JP2013098796A - Radiation image detector and irradiation detection method used for radiation image detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To speedily and accurately detect irradiation of radiation.SOLUTION: An electronic cassette self detects start of irradiation of X-ray from an X-ray source using a pixel of an FPD. The FPD can read a signal charge from a pixel in a nondestruction manner, and can change gain of a signal output from the pixel. In irradiation start detection operation, first, the electronic cassette sets the gain of the pixel to high gain GnL to read an output signal, and performs primary determination to determine whether or not the signal value SV is threshold Th or more. When the primary determination is affirmed, the gain is changed to low gain GnL, the output signal is read and secondary determination is performed with the signal value SV. Since primary determination is performed at high gain GnH, start of irradiation can be detected at a stage where intensity of x-ray is low. Since secondary determination is performed at low gain GnL, accurate detection is performed.

Description

  The present invention relates to a radiation image detection apparatus for detecting a radiation image and an irradiation detection method used in the radiation image detection apparatus.

  In recent years, in the field of radiography, for example, X-ray imaging, X-ray image detection apparatuses using flat panel detectors (hereinafter referred to as FPD) using semiconductor elements instead of X-ray films and imaging plates (IP) have become widespread. ing. The FPD is an image sensor formed using a semiconductor element, and has an imaging region in which pixels that accumulate signal charges corresponding to the amount of incident X-rays are arranged in a two-dimensional matrix of rows and columns. The FPD detects an X-ray image representing image information of a subject by converting a signal charge accumulated for each pixel upon incidence of X-rays into a voltage signal. An X-ray image detected by the FPD is output as digital image data.

  There are two types of FPDs, a destructive read type and a non-destructive read type, depending on the signal read type. In the destructive readout method, a signal charge accumulated in a pixel having a photoelectric conversion element such as a photodiode is transferred to an integration amplifier via a signal line, and an output signal (voltage signal) corresponding to the signal charge integrated by the integration amplifier ) And is called destructive readout because the signal charge in the pixel is emptied by the transfer of the signal charge. On the other hand, the nondestructive readout method is a method in which an amplifier that converts signal charge into an output signal is provided for each pixel, and an output signal corresponding to the signal charge is read with the signal charge held in the pixel. The signal charge accumulated in the pixel is also held without being emptied, and is called nondestructive readout.

  Patent Document 1 describes an FPD of a non-destructive readout method, and an irradiation detection sensor for detecting that irradiation with an X-ray source has started (irradiation start) and that irradiation has ended (irradiation end). It is disclosed that an FPD pixel is used. The irradiation detection is performed in order to synchronize the X-ray irradiation period by the X-ray source and the signal accumulation period of the FPD, and the FPD is self-detected by using the FPD pixels to thereby detect the X-ray source and the FPD. The synchronization control for communicating the synchronization signal between the two becomes unnecessary.

  As described in paragraph 0065 of Patent Document 1, in the case of the non-destructive readout method, the signal charge in the pixel is retained even if the output signal is read out while the X-ray is irradiated. Can be reflected in the X-ray image without wasting. Generally, in order to ensure the image quality of an X-ray image, a higher X-ray irradiation dose is better. If unnecessary X-rays that are not reflected in the X-ray image despite being irradiated on the subject can be reduced, the image quality of the X-ray image can be secured even if the irradiation dose of unnecessary X-rays is reduced. As a result, the exposure dose of the subject (patient) can be reduced.

  The FPD described in Patent Document 1 repeats signal readout from a pixel during standby waiting for X-ray irradiation start in irradiation start detection, and the difference between signal values of output signals that are continuously read out a plurality of times. Based on the above, the start of irradiation is determined. Since the difference increases when X-rays are incident, it is determined that irradiation starts when the difference becomes equal to or greater than the threshold value.

  When the difference is less than the threshold value, a reset for sweeping out the charge accumulated in the pixel is performed. This is to remove noise components caused by dark current accumulated in the pixels while no X-rays are incident. This reset is not performed when the difference is greater than or equal to the threshold value (when it is determined that irradiation has started), and the process proceeds to a signal accumulation operation.

  In the irradiation end detection, the signal reading from the pixel is repeatedly performed during the X-ray irradiation, and it is determined that the irradiation is ended when the signal value of the output signal continuously read out a plurality of times becomes less than the threshold value. . The X-ray image is read after the end of irradiation is detected.

JP 2003-126072 A

  As described in Patent Document 1, it is a very important issue from the viewpoint of reducing the exposure dose of a subject to reflect irradiated X-rays in an X-ray image without wasting them. Therefore, further contrivance for reducing the exposure dose as much as possible is required.

  The intensity of X-rays irradiated by the X-ray source (dose per unit time) gradually increases after receiving the irradiation start command and rises to a peak. The response of the X-ray source at the beginning of the start is slow, and the slope of the rise is gentle particularly at the initial stage immediately after receiving the start command. When the irradiation start is detected by the FPD, if the irradiation start can be detected at a stage where the intensity of the X-ray is low, the time lag from when the X-ray source starts irradiation until the FPD detects the irradiation start becomes short. Wasteful X-rays that are not reflected in the X-ray image are reduced, and the exposure dose can be reduced.

  At the stage where the intensity of the X-ray is low, the amount of signal charge accumulated is small, and the corresponding output signal is also small. If the output signal is small, there are many detection misses that the irradiation start cannot be detected despite the X-ray irradiation, and the time lag becomes long. Therefore, if a large output signal can be obtained even with a small amount of signal charge, detection misses are reduced, so that the time lag is shortened and useless X-rays can be reduced.

  As a countermeasure, it is conceivable to increase the sensitivity of the pixel. Specifically, the sensitivity of the pixel can be increased by increasing the amplification factor (gain) of the amplifier when reading the output signal of the pixel. If the gain is increased, a large output signal can be obtained even if the charge accumulation amount is small. In addition, since the signal value of the output signal is relatively large with respect to circuit noise (for example, circuit noise generated in the ASIC constituting the readout circuit) generated regardless of the magnitude of the gain, the S / N ratio is increased. Will also improve.

  However, in the pixel, charges caused by dark current are generated regardless of whether X-rays are incident. Dark current noise caused by dark current is amplified by an amplifier and appears as an output signal. Therefore, unlike circuit noise that is unrelated to the magnitude of the gain, dark current noise increases when the gain is high. In addition to dark current noise, there is vibration noise caused by vibration or impact, and the vibration noise is amplified by an amplifier and appears as an output signal. For this reason, when the gain is increased to increase the sensitivity of the pixel, there is a concern that dark current noise and vibration noise are picked up and erroneously detected.

  In addition, as soon as the gain is increased, the output signal is saturated. When the output signal is saturated, it becomes impossible to determine whether or not the X-ray intensity has increased. If the sensitivity of the pixel is low, there is little risk of such false detection or determination being impossible. However, as described above, if the sensitivity is low, the start of irradiation cannot be detected when the X-ray intensity is low, so the time lag cannot be shortened and wasted. Can't reduce X-rays.

  Patent Document 1 does not clearly indicate or suggest these problems and solutions. Also in the configuration of Patent Document 1, there is a problem of false detection or output signal saturation if the pixel sensitivity is high. Further, if the sensitivity of the pixel is low, the above-described detection miss is likely to occur. This is because, in the FPD of Patent Document 1, the start of irradiation is determined based on the difference between two output signals that are continuously read out, but at the initial stage of the start of irradiation where the X-ray intensity gradually increases. The difference does not increase unless the difference is small and the slope of the increase is increased to some extent. In addition, since the difference is amplified by the gain, the difference is smaller as the sensitivity of the pixel is lower, and detection is likely to be missed when the intensity of the X-ray at the beginning of irradiation is low.

  In paragraph 0064 of Patent Document 1, when the start of irradiation is determined (difference is greater than or equal to the threshold), the process proceeds to the accumulation operation without resetting, thereby delaying the irradiation determination timing from the X-ray irradiation start timing. Even when a time lag occurs, it is described that unnecessary X-rays are reduced. However, as described in paragraph 0065 of Patent Document 1, when it is not determined that irradiation is started (difference is less than the threshold value), resetting is performed to discard the signal charge. If a miss occurs, X-rays are wasted due to the time lag.

  Although the detection of the start of irradiation has been described, the same problem as that of the start of irradiation occurs in the detection of the end of irradiation. That is, in order to quickly detect the end of irradiation, it is necessary to detect a small change in the output signal at an early stage. The higher the pixel sensitivity, the higher the pixel sensitivity. Concerns arise.

  The present invention has been made in view of the above problems, and an object of the present invention is to quickly and accurately perform irradiation detection using a radiological image detection apparatus.

  The radiological image detection apparatus of the present invention is an image detection panel that receives a radiation irradiated from a radiation source and transmitted through a subject to detect a radiographic image, and a plurality of pixels that accumulate signal charges according to the amount of incident radiation. An image detection panel having an imaging region arranged two-dimensionally in rows and columns, and an irradiation detection region for detecting at least one of the start of irradiation and the end of irradiation, An output detection signal corresponding to the signal charge is read from the irradiation detection area including at least one pixel set in the imaging area and the pixel in the irradiation detection area in a state where the signal charge is held in the pixel. When reading the output signal, an output signal reading unit, an irradiation detection unit that detects at least one of the start and end of irradiation based on the signal value of the output signal, and Which is the amplification factor gain, characterized in that it comprises at least high gain and low gain two gain changeable gain setting part.

  It is preferable to have an area setting unit that can set the irradiation detection area at an arbitrary position in the imaging area. Moreover, it is preferable that the area setting unit can set the irradiation detection areas at a plurality of locations in the imaging area. Moreover, it is preferable that the said area setting part can set the said irradiation detection area in the whole region of the said imaging area.

  The imaging apparatus may further include an imaging condition receiving unit that receives an input of imaging conditions including at least one of an irradiation dose irradiated by the radiation source and an imaging region of the subject. In this case, it is preferable that the area setting unit sets the irradiation detection area based on the imaging region included in the imaging conditions. Further, it is preferable that the gain setting unit sets an initial value of the gain used in detecting at least one of the start and end of the irradiation based on the irradiation dose included in the imaging condition.

  For example, the irradiation detection unit detects the start of the irradiation. It is preferable that the irradiation detection unit includes an irradiation determination unit that determines whether or not the irradiation is started based on at least two types of output signal values read at high gain and low gain.

  The irradiation determination unit performs a primary determination as to whether or not the signal value of the output signal read at high gain is equal to or greater than a preset threshold value, and is determined that the signal value is equal to or greater than the threshold value in the primary determination. In this case, it is preferable to perform secondary determination based on the signal value of the output signal read at the low gain.

  The irradiation determination unit determines whether the signal value read at the low gain is equal to or higher than the threshold in the secondary determination, and determines that the irradiation is started when the determination result is affirmative. It is preferable to do.

  The output signal reading unit reads the output signal with a high gain from the irradiation detection region, and after reading the high gain, the gain setting unit changes the gain of the same irradiation detection region to a low gain, and the low gain It is preferable to read the output signal.

  Further, the irradiation detection area has at least two areas of a high sensitivity area where the gain is set to a high gain and a low sensitivity area where the gain is set to a low gain by the gain setting unit, and the output The signal readout unit may read out two types of output signals of high gain and low gain from the high sensitivity region and the low sensitivity region, respectively.

  The gain setting unit preferably decreases the gain value stepwise when the signal value of the output signal is saturated.

  The irradiation detection unit detects, for example, the end of the irradiation. Preferably, the irradiation detection unit determines that the irradiation has ended when a signal value of the output signal read out a plurality of times continuously with the same gain becomes substantially constant. The irradiation detection unit preferably detects both the start and end of the irradiation.

  The gain setting unit initially sets at least one of the irradiation detection area and the initial gain value when detecting the end of the irradiation based on a result when the irradiation detecting unit detects the start of the irradiation. It is preferable to do.

  The image detection panel is, for example, a CMOS type capable of reading an X-ray image while holding the signal charge in the pixel. The radiological image detection apparatus is, for example, an electronic cassette in which the image detection panel is accommodated in a portable housing.

  The irradiation detection method used in the radiation image detection apparatus of the present invention is an image detection panel that can detect a radiation image by receiving radiation irradiated from a radiation source and transmitted through a subject, and a signal charge corresponding to the amount of incident radiation. Using an image detection panel that has an imaging region in which a plurality of pixels that store two-dimensionally are arranged in rows and columns, and that can read an output signal corresponding to the signal charge while holding the signal charge in the pixel In the irradiation detection method for detecting at least one of the start and end of the radiation irradiation, the irradiation detection method includes at least one of the pixels, and the irradiation start and the irradiation end. In the irradiation detection area set in the imaging area in order to detect at least one of them, an output signal readout scan for reading out the output signal from the pixel. And an irradiation detection step for detecting at least one of the start and end of the irradiation based on the signal value of the output signal. In the irradiation detection step, an amplification factor when reading the output signal And a gain setting step for changing the gain to at least two types of gains, a high gain and a low gain.

  In the present invention, when the irradiation detection is performed based on the output signal of the pixel, the gain at the time of reading the output signal is read by at least two types of gains, a high gain and a low gain. It can be done quickly and accurately.

It is the schematic which shows the structure of a X-ray imaging system. It is explanatory drawing which shows the operation timing at the time of imaging | photography. It is a block diagram which shows the electric structure of FPD. It is explanatory drawing of an irradiation detection area | region. It is explanatory drawing of the determination process at the time of the irradiation start detection. It is explanatory drawing of the determination process at the time of the completion | finish detection of irradiation. It is a flowchart which shows the operation | movement procedure at the time of imaging | photography. It is a flowchart which shows the procedure of irradiation start detection operation | movement. It is a flowchart which shows the procedure of irradiation completion | finish detection operation | movement. It is explanatory drawing which shows the example which set the irradiation detection area | region to the imaging area. It is explanatory drawing of the example which connected the variable gain amplifier to the output line. It is explanatory drawing of the example which set two area | regions, a high sensitivity area | region and a low sensitivity area | region, as an irradiation detection area | region. It is a flowchart which shows the operation | movement procedure of the example of FIG.

[First Embodiment]
In FIG. 1, the X-ray imaging system 10 includes an X-ray generation device 11 and an X-ray imaging device 12. The X-ray generator 11 includes an X-ray source 13, a radiation source controller 14 that controls the X-ray source 13, and an irradiation switch 15. The X-ray source 13 includes an X-ray tube 13a that irradiates X-rays, and an irradiation field limiter (collimator) 13b that limits an X-ray irradiation field irradiated by the X-ray tube 13a.

  The X-ray tube 13a has a cathode made of a filament that emits thermoelectrons, and an anode (target) that emits X-rays when the thermoelectrons emitted from the cathode collide. The irradiation field limiter 13b has, for example, a plurality of lead plates that shield X-rays arranged in a cross pattern, and an irradiation opening that transmits X-rays is formed in the center, and moves the position of the lead plate. By changing the size of the irradiation aperture, the irradiation field is limited.

  The radiation source control device 14 includes a high voltage generator that supplies a high voltage to the X-ray source 13, a tube voltage that determines the quality (energy spectrum) of the X-rays that the X-ray source 13 irradiates, and a unit time. It consists of a tube current that determines the dose and a controller that controls the irradiation time of the X-rays. The high voltage generator boosts the input voltage with a transformer to generate a high voltage tube voltage, and supplies driving power to the X-ray source 13 through a high voltage cable. Imaging conditions such as tube voltage, tube current, and irradiation time are manually set by an operator such as a radiographer through the operation panel of the radiation source control device 14.

  The irradiation switch 15 is connected to the radiation source control device 14 by a signal cable, and is operated by an operator. The irradiation switch 15 is a two-stage push switch, which generates a warm-up start signal for starting the warm-up of the X-ray source 13 by pressing the single stage, and for starting the irradiation of the X-ray source 13 by pressing the two-stage. The irradiation start signal is generated. These signals are input to the radiation source controller 14 through a signal cable.

  The X-ray imaging apparatus 12 includes an electronic cassette 21, an imaging table 22, an imaging control apparatus 23, and a console 24. The electronic cassette 21 includes an FPD 25 and a portable housing 26 that accommodates the FPD 25. The electronic cassette 21 receives X-rays irradiated from the X-ray source 13 and transmitted through the subject H such as a patient, and receives X-rays from the subject H. It is a portable radiographic image detection device that detects an image. The electronic cassette 21 has a flat housing whose planar shape is substantially rectangular, and the planar size is substantially the same as that of a film cassette or an IP cassette.

  As will be described later, the electronic cassette 21 has a function of receiving X-rays emitted from the X-ray source 13 by the FPD 25 and self-detecting the start and end of X-ray irradiation. Therefore, synchronization control by communication of a synchronization signal that synchronizes the irradiation timing of the X-ray source 13 and the operation timing of the FPD 25 is not required between the X-ray generation device 11 and the X-ray imaging device 12.

  The imaging table 22 has a slot in which the electronic cassette 21 is detachably attached, and holds the electronic cassette 21 with an incident surface on which X-rays are incident facing the X-ray source 13. The electronic cassette 21 can be attached to a film cassette or an IP cassette photographing stand because the size of the casing is substantially the same as that of the film cassette or the IP cassette. In addition, although the standing position imaging stand which image | photographs the subject H with a standing posture is illustrated as the imaging stand 22, the supine position imaging stand which image | photographs the subject H with a standing posture may be sufficient.

  The imaging control device 23 is connected to the electronic cassette 21 so as to be communicable by a wired method or a wireless method, and controls the electronic cassette 21. Specifically, the imaging conditions are transmitted to the electronic cassette 21 to set the signal processing conditions of the FPD 36 and the like. Further, the imaging control device 23 receives the image data output from the electronic cassette 21 and transmits it to the console 24.

  The console 24 receives an input of an examination order including information such as the patient's sex, age, imaging region, and imaging purpose, and displays the examination order on the display. The examination order is input through a network from an ordering system that manages patient information such as HIS (Hospital Information System) and RIS (Radiation Information System) and examination information related to radiation examination, or a keyboard or mouse of the console 24, etc. Manually input by the operator from the operation unit.

  The operator confirms the contents of the inspection order on the display and determines the imaging conditions according to the contents. Specifically, the X-ray quality and radiation dose to be irradiated are determined according to the patient's sex, age, imaging region, and imaging purpose, and the tube voltage and radiation dose that define the radiation quality are defined. The tube current and irradiation time are determined as imaging conditions. The shooting conditions are set through the operation screen of the console 24. The console 24 sets imaging conditions including the imaging region in addition to the tube voltage, the tube current, and the irradiation time for the electronic cassette 21 through the imaging controller 23. The setting of imaging conditions for the X-ray generator 11 is also performed based on the imaging conditions input to the console 24.

  The console 24 performs various image processing such as gamma correction and frequency processing on the X-ray image data transmitted from the imaging control device 23. The X-ray image subjected to the image correction processing is displayed on the display of the console 24, and the data is stored in a data storage device such as a hard disk or memory in the console 24 or an image storage server connected to the console 24 via a network.

  As shown in FIG. 2, the radiation source control device 14 controls the operation of the X-ray source 13 based on a control signal from the irradiation switch 15. When receiving an irradiation start signal (ON signal) from the irradiation switch 15, the radiation source control device 14 issues a start command to the X-ray source 13 and starts supplying power. Thereby, the X-ray source 13 starts irradiation. The radiation source control device 14 starts the measurement of the X-ray irradiation time by operating a timer together with the start of power supply. Then, the timer is monitored, and when the irradiation time set in the imaging conditions has elapsed, a stop command is issued to the X-ray source 13 to stop the power supply. When receiving the stop command, the X-ray source 13 stops the X-ray irradiation. Although the X-ray irradiation time varies depending on the imaging conditions, in the case of still image shooting, the maximum X-ray irradiation time is often set to a range of about 500 msec to about 2 s, and the irradiation time Is set with this maximum irradiation time as the upper limit.

  Before the irradiation switch 15 is pressed by the operator, a standby instruction for waiting for the start of X-ray irradiation is input to the electronic cassette 21 through the console 24. When the standby instruction is input, the electronic cassette 21 starts an irradiation detection operation for detecting the start of X-ray irradiation by the FPD 25. In this state, when the X-ray source 13 starts X-ray irradiation, the start of irradiation is detected based on the X-rays that the FPD 25 enters. When the electronic cassette 21 detects the start of irradiation, the electronic cassette 21 proceeds to an accumulation operation for accumulating signal charges according to the amount of incident X-rays. Then, when shifting to the accumulation operation, an irradiation end detection operation for detecting the end of X-ray irradiation is started in parallel with the accumulation operation. When the X-ray source 13 stops the X-ray irradiation, the electronic cassette 21 detects the end of the irradiation, ends the accumulation operation, and reads out the X-ray image.

  As shown in FIG. 3, the FPD 25 has an image detection panel 36 having an imaging region 36a in which a plurality of pixels PX that accumulate signal charges corresponding to the amount of incident X-rays are arranged. The plurality of pixels PX are two-dimensionally arranged in a matrix of n rows (x direction) × m columns (y direction) at a predetermined pitch. Reference numerals 11, 12, 21, and 22 assigned to the pixel PX represent addresses in the imaging region 36a of the pixel PX. Except for the case of describing a specific pixel, the address is omitted and the description will be made simply as the pixel PX.

  The FPD 25 is an indirect conversion type that performs photoelectric conversion after converting X-rays into visible light. A scintillator (not shown) is disposed on the image detection panel 36 so as to face the entire surface of the imaging region 36a. The The scintillator is made of a phosphor such as CsI (cesium iodide) or GOS (gadolinium oxysulfide) that converts X-rays into visible light. The image detection panel 36 is, for example, a non-destructive readout type image sensor capable of reading a voltage signal corresponding to a signal charge while holding the signal charge in the pixel PX. For example, it is an image sensor in which the imaging region 36a is enlarged by bonding a plurality of CMOS sensor chips each having a pixel PX formed on a single crystal silicon substrate.

  The pixel PX includes circuit elements such as a photodiode PD, a capacitor CP, an amplifier AP, a reset transistor Tr1, and a pixel selection transistor Tr2. The photodiode PD photoelectrically converts visible light converted by the scintillator and accumulates signal charges corresponding to the amount of incident X-rays. The capacitor CP is an auxiliary capacitor for holding signal charges. The capacitor CP is not necessary when the charge holding capacity of the photodiode PD is sufficient. The amplifier AP amplifies and outputs a voltage signal corresponding to the signal charge accumulated in the photodiode PD and the capacitor CP. The amplifier AP outputs a voltage signal with the signal charge held in the pixel PX. The pixel PX retains signal charges unless reset is performed. The amplifier AP is a variable gain amplifier that can change the gain (amplification factor).

  When the gain value is changed, the sensitivity of the pixel PX changes, and the sensitivity of the pixel PX increases as the gain value increases. Comparing with the same charge accumulation amount, the higher the sensitivity, the larger the signal value of the output voltage signal, and the S / N ratio increases for circuit noise that occurs regardless of the magnitude of the gain.

  The amplifier AP is an amplifier that, for example, feeds back the output of the operational amplifier to the input side to amplify and output the input voltage. The amplifier AP is connected to the input terminal of the operational amplifier (not shown), and the operational amplifier input terminal and output terminal. This is a variable gain amplifier capable of changing the gain by changing the ratio of the resistance value to a feedback resistor (not shown) connected between the two. A gain control line GC is connected to the amplifier AP, and the gain value is set by changing the resistance value of the input resistance or feedback resistance based on the gain setting signal input from the gain control line GC. The Compared to the imaging area of an image sensor used in a general-purpose digital camera, the imaging area of an X-ray FPD is large, so that there is a spatial margin for making such a variable gain amplifier for each pixel.

  The reset transistor Tr1 is a transistor for resetting the pixel PX. A reset line RST is connected to the gate electrode of the reset transistor Tr1, and when a reset signal is input from the reset line RST, the reset transistor Tr1 drains the signal charges accumulated in the photodiode PD and the capacitor CP. To reset the pixel PX.

  The pixel selection transistor Tr2 has a source electrode connected to the output terminal of the amplifier AP, a gate electrode connected to the row selection line SR, and a drain electrode connected to the column signal line SC. The pixel selection transistor Tr2 is turned on when a row selection signal is input to the gate electrode, and applies a voltage signal of the amplifier AP to the column signal line SC. The source electrode of the column selection transistor 37 is connected to the output end on the downstream side of the column signal line SC. The column selection transistor 37 has a drain electrode connected to the output line 38 and is turned on when a column selection signal is input to the gate electrode of the column line selection transistor 37, and a voltage applied from the column signal line SC. A signal is output to the output line 38.

  The vertical scanning circuit 41 and the horizontal scanning circuit 42 are scanning circuits for selecting a pixel PX that outputs a voltage signal to the output line 38. The vertical scanning circuit 41 inputs a row selection signal to the row selection line SR. When a row selection signal is input to the row selection line SR, the plurality of pixel selection transistors Tr2 connected to the row selection line SR are turned on. The horizontal scanning circuit 42 turns on the column selection transistor 37 by inputting a column selection signal to the column selection transistor 37. In each pixel PX, when each pixel selection transistor Tr2 and each column selection transistor 37 connected via the column signal line SC are simultaneously turned on, the voltage signal of each pixel PX is output to the output line 38. Is done.

  In the image reading operation, the vertical scanning circuit 41 sequentially transmits row selection signals to the row selection lines SR1, SR2,... From the first row to the last row. The horizontal scanning circuit 42 sequentially turns on the column selection transistors 37 of each column while the pixel selection transistors Tr2 of the pixels PX11, PX12,... For one row are turned on by the row selection signal. Such a procedure is repeated until the last row, and all the pixels PX in the imaging region 36a are sequentially selected. The voltage signal of each pixel PX is sequentially output to the output line 38. Thereby, a voltage signal representing an X-ray image for one screen is read out.

  The FPD 25 includes a control unit 46, a timing generator (TG) 47, an A / D converter 48, a memory 49, an irradiation detection unit 51, and a communication unit 52 in addition to the image detection panel 36, the vertical scanning circuit 41, and the horizontal scanning circuit 42. It has.

  The control unit 46 comprehensively controls each unit of the FPD 25. The TG 47 generates a clock signal based on the control signal from the control unit 46 and inputs it to the vertical scanning circuit 41 and the horizontal scanning circuit 42. The vertical scanning circuit 41 and the horizontal scanning circuit 42 generate a row selection signal and a column selection signal, respectively, in synchronization with the clock signal.

  The A / D converter 48 converts analog voltage signals sequentially output from the output line 38 into digital signals. The memory 49 temporarily stores the digital signal output from the A / D converter 48. When image reading is performed, the memory 49 stores a digital signal representing an X-ray image. The communication unit 52 receives control signals and shooting conditions from the shooting control device 23 and inputs them to the control unit 46. The control unit 46 receives the input control signal and imaging conditions. In addition, the communication unit 52 transmits the X-ray image read from the memory 49 to the imaging control device 23.

  The irradiation detection unit 51 detects the start and end of irradiation of X-rays emitted from the X-ray source 13 using the pixels PX. The irradiation detection unit 51 includes an area setting unit 51b, a gain setting unit 51a, and an irradiation determination unit 51c.

  As shown in FIG. 4, the area setting unit 51b sets an irradiation detection area 56 including at least one pixel PX used for irradiation detection in the imaging area 36a. A plurality of irradiation detection areas 56 are set in the imaging area 36a. In this example, a state is shown in which the irradiation detection areas 56 are set at a total of five locations including the center position of the imaging area 36a and the four corners centered on the center position. Each irradiation detection area 56 is composed of four adjacent pixels PX.

  The irradiation detection area 56 can be set at an arbitrary size (number of pixels PX) at an arbitrary position in the imaging area 36a. The area setting unit 51b includes table data in which imaging parts such as a head, a chest, and an abdomen are associated with position information and size information of the irradiation detection area 56, and the imaging conditions accepted by the control unit 46 are included. The position and size of the irradiation detection area 56 are determined in accordance with the imaging regions such as the head, chest, and abdomen that are included. Of course, input of position information and size information may be directly accepted, and the position and size of the irradiation detection region 56 may be determined based on the input position information and size information. In this way, the position and size of the irradiation detection area 56 can be set regardless of preset table data, and the position and size of the irradiation detection area 56 determined by the table data can be finely adjusted.

  The area setting unit 51 b inputs the determined position and size information in the irradiation detection area 56 to the control unit 46 in each detection operation of irradiation start and end. The control unit 46 determines the irradiation detection area 56 based on the input position and size information, and specifies the pixel PX in the irradiation detection area 56. In each detection operation, the control unit 46 controls the vertical scanning circuit 41 and the horizontal scanning circuit 42 to sequentially select the specified pixels PX, and outputs a voltage signal from the pixels PX in the irradiation detection region 56. .

  For example, in FIG. 3, when the pixel PX22 is specified as the pixel PX in the irradiation detection region 56, the control unit 46 selects the row to which the pixel PX22 is connected through the vertical scanning circuit 41 and the horizontal scanning circuit 42. A row selection signal and a column selection signal are input to the line SR2 and the column signal line SC2, respectively. As a result, the pixel selection transistor Tr2 and the column selection transistor 37 of the pixel PX22 are turned on, and the voltage signal of the pixel PX22 is output to the output line 38. Thus, the control unit 46, the vertical scanning circuit 41, and the horizontal scanning circuit 42 function as an output signal reading unit.

  In each detection operation of irradiation start and irradiation end, the voltage signal read from the pixel PX22 in the irradiation detection region 56 is converted into a digital signal by the A / D converter 48 and stored in the memory 49. The irradiation determination unit 51c reads the digital signal stored in the memory 49 as an output signal for performing irradiation detection, and determines the irradiation start and irradiation end based on the read output signal. Reading of the output signal from each irradiation detection area 56 is repeatedly executed at a predetermined sampling rate.

  In this example, in each detection operation, the output signal is read out by selecting the pixel PX in each irradiation detection region 56 instead of all the pixels PX in the imaging region 36a, but the output signal is output from all the pixels PX. You may extract the output signal in each irradiation detection area | region 56 from the output signal for 1 screen after reading and converting into a digital signal.

  The gain setting unit 51a sets the gain of the amplifier AP. The gain setting unit 51 a inputs designation information for designating a gain value to the control unit 46. Based on the position information and size information of the irradiation detection region 56 input from the region setting unit 51b and the designation information input from the gain setting unit 51a, the control unit 46 passes the irradiation detection region 56 through the horizontal scanning circuit 42. The gain of the pixel PX is changed. The horizontal scanning circuit 42 selects a gain control line GC to which the pixel PX in the irradiation detection area 56 is connected, and inputs a gain setting signal to the amplifier AP through the selected gain control line GC. Thereby, the gain of the pixel PX in the irradiation detection area 56 is set.

  In this example, since the gain setting signal is input by the gain control line GC connected to the horizontal scanning circuit 42, the gain can be set only for each column. For example, the gain control line GC is vertically scanned. It is also possible to connect to the circuit 41 so that the gain can be set for each pixel PX by specifying a row and a column.

  As shown in FIG. 5 (A), the intensity of X-rays irradiated by the X-ray source 13 gradually rises when a start command is received at time t1. The rising slope is gentle at the initial stage of irradiation, gradually increasing, and rising to a peak corresponding to the tube current at time t2. Then, a substantially steady state is maintained near the peak until a stop command is received at time t3, and when the stop command is received, it gradually decreases and becomes zero at time t4. Thus, the profile of the curve representing the intensity change of the X-rays irradiated in one imaging is substantially trapezoidal.

  Corresponding to such an X-ray intensity profile, the change in the amount of signal charge accumulated in the pixel PX becomes a profile as shown in FIG. The charge accumulation amount starts increasing from time t1 in response to the increase in X-ray intensity, and increases while increasing the slope (increase rate) as it approaches time t2 when the X-ray intensity reaches the peak. Between time t2 and time t3 when the X-ray intensity reaches a steady state, the intensity increases at a substantially constant rate. Then, the increase rate starts decreasing from time t3 corresponding to the decrease in the intensity of X-rays, but the charge accumulation amount continues to increase gradually until time t4 when the intensity of X-rays becomes zero. After time t4, the charge accumulation amount does not increase and becomes constant. To be precise, since the charge due to the dark current is generated in the pixel PX even when the X-rays are not incident, the charge accumulation amount slightly increases before time t1 and after time t4.

  In the irradiation start detection operation, output signals are read from the pixels PX in each irradiation detection region 56 at a predetermined sampling rate. The irradiation detection unit 51 sets the initial value of the gain of the pixel PX in the irradiation detection area 56 to a high value (high gain GnH), and starts the detection operation with the sensitivity of the pixel PX increased. The irradiation detection unit 51 causes the irradiation determination unit 51c to perform primary determination in a state where the sensitivity of the pixel PX is increased.

  In the primary determination, the irradiation determination unit 51c compares the signal value SV of the output signal read from the pixel PX with a high gain with a preset threshold Th, and starts irradiation when the signal value becomes equal to or higher than the threshold Th. judge. As the signal value SV to be compared with the threshold value Th, for example, an average value or an intermediate value of each pixel PX in each irradiation detection region 56 is used. An abnormal value can be eliminated by using an average value or an intermediate value of a plurality of pixels PX. And the irradiation determination part 51c compares the signal value SV of each irradiation detection area | region 56 with threshold value Th sequentially.

  In a state where the sensitivity is high, a slight increase in the charge accumulation amount appears as a large increase in the signal value SV of the output signal. Therefore, even immediately after the time t1 when the X-ray irradiation is started, the signal value SV of the output signal read with the high gain GnH as shown in FIG. Since the difference from the threshold value Th appears larger compared to the low gain GnL state, the signal value SV and the threshold value Th can be accurately compared.

  On the other hand, as shown in FIG. 5C, before the time t1 when the irradiation is started, the charge accumulation amount is only slightly increased due to the charge caused by the dark current, but the gain is set to the high gain GnH. In such a case, the signal value SV may exceed the threshold Th even with such a slight increase. In addition, vibration noise due to impact or vibration appears as a large signal value SV when the gain is high, and thus may exceed the threshold Th.

  In such a case, since it is erroneously detected, the irradiation detection unit 51 verifies the determination result of the primary determination. When the signal value SV is determined to be greater than or equal to the threshold Th in the primary determination, the gain detection unit 51 The value is decreased from the high gain GnH to the low gain GnL, and the secondary determination is performed. The secondary determination is performed on the irradiation detection region 56 in which the signal value SV is determined to be greater than or equal to the threshold Th.

  In the secondary determination, the irradiation determination unit 51c determines whether the signal value SV of the output signal read out with the low gain GnL has increased to the threshold value Th or more. The irradiation determination unit 51c compares the signal values SV read a plurality of times with the low gain GnL, and checks whether the signal values SV are increased or not and whether the signal value SV is equal to or higher than the threshold Th. As shown in FIG. 5D, when the signal value SV read with the low gain GnL increases and becomes equal to or greater than the threshold Th, it is determined that the result of the primary determination is not a false detection, and the primary determination Confirm the judgment result.

  On the other hand, as shown in FIG. 5C, in the secondary determination, if the signal value SV read at the low gain GnL does not increase and does not exceed the threshold Th, the result of the primary determination is false detection. to decide. In the secondary determination, an upper limit value of the number of times of reading the low gain GnL (for example, three times) is determined, and if the signal value SV does not reach the threshold value Th within the range of the number of times, it is determined that the detection is false. The upper limit value may be determined by time. If it is determined that the detection is erroneous, the gains of all the pixels PX in the irradiation detection area 56 are returned to the high gain GnL, and the primary determination is performed again.

  When the determination result is confirmed in the secondary determination, the irradiation detection unit 51 sends a detection signal indicating that the start of irradiation has been detected to the control unit 46. Upon receiving the detection signal, the control unit 46 stops reading the output signal of the irradiation detection region 56 and starts the signal charge accumulation operation without resetting the pixel PX. Then, the irradiation end detection operation is started in parallel.

  Further, the irradiation detection unit 51c stores in the internal memory the signal value SV at the time when the irradiation start is detected and the position information of the irradiation detection area 56 from which the signal value SV is read. As will be described later, the signal value SV and the position information stored in the internal memory are used in the irradiation detection end operation.

  6A and 6B are the same as FIGS. 5A and 5B, respectively, and show the profile of the X-ray intensity change and the profile of the change in charge accumulation over time. In the irradiation end detection operation, based on the signal value SV of the output signal, it is detected that the irradiation is completely stopped and the X-ray intensity becomes zero. As shown in FIG. 6B, the rate of increase of the charge accumulation amount starts decreasing from time t3 when the X-ray source 13 receives a stop command in response to the decrease in the intensity of the X-ray. The increase continues slowly until time t4 when the intensity of becomes zero. After time t4, the charge accumulation amount does not increase and becomes constant.

  In the irradiation end detection operation, first, the irradiation detection unit 51 sets the irradiation detection region 56 used in the irradiation end detection and the initial value of the gain based on the detection result of the irradiation start detection. Specifically, in the irradiation start detection operation, the irradiation detection region 56 used for the irradiation end detection operation is determined based on the position information stored in the internal memory of the irradiation detection unit 51. The region where the signal value SV rises quickly in the detection of the start of irradiation is considered to be a so-called blank region where X-rays directly enter the imaging region 36a without passing through the subject H. Since the unexposed region has little attenuation of X-rays by the subject H, it is suitable not only for the start of irradiation detection but also for the end of irradiation detection. Therefore, the irradiation detection unit 51 stores the position information of the irradiation detection area 56 at the time of irradiation start detection, and uses the corresponding irradiation detection area 56 for irradiation end detection.

  Further, the irradiation detection unit 51 sets an initial gain value Gn1 based on the signal value SV stored in the internal memory of the irradiation detection unit 51. Although the signal value SV is the signal value SV at the initial stage of X-ray irradiation start, an approximate peak of X-ray intensity can be predicted based on the magnitude. If the peak of the X-ray intensity is known, the initial gain value can be set to a high value in a range where the output signal is not saturated in the detection of the end of irradiation. The irradiation detection unit 51 sets an initial value of the gain of the irradiation detection region 56 and the pixel PX in the irradiation detection region 56 through the control unit 46.

  As described above, in the irradiation end detection, the irradiation detection unit 51 performs the initial setting for the irradiation detection region 56 and the initial gain value Gn1 based on the detection result of the irradiation start detection. After completing the initial setting, the control unit 46 starts reading the output signal from the pixels PX in the irradiation detection area 56 at a predetermined sampling rate.

  As shown in FIG. 6B, the charge accumulation amount does not increase and becomes constant after time t4. As shown in FIGS. 6C and 6D, the irradiation determination unit 51c compares the signal values SV of two output signals that are successively read out with the same gain Gn1, and increases due to dark current noise. Except for minutes, it is determined whether or not the signal value SV has become substantially constant. When the signal value SV becomes substantially constant, it is checked whether the output signal is saturated. Since the initial value Gn1 of the gain is a value predicted based on the result of the irradiation start detection, the output signal may be saturated beyond the prediction. Therefore, when the output signal is saturated as shown in the signal value SV read with the gain Gn1 in FIG. 6C, the irradiation detection unit 51 gain value through the gain setting unit 51a and the control unit 46. Is changed to a gain Gn2.

  After changing to the gain Gn2, the irradiation determination unit 51c compares the signal values SV of two output signals that are successively read out with the same gain Gn2, and determines whether or not the signal value SV is substantially constant. . When the output signal read with the gain Gn2 is also saturated, as shown in FIG. 6D, the gain is further lowered step by step with the gains Gn3 and Gn4. Then, when the signal value SV becomes substantially constant in a state where the output signal is not saturated, like the signal value SV of the gain Gn4 in FIG. 6D, the irradiation determination unit 51c determines that the irradiation has ended. judge. When the irradiation determination unit 51c determines that the irradiation has ended, the irradiation detection unit 51 sends a detection signal indicating that the irradiation end has been detected to the control unit 46. When receiving the detection signal, the controller 46 starts an image reading operation.

  The effect | action by the said structure is demonstrated, referring the flowchart shown in FIGS. When imaging the subject H, the relative position between the imaging region of the subject H and the electronic cassette 21 is adjusted by adjusting the height of the imaging table 22 on which the electronic cassette 21 is set, the imaging conditions are set, and the like. The shooting preparation work is performed. As shown in FIG. 7, when an imaging condition is set and an input of a standby instruction is accepted (S100), the FPD 25 of the electronic cassette 21 starts an irradiation start detection operation (S101). When the irradiation switch 15 is pressed after completion of the imaging preparation work, the X-ray source 13 starts X-ray irradiation.

  As shown in FIG. 8, in the irradiation start detection operation, the control unit 46 of the electronic cassette 21 resets all the pixels in the imaging region 36a (S111). As a result, charges due to dark current noise are swept out of the pixel PX. The electronic cassette 21 sets the irradiation detection area 56 in the imaging area 36a, and sets the gain values of all the pixels PX in the irradiation detection area 56 to the high gain GnH (S112). After setting the gain, the electronic cassette 21 starts reading the output signal from the pixel PX in each irradiation detection area 56 at a predetermined sampling rate (S113).

  In the irradiation detection unit 51, the electronic cassette 21 compares the signal value SV of the output signal of each irradiation detection region 56 with the threshold value Th, and performs primary determination with the high gain GnH. In the primary determination, the electronic cassette 21 monitors whether or not the signal value SV is equal to or greater than the threshold value Th (S114). If there is an irradiation detection region 56 in which the signal value SV is equal to or greater than the threshold Th (Y in S114), the gain of the irradiation detection region 56 is changed to the low gain GnL (S115). And the irradiation detection part 51 performs the secondary determination which monitors whether the signal value SV of the output signal read by low gain GnL became more than threshold value Th (S116).

  In the secondary determination, as shown in FIG. 5C, when the signal value SV of the output signal read with the low gain GnL is less than the threshold, the irradiation detection unit 51 determines that the primary determination is a false detection. To do. If it is determined that there is a false detection (N in S116), the electronic cassette 21 returns to S111, resets all the pixels PX in the imaging region 36a, and sets all the gain values of each irradiation detection region 56 to the high gain GnH. (S112), and the primary determination is resumed (S113 to S116).

  On the other hand, as shown in FIG. 5D, when the signal value SV of the output signal read with the low gain GnL is equal to or larger than the threshold value (Y in S116), the irradiation detection unit 51 determines the primary determination result. Is determined, and it is determined that irradiation has started (S117). The irradiation detection unit 51 stores the final signal value SV determined to start irradiation and the position information of the irradiation detection area 56 in the internal memory. If the irradiation detection unit 51 determines that the irradiation has been started in the secondary determination, the irradiation detection unit 51 sends a detection signal indicating that the irradiation start has been detected to the control unit 46. When detecting the start of irradiation, the electronic cassette 21 ends reading of the output signal from the irradiation detection area 56 and ends the irradiation start detection operation (S118).

  As described above, since the electronic cassette 21 performs the primary determination with the high gain GnL, a large signal value SV can be obtained even at a stage where the charge accumulation amount is small. Therefore, it is determined that the irradiation starts even though the irradiation is started. It is possible to reduce missed detection. Further, in the case of the low gain GnL, the signal value SV does not exceed the threshold value Th in the case of the low gain GnL as compared with the case of the high gain GnL. Thus, it is possible to accurately determine the presence or absence of the start of irradiation.

  In addition, when X-ray irradiation is started in the primary determination, the amount of accumulated charge greatly increases until the next signal value SV is read. Then, when the output signal is read with the high gain GnH after the primary determination, the signal value SV immediately reaches the upper limit ST and becomes saturated. When saturated, it becomes impossible to determine whether or not the signal value has increased. The electronic cassette 21 can avoid such a situation where it cannot be determined by setting the gain GnL after the primary determination. For this reason, the electronic cassette 21 can detect the irradiation start quickly and accurately compared to the conventional case.

  Further, the electronic cassette 21 uses a non-destructive readout type FPD 25, and when it is determined that irradiation is started in the secondary determination, the pixel PX is not reset. Therefore, from the primary determination to the secondary determination. X-rays irradiated in the meantime are not wasted.

  In FIG. 7, when the electronic cassette 21 finishes the irradiation start detection operation, the electronic cassette 21 starts an accumulation operation (S120). Then, an irradiation end detection operation is started in parallel with the accumulation operation (S130).

  As shown in FIG. 9, in the irradiation end detection operation, the electronic cassette 21 performs initial setting based on the detection result of the irradiation start detection operation. In the initial setting, the irradiation detection unit 51 uses the irradiation detection region 56 and the initial gain value Gn1 used in the irradiation end detection operation through the control unit 46 based on the information of the irradiation detection region 56 stored in the internal memory and the signal value SV. Is set (S131). In this way, it is possible to set the irradiation detection region 56 that is considered to be a missing region, and to set the initial gain value Gn1 higher within a range that is predicted not to be saturated.

  After the initial setting, the electronic cassette 21 starts reading output signals from the pixels PX in the irradiation detection area 56 set at a predetermined sampling rate (S132). The electronic cassette 21 monitors whether or not the signal values SV of the two output signals read continuously are substantially constant in the irradiation determination unit 51c (S133). As shown in FIG. 6C, when the signal value SV is constant like the signal value SV of the gain Gn1, it is checked whether or not the output signal is saturated (S134). When the output signal is saturated (Y in S134), the electronic cassette 21 decreases the gain Gn1 by one step and changes it to the gain Gn2 (S135). Then, the process returns to S133 to monitor whether the signal value SV becomes constant. As shown in FIG. 6D, when it is determined that the signal value SV is constant and not saturated (N in S134), the irradiation determination unit 51c determines that the irradiation has ended (S136). The irradiation detection unit 51 sends a detection signal indicating that the end of irradiation has been detected to the control unit 46. The control unit 46 finishes reading the output signal from the irradiation detection area 56 (S137), and sets the gains of all the pixels PX in the imaging area 36a to gain values for image reading (S138).

  In this way, in the irradiation end detection, a higher gain is set as the initial gain value, and when the output signal is saturated, the gain value is lowered step by step, so a relatively large value (S / It is possible to determine whether or not the signal value SV becomes constant in a state where the N ratio is high. For this reason, it is possible to quickly and accurately determine where the X-ray irradiation completely stops and the X-ray intensity becomes zero. Thereby, useless X-rays that are not reflected in the X-ray image can be reduced. Further, dark current noise generated after the X-ray intensity becomes zero can be reduced.

  In FIG. 7, when the irradiation detection operation ends, the electronic cassette 21 executes an image read operation to read an X-ray image for one screen (S140). The X-ray image is transmitted from the communication unit 52 to the console 24. If there is a next shooting, the process returns to S100 and the above procedure is repeated. If there is no next shooting, the process ends (S150). Since the X-ray image reflects the irradiated X-rays without waste, good image quality is ensured. Further, in the present invention, irradiated X-rays can be reflected without waste, so that the irradiation dose set in advance under the imaging conditions can be set to be small, which contributes to a reduction in the exposure dose of the subject H.

  In this example, in the irradiation start detection operation, the irradiation start is detected by performing a secondary determination on the signal value SV of one irradiation detection area 56 that is equal to or greater than the threshold value Th in the primary determination. For example, the determination up to the secondary determination is performed based on the signal values SV of the irradiation detection areas 56 at two or more locations, and the irradiation start is detected when the secondary determination is passed at two or more locations. May be. Thus, even if it takes time for the irradiation start determination, the signal charge of the pixel PX is held unless resetting is performed, so there is no waste of X-rays.

  In the primary determination, the secondary determination is performed by passing the primary determination when the signal value SV of the output signal read once with the high gain GnH is equal to or greater than the threshold value Th. In this case, for example, when the signal value SV of the high gain GnH is continuously equal to or more than the threshold Th a plurality of times, the primary determination may be passed and the secondary determination may be performed.

  Further, in the irradiation end detection operation, the example in which the irradiation detection area 56 that is considered to be a blank area is used for the irradiation end detection based on the result of the irradiation start detection has been described. You may use the irradiation detection area | region 56 located for an irradiation end detection. Since there is X-ray attenuation in the region where the subject H is located, the amount of incident X-rays is small. For this reason, since the charge accumulation amount is also relatively small, the gain value can be set higher than that in the blank region. When emphasizing not to saturate the output signal and to reduce the number of times the gain is lowered stepwise, the region where the subject H is located may be set as the irradiation detection region 56 at the time of detecting the end of irradiation.

  In this case, during the irradiation start detection operation, the irradiation detection unit 51 examines, for example, the region where the signal value SV is less than the threshold Th or the irradiation detection region 56 that takes a long time to reach the threshold Th. The irradiation detection area 56 where H is considered to be located is specified, and the position information and signal value SV are stored in the internal memory. Then, when the irradiation end operation is started, the irradiation detection region 56 and the gain are initially set based on the position information and the signal value SV.

  In this example, the value of the high gain GnH that is set as the initial value at the start of irradiation detection is constant regardless of the imaging conditions, but the irradiation dose (tube current and irradiation time) included in the imaging conditions. The value of the high gain GnH may be changed based on

  In this case, the gain setting unit 51b reads the tube current and the irradiation time from the imaging conditions received by the control unit 46, and sets the value of the high gain GnH based on the read tube current and the irradiation time. The value of the high gain GnH is calculated by calculation or reference using a function expression or table data representing a correspondence relationship between the tube current and the irradiation time and the gain value stored in advance in the internal memory.

  For example, the gain setting unit 51b sets the value of the high gain GnH to be lower as the irradiation dose increases. This is because, as the irradiation dose increases, the charge accumulation amount increases, so that a relatively large value can be obtained as the signal value SV even if the gain is lowered. Further, since the concern about the saturation of the signal value SV increases as the irradiation dose increases, there is an effect of suppressing the saturation by lowering the gain.

  The gain setting unit 51b may set the value of the high gain GnH based on the total dose that is the product of the tube current and the irradiation time (referred to as mAs value), or a tube that defines the dose per unit time. You may set based on only an electric current.

  Further, in addition to or instead of the value of the high gain GnH, the value of the low gain GnL may be set according to the irradiation dose. For the low gain GnL, the effect as described for the case of the high gain GnH can be expected.

  Furthermore, the example of setting the initial gain value at the time of irradiation end detection based on the result of the irradiation start detection has been described. However, if the result of the irradiation start detection is not used, the initial value at the time of irradiation end detection is described. The value of the gain Gn1 set as may be set according to the irradiation dose included in the imaging conditions. Also in this case, the effect as described for the case of the high gain GnH at the start of irradiation can be expected.

  Further, in this example, in the irradiation start detection operation, the irradiation detection areas 56 are first set at a plurality of locations in the imaging area 36a, and the irradiation start detection is performed based on the signal value SV of each irradiation detection area 56. However, as shown in FIG. 10, the entire imaging region 36 a may be set as the irradiation detection region 56.

  In this case, in the irradiation start detection operation, the irradiation detection unit 51 reads the output signals of all the pixels PX in the imaging region 36a and compares the signal value SV with the threshold value Th. The signal value SV to be compared with the threshold Th may be a signal value SV for each pixel PX, or an average value or an intermediate value of signal values SV of a plurality of adjacent pixels PX (for example, a total of nine pixels PX of 3 × 3). May be a signal value SV that is compared with the threshold Th.

  In the irradiation end detection, the irradiation detection area 56 used for the irradiation end detection is set with reference to the position of the pixel PX that has output the signal value SV finally determined as the irradiation start. For example, when the pixel PX that has output the signal value SV that is finally determined as the start of irradiation is the pixel PX22, the region 59 centered on the pixel PX22 is set as the irradiation detection region 56 used for detection of the end of irradiation.

  Since the electronic cassette 21 is portable, it is often used without using the photographing stand 22. When photographing limbs such as hands and feet, only a part of the imaging region 36a may be used as an irradiation region. The irradiation area is set by the irradiation field limiter 13 b of the X-ray source 13. Since the position and size of the irradiation region vary depending on the posture of the subject H and the size of the imaging region, the irradiation region is not constant within the imaging region 36a. As shown in FIG. 10, the method of setting the entire imaging region 36a as the irradiation detection region 56 is very effective for the electronic cassette 21 used in this way.

  Further, instead of configuring the amplifier AP in the pixel PX as a variable gain amplifier, a variable gain amplifier 60 may be provided in the subsequent stage of the output line 38 and in the previous stage of the A / D converter 48 as shown in FIG. . In this case, the gain control line GC connected to the horizontal scanning circuit 42 and the amplifier AP of the pixel PX becomes unnecessary. Since the other configuration is the same as that of the above example, the same portions are denoted by the same reference numerals and description thereof is omitted.

[Second Embodiment]
In the first embodiment, in the irradiation start detection, all the pixels PX in the set irradiation detection area 56 are set to the high gain GnH in the primary determination, and all the pixels PX in the irradiation detection area 56 are set to the low gain in the secondary determination. As described in the example of changing to GnL, as in the second embodiment shown in FIG. 11, the high sensitivity region 56 a set to the high gain GnH and the low gain GnL are set in one irradiation detection region 56. A low sensitivity region 56b (shown by hatching) may be provided. The gain values of the high sensitivity region 56a and the low sensitivity region 56b are not changed during the irradiation start detection operation. The second embodiment is the same as the first embodiment except that the irradiation detection region 56 is divided into a high sensitivity region 56a and a low sensitivity region 56b. Only will be described.

  As shown in the flowchart of FIG. 12, the electronic cassette 21 according to the second embodiment includes a high sensitivity region 56 a and a low sensitivity region in each irradiation detection region 56 in addition to the setting of the irradiation detection region 56 in the irradiation start detection operation. 56b is set, and the respective gain values are set to the high gain GnH and the low gain GnL (S211). After the setting, as in S111 (see FIG. 8) of the first embodiment, all the pixels PX in the imaging region 36a are reset (S212), and further, similarly to S113 (see FIG. 8) of the first embodiment, Reading of the output signal from the irradiation detection area 56 is started at a predetermined sampling rate (S213).

  Then, the electronic cassette 21 performs primary determination based on the output signal read from the high sensitivity region 56a in the irradiation detection unit 51, and compares the signal value SV output from the high sensitivity region 56a with the threshold Th (S214). When it is determined that there is a signal value SV equal to or greater than the threshold Th (Y in S214), the signal value SV of the low sensitivity region 56b corresponding to the high sensitivity region 56a that has output the signal value SV is compared with the threshold Th. A secondary determination is performed (S215).

  In the secondary determination, when the signal value SV of the low sensitivity area 56b is less than the threshold Th (N in S215), the process returns to S212 to reset all the pixels PX in the imaging area 36a and resume the primary determination. If the signal value SV of the low sensitivity region 56b is equal to or greater than the threshold Th (Y in S215), it is determined that irradiation is started (S216). The irradiation detection unit 51 finishes reading the output signal (S217), and transmits an irradiation detection signal indicating that irradiation has started to the control unit 46. The electronic cassette 21 ends the irradiation detection operation, and executes the accumulation operation and the image reading operation in the same procedure as shown in FIG. 7 of the first embodiment.

  As in the second embodiment, by dividing one irradiation detection region 56 into a high sensitivity region 56a and a low sensitivity region 56b, primary determination and secondary determination can be performed based on output signals output almost simultaneously. .

  In the first embodiment and the second embodiment, the example in which the irradiation start is detected based on the two types of output signals read with the two types of high and low gains has been described. However, the high gain, the medium gain, and the low gain are used. You may use the output signal read by the gain of three or more types from which the value of a gain differs. In the case of three or more gains, primary determination is performed with high gain and medium gain and secondary determination is performed with low gain, or primary determination is performed with high gain and secondary determination is performed with medium gain and low gain. As described above, at least one of the primary determination and the secondary determination is performed with two or more types of gains. Alternatively, as shown in the detection of the end of irradiation, in the detection of the start of irradiation, when the output signal is saturated, the gain may be decreased stepwise from high gain to medium gain. These are appropriately changed depending on the degree of emphasizing the accuracy of detection of irradiation start.

  In the above-described embodiment, the threshold value Th to be compared with the signal value SV has been described as an example in which both the primary determination and the secondary determination use the same threshold Th in the irradiation start detection. Th may be set to another value. For example, since the gain GnH in the primary determination is higher than the gain GnL in the secondary determination, the signal value SV is also a large value. For this reason, the threshold value Th for primary determination is set higher than the threshold value Th for secondary determination. Further, as described in Patent Document 1, the signal value SV to be compared with the threshold Th may be a difference between two output signals that are read continuously. Moreover, although the example which performs both the detection of irradiation start and the completion | finish of irradiation was demonstrated, you may detect only one.

  Further, the signal value SV has been described as an example in which the signal value SV is converted into a digital signal by A / D conversion and then compared with the threshold value Th. For example, by using a comparator that outputs a comparison result between the reference voltage and the input voltage, The signal value SV and the threshold value Th may be compared with the analog signal before D conversion.

  The above embodiment is an example, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, an example using a CMOS type FPD has been described. However, if non-destructive reading is possible, a TFT matrix substrate in which pixels including TFTs are formed on a glass substrate is used. Other than the CMOS type may be used. Further, although an indirect conversion type FPD that once converts X-rays into visible light using a scintillator is used as an example, a direct conversion type FPD that converts X-rays directly into an electrical signal may be used.

  In each of the above embodiments, the electronic cassette as the X-ray image detection device and the imaging control device and the console have been described separately. However, the functions of the imaging control device are incorporated in the electronic cassette. The electronic cassette may be integrated with the imaging control device and the console, for example, the console function is built in the electronic cassette in addition to the imaging control device.

  The X-ray image detection apparatus is not limited to an electronic cassette, and may be a stationary type in which an FPD is built in an imaging table.

  In addition, the present invention is not limited to X-rays but can be applied to a radiation imaging system that uses other radiation such as γ rays.

DESCRIPTION OF SYMBOLS 10 X-ray imaging system 11 X-ray generator 12 X-ray imaging apparatus 13 X-ray source 14 Radiation source control apparatus 21 Electronic cassette 23 Imaging control apparatus 24 Console 25 FPD
36 Image Detection Panel 36a Imaging Area PX Pixel AP Amplifier 46 Control Unit 51 Irradiation Detection Unit 51a Gain Setting Unit 51b Area Setting Unit 56 Irradiation Detection Area 56a High Sensitivity Area 56b Low Sensitivity Area 60 Variable Gain Amplifier

Claims (21)

An image detection panel that receives radiation transmitted from a radiation source and passes through a subject to detect a radiation image, and a plurality of pixels that accumulate signal charges according to the amount of incident radiation are arranged in two rows and columns. An image detection panel having an image pickup area;
An irradiation detection region for detecting at least one of the start of irradiation and the end of irradiation, and an irradiation detection region set in the imaging region including at least one pixel;
An output signal reading unit that reads an output signal corresponding to the signal charge in a state where the signal charge is held in the pixel from the pixel in the irradiation detection region;
An irradiation detector that detects at least one of the start and end of the irradiation based on the signal value of the output signal;
A radiation image detection apparatus comprising: a gain setting unit capable of changing a gain, which is an amplification factor when reading the output signal, into at least two types of gains, a high gain and a low gain.   The radiographic image detection apparatus according to claim 1, further comprising an area setting unit capable of setting the irradiation detection area at an arbitrary position in the imaging area.   The radiographic image detection apparatus according to claim 2, wherein the region setting unit is capable of setting the irradiation detection regions at a plurality of locations in the imaging region.   The radiographic image detection apparatus according to claim 2, wherein the region setting unit is capable of setting the irradiation detection region over the entire imaging region.   5. The imaging condition receiving unit that receives an input of an imaging condition including at least one of an irradiation dose irradiated by the radiation source and an imaging region of the subject. The radiological image detection apparatus of description.   The radiographic image detection apparatus according to claim 5, wherein the region setting unit sets the irradiation detection region based on the imaging region included in the imaging condition.   The gain setting unit sets an initial value of the gain used when detecting at least one of the start and end of the irradiation based on the irradiation dose included in the imaging condition. Item 7. The radiological image detection apparatus according to Item 5 or 6.   The radiographic image detection apparatus according to claim 1, wherein the irradiation detection unit detects the start of the irradiation.   The irradiation detection unit includes an irradiation determination unit that determines whether or not the irradiation is started based on signal values of two or more kinds of output signals read at least with a high gain and a low gain. The radiological image detection apparatus of Claim 8.   The irradiation determination unit performs a primary determination as to whether or not the signal value of the output signal read at high gain is equal to or greater than a preset threshold value, and is determined that the signal value is equal to or greater than the threshold value in the primary determination. The radiological image detection apparatus according to claim 9, wherein secondary determination is performed based on a signal value of the output signal read at the low gain.   The irradiation determination unit determines whether the signal value read at the low gain is equal to or higher than the threshold in the secondary determination, and determines that the irradiation is started when the determination result is affirmative. The radiological image detection apparatus according to claim 10.   The output signal reading unit reads the output signal with a high gain from the irradiation detection region, and after reading the high gain, the gain setting unit changes the gain of the same irradiation detection region to a low gain, and the low gain The radiological image detection apparatus according to any one of claims 9 to 11, wherein an output signal is read out. The irradiation detection area has at least two areas of a high sensitivity area where the gain is set to a high gain and a low sensitivity area where the gain is set to a low gain by the gain setting unit.
The said output signal read-out part reads the said 2 types of said output signal of a high gain and a low gain, respectively from the said high sensitivity area | region and the said low sensitivity area | region, The any one of Claims 9-11 characterized by the above-mentioned. Radiation image detection device.   The radiographic image according to any one of claims 1 to 13, wherein the gain setting unit lowers the gain value stepwise when a signal value of the output signal is saturated. Detection device.   The radiographic image detection apparatus according to claim 1, wherein the irradiation detection unit detects the end of the irradiation.   16. The radiation according to claim 15, wherein the irradiation detection unit determines that the irradiation has ended when a signal value of the output signal read out a plurality of times continuously with the same gain becomes substantially constant. Image detection device.   The radiographic image detection apparatus according to claim 15, wherein the irradiation detection unit detects both the start and end of the irradiation.   The gain setting unit initially sets at least one of the irradiation detection area and the initial gain value when detecting the end of the irradiation based on a result when the irradiation detecting unit detects the start of the irradiation. The radiological image detection apparatus according to claim 17, wherein:   The radiographic image detection apparatus according to claim 1, wherein the image detection panel is a CMOS type capable of reading an X-ray image while holding the signal charges in the pixels. .   The radiographic image detection apparatus according to claim 1, wherein the image detection panel is an electronic cassette housed in a portable housing. An image detection panel that can detect a radiation image by receiving radiation that has been irradiated from a radiation source and transmitted through a subject, and a plurality of pixels that accumulate signal charges according to the amount of incident radiation are arranged in two-dimensional rows and columns Using an image detection panel that has an imaged area that can read out an output signal corresponding to the signal charge while holding the signal charge in the pixel, and that the radiation irradiation has started and ended In the irradiation detection method for detecting at least one of them,
In the irradiation detection area set in the imaging area for detecting at least one of the start of irradiation and the end of irradiation including at least one of the pixels, the output from the pixel An output signal read step for reading a signal; and
An irradiation detection step of detecting at least one of the start and end of the irradiation based on the signal value of the output signal,
The radiation detection step comprising: a gain setting step of changing a gain, which is an amplification factor when reading the output signal, into at least two types of gains, a high gain and a low gain, in the irradiation detection step. Irradiation detection method used in the apparatus.

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