JP5509032B2 - Radiation image detector - Google Patents

Description

  Embodiments described herein relate generally to a radiation image detector that suppresses image deterioration due to noise mixing during continuous imaging.

  As a new generation X-ray diagnostic detector, a radiological image detector using an active matrix has attracted much attention. By irradiating the radiation image detector with X-rays, an X-ray image or a real-time X-ray image is output as a digital signal. In addition, since it is a flat solid detector, it is highly expected in terms of image quality and stability. For this reason, many universities and manufacturers are working on research and development.

  Radiation image detectors are roughly classified into two methods, a direct method and an indirect method.

  The direct method is a method in which X-rays are directly converted into a charge signal by a photoconductive film such as a-Se and led to a charge storage capacitor.

  One indirect method is a method in which X-rays are received by a phosphor layer and converted to visible light, and then the visible light is converted into a charge signal by an a-Si photodiode or the like and led to a charge storage capacitor.

  Many of the radiation image detectors currently in practical use employ the indirect method.

  A conventional indirect radiation image detector forms an image detection unit by two-dimensionally arranging pixels including a photodiode that converts visible light into a charge signal and a TFT transistor as a switching element on a glass substrate, On top of that, a fluorescence conversion film for converting radiation into visible light is provided.

  In this indirect radiation image detector, X-rays incident from the outside are converted into visible light inside the fluorescence conversion film, and this visible light enters a photodiode in a pixel of the image detection unit. Visible light is converted into electric charge, and the electric charge is accumulated in the photodiode or in the capacitor element connected in parallel.

  The X-ray information converted into electric charges passes through each switching element (TFT transistor) connected to the photodiode for each pixel group connected to the row selection line arranged in the row direction in each pixel and column direction. The signal is transmitted to the outside of the image detection unit through the connected signal line.

  The potential of the row selection line for driving the TFT transistor is normally changed to the potential of only one row selection line, thereby bringing the TFT transistors in all pixels corresponding to a specific row into a conductive state. By sequentially changing the row selection line for changing the potential, a signal from a pixel corresponding to a specific row is discharged to the outside. By referring to the position of the signal line from which the charge has been discharged and the position of the selection line whose potential has fluctuated at that time, it is possible to calculate the X-ray incident position and intensity.

  The charge signal discharged to the outside of the glass substrate is input to an integrating amplifier connected to each signal line. The charge information input to the integrating amplifier is amplified, converted into a potential signal, and output. The potential signal output from the integrating amplifier is converted into a digital value by an analog / digital converter, and finally edited as an image signal and output to the outside of the radiation image detector.

  Radiation image detectors are often used mainly for medical purposes, and weak X-rays are often used for imaging in order to reduce the amount of X-ray exposure that is harmful when exposed in large quantities. For this reason, the influence of leakage current and offset current peculiar to the TFT circuit and the integration amplifier is large, and it is important to remove the influence in X-ray imaging.

  To eliminate the effects of leakage current and offset current of TFT circuits and integrating amplifiers, which are problematic when taking weak X-ray images, take an image in the absence of X-ray incidence, and obtain the dark image obtained at that time. Will be used. By using this dark image data and subtracting the dark image component from the image obtained when X-rays are incident, it is possible to remove the influence of the leakage current and offset current of the TFT circuit and the integrating amplifier.

  In a normal imaging operation, the values of the leakage current and offset current of the TFT circuit and the integrating amplifier do not change unless the driving conditions such as the temperature, amplification factor, and imaging time of the radiation image detector change. Therefore, as long as the temperature and driving conditions of the radiation image detector do not change, one kind of dark image data is taken in advance and the data is used to eliminate the influence of leakage current and offset current of the TFT circuit and integrating amplifier. Is possible.

JP 2009-128023 A

  On the other hand, continuous imaging may be required in imaging X-ray images for the purpose of inspection. This is to perform a plurality of radiographs with different X-ray quality, and to obtain information on the difference in the substance of the subject, the difference in density, and the difference in thickness from the difference in the X-ray images obtained in each radiographing. .

  In particular, in the case of an examination for a human body, it is necessary to stop the movement of the subject during consecutive photographing, and therefore it is necessary to perform a plurality of photographings in the shortest possible time. Therefore, a normal radiographic image inspection apparatus performs only one imaging with respect to one electrical signal. However, when a plurality of images are continuously captured, a plurality of imaging with respect to one electrical signal is performed. By doing so, it is necessary to reduce the processing time loss of the apparatus accompanying the processing of electrical signals and shorten the imaging interval.

  In a radiographic image detector intended for imaging, when an imaging operation is not performed, it is necessary to always return the state of the pixels in the TFT panel and the integration amplifier to the initial state.

  In the normal shooting operation, there is a sufficient interval between shooting, so the TFT panel pixels and the integration amplifier are initialized with a sufficient period until the next shooting starts. And the integration amplifier can be completely initialized.

  However, in the case of continuous shooting operations, sufficient initialization is performed in the first shooting operation, but in the second and subsequent shooting, the period from the last shooting operation is extremely short and sufficient. In many cases, initialization time cannot be taken. For this reason, when the shooting operation for the second and subsequent sheets is performed, the influence of the voltage supply state to the TFT peculiar to the shooting operation remains, and the dark image level is different from that during the shooting of the first sheet.

  Therefore, in image processing using only one type of dark image data, the leakage current and offset current of the TFT circuit and the integration amplifier, and the charge remaining in the thin film transistor from the first and second captured images. The effects of the components cannot be removed sufficiently.

  Accordingly, an object of the present invention is to provide a radiographic image detector that can improve noise mixed in an X-ray image during continuous imaging.

  In order to achieve the above-described object, the radiation image detector is arranged in a two-dimensional manner on a substrate, and a plurality of pixels that enter visible light and convert it into signal information of one image as a whole extend in the row direction. Provided on one of the plurality of row selection lines and one of the plurality of signal lines extending in the column direction, respectively, and on the plurality of pixels of the image detection unit A radiation sensor having a fluorescence conversion film that converts radiation into visible light, a gate drive circuit that sequentially applies a drive voltage to each row selection line, and an initialization of the charge state in the pixel every time imaging is completed Any one of the row selection lines to which the driving voltage is applied in a state where the radiation is irradiated during the photographing of an arbitrary number of images when performing continuous imaging of n images through the refresh operation. The signal line connected to the pixel connected to A first reading control circuit that issues a reading signal for reading image signal information from the pixels via the non-irradiation continuous photographing in the same manner as the continuous photographing in a state where the radiation is not irradiated. During non-irradiation imaging of the same number as the arbitrary number of sheets, the signal from the pixel is connected via the signal line connected to the pixel group connected to the arbitrary one row selection line to which the driving voltage is applied. A second read control circuit that issues a read signal for reading noise signal information; and the image signal information is read by a read signal from the first read control circuit, and the noise is read by a read signal from the second read control circuit. A reading circuit that reads signal information and stores the image signal information and the noise signal information, a correction circuit that corrects the image signal information based on the noise signal information, and the correction Characterized in that it comprises a display device for displaying the image signal information, the.

  The radiological image detector also includes one of a plurality of row selection lines arranged in a two-dimensional array on the substrate and extending in the row direction, and a column direction. A radiation sensor having an image detection unit connected to one of a plurality of signal lines extending in a row, a gate drive circuit for sequentially applying a drive voltage to each row selection line, and each time imaging is completed In addition, the drive voltage is applied in a state in which the radiation is irradiated during imaging of an arbitrary number of images when performing continuous imaging of n images through a refresh operation that initializes the state of charge in the pixel. A first read control circuit for emitting a read signal for reading image signal information from the pixel via the signal line connected to the pixel connected to any one of the row selection lines, and the radiation The above continuous shooting without irradiation In the same manner as described above, pixels connected to any one of the row selection lines to which the drive voltage is applied during the non-irradiation shooting of the same number as the arbitrary number when performing non-irradiation continuous shooting. A second reading control circuit for issuing a reading signal for reading noise signal information from the pixels via the signal line connected to the group; and reading the image signal information by a reading signal from the first reading control circuit. A read circuit that reads the noise signal information by a read signal from the second read control circuit and stores the image signal information and the noise signal information; and corrects the image signal information based on the noise signal information And a display device for displaying the corrected image signal information.

The schematic diagram which shows the structure of the radiographic image detector which concerns on one embodiment of this invention. The perspective view which shows an example of the radiation sensor used for the radiographic image detector of FIG. FIG. 3 is an equivalent circuit diagram of the image detection unit in FIG. 2. The equivalent circuit diagram inside a pixel. FIG. 2 is a plan view illustrating an example of a gate drive circuit and a read circuit in FIG. 1. 6 is a timing chart illustrating an example of a driving method when performing a shooting operation (single shooting) of only one image. 6 is a timing chart illustrating an example of a driving method when performing continuous shooting.

  Embodiments of the present invention will be described below with reference to the drawings.

(Overall configuration of radiation image detector)
FIG. 1 shows an embodiment of a radiation image detector according to the present invention.

  The radiation image detector 10 is two-dimensionally arranged in a radiation sensor 11 having a phosphor layer that converts incident X-rays into light and an image detection unit that converts incident light into an electrical signal, and the image detection unit. A gate drive circuit 13 for sequentially applying a drive voltage for each scan line for each pixel, a drive control circuit 15 for generating a drive signal for determining a scan timing in the column direction for the gate drive circuit 13, and a pixel in a selected row A reading circuit 17 that reads and amplifies the electric signal, a first reading control circuit 18a that generates a reading signal for determining a reading timing in the reading circuit 17, a second reading control circuit 18b, and an image mixed with a noise signal A correction circuit 19 that performs noise correction by subtracting a noise signal from the signal, and a display device 20 that displays an image signal after performing the noise correction are provided.

(Radiation sensor 11)
FIG. 2 shows an example of a specific configuration of the radiation sensor 11.

  The radiation sensor 11 includes a fluorescence conversion film 23 that converts incident X-rays 21 into fluorescence, and an image detection unit 25 that converts the fluorescence into image information using an electrical signal.

  The image detection unit 25 is formed by providing a circuit layer 29 on which a large number of pixels 28 including photodiodes and thin film transistors (TFTs) are arranged on a holding substrate 27 mainly composed of a glass substrate.

  FIG. 3 shows an equivalent circuit of the image detection unit 25, and FIG. 4 shows an equivalent circuit inside the pixel 28.

  The pixel 28 includes a thin film transistor 31 and a photodiode 33, a row selection line (gate line) 35 is connected to the gate of the thin film transistor 31, a signal line 37 is connected to the source of the thin film transistor 31, and the drain of the thin film transistor 31 is connected to the drain. A photodiode 33 and a capacitor 36 are connected in parallel. The capacitor 36 is a capacitance between the electrodes of the photodiode 33.

  Further, an integrating amplifier 41 having a function of amplifying a charge signal transmitted through the signal line 37 and outputting the amplified signal to the outside is connected to the signal line 37 on a one-to-one basis.

  Furthermore, the row selection line (gate line) 35 is connected to a specific signal line of the gate driver 63 shown in FIG.

(Gate drive circuit 13 and reading circuit 17)
FIG. 5 shows an example of a specific configuration of the gate driving circuit 13 and the reading circuit 17 of FIG.

  The gate driving circuit 13 includes a gate driver 63 and a row selection circuit 65, and the reading circuit 17 includes an integrating amplifier 41, an A / D (analog / digital) converter 67, and a driver 69.

  The gate driver 63 has a function of sequentially changing the voltages of a large number of signal lines connected to the radiation sensor 11 when receiving a signal from the outside. A row selection circuit 65 is connected to the gate driver 63.

  The row selection circuit 65 has a function of sending a signal to the corresponding gate driver 63 according to the scanning direction of the X-ray image, and is connected to the drive control circuit 15 of FIG.

  Further, the integrating amplifier 41 is connected to a driver 69 via an A / D converter 67. The driver 69 is connected to the first reading control circuit 18a and the second reading control circuit 18b shown in FIG. 1, and in response to reading signals from the first reading control circuit 18a and the second reading control circuit 18b. The A / D converter 67 reads the digitized signal.

(First reading control circuit 18a, second reading control circuit 18b)
The first reading control circuit 18a is performing any number of images during continuous shooting of n images through a refresh operation that initializes the state of charge in the pixels 28 every time shooting is completed. A read signal for reading image signal information from the pixel 28 via a signal line 37 connected to the pixel 28 connected to any one row selection line 35 to which a driving voltage is applied in a state where radiation is applied. To emit.

  Further, the second reading control circuit 18b is driven during the non-irradiation imaging of the same number as the arbitrary number when the non-irradiation continuous imaging is performed in the same manner as the continuous imaging without radiation. A read signal for reading noise signal information from the pixel 28 is generated via a signal line 37 connected to the pixel 28 connected to any one row selection line 35 to which a voltage is applied.

(Correction circuit 19)
The correction circuit 19 performs the X-ray image signal read by the reading circuit 17 when the continuous imaging operation is performed under the condition where X-rays are incident and the similar continuous imaging operation when the X-rays are not incident. A plurality of read dark image data (noise signals) are individually recorded, the noise signal is subtracted from the X-ray image signal, and only the image signal from which the noise signal has been removed is output to the display device 20.

(Operation of Radiation Image Detector 10)
Below, operation | movement of said radiographic image detector 10 is demonstrated.

  In the initial state, electric charge is stored in the capacitor 36 in FIG. 4, and a reverse bias voltage is applied to the photodiodes 33 connected in parallel. The voltage at this time is the same as the voltage applied to the signal line 37. Since the photodiode 33 is a kind of diode, even if a reverse bias voltage is applied, almost no current flows. Therefore, the electric charge stored in the capacitor 36 is held without decreasing.

  In this state, when the incident X-ray 21 shown in FIG. 2 enters the fluorescence conversion film 23, the high-energy X-rays are converted into a large number of low-energy visible light inside the fluorescence conversion film 23. Part of the fluorescence generated inside the fluorescence conversion film 23 reaches the photodiode 33 disposed on the surface of the image detection unit 25.

  Fluorescence incident on the photodiode 33 shown in FIG. 4 is converted into charges consisting of electrons and holes inside the photodiode 33, and reaches both terminals of the photodiode 33 along the direction of the electric field applied to the capacitor 36. Thus, the current flowing through the photodiode 33 is observed.

  The current generated inside the photodiode 33 flows into the capacitor 36 connected in parallel, and acts to cancel the electric charge stored in the capacitor 36. As a result, the charge stored in the capacitor 36 decreases, and the potential difference generated between the terminals of the capacitor 36 also decreases compared to the initial state.

  In FIG. 5, the gate driver 63 has a function of sequentially changing the potentials of a large number of control lines, but there is only one control line whose potential changes at a specific time. The source-drain terminal of the thin film transistor 31 connected in parallel to the row selection line 35 connected to the control line changes from an insulating state to a conductive state.

  A specific voltage is applied to each signal line 37 in FIG. 4, and the voltage is applied to the capacitor 36 connected through the source and drain terminals of the thin film transistor 31 connected to the row selection line 35 whose potential has changed. Will be.

  Since the capacitor 36 is in the same potential state as the signal line 37 in the initial state, if the charge amount of the capacitor 36 is not changed from the initial state, the capacitor 36 does not move the charge from the signal line 37. . However, in the capacitor 36 connected in parallel with the photodiode 33 into which the fluorescence generated inside the fluorescence conversion film 23 from the incident X-ray 21 from the outside is incident, the charge stored inside is decreased, The potential of the state has changed. For this reason, a movement of electric charge is generated from the signal line 37 through the thin film transistor 31 in the conductive state, and the amount of electric charge stored in the capacitor 36 returns to the initial state. In addition, the amount of electric charge that has moved becomes a signal flowing through the signal line 37 and is transmitted to the outside.

  The current flowing through the signal line 37 in FIG. 4 is input to the corresponding integrating amplifier 41, and the integrating amplifier 41 integrates the current flowing within a predetermined time and outputs a voltage corresponding to the integrated value to the outside. By performing this operation, the amount of charge flowing through the signal line within a certain time can be converted into a voltage value. As a result, a charge signal corresponding to the intensity distribution of the fluorescence generated inside the fluorescence conversion film 23 by the incident X-ray 21 is generated inside the photodiode 33, and this charge signal is converted into potential information by the integrating amplifier 41. Is done.

  The potential generated by the integrating amplifier 41 is sequentially converted into a digital signal by the A / D converter 67 shown in FIG. The signal having the digital value is subjected to noise signal removal by the correction circuit 19 shown in FIG. 1 via the driver 69, and then the rows and columns of pixels arranged in the circuit layer 29 by an image synthesis circuit (not shown). Are sequentially arranged and output to the outside as an image signal.

  Image information based on electrical signals output to the outside can be easily imaged by a display device 20 such as a normal display device, and an X-ray image can be observed as an image by visible light.

(Various signal operations during shooting)
FIG. 6 is a timing chart illustrating an example of a driving method when performing a normal photographing operation (single shooting), and FIG. 7 is a timing chart illustrating an example of a driving method when performing continuous shooting.

  Hereinafter, various signal operations will be described with reference to FIG.

  When the “shooting signal” in FIG. 6 is input, the refresh period 70 ends and the shooting operation is started. The imaging operation includes a pair of X-ray irradiation periods 72 and an image reading period 74.

  The “row selection line 1” is connected to the pixel group in the uppermost row of the plurality of pixels 28 arranged in a matrix in the image detection unit 25 in FIG. 2, and the “row selection line 2” is 2 from the upper end. Connected to the pixel group in the second row. Similarly, the “row selection line 3” is connected to the pixel group of the third row, and the row selection line N is connected to the pixel group of the Nth (lower end) row.

  As shown in FIG. 4, each row selection line is connected to a gate terminal of a thin film transistor (TFT) 31 disposed inside the pixel 28 of the corresponding row.

  Based on the timing signals (Lo state → Hi state) of the row selection lines 1 to N shown in FIG. 6 generated by the drive control circuit 15 of FIG. 1, the gate drive circuit 13 applies a voltage to the row selection lines 1 to N. . Here, as shown in FIG. 6, a voltage (Lo state) for insulating the connected TFTs is applied to the row selection lines 1 to N in the most part, but only in a specific period. A voltage (Hi state) for making the thin film transistor (TFT) 31 conductive is applied.

  Further, the period during which the gate driving circuit 13 applies a voltage for bringing the thin film transistor (TFT) 31 into a conductive state to each of the row selection lines 1 to N is different, and from the row selection line 1 as shown in FIG. Voltage application is performed with the period shifted to the row selection line N in order. In this way, the charge signals from the pixels 28 belonging to different rows are input to the integrating amplifier 41 through the common signal line 37 without being mixed with each other.

  Next, the “integration amplifier reset” operation in FIG. 6 resets the charge information accumulated in the integration amplifier 41 based on the timing signal generated by the first read control circuit 18a in FIG. Is. The integrating amplifier 41 accumulates the charge flowing through the signal line 37 and converts it into a voltage. In performing this operation, in the case of accumulating charge information from the pixels 28 belonging to a specific row, a reset operation for releasing the accumulation of charge information from the pixels 28 belonging to a different row in the previous operation and returning to the initial state. I do. By performing this reset operation immediately before each row selection line is turned on, only the signals from the pixels 28 belonging to the target row can be accumulated in the integrating amplifier 41.

  Further, the operation of “integrating amplifier accumulation” performs the charge accumulating operation of all the integrating amplifiers 41 connected to the signal line 37 when the signal from the first reading control circuit 18 a is in the Hi state. Accumulate the flowing charge. On the other hand, when the Lo state is set, the charge accumulation operation is paused and the accumulation of the charge flowing through the signal line 37 is paused.

  Further, the “AD conversion” operation is an analog / digital conversion performed by an A / D converter 67 connected after the integrating amplifier 41 shown in FIG. At the time when the timing signal from the first reading control circuit 18a changes from Hi to Lo, the A / D converter 67 converts the analog signal accumulated in the integrating amplifier 41 and converted into a voltage into a digital signal. As shown in FIG. 6, immediately after the “integration amplifier accumulation” timing signal is changed from the Hi state to the Lo state, the “AD conversion” timing signal is changed to the Hi state, thereby completing the charge accumulation of the integration amplifier 41. A / D conversion is performed at the timing.

(Refresh operation)
During the period from the end of the shooting operation to the arrival of the next shooting command, as the refresh periods 70 and 76, the thin film transistor 31 in the pixel 28 is turned on, and the amount of charge stored in the capacitor 36 in the pixel 28 is determined. An operation for maintaining the initial state (refresh operation) is performed.

  Usually, the amount of charge in the capacitor 36 decreases with time due to leakage current, which is a phenomenon peculiar to the photodiode 33 connected in parallel to the capacitor 36. This phenomenon occurs even when X-rays are not incident from the outside. For this reason, if the refresh operation is not performed between imaging, the amount of electric charge stored in the capacitor 36 is reduced at the next imaging, and is mixed as noise in the X-ray image incident at the next imaging. It will end up.

  In a normal operation, it is necessary to make all the thin film transistors 31 conductive by a refresh operation, and photographing cannot be performed during this period. Therefore, in the refresh operation, the voltages of the plurality of row selection lines 35 shown in FIG. 3 are simultaneously changed, and the thin film transistors 31 connected to the row selection lines 35 are turned on. By simultaneously changing the voltages of the plurality of row selection lines 35 and sequentially changing the voltages of the other row selection lines 35, all of the pixels 28 of the radiation image detector 10 having a large number of rows are refreshed. Can be shortened. By performing this operation, it is possible to minimize the delay of the next photographing operation.

(Driving method for single shooting)
As described above, in FIG. 6, the imaging operation period includes a pair of X-ray irradiation periods 72 and an image reading period 74.

  Before the imaging signal is input, the refresh operation is repeated in the refresh period 70, but when the imaging signal is input using an electric pulse or a command using a signal line, the refresh operation is terminated.

  When the repeated refresh operation in the refresh period 70 is completed, an X-ray irradiation period 72 is reached. During the X-ray irradiation period 72, all the row selection lines 35 are applied with a voltage for turning off the thin film transistors 31 stored therein, and the capacitors 36 and the photodiodes 33 in all the pixels 28 are separated from the circuit. Is done.

  In the X-ray irradiation period 72, current is generated inside the photodiode 33 due to fluorescence caused by X-rays incident from the outside, and the charge of the capacitors 36 connected in parallel is reduced. This decrease in charge is due to the amount of incident X-rays, the leakage current component peculiar to the photodiode 33, the drain-source leakage current generated in the thin film transistor 31, and the charge component remaining inside the thin film transistor 31. It will be synthesized.

  When the X-ray irradiation period 72 ends, the next is an image reading period 74. In the image reading period 74, voltages are sequentially applied to the respective row selection lines 35 shown in FIG. This voltage brings the thin film transistor 31 connected to the row selection line 35 into a conductive state, so that the capacitor 36 and the signal line 37 connected to the thin film transistor 31 in the conductive state are electrically connected. As a result, charges are transferred from the signal line 37 to the capacitor 36 connected to the thin film transistor 31 that is in a conductive state. This amount of charge is the photoelectric of the photodiode 33 due to fluorescence by incident X-rays in the X-ray irradiation period 72. The total amount of the leakage current of the photodiode 33 and the thin film transistor 31 in the period until the thin film transistor 31 becomes conductive in the X-ray irradiation period 72 and the image reading period 74.

  The charge flowing through the signal line 37 flows into the integrating amplifier 41 connected to the signal line 37. During the period in which the voltage is applied to each row selection line 35 in FIG. 3, charges are accumulated by the Hi signal in the “integration amplifier accumulation” generated by the first read control circuit 18a shown in FIG. Since the integration amplifier 41 releases the charge accumulated until immediately before by the Hi signal of “integration amplifier reset” immediately before the accumulation starts, the integration amplifier 41 is only in a period during which the thin film transistor 31 is in a conductive state. Charges flowing through the signal line 37 are accumulated.

  When “integrating amplifier accumulation” is completed, A / D conversion by the A / D converter 67 connected to the integrating amplifier 41 is started. This is started by an A / D conversion timing signal, and information on the amount of charge converted into a digital value is input to an image synthesis circuit (not shown).

  That is, by sequentially applying a voltage to the row selection line 35 in the image reading period 74, charges from the pixels 28 connected to the row selection lines 35 are sequentially amplified by the integrating amplifier 41 through the signal lines 37. The digital signal is converted by the A / D converter 67. By this operation, the amount of electrification from all the pixels 28 is converted into a digital signal and input to the image composition circuit, and the digital signal is converted into an image signal by this image composition circuit.

  When the image reading period 74 ends, a refresh period 76 starts. In the example shown in FIG. 6, a voltage is simultaneously applied to all the row selection lines 35 in the refresh period 76, and the thin film transistors 31 in all the pixels 28 are turned on by this operation. For this reason, the capacitors 36 and the signal lines 37 in all the pixels 28 are in a connected state, and charge is transferred to the capacitors 36 through the signal lines 37. This refresh operation continues for a long time until the next photographing signal is input, and the capacitor 36 in the pixel 28 can accumulate sufficient charges. When the next shooting signal is input, the above-described shooting operation is resumed.

  In addition, in the image signal output as an image signal by the above-described image composition circuit, the offset component of the integrating amplifier 41, the leakage current of the thin film transistor 31 inside the pixel 28 during the photographing period, and the leakage current of the photodiode 33 Then, the charge component remaining inside the thin film transistor 31 is included. Since these signals are mixed as noise in the image signal, it is necessary to remove them by image calculation.

  In this image calculation, dark image data is usually obtained by performing a photographing operation without incident X-rays immediately before or immediately after photographing, and the dark image is obtained from the image data obtained at the time of X-ray incidence by the correction circuit 19. This can be done by subtracting the data. The acquisition of the dark image data is performed in the same manner as normal imaging, except that no X-ray is input to the radiation image detector 10.

(Driving method for continuous shooting)
FIG. 7 shows a timing chart of a driving method when performing continuous shooting.

  First, when a shooting signal for starting continuous shooting is input, the first shooting operation is started. The period of the first imaging operation includes a first X-ray irradiation period 82 and a first image reading period 84.

  The first shooting operation is the same as the normal single shooting operation shown in FIG. That is, the first refresh period 80 having a sufficient period is completed by the input of the imaging signal, and the process proceeds to the first X-ray irradiation period 82. The operation in the first X-ray irradiation period 82 is the same as that in the X-ray irradiation period 72 during single-shot imaging shown in FIG. Next, when the first X-ray irradiation period 82 ends, the process proceeds to the first image reading period 84. The operation in the first image reading period 84 is the same as that in the image reading period 74 shown in FIG.

  Similarly, the operations of the second X-ray irradiation period 88 and the second image reading period 90 in the second imaging are the same as those of the X-ray irradiation period 72 and the image reading period 74 shown in FIG.

  However, in the case of this continuous shooting, a second refresh period 86 different from the refresh period 76 in FIG. 6 exists between the first shooting and the second shooting. The time of the second refresh period 86 is significantly shorter than the refresh period 76 of FIG. 6 because a plurality of images are taken in a short time.

  In the example shown in FIG. 7, the voltage application to all the row selection lines 35 is performed only once in the second refresh period 86.

  In normal X-ray imaging, when a single X-ray image is taken, an imaging engineer performs an operation of judging by looking at the X-ray image, so the next imaging operation is performed after a period of at least several seconds. On the other hand, in the continuous shooting operation, the continuous shooting operation must be completed in as short a time as possible in order to suppress image blurring in each shooting due to the movement of the human body that is the subject. For this reason, it is necessary to reduce the borrow period of the refresh period between each shooting in continuous shooting.

  In general, in the thin film transistor 31 in each pixel 28, in most of the X-ray irradiation period 72 and the image reading period 74, a state in which no voltage is applied to the gate terminal is very long. Residual charge is generated inside. This is a problem peculiar to the thin film transistor 31 composed of an amorphous semiconductor, and it is very difficult to solve this problem.

  However, in normal single shooting, the refresh periods 70 and 76 are very long, and a voltage is frequently applied to the gate terminal of the thin film transistor 31. Therefore, the charge remaining in the semiconductor film inside the thin film transistor 31 can be sufficiently initialized during the refresh periods 70 and 76.

  However, in the second imaging in the continuous imaging of FIG. 7, the second refresh period 86 immediately before the second X-ray irradiation period 88 is short, and therefore during the first X-ray irradiation period 82 and the first image reading period 84. It is not possible to sufficiently initialize the residual charge in the thin film transistor 31 that is generated in the above. The residual charge generated in the thin film transistor 31 is mixed into a current component accompanying the movement of the charge to the capacitor 36 in the pixel 28 in the next image reading period, and finally appears as noise in the output image. Become.

  At this time, if the numerical value consisting of one kind of dark image component is subtracted from the image signal output from the image synthesis circuit by the same method as the conventional single shooting, the dark image component is different between the first and second images. It becomes impossible to perform optimal image processing on both images at the same time.

  Therefore, in the present embodiment, a plurality of dark image data is prepared, and an optimum X-ray image can be obtained by performing a difference process from the X-ray image obtained in continuous imaging. Specifically, the same continuous imaging operation as usual is executed under the condition where X-rays are not incident, and reading is performed by the reading circuit 17 by the reading signal generated by the second reading control circuit 18b shown in FIG. A plurality of pieces of dark image data obtained are individually recorded in the correction circuit 19. At this time, continuous imaging is performed a plurality of times, and dark image data obtained at that time is individually averaged, whereby dark image data with higher accuracy can be obtained.

  Further, the accumulation time of dark image data used for image correction may be set to n times the normal time. In this case, the amount of charge obtained is n times, but the amount of noise generated from the integrating amplifier 41 and the A / D converter 67 does not change. Therefore, by setting the obtained charge amount to 1 / n, it can be made equal to the charge amount of the normal accumulation time, and the noise amount generated from the integrating amplifier 41 and the A / D converter 67 is set to 1 / n. be able to. Therefore, in the correction of the X-ray image, it is possible to use a correction charge amount with less noise from the integrating amplifier 41 and the A / D converter 67, so it is possible to use highly accurate dark image data. In addition, it is preferable to make n into the range of 2-10 from a viewpoint of reducing noise.

  Next, using the dark image data obtained by the above-described operation, the correction circuit 19 performs a subtraction process on the X-ray image data obtained by the continuous imaging operation when X-rays are incident. At this time, the first X-ray image is subjected to subtraction processing using the first dark image data, and similarly, the second dark image data is obtained from the second X-ray image data. To perform subtraction processing. When there is X-ray image data for the third and subsequent images in the continuous imaging operation, optimal processing can be performed by performing the same processing.

  As a result of actual verification using a radiographic image detector, 2.63 LSB noise is mixed with the output X-ray image in the normal image processing using only one type of dark image data. In the subtraction process according to the present embodiment, 2.26 LSB noise is mixed in the output X-ray image, and the effect of noise reduction can be confirmed.

  In the above embodiment, an example of the indirect system in which the photodiode 33 is provided as the radiation sensor 11 is shown. However, the X-ray is directly converted into a charge signal by the photoconductive film and led to the charge storage capacitor. The same applies to the direct method. In this case, the same effect can be obtained by omitting the fluorescence conversion film 23 and using a photoconductive film such as a-Se instead of the photodiode 33 of the image detection unit 25.

10: Radiation image detector 11: Radiation sensor 13: Gate drive circuit 15: Drive control circuit 17: Reading circuit 18a: First reading control circuit 18b: Second reading control circuit 19: Correction circuit 20: Display device 21: Incident X-ray 23: Fluorescence conversion film 25: Image detection unit 27: Holding substrate 28: Pixel 29: Circuit layer 31: Thin film transistor 33: Photo diode 35: Row selection line 36: Capacitor 37: Signal line 41: Integration amplifier 63: Gate Driver 65: Row selection circuit 67: A / D converter 69: Driver 70, 76: Refresh period 72, 78: X-ray irradiation period 74: Image reading period 80: First refresh period 82: First X-ray irradiation period 84 : First image reading period 86: Second refresh period 88: Second X-ray irradiation period 90: Second image reading period

Claims (5)

A plurality of pixels arranged in a two-dimensional manner on a substrate and incident on visible light and converted into signal information of one image as a whole extend in the row direction in one and a plurality of row selection lines. Radiation sensor comprising an image detection unit connected to each of a plurality of signal lines to be stretched, and a fluorescence conversion film provided on the plurality of pixels of the image detection unit for converting radiation into visible light When,
A gate driving circuit for sequentially applying a driving voltage to each row selection line;
In a state in which the radiation is irradiated during an arbitrary number of images when performing continuous imaging of n images through a refresh operation that initializes the state of charge in the pixel every time imaging is completed. A first reading control circuit that issues a reading signal for reading image signal information from the pixel via the signal line connected to a pixel connected to any one of the row selection lines to which the driving voltage is applied When,
Arbitrary image to which the drive voltage is applied during non-irradiation imaging of the same number as the arbitrary number of images when continuous non-irradiation imaging is performed in the same manner as the continuous imaging in a state where the radiation is not irradiated. A second read control circuit for issuing a read signal for reading noise signal information from the pixel via the signal line connected to the pixel group connected to one row selection line;
The image signal information is read by a read signal from the first read control circuit, the noise signal information is read by a read signal from the second read control circuit, and the image signal information and the noise signal information are stored. A reading circuit to
A correction circuit for correcting the image signal information based on the noise signal information;
A display device for displaying the corrected image signal information;
A radiation image detector comprising: One of a plurality of row selection lines extending in the row direction and a plurality of signal lines extending in the column direction, which are two-dimensionally arranged on the substrate and convert the radiation into electrical signals. A radiation sensor having an image detection unit connected to each one of them,
A gate driving circuit for sequentially applying a driving voltage to each row selection line;
In a state in which the radiation is irradiated during an arbitrary number of images when performing continuous imaging of n images through a refresh operation that initializes the state of charge in the pixel every time imaging is completed. A first reading control circuit that issues a reading signal for reading image signal information from the pixel via the signal line connected to a pixel connected to any one of the row selection lines to which the driving voltage is applied When,
Arbitrary image to which the drive voltage is applied during non-irradiation imaging of the same number as the arbitrary number of images when continuous non-irradiation imaging is performed in the same manner as the continuous imaging in a state where the radiation is not irradiated. A second read control circuit for issuing a read signal for reading noise signal information from the pixel via the signal line connected to the pixel group connected to one row selection line;
The image signal information is read by a read signal from the first read control circuit, the noise signal information is read by a read signal from the second read control circuit, and the image signal information and the noise signal information are stored. A reading circuit to
A correction circuit for correcting the image signal information based on the noise signal information;
A display device for displaying the corrected image signal information;
A radiation image detector comprising:   The radiation image detector according to claim 1, wherein the correction circuit subtracts the noise signal information from the image signal information.   The noise signal information uses an average value of the plurality of noise signal information for each arbitrary number of images when the non-irradiation continuous imaging is repeated a plurality of times in a state where the radiation is not irradiated. The radiation image detector according to claim 1, wherein the radiation image detector is a radiographic image detector.   The noise signal information uses a charge signal obtained by accumulating a charge amount by multiplying the normal accumulation time by n times in a state where the radiation is not irradiated by the reading circuit and reducing the charge amount to 1 / n. The radiation image detector according to any one of claims 1 to 4, wherein the radiation image detector is provided.

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