JP4539426B2 - Radiation imaging apparatus and offset correction method used therefor - Google Patents

Description

  The present invention relates to a radiation imaging apparatus for detecting a radiation image and an offset correction method used therefor, which are used in the medical field, industrial field, nuclear power field, and the like, and more particularly to a technique for improving the image quality of a radiation image.

  A radiation detector (hereinafter simply referred to as a detector) mounted on a radiation imaging apparatus converts radiation into charges by a detection element formed on the active matrix substrate, and the converted charge is a signal connected to the active matrix substrate. Read sequentially from the reading unit, and output data corresponding to radiation.

  This detector is roughly classified into two types depending on the configuration of the detection element. One is a direct conversion type detector that directly converts radiation into electric charge by a radiation-sensitive semiconductor layer, and the other is a photoelectric conversion element such as a photodiode after the radiation is once converted into light. This is an indirect conversion type detector that converts light into electric charge.

  The direct conversion type detector applies a predetermined bias voltage to the application electrode formed on the surface of the semiconductor layer, and collects the charges converted by the semiconductor layer by the carrier collection electrode formed on the back surface thereof. As the semiconductor layer, an amorphous semiconductor capable of easily forming a thick film easily by a method such as vacuum deposition is suitable for a radiation detector having a large detection surface.

  However, a residual output is generated in an amorphous semiconductor used for the direct conversion type and a photodiode used for the indirect conversion type, which causes a problem of causing an afterimage (false image) in the radiation image. This residual output is different from so-called dark current in that it is caused by charges remaining in the detection element.

  Therefore, in order to suppress the residual output, a method of constantly irradiating the detector with light (for example, refer to Patent Document 1) or a method of irradiating the detector with light only during a period in which no radiation is incident (for example, Patent Document 2). Further, there is a method (for example, see Patent Documents 3 and 4) in which the temporal change of the residual output is expressed as a mathematical expression and the residual output is arithmetically removed based on the mathematical expression.

JP 2004-146769 A JP 2000-214297 A JP 2003-185752 A JP 2000-070250 A

However, the conventional example having such a configuration has the following problems.
That is, the cause of residual output is not simple. A short-term residual output component that decays with a time constant of 1 second or less, a long-term residual output component that decays with a time constant of 1 minute or more, and various components that decay with a time constant in between. I understand that.

  Therefore, the output data obtained from the detection element after the radiation irradiation is stopped will generate a residual output mainly composed of a component that decays with a relatively short time constant immediately after the radiation irradiation is stopped. As the value continues, the long-term residual output component becomes dominant.

  Such a long-term residual output component cannot be removed no matter how the above methods are combined. In any of the above-described methods, a component that has a relatively large residual output and is attenuated in a short period, whose influence is serious, is targeted for removal. For this reason, it is difficult to completely suppress the long-term residual output component having a relatively small absolute amount even if the detector is irradiated with light, and it is also difficult to formulate it.

  In this case, when outputting a radiographic image even in a non-radiation state, such as when outputting a radiographic image as a moving image, an afterimage is formed over a long period of time, such as an increase in luminance only at a portion where strong radiation is incident. The

  The present invention has been made in view of such circumstances, and a radiation imaging apparatus capable of removing an afterimage formed by the influence of a long-term residual output component in non-radiation and an offset correction method used therefor The purpose is to provide.

In order to achieve such an object, the present invention has the following configuration.
That is, the invention described in claim 1 includes a plurality of detection elements that detect radiation, a correction unit that performs offset correction of output data of the detection element using offset correction data, and the non-radiation irradiation. Update means for updating the offset correction data based on output data of the detection element, and in a radiation imaging apparatus that outputs an image output signal after passing through the correction means, at least after radiation irradiation is stopped, radiation irradiation is performed again. In the period up to the transition, a signal monitoring unit that monitors a statistic between detection elements of the image output signal is provided, and the statistic or a temporal change of the statistic becomes a reference value or less by the signal monitoring unit. The update means updates the offset correction data when it is determined that the correction has been made.

  [Operation / Effect] According to the invention described in claim 1, by providing the signal monitoring means for monitoring the statistic between the detection elements of the image output signal, the statistic or the temporal change of the statistic is the reference value. It is determined whether or not the following has occurred. For this reason, it is possible to accurately obtain the timing at which the long-term residual output component can be approximately regarded as a constant component. Then, when it is determined that this statistic or the temporal change of the statistic is equal to or less than the reference value, the offset correction data is updated, so that an afterimage formed due to the influence of the long-term residual output component can be removed.

The invention according to claim 2 is the radiation imaging apparatus according to claim 1, wherein the statistic is any one of variance, standard deviation, average, and range of an image output signal. It is what.

[Operation / Effect] According to the invention described in claim 2, the signal monitoring means is preferably realized by setting the statistic to any one of variance, standard deviation, average, and range of the image output signal. be able to. The range is a difference between the maximum value and the minimum value of the image output signal, and includes a so-called light / dark difference.

  According to a third aspect of the present invention, in the radiation imaging apparatus according to the first or second aspect, the time after the correction data update in which the radiation non-irradiation state is continued after the offset correction data is updated. A post-update time monitoring means for measuring and monitoring when the time after the correction data update passes a predetermined period, the update means when the time after the correction data update passes a predetermined period Is updated.

  [Operation / Effect] According to the invention described in claim 3, when the non-irradiation state continues after updating the offset correction data by providing the post-update time monitoring means, a predetermined time elapses. The offset correction data can be updated every time. As a result, minute fluctuations in the long-term residual output component can be removed.

  According to a fourth aspect of the present invention, there are provided a plurality of detection elements for detecting radiation, a correction means for performing offset correction of output data of the detection element using offset correction data, and the non-radiation irradiation Update means for updating the offset correction data based on output data of the detection element, and in a radiation imaging apparatus that outputs an image output signal after passing through the correction means, and when shifting to radiation non-irradiation, and / or Alternatively, it includes non-irradiation time monitoring means for measuring a non-irradiation time during which the radiation non-irradiation state continues from when the offset correction data is updated, and monitoring when the non-irradiation time passes a predetermined period. The updating means updates offset correction data when the non-irradiation time passes a predetermined period.

  [Operation / Effect] According to the invention described in claim 4, by providing the non-irradiation time monitoring means, the non-irradiation time in which the radiation non-irradiation state is continued from the transition to the radiation non-irradiation, and offset correction data When is updated, the non-irradiation time in which the radiation non-irradiation state is continued is measured from that time, and the time when the non-irradiation time passes a predetermined period is monitored. A long-term residual output component having a time constant of 1 minute or more can be approximately regarded as a constant component after a predetermined period has elapsed by measuring afterimage characteristics in advance. By updating the offset correction data when such a predetermined period has elapsed, an afterimage formed due to the influence of the long-term residual output component can be removed. In addition, minute fluctuations in the image output signal after updating the offset correction data can be removed.

  The invention according to claim 5 is the radiation imaging apparatus according to claim 4, wherein the predetermined period is set such that an interval for updating the offset correction data is gradually increased. To do.

  [Operation / Effect] According to the invention described in claim 5, the fluctuation of the long-term residual output component decreases with time, and the value itself also decreases. For this reason, it is preferable to gradually increase the time interval for the second and subsequent offset correction data updates that are repeated when the radiation non-irradiation state is continued after the offset data is first updated after the radiation irradiation is stopped. .

  According to a sixth aspect of the present invention, in the radiation imaging apparatus according to the first to fifth aspects, the detection element includes at least a semiconductor layer and an electrode formed on an active matrix substrate. It is characterized by being.

  [Operation / Effect] According to the invention described in claim 6, a radiation imaging apparatus can be suitably realized by configuring the detection element by at least a semiconductor layer and an electrode formed on the active matrix substrate.

  According to a seventh aspect of the present invention, in the offset correction method for performing offset correction on the output data of a plurality of detection elements for detecting radiation, the offset correction data based on the output data of the detection elements when no radiation is irradiated. Using the output data of the detection element to perform offset correction, and outputting the image output signal corresponding to each detection element and the period from the stop of radiation irradiation to the transition to radiation irradiation. A process of determining whether a statistic between detection elements or a temporal change in the statistic is less than or equal to a reference value, and when determining that a temporal change in the statistic or the statistic is less than or equal to a reference value And a step of updating the offset correction data.

  [Operation / Effect] According to the invention described in claim 7, in the period from the stop of radiation irradiation to the transition to radiation irradiation again, the statistics between the detection elements of the image output signal, or the time of the statistics By providing a process for determining whether or not the change is equal to or less than the reference value, it is possible to accurately obtain a timing at which the long-term residual output component can be approximately regarded as a constant component. When it is determined that the statistic or the temporal change of the statistic is equal to or less than the reference value, the offset correction data is updated, so that an afterimage formed due to the influence of the long-term residual output component can be removed. .

  According to an eighth aspect of the present invention, in the offset correction method for performing offset correction on the output data of a plurality of detection elements for detecting radiation, the offset correction data based on the output data of the detection elements when no radiation is irradiated. Using the process of performing offset correction of the output data of the detection element and outputting an image output signal corresponding to each detection element, when shifting to radiation non-irradiation, and / or when the offset correction data is updated , A process of monitoring when the non-irradiation time during which the radiation non-irradiation state is continued passes a predetermined period, and a process of updating the offset correction data when the non-irradiation time passes a predetermined period It is characterized by comprising.

  [Operation / Effect] According to the invention described in claim 8, the non-irradiation time in which the state of non-irradiation continues from the time when the radiation non-irradiation is continued, and / or when the offset correction data is updated at that time. The non-irradiation time during which the radiation non-irradiation state is continued includes a process of monitoring when a predetermined period elapses. A long-term residual output component having a time constant of 1 minute or more can be approximately regarded as a constant component after a predetermined period has elapsed by measuring afterimage characteristics in advance. By updating the offset correction data when such a predetermined period has elapsed, an afterimage formed due to the influence of the long-term residual output component can be removed. In addition, minute fluctuations in the image output signal after updating the offset correction data can be removed.

  According to a ninth aspect of the present invention, in the offset correction method according to the seventh or eighth aspect, the detection element includes at least a semiconductor layer and an electrode formed on an active matrix substrate. It is characterized by being.

  [Operation / Effect] According to the invention described in claim 9, the radiation imaging apparatus can be suitably realized by configuring the detection element by at least a semiconductor layer and an electrode formed on the active matrix substrate.

  The present specification also discloses an invention relating to the following radiation imaging apparatus.

  (1) The radiation imaging apparatus according to any one of claims 1 to 3, further comprising an irradiation control unit that controls radiation irradiation, wherein the signal monitoring unit is configured to perform radiation irradiation. A radiation imaging apparatus characterized by starting monitoring between detection elements of an image output signal when detecting stoppage.

  According to the invention described in (1) above, the signal monitoring means can easily start monitoring.

  (2) The radiation imaging apparatus according to claim 4 or 5, further comprising an irradiation control unit that controls irradiation of radiation, wherein the non-irradiation time monitoring unit stops irradiation of the radiation by the irradiation control unit. A radiation imaging apparatus characterized by starting measurement of the non-irradiation time when it is detected.

  According to the invention described in (2) above, the time monitoring means can easily start monitoring.

  According to the radiation imaging apparatus according to the present invention, by providing the signal monitoring means for monitoring the statistic between the detection elements of the image output signal, whether or not the statistic or the temporal change of the statistic is equal to or less than the reference value. Determine whether. For this reason, it is possible to accurately obtain the timing at which the long-term residual output component can be approximately regarded as a constant component. Then, when it is determined that this statistic or the temporal change of the statistic is equal to or less than the reference value, the offset correction data is updated, so that an afterimage formed due to the influence of the long-term residual output component can be removed.

Embodiment 1 of the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram illustrating the overall configuration of the X-ray imaging apparatus according to the first embodiment. In the present embodiment, a medical X-ray imaging apparatus will be described as an example.

  The X-ray imaging apparatus includes an X-ray tube 1 that irradiates a subject M with X-rays, and a flat panel X-ray detector (hereinafter referred to as “FPD” as appropriate) 3 that detects X-rays transmitted through the subject M. They are arranged facing each other across the top plate 5 on which the subject M is placed.

  In addition, the X-ray imaging apparatus includes an X-ray irradiation control unit 7 that controls X-ray irradiation on the X-ray tube 1, an FPD control unit 9 that controls reading of output data from the FPD 3, and output data. A signal processing unit 13 that processes and outputs an image output signal, a signal monitoring unit 15 that monitors a statistic between detection elements of the image output signal, and a correction in which the non-irradiation state is continued from when the offset correction data is updated A post-update time monitoring unit 16 that monitors the post-data update time, and a main control unit 17 that performs overall control thereof are provided. Further, a display monitor 19 that displays an X-ray image corresponding to the image output signal is provided. The X-ray irradiation control unit 7 corresponds to the X-ray irradiation control means in this invention.

  The signal processing unit 13 further includes an offset correction unit 21 that performs offset correction, and a correction data update unit 23 that updates offset correction data used for offset correction.

  Hereinafter, each part will be described.

  FIG. 2 is a vertical cross-sectional view of the main part of the FPD 3, and FIG. 3 is a plan view of the FPD 3. The FPD 3 includes an application electrode 31, an X-ray sensitive semiconductor layer 33, a carrier collection electrode 35, and an active matrix substrate 37, which are sequentially stacked from the X-ray incident side. That is, the FPD 3 is a direct conversion type that directly converts X-rays into electric charges.

  Examples of the semiconductor layer 33 include amorphous selenium. The active matrix substrate 37 is exemplified by a glass substrate having electrical insulation.

  In addition, the semiconductor layer 33 is provided with a carrier-selective intermediate layer (not shown) on the application electrode 31 side and the carrier collection electrode 35 side, respectively. Carrier selectivity refers to the property that the contribution rate to the charge transfer action differs significantly between electrons and holes. By providing such an intermediate layer, dark current can be reduced. In addition, you may change the design of these intermediate | middle layers suitably so that the one or both may be omitted.

  The carrier collecting electrodes 35 are separately formed in a two-dimensional matrix in plan view. On the active matrix substrate 37, for each carrier collection electrode 35, a capacitor Ca that accumulates charge information, and a thin film transistor that is a switch element that connects the carrier collection electrode 35 and the capacitor Ca to the source S and extracts the charge information. (Thin Film Transistors) Tr is formed separately. The one set of carrier collection electrode 35, capacitor Ca, and thin film transistor Tr constitute one detection element d together with the semiconductor layer 33 and the application electrode 31 in a region corresponding thereto. The application electrode 31 and the carrier collection electrode 35 are included in the electrodes in the present invention.

  Therefore, the detection elements d are arranged in a two-dimensional matrix as shown in FIG. For example, 1536 vertical × 1536 horizontal detection elements d are arranged on a detection surface with a width of about 30 cm × 30 cm.

  Furthermore, on the active matrix substrate 37, a gate bus line 41 is laid for each row of the detection elements d, and a data bus line 43 is laid for each column of the detection elements d. Each gate bus line 41 is commonly connected to the gates of the thin film transistors Tr in each row. Each data bus line 43 is commonly connected to the drains of the thin film transistors Tr in each column.

  The FPD 3 further includes a plurality of amplifiers 45 and an A / D converter 47 on one end side of the active matrix substrate 37. Each of the data bus lines 43 drawn from the active matrix substrate 37 is connected to the plurality of amplifiers 45 one by one. The output side of each amplifier 45 is connected to the A / D converter 47.

  The gate driver 49 is provided on the other end side of the active matrix substrate 37, and each gate bus line 41 is connected thereto.

  The operation in the FPD 3 will be described. When X-rays enter the FPD 3 with a bias voltage applied to the application electrode 31, charges are generated in the semiconductor layer 33, and the charges are accumulated in the capacitor Ca via the carrier collection electrodes 35. The gate bus line 41 transmits a scanning signal from the gate driver 49 and applies it to the gate of the thin film transistor Tr. As a result, the charge information stored in the capacitor Ca is read out to the data bus line 43 via the thin film transistor Tr that has been turned on. The charge information read through each data bus line 43 is amplified by an amplifier 45. Thereafter, it is digitized by an A / D converter 47 to obtain output data.

  Further, the FPD control unit 9 controls an operation of reading detection data from the FPD 3 described above.

  The offset correction unit 21 performs offset correction of the output data of the detection element d using the offset correction data. An image output signal is output through the offset correction unit 21. The offset correction unit 21 corresponds to the correction unit in the present invention.

  The correction data update unit 23 acquires and updates offset correction data used for offset correction based on output data of the detection element d when X-rays are not irradiated. Immediately after starting the X-ray imaging apparatus, the correction data updating unit 23 acquires and sets the initial value of the offset correction data. Note that the initial value of the offset correction data may be configured to be given in advance so that the processing related to acquisition and setting of the initial value may be omitted.

  The offset correction data is updated based on a command from the main control unit 17. The correction data updating unit 23 corresponds to the updating unit in the present invention.

  A specific configuration of offset correction data and an example of offset correction using this offset correction data will be described with reference to FIG. Here, for convenience of explanation, it is assumed that the number of detection elements d is nine and is arranged in three rows and three columns. In particular, when each detection element d is distinguished and called, the number of columns i and the number of rows j are added and described as dij. FIG. 4A is a diagram schematically showing the configuration of output data E (i, j) of each detection element d, and FIG. 4B schematically shows offset correction data F (i, j). FIG.

If the output data E (i, j) of the detection element dij is e (i, j) particularly when X-rays are not irradiated, the offset correction data F (i, j) is obtained when the X-rays are not irradiated. The average value ea is subtracted from the output data e (i, j) of the detection element dij and can be expressed as the following equation.
F (i, j) = e (i, j) −ea (1)

  The offset correction data F (i, j) is associated with each detection element dij.

Further, the offset correction is a process of subtracting the offset correction data F (i, j) from the output data E (i, j) of the detection element dij. Therefore, the image output signal G (i, j) can be expressed as follows.
G (i, j) = E (i, j) −F (i, j) (2)

  Therefore, the image output signal G (i, j) is associated with each detection element dij.

When performing such offset correction, the image output signal G (i, j) may be increased or decreased as a whole and shifted to a certain value. For example, an amount H corresponding to the difference between the average value ea of the non-irradiation output data e (i, j) and a constant value may be further reduced from the output data E (i, j). Good.
G ′ (i, j) = E (i, j) −F (i, j) −H (3)

  In this case, the image output signal G ′ (i, j) can be lowered to a certain fixed value as a whole. Thereby, the detection range of the detection element dij can be expanded.

  Note that the process of shifting the image output signal G (i, j) to a certain fixed value as a whole may be performed separately from the offset correction. For example, just the processing may be performed immediately before the display monitor 19 outputs the data.

  The signal monitoring unit 15 monitors the statistic between the detection elements d of the image output signal output from the signal processing unit 13 during a period from the stop of X-ray irradiation to the transition to X-ray irradiation again. In this embodiment, the average value between the detection elements d of the image output signal is used as a statistic. Then, it is determined whether or not the average value is equal to or less than a reference value.

  The signal monitoring unit 15 is connected to the X-ray irradiation control unit 7 and the main control unit 17. Then, monitoring is started in cooperation with control for stopping X-ray irradiation by the X-ray irradiation control unit 7, and monitoring is ended in cooperation with control for restarting X-ray irradiation by the X-ray irradiation control unit 7. Further, by outputting the monitoring result of the signal monitoring unit 15 to the main control unit 17, a timing at which the average value becomes equal to or less than the reference value is given to the main control unit 17. The signal monitoring unit 15 corresponds to the signal monitoring means in this invention.

  The post-update time monitoring unit 16 measures the time during which the X-ray non-irradiation state has continued since the correction data update unit 23 updated the offset correction data. In this specification, this time is referred to as a time after correction data update. Then, the time after the correction data update is over a predetermined period is monitored.

  The updated time monitoring unit 16 is connected to the correction data updating unit 23, the X-ray irradiation control unit 7, and the main control unit 17. Then, monitoring is started when the correction data updating unit 23 updates the offset correction data, and the monitoring is finished in cooperation with the control for restarting the X-ray irradiation by the X-ray irradiation control unit 7. When the offset correction data is updated a plurality of times while the X-ray non-irradiation state is continued, the measurement of the time after the correction data update is started each time. In addition, by outputting the monitoring result of the post-update time monitoring unit 16 to the main control unit 17, the main control unit 17 is given a timing at which the time after the correction data update passes a predetermined period.

  The predetermined period is set to be gradually longer according to the number of times the offset correction data is updated while one X-ray non-irradiation state is continued. The post-update time monitoring unit 16 corresponds to the post-update time monitoring means in the present invention.

  The main control unit 17 operates the X-ray irradiation control unit 7 and the FPD control unit 9 to control X-ray irradiation. Further, the signal processing unit 13 (the offset correction unit 21 and the correction data update unit 23) is operated to control the offset correction of the output data of the detection element d. More specifically, based on the input of the signal monitoring unit 15, when the average value between the detection elements d of the image output signal becomes equal to or less than the reference value, the correction data update unit 23 updates the offset correction data. Also, based on the input from the post-update time monitoring unit 16, when the post-update data update time has passed a predetermined period, the correction data update unit 23 is made to update the offset correction data.

  Next, the operation of the X-ray imaging apparatus according to Embodiment 1 will be described. FIG. 5 is a flowchart showing the operation of the X-ray imaging apparatus, and FIG. 6 is a flowchart mainly showing the monitoring operation by the signal monitoring unit 15 and the post-update time monitoring unit 16. 7A and 7B are time charts showing the characteristics of the image output signal. FIG. 7A is a reference diagram in the case where the offset correction data is not updated after the X-ray irradiation is stopped, and FIG. It is a figure which shows the case by an X-ray imaging device. In FIG. 7, the image output signal corresponding to the detection element A on which the X-rays transmitted through the subject M are incident is represented by a solid line, and the image output signal corresponding to the detection element B that is not transmitted through the subject M and is directly incident on the X-rays. The image output signal is represented by a broken line.

<Step S1> Setting Initial Offset Correction Data The X-ray imaging apparatus is activated (at time t0 in FIG. 7. Only the time is abbreviated as appropriate below). X-rays have not been irradiated yet. From the FPD 3, output data of each detection element d when X-rays are not irradiated is read out.

  FIG. 7 clearly shows that the image output signals corresponding to the detection elements A and B vary during the period from time t0 to time t1.

  The correction data update unit 23 acquires and sets offset correction data based on the output data of each detection element d. The offset correction unit 21 performs offset correction on the output data of the detection element d using the set offset correction data (time t1).

  FIG. 7 clearly shows that the image output signals corresponding to the detection elements A and B are uniform during the period from time t1 to time t2.

<Step S2> Start X-ray Irradiation The subject M starts X-ray irradiation under the control of the X-ray irradiation control unit 7 (time t2). The FPD 3 detects X-rays that have passed through the subject M. Therefore, the output data of each detection element d is for X-ray irradiation. Each output data is offset-corrected by the offset correction unit 21. At this time, the offset correction data used is the offset correction data set at time t1.

  In FIG. 7, it is clearly shown that the detection element A has lower incident X-ray intensity than the detection element B in the period from time t2 to time t3.

<Step S3> Stop X-ray irradiation The X-ray irradiation control unit 7 performs control to stop X-ray irradiation. As a result, X-rays are not emitted from the X-ray tube 1 (time t3). From the FPD 3, the output data of the detection element d at the time of non-X-ray irradiation is read again. These output data should ideally return to the state before the start of X-ray irradiation (before time t2), but actually a residual output is generated.

  FIG. 7 clearly shows that the residual output is generated in the period after time t3. In particular, as shown in FIG. 7A, immediately after the X-ray irradiation is stopped, the residual output is sharply attenuated. Thereafter, as the time elapses after the X-ray irradiation is stopped, the attenuation of the residual output becomes gentle, and the residual output remains for a long time. Here, the residual output remaining for a long time is mainly due to the long-term residual output component.

  In addition, the detection element B on which strong X-rays are incident generates a larger residual output than the detection element A. An afterimage is formed under the influence of such a long-term residual output component.

<Step S <b>4> Starting Monitoring The signal monitoring unit 15 starts monitoring in cooperation with control for stopping X-ray irradiation by the X-ray irradiation control unit 7. Details will be described later.

<Step S5> Do you want to resume X-ray irradiation?
When resuming X-ray irradiation, the process proceeds to step S6. If not restarted, the operation of the X-ray imaging apparatus is terminated.

<Step S6> Ending Monitoring When resuming X-ray irradiation, control is performed to restart X-ray irradiation by the irradiation controller 7. In cooperation with this control, the signal monitoring unit 15 ends the monitoring. In addition, when the post-update time monitoring unit 16 has already started monitoring, the post-update time monitoring unit 16 also ends monitoring in cooperation with the control of the irradiation control unit 7. Then, the process returns to step S2.

  As a result of the monitoring performed after the X-ray irradiation is stopped, the monitoring may be terminated without updating the offset correction data. This is because the residual output is sufficiently small so as not to affect the X-ray image obtained by the restarted X-ray irradiation.

  Next, the operation in step S4 (start monitoring) will be described in more detail with reference to FIG.

<Step T1> Calculate the Average Value The signal monitoring unit 15 calculates the average value between the detection elements d of the image output signal after passing through the offset correction unit 21.

<Step T2> Is the average value below the reference value?
Next, the signal monitoring unit 15 compares the average value with the reference value. And it is judged whether an average value is below a standard value. If it is determined that the average value is less than or equal to the reference value, the process proceeds to step T3. If it is determined that the average value is larger, the process returns to step T1, and monitoring is repeated for the image output signal of the next frame.

<Step T3> Update offset correction data The monitoring result of the signal monitoring unit 15 is output to the main control unit 17. Thus, the main control unit 17 causes the correction data update unit 23 to update the offset correction data when the average value between the detection elements d of the image output signal becomes equal to or less than the reference value. The correction data update unit 23 acquires and updates offset correction data based on the output data of the detection element d. The offset correction unit 21 uses the new updated offset correction data to offset-correct output data from the subsequent detection element d (time t4).

  FIG. 7B clearly shows that the image output signal is uniform for a while after time t4.

<Step T4> The time after correction data update is monitored In cooperation with the operation in which the correction data update unit 23 updates the offset correction data at time t4, the post-update time monitoring unit 16 starts monitoring. That is, the time after the correction data update in which the X-ray non-irradiation state has been continued since the offset correction data was updated is measured, and the time when the time after the correction data update passes a predetermined period is monitored. Then, the monitoring result is output to the main control unit 17.

  The main control unit 17 causes the correction data update unit 23 to update the offset correction data when the time after the correction data update passes a predetermined period. The correction data update unit 23 acquires and updates the offset correction data again based on the output data of the detection element d. The offset correction unit 21 uses the new updated offset correction data to offset-correct the output data of the subsequent detection element d (time t5).

  Then, the process returns to step T4. In cooperation with the operation in which the correction data update unit 23 updates the offset correction data at time t5, the post-update time monitoring unit 16 repeats monitoring again. Then, every time a predetermined period elapses, the offset correction data is updated, and monitoring of the time after new correction data update is started (in FIG. 7B, the time t5 and later are omitted).

  Note that a predetermined period (period tb starting from time t5) for determining whether or not the time after the correction data update starting from time t5 elapses is a predetermined period (time) used for the previous monitoring. It is set longer than the period ta) from t4 to time t5. Therefore, when the offset correction data is updated repeatedly, the time interval gradually increases.

  As described above, according to the X-ray imaging apparatus according to the first embodiment, the signal monitoring unit 15 is provided to monitor when the average value between the detection elements d of the image output signal is equal to or less than the reference value. As a result, it is possible to accurately obtain a timing at which the long-term residual output component can be approximately regarded as a constant component. Then, under the control of the main control unit 17, the correction data update unit 23 updates the offset correction data when the average value between the detection elements d of the image output signal is equal to or less than the reference value. It is possible to remove the afterimage formed by the influence of the above.

  Further, by providing the post-update time monitoring unit 16, the predetermined period elapses even when the non-irradiation state continues after the offset correction data is updated based on the monitoring result of the signal monitoring unit 15. The offset correction data is repeatedly updated every time. Therefore, minute fluctuations in the long-term residual output component can be removed.

  Further, since the time interval for updating the offset correction data is set to be gradually longer during the predetermined period, it can be updated efficiently.

  Further, the signal monitoring unit 15 is configured to start monitoring in cooperation with the control in which the X-ray irradiation control unit 7 stops the X-ray irradiation, thereby hindering the operation of acquiring the output data or the image output signal. Therefore, it is possible to monitor the image output signal.

  As described above, according to the X-ray imaging apparatus according to the first embodiment, after the X-ray irradiation is stopped, the afterimage formed due to the influence of the long-term residual output component can be removed, and a higher quality X-ray image is obtained. Can be obtained.

Next, Embodiment 2 of the present invention will be described with reference to the drawings.
FIG. 8 is a block diagram illustrating the overall configuration of the X-ray imaging apparatus according to the second embodiment. In addition, about the same structure as Example 1, detailed description is abbreviate | omitted by attaching | subjecting the same code | symbol.

  In the X-ray imaging apparatus according to the second embodiment, the signal monitoring unit 15 provided in the first embodiment is omitted, and a non-irradiation time monitoring unit 18 is newly provided.

  The non-irradiation time monitoring unit 18 sets the time for which the X-ray non-irradiation state is continued from the time of shifting to X-ray non-irradiation and the time for which the X-ray non-irradiation state is continued after the offset correction data is updated measure. These times are collectively referred to as non-irradiation times in this specification. And the time when this non-irradiation time passes a predetermined period is monitored.

  Further, the non-irradiation time monitoring unit 18 is connected to the X-ray irradiation control unit 7, the correction data update unit 23, and the main control unit 17. Then, monitoring is started in cooperation with control for stopping X-ray irradiation by the X-ray irradiation control unit 7. Monitoring is also started when the correction data update unit 23 updates the offset correction data. When the offset correction data is updated a plurality of times while the X-ray non-irradiation state is continued, the measurement of the time after the correction data update is started each time. On the other hand, the monitoring is finished in cooperation with the control for restarting the X-ray irradiation by the X-ray irradiation control unit 7. In addition, by outputting the monitoring result of the post-update time monitoring unit 16 to the main control unit 17, the main control unit 17 is given a timing at which the time after the correction data update passes a predetermined period.

  Here, the predetermined period is set as a period in which a long-term residual output component is obtained in advance by measuring afterimage characteristics and the long-term residual output component can be approximately regarded as a constant component. This “predetermined period” has nothing to do with the “predetermined period” used by the post-update time monitoring unit 16 described in the first embodiment. Although it is possible to distinguish one as the first period and the other as the second period, the following description will be continued assuming that the period used by the non-irradiation time monitoring unit 18 is a “predetermined period”.

  In addition, this predetermined period is set so that the time interval gradually increases when the offset correction data is updated a plurality of times within one period in which the X-ray non-irradiation state is continued. . The non-irradiation time monitoring unit 18 corresponds to the non-irradiation time monitoring means in this invention.

Next, the operation of the X-ray imaging apparatus according to Embodiment 2 will be described. The overall operation is the same flow as in the first embodiment. Therefore, a more detailed description of the overall operation is omitted. In FIG. 5 , monitoring is started or ended in step S4 and step S6, except that the non-irradiation time monitoring unit 18 is used in the second embodiment. Hereinafter, the monitoring operation will be mainly described.

  First, the non-irradiation time monitoring unit 18 starts monitoring in cooperation with control for stopping X-ray irradiation by the X-ray irradiation control unit 7. That is, the non-irradiation time during which the X-ray non-irradiation state has been continued since the transition to X-ray non-irradiation is measured, and the time when the non-irradiation time passes a predetermined period is monitored. Then, the monitoring result is output to the main control unit 17.

  The main control unit 17 causes the correction data update unit 23 to update the offset correction data when the non-irradiation time passes a predetermined period. The correction data update unit 23 acquires and updates offset correction data based on the output data of the detection element d. The offset correction unit 21 uses the updated new offset correction data to offset-correct the output data of the subsequent detection element d.

  In cooperation with the operation in which the correction data update unit 23 updates the offset correction data, the non-irradiation time monitoring unit 18 starts monitoring again. A new non-irradiation time in which the X-ray non-irradiation state has been continued since the offset correction data was updated is measured, and the time when the time after the correction data update has passed a predetermined period is monitored. And based on this monitoring result, the update of offset correction data is repeated.

  When resuming X-ray irradiation, the non-irradiation time monitoring unit 18 ends the monitoring in cooperation with the control by the irradiation control unit 7.

  In Example 2, as a result of monitoring performed after X-ray irradiation is stopped, monitoring may be terminated without updating the offset correction data.

  As described above, according to the X-ray imaging apparatus according to the second embodiment, since the predetermined period is set in advance based on the measured afterimage characteristics, the non-irradiation time monitoring unit 18 has a long-term residual output component. The timing that can be regarded as a constant component approximately can be monitored. An afterimage formed due to the influence of the long-term residual output component can be removed. In addition, minute fluctuations in the image output signal after updating the offset correction data can be removed.

  The fluctuation of the long-term residual output component decreases with time, and the value itself decreases. For this reason, after updating the offset data for the first time after stopping the X-ray irradiation, the second and subsequent offset correction data updates that are repeated when the X-ray non-irradiation state is continued are set to gradually increase the time interval. And efficient.

  The non-irradiation time monitoring unit 18 is configured so as to start monitoring in cooperation with the X-ray irradiation control unit 7 and the correction data update unit 23, so that the acquisition operation of the output data or the image output signal is not hindered. The image output signal can be monitored.

  As described above, even with the X-ray imaging apparatus according to the second embodiment, the X-ray imaging apparatus according to the first embodiment removes afterimages formed by the influence of the long-term residual output component after the X-ray irradiation is stopped. And a higher quality X-ray image can be acquired.

  The present invention is not limited to the above-described embodiment, and can be modified as follows.

  (1) In the first embodiment described above, the signal monitoring unit 15 is configured to compare the average value of each image output signal with the reference value, but is not limited thereto. For example, statistics such as variance, standard deviation, and range of each image output signal may be compared with a predetermined reference value. The range is a so-called light / dark difference and is a difference between the maximum value and the minimum value of the image output signal.

  Furthermore, the signal monitoring unit 15 may be configured to calculate a temporal change of a statistic such as variance, standard deviation, average, or range of each image output signal and compare it with a predetermined reference value.

  Further, each image output signal is divided into a plurality of blocks (for example, a plurality of blocks divided into m × n) according to the position of the detection element d, and an average density value of each block is calculated. The determination may be made using the difference between the maximum value and the minimum value. Alternatively, the determination may be made using a temporal change in the difference between the maximum value and the minimum value between the blocks. Such values are also included in the statistics.

  (2) In the first embodiment described above, the offset correction data updated after the second time after stopping the X-ray irradiation is configured based on the monitoring result of the post-update time monitoring unit 16. I can't. For example, the second and subsequent updates may be performed based on the monitoring result of the signal monitoring unit 15. In this case, the design can be changed as appropriate, such as using a different reference value depending on the number of updates. Alternatively, the second and subsequent updates may be performed based on an external input by an operator operation or the like.

  (3) The signal monitoring unit 15 of Example 1 or the non-irradiation time monitoring unit 18 of Example 2 described above is configured to start monitoring in cooperation with the X-ray irradiation control unit 7 or the correction data update unit 23. However, it is not limited to this. A similar function may be realized by appropriately linking with the X-ray tube 1, the FPD 3, the FPD control unit 9, the main control unit 17, or the like.

  (4) In each of the above-described embodiments, the offset correction data is obtained by subtracting the average value from the output data of each detection element d when X-rays are not irradiated. However, the present invention is not limited to this. . The offset correction data can be appropriately changed and defined by a known method.

  (5) Each of the above-described embodiments is mainly intended to remove an afterimage formed due to the influence of a long-term residual output component of the residual output. It is possible to use a known technique for the purpose of, for example, a technique for irradiating light to the FPD 3 or a technique for formulating the temporal change of the residual output and arithmetically removing the residual output based on the mathematical expression.

  (6) In each of the above-described embodiments, the description has focused on performing offset correction on the output data of the detection element d. However, an X-ray image may be generated by appropriately combining other processes. For example, the sensitivity correction of each detection element d may be performed after the offset correction.

  (7) In each of the above-described embodiments, the FPD 3 has been described as an example. However, the present invention can be applied to any X-ray detector having a plurality of detection elements on the detection surface.

  In the above-described embodiment, the FPD 3 has been described by taking a direct conversion type detector as an example. However, the present invention is not limited to this. For example, an indirect detection element that converts incident X-rays into light with a scintillator and converts the light into charge information with a semiconductor layer formed of a photosensitive material may be used.

  (8) In the above-described embodiment, the FPD 3 is a detector that detects the incidence of X-rays, but what is incident is not limited to X-rays. The present invention can also be applied when radiation other than X-rays or light is incident.

  (9) In each of the above-described embodiments, the medical X-ray imaging apparatus is used. However, the present invention is not limited to this. For example, the present invention can be applied to a radiation imaging apparatus used in industrial fields such as non-destructive inspection, RI (Radio Isotope) inspection, and optical inspection, and in the nuclear field.

1 is a block diagram illustrating an overall configuration of an X-ray imaging apparatus according to Embodiment 1. FIG. It is a vertical sectional view of the principal part of FPD. It is a top view of FPD. (A) is a figure which shows typically the structure of the output data of a detection element, (b) is a figure which shows typically offset correction data. It is a flowchart which shows operation | movement of an X-ray imaging device. It is a flowchart which mainly shows the monitoring operation | movement by a signal monitoring part and the time monitoring part after an update. FIG. 2 is a time chart showing characteristics of an image output signal, where (a) is a reference diagram when offset correction data is not updated after X-ray irradiation is stopped, and (b) is an X-ray imaging apparatus according to the first embodiment. It is a figure which shows the case by. FIG. 3 is a block diagram illustrating an overall configuration of an X-ray imaging apparatus according to a second embodiment.

Explanation of symbols

1 ... X-ray tube 3 ... Flat panel X-ray detector (FPD)
DESCRIPTION OF SYMBOLS 7 ... X-ray irradiation control part 13 ... Signal processing part 15 ... Signal monitoring part 16 ... Post-update time monitoring part 17 ... Main control part 18 ... Non-irradiation time monitoring part 21 ... Offset correction part 23 ... Correction data update part M ... Subject d: detecting element

Claims (9)

  A plurality of detection elements for detecting radiation, a correction means for performing offset correction of output data of the detection element using offset correction data, and the offset correction based on output data of the detection element when radiation is not irradiated Update means for updating the data, and in the radiation imaging apparatus that outputs the image output signal after passing through the correction means, at least in the period from the stop of radiation irradiation to the transition to radiation irradiation, A signal monitoring unit that monitors a statistic between detection elements, and when the signal monitoring unit determines that the statistic or a temporal change in the statistic is equal to or less than a reference value, the updating unit includes: A radiation imaging apparatus, wherein offset correction data is updated. The radiation imaging apparatus according to claim 1, wherein the statistic is any one of a variance, a standard deviation, an average, and a range of an image output signal.   The radiation imaging apparatus according to claim 1 or 2, wherein the time after the correction data update in which the radiation non-irradiation state is continued is measured after the offset correction data is updated, and the time after the correction data update is predetermined. A post-update time monitoring means for monitoring when a period of time elapses, wherein the update means updates offset correction data when the time after the correction data update passes a predetermined period .   A plurality of detection elements for detecting radiation, a correction means for performing offset correction of output data of the detection element using offset correction data, and the offset correction based on output data of the detection element when radiation is not irradiated Update means for updating data, and in a radiation imaging apparatus that outputs an image output signal after passing through the correction means, when shifting to radiation non-irradiation and / or from when offset correction data is updated, Non-irradiation time monitoring means for measuring a non-irradiation time during which each radiation non-irradiation state is continued and monitoring when the non-irradiation time passes a predetermined period, and the updating means includes the non-irradiation time A radiation imaging apparatus, wherein offset correction data is updated when a predetermined period elapses.   5. The radiation imaging apparatus according to claim 4, wherein the predetermined period is set such that an interval for updating the offset correction data is gradually increased.   6. The radiation imaging apparatus according to claim 1, wherein the detection element includes at least a semiconductor layer and an electrode formed on an active matrix substrate.   In an offset correction method for performing offset correction on output data of a plurality of detection elements that detect radiation, an offset of output data of the detection element using offset correction data based on output data of the detection element when radiation is not irradiated In the process of performing the correction and outputting the image output signal corresponding to each detection element and the period from the stop of the radiation irradiation to the transition to the radiation irradiation, the statistic between the detection elements of the image output signal or the statistic A process of determining whether a temporal change is less than a reference value, and a process of updating the offset correction data when it is determined that the statistic or the temporal change of the statistic is less than a reference value. An offset correction method comprising:   In an offset correction method for performing offset correction on output data of a plurality of detection elements that detect radiation, an offset of output data of the detection element using offset correction data based on output data of the detection element when radiation is not irradiated The state of non-irradiation is continued from the process of performing the correction and outputting the image output signal corresponding to each detection element, and when shifting to radiation non-irradiation and / or updating the offset correction data. An offset correction comprising: a process of monitoring when a non-irradiation time passes a predetermined period; and a process of updating the offset correction data when the non-irradiation time passes a predetermined period Method. 9. The offset correction method according to claim 7, wherein the detection element includes at least a semiconductor layer and an electrode formed on an active matrix substrate.

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