JP2006101396A - Radiation image detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable properly eliminating a noise signal generated due to parasitic capacitance formed near a cross point between a gate control signal line and an electric charge signal line with a simple configuration in the radiation image detector of a TFT reading system. <P>SOLUTION: The radiation image detector is provided with electric charge detecting elements 12c for accumulating electric charges generated by irradiation of radiation, gate control signal lines 13 to which a gate control signal for controlling switch elements 12b of the elements 12c is applied, electric charge signal lines 14 from which the electric charge signals accumulated in charge accumulating units 12a of the elements 12c flow, and a charge amplifier 21 for integrating the electric charge signals flowing to the signal lines 14. In the detector, a gate control signal for bringing the switch element 12b into an off-state is outputted to the signal line 13 during the integration period of the charge amplifier 21, the charge amplifier 21 integrates the signals including a noise compensation signal Q<SB>N2</SB>having a polarity reverse to that of the noise signal Q<SB>N1</SB>generated by output of the gate control signal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a radiographic image detector that generates and accumulates charges upon irradiation with radiation carrying a radiographic image and detects the accumulated charges as an image signal.

  Conventionally, in the medical field and the like, various radiation image detectors that record radiation images related to a subject by receiving radiation that has passed through the subject and detect an image signal corresponding to the recorded radiation image have been proposed and put into practical use. ing.

  As the radiation image detector, for example, there is a radiation image detector using a semiconductor material that generates an electric charge by irradiation of radiation, and a so-called TFT reading type is proposed as such a radiation image detector. .

  As a radiographic image detector of a TFT reading system, for example, a radiation image recording medium in which a charge generation layer that generates charges upon irradiation with radiation and a charge detection layer that accumulates charges generated in the charge generation layer are laminated And a detection unit including a charge amplifier that detects a charge signal flowing out from the radiation image recording medium has been proposed.

  The charge detection layer in the radiation image recording medium is specifically a charge detection element having a power storage unit that stores the charges generated in the charge generation layer and a TFT switch element that reads a charge signal stored in the power storage unit. The charge detection elements are two-dimensionally arranged in the orthogonal direction. Further, the charge detection layer is provided in parallel to each of the charge detection elements in a row perpendicular to the charge signal lines and a large number of charge signal lines provided in parallel for each column of the charge detection elements. And a large number of gate control signal lines.

  When recording a radiographic image using the radiographic image detector configured as described above, first, the charge generation layer is irradiated with radiation carrying the radiographic image, and the charge generated in the charge generation layer is generated. Is stored in the power storage unit in the charge detection layer, whereby a radiographic image is recorded. When the radiation image is read, a gate control signal is selectively output from the gate driver to the gate control signal line, and the TFT of the charge detection element connected to the gate control signal line according to the gate control signal The switch element is turned on, and a charge signal flows from the power storage unit of the charge detection element to the charge detection element. The charge signal that has flowed out to the charge signal line is detected as an image signal by a charge amplifier connected to the charge signal line, and the radiation image is read.

  Here, in the radiation image detector as described above, since the gate control signal line and the charge detection signal line are provided orthogonally via the insulating layer, the gate control signal line and the charge detection signal line are separated from each other. Parasitic capacitance is formed in the vicinity of the intersection. When a radiographic image is read as described above, if a gate control signal is passed through the gate control signal line, a potential difference is generated between the gate control signal line and the charge detection signal line, and the parasitic capacitance is charged. Is accumulated, and the charge flows out to the charge signal line as a noise signal, and this noise signal is added to the charge signal flowing out from the power storage unit of the charge detection element.

  Therefore, for example, in Patent Document 1, a noise compensation signal line orthogonal to the charge signal line is provided via an insulating layer separately from the gate control signal line, and is connected to the noise compensation signal line and the charge signal line. TFT switching element and a dummy capacitor connected to the TFT switching element are provided, and when a gate control signal is output to the gate control signal line, a signal having a polarity opposite to that of the gate control signal is also applied to the noise compensation signal line. The noise compensation signal generated near the intersection of the charge signal line and the noise compensation signal line according to the signal of the opposite polarity is accumulated in the dummy capacitor, and the accumulated noise compensation signal is passed through the TFT switch element. There has been proposed a method of removing the noise signal by flowing it through a charge signal line.

JP 2004-37382 A

  However, the noise compensation signal line described in Patent Document 1 is provided in a region different from the region where the gate control signal line is disposed, and the gate control signal line, the noise compensation signal line, and the charge signal line are not provided. Because there is in-plane variation in the thickness of the insulating layer between them, parasitic capacitance formed near the intersection between the gate control signal line and the charge signal line and formed near the intersection between the noise compensation signal line and the charge signal line The size of the parasitic capacitance is different. Therefore, the magnitudes of the noise signal and the noise compensation signal are different, and the noise signal cannot be appropriately removed, and the noise signal remains in the charge signal.

  In view of the above circumstances, the present invention provides a radiation image detector capable of appropriately removing the noise signal generated by the parasitic capacitance as described above in the above-described TFT reading type radiation image detector. It is the purpose.

  The radiation image detector of the present invention includes a charge generation unit that generates charges upon irradiation with radiation carrying a radiographic image, a power storage unit that stores charges generated in the charge generation unit, and a charge signal stored in the power storage unit. A plurality of charge detection elements arranged two-dimensionally in an orthogonal direction and parallel to each column of charge detection elements arranged in one of the orthogonal directions. ON / OFF control of switching elements provided in parallel for each row of charge detection elements arranged in the other of the orthogonal directions and a large number of charge signal lines from which charge signals flow out is provided. Radiation image recording medium having a large number of gate control signal lines from which gate control signals are output, a gate control signal output unit for outputting gate control signals to the gate control signal lines, and a charge signal line In a radiographic image detector including a detection unit having a charge amplifier that detects an image signal representing a radiographic image by integrating the generated charge signal, the switch element is turned on within the integration period of the charge signal in the charge amplifier A noise compensation signal output unit for outputting a signal having a voltage opposite to the direction of the voltage of the gate control signal to each gate control signal line is provided.

  In the radiation image detector, the gate control signal output unit can be used as a noise compensation signal output unit, and the signal of the reverse voltage can be output from the gate control signal output unit.

  Further, the first signal integrated by the charge amplifier is acquired by the first time point in the period from the start of integration by the charge amplifier to the output of the gate control signal for turning on the switch element, and the reverse voltage is obtained. The second signal integrated by the charge amplifier up to the second time point within the integration period after the output of the signal is acquired, and the difference between the second signal and the first signal is acquired as an image signal A correlated double sampling control unit may be provided.

  Here, “the direction of the voltage of the gate control signal for turning on the switch element” means, for example, the direction of the rise when the switch element is turned on by the rise of the voltage. When the element is turned on by a voltage fall, it means the direction of the fall. The “reverse voltage signal” means, for example, a voltage signal that falls when the switch element is turned on when the voltage rises, and the switch element is turned on when the voltage falls. In the case of a state, it means a voltage signal in a rising direction.

  Further, a gate control signal for turning off the switch element can be used as the “reverse voltage signal”.

  According to the radiographic image detector of the present invention, a signal having a voltage opposite to the direction of the voltage of the gate control signal for turning on the switch element is supplied to each gate control signal line within the integration period of the charge signal in the charge amplifier. Therefore, it is possible to generate a noise compensation signal having a reverse polarity corresponding to the signal having the reverse voltage in the parasitic capacitance, and integrating the charge amplifier including the noise compensation signal having the reverse polarity. Therefore, the noise signal at the time of outputting the gate control signal can be appropriately removed by the noise compensation signal having the reverse polarity without being affected by the in-plane variation of the insulating layer as described above.

  Further, in the radiological image detector, when the gate control signal output unit is used as a noise compensation signal output unit and the signal of the reverse voltage is output from the gate control signal output unit, the patent document As compared with the case where the gate driver is provided separately as in the method described in 1, the noise compensation signal output unit can be configured easily and inexpensively.

  Hereinafter, an embodiment of the radiation image detector of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the radiation image detector includes a charge generation layer 11 that generates a charge upon irradiation of radiation and a charge detection layer 12 that stores a charge generated in the charge generation layer 11. A medium 10 and a detection unit 20 that detects a charge signal flowing out from the radiation image recording medium 10 are described below.

  The charge generation layer 11 may be formed of any material as long as it is a material that generates charges when irradiated with radiation. For example, the charge generation layer 11 is made of a material such as a-Se having high quantum efficiency and low dark current. It is desirable to form. The charge generation layer 11 is composed of two layers: a phosphor layer that emits fluorescence when irradiated with radiation, and a photoconductive layer that generates charge when irradiated with fluorescence emitted from the phosphor layer. You may do it.

  Specifically, as shown in FIG. 2, the charge detection layer 12 includes a power storage unit 12 a that stores the charge generated in the charge generation layer 11 and a switch element 12 b that reads a charge signal stored in the power storage unit 12 a. A large number of detection elements 12c are provided, and the charge detection elements 12c are two-dimensionally arranged in the orthogonal direction. The power storage unit 12a is a capacitor, and the switch element 12b is configured by a TFT switch.

  As shown in FIG. 2, the charge detection layer 12 is arranged in the Y direction with a large number of gate control signal lines 13 provided in parallel for each row of the charge detection elements 12c arranged in the X direction. And a large number of charge signal lines 14 provided in parallel for each column of the charge detection elements 12c. A gate control signal is supplied to the gate control signal line 13 in order to turn on / off the switch element 12b connected to each gate control signal line 13. In addition, a charge signal accumulated in the power storage unit 12 a connected to each charge signal line 14 flows out to each charge signal line 14. The gate control signal is output from a gate driver described later.

  The gate control signal line 13 and the charge signal line 14 are provided orthogonally as described above, but they are not in contact at the intersection 15 and are spaced at a predetermined interval as shown in FIG. Is provided. In addition, an insulating layer is provided at the interval.

  The detection unit 20 includes a number of differential amplifiers 21, a gate driver 30, a sampling circuit 40, a multiplexer 50, and an AD converter 60.

  Each charge signal line 14 is connected to each differential amplifier 21. The differential amplifier 21 is a charge amplifier, and includes a reset switch 21a and an integrating capacitor 21b.

  The gate driver 30 selectively outputs gate control signals sequentially to each gate control signal line 13 of the radiation image recording medium 10. Further, the gate driver 30 in the present embodiment outputs a gate control signal for controlling on / off of the switch element 12b within the integration period of the differential amplifier 21, and the operation thereof will be described in detail later.

  The sampling circuit 40 samples the signal output from each differential amplifier 21 at a predetermined timing, and performs so-called correlated double sampling. The operation will be described in detail later.

  The multiplexer 50 selectively switches the analog image signal output from the sampling circuit 40 and outputs the analog image signal to the AD converter 60.

  The AD converter 60 converts the analog image signal output from the multiplexer 50 into a digital image signal.

  Next, the operation of recording and reading a radiographic image by the radiographic image detector will be described. The voltage waveform of the gate control signal line shown in FIG. 4 shows the voltage waveform at point A in FIG. 2, and the voltage waveform of the charge signal line shows the voltage waveform at point B in FIG. The voltage waveform of the output voltage of the differential amplifier shows the voltage waveform at point C in FIG.

  When a radiographic image is recorded using this radiographic image detector, first, radiation that has passed through the subject is irradiated from the side of the charge generation layer 11 of the radiographic image recording medium 10. Then, charges are generated in the charge generation layer 11 in response to the irradiation of the radiation, and the charges are accumulated in the power storage unit 12a in the charge detection layer 12, whereby a radiographic image is accumulated and recorded.

  Next, the operation of reading the radiation image stored and recorded as described above will be described with reference to the timing chart shown in FIG.

When reading a radiation image, first, as shown in the timing chart of FIG. 4, the reset switch 21 a in the differential amplifier 21 is turned off, and integration in the differential amplifier 21 is started. Then, at time t1 immediately after the reset switch 21a is turned off, the output voltage S1 output from each differential amplifier 21 is sampled by the sampling circuit 40. Then, at a time point t2 immediately after the time point t1, a gate control signal for turning on the switch element 12b is output from the gate driver 30 to one of the gate control signal lines 13. Specifically, the gate control signal for turning on the switch element 12b is a voltage rising signal as shown in FIG. Then, each switch element 12b of the charge detection device 12c connected to the gate control signal line 13 in response to the gate control signal is turned on, the charge detection device 12c charge signal Q _S stored in the storage unit 12a of each It is output to the charge signal line 14 connected to the power storage unit 12a.

Here, when the gate control signal from the gate driver 30 to the gate control signal line 13 as described above is output, not only the charge signal Q _S from the power storage unit 12a as described above flows, the gate control signal lines 13 noise signal Q _N1 accumulated in the parasitic capacitance C _P that is formed at the intersection 15 near the charge signal line 14 also flows to the charge signal line 14 at the same time. That is, as shown in FIG. 4, a signal obtained by adding the charge signal Q _S and the noise signal Q _N1 flows out to the charge signal line 14. Noise signal Q _N1 as described above, particularly since the large signal when compared to the charge signal Q _S in the low-dose region, unless it is impossible to obtain an appropriate image signal remove it.

Therefore, in the present radiation image detector, in order to remove the noise signal Q _N1 as described above, as shown in FIG. 4, each gate control signal is output from the gate driver 30 at the time t <b> 3 within the integration period of the differential amplifier 21. A gate control signal for turning off the switch element 12b is output to the line 13. The gate control signal for turning off the switch element 12b is specifically a voltage fall signal as shown in FIG. When the voltage falling signal to the gate control signal line 13 as described above is output, the parasitic capacitance C _P that the gate control signal line 13 is formed near the intersection of the charge signal line 14, and the noise signal Q _N1 Generates a noise compensation signal Q _N2 having a reverse polarity, and this noise compensation signal Q _N2 also flows out to the charge signal line 14. If the magnitude of the voltage fall signal is the same as the voltage rise signal, generally, Q _N2 = −Q _N1 . Then, the noise compensation signal Q _N2 of the opposite polarity is integrated from the differential amplifier 21, the output voltage value from that amount differential amplifier 21 is reduced. That is, as described above, the noise compensation signal Q _N2 having the opposite polarity is integrated by the differential amplifier 21 so that the noise signal Q _N1 is substantially removed.

Thus, the result, the noise signal charge signal Q _S where Q _N1 is removed is integrated in the differential amplifier 21, at time t4, immediately before completion predetermined integration time, again the output of the differential amplifier 21 by the sampling circuit 40 The voltage S2 is sampled, and immediately thereafter, the reset switch 21a of the differential amplifier 21 is turned on.

  Then, in the sampling circuit 40, the output voltage S1 is subtracted from the output voltage S2 sampled as described above, and the subtracted voltage value is acquired as an analog image signal. Then, the analog image signal acquired as described above is switched for each differential amplifier 21 by the multiplexer 50 and input to the AD converter 60, and is sequentially digitized to output a digital image signal.

  After that, gate control signals are sequentially and selectively output from the gate driver 30 to the gate control signal line 13, and the same operation as described above is repeatedly performed, so that the entire digital image signal of the radiation image recording medium 10 is To be acquired.

In the radiation image detector of the above embodiment, the noise compensation signal _QN2 is generated using the gate control signal that turns off the switch element 12b. In addition, a noise compensation signal output unit that outputs a signal having a voltage opposite to the direction of the voltage of the gate control signal that turns on the switch element 12b to the gate control signal line 13 is provided. The noise compensation signal Q _N2 may be generated by outputting a signal having a reverse voltage to the gate control signal line 13. When the noise compensation signal output unit as described above is provided, the gate control signal for turning off the switch element 12b needs to be output to the gate control signal line 13 after the integration is completed.

  In the radiation image detector of the above-described embodiment, the potential difference of the gate control signal when the switch element 12b is turned on is the same as the potential difference of the gate control signal when the switch element 12b is turned off. However, the present invention is not limited to this. For example, when the amount of noise signal generated varies depending on the direction of potential change of the gate control signal, the potential difference may be different. For example, the switch element 12b is turned off. The potential of the gate control signal at the time may be further lowered.

The perspective view of the radiographic image recording medium of one Embodiment of the radiographic image detector of this invention The figure which shows schematic structure of the detection part of one Embodiment of the radiographic image detector of this invention, and the electric charge detection layer of a radiographic image recording medium. The figure for demonstrating arrangement | positioning of a gate control signal line and an electric charge signal line Timing chart for explaining the operation of one embodiment of the radiation image detector of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Radiation image detector 11 Charge generation layer 12 Charge detection layer 12a Power storage part 12b Switch element 12c Charge detection element 13 Gate control signal line 14 Charge signal line 21 Differential amplifier _CP Parasitic capacitance

Claims (3)

A charge generation unit that generates a charge upon irradiation with radiation carrying a radiographic image; a storage unit that stores the charge generated in the charge generation unit; and a switch element that reads a charge signal stored in the storage unit A plurality of charge detection elements arranged two-dimensionally in an orthogonal direction and provided in parallel for each column of the charge detection elements arranged in any one of the orthogonal directions, A gate control signal for controlling on / off of the switch elements provided in parallel for each row of the charge detection elements arranged in the other of the orthogonal directions and a large number of charge signal lines from which charge signals flow. A radiographic image recording medium having a plurality of gate control signal lines to be output, a gate control signal output unit for outputting the gate control signal to the gate control signal line, and the charge signal line In the radiation image detector having a detecting portion having a charge amplifier for detecting an image signal by integrating the charge signal out is representative of the radiation image,
A noise compensation signal for outputting a signal having a voltage opposite to the voltage direction of the gate control signal for turning on the switch element to each gate control signal line within the integration period of the charge signal in the charge amplifier. A radiation image detector comprising an output unit.   The radiographic image detector according to claim 1, wherein the gate control signal output unit outputs the reverse voltage signal as the noise compensation signal output unit.   A first signal integrated by the charge amplifier is acquired by a first time point within a period from the start of integration by the charge amplifier to the output of a gate control signal for turning on the switch element. A second signal integrated by the charge amplifier after the output of the voltage signal and before the second time point in the integration period is obtained, and a difference between the second signal and the first signal is obtained. The radiographic image detector according to claim 1, further comprising a correlated double sampling control unit that acquires the image signal.

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