JP2015204921A - Image processor, image processing method, radiographic apparatus, radiographic system and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processor for reducing artifacts by correction processing using an image after offset correction and gain correction.SOLUTION: The image processor for processing an image outputted from a radiation detection part including a plurality of pixels of a radiation detector used in a radiographic apparatus, or the like includes a determination part for determining whether pixels are saturated due to a distribution of pixel values of an image, an acquisition part for acquiring a saturation pixel value of a pixel in the case that the pixel is saturated, and a correction part for correcting the image by using the saturation pixel value.

Description

  The present invention relates to an image processing apparatus, an image processing method, a radiation imaging apparatus, a radiation imaging system, and a program.

  As a radiation detector used in a radiography apparatus or the like, a configuration in which a radiation distribution is acquired by a large image sensor using a solid-state image sensor called a flat panel detector (hereinafter abbreviated as FPD) is becoming common. .

  In the FPD, a plurality of pixels are arranged on a plane, and the energy distribution on the plane can be directly spatially sampled and converted into a signal. However, a plurality of pixels for spatial sampling are basically independent elements and have different characteristics. For this reason, in order to acquire an image having uniform characteristics, it is necessary to correct variations in characteristics among pixels.

  Patent Document 1 discloses a technique for detecting artifacts by detecting the presence or absence of a saturated state in a radiographic image before correction, and by making no correction for pixels in a saturated state, thereby making the saturated pixel value constant and reducing artifacts. Have been described.

JP 2003-000576 A

  However, many existing FPDs are mounted so as to perform offset correction by an internal arithmetic unit, and it has not been considered to newly add processing for reducing artifacts after offset correction and gain correction.

  An object of the present invention is to provide an image processing technique capable of reducing artifacts by correction processing using an image after offset correction and gain correction.

  An image processing apparatus according to an aspect of the present invention is an image processing apparatus that processes an image output from a radiation detection unit including a plurality of pixels, and determines whether a pixel is saturated by a distribution of pixel values of the image A determination unit that performs correction, an acquisition unit that acquires a saturated pixel value of the pixel when the pixel is saturated, and a correction unit that corrects the image using the saturated pixel value. .

  According to the present invention, artifacts can be reduced by correction processing using an image after offset correction and gain correction.

The figure which shows the structure of an image processing apparatus. The figure explaining the flow of a correction process. The figure explaining the flow of a saturation artifact correction process. The figure explaining offset correction. The figure explaining a saturation artifact. The figure explaining the process of a profile acquisition part. The figure which shows the example of a profile. The figure which shows the comparative example of the pixel from which a saturation characteristic differs. The figure which illustrates the artifact between the areas where saturation characteristics differ. The figure which shows the comparative example of the area | region A and the area | region B from which a saturation characteristic differs. The figure explaining the process of a profile acquisition part. The figure which illustrates the image and histogram used as the object of a correction process.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in this embodiment are merely examples, and the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments. Absent.

  FIG. 1A is a diagram illustrating a schematic configuration of the image processing apparatus according to the present embodiment, and FIG. 1B is a diagram illustrating a functional configuration based on the processing of the CPU 302 of the image processing apparatus 300.

  The FPD 200 detects the radiation irradiated from the radiation generation unit 100 and transmitted through the subject S, converts the radiation into a radiation image (subject image), and outputs the radiation image. The image processing apparatus 300 acquires the radiation image transmitted from the FPD 200 via the communication unit 301. The CPU 302 functions as a control unit (control device) that controls the operation of each unit of the image processing apparatus 300. The CPU 302 stores the acquired radiation image in the memory 303 or the storage medium 304 inside the image processing apparatus 300. The CPU 302 executes an image processing program stored in the memory 303 or the storage medium 304 to perform image processing on the radiation image. The CPU 302 performs display control so that the image after image processing is displayed using the display device 400.

  FIG. 1B is a diagram illustrating a functional configuration of the image processing apparatus 300. It is a figure which shows the function structure of CPU302, The function of each part is implement | achieved when CPU302 runs the control program memorize | stored in the memory 303 or the storage medium 304. FIG. The control unit 110 has a configuration corresponding to the function provided by the CPU 302. An image processing apparatus 300 that processes an image output from a radiation detection unit including a plurality of pixels includes a determination unit 101, an acquisition unit 102, and a correction unit 103. Here, the determination unit 101 determines whether a pixel is saturated based on a distribution of pixel values of an image output from the FPD 200 (radiation detection unit). The acquisition unit 102 acquires the saturated pixel value of the pixel when the pixel is saturated. Then, the correction unit 103 (artifact correction unit) corrects the image acquired from the FPD 200 using the saturated pixel value.

  The determination unit 101 includes an area deriving unit 104, a profile acquisition unit 105, and a profile determination unit 106 as specific configurations. The region deriving unit 104 derives from the image a direct line region in which radiation directly reaches the FPD 200 (radiation detection unit) from the radiation generation unit 100. The profile acquisition unit 105 acquires a profile indicating the distribution of pixel values for the direct line region. The profile determination unit 106 analyzes the acquired profile and determines whether the profile is a saturated region profile in which the pixel value is saturated.

  Based on the determination result, the acquisition unit 102 acquires, from the saturation region profile, a pixel value that is a minimum value (minimum) in the saturation region as a saturation pixel value. The correction unit 103 corrects the image using the saturated pixel value acquired by the acquisition unit 102.

  The image processing apparatus 300 includes an offset correction unit 107 and a gain correction unit 108 as a configuration for performing preprocessing of correction by the correction unit 103. The offset correction unit 107 performs offset correction for removing the dark current component from the radiation image. The gain correction unit 108 corrects the pixel value output for the radiation dose incident on the pixel for the offset-corrected image. The correction unit 103 corrects the image using the saturated pixel value with respect to the image corrected by the gain correction unit.

(Contents of correction processing)
Prior to specific description of the correction processing according to the embodiment, correction that is a premise will be described. In the FPD 200, a plurality of pixels are arranged on a plane, and the plurality of pixels are basically independent elements, each having different characteristics. Therefore, the correction unit 103 corrects the variation in characteristics for each pixel in order to acquire an image having uniform characteristics.

  The FPD 200 is an energy conversion element that can directly sample an energy distribution on a plane and spatially sample it. Variations in offset (dark current) and gain (conversion efficiency) are the main characteristic variations of pixel elements of the energy conversion element. When the FPD 200 is used, the offset correction unit 107 performs offset correction, and the gain correction unit 108 gains gain. Make corrections. An outline of offset correction and gain correction will be described below.

  The offset correction unit 107 performs offset correction on the radiation image in order to remove the dark current component of the solid-state imaging device that is generated regardless of radiation irradiation. The dark current component used in the offset correction is acquired by, for example, accumulating charges without irradiating radiation for the same time as the charge accumulation time when the image information of the subject S is acquired immediately after the acquisition of the radiation image. be able to. The image acquired at this time is hereinafter referred to as an offset image. Offset correction is performed by subtracting the offset image from the radiation image.

  Next, the gain correction unit 108 performs gain correction on the image after offset correction in order to correct variations in conversion efficiency between pixels. An image used for gain correction can be obtained as an image in the same manner as a radiographic image by irradiating radiation without placing the subject S. The image acquired at this time is hereinafter referred to as a gain image.

  The gain correction is performed by dividing the radiation image after the offset correction by the gain image and then multiplying by an appropriate coefficient such as an average value of the entire gain image. The correction unit 103 performs processing for converting the image after offset correction and gain correction into a diagnostic image such as gradation processing, dynamic range processing, and spatial frequency processing. Then, the CPU 302 of the image processing apparatus 300 displays the image after image processing so as to be presented to the user via the display apparatus 400 by transferring the image to an external device represented by a filing apparatus or performing a hard copy. Take control.

  One of the features of the radiation imaging apparatus is that the dynamic range of the radiation dose reaching the FPD 200 is very wide. This is because when the amount of radiation to be irradiated is adjusted so that the inside of the subject S, which is the region of interest, is appropriately depicted, the amount of reaching radiation (hereinafter referred to as a direct line) that directly reaches the FPD 200 without passing through the subject. This is because it becomes larger.

  If the radiation dose reaching the FPD 200 of the solid-state imaging device is too large, two phenomena as shown in FIG. The first phenomenon is a phenomenon in which the output value of the A / D converter of the FPD 200 is saturated and the pixel value of the radiation image is fixed to a constant value. Hereinafter, the radiation dose causing this phenomenon is referred to as a saturated dose. The A / D output pixel value at the saturated dose is called a saturated pixel value, and a state exceeding the saturated dose is called a saturated state.

  The second phenomenon is that an afterimage component proportional to the radiation dose is added to the offset image as a component other than the dark current component, and the pixel value (offset pixel value) of the offset image monotonously increases as the radiation dose reaching the FPD 200 increases. It is a phenomenon that increases.

  This afterimage component is a component that can occur when a capacitor in the pixel of the FPD 200 cannot be released when acquiring a radiation image. This phenomenon occurs because the afterimage component is acquired in a form included in a part of the offset image acquired thereafter.

  When offset correction and gain correction are performed in a saturated state, the following two artifacts may occur. As shown in FIG. 4 (b), the first artifact is an artifact that appears as if the amount of radiation reached is low at the pixel where the radiation exceeding the saturation dose has arrived, the pixel value after offset correction decreases. It is. This is because the offset pixel value monotonically increases while the saturated pixel value for generating the radiographic image is constant, and therefore, in the region corresponding to the radiation dose exceeding the saturated dose, the radiographic image after offset correction This is because the pixel value (radiation pixel value) decreases.

  As an example, FIG. 5 shows a radiographic image after offset correction obtained by radiography without placing the subject S. In this image, a region having a high pixel value is expressed in white. Usually, since the radiation dose reaching the radiation irradiation center is the largest, when the saturation dose is not exceeded, the pixel value decreases as the distance from the radiation irradiation center increases as shown in FIG.

  However, when the saturation dose is exceeded at the pixel near the radiation irradiation center, as described above, the pixel value after the offset correction decreases, resulting in a donut-shaped artifact as shown in FIG. That is, the pixel value at the center of radiation irradiation becomes low (region expressed in black). A region with a high pixel value (a region expressed in white) is distributed around a region with a low pixel value, and the image value becomes as the distance increases from the periphery of the region with a high pixel value toward the edge of the image. It becomes lower (area that is expressed in black).

  FIG. 5C shows the profile of the pixel value at the location indicated by the dotted line in FIG. The pixel value of the radiation image is fixed at a constant value near the radiation irradiation center exceeding the saturation dose, and the offset correction is performed using the offset image including the afterimage component. As a result, the pixel value of the radiation image after the offset correction has a radiation dose. It can be seen that it decreases as it increases.

  The second artifact is that the saturation dose varies due to the difference in dark current characteristics and conversion efficiency (hereinafter collectively referred to as saturation characteristics) of each pixel due to individual differences such as A / D converters and amplifiers. This is an artifact caused by This phenomenon will be described with reference to FIG. FIG. 8A is a diagram showing the relationship between the amount of radiation reached (input radiation dose) of two pixels A and B and the pixel value. Comparing the pixel value (dark current component) at zero input radiation dose shows that the dark current component of pixel B is smaller than the dark current component of pixel A. Further, since the saturated pixel value is constant at the maximum value of A / D conversion, it is indicated that the saturated dose of the pixel A is smaller than that of the pixel B. Comparing pixel A and pixel B shows that pixel B has higher conversion efficiency than pixel A.

FIG. 8B is a diagram showing pixel values after offset correction of the pixel A and the pixel B, respectively. As described with reference to FIG. 4B, the dark current component that is not proportional to the radiation dose is removed by the offset correction. Therefore, the pixel A and the pixel B each have a intercept near the pixel value 0, but the radiation dose exceeding the respective saturation doses. When the value reaches, a phenomenon occurs in which the pixel value gradually decreases. Further, because of the difference in dark current components, the saturated pixel value AMMax _offset after offset correction of the pixel A is smaller than the saturated pixel value BMax _offset after offset correction of the pixel B.

FIG. 8C is a diagram illustrating pixel values after performing gain correction in order to correct variations in conversion efficiency between the two pixels A and B. At this time, the saturated pixel value after gain correction of each of the pixel A and the pixel B becomes a value different from AMMax _gain and BMax _gain due to a difference in saturated dose, and this is observed as an artifact.

  Since A / D converters, amplifiers, and the like, which can cause different saturation pixel values, are generally mounted in the FPD, as shown in FIG. A step occurs at the boundary between the A / D converter and the amplifier. 9A, it is assumed that pixels having the characteristics of the pixel A described in FIG. 8 are arranged in the A area, and pixels showing the characteristics of the pixel B are arranged in the B area. The distribution of pixel values is discontinuous at the portion indicated as a step.

FIG. 9B shows a profile of the pixel value at the location indicated by the dotted line in FIG. It becomes a value different from AMax _gain and BMax _gain due to the difference in saturation dose, and the distribution tendency of pixel values is different in the A region and the B region. At the boundary between the A area and the B area (CC in FIG. 9B), the pixel values are discontinuous. Differences in the distribution tendency of pixel values are observed as artifacts. CC in FIG. 9B corresponds to the position of the step in FIG.

(First embodiment)
Specific contents of the image processing for reducing the artifact will be described. In the present embodiment, image processing for reducing artifacts in which the pixel value in the saturated region described with reference to FIGS. 4 and 5 is reduced is taken as an example. That is, an example of image processing that reduces artifacts that appear as if the amount of radiation that has reached the radiation dose that exceeds the saturation dose decreases as the pixel value after the offset correction is reduced, will be described. Although FIG. 5B illustrates the case where the subject S is not present, the configuration of the embodiment is not limited to this example, and the case where the subject S exists can also be handled.

  FIG. 2 is a flowchart for explaining the flow of correction processing according to this embodiment. First, in step S110, the communication unit 301 of the image processing apparatus 300 acquires an image from the FPD 200 (radiation detection unit). The image includes a radiation image and dark current image for performing offset correction, which will be described later, and a gain image for performing gain correction.

  Under the control of the control unit 110, the radiation generation unit 100 emits radiation while the subject S is arranged. The FPD 200 detects the radiation transmitted through the subject S, converts it to a radiation image (subject image), and outputs it.

  After acquiring the radiation image of the subject S, the communication unit 301 acquires a dark current image from the FPD 200 in a state where no radiation is irradiated. Note that the timing of acquiring the dark current image is not limited to after the subject S is captured, and the dark current image may be acquired before the subject S is captured. Further, under the control of the control unit 110, the radiation generation unit 100 emits radiation in a state where the subject S is not arranged, and the FPD 200 detects radiation that does not pass through the subject S, converts it to a gain image, and outputs it. To do. The image processing apparatus 300 acquires the gain image transmitted from the FPD 200 via the communication unit 301. Note that the timing of acquiring the gain image is not limited to after the subject S is captured, and the gain image may be acquired before the subject S is captured.

  In step S120, the offset correction unit 107 performs offset correction to remove the dark current component from the radiation image of the subject S. The offset correction unit 107 performs offset correction by subtracting the dark current image from the radiation image of the subject S acquired in step S110.

  In step S <b> 130, the gain correction unit 108 performs gain correction in order to correct variation in conversion efficiency between pixels of the image after offset correction (offset correction image). The gain correction unit 108 divides the pixel value of the offset corrected image (corrected image) by the pixel value of the gain image, and then multiplies the coefficient (correction coefficient) such as the average value of the pixel values of the entire gain image. Perform gain correction. Note that the timing of acquiring the gain image is not limited to the case of acquiring in the previous step S110. For example, the gain is applied without irradiating the subject S at the timing before the gain correction in step S130. An image may be acquired.

  Next, in step S140, the correction unit 103 performs a saturation artifact correction process on the gain-corrected image.

  FIG. 3A is a diagram illustrating a specific processing flow of the saturation artifact correction processing. In step S141, the area deriving unit 104 of the image processing apparatus 300 derives a direct line area. For example, the region deriving unit 104 performs logarithmic conversion on an image after gain correction (gain-corrected image), and creates a histogram of pixel values of the image after logarithmic conversion. FIG. 12 is a diagram exemplifying an image 1200 to be corrected and a created histogram 1210. Reference numeral 1202 denotes a subject portion and a histogram portion corresponding to the subject portion, and 1203 denotes a direct line region portion outside the subject and a direct line region histogram portion. The region deriving unit 104 can extract a region showing a narrow frequency distribution on the highest dose side (right side in FIG. 12) from the frequency distribution of the histogram, and set the extracted region as a direct line region.

  In step S142, the profile acquisition unit 105 performs profile acquisition processing on the derived direct line area. In the profile acquisition process, the profile acquisition unit 105 searches for the maximum value pixel and the minimum value pixel of the direct line region. The profile acquisition unit 105 derives the end point of the direct line area on the extension line of the line segment from the coordinate of the minimum value pixel to the coordinate of the maximum value pixel. Then, the profile acquisition unit 105 generates a profile indicating the distribution of pixel values between the maximum value pixel and the minimum value pixel and a profile indicating the distribution of pixel values between the maximum value pixel and the end point of the direct line area. To do.

  Specifically, the profile acquisition unit 105 first searches the maximum value pixel and the minimum value pixel from the direct line area as shown in FIG. Further, the profile acquisition unit 105 derives the end point of the direct line region on the extension line of the line segment from the coordinate of the minimum value pixel to the coordinate of the maximum value pixel. Further, the profile acquisition unit 105 generates a profile indicating the distribution of pixel values between the searched maximum value pixel and the minimum value pixel, and a profile indicating the distribution of pixel values between the maximum value pixel and the end point of the direct line area. Generate. The profile is plotted with the pixel value against the distance from the maximum value pixel in the direct line region. FIG. 7 is a diagram illustrating an example of a profile. When acquiring the profile, a low-pass filter or various noise filters may be used to reduce noise in the direct line region.

  In step S143, the profile determination unit 106 performs a saturation region profile determination process on the acquired profile. In the saturation region profile determination process, the profile determination unit 106 determines whether the region is a saturated region or a non-saturated region, and specifies a saturated region profile. As shown in FIG. 4B, in the saturation region, the pixel value decreases as the radiation dose increases, and the radiation dose decreases as the distance from the radiation irradiation center increases. From these viewpoints, if the pixel value distribution is a saturated region profile as shown in FIG. Moreover, if it is a non-saturation area | region profile, an upward convex shape is shown.

The profile determination unit 106 can specify the saturation region profile based on the approximate expression using the fact that the shape of the saturation region profile is different from the shape of the non-saturation profile. As a specific example, the profile determination unit 106 fits a profile to a quadratic function (y = a * x ² + b * x + c) indicating a two-dimensional model. If the coefficient a is positive, the profile of the quadratic function has a downwardly convex shape, and the profile determination unit 106 determines that the profile is a saturated region profile. On the other hand, if the coefficient a is negative, the profile of the quadratic function has an upwardly convex shape, and the profile determination unit 106 determines that the profile is an unsaturated profile. At this time, the fitting may use the least square method, M estimation, or the like.

In step S144, the acquisition unit 102 performs an acquisition process for acquiring a saturated pixel value for the profile of the saturated region. The saturation pixel value acquisition process can be obtained by deriving the pixel value of the radiation irradiation center using the profile of the saturation region. The pixel value at the center of radiation irradiation is a minimum value in the saturation region profile. As a specific example, the acquisition unit 102 performs fitting on a quadratic function in the same manner as the saturation region profile acquisition process, and acquires the minimum value V _min of the quadratic function by the following Expression 1. At this time, the fitting may use the least square method, M estimation, or the like. In Equation 1 below, a, b, and c indicate coefficients when fitting the saturation region profile to a quadratic function (fitting).

V _min = c− (b ² / 4a) Equation 1
V _min obtained by Equation 1 is the minimum value of the pixel value in the saturation region profile.

  As a method for obtaining the minimum value of the pixel value, Equation 1 has exemplified the method using the saturation region profile, but is not limited to this example. For example, it is possible to take a histogram of pixel values in the saturation region profile and use a certain percentage (%) of pixel values from the bottom as the lowest pixel value among all the pixels.

  Next, in step S145, the correction unit 103 performs correction processing using the acquired saturated pixel value. The correction unit 103 clips a pixel value that is equal to or higher than the saturated pixel value of the image after gain correction as a correction process of the saturated pixel value. With the above processing, the saturation artifact correction processing in step S140 in FIG. 2 is completed.

  Next, in step S150 of FIG. 2, the correction unit 103 of the image processing apparatus 300 performs gradation processing and frequency processing to obtain an image suitable for diagnosis using the image on which saturation artifact correction has been performed. In step S160, the CPU 302 of the image processing apparatus 300 performs display control so that the image after gradation processing / frequency processing is displayed on the display device 400 such as a monitor or a film.

(Second Embodiment)
In the present embodiment, specific contents of image processing for reducing artifacts due to different saturation characteristics of pixels in addition to the artifacts in which the pixel value of the saturation region is reduced as described in the first embodiment will be described.

  As a specific example, FIG. 9A shows an image after gain correction in a case where an artifact due to a decrease in the pixel value in the saturation region and an artifact due to a difference in pixel saturation characteristics occur simultaneously. In this image, a region having a high pixel value is expressed in white. It can be seen from the figure that the pixel value is reduced near the radiation irradiation center and that a step is generated at the boundary between the A region and the B region having different saturation characteristics.

FIG. 9B shows a profile of the pixel value at the location indicated by the dotted line in FIG. The saturated pixel value AMax _gain after gain correction in the A region is lower than the saturated pixel value BMax _gain after gain correction in the B region, and further, the pixel value greatly changes at the boundary between the A region and the B region. I understand that.

  As a main factor that the saturated pixel value is different, there is a variation in hardware performance of the FPD 200 such as an A / D converter or an amplifier that amplifies a signal when reading the charge accumulated in the FPD 200. In this case, the same saturated pixel value is obtained between pixels using the same amplifier or A / D converter, but different saturated pixel values are used between pixels using different A / D converters or amplifiers. Can occur. As a conceptual diagram, as shown in FIG. 9A, different A / D converters and amplifiers are used in the A area and the B area, there are variations in hardware performance, and the A area and When the B region is saturated, a step is generated in the saturated region at the boundary between the A region and the B region. In the example of FIG. 9, the case where there is no subject S has been described. However, the configuration of the embodiment is not limited to this example, and the case where the subject S exists can also be handled.

  The image processing in this embodiment is different from the first embodiment only in the saturation artifact correction processing in step S140 of FIG. 2, and the other processing is the same processing flow as in the first embodiment. In the present embodiment, only the content of the saturation artifact correction process in step S140 will be described, and the description of other processes will be omitted to avoid duplication.

  As a functional configuration of the image processing apparatus 300, the determination unit 101 of the present embodiment includes an area deriving unit 104, a profile acquisition unit 105, a profile determination unit 106, and a dividing unit 109 as specific configurations. The functions of the area deriving unit 104, the profile acquiring unit 105, and the profile determining unit 106 are the same as those in the first embodiment.

  The dividing unit 109 divides the direct line region obtained by the region deriving unit 104 into regions (small regions) having different pixel value distributions.

  The profile acquisition unit 105 acquires a profile indicating the distribution of pixel values for each region. The profile determination unit 106 analyzes the profile acquired for each region, and determines whether the profile is a saturated region profile in which the pixel value is saturated.

  The acquisition unit 102 acquires a pixel value that is a minimum value from the profile of the saturation region for a plurality of regions, and acquires the minimum value among the acquired minimum values as a saturation pixel value. Then, the correction unit 103 corrects the image using the saturated pixel value.

  FIG. 3B is a diagram illustrating a specific processing flow of the saturation artifact correction processing. In step S141, the area deriving unit 104 of the image processing apparatus 300 derives a direct line area. For example, the area deriving unit 104 acquires a line area directly from an image after gain correction (gain correction image). This process is the same as step S141 in FIG.

  In step S146, the dividing unit 109 performs area division processing. In the area dividing process, the dividing unit 109 directly divides a line area between areas having different saturation characteristics according to differences in known hardware characteristics. An area obtained by dividing the direct line area is referred to as a partial area. FIG. 10 is a diagram illustrating a region A and a region B having different saturation characteristics according to differences in known hardware characteristics. For example, when the saturation characteristics differ from the center of the FPD 200 due to the difference in hardware characteristics as shown in FIG. 10, the dividing section 109 divides the line area directly like the A area and the B area.

  In step S142, the profile acquisition unit 105 performs profile acquisition processing on the direct line area in each divided area. In this profile acquisition process, the profile acquisition unit 105 performs the same process as the process described in the first embodiment on the direct line area in each area. In the profile acquisition process, the profile acquisition unit 105 first searches for the maximum value pixel and the minimum value pixel from the direct line area for each of the A area and the B area as shown in FIG. Further, the profile acquisition unit 105 derives the end point of the direct line region on the extension line of the line segment from the coordinate of the minimum value pixel to the coordinate of the maximum value pixel. Further, the profile acquisition unit 105 generates a profile indicating the distribution of pixel values between the searched maximum value pixel and the minimum value pixel, and a profile indicating the distribution of pixel values between the maximum value pixel and the end point of the direct line area. Generate. The profile is plotted with the pixel value against the distance from the maximum value pixel in the direct line region. The profile is a diagram as illustrated in FIG. 7 described in the first embodiment. When acquiring the profile, a low-pass filter or various noise filters may be used to reduce noise in the direct line region.

  In step S143, the profile determination unit 106 performs a saturation region profile determination process on the derived profile of each region. In this saturation region profile determination processing, the profile determination unit 106 performs processing similar to the processing described in the first embodiment, and determines the saturation region profile for each region profile.

In step S144, the acquisition unit 102 performs an acquisition process for acquiring a saturated pixel value for the saturated region profile of each region. As described in the first embodiment, the acquisition unit 102 fits a profile to a quadratic function (fitting) with respect to the saturation region profile of each region, and obtains the minimum value V _min of the quadratic function by Expression 1. get. The acquisition unit 102 further derives a minimum value from the minimum values of the obtained pixel values, and acquires this as a saturated pixel value used in the saturation artifact correction process.

  When there are a plurality of regions, the saturation region profiles are also different, and the saturation pixel values of the regions are also different. For this reason, when correction processing is performed using the saturated pixel value of each region, a step may be generated between the regions. For this reason, when there are a plurality of regions, the acquiring unit 102 acquires the minimum value as the saturated pixel value used for the saturation artifact correction process among the minimum values of the pixel values acquired from the saturated region profile for all the regions.

  In step S145, the correction unit 103 performs correction processing using the acquired saturated pixel value. The correction unit 103 clips a pixel value equal to or greater than the saturated pixel value for each area image. Through the above processing, the saturation artifact correction processing according to the second embodiment is completed.

  The configuration of each of the above embodiments can be used for, for example, a radiation imaging apparatus or a radiation imaging system that performs an offset correction process or a gain correction process on a radiographic image using an imaging element including a plurality of pixels. is there. For example, a radiation imaging apparatus having a radiation generation unit 100 that emits radiation and an FPD 200 (radiation detection unit) that includes a plurality of pixels that detect radiation includes an image processing unit that processes an image output from the radiation detection unit. Prepare. The image processing unit includes a determination unit 101 that determines whether a pixel is saturated based on a distribution of pixel values of the image, an acquisition unit 102 that acquires a saturated pixel value of the pixel when the pixel is saturated, and a saturation pixel value. And a correction unit 103 that corrects the image using the correction unit 103. Further, the radiation imaging system can be configured by including a radiation imaging apparatus.

  According to each of the embodiments described above, it is possible to reduce artifacts that appear due to different saturation pixel values using the radiation image after offset correction and gain correction.

(Other embodiments)
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

101 determination unit, 102 acquisition unit, 103 correction unit, 300 image processing apparatus

Claims (13)

An image processing apparatus that processes an image output from a radiation detection unit including a plurality of pixels,
A determination unit for determining whether a pixel is saturated based on a distribution of pixel values of the image;
An acquisition unit that acquires a saturated pixel value of the pixel when the pixel is saturated;
A correction unit that corrects the image using the saturated pixel value;
An image processing apparatus comprising: The determination unit
A region deriving unit for deriving from the image a direct line region where radiation directly reaches the radiation detecting unit from a radiation generating unit;
A profile acquisition unit that acquires a profile indicating a distribution of pixel values for the direct line region;
A profile determination unit that analyzes the profile and determines whether the profile is a saturated region profile in which pixel values are saturated,
The acquisition unit
Obtaining a minimum pixel value of the saturated region from the profile of the saturated region as the saturated pixel value;
The correction unit is
The image processing apparatus according to claim 1, wherein the image is corrected using the saturated pixel value. The profile acquisition unit
Search for the maximum value pixel and the minimum value pixel of the direct line region,
Deriving the end point of the direct line region on the extension line of the line from the coordinate of the minimum pixel to the coordinate of the maximum pixel,
Generating a profile indicating a distribution of pixel values between the maximum value pixel and the minimum value pixel, and a profile indicating a distribution of pixel values between the maximum value pixel and an end point of the direct line region. The image processing apparatus according to claim 2. The profile determination unit
Apply the generated profile to a quadratic function to obtain the coefficient of the quadratic function;
The image processing apparatus according to claim 3, wherein whether or not the profile is a saturation region profile is determined from the coefficient. The correction unit is
An offset correction unit that performs offset correction to remove dark current components from the radiation image;
A gain correction unit that corrects a pixel value that is output with respect to the amount of radiation incident on the pixel with respect to the offset-corrected image;
The correction unit is
5. The image processing apparatus according to claim 1, wherein the image is corrected using the saturated pixel value with respect to the image corrected by the gain correction unit. 6. A communication unit for acquiring an image from the radiation detection unit;
The offset correction unit
Offset correction for acquiring a radiation image based on the irradiated radiation and a dark current image in a state where no radiation is irradiated via the communication unit, and removing a dark current component of the dark current image from the radiation image The image processing apparatus according to claim 5, wherein: The gain correction unit
A gain image based on the irradiated radiation is acquired via the communication unit, and the pixel value output for the radiation dose incident on the pixel is corrected using the offset corrected image and the gain image. The image processing apparatus according to claim 6. A division unit that divides the direct line region into regions having different pixel value distributions;
The profile acquisition unit
For each of the regions, obtain a profile indicating the distribution of pixel values,
The profile determination unit
The image processing apparatus according to claim 2, wherein the profile acquired for each of the regions is analyzed to determine whether the profile is a saturated region in which a pixel value is saturated. The acquisition unit
For a plurality of regions, obtain a pixel value that is a minimum value from the profile of the saturation region, obtain a minimum value among the obtained minimum values as the saturated pixel value,
The correction unit is
The image processing apparatus according to claim 8, wherein the image is corrected using the saturated pixel value. An image processing method of an image processing apparatus for processing an image output from a radiation detection unit including a plurality of pixels,
A determination step of determining whether a pixel is saturated by a distribution of pixel values of the image;
An acquisition step in which the acquisition unit of the image processing apparatus acquires the saturated pixel value of the pixel when the pixel is saturated;
A correction step in which a correction unit of the image processing apparatus corrects the image using the saturated pixel value;
An image processing method comprising:   The program for functioning a computer as each part of the image processing apparatus of any one of Claims 1 thru | or 9. A radiation imaging apparatus having a radiation generation unit for irradiating radiation and a radiation detection unit including a plurality of pixels for detecting the radiation,
An image processing unit for processing an image output from the radiation detection unit;
The image processing unit
A determination unit for determining whether a pixel is saturated based on a distribution of pixel values of the image;
An acquisition unit that acquires a saturated pixel value of the pixel when the pixel is saturated;
A correction unit that corrects the image using the saturated pixel value;
A radiation imaging apparatus comprising:   A radiation imaging system comprising the radiation imaging apparatus according to claim 12.