JP2006101395A - 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 and inexpensive configuration, in a radiation image detector of an 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, and electric charge signal lines 14 from which the electric charge signals accumulated in charge accumulating units 12a of the element 12c flow. In this detector, a delay circuit 16 for delaying the rise time of the gate control signal is provided for each switch element 12b of the element 12c, and an electric charge signal Q<SB>S</SB>flowing to the lines 14 after the rise time elapses is detected, and thus, the electric charge signal Q<SB>S</SB>is detected at a timing different from that of a noise signal Q<SB>N</SB>generated at an output start time point 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. Further, since it is necessary to separately provide a gate driver for outputting a signal having the reverse polarity as described above to the noise compensation signal line, the circuit becomes complicated and the cost is increased.

  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 with a simple and inexpensive circuit configuration in the above-described TFT reading type radiation image detector. An object of the present invention is to provide an image detector.

  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 comprising a radiation image recording medium having a number of gate control signal lines from which a gate control signal is output, and a detector for detecting a charge signal flowing out to the charge signal lines of the radiation image recording medium In the image detector, the radiographic image recording medium has a plurality of delay circuits provided for each switch element of the charge detection element and delays the rising time of the gate control signal for turning on each switch element, Is characterized in that it detects a charge signal that has flowed out to the charge signal line after the rise time has elapsed.

  In the radiation image detector, the delay circuit can be constituted by a low-pass filter circuit.

  In addition, the charge amplifier that integrates the charge signal that has flowed out to the charge signal line, and the integration by the charge amplifier is started before the rise time elapses after the start of the output of the gate control signal to the gate control signal line. After that, a first signal integrated by the charge amplifier until a predetermined time point before the rise time elapses is acquired, and a second signal integrated by the charge amplifier immediately before the predetermined integration time elapses after the integration starts. It is possible to include a correlated double sampling control unit that acquires a signal and acquires a difference between the second signal and the first signal as an image signal representing a radiographic image.

  Here, the “rise time” means the time from when the gate control signal is applied to the switch element until the switch element is turned on. The above “delay the rise time” means the gate control. This means that a predetermined time constant is given to the rising edge of the signal.

  The “column” and the “row” are used to distinguish two orthogonal directions, and do not define a specific direction such as a horizontal direction or a vertical direction.

  According to the radiation image detector of the present invention, a delay circuit that delays the rise time of the gate control signal is provided for each switch element of the charge detection element, and the charge signal that has flowed out to the charge signal line after the rise time elapses is detected. As a result, when the gate control signal starts to be output to the gate control signal line, the noise signal generated by the parasitic capacitance and the charge signal accumulated in the power storage unit of the charge detection element are charged at different timings. A charge signal that does not include the noise signal can be detected.

  In the radiation image detector, when the delay circuit is a low-pass filter circuit, the delay circuit can be configured by an inexpensive CR circuit including a resistance element and a capacitor, for example.

  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.

  Here, as shown in FIG. 2, the radiation image recording medium 10 of this embodiment is further provided with a delay circuit 16 for each charge detection element 12c. The delay circuit 16 is a low-pass filter circuit (LPF), and includes a resistance element and a capacitor. Any known configuration may be adopted for the configuration of the low-pass filter. In this embodiment, the low-pass filter circuit is employed as the delay circuit. However, any known circuit may be employed as long as the circuit delays the rise time of the gate control signal. The delay circuit 16 is provided at a position where the gate control signal line 13 and the charge signal line 14 do not intersect, as shown in FIG. The operation of the delay circuit 16 will be described later.

  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.

  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.

  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. 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 switch element shows the voltage waveform at point B in FIG. The voltage waveform of the charge signal line shows the voltage waveform at point C in FIG. 2, and the voltage waveform of the output voltage of the differential amplifier shows the voltage waveform at point D in FIG.

  When reading a radiation image, first, as shown in the timing chart of FIG. 4, a gate control signal for turning on the switch element 12 b is output from the gate driver 30 to one of the gate control signal lines 13. The gate control signal supplied to the gate control signal line 13 is applied to the switch element 12b connected to the gate control signal line. In the radiographic image recording medium 10 of the present embodiment, each switch as described above. Since the delay circuit 16 is provided in the element 12b, as shown in the timing chart of FIG. 4, the voltage of the gate control signal applied to the gate terminal of the switch element 12b has a time constant at the rise, and the rise time Is delayed.

Accordingly, the switch element 12b is not turned on at the start t1 of the output of the gate control signal from the gate driver 30 to the gate control signal line 13. In this embodiment, the switch element 12b is turned on when the threshold value th shown in FIG. 4 is reached. On the other hand, at the start t1 of the output of the gate control signal from the gate driver 30 to the gate control signal line 13, the parasitic capacitance _CP1 formed near the intersection of the gate control signal line 13 and the charge signal line 14 as described above. noise signal _{Q N} to the charge signal line 14 from the flow-out.

Then, even after the noise signal Q _N as described above flew to charge the signal line 14, at time t2 before the gate terminal voltage of the switching element 12b is the threshold value th, the reset switch 21a in the differential amplifier 21 It is turned off and integration in the differential amplifier 21 is started. The output voltage S1 output from each differential amplifier 21 by the sampling circuit 40 at time t3 after the reset switch 21a is turned off and before the gate terminal voltage of the switch element 12b reaches the threshold th. Sampled. Then, after the output voltage S1 is sampled as described above, the gate terminal voltage of the switch element 12b becomes the threshold value th, and the switch element 12b is turned on. The time constant of the delay circuit 16 is selected so as to have the timing as described above.

Then, each switch element 12b is turned on, and output to the charge detection device 12c of the storage unit 12a accumulated charge signal Q _S to be connected to each power storage unit 12a charges the signal line 14.

The charge signal Q _S flowing out to the charge signal line 14 as described above is input from the inverting input of the differential amplifier 21 is integrated in the differential amplifier 21. Then, at the time t4 immediately before the end of the predetermined integration time, the output voltage S2 of the differential amplifier 21 is sampled again by the sampling circuit 40.

  Immediately after sampling as described above, the reset switch 21a of the differential amplifier 21 is turned on, and thereafter, a gate control signal for turning off the switch element 12b is output from the gate driver 30 to the gate control signal line 13, Each switch element 12b is turned off in response to the gate control signal.

  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.

According to this radiation image detector, the delay circuit 16 that delays the rise time of the gate control signal is provided for each switch element 12b of the charge detection element 12c, and the charge signal that has flowed out to the charge signal line 14 after the rise time has elapsed. since to detect the Q _S, and the noise signal Q _N which is generated by the parasitic capacitance C _P at the time when the gate control signal is started outputted to the gate control signal line 13, accumulated in the storage unit 12a of the charge detection device 12c It has been a charge signal Q _S can flow to charge the signal line 14 at different timings, it is possible to detect the charge signal Q _S free from said noise signal Q _N.

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 16 Delay circuit 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. Radiation comprising: a radiation image recording medium having a plurality of gate control signal lines from which the signal is output; and a detection unit for detecting the charge signal flowing out to the charge signal lines of the radiation image recording medium In the image detector,
The radiation image recording medium includes a plurality of delay circuits provided for each of the switch elements of the charge detection element, for delaying a rise time of the gate control signal for turning on each of the switch elements,
The radiation image detector, wherein the detector detects a charge signal that has flowed out to the charge signal line after the rise time has elapsed.   The radiation image detector according to claim 1, wherein the delay circuit is a low-pass filter circuit. A charge amplifier that integrates the charge signal that has flowed out to the charge signal line;
The integration by the charge amplifier is started before the rise time elapses after the start of the output of the gate control signal to the gate control signal line, and the charge is performed by a predetermined time after the start of the integration and before the rise time elapses. A first signal integrated by an amplifier is acquired, a second signal integrated by the charge amplifier is acquired immediately after a predetermined integration time has elapsed after the start of integration, and the second signal and the first signal The radiological image detector according to claim 1, further comprising: a correlated double sampling control unit that acquires a difference from the signal of 1 as an image signal representing the radiographic image.

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