JP2010240259A - Radiation image pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radiation image pickup device capable of removing the remainder of grid stripes which can not be removed until now in an irradiation image from which a grid image is subtracted. <P>SOLUTION: The radiation image pickup device is provided with a grid frequency component removal processing means for removing grid frequency components from an original image detected by a radiation detector, wherein the means consists of: a BPF 21 and a HPF 22 which obtain a bandpass grid image and a high pass grid image by bandpass or high pass extraction from the original image; a subtractor 23 which takes difference between both grid images; an LPF 25 which performs low pass processing to a differential image in a direction parallel with a grid pattern to obtain a differential grid image consisting of grid frequency components which are not completely extracted by the bandpass; and adder 26 which obtains a bandpass and the differential grid image by adding the differential grid image to the bandpass grid image; and a subtractor 24 which subtracts the bandpass and the differential grid image from the original image. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a radiological image diagnostic apparatus using a radiation plane detector.

As a radiation imaging apparatus, an X-ray imaging apparatus will be described as an example.
In an X-ray imaging apparatus, X-rays are emitted from an X-ray tube toward a subject, X-rays transmitted through the subject are detected, and an X-ray image is obtained based on the detected X-rays. Take a picture. When X-rays pass through the subject, scattered rays are generated when the X-rays collide with the subject, and the image is blurred due to the scattered rays.

  Therefore, in order to remove scattered rays scattered by the subject, an X-ray imaging apparatus uses a grid in which substances having a high X-ray absorption rate and substances having a low X-ray absorption rate are alternately arranged in parallel at regular intervals. Moire fringes occur in an image obtained by the X-ray detector due to the difference between the spatial frequency (array interval) and the spatial sampling period (detection pixel interval) of the X-ray detector. For this reason, conventionally, various countermeasures have been taken in order to remove and reduce moire fringes in such a photographed image.

As one of the measures,
(1) It is known to use a grid having the same frequency characteristic as that of an X-ray detector so as not to cause moiré fringes.
(2) Further, the moire fringes may be erased by moving the grid during X-ray irradiation.

(3) In addition, when a grid is used, moire fringes (grid fringes or grids) composed of grid frequency components (see FIG. 5 (e)) appearing in an X-ray irradiation image (= original image; see FIG. 6 for example) In order to remove a high frequency component such as a pattern),
[A] Applying a low pass filter (LPF) in a direction perpendicular to the grid pattern (that is, a direction perpendicular to the grid eyes), or
[B] After extracting high frequency components including grid frequency components by applying a high pass filter (HPF: High Pass Filter) in a direction perpendicular to the grid pattern, the extracted high frequency components are extracted from the original image. Remove the grid frequency component by subtracting,
What is illustrated by the process is mentioned.

Furthermore, [C] band frequency extraction of the grid frequency component from the original image through a band pass filter (BPF: Band Pass Filter) (see FIG. 5A), and a grid image (band pass grid image) is performed. create. An image processing technique for removing a grid frequency component from an original image by subtracting a bandpass grid image from the original image has also been proposed.
More specifically, when the original image is subjected to Fourier transform and the spectrum intensity is obtained, as shown in FIG. 12, in the case of an image having a horizontal binning size of × 1, it is about 1/2 and 1 near the Nyquist frequency Nq. A grid frequency peak appears (see FIG. 13A). Similarly, in the case of an image with a horizontal binning size of × 2, a grid frequency peak appears near 1 of the Nyquist frequency Nq (see FIG. 13B).
With regard to the frequency at which the peak appears, when the grid arrangement interval is lg, the detection pixel interval is lp, and lg <lp, the pitch lm of the moire fringes is
lm = lp × lg / (lp−n × lg)
Here, n = 0, 1, 2,...
If lg is less than 2 times lp, n = 1; if it is less than 3 times, n = 2

Therefore, the frequency fm corresponding to the peak of the grid frequency is
fm = (lp−n × lg) / lp × lg (second harmonic is twice this)
As obtained.
Note that the right side, that is, the second peak shown in FIG. 5A corresponds to the second harmonic of the left grid frequency peak (frequency at which the peak appears = fm).
The above bandpass extraction is a technique for detecting these peaks and extracting the corresponding frequencies from the original image. However, the problem that the moire fringes remain as described below still remains in the frequency region other than the peak due to the dispersion of the grid and the like.

  This can be understood at a glance by referring to FIG. 8 according to the following embodiment and the description thereof, and it is understood that vertical lines remain as a whole in the image of FIG. As shown in FIG. 8, under the prior art, the grid frequency component can be removed only up to this level, and there is no moiré fringe left in the X-ray irradiation image obtained by subtracting the grid image (bandpass grid image). Resulting in.

JP 2002-325755 A

Here, (1) when using a grid having the same frequency characteristics as the X-ray detector, there is no effect unless they are exactly matched, so that the grid position accuracy and manufacturing accuracy are very high. It is required and cannot be obtained inexpensively. Even if such a grid is obtained, if the SID (distance between the X-ray source and the X-ray detector) changes, the coincidence of the frequency characteristics will be lost, and the frequency even if the SID changes slightly. A difference occurs and moire fringes appear.
In addition, (2) when the grid is moved during X-ray irradiation, a separate moving mechanism / device is required, leading to an increase in the size of the entire device and an increase in manufacturing cost.

  Furthermore, (3) when performing image processing to remove high-frequency components including the grid frequency, for example, the stripes reflected in the original image by the grid cannot be extracted with only the [C] bandpass. As a result, there is a problem that there is an unstriped stripe in the original image obtained by subtracting the grid image.

  Thus, in view of the above, the present invention has a spatial frequency characteristic of a grid and a spatial frequency characteristic of an X-ray detector that can be realized by image processing with a very simple configuration and no cost problem. An object of the present invention is to provide an X-ray imaging apparatus improved so as to eliminate the remaining moiré fringes based on the difference between the two.

  As a result of various studies to solve the above problems, the present inventor creates a grid image (high-pass grid image) by high-pass extraction separately from the band-pass extraction grid image (band-pass grid image). It was found that the moire fringe component that could not be extracted in the bandpass extraction grid image can be extracted through the image difference (difference grid image), and this difference grid image is converted into the previous bandpass grid image. By further subtracting the added image (bandpass + difference grid image) from the original image, the moire fringes can be removed without leaving the original image (bandpass + highpass processed image), and the present invention has been completed. .

The radiation imaging apparatus of the present invention capable of achieving the above object is (1) a radiation generator, arranged so that the radiation transmitted through the subject is incident, and a high absorption rate portion and a low absorption rate portion of the radiation are alternately arranged. Are arranged so that the radiation that has passed through the scattered radiation removing grid is incident, and the spatial sampling period in at least one direction in the vertical or horizontal direction perpendicular to the grid pattern is equal. A radiation imaging apparatus comprising: a radiation detector; and a grid frequency component removal processing unit that performs a process of removing a grid frequency component from an original image that is transmitted through the scattered radiation removal grid and detected and collected by the radiation detector. Because
The band for performing a process of obtaining a bandpass grid image by bandpass extraction from the original image by performing a bandpass process on the original image in a direction perpendicular to the grid pattern with respect to the original image. A high pass process is performed on the pass filter and the original image in a direction perpendicular to the grid pattern, and the frequency component of the grid pattern extracted by the band pass process by the band pass filter is passed. A high-pass filter that performs processing for obtaining a high-pass grid image by high-pass extraction from the original image, first difference means for taking a difference between the band-pass grid image and the high-pass grid image, and the difference image Then, low-pass processing is performed in a direction parallel to the grid pattern, and extraction is performed with the band pass. A low-pass filter that performs processing for obtaining a difference grid image composed of grid frequency components that are not fully divided, and an adding unit that adds the difference grid image to the bandpass grid image and creates a bandpass + difference grid image separately And second difference means for subtracting the bandpass + difference grid image from the original image.

The present invention also provides (2) a radiation generator and a scattered radiation that is arranged so that the radiation that has passed through the subject enters, and in which high-absorption rate portions and low-absorption rate portions of radiation are alternately arranged in parallel. A removal grid, and a radiation detector arranged so that the radiation having passed through the scattered radiation removal grid is incident, and having a spatial sampling period equal to at least one direction in the vertical or horizontal direction that vertically traverses the grid pattern. In a radiation imaging apparatus, a method of removing a grid frequency component from an original image transmitted through the scattered radiation removal grid and detected and collected by the radiation detector,
A process of obtaining a bandpass grid image by bandpass extraction from the original image by performing a bandpass process using a bandpass filter in a direction perpendicular to the grid pattern on the original image; High-pass processing by a high-pass filter that can also pass the frequency component of the grid pattern extracted by the band-pass processing by the band-pass filter in a direction perpendicular to the grid pattern To obtain a high-pass grid image by high-pass extraction from the original image, take a difference between the band-pass grid image and the high-pass grid image, and reduce the difference image in a direction parallel to the grid pattern. Perform low-pass processing with a band-pass filter and extract with the bandpass. Processing for obtaining a difference grid image composed of non-grid frequency components, processing for separately creating a band pass + difference grid image obtained by adding the difference grid image to the band pass grid image, and the band pass + difference grid And subtracting the image from the original image to obtain a band pass + high pass processed image.

According to the present invention, a grid image (high-pass grid image) is created by high-pass extraction separately from a band-pass extraction grid image (band-pass grid image), and a band-pass extraction grid is obtained by taking the difference between these two grid images. Moire fringe components that could not be extracted from the image can be extracted (difference grid image).
In addition, the extracted moire fringe image component (difference grid image) that has not been completely extracted by bandpass is added to the bandpass extraction grid image (bandpass grid image) to generate a so-called composite grid image (bandpass). + Difference grid image), and by subtracting this band pass + difference grid image from the original image, it is possible to remove moire fringes (band pass + high pass processed image) without leaving the original image. Become.
That is, according to the present invention, it is possible to remove a moire fringe residue that has not been removed in the original image obtained by subtracting the grid image and provide a clear processed image.

  Further, according to the present invention, it is possible to remove the grid frequency component without leaving it while maintaining the high frequency component from the original image.

  Further, according to the present invention, it is possible to remove the grid frequency component without leaving the image while maintaining the resolution of the original image.

  As described above, according to the present invention, the spatial frequency characteristics of the grid and the spatial frequency characteristics of the X-ray detector can be realized by image processing with a very simple configuration and no problem in cost. It is possible to provide an X-ray imaging apparatus improved so as to eliminate the remaining moiré fringes based on the difference between the X-ray imaging apparatus and the moiré pattern.

In the present specification, “vertical low-pass (or grid direction low-pass) processing” refers to performing low-pass processing using a low-pass filter (low-pass filter) in a direction parallel to the grid pattern.
With regard to binning or binning size in this specification, binning refers to handling several pixels together. For example, corresponding to binning aspect 2 × 2 in total four pixels P _A to be handled as combined into a single pixel P _B as shown in FIG. 14 (b) 2 × 2 as shown in FIG. 14 (a) To do. By combining the pixels into one pixel, for example, an X-ray image composed of 1024 × 1024 pixels in the vertical and horizontal directions as shown in FIG. 14A is composed of 512 × 512 pixels in the vertical and horizontal directions as shown in FIG. 14B. X-ray image (data size can be saved to 1/4).
By the way, when the binning size is x1, this corresponds to 1x1 binning, but in this image, it means that individual pixels are used as they are. On the other hand, if the binning size is x2, this is equivalent to 2x2 binning, which means that the area of four adjacent pixels in total in the vertical and horizontal directions are combined into one larger pixel. To do. In this case, the sensitivity to light is quadrupled (for four pixels), but the image resolution is halved. FIG. 14 also shows this effect.
The Nyquist frequency Nq in this specification, when sampling certain signals, refers to half of the frequency of f _s of the sampling frequency. Frequency components exceeding the Nyquist frequency Nq cause a phenomenon of aliasing (also referred to as aliasing) when sampled, and are not faithfully reproduced as the original signal during reproduction.

It is a block diagram of the radiation imaging device concerning this embodiment. It is the block diagram which showed the specific structure of the image process part. It is a system configuration diagram showing an outline of each processing process performed in image processing in the present embodiment and their connection relationship. It is a flowchart which shows the flow of the image processing of the radiation imaging device which concerns on this embodiment. It is a schematic diagram which shows the frequency characteristic obtained in each process performed by the image processing in this embodiment. It is a figure which shows an example of an original image. It is a figure which shows an example of a band pass grid image. It is a figure which shows an example of a band pass process image. It is a figure which shows an example of a high pass grid image. It is a figure which shows an example of a difference grid image. It is a figure which shows an example of a band pass + difference grid image. It is a figure which shows an example of a band pass + high pass process image. It is a schematic diagram of an image for explanation of binning size, band pass extraction, and high pass extraction. It is a schematic diagram of the image used for description of a binning process.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
In the following description, an X-ray imaging apparatus will be described as an example of the radiation imaging apparatus.

[System outline configuration]
As shown in FIG. 1, the X-ray imaging apparatus 100 passes through the subject M, the top plate 1 on which the subject M is placed, the X-ray tube 2 that irradiates the subject M with X-rays, and the subject M. A flat panel X-ray detector (hereinafter abbreviated as “FPD”) 3 for detecting X-rays is provided. In addition to the FPD, the X-ray detector may be an image intensifier or an X-ray film. The X-ray tube 2 corresponds to the radiation irradiation means in this invention.

  In addition, the X-ray imaging apparatus also includes the top panel control unit 4 that controls the elevation and horizontal movement of the top panel 1, the FPD control unit 5 that controls the scanning of the FPD 3, and the tube voltage and tube current of the X-ray tube 2. An X-ray tube control unit 7 having a high voltage generation unit 6 to be generated, an A / D converter 8 that digitizes and extracts an X-ray detection signal that is a charge signal from the FPD 3, and an A / D converter 8 An image processing unit 9 that performs various processes based on the X-ray detection signal, a controller 10 that controls each of these components, a memory unit 11 that stores processed images, and an input that is input by an operator And a monitor 13 for displaying the processed image and the like.

  The top board control unit 4 horizontally moves the top board 1 to accommodate the subject M up to the imaging position, moves the top and bottom, rotates and horizontally moves the subject M to a desired position, or horizontally moves the subject M. Then, the image is picked up, or the image is moved horizontally after the image pickup is finished, and the control is performed to retract from the image pickup position.

  The FPD control unit 5 performs control related to scanning by moving the FPD 3 horizontally or rotating around the body axis of the subject M.

The high voltage generation unit 6 generates a tube voltage and a tube current for irradiating X-rays and applies them to the X-ray tube 2.
The X-ray tube control unit 7 controls the scanning by horizontally moving the X-ray tube 2 or rotating around the body axis of the subject M, and a collimator (not shown) on the X-ray tube 2 side. Control the setting of the illumination field.
When scanning the X-ray tube 2 or the FPD 3, the X-ray tube 2 and the FPD 3 move while facing each other so that the FPD 3 can detect the X-rays emitted from the X-ray tube 2.

  The A / D converter 8 converts the charge signal output from the FPD 3 from analog to digital, and outputs a digitized X-ray detection signal. The controller 10 is configured by a central processing unit (CPU) and the like, and the memory unit 11 is configured by a storage medium represented by ROM (Read-only Memory), RAM (Random-Access Memory), and the like. Yes.

  The input unit 12 includes a pointing device represented by a mouse, a keyboard, a joystick, a trackball, a touch panel, and the like. In the X-ray imaging apparatus, the FPD 3 detects X-rays transmitted through the subject M, and the image processing unit 9 performs image processing based on the detected X-rays, thereby imaging the subject M.

In order to remove scattered radiation, an X-ray grid 14 is provided on the incident surface side of the FPD 3. The X-ray grid 14 is configured by, for example, alternately arranging lead and aluminum. When scattered rays are incident on the X-ray grid 14, they travel obliquely to lead and are absorbed and removed by lead.
On the other hand, when X-rays other than scattered rays enter the X-ray grid 14, the X-rays travel almost parallel to aluminum or lead, so that they pass through the aluminum and enter the FPD 3 to be detected. The X-ray grid 14 corresponds to the scattered radiation removal grid in this invention.

[Configuration of image processing unit]
As shown in FIG. 2, the image processing unit 9 corresponding to the grid frequency component removal processing means in the present invention includes a band-pass filter 21 (a band-pass filter, expressed as “BPF” in FIG. 2), and a high-pass filter. A filter 22 (high-pass filter; expressed as “HPF” in FIG. 2), subtractors 23 and 24 corresponding to difference means, an adder 26 corresponding to addition means, and a low frequency band for performing vertical low-pass processing described later And a pass-type filter (low-pass filter) 25. The connection relationship of each component is as shown in FIG. In the following description, the configuration related to the binning process and the process of the binning process are omitted.
The process of image processing will be described later again with reference to FIGS. 3, 4, and 5. Each frequency characteristic diagram of (a) to (e) shown in FIG. 5 indicates the level [dB] of the frequency component extracted by the following filter processing or the like.

  The low-pass filter 25 for performing the above-described vertical low-pass processing is required when low-pass processing (vertical low-pass processing) by the low-pass filter 25 is performed in a direction parallel to the grid pattern.

  The band-pass filter 21 performs the band-pass process (BPF process) by the band-pass filter 21 in the direction perpendicular to the grid pattern on the original image detected and collected by the FPD 3 as before. The grid frequency component is bandpass extracted to create a grid image (bandpass grid image) (see FIG. 5A and FIG. 13 for an image of bandpass extraction).

The high-pass filter 22 includes a grid frequency from the original image by performing high-pass processing (HPF processing) by the high-pass filter 24 on the original image detected and collected by the FPD 3 in a direction perpendicular to the grid pattern. A high-frequency component is high-pass extracted to create a grid image (high-pass grid image) (see FIG. 5B and FIG. 13 for an image of high-pass extraction). The cutoff frequency of the high pass filter 22 is a frequency corresponding to the peak of the grid frequency extracted by the band pass processing by the band pass filter 21.
fm = (lp−n × lg) / lp × lg
It is said. Here, n = 0, 1, 2,..., Lg: grid array interval, lp: detection pixel interval, lg <lp, where lg is less than 2 times lp, n = 1, 3 times or less In this case, n = 2.

  The subtractor 23, the adder 26, and the subtractor 24 each take a difference between the band pass grid image and the high pass grid image (reference: FIG. 5C), and an image obtained by adding a vertical low pass process to this difference (difference) Grid image (reference: FIG. 5 (d)) added to bandpass grid image (bandpass + difference grid image; reference: FIG. 5 (e)) and bandpass + difference grid from original image An image obtained by subtracting the image (band pass + high pass processed image) is obtained.

[Image processing process]
Next, a series of image processing by the image processing unit 9 will be described with reference to a block diagram of the image processing unit in FIG. 2, a system configuration diagram in FIG. 3, a flowchart in FIG. 4, and an explanatory diagram in FIG.

First, an overview of image processing performed in the present invention will be described.
In the present invention, a grid image (bandpass grid image) for bandpass extraction (see FIG. 5A) and a grid image (highpass grid image) for highpass extraction (see FIG. 5B) from the original image are used. Create a grid image.
Here, as shown in FIG. 5B, moire fringes composed of grid frequency components that cannot be completely extracted by the band pass can be extracted by the high pass, so the difference between the two grid images (reference: FIG. 5 ( c)) to extract the fringes.

By the way, as can be understood from FIG. 5C, the obtained difference image is an image in which only the high-frequency component of the original image composed of the component resulting from the variation of the grid and the other components remains. In the present invention, low-pass processing (vertical low-pass processing) by the low-pass filter 25 is added to the difference image in a direction parallel to the grid pattern (reference: FIG. 5D). This is due to the fact that only the fringes composed of the components resulting from the grid variation, that is, the grid frequency components that cannot be extracted by the band pass, have the vertical directionality. Through this vertical low-pass processing, only the fringes composed of frequency components resulting from the variation of the grid (not completely extracted by the band pass) remain in the difference image (reference: FIG. 5D). In the present invention, as described above, fringes composed of grid frequency components that are not completely extracted by the band pass are extracted (difference grid image).
In the present invention, the obtained difference grid image (reference: FIG. 5 (d)) is added to the bandpass grid image of the bandpass extraction (see FIG. 5 (a)) to obtain a so-called composite grid image (bandpass + (Difference grid image) is created (reference: FIG. 5E). This is data indicating a grid frequency component including a component resulting from the grid variation.

Finally, by subtracting this bandpass + difference grid image (reference: FIG. 5 (e)) from the original image, it is possible to remove moire fringes (bandpass + high-pass processed image) without leaving an original image. It becomes possible.
As described above, according to the present invention, it is possible to perform a moire fringe removal process that leaves no moire fringes composed of grid frequency components and leaves other frequency components.
Hereinafter, the detailed flow of image processing according to the present embodiment will be described step by step.

(Premise stage) Collection of original image X-ray is irradiated from the X-ray tube 2 toward the subject M, and the X-ray transmitted through the subject M is detected by the flat panel X-ray detector (FPD) 3. Scattered rays are removed by passing through the X-ray grid 14 before entering the FPD 3. As a result, an original image in which a grid pattern (moire fringes) by the X-ray grid 14 is reflected is collected.

(Step S1) Bandpass filter processing [Reference: FIG. 5 (a)]
As shown in FIG. 5A, the bandpass process (BPF process) by the bandpass filter 21 is performed on the original image detected and collected by the FPD 3 in a direction perpendicularly crossing the grid pattern. The grid frequency component is bandpass extracted to create a grid image (bandpass grid image). Here, the pass band of the band-pass filter 21 is set around the frequency fm corresponding to the band where the peak of the grid frequency appears and the band where the second harmonic appears.
In addition, regarding each process described below with reference to the explanatory diagram of FIG. 5, the extraction in steps S1 and S2 shown in FIGS. 5A and 5B is performed for each pixel line in the direction perpendicular to the grid and continues. In addition to the vertical low-pass processing shown in FIG. 5D, the process of obtaining a corrected image (band pass + high pass processed image) after removing the moire fringes without leaving the original image through each step is also included. Is processed for each pixel line.

(Step S2) High-pass filter processing [Reference: FIG. 5 (b)]
As in step S1, as shown in FIG. 5B, the high-pass filter 22 performs high-pass processing (HPF processing) on the original image detected and collected by the FPD in a direction perpendicular to the grid pattern. Then, high frequency components including grid frequency components are passed through and extracted from the original image (high pass grid image). The cutoff frequency of the high pass filter 22 is a frequency corresponding to the peak of the grid frequency extracted by the band pass processing by the band pass filter 21.
fm = (lp−n × lg) / lp × lg
It is said. Here, n = 0, 1, 2,..., Lg: grid array interval, lp: detection pixel interval, lg <lp, where lg is less than 2 times lp, n = 1, 3 times or less In this case, n = 2.

(Step S3-1) Difference between Images [Reference: FIG. 5 (c)]
Next, the difference between the obtained band-pass grid image and high-pass grid image is obtained by the subtractor 23.
A vertical low-pass process is added to this (step S3-2, difference grid image).

(Step S3-2) Longitudinal Low Pass Processing [Reference: FIG. 5 (d)]
After the difference is taken in step S3-1, low-pass processing by the low-pass filter 25 (vertical low-pass processing, see FIG. 5D) is added to the difference image in a direction parallel to the grid pattern. . This is due to the fact that only the fringes composed of the components resulting from the grid variation, that is, the grid frequency components that cannot be extracted by the band pass, have the vertical directionality. By performing the vertical low-pass process, only the grid stripes (not completely extracted by the band pass) remain in the difference image. As described above, grid stripes that are “not completely extracted by bandpass” are extracted (difference grid image).
Note that, even in this step according to FIG. 5 (d), as in the case of FIG. 5 (c), there may be a grid frequency component that is very small than the second harmonic in FIG. 5 (c). . As can be seen from FIG. 5 (d), even if it exists, it is at a level that is almost negligible.

(Step S4) Add the difference grid image to the bandpass grid image [Reference: FIG. 5 (e)]
Thereafter, the obtained difference grid image is added to the previously obtained band pass grid image to obtain a band pass + difference grid image. This is data indicating a grid frequency component including a component resulting from the grid variation.

(Step S5) Subtracting the bandpass + difference grid image from the original image Then, by subtracting the bandpass + difference grid image from the original image, a bandpass + high-pass processed image to be finally obtained is acquired.
In this band pass + high pass processed image obtained through each of the above processing steps, the grid frequency component is reliably removed from the original image while maintaining the high frequency component. I can understand that it has been resolved.

Based on the above, the contents of one embodiment of the present invention will be described in order while showing actual grid images and other examples. The structure of the radiation imaging apparatus according to the present embodiment, the control system and system configuration, and the outline of the flow of image processing are as described above.
FIGS. 6-12 is an example of the reference image which shows the flow of the process performed in the image process part 9 corresponded to the grid frequency component removal process means of the radiation imaging device which concerns on this invention. The images according to FIGS. 6 to 12 are images for each processing step of the image processing performed in the radiation imaging apparatus 100 according to the present embodiment. These are all examples of images obtained when a subject sample M is imaged, and are recorded to explain each processing step of image processing performed in the radiation imaging apparatus according to the present embodiment. is there. Therefore, it is important to note that the image (bandpass + highpass processed image 57) shown in FIG. 12 is obtained when the subject sample M is actually captured using the radiation imaging apparatus according to the present embodiment. I refuse.

Here, in describing each processing process of the radiation imaging apparatus according to the present embodiment, which state the image according to FIGS. 6 to 12 corresponds to (in which process process the image is recorded) Is briefly explained.
FIG. 6 shows the original image 51. That is, FIG. 6 shows the X-ray irradiation image itself (= the image without any image processing) that is transmitted through the subject M and the X-ray grid 14 and detected and collected by the FPD 3. In the original image of FIG. 6, it is understood that moire fringes by the grid are reflected.
FIG. 7 is a grid image (bandpass grid image 52) extracted by the bandpass filter 21 from the original image 51 shown in FIG. 6 (reference: step S1 and FIG. 5A).
8 is an image obtained by subtracting the bandpass grid image 52 shown in FIG. 7 from the original image shown in FIG. 6 (bandpass processed image 53). In this image, it is understood that vertical lines remain as a whole.
As shown in FIG. 8, under the prior art, the grid frequency component can be removed only up to this level, and there is a problem that moiré fringes remain in the X-ray irradiation image obtained by subtracting the grid image. there were. As described above, the present invention solves this problem by including the following image processing steps.

FIG. 9 is a grid image (high-pass grid image 54) extracted from the original image 51 shown in FIG. 6 by the high-pass filter 22 (reference: step S2 and FIG. 5B).
FIG. 10 is an image (difference grid image 55) obtained by adding a vertical low-pass process to the difference between the band-pass grid image 52 according to FIG. 7 and the high-pass grid image 54 according to FIG. 9 (reference: steps S3-1 and S3-). 2 and FIGS. 5C and 5D).
11 is an image (band pass + difference grid image 56) obtained by adding the difference grid image 55 shown in FIG. 10 to the band pass grid image 52 shown in FIG. 7 (reference: step S4 and FIG. 5 (e)).
FIG. 12 is a result (band pass + high pass processed image 57) finally obtained by subtracting the band pass + difference grid image 56 shown in FIG. 11 from the original image shown in FIG. 6 (see step S5). In this image, it can be seen that the vertical lines have disappeared.

[Image processing process]
Hereinafter, the flow of image processing performed in the present embodiment will be described in order based on FIGS. 3, 4, and 5. As a premise for explanation, the FPD 3 collects the original image 51 shown in FIG. 6 in which a grid pattern (moire fringes) by the X-ray grid 14 is reflected.

  First, grid images (the bandpass grid image 52 shown in FIG. 7 and the highpass grid image 54 shown in FIG. 9) extracted by the bandpass filter 21 or the highpass filter 22 from the original image 51 shown in FIG. 6 are acquired (step S1). And S2. Reference: Fig. 5 (a) and (b)).

Next, a vertical low-pass process is performed on the difference (step S3-1) between the obtained bandpass grid image 52 and the highpass grid image 54 to obtain a difference grid image 55 according to FIG. 10 (step S3-2. Reference: FIG. 5 (c) and (d)).
.
Then, the obtained difference grid image 55 according to FIG. 10 is added to the bandpass grid image 52 according to FIG. 7 to obtain the bandpass + difference grid image 56 according to FIG. 11 (step S4. Reference: FIG. 5 ( e)).

Finally, the bandpass + high-pass processed image 57 shown in FIG. 12 is obtained by subtracting the bandpass + difference grid image 56 shown in FIG. 11 from the original image 51 shown in FIG. 6 (step S5).
This bandpass + highpass processed image 57 according to FIG. 12 is the result to be obtained through the present embodiment. It can be understood that the vertical line disappears in the band pass + high pass processed image 57 according to FIG. 12 as compared with the band pass processed image 53 according to FIG.

[Modification]
The present invention is not limited to the above embodiment, and can be modified as follows.

  (1) In the above-described embodiment, an X-ray imaging apparatus has been described as an example of a radiation imaging apparatus. However, an ECT represented by a PET (Positron Emission Tomography) apparatus, a SPECT (Single Photon Emission CT) apparatus, or the like. A radiation imaging apparatus that performs radiation imaging by detecting radiation other than X-rays (γ-rays in the case of a PET apparatus) and obtaining a radiation image based on the detected radiation, such as an (Emission Computed Tomography) apparatus You may apply to.

  (2) In the above-described embodiment, the radiation imaging apparatus 100 as shown in FIG. 1 has been described as an example, but the present invention is also applied to, for example, an X-ray imaging apparatus disposed on a C-arm. May be. The present invention may also be applied to an X-ray CT apparatus.

  (3) In the above-described embodiment, the binning process is omitted and described. However, the present invention is not limited to this, and the binning process may be appropriately performed.

M Subject 1 Top plate 2 X-ray tube 3 FPD
4 Top plate control unit 5 FPD control unit 6 High voltage generation unit 7 X-ray tube control unit 8 A / D converter 9 Image processing unit 10 Controller 11 Memory unit 12 Input unit 13 Monitor 14 X-ray grid 21 Band pass filter 22 High pass Filters 23 and 24 Subtractor 25 Low pass filter 26 Adder 100 Radiation imaging device

Claims (2)

A radiation generator;
A scattered radiation removal grid that is arranged so that the radiation that has passed through the subject enters, and in which high-absorption rate portions and low-absorption rate portions of the radiation are alternately arranged in parallel,
A radiation detector that is arranged so that the radiation that has passed through the scattered radiation removal grid is incident, and that has a spatial sampling period equal to at least one direction in the vertical or horizontal direction that vertically crosses the grid pattern;
Grid frequency component removal processing means for performing processing for removing the grid frequency component from the original image detected and collected by the radiation detector through the scattered radiation removal grid;
A radiation imaging apparatus comprising:
The grid frequency component removal processing means includes
A band-pass filter that performs processing for obtaining a band-pass grid image by band-pass extraction from the original image by performing band-pass processing in a direction that crosses the grid pattern vertically with respect to the original image;
The original image is subjected to high-pass processing in a direction perpendicularly crossing the grid pattern, and the original image is passed by passing the frequency component of the grid pattern extracted by band-pass processing by the band-pass filter. A high-pass filter that performs processing to obtain a high-pass grid image by high-pass extraction from,
First difference means for taking a difference between the bandpass grid image and the highpass grid image;
A low-pass filter that performs a low-pass process on the difference image in a direction parallel to the grid pattern to obtain a difference grid image composed of grid frequency components that cannot be extracted by the band pass; and
Adding means for adding the difference grid image to the bandpass grid image and creating a bandpass + difference grid image separately; and
Second difference means for subtracting the bandpass + difference grid image from the original image;
A radiation imaging apparatus comprising: A radiation generator;
A scattered radiation removal grid that is arranged so that the radiation that has passed through the subject enters, and in which high-absorption rate portions and low-absorption rate portions of the radiation are alternately arranged in parallel,
A radiation detector that is arranged so that the radiation that has passed through the scattered radiation removal grid is incident, and that has a spatial sampling period equal to at least one direction in the vertical or horizontal direction that vertically crosses the grid pattern;
In a radiation imaging apparatus comprising:
A method of removing a grid frequency component from an original image detected and collected by the radiation detector through the scattered radiation removal grid,
A process of obtaining a bandpass grid image by bandpass extraction from the original image by performing a bandpass process by a bandpass filter in a direction perpendicular to the grid pattern on the original image;
A high-pass filter in a direction perpendicularly crossing the grid pattern with respect to the original image, which can also pass the frequency component of the grid pattern extracted by band-pass processing by the band-pass filter Processing to obtain a high-pass grid image by high-pass extraction from the original image by performing high-pass processing by
The difference between the band-pass grid image and the high-pass grid image is taken, and the difference image is subjected to low-pass processing by a low-pass filter in a direction parallel to the grid pattern, and can be extracted by the band pass. Processing to obtain a difference grid image composed of non-grid frequency components,
A process of separately creating a bandpass + difference grid image obtained by adding the difference grid image to the bandpass grid image; and
A process of subtracting the bandpass + difference grid image from the original image to obtain a bandpass + highpass processed image;
A method for removing a grid frequency component, comprising: