation. . On the other hand, in section B where the amount of change from the reset voltage is large, the S / N ratio is high, so the measurement interval is increased. Thereby, the measurement time and the data amount can be reduced. When the measurement interval is large, the relationship between the input voltage and the output current is not linear, and nonlinear interpolation is performed.

  In general, input-output characteristics (and vice versa) vary with time. Therefore, in order to use an appropriate characteristic, it is desirable to derive the input-output characteristic using a value measured immediately before imaging as much as possible. However, if there are many measurement locations, it takes time to measure and it is difficult to derive appropriate characteristics.

  In the present embodiment, it is possible to derive appropriate input-output characteristics with a small number of measurement points, and therefore appropriate input-output characteristics can be derived even immediately before imaging.

  In FIG. 11, only two points 1101 and 1102 are measured in the section A where linear interpolation is performed, but the number of points measured in the section where linear interpolation is performed is not limited to two points. Three or more points may be used.

[Embodiment 7]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  In the above-described sixth embodiment, the input-output characteristics are derived by changing the interpolation formula used according to the section. In this embodiment, the input-output characteristic is derived using the same interpolation formula as in the sixth embodiment, but the measurement frequency is varied depending on the section.

  Specifically, the calibration setting unit 100 according to the present embodiment performs measurement at a higher frequency for the section A than for the section B, and derives input-output characteristics. For example, the measurement points included in the section A are always measured immediately before and after imaging, and the measurement points included in the section B are measured after a predetermined time has elapsed since the previous measurement.

  As described above, since the S / N ratio is low in the section A, the influence of the error between the time point when the input-output characteristic is derived and the time point of imaging is large as time passes. Therefore, for section A, measurement is performed immediately before imaging as much as possible to derive input-output characteristics. On the other hand, in the section B, since the S / N ratio is high, the influence of the error between the time point when the input-output characteristics are derived and the time point of imaging is smaller in the section A.

[Embodiment 8]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

In the above-described sixth embodiment, the input-output characteristics are derived using the relationship between the input voltage and the output current at the measurement point. In the present embodiment, input-output characteristics are derived based on the amount of change from the reset voltage V _RESET and the reset current I _RESET that is the current value at that time, not the relationship between the input voltage and the output current at the measurement point.

  This will be specifically described with reference to FIG. FIG. 12 is a diagram for explaining an example in which the input-output characteristic is derived based on the change amount. In FIG. 12, the vertical axis represents the difference from the reset voltage of the input voltage, and the horizontal axis represents the difference from the reset current of the output current.

In the present embodiment, the calibration setting unit 100 performs linear interpolation on the section A ′ (ΔVg <V _border , ΔId <I _border ) from the origin corresponding to the section A to the point 1201, and For the corresponding section B ′ (ΔVg> V _border , ΔId> I _border ) where the input voltage difference is larger than the point 1201, nonlinear interpolation is performed to derive the input-output characteristics. For example, for section A ′, interpolation is performed using ΔVg = a ′ · ΔId + b ′ (a ′ and b ′ are coefficients), and for section B ′, ΔVg = f ′ _nl (ΔId) (f ′ _nl () is , Interpolation is performed by a non-linear function (second-order or higher polynomial, exponential function, etc.).

  As described above, the input-output characteristic is derived in order to derive the calibration characteristic. The derived calibration characteristics are used to derive the X-ray dose corresponding to the input voltage from the output current. Therefore, finally, it is only necessary to derive the X-ray dose corresponding to the output current. In order to derive the X-ray irradiation amount, the values of the input voltage and the output current are not necessarily required, and it is only necessary to know the amount of change since the reset (no X-ray irradiation). Therefore, the input-output characteristic may be derived using the characteristic of the amount of change from the reset time as in the present embodiment.

[Embodiment 9]
Still another embodiment of the present invention will be described below. For convenience of explanation, members having the same functions as those described in the above embodiment are given the same reference numerals, and the description thereof will not be repeated.

  Generally, it is known that the threshold voltage of a TFT (thin film transistor) or MOSFET (MOS type field effect transistor) changes with time. Since the input-output characteristics change before and after the threshold voltage fluctuates, the calibration characteristics derived using the input-output characteristics before the threshold voltage fluctuates are the threshold voltage changes. Later it would be inappropriate.

  With reference to FIG. 13, a change in input-output characteristics due to a change in threshold voltage will be described. FIG. 13A is a diagram showing changes in input-output characteristics when the threshold voltage fluctuates, and FIG. 13B is a diagram showing the inverse characteristics.

  As shown in FIG. 13A, for example, when the threshold voltage increases by 0.5 V, the input-output characteristic after the threshold fluctuation is the input voltage in the input-output characteristic before the threshold fluctuation. The characteristics are as if an offset of 0.5V was applied to the current. Further, as shown in FIG. 13B, the inverse characteristic of the input-output characteristic is a characteristic in which an offset of 0.5 V is applied to the input voltage in the output-input characteristic before the threshold fluctuation.

  Therefore, when the calibration value (estimated value of the input corresponding to the output value) is derived using the calibration characteristic derived using the input-output characteristic before the threshold voltage fluctuates, the actual input value and The deviation will occur.

  Therefore, the calibration unit 120 according to the present embodiment sets the output value as the difference value between the calibration values before and after the X-ray irradiation, not the calibration value. Thereby, the influence by the fluctuation | variation of the threshold voltage mentioned above can be excluded. Hereinafter, the reason why the influence due to the fluctuation of the threshold voltage can be eliminated by using the difference value will be described.

  As described above, the influence on the input-output characteristics due to the fluctuation of the threshold voltage is only an offset in the input voltage. Therefore, the difference between the calibration value for the output before X-ray irradiation derived using the calibration characteristic derived before the threshold voltage fluctuation and the calibration value for the output after X-ray irradiation is the threshold voltage fluctuation. The difference between the calibration value for the output before X-ray irradiation derived using the calibration characteristics derived later and the calibration value for the output after X-ray irradiation, that is, the difference when there is no deviation from the actual value It becomes.

  The difference between the calibration values before and after the X-ray irradiation corresponds to the received light amount (dose). Therefore, the calibration unit 120 uses the difference between the calibration values before and after the X-ray irradiation as data to be transmitted to the output unit. As a result, an accurate value can be used as output data without being affected by fluctuations in the threshold voltage.

  A specific description will be given with reference to FIG. FIG. 14 is a diagram for explaining the reason why the difference value is not affected by the fluctuation of the threshold voltage. In FIG. 14, the calibration characteristic derived based on the input-output characteristic before the threshold fluctuation is clearly indicated as “calibration characteristic”, and the actual characteristic is indicated as “true output-input characteristic”. Yes.

The output value (current) before X-ray irradiation is I _pre and the output value (current) after X-ray irradiation is I _post . When the threshold voltage fluctuates, the calibration value V _{calib_pre} for the I _pre and the actual input value V _{true_pre} are different, and the calibration value V _{calib_post} for the I _post and the actual input value V _{true_post} are also different. However, the difference between the calibration values before and after the X-ray irradiation ( _{Vcalib_pre} - _{Vcalib_post} ) and the difference between the actual input values ( _{Vtrue_pre} - _{Vtrue_post} ) are equal. Therefore, by using the difference value, it is possible to eliminate the influence of fluctuations in the threshold voltage.

[Example of software implementation]
The control block (particularly the calibration setting unit 100 (input voltage setting unit 101, setting output value acquisition unit 102, calibration characteristic deriving unit 103)) of the calibration apparatus 10 is formed on an integrated circuit (IC chip) or the like. It may be realized by a logic circuit (hardware) or may be realized by software.

  In the latter case, the calibration apparatus 10 includes a computer that executes instructions of a program that is software for realizing each function. The computer includes, for example, at least one processor (control device) and at least one computer-readable recording medium storing the program. In the computer, the processor reads the program from the recording medium and executes the program, thereby achieving the object of the present invention. As the processor, for example, a CPU (Central Processing Unit) can be used. As the recording medium, a “non-temporary tangible medium” such as a ROM (Read Only Memory), a tape, a disk, a card, a semiconductor memory, a programmable logic circuit, or the like can be used. Further, a RAM (Random Access Memory) for expanding the program may be further provided. The program may be supplied to the computer via an arbitrary transmission medium (such as a communication network or a broadcast wave) that can transmit the program. Note that one embodiment of the present invention can also be realized in the form of a data signal embedded in a carrier wave, in which the program is embodied by electronic transmission.

[Summary]
An imaging system (1) according to aspect 1 of the present invention includes a sensor element (photodiode 252) that generates an electrical signal based on a dose of incident radiation, and an amplifier transistor (Amp transistor 253) that amplifies the electrical signal. A voltage application unit (input voltage setting unit 101) for applying an input voltage to the amplifier transistor at a predetermined interval, and an output corresponding to the input voltage. An obtaining unit (setting output value obtaining unit 102) that obtains each pixel, and an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage is derived for each pixel, and the input output Calibration characteristic deriving unit for deriving calibration characteristics used for calibration based on the inverse characteristics And 103), and a.

  According to the above configuration, for each pixel, an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage is derived, and calibration is used for calibration based on an inverse characteristic of the input output characteristic. To derive the application characteristics. Thereby, the calibration characteristic corresponding to the pixel can be derived without causing radiation to enter the sensor element. Therefore, it is possible to easily calibrate the output of the pixel.

  In the imaging system according to aspect 2 of the present invention, in the aspect 1, the calibration characteristic deriving unit is based on an inverse characteristic of the input output characteristic obtained by thinning out a part of the input voltage applied by the voltage application unit. The calibration characteristic may be derived.

  According to the above configuration, since the calibration characteristic is derived based on the inverse characteristic of the input output characteristic obtained by thinning out a part of the input voltage, the amount of processing required for deriving the inverse characteristic can be reduced.

  In the imaging system according to aspect 3 of the present invention, in the aspect 1, the calibration characteristic deriving unit derives the calibration characteristic by expressing the inverse characteristic by an expression including three parameters. Also good.

  According to the above configuration, the calibration characteristic can be derived from an equation including three parameters.

  In the imaging system according to aspect 4 of the present invention, in the aspect 1, the imaging system may include a reset voltage determination unit that determines a reset voltage to be applied to the amplifier transistor based on the input output characteristics.

  According to the above configuration, since the reset voltage is determined based on the input / output characteristics, the input / output characteristics can be effectively used for each pixel as compared with the case where the same reset voltage is used for all the pixels.

  In the imaging system according to Aspect 5 of the present invention, in any one of Aspects 1 to 3, the calibration characteristic deriving unit corresponds to the predetermined input voltage after a lapse of a predetermined period after deriving the calibration characteristic. When the output is acquired and the difference between the output and the output corresponding to the input voltage when the calibration characteristic was derived last time exceeds a threshold value, the calibration characteristic may be derived again. .

  According to the above configuration, when the previously derived calibration characteristic becomes inappropriate, an appropriate calibration characteristic can be derived again.

  An imaging system control method according to aspect 6 of the present invention is an imaging system including a plurality of pixels each including a sensor element that generates an electrical signal based on a dose of incident radiation and an amplifier transistor that amplifies the electrical signal. In the control method, a voltage application step of applying an input voltage to the amplifier transistor at a predetermined interval, an acquisition step of acquiring an output corresponding to the input voltage for each pixel, and the input voltage for each pixel And a calibration characteristic deriving step of deriving an input output characteristic indicating a correspondence relationship between the outputs corresponding to the input voltage and deriving a calibration characteristic used for calibration based on an inverse characteristic of the input output characteristic. It is characterized by that.

  In the imaging system according to aspect 7 of the present invention, in the aspect 1, the calibration characteristic deriving unit includes a first section corresponding to the input voltage equal to or lower than a threshold value, and a second interval corresponding to the input voltage higher than the threshold value. The input / output characteristics may be derived using an interpolation formula that differs from section to section.

  According to the above configuration, since the interpolation formulas used before and after the threshold value are made different, the input / output characteristics can be derived using the interpolation formulas appropriately corresponding to the respective sections. Since the input / output characteristics can be derived by interpolating with the interpolation formula, the processing amount can be reduced and the processing time can be shortened.

  In the imaging system according to aspect 8 of the present invention, in the aspect 7, the calibration characteristic deriving unit varies the frequency of deriving the input output characteristic between the first interval and the second interval. Also good.

  According to the above configuration, since the input / output characteristics can be derived at an appropriate frequency according to the section, it is possible to suppress the useless input / output characteristics from being derived.

  In the imaging system according to aspect 9 of the present invention, in the aspect 1, the calibration characteristic deriving unit may derive the input output characteristic based on a change amount of the input voltage from a predetermined value. .

  According to the above configuration, the input / output characteristics can be derived based on the amount of change from the predetermined value.

  In the imaging system according to Aspect 10 of the present invention, in any one of Aspects 1 to 5 and 7 to 9, the calibration value derived using the calibration characteristic before imaging and the calibration value after imaging. A calibration unit that outputs a difference value between the two as a result of calibration may be provided.

  According to the above configuration, an appropriate value can be output even when the threshold voltage of the amplifier transistor varies.

  The imaging system according to each aspect of the present invention may be realized by a computer. In this case, the imaging system is realized by the computer by operating the computer as each unit (software element) included in the imaging system. An imaging system control program and a computer-readable recording medium on which the control program is recorded also fall within the scope of the present invention.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention. Furthermore, a new technical feature can be formed by combining the technical means disclosed in each embodiment.

DESCRIPTION OF SYMBOLS 1 Imaging system 10 Calibration apparatus 20 Imaging sensor 21 Imaging sensor main body 22 Voltage generation part 23 Row selection part 24 Reading part 30 Display apparatus 100 Calibration setting part 101 Input voltage setting part (voltage application part)
102 Setting output value acquisition unit (acquisition unit)
103 Calibration characteristic deriving unit 104 Calibration characteristic deriving data 110 Imaging output value acquiring unit 120 Calibration unit 130 Output unit 211 Pixel 241 AFE
250 In-pixel circuit 251 Calibration / reset switch 252 Photodiode (sensor element)
253 Amp transistor (amplifier transistor)
254 Reed switch 910 Calibration / reset voltage line 911 Calibration / reset control line

It is a figure which shows the outline | summary of the imaging system which concerns on this embodiment. It is a figure which shows the circuit structure in the pixel contained in an imaging sensor main body. It is a block diagram which shows the principal part structure of a calibration apparatus. (A) is a graph which shows the relationship between the input voltage which the input voltage setting part set, and the output which the output value acquisition part for setting acquired, (b) is a graph which shows the reverse characteristic of the graph shown to (a) It is. It is a flowchart which shows the flow of the process in which a calibration setting part derives | leads-out a calibration characteristic. It is a circuit diagram for demonstrating the process which derives | leads-out a calibration characteristic. It is a figure for demonstrating the example which thins out a part and derives the reverse characteristic. It is a figure for demonstrating the example which expresses an inverse characteristic with the type | formula containing three parameters. It is a circuit diagram for giving a different reference potential for every pixel. It is a figure for demonstrating the reason that the characteristic of a pixel can be used effectively by varying a reference potential for every pixel, and (a) is a figure showing an example at the time of making a reference potential of a pixel into a specific potential. There is a diagram illustrating an example of a case having different reference potential (b) is Pikuse Le basis. It is a figure for demonstrating the derivation | leading-out example of the input-output characteristic which concerns on other embodiment. It is a figure for demonstrating the derivation | leading-out example of the input-output characteristic which concerns on other embodiment. (A) is a figure which shows the change of the input-output characteristic when a threshold voltage fluctuates, (b) is a figure which shows the reverse characteristic. It is a figure for demonstrating the reason which is not influenced by the fluctuation | variation of a threshold voltage by using a difference value.

Specifically, this will be described with reference to FIG. FIG. 11 is a diagram for explaining an example of deriving input-output characteristics using different interpolation formulas depending on sections. In FIG. 11, the vertical axis represents the input voltage V _g (V), and the horizontal axis represents the output current I _d (μA) corresponding to the input voltage V _g . In the present embodiment, the calibration setting unit 100 determines that the input voltage V _g is an interval A (V _RESET > V _g > V _border , I _RESET from the reset voltage V _RESET (point 1101) to the threshold V _border (point 1102). > I _d > I _border ) (first interval), the measurement interval is shortened and interpolation is performed by linear (primary expression) interpolation. For the section B (V _g <V _border , I _d <I _border ) (second section) smaller than the threshold value V _border (point 1102), the measurement interval is increased and nonlinear (second-order or higher-order polynomial or exponent) Function etc.) Interpolate by interpolation. For example, in section A, interpolation is performed using V _g = a · I _d + b (a and b are coefficients), and in section B, V _g = f _nl (I _d ) (f _nl () is a nonlinear function (second order). The above polynomial, exponential function, etc.) are interpolated. Examples of second-order or higher polynomials include Lagrange interpolation, Spline interpolation, Hermite interpolation, and the like.

Section in the present embodiment, the calibration setting section 100, section A from the origin corresponding to the section A described above to point 1201 'that _{(ΔV g <V border, ΔI} d <I border) performs a linear interpolation for, the above-described For the section B ′ (ΔV _g > V _border , ΔI _d > I _border ) corresponding to B where the input voltage difference is larger than the point 1201, nonlinear interpolation is performed to derive the input-output characteristics. For example, for the section A ′, interpolation is performed by ΔV _g = a ′ · ΔI _d + b ′ (a ′ and b ′ are coefficients), and for the section B ′, ΔV _g = f ′ _nl (ΔI _d ) (f ' _nl () interpolates using a nonlinear function (second-order or higher polynomial, exponential function, etc.)).

Claims (11)

An imaging system comprising a plurality of pixels including a sensor element that generates an electrical signal based on a dose of incident radiation and an amplifier transistor that amplifies the electrical signal,
A voltage application unit for applying an input voltage to the amplifier transistor at a predetermined interval;
An acquisition unit that acquires an output corresponding to the input voltage for each pixel;
Calibration for deriving an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage for each pixel, and deriving a calibration characteristic to be used for calibration based on an inverse characteristic of the input output characteristic An application characteristic deriving unit;
An imaging system comprising:   The calibration characteristic deriving unit derives the calibration characteristic based on an inverse characteristic of the input output characteristic obtained by thinning a part of the input voltage applied by the voltage application unit. The imaging system described.   The imaging system according to claim 1, wherein the calibration characteristic deriving unit derives the calibration characteristic by expressing the inverse characteristic by an expression including three parameters.   The calibration characteristic deriving unit derives the input output characteristic using different interpolation formulas in a first interval corresponding to the input voltage equal to or lower than a threshold and a second interval corresponding to the input voltage higher than the threshold. The imaging system according to claim 1.   The imaging system according to claim 4, wherein the calibration characteristic deriving unit varies the frequency of deriving the input output characteristic between the first section and the second section.   The imaging system according to claim 1, wherein the calibration characteristic deriving unit derives the input output characteristic based on a change amount of the input voltage from a predetermined value.   A calibration unit is provided that outputs a difference value between a calibration value derived using the calibration characteristic before imaging and the calibration value after imaging as a calibration result. Item 7. The imaging system according to any one of Items 1 to 6.   The imaging system according to claim 1, further comprising a reset voltage determining unit that determines a reset voltage to be applied to the amplifier transistor based on the input / output characteristics.   The calibration characteristic deriving unit obtains an output corresponding to the predetermined input voltage after the predetermined period has elapsed after deriving the calibration characteristic, and the output and the calibration characteristic derived when the calibration characteristic was derived last time. The imaging system according to any one of claims 1 to 8, wherein a calibration characteristic is derived again when a difference from an output corresponding to an input voltage exceeds a threshold value. An imaging system control method comprising a plurality of pixels including a sensor element that generates an electrical signal based on a dose of incident radiation and an amplifier transistor that amplifies the electrical signal,
A voltage application step of applying an input voltage to the amplifier transistor at a predetermined interval;
Obtaining an output corresponding to the input voltage for each pixel;
Calibration for deriving an input output characteristic indicating a correspondence relationship between the input voltage and the output corresponding to the input voltage for each pixel, and deriving a calibration characteristic to be used for calibration based on an inverse characteristic of the input output characteristic A step of deriving the application characteristics,
An imaging system control method comprising:   A control program for causing a computer to function as the imaging system according to claim 1, wherein the control function causes the computer to function as the voltage application unit, the acquisition unit, and the calibration characteristic deriving unit.

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