Radiation imaging apparatus, image processing apparatus, and image processing method

An image processing apparatus that processes an image obtained from an imaging sensor having a plurality of pixels arranged in a matrix pattern, the pixels including a first pixel group for obtaining a pixel value corresponding to a radiation dose and a second pixel group for obtaining an offset value even with irradiation with radiation, performs offset correction of a radiation image obtained from the plurality of pixels by an imaging operation with irradiation with radiation based on a dark image obtained from the plurality of pixels by an imaging operation without irradiation with radiation, calculates a statistic value of pixel values obtained from the second pixel group of the corrected radiation image, and corrects pixel values obtained from the first pixel group, which have been offset-corrected, based on a temporal variation in the statistic value.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a radiation imaging apparatus using a solid-state imaging device, an image processing apparatus, and an image processing method.

Description of the Related Art

As a radiation imaging apparatus used for medical image diagnosis and non-destructive inspection using radiation such as X-rays, a radiation imaging apparatus having pixels as combinations of switches such as TFTs (thin-film transistors) and conversion elements such as photoelectric conversion elements arranged in a matrix pattern has been put into practice. In such a type of radiation imaging apparatus, manufacturing variations, environmental variations such as temperature variations, and the like cause variations in the value of an output without any irradiation (that is, an offset output) for each pixel. Accordingly, offset correction is performed to correct these variations.

As one method of performing the above offset correction, a method of calculating the difference between an offset signal (offset image) and an output at the time of imaging is used. This offset signal is obtained by obtaining a plurality of images without irradiation with radiation before imaging and calculating the average value of the resultant pixel values. This offset correction method need not obtain any offset image before and after radiation imaging, and hence is suitable for high-speed imaging such as moving-image imaging that requires a high frame rate. In addition, because an offset image is generated from a plurality of obtained images, this method has an advantage in reducing noise in the offset image.

Japanese Patent Laid-Open No. 2007-019820 (to be referred to as patent literature 1) discloses a method of providing each pixel with an optical black pixel that is light-shielded to obtain an offset signal in addition to a photoelectric conversion element for obtaining a radiation signal and using an output from the optical black pixel. Patent literature 1 discloses a radiation detection apparatus having a function of grasping the shading of an offset signal in an effective pixel region from outputs from optical black pixels in an effective pixel region and suppressing shading caused in an image obtained from the effective pixel region.

In the above offset correction method, a time difference occurs between the time of obtaining an offset image and the time of imaging, and the time difference causes a temperature distribution having a temperature difference between a region near a heating element in the apparatus and a region remote from the heating element. This temperature distribution changes an offset output, and hence an offset value shifts in the pixel matrix plane. This shift is visually recognized as an image artifact. In recent years, there has been a trend toward thinner and lighter radiation imaging apparatuses. This tends to increase the influence of heating elements such as a power supply and a high-density electrical component in the radiation imaging apparatus housing. Patent literature 1 makes some reference to offset correction of shading but makes no reference to a temporal change in offset components like that described above.

SUMMARY OF THE INVENTION

The present invention provides a technique of reducing a deterioration in image quality caused by a temporal change in offset component caused during imaging in a radiation imaging apparatus.

According to one aspect of the present invention, there is provided an image processing apparatus that processes an image obtained from an imaging sensor having a plurality of pixels arranged in a matrix pattern, the pixels including a first pixel group for obtaining a pixel value corresponding to a radiation dose and a second pixel group for obtaining an offset value even with irradiation with radiation, the apparatus comprising: a first correction unit configured to perform offset correction of a radiation image obtained from the plurality of pixels by an imaging operation with irradiation with radiation based on a dark image obtained from the plurality of pixels by an imaging operation without irradiation with radiation; a calculation unit configured to calculate a statistic value of pixel values obtained from the second pixel group of the radiation image corrected by the first correction unit; and a second correction unit configured to correct pixel values obtained from the first pixel group, which are corrected by the first correction unit, based on a temporal variation in the statistic value obtained by the calculation unit.

According to another aspect of the present invention, there is provided a radiation imaging apparatus comprising: an imaging sensor having a plurality of pixels arranged in a matrix pattern, the pixels including a first pixel group for obtaining a pixel value corresponding to a radiation dose and a second pixel group for obtaining an offset value even with irradiation with radiation; and an image processing apparatus configured to process an image obtained from the imaging sensor, wherein the image processing apparatus includes a first correction unit configured to perform offset correction of a radiation image obtained from the plurality of pixels by an imaging operation with irradiation with radiation based on a dark image obtained from the plurality of pixels by an imaging operation without irradiation with radiation, a calculation unit configured to calculate a statistic value of pixel values obtained from the second pixel group of the radiation image corrected by the first correction unit, and a second correction unit configured to correct pixel values obtained from the first pixel group, which are corrected by the first correction unit, based on a temporal variation in the statistic value obtained by the calculation unit.

According to another aspect of the present invention, there is provided an image processing method for processing an image obtained from an imaging sensor having a plurality of pixels arranged in a matrix pattern, the pixels including a first pixel group for obtaining a pixel value corresponding to a radiation dose and a second pixel group for obtaining an offset value even with irradiation with radiation, the method comprising: executing first correction processing of performing offset correction of a radiation image obtained from the plurality of pixels by an imaging operation with irradiation with radiation by using an offset image based on a dark image obtained from the plurality of pixels by an imaging operation without irradiation with radiation; calculating a statistic value of pixel values obtained from the second pixel group of the radiation image corrected by the first correction processing; and executing second correction processing of correcting pixel values obtained from the first pixel group, corrected by the first correction, based on a temporal variation in the statistic value.

DESCRIPTION OF THE EMBODIMENTS

Each embodiment of the present invention will exemplify a case in which an X-ray imaging apparatus that images an object by using X-rays as one type of radiation is used as a radiation imaging apparatus according to the present invention. Note that the present invention is not limited to an X-ray imaging apparatus and can be applied to a radiation imaging apparatus that images an object by using other types of radiation (for example, α-rays, β-rays, and γ-rays).

First Embodiment

FIG. 1shows the overall arrangement of an X-ray imaging apparatus100as an example of a radiation imaging apparatus according to the first embodiment. The X-ray imaging apparatus100is for medical use. The X-ray imaging apparatus100includes an X-ray generation apparatus101, an X-ray detection apparatus104, and an imaging control apparatus102. Users such as medical care personnel (to be simply referred to as a user hereinafter) set imaging conditions via an operation panel111of the imaging control apparatus102. The imaging control apparatus102including a CPU110and a storage unit113sets the set imaging conditions in the X-ray generation apparatus101and the X-ray detection apparatus104. The X-ray generation apparatus101irradiates an object1003with X-rays1002. The object1003is, for example, the human body. The X-ray detection apparatus104includes an imaging sensor151and generates X-ray image data and an offset signal. The imaging sensor151is, for example, an FPD (Flat Panel Detector) having many pixels arranged on a large flat wafer. A driving unit152drives the imaging sensor151to read out a signal from each pixel and output image data (radiation image).

FIG. 4shows an example of the arrangement of the X-ray detection apparatus104. As described above, the X-ray detection apparatus104includes the imaging sensor151and the driving unit152. The imaging sensor151includes a detection panel having a plurality of pixels arranged in a matrix pattern. The plurality of pixels include a first pixel group for obtaining pixel values corresponding to radiation doses and a second pixel group for also obtaining offset values with irradiation with radiation. The driving unit152includes a gate driving circuit482, a power supply circuit483, a readout circuit484, and a signal conversion circuit485. These circuits will be described in more detail below.

An effective pixel region401of the imaging sensor151has about 3000 row×3000 column photoelectric conversion elements arrayed two-dimensionally. The imaging sensor151detects the two-dimensional distribution of X-ray doses reaching the effective pixel region401, and generates X-ray image data. Photoelectric conversion elements that are not light-shielded (to be referred to as effective elements421hereinafter) in the effective pixel region401constitute the first pixel group described above. An ineffective pixel region402is provided around the effective pixel region401. The ineffective pixel region402contains photoelectric conversion elements provided with light-shielding members (to be referred to as light shielded elements420hereinafter). Note that the light shielded elements420may exist in the effective pixel region401. The light shielded elements420constitute the second pixel group described above. Note that all the photoelectric conversion elements in the ineffective pixel region402may be the light shielded elements420. As shown inFIG. 4, the light shielded elements420can be provided in the effective pixel region401. The light shielded elements420detect offset signals contained in the X-ray image data generated by the X-ray detection apparatus104.

At the time of moving-image imaging, binning readout, that is, collectively reading out a plurality of pixels, is sometimes performed. For example, in the case of 3×3 binning, the overall image size is about 1000 rows×1000 columns. In the case of moving-image imaging, an X-ray irradiation range450is sometimes set to be narrower than the effective pixel region401. In moving-image imaging, because radiation doses are accumulated after long-time irradiation with radiation, the exposure dose is reduced by spatially narrowing down the aperture. It is possible to achieve a higher frame rate and power saving by not only performing full readout driving but also limiting the drive of the imaging sensor151in accordance with the range including a trimming drive range451set in advance by the operation panel111.

In order to read out a signal from each pixel, the gate driving circuit482selectively drives a row in the matrix constituted by the plurality of pixels of the imaging sensor151. The readout circuit484reads out analog signals from effective elements421and the light shielded elements420which are connected to the row selected and driven by the gate driving circuit482, and transmits the signals to the signal conversion circuit485. The readout circuit484includes an amplifier for amplifying analog signals output from the effective elements421and the light shielded elements420. The signal conversion circuit485includes an A/D converter for converting the analog signals amplified by the readout circuit484into digital signals. The X-ray detection apparatus104transmits the digital signal obtained by the driving unit152to an information processing apparatus. The amplifier and the A/D converter are examples of heat generating members in the X-ray detection apparatus104.

Referring back toFIG. 1, phosphors convert the X-rays1002that have entered the X-ray detection apparatus104into visible light. The effective elements421photoelectrically convert the visible light into electrical signals. The light shielded element420includes a light-shielding member such as a metal member which is located between the phosphor and the photoelectric conversion element and light-shields a part of the adjacent pixel. X-rays and visible light applied to the light shielded element420do not reach the photoelectric conversion element. The X-ray image obtained by the X-ray detection apparatus104in the above manner is transmitted to a data collecting unit106of the imaging control apparatus102.

The imaging control apparatus102can be constituted by an information processing apparatus such as a personal computer. An irradiation control unit105controls irradiation with X-rays by the X-ray generation apparatus101. The data collecting unit106receives an X-ray image transmitted from the X-ray detection apparatus104. A preprocessing unit108performs, for example, offset correction processing of an X-ray image received from the X-ray detection apparatus104by the data collecting unit106. The operation of each unit of the preprocessing unit108will be described later with reference to the flowchart ofFIG. 2. An image processing unit109generates a diagnosis image by performing QA processing such as gradation processing and highlighting processing with respect to the X-ray image preprocessed by the preprocessing unit108. The CPU110implements the respective functional units of the imaging control apparatus102(the preprocessing unit108, the image processing unit109, the irradiation control unit105, the data collecting unit106, an ABC/AEC unit120, and the like) by executing predetermined programs stored in the storage unit113. The operation panel111accepts various types of user operations. A display unit112performs various types of display under the control of the CPU110. For example, the display unit112displays the diagnosis image generated by the image processing unit109. The storage unit113stores various types of programs executed by the CPU110.

The X-ray image preprocessed by the preprocessing unit108is also transmitted to the ABC/AEC unit120. ABC stands for “Auto Brightness Control”. AEC stands for “Auto Exposure Control”. It is possible to make setting in advance via the operation panel111to stop irradiation by the X-ray generation apparatus101at a predetermined X-ray dose. The AEC function of the ABC/AEC unit120stops irradiation with X-rays by the X-ray generation apparatus101by sending an exposure permission end signal to the irradiation control unit105upon determining from an image signal that the X-ray dose has exceeded a predetermined X-ray dose. Alternatively, it is possible to make setting in the ABC/AEC unit120in advance via the operation panel111to stop irradiation by the X-ray generation apparatus101when a predetermined luminance is obtained. The ABC/AEC unit120stops irradiation with X-rays by the X-ray generation apparatus101by sending an exposure permission end signal to the irradiation control unit105upon determining that the luminance of an image signal has exceeded a set luminance. In this manner, the irradiation dose of X-rays is properly controlled.

AlthoughFIG. 1shows the case in which the preprocessing unit108is mounted outside the X-ray detection apparatus104, the preprocessing unit108may be mounted inside the X-ray detection apparatus104. A merit in mounting the preprocessing unit108outside the X-ray detection apparatus104is to allow a memory unit170and the storage unit113to have larger capacities, because a sufficient space can be secured, and hence to be able to implement image saving for preprocessing and perform sequential correction processing. In contrast to this, a merit in mounting the preprocessing unit108inside the X-ray detection apparatus104is to be able to handle signals simply by executing preprocessing in the X-ray detection apparatus104because only appropriate signals after the processing need to be handled on the subsequent stage. In addition, because an X-ray image after correction is transmitted, the X-ray image can be transferred after being compressed. This can reduce the load on communication. Note that some units of the preprocessing unit108may be mounted inside the X-ray detection apparatus104, and the remaining units may be mounted outside the X-ray detection apparatus104.

The respective functional units such as the preprocessing unit108and the image processing unit109may be implemented by causing the CPU110to execute programs stored in the storage unit113, may be implemented by dedicated hardware, or may be implemented by cooperation between programs and hardware. In addition, other functional units of the preprocessing unit108may be implemented either inside or outside the imaging control apparatus102. For example, the ABC/AEC unit120may be implemented as an FPGA (Field-Programmable Gate Array) inside the X-ray detection apparatus104.

An operation from the start of imaging an object to the end of preprocessing by the preprocessing unit108according to the first embodiment will be described below with reference to the flowchart ofFIG. 2. A first correction unit181uses the dark image obtained from a plurality of pixels obtained by an imaging operation without irradiation with radiation to perform offset correction of the radiation image obtained from the plurality of pixels obtained by an imaging operation with irradiation with radiation (first offset correction).

First of all, the first correction unit181obtains a dark image by executing an imaging operation using the X-ray detection apparatus104without irradiation with radiation before an object imaging operation to generate an offset image used for the first offset correction (step S201). The obtained dark image is saved in the memory unit170. Upon obtaining a predetermined number of dark images (step S202), the first correction unit181averages the obtained dark images and saves the resultant image as an offset image in the memory unit170(step S203). In this manner, the first correction unit181performs the first offset correction by using the image obtained by averaging a predetermined number of dark images as an offset image. For example, the first correction unit181obtains an offset image by averaging 32 dark images. Note that the obtained offset image contains offset correction values for the pixel values obtained from the light shielded elements420.

It is necessary to complete generating an offset image in steps S201to S203before the start of X-ray imaging. The first correction unit181performs control so as to cause the display unit112to display a message such as “activating” to inhibit the operator from performing X-ray imaging while the obtaining of a predetermined number of dark images and the generation of an offset image are completed. The obtained offset image is saved in the memory unit170. In addition, during “activating” described above (during the execution of steps S201to S203), a third correction unit185loads a gain image used for gain correction and a defect map used for defect correction from the X-ray detection apparatus104and save them in the memory unit170.

When the obtaining of an offset image is completed, target X-ray imaging can be performed, and the operator starts obtaining an X-ray image. When X-ray imaging is executed, the X-ray image obtained by the imaging is transmitted from the data collecting unit106to the preprocessing unit108(step S204), and preprocessing in and after step S205is executed.

The first correction unit181performs first offset correction of an X-ray image by using the offset image obtained in steps S201to S203(step S205). A second correction unit182, a determination unit183, and a calculation unit184perform the second offset correction of the X-ray image having undergone the first offset correction (step S206). The second offset correction will be described in detail later with reference to the flowchart ofFIG. 3.

The third correction unit185performs offset uniform addition (step S207). Performing this offset uniform addition makes it possible to suppress the occurrence of negative values caused by local drops in value in an image having a portion difficult to handle by offset correction using statistic processing based on outputs from photoelectric conversion elements. In this processing, for example, an offset value of about 50 LSBs is uniformly added. This technique has the merit of reducing the probability of occurrence of negative values as the offset value to be added increases. In contrast to this, as the offset value to be added increases, when the preprocessing unit108or the image processing unit109performs image processing based on the premise that when the dose is zero, the pixel value is zero, the above technique tends to have the demerit of causing an inverse image in calibration, reducing contrast, and the like. An inverse image in calibration may be caused as the pixel value becomes positive when the dose is zero. For this reason, it is preferable to uniformly add, for example, a relatively small offset value of 50 LSBs. Alternatively, the user may select an offset value to be uniformly added based on, for example, the method used in a clinical case. Note that offset uniform addition is an option and may be omitted.

The third correction unit185performs negative value clipping (step S208). In negative value clipping, the third correction unit185clips each pixel having a negative value or 0 as a pixel value to a minimum fixed value such as 1 LSB as a pixel value. When, for example, the preprocessing unit108or the image processing unit109uses division, a denominator including a pixel value of 0 may cause a trouble. In addition, in logarithmic conversion of a pixel value, a negative value may cause a trouble. Accordingly, negative value clipping is performed to fix a negative pixel value or a pixel value of 0 to 1 LSB. In this embodiment, before the execution of this negative value clipping, the second offset processing (step S206) and offset uniform addition (step S207) are performed to reduce the number of pixel values that become negative values. Performing these processes will reduce the possibility of pixel values being negative values in an image region having distribution information transmitted through an object.

The third correction unit185performs gain correction by using a gain image saved in the memory unit170(step S209). In general, gain correction is performed by using division or subtraction after logarithmic conversion. The third correction unit185performs defect correction by using the defect map saved in the memory unit170(step S210). Note that a known method can be used for such gain correction and defect correction.

The CPU110checks the state of the operation panel111and determines whether to continue exposure (step S211). When finishing imaging, the CPU110simultaneously finishes exposure by the X-ray generation apparatus101. At the time of tomographic imaging, the computer determines the continuation of imaging based on an imaging instruction saved in the storage unit113in advance instead of checking the state of the operation panel111. If the preprocessing unit108determines whether to finish imaging and determines not to finish imaging, the process returns to step S203to perform the above processing for a next X-ray image (NO in step S211). Upon determining to finish X-ray imaging, the CPU110terminates this processing (YES in step S211).

The second offset correction in step S206will be described next.FIG. 3is a flowchart for explaining the second offset correction processing according to the first embodiment.

In still-image imaging or the like, it is possible to perform the first offset correction by calculating differences between the data obtained by imaging with irradiation with radiation and the data obtained by imaging without irradiation with radiation for each imaging operation. That is, in still-image imaging, because the offset image obtained at a timing near imaging can be used, the necessity of the second offset correction processing according to this embodiment is low. In high-speed imaging such as moving-image imaging, in order to secure a high frame rate, the first offset correction is performed by repeatedly using the offset image obtained in steps S201and S202during moving-image imaging. That is, offset correction is performed by obtaining a plurality of images without irradiation with radiation before moving-image imaging, holding the average value of the resultant values as an offset image, and calculating the difference between an output at the time of moving-image imaging and the offset image.

Moving-image imaging is generally performed with a low radiation dose. The above first offset correction is advantageous in reducing noise by generating an offset image using a plurality of dark images. In contrast to this, in moving-image imaging, there is a time difference between the time of obtaining an offset image and the time of imaging. This time difference causes a temperature distribution including different temperatures in a region near a heating element in the apparatus and a region remote from the heating element. Changes in temperature will change an offset output. For this reason, as the imaging time prolongs, an offset value shifts within a plane due to a temperature distribution, resulting in image artifacts. The second correction unit182reduces offset value shifts caused by a temperature distribution by performing the second offset correction processing.

First of all, the determination unit183determines whether to perform the second offset correction processing (step S301). As described above, as the elapsed time since the time of obtaining an image for the first offset correction increases, the temperature distribution changes from the time of obtaining the first offset image, resulting in a different offset output distribution. Accordingly, the determination unit183determines whether to execute the second offset correction, based on the value of a temperature sensor provided on an electric board in the X-ray detection apparatus104or the elapsed time since the time of obtaining the image.

For example, the determination unit183compares an output value from the temperature sensor at the time of obtaining an image for the first offset correction with an output value from the temperature sensor during imaging of an object. If these output values differ from each other by a predetermined value (for example, 2° C.) or more, the determination unit183determines to execute the second offset correction. If the difference is less than the predetermined value, the determination unit183determines not to execute the second offset correction. In addition, if, for example, the elapsed time since the time of obtaining an image for the first offset correction exceeds a predetermined value (for example, 30 min), the determination unit183determines to execute the second offset correction. If the elapsed time does not exceed the predetermined value, the determination unit183determines not to execute the second offset correction. Note that a temperature change often occurs in a short time immediately after power-on, at the start of charging, or after a change in charging rate. For this reason, at these timings, the determination unit183may perform determination upon changing the above time from 30 min to a short time, for example, 10 min. Note that the determination unit183may determine whether to execute the second offset correction processing, based on both determinations using a value from the temperature sensor and the elapsed time.

If the determination unit183determines to execute the second offset correction (YES in step S301), the second offset correction indicated in and after step S302is executed. In the second offset correction, first of all, the calculation unit184calculates the statistic value of pixel values obtained from the second pixel group of the radiation image (X-ray image) corrected by the first correction unit181(step S302). The second pixel group is a pixel group constituted by the light shielded elements420. The calculation unit184according to this embodiment performs recursive processing of integrating, using weighting, the statistic value obtained before the previous radiation imaging (X-ray imaging) and the statistic value obtained by current radiation imaging (X-ray imaging) to obtain a statistic value used by the second correction unit182.

More specifically, first of all, the calculation unit184calculates a statistic value An−1of pixel values obtained from light shielded elements by previous X-ray imaging using values from the light shielded elements after the first offset correction. As the statistic value used in step S302, for example, the average value of pixel values obtained from the light shielded elements420constituting the second pixel group can be used. Note that the statistic value used in step S302may be a median value, mode value, top % value, or the like. The details of calculation of a statistic value in step S302will be described in detail later with reference to the flowchart ofFIG. 5. Note that for a correction target image (n) obtained in step S204, the statistic value An−1of an immediately preceding image (n−1) is obtained in step S302. This is because a statistic value cannot be obtained in time due to limitations in terms of frame rates.

The calculation unit184then executes recursive processing An−1=αAn−1+(1−α)An−2(step S303). In this case, a statistic value An−2is the statistic value obtained by recursive processing in the previous X-ray imaging (step S303previously executed). Note that recursive processing is used because flicker, that is, a variation in pixel value, sometimes occurs every several frames in moving-image imaging depending on a time period, and hence the use of values from the light shielded elements420in a plurality of images improves the reliability. Note that a statistic value may be calculated in steps S302and S303regardless of the determination result obtained in step S301.

Note that a weight value α used in step S303may be dynamically changed by using the elapsed time since the obtaining of an offset image or a temperature sensor output value used in step S301. For example, increasing the weight value α when the elapsed time since the obtaining of an image for the first offset correction is short will obtain the merit of performing proper correction at the occurrence of a rapid characteristic change. When the elapsed time is long, reducing the weight value α makes it possible to perform correction by using image values obtained from more imaging operations with stable characteristics. In addition, as in the above case, in correction using an output value from the temperature sensor, when a temperature change is large, increasing the weight value α can obtain the merit of performing proper correction at the occurrence of a rapid temperature change. When a temperature change is small, it is possible to perform correction by using image values obtained by more imaging operations while the temperature is stable.

The determination unit183performs positive/negative determination of the statistic value Ann−1of light shielded elements (step S304). The statistic value An−1sometimes becomes a negative value as a result of the first offset correction. That a statistic value from light shielded elements becomes negative indicates that when the radiation dose is low, in particular, negative values occur in many pixels, and an image signal of an object is cut to a predetermined value in subsequent negative value clipping (step S208). When the statistic value An−1is positive, a low-contrast image may be obtained as a result of performing image processing based on the premise that a pixel value without irradiation with X-rays is 0. If the statistic value An−1of values from light shielded elements is negative, the process advances to step S305. If the statistic value is positive, the process advances to step S308. Note that if the statistic value An−1is 0 (zero), the process may advance to either step S305or step S309.

Based on the statistic value, the second correction unit182performs offset correction of the pixel values obtained from the first pixel group (effective elements421) of the radiation image (X-ray image) corrected by the first correction unit181. The operation of the second correction unit182will be described in detail with reference to steps S305to S310. Note that in this embodiment, if the absolute value of a statistic value is larger than a predetermined threshold, the second offset correction is executed by using the threshold. If the absolute value of a statistic value is equal to or less than the predetermined threshold, the second offset correction is executed by using the statistic value.

If the value of the statistic value An−1is negative, the second correction unit182determines whether the absolute value of the statistic value An−1is larger than a predetermined threshold B (B>0) (step S305). If the absolute value of the statistic value An−1is larger than the threshold B (|An−1|>B, that is, An−1<-B), some kind of abnormality such as wiring disconnection may have occurred.

If statistic value An−1<-B (YES in step S305), the second correction unit182adds B as a limiter value to the pixel values of all the effective pixels (step S306). This serves as a predetermined limiter with respect to the influence on an overall image even if abnormality has occurred in part of the image. If statistic value An−1≥-B (NO in step S305), the second correction unit182determines that the value of the statistic value An−1falls within a proper range. The second correction unit182then uniformly adds the absolute value of the statistic value An-31 1to all effective pixels (step S307).

Upon determining in step S304that the statistic value An−1is 0 or positive, the second correction unit182determines whether the statistic value An−1is larger than a predetermined threshold C (step S308). If the positive value is larger than the threshold by a predetermined value or more, some kind of abnormality such as disconnection may have occurred. If the statistic value Ann−1is larger than the threshold C, the process advances to step S310. If the statistic value An−1is equal to or less than the threshold C, the process advances to step S309.

If the statistic value An−1is equal to or less than the threshold C (NO in step S308), the second correction unit182determines that the statistic value An−1falls within the proper range, and uniformly subtracts the statistic value An−1from all the effective pixels (step S309). If the statistic value An−1is larger than the threshold C (YES in step S308), the second correction unit182subtracts C, which is a limiter value, from all the effective pixels (step S310). This operation serves as a certain kind of limiter when, for example, abnormality has occurred in part of the image with respect to the influence of the abnormality on an overall image.

Note that steps S308to S310may be omitted from the flowchart ofFIG. 3which explains the second offset correction. In this case, only when the statistic value An−1is smaller than a predetermined threshold (in this case, the statistic value An−1is a negative value), the determination unit183determines to execute the second offset correction. This arrangement has the merit of being able to suppress the clipping of an image with a low radiation dose and simplifying the arrangement.

If the calculation of a statistic value Anobtained from a currently obtained image (n) in step S302can be done in time for recursive processing in step S303, the statistic value Anmay be used. In this case, An−1in steps S302to S305and steps S307to S309may be read as An, and An−2in step S303may be read as An−1. In addition, the statistic value to be obtained in step S302may be switched between Anand An−1depending on the frame rate of a moving image. That is, control may be performed in step S302such that if the frame rate is higher than a predetermined value, the statistic value An−1is obtained, whereas if the frame rate is equal to or less than the predetermined value, the statistic value Anis obtained.

FIG. 5is a flowchart for explaining statistic processing by the calculation unit184. Note that the calculation unit184performs statistic processing described below by using the image saved in the memory unit170and having undergone the first offset correction. However, this is not exhaustive. Statistic processing is executed during moving-image imaging and hence may be executed concurrently with the readout operation of the readout circuit484. In this case, the first offset correction and statistic processing are executed for each row. Note that the X-ray detection apparatus104may execute statistic processing by using, for example, an FPGA. If most processing to be executed by the preprocessing unit108is executed inside the X-ray detection apparatus104, for example, information from the second pixel group need not be transferred to the imaging control apparatus102. This enables efficient transfer. It is also obvious to those skilled in the art that providing the calculation unit184in the X-ray detection apparatus104can implement the following statistic processing. Note, however, that the arrangement having the calculation unit184provided in the X-ray detection apparatus104is preferably configured to perform the first offset correction and statistic processing for each row read out by the readout circuit484.

The readout circuit484reads out output signals from the photoelectric conversion elements connected to the row driven by the gate driving circuit482. In this case, the readout circuit484also reads out output signals from the photoelectric conversion elements in the ineffective pixel region402as well as the photoelectric conversion elements in the effective pixel region401. That is, the readout circuit484reads out output signals from the first pixel group constituted by effective elements and the second pixel group constituted by the light shielded elements420. The signal conversion circuit485converts the output signals read out by the readout circuit484into digital pixel values and outputs them. The calculation unit184obtains pixel values from each row output from the signal conversion circuit485. As described above, the first correction unit181performs offset correction using an offset image with respect to the pixel values obtained from the signal conversion circuit485and saves the pixel values after the correction in the memory unit170.

Upon starting statistic processing with respect to outputs from a plurality of pixels which are obtained from the light shielded elements420, the calculation unit184obtains pixel values from each row of an image after the first offset correction, which are saved in the memory unit170(step S501). The calculation unit184saves, in the memory unit170, pixel values obtained from the light shielded elements420of pixel values from each row which are obtained in step S501(step S502). For example, output signals from photoelectric conversion elements are divided into output signals from the effective elements421and output signals from the light shielded elements420on the left side and the light shielded elements420on the right side. Output signals from the effective elements421are used as pixel values at the respective positions and formed into an image. In contrast, the light shielded elements420outside the effective pixel region401are used to execute the second offset correction.

The calculation unit184performs the first statistic processing with respect to pixel values obtained from the light shielded elements420(step S503). The calculation unit184performs the first statistic processing upon dividing the region constituted by the light shielded elements420into a plurality of regions. For example, referring toFIG. 6, when P column is read out, the calculation unit184extracts horizontal 3 pixels×vertical 5 pixels, that is, a total of 15 pixels (a region601), from the light shielded elements420on the left side on P row, P-1row, P-2row, P-3row, and P-4row, and calculates a median value. Likewise, the calculation unit184calculates a median value concerning 15 pixels in a region602. The calculation unit184saves the obtained statistic values (the median values in this case) as the first statistic values in the memory unit170(step S504). The processing in steps S501to S504is repeated until the above processing is executed for all the rows in a planned region (step S505). The first statistic processing obtains a plurality of first statistic values concerning the light shielded elements420on the left side of the effective pixel region401and the light shielded elements420on the right side of the effective pixel region401.

Upon completing the first statistic processing concerning the planned region, the calculation unit184performs the second statistic processing with respect to a plurality of first statistic values saved in the memory unit170(step S506). In this embodiment, the calculation unit184calculates a second statistic value (An) by performing median processing of the plurality of first statistic values. Performing the second statistic processing can handle problems such as locally excessive increases and decreases in output. Note that the second statistic processing may use a plurality of types of statistic processing. For example, the second statistic processing may be performed to obtain four median values by performing median processing of the first statistic values in four regions, that is, upper, lower, left, and right regions, in the ineffective pixel region402shown inFIG. 4and calculate the statistic value (An) by performing averaging processing of the four obtained median values.

As shown inFIG. 4, although the upper and lower ineffective pixel regions402each do not include the effective pixel region401in the center as shown inFIG. 4, statistic processing may be performed by using left and right end regions as in the left and right ineffective pixel regions402. This eliminates the necessity to change the regions to be used for statistic processing among the upper and lower ineffective pixel regions and the left and right ineffective pixel regions, thereby simplifying calculation processing by the calculation unit184and suppressing an increase in the circuit size of the FPGA.

Note that the above description has exemplified median processing as the first statistic processing and the second statistic processing. However, this is not exhaustive. Median processing or mode processing without division is suitable for sequential calculation by the FPGA. However, when, for example, an information processing apparatus including a CPU is used, average values may be used in the first statistic processing and the second statistic processing.

Note that the calculation unit184described above performs statistic processing for each row. However, this is not exhaustive. For example, in the first statistic processing, the calculation unit184may perform statistic processing with respect to the entire light shielded elements420on the left side and the entire light shielded elements420on the right side. In this case, more complicated statistic processing such as density ratio estimation may be performed.

FIG. 7is a view for explaining a problem that can occur when the second offset correction according to this embodiment is not used.FIG. 7shows images obtained when stomach fluoroscopy is performed with moving images. The readout circuit484of the X-ray detection apparatus104includes an amplifier IC that amplifies a signal read out from each photoelectric conversion element. The amplifier IC is a component that tends to generate heat, and is arranged near the photoelectric conversion elements arranged in a matrix pattern (array pattern) in the X-ray detection apparatus104to prevent noise from mixing in a signal path before amplification. When, for example, the amplifier IC is arranged near the upper end portion of the imaging sensor151, the temperature of the upper end portion of the imaging sensor151rises. This also causes a great change in pixel value (offset) in a dark image.

FIG. 7Ashows a stomach fluoroscopic image immediately after the start of imaging. Before imaging of an object, the first offset image is obtained and saved in the memory unit170. In a stomach fluoroscopic image, because the elapsed time since the obtaining of the first offset image is short, an offset distribution can be properly corrected without applying the second offset correction at the start of imaging.

FIG. 7Bshows a stomach fluoroscopic image one hour after the start of imaging. As the temperature of the imaging sensor151rises, pixel values decrease in an upper portion of an image near a component with a high degree of heat generation, such as an amplifier IC. Consequently, when the first correction unit181subtracts an offset image obtained in advance from the obtained image (executes offset correction), negative pixel values appear. When pixel values become negative, the pixel values are made uniform by image processing such as negative value clipping, resulting in substantial signal defects. According to this embodiment, the application of the second offset correction described above can reduce image signal defects, thereby solving the problem shown inFIGS. 7A and 7B.

This embodiment has exemplified the case in which the second pixel group from which offset values are also obtained with irradiation with radiation is constituted by the light shielded photoelectric conversion elements (light shielded elements). However, this is not exhaustive. As pixels constituting the second pixel group, elements without photoelectric conversion portions may be used. The optical black pixels according to the embodiment may be pixels with light shielded photoelectric conversion elements or pixels without photoelectric conversion elements (pixels constituted by only capacitors).

Second Embodiment

The second embodiment will be described next. In the second embodiment, the driving method is changed to increase the imaging speed and reduce the exposure dose, thereby more properly correcting an in-plane change in offset caused by temperature and environmental changes. Note that an X-ray imaging apparatus100according to the second embodiment has the same arrangement such as that of the first embodiment (FIG. 1). Differences from the first embodiment will be mainly described below.

FIGS. 8A to 8Care views for explaining variations of the second offset correction when imaging is performed with a narrowed-down irradiation field. Referring toFIGS. 8A to 8C, an X-ray irradiation range450is a range in which radiation is applied from an imaging sensor151of an X-ray detection apparatus104. The user sets a trimming drive range451by designating an imaging region as a 9-inch square region, 12-inch square region, or the like via an operation panel111. When the trimming drive range451is set, a gate driving circuit482and a readout circuit484perform partial readout driving of reading out pixel values by driving a row including the trimming drive range451. The second embodiment will exemplify the following three cases.

An example of a first driving method will be described first with reference toFIG. 8A. The first driving method performs the second offset correction by using light shielded elements located in a lateral direction of the range driven by the gate driving circuit482. The gate driving circuit482drives gate lines820of rows including the designated trimming drive range451and does not drive other gate lines821. A calculation unit184performs the first statistic processing and the second statistic processing described above with respect to a right region810and a left region811constituted by light shielded elements420existing on the gate lines driven by the gate driving circuit482. The first driving method has the merit of increasing the frame rate because the number of gate lines to be driven decreases. In addition, this can reduce the driving change branches based on the trimming drive range451, thereby simplifying drive control by the FPGA and the like.

The second driving method will be described next with reference toFIG. 8B. Unlike in the first driving method described above, in the second driving method, an upper region812and a lower region813are added as regions of the light shielded elements420used for the second offset correction processing. Accordingly, in the second driving method, the gate driving circuit482drives the gate lines on the upper and lower ends of the imaging sensor151in addition to the trimming drive range451. The second driving method has the merit of being able to grasp each side by a uniform method with respect to an end portion where characteristics tend to change, and hence suppressing the occurrence of troubles caused because different methods are used depending on direction.

The third driving method will be described next with reference toFIG. 8C. The third driving method uses partial regions of the upper region812and the lower region813as the regions of light shielded elements used for offset correction processing in the second driving method. That is, the third driving method uses an upper left region851, an upper right region852, a lower right region853, and a lower left region854in addition to the left region811and the right region810. In the third driving method, the width (the number of pixels) of the upper left region851, the upper right region852, the lower right region853, and the lower left region854in the row direction is matched with the width of the left region811and the right region810in the row direction to fix the number of light shielded elements in the row direction in the first statistic processing. Accordingly, as compared with the second driving method, the third driving method has the merit of being able to simplify the calculation performed by the FPGA. In contrast to this, when characteristics are distributed in the lateral direction inFIG. 8C, the second driving method is superior to the third driving method. The third driving method is suitable for a case in which it is obvious from various environmental tests that the readout circuit484is the main cause of heat generation and the difference in temperature distribution in the lateral direction is small.

Note that the application of correction values used for the second offset correction is not limited to the correction of image signals. For example, such correction values may be used for the correction of pixel values provided by an ABC/AEC unit120. Using the pixel values obtained by the second offset correction can improve the control accuracy of ABC/AEC and prevent excessive irradiation and insufficient irradiation accompanying output variation.

The second offset correction applies the first offset correction processing to pixel values from the second pixel group constituted by the light shielded elements420and uses the resultant values for the calculation of statistic values (steps S205and S206). However, this is not exhaustive. The first offset correction is applied to pixel values from the second pixel group to evaluate variations in values from the second pixel group since the obtaining of an offset image used for the first offset correction. That is, the second offset correction is only required to enable processing in steps S302to S310based on temporal variations in pixel values obtained from the second pixel group, and is not limited to the use of the result of the first offset correction. For example, “temporal variations in pixel values obtained from the second pixel group” may be obtained from one of dark images repeatedly obtained in steps S201and S202.

The above embodiment uses pixel values from the light shielded elements420existing in the ineffective pixel region402. However, this is not exhaustive. For example, pixel values from the light shielded elements420arranged in the effective pixel region401inFIG. 4may be used for the above calculation of a statistic value.

The X-ray imaging apparatus100(the X-ray generation apparatus101and the X-ray detection apparatus104) can be mounted on various types of imaging systems. For example, the X-ray imaging apparatus100can be mounted on a head imaging apparatus, a Bucky stand, a top lifting type Bucky table D, and a U-arm Bucky imaging apparatus.

As has been described above, according to each embodiment described above, it is possible to reduce a deterioration in image quality caused by a temporal change in offset component caused during imaging in a radiation imaging apparatus.

Other Embodiments

This application claims the benefit of Japanese Patent Application No. 2019-045136, filed on 12 Mar. 2019, which is hereby incorporated by reference herein in its entirety.