JP2014008069A - X-ray diagnostic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray diagnostic apparatus in which visual recognition can be improved in an image after a moire pattern of a grid is removed.SOLUTION: In an X-ray diagnostic apparatus 1, a frequency band of a narrow band filter is narrowed while a frequency band of a broad band filter is widened. As a result, an image P1 with a narrow band moire pattern removed is suppressed in the deterioration of visual recognition in a region other than a flat region, while an image P2 with a broad band moire pattern removed has removal of the moire pattern more completely in the flat region. A flatness calculation part 29 calculates flatness in the image P1 with the narrow band moire pattern removed and acquires a flatness map. An output selection part 31, on the basis of the flatness map, outputs the image P2 with the broad band moire pattern removed, in a flat region and outputs the image P1 with the narrow band moire pattern removed, in a region other than the flat region. Consequently, visual recognition can be improved in the image after the moire pattern of a grid 5 is removed.

Description

  The present invention relates to an X-ray diagnostic apparatus that removes a moire pattern of a grid reflected in an acquired image.

  A conventional X-ray diagnostic apparatus includes an X-ray tube that irradiates a subject with X-rays, and a flat panel X-ray detector (hereinafter referred to as an X-ray detector that is provided facing the X-ray tube and detects X-rays transmitted through the subject). Appropriately referred to as “FPD”). The FPD is provided with a scattered radiation removal grid (hereinafter referred to as “grid” as appropriate) so as to cover its X-ray detection surface.

  The grid is for removing scattered X-rays generated when X-rays pass through the subject. The grid is configured by alternately arranging an absorber (for example, lead) that absorbs X-rays and a transmissive body (for example, aluminum or air) that transmits X-rays. The scattered X-rays are removed by the grid, and the X-rays from which the scattered X-rays are removed are incident on the FPD. As a result, the FPD can output a clear image with reduced blur.

  On the other hand, in the conventional X-ray diagnostic apparatus, there is a problem that a moire pattern of a grid is reflected in an image acquired by FPD. The moire pattern of the grid is generated due to the difference between the resolution (density) of the FPD and the density of the absorber and the transparent body arranged alternately in the grid. Various proposals have been made for a method of removing a moire pattern from an image (see, for example, Patent Document 1).

  Next, an example of a conventional method for removing a grid moire pattern from an image will be described with reference to FIG. First, an image is acquired by the FPD 104. The acquired image (original image) includes a moire pattern of the grid. The frequency characteristic generation unit 121 performs Fourier transform (FFT) line by line in a direction crossing the grid moire pattern reflected in the original image. The peak frequency is detected by the peak frequency detector 123 from the frequency characteristics after the Fourier transform. The filter creation unit 125 creates a band pass filter for removing the moire pattern from the peak frequency. The moire pattern extraction unit 142 performs filtering on the original image based on the bandpass filter, and extracts an image having only a moire pattern. Then, the subtracting unit 144 subtracts and removes the moire pattern image from the original image. In this way, the moire pattern is removed from the original image. The band-pass filter described above has a preset fixed shape.

JP 2011-010873 A

  However, the conventional apparatus has a problem that the visibility of an image (moire pattern removed image) from which the moire pattern of the grid is removed is deteriorated by the acquired image. That is, in the conventional apparatus, when the frequency band of the bandpass filter is widened, the moire pattern can be more completely removed. However, a filter in a wide frequency band also removes patterns other than the moire pattern from the original image, thereby degrading the visibility of the image. Therefore, there is a demand for making the frequency band of the filter as narrow as possible. However, simply by narrowing the frequency band of the filter, the moire pattern in the flat area in the moire pattern removal image cannot be completely removed.

  Further, in the apparatus of Patent Document 1, a slow characteristic filtering unit and a sudden characteristic filtering unit having two different characteristics, a differential value map generating unit that generates a differential value map in which the intensity of change in pixel values in the original image is mapped, It has. Further, this apparatus refers to the differential value map, and removes moire (pattern) reflected in the original image while switching the steep characteristic moire image and the moderate characteristic moire image for each pixel with respect to the original image. A removal image generation unit is provided. The differential value map is generated based on the original image. However, from the original image in which the moiré pattern is reflected, it is difficult to generate a removed image by accurately switching due to the moiré pattern.

  The present invention has been made in view of such circumstances, and an object thereof is to provide an X-ray diagnostic apparatus capable of improving the visibility of an image after removing a moire pattern from a grid.

In order to achieve such an object, the present invention has the following configuration.
That is, an X-ray diagnostic apparatus according to the present invention includes an X-ray irradiation unit that irradiates a subject with X-rays, an X-ray detector that detects X-rays transmitted through the subject, and the X-rays of the X-ray detector. A grid arranged on the incident side for removing scattered X-rays, and a peak frequency detection unit for detecting the peak frequency of the first harmonic of the moire pattern of the grid reflected in the original image output from the X-ray detector And narrowing the original image based on a narrowband filter including a frequency band formed in a preset width between a reference frequency determined based on the peak frequency and a frequency lower than the reference frequency. Based on a narrowband moire pattern removal unit that removes a band moire pattern and obtains a narrowband moire pattern removal image, and a wideband filter in which the frequency band is formed wider than the narrowband filter, the original A wideband moire pattern removal unit that obtains a wideband moire pattern removal image by removing a wideband moire pattern from either the image or the narrowband moire pattern removal image, and calculates flatness in the narrowband moire pattern removal image A flatness calculation unit that acquires a flatness map, and outputs the wideband moire pattern removal image in a flat region of the flatness map, and the narrowband moire pattern removal image in a region other than the flat region of the flatness map. And an output selection unit that outputs the signal.

  According to the X-ray diagnostic apparatus of the present invention, the narrow-band moire pattern removing unit is formed to have a preset width between a reference frequency determined based on the peak frequency and a frequency lower than the reference frequency. Based on the narrow band filter including the frequency band, the narrow band moire pattern is removed from the original image to obtain the narrow band moire pattern removed image. The wideband moire pattern removing unit removes the wideband moire pattern from either the original image or the narrowband moire pattern removed image based on the wideband filter having a wider frequency band than the narrowband filter. A moire pattern removal image is acquired. Further, the flatness calculation unit calculates the flatness in the narrowband moire pattern removed image and acquires a flatness map. The output selection unit outputs a wideband moire pattern removed image in a flat region of the flatness map, and outputs a narrowband moire pattern removed image in a region other than the flat region of the flatness map.

  That is, the frequency band of the narrow band filter is narrowed, and the frequency band of the wide band filter is wider than that of the narrow band filter. Thereby, the narrow-band moire pattern-removed image can suppress deterioration of visibility in the structure region other than the flat region, and the wide-band moire pattern-removed image can more completely remove the moire pattern in the flat region. . Furthermore, the flatness calculation unit calculates the flatness in the narrowband moire pattern removed image and obtains the flatness map. Since the narrow-band moire pattern-removed image has the narrow-band moire pattern removed, a flatness map can be acquired with higher accuracy than the original image. Then, based on the flatness map, the output selection unit outputs a wideband moire pattern image in a flat region, and outputs a narrowband moire pattern image in a region other than the flat region. For this reason, in the selectively output image, the moire pattern in the flat area is more completely removed, and the deterioration of the visibility of the structure area other than the flat area is suppressed. Therefore, the visibility in the image after removing the moire pattern from the grid can be improved.

  In the X-ray diagnostic apparatus according to the present invention, the flatness calculation unit calculates a luminance change amount between the target pixel to be processed and its surrounding pixels, and acquires a flatness map. It is preferable to provide. Accordingly, the luminance change amount calculation unit can acquire a flatness map indicating the flatness in the narrowband moire pattern-removed image by calculating the luminance change amount between the pixel of interest and the surrounding pixels.

  In the X-ray diagnostic apparatus according to the present invention, the luminance change amount calculation unit subtracts an addition average value of the target pixel and surrounding pixels from a pixel value of the target pixel to obtain a flatness map. It is preferable. Thereby, the luminance change amount calculation unit can obtain the flatness map by calculating the luminance change amount by subtracting the addition average value of the target pixel and surrounding pixels from the pixel value of the target pixel. .

  Further, in the X-ray diagnostic apparatus according to the present invention, the flatness calculation unit normalizes the luminance change amount of each pixel of the flatness map by dividing by a pixel value of the corresponding narrowband moire pattern removed image. It is preferable to provide a normalization processing unit for processing. That is, a pixel having a high pixel value has a large luminance change amount, and a pixel having a low pixel value tends to have a small luminance change amount. Even if the brightness change amount is the same, the weight change is different between the brightness change amount of the high brightness pixel and the brightness change amount of the low brightness pixel. Therefore, by dividing the luminance change amount of each pixel by the pixel value of the corresponding pixel, the luminance change amounts of the high luminance pixel and the low luminance pixel can be compared on the same basis.

  Further, in the X-ray diagnostic apparatus according to the present invention, the flatness calculation unit performs an average of luminance change amounts for each region obtained by dividing the flatness map by a plurality of pixels, and divides the average value. It is preferable to include a smoothing processing unit that applies the smoothing to each pixel in the region. By smoothing the luminance change amount, output selection by the output selection unit can be performed in an arbitrary region. In addition, a region determined as a flat region scattered in a region other than the flat region can be processed as a region other than the flat region. If the wideband moire pattern-removed image is output up to flat regions scattered in regions other than the flat region, visibility may be deteriorated.

  According to the X-ray diagnostic apparatus of the present invention, the frequency band of the narrow band filter is narrowed and the frequency band of the wide band filter is wider than that of the narrow band filter. Thereby, the narrow-band moire pattern-removed image can suppress deterioration of visibility in the structure region other than the flat region, and the wide-band moire pattern-removed image can more completely remove the moire pattern in the flat region. . Furthermore, the flatness calculation unit calculates the flatness in the narrowband moire pattern removed image and obtains the flatness map. Since the narrow-band moire pattern-removed image has the narrow-band moire pattern removed, a flatness map can be acquired with higher accuracy than the original image. Then, based on the flatness map, the output selection unit outputs a wideband moire pattern image in a flat region, and outputs a narrowband moire pattern image in a region other than the flat region. For this reason, in the selectively output image, the moire pattern in the flat area is more completely removed, and the deterioration of the visibility of the structure area other than the flat area is suppressed. Therefore, the visibility in the image after removing the moire pattern from the grid can be improved.

It is a block diagram which shows schematic structure of the X-ray diagnostic apparatus which concerns on an Example. It is a figure which shows an example of the moire pattern reflected in the original image. It is a figure where it uses for description of the peak frequency detection in a frequency characteristic. (A) is a figure which shows the detailed structure of a narrow-band moire pattern removal part, (b) is a figure which shows the detailed structure of a wideband moire pattern removal part. (A) is a figure which shows an example of the shape of a narrow-band filter, (b) is a figure which shows an example of the shape of a wideband filter. It is a figure where it uses for description of FIR filter processing. It is a figure which shows the image (narrowband moire pattern image) containing a narrowband moire pattern. (A), (b) is a figure where it uses for description of a vertical low pass filter (LPF) process. It is a figure where it uses for description of narrow-band moire pattern removal. It is a figure where it uses for description of broadband moiré pattern removal. It is a figure which shows the detailed structure of a flatness calculation part. It is a figure where it uses for description of luminance variation calculation. It is a figure where it uses for description of the smoothing process. It is a figure where it uses for description of output selection. It is a flowchart which shows operation | movement of the X-ray diagnostic apparatus of an Example. (A), (b) is a figure which shows an example of the wideband filter which concerns on a modification. It is a block diagram which shows schematic structure of the X-ray diagnostic apparatus which concerns on a modification. It is a block diagram which shows schematic structure of the conventional X-ray diagnostic apparatus.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a schematic configuration of the X-ray diagnostic apparatus according to the embodiment. FIG. 2 is a diagram illustrating an example of a moire pattern reflected in the original image. FIG. 3 is a diagram for explaining the peak frequency detection in the frequency characteristics.

  Please refer to FIG. The X-ray diagnostic apparatus 1 is arranged to face the X-ray tube 3 with the subject M sandwiched between the top plate 2 on which the subject M is placed, the X-ray tube 3 that irradiates the subject M with X-rays, and the subject M. And an X-ray detector 4 for detecting X-rays transmitted through the subject M, and a grid 5 disposed on the X-ray incident side of the X-ray detector 4 for removing scattered X-rays. The X-ray tube 3 corresponds to the X-ray irradiation unit in the present invention.

  The X-ray tube 3 is subjected to control necessary for X-ray irradiation by the X-ray tube controller 6. The X-ray tube controller 6 has a high voltage generator 7 that generates the tube voltage and tube current of the X-ray tube 3. The X-ray tube control unit 6 irradiates X-rays from the X-ray tube 3 in accordance with X-ray conditions such as a tube voltage, a tube current, and an irradiation time set by an input unit 12 described later.

  The X-ray detector 4 detects X-rays emitted from the X-ray tube 3 and outputs an X-ray detection signal that is a signal corresponding to the X-ray intensity distribution. The X-ray detector 4 is composed of a flat panel X-ray detector (FPD) or the like. The X-ray detector 4 is configured with, for example, a 0.15 um (micrometer) pixel pitch and has a resolution of 3.33 lp / mm. The grid 5 is configured with a grid density of 50 lines / cm (5 lp / mm), for example.

  An A / D converter 8 and an image processing unit 9 are provided in the subsequent stage of the X-ray detector 4. The A / D converter 8 converts the analog X-ray detection signal output from the X-ray detector 4 into a digital signal. The image processing unit 9 performs various image processing on the image based on the digital X-ray detection signal. The X-ray diagnostic apparatus 1 includes a main control unit 10 that comprehensively controls each component of the apparatus 1, a display unit 11 that displays an image processed by the image processing unit 9, and an operator that performs input settings and An input unit 12 for performing various operations and a storage unit 13 for storing processed images are provided.

  The main control unit 10 is composed of a central processing unit (CPU) and the like, and executes various programs. For example, the main control unit 10 performs control to move the top 2, the X-ray tube 3 or the X-ray detector 4 on which the subject M is placed to a predetermined position. The display unit 11 includes a monitor or the like. The input unit 12 includes a keyboard, a mouse, and the like. The storage unit 13 is configured by a storage medium including a removable medium such as a ROM (Read-only Memory), a RAM (Random-Access Memory), or a hard disk.

  As described above, the image processing unit 9 performs various image processes on the image. The moiré pattern of the grid 5 is reflected in the original image P0 sent to the image processing unit 9. The image processing unit 9 removes the moire pattern of the grid 5 that is actually reflected in the original image P0 acquired based on the output of the X-ray detector 4.

  An outline of image processing performed by the image processing unit 9 will be described. The original image P0 is subjected to frequency analysis by the frequency characteristic generation unit 21. Next, the peak frequency detection unit 23 detects a peak indicating a moire component included in the original image P0 based on the frequency analysis. The detected peak frequency is sent to each of the narrowband moire pattern removal unit 25 and the wideband moire pattern removal unit 27. The narrow-band moire pattern removing unit 25 creates the narrow-band filter 40 based on the peak value and applies it to the original image P0 to generate the narrow-band moire pattern removed image P1 from which the moire pattern is removed. Similarly, the wideband moire pattern removal unit 27 creates a wideband filter 50 based on the peak value, and applies this to the narrowband moire pattern removal image P1 to generate a wideband moire pattern removal image P2 from which the moire pattern is further removed. . The output selection unit 31 generates an output selection image P7 based on the narrowband moire pattern removed image P1 and the wideband moire pattern removed image P2. The output selection image P7 is a partially different image so that the degree of moire remaining does not reduce the visibility. The output selection unit 31 operates with reference to the flatness map P6 created by the flatness calculation unit 29 based on the narrow-band moire pattern removed image P1, which will be described later.

<Configuration of frequency characteristic generator>
The frequency characteristic generation unit 21 converts the frequency of the original image P0. Please refer to FIG. The moire pattern mp0 is reflected in the original image P0. A symbol f indicates a flat region of the subject M in the image. For example, the frequency characteristic generation unit 21 performs one-dimensional Fourier transform (FFT) for each pixel row L of the original image P0 in which a plurality of pixels are configured in a two-dimensional matrix. Thereby, the frequency characteristic as shown in FIG. 3 is acquired. The pixel row L is arranged in the X direction orthogonal to the stripes of the moire pattern mp0. The frequency characteristic generation unit 21 acquires the frequency characteristics for each pixel row L in order from the upper side of the original image P0 in FIG.

<Configuration of peak frequency detector>
The peak frequency detector 23 detects the peak frequency Pk1 of the first harmonic of the moire pattern mp0 of the grid 5 reflected in the original image P0. That is, the peak frequency detection unit 23 detects the peak frequency Pk1 of the first harmonic (also referred to as a fundamental wave) indicating the wave component having the longest period of the moire pattern mp0 from the frequency characteristics as shown in FIG. Note that the symbol Pk2 indicates the peak frequency of the second harmonic. The peak frequency Pk2 of the second harmonic is a frequency twice the peak frequency Pk1 of the first harmonic, and is obtained from the peak frequency Pk1 of the first harmonic. An arbitrary peak frequency Pk1 is determined from the frequency characteristics for each pixel row L of the original image P0.

<Configuration of narrowband moire pattern removal unit>
Next, the narrow band moire pattern removing unit 25 will be described. FIG. 4A is a diagram illustrating a detailed configuration of the narrowband moire pattern removing unit. The narrowband moire pattern removal unit 25 removes the narrowband moire pattern mp1 from the original image P0 based on the narrowband filter 40, and acquires the narrowband moire pattern removed image P1. Specifically, the narrowband moire pattern removing unit 25 includes a narrowband filter creating unit 41 that creates the narrowband filter 40, a narrowband moire pattern extracting unit 42 that extracts the narrowband moire pattern mp1 from the original image P0, The vertical LPF processing unit 43 that removes noise from the extracted narrowband moire pattern mp1 and the narrowband moire pattern (more precisely, the pattern after the vertical LPF processing) mp1 is subtracted from the original image P0 and reflected in the original image P0. And a narrow-band moire pattern subtracting unit 44 for removing the moire pattern.

  The narrowband filter creation unit 41 creates the narrowband filter 40. FIG. 5A is a diagram illustrating an example of the shape of the narrow band filter 40. The narrow band filter 40 includes a frequency band 201a formed in a preset width between a peak frequency Pk1 and a frequency fr1 lower than the peak frequency Pk1. The frequency bands including the peak frequency Pk1 are defined as frequency bands 201a and 201b, and the frequency bands including the peak frequency Pk2 are defined as wideband frequency bands 201c and 201d.

  The widths of the frequency bands 201a to 201d are determined by, for example, the frequencies fr1 to fr4 at positions where the filter coefficient is 0.5. The frequency bands 201a to 201d are configured with the same width, but may have different widths. The frequency band 201a corresponds to the frequency band of the present invention.

  In this embodiment, the narrowband moire pattern extraction unit 42 performs FIR (Finite Impulse Response) filter processing as filter processing for the original image P0. In this case, the narrowband filter creation unit 41 performs a one-dimensional inverse Fourier transform (inverse FFT) on the narrowband filter 40 to calculate FIR filter coefficients. Therefore, the FIR filter coefficient is sent to the narrowband moire pattern extraction unit 42. For example, the FIR filter coefficients are represented by fc1 to fc15.

  The narrowband moire pattern extraction unit 42 performs FIR filter processing on the original image P0 based on the narrowband filter 40, and extracts a narrowband moire pattern mp1. FIG. 6 is a diagram for explaining the FIR filter processing. In FIG. 6, the FIR filter is a pixel row a1 to a29 in the horizontal direction (X direction), and is weighted by FIR filter coefficients fc1 to fc15. Note that the number of FIR filter coefficients is smaller than the number of pixel columns because the same FIR filter coefficients are used for a plurality of pixels.

  In the FIR filter process, when the target pixel is a15, the pixel r15 at the corresponding position after the FIR filter process is calculated as follows. r15 = a15 × fc1 + (a14 + a16) × fc2 + (a13 + a17) × fc3 + (a12 + a18) × fc4 +... + (a1 + a29) × fc15. The FIR filter process is performed on each pixel of the original image P0, and a narrowband moire pattern mp1 is extracted. FIG. 7 is a diagram illustrating an image (narrowband moire pattern image) P1a including the narrowband moire pattern mp1.

  The vertical LPF processing unit 43 performs one-dimensional low-pass filter processing (smoothing processing) in the vertical direction (Y direction) along the stripes of the narrow-band moire pattern mp1 extracted by the narrow-band moire pattern extraction unit 42. The noise is removed from the narrow-band moire pattern mp1. Reference is made to FIG. The noise superimposed on the narrowband moire pattern mp1 will be described. A narrowband moire pattern mp1 is extracted from the narrowband moire pattern image P1a. However, in addition to this, for example, a pattern 203 having the same cycle as the narrowband moire pattern mp1 is extracted from the narrowband moire pattern image P1a. This pattern 203 intersects with the narrow band moire pattern mp1, and is not related to the narrow band moire pattern mp1. The vertical LPF processing unit 43 removes the oblique pattern 203 so that only the narrow-band moire pattern mp1 is displayed in the narrow-band moire pattern image P1a.

  The vertical LPF processing unit 43 performs one-dimensional low-pass filter processing on the narrowband moire pattern image P1a with the filter illustrated in FIG. The low-pass filter process is a pixel row in the vertical direction (Y direction) along the narrow-band moire pattern mp1, and for example, 17 pixels including the upper and lower 8 pixels of the target pixel to be processed are added and averaged. The added average value is a new pixel value of a pixel at a position corresponding to the target pixel in the new image. The low-pass filter process is performed on each pixel of the narrow-band moire pattern image P1a to obtain the narrow-band moire pattern image P1b after the low-pass filter process.

  As shown in FIG. 9, the narrowband moire pattern subtraction unit 44 subtracts the narrowband moire pattern image P1b from the original image P0 to obtain a narrowband moire pattern removed image P1 from which the moire pattern is removed from the original image P0. . The narrow band filter 40 has a frequency band 201a narrower than a wide band filter 50 described later. By forming the narrow band filter 40 relatively narrow, it is possible to suppress the removal of patterns other than the moire pattern mp0 reflected in the original image P0. Therefore, it is possible to suppress deterioration in image quality in a structure region such as a bone. However, as shown in FIG. 9, the moire pattern mp0 cannot be completely removed in the narrow-band moire pattern removed image P1, and a part of the moire pattern mp0 remains. The moire pattern remaining in the image is referred to as a moire pattern mp0a.

  Returning to FIG. The narrowband moire pattern removal image P1 is sent to the wideband moire pattern removal unit 27, the flatness calculation unit 29, and the output selection unit 31.

<Configuration of Broadband Moire Pattern Removal Unit>
Next, the broadband moiré pattern removal unit 27 will be described. FIG. 4B is a diagram illustrating a detailed configuration of the wideband moire pattern removing unit. The wideband moire pattern removing unit 27 removes the wideband moire pattern mp2 from the narrowband moire pattern removed image P1 based on the wideband filter 50, and acquires the wideband moire pattern removed image P2. The broadband moiré pattern removal unit 27 includes a broadband filter creation unit 51, a broadband moiré pattern extraction unit 52, a vertical LPF processing unit 53, and a broadband moiré pattern subtraction unit 54.

  The narrowband moire pattern removing unit 25 and the wideband moire pattern removing unit 27 are different in the shape of the filter to be created. FIG. 5B is a diagram illustrating an example of the shape of the broadband filter 50. The wideband filter 50 is formed such that the frequency band 202 a is wider than the frequency band 201 a of the narrowband filter 40. Further, unlike the bandpass filter of the narrowband filter 40, the wideband filter 50 is composed of a highpass filter. The frequency band 202a corresponds to the frequency band of the present invention.

  The wideband moire pattern removing unit 27 has the same configuration as the narrowband moire pattern removing unit 25 except for the filter shape and the processing target image. That is, the wideband filter creation unit 51 creates the wideband filter 50 as shown in FIG. The wideband moire pattern extraction unit 52 performs filter processing (for example, FIR filter processing) on the narrowband moire pattern removal image P1 based on the wideband filter 50, and extracts the wideband moire pattern mp2. Thereby, an image (broadband moire pattern image) P2a including the wideband moire pattern mp2 is acquired.

  The vertical LPF processing unit 53 performs one-dimensional low-pass filter processing (smoothing processing) in the vertical direction (Y direction) along the stripes of the wideband moire pattern mp2 extracted by the wideband moire pattern extraction unit 52, and performs wideband processing. Remove noise from moire patterns. Thereby, the broadband moiré pattern image P2b in which only the broadband moiré pattern mp2 is reflected is acquired from the broadband moiré pattern image P2a. As shown in FIG. 10, the wideband moire pattern subtraction unit 54 subtracts the wideband moire pattern image P2b from the narrowband moire pattern removed image P1 to obtain a wideband moire pattern removed image P2. The wideband moire pattern removed image P2 can be obtained by removing the moire pattern mp0 more completely. The wideband moire pattern removed image P2 is sent to the output selection unit 31.

  In the configuration of the embodiment, the narrow-band moire-removed image P1 and the wide-band moire-removed image P2 are combined to generate one output selection image P7. Therefore, it becomes a problem which image P1, P2 is used in each part of the output selection image P7. This image selection is performed based on the flatness map P6 generated by the flatness calculator 29. The flatness map P6 represents how flat the luminance change is. That is, the flatness of the area on the image composed of similar pixel values is high. On the contrary, the flatness is low in a region where the luminance change on the image is severely rough.

<Configuration of flatness calculation unit>
Next, the flatness calculation unit 29 will be described. The flatness calculation unit 29 is processed in parallel with the wideband moire pattern removal unit 27, calculates the flatness in the narrowband moire pattern removal image, and acquires a flatness map. The flatness map is for identifying a flat area in the narrow-band moire pattern removal image and a structure area other than the flat area. For example, in imaging of the subject M, the flat region is a region such as a muscle, and the structure region is a region such as a bone.

  FIG. 11 is a diagram illustrating a detailed configuration of the flatness calculation unit. The flatness calculation unit 29 calculates a luminance change amount C1 from the narrowband moire pattern removed image P1, and obtains an image P3 composed of the luminance change amount C1, and a normalization process for the image P3 Thus, a normalization processing unit 62 that acquires the intermediate map P4 and a smoothing processing unit 63 that smoothes the luminance change amount C2 for each region obtained by dividing the intermediate map P4 by a plurality of pixels are provided. The intermediate map P4 is composed of the normalized luminance change amount C2.

  The luminance change amount calculation unit 61 calculates the luminance change amount between the target pixel to be processed and its surrounding pixels, and acquires an image P3 that is a kind of flatness map. That is, the luminance change amount calculation unit 61 subtracts the addition average value A of the target pixel and the surrounding pixels from the pixel value B of the target pixel on the narrowband moire pattern removed image P1 to generate an image P3 (luminance change amount C1 Is mapped). Specifically, as shown in FIG. 12, the luminance change amount calculation unit 61 calculates the average value A by adding and averaging 21 × 21 pixels including the pixel of interest and the surrounding 10 pixels, for example, up, down, left, and right. To do. In FIG. 12, it is assumed that pixels around the pixel of interest are indicated by six pixels in the vertical and horizontal directions for drawing.

  As described above, the luminance change amount calculation unit 61 subtracts the pixel value B of the target pixel from the addition average value A to calculate the luminance change amount C1. The luminance change amount C1 is expressed by C1 = | B−A |. The luminance change amount C1 becomes a new pixel value of the pixel at the position corresponding to the target pixel in the new image. This process is performed for each pixel of the narrowband moire pattern removal image P1, and a new image represented by the luminance change amount C1 is acquired as a flatness map. This image is set as an image P3. In the present invention, the image P3 is a kind of flatness map. The same applies to the intermediate map P4 and the reduced map P5 in the following description. The flatness map P6 is a final flatness map that has been adjusted by performing various image processes on the image P3.

  The normalization processing unit 62 performs normalization processing on the image P3. That is, normalization is performed by dividing each pixel (luminance change amount C1) of the image P3 flatness map by the pixel value of the narrowband moire pattern removed image P1 at the corresponding position. That is, if the luminance change amount after the normalization processing is C2, it is expressed as C2 = | B−A | / B.

  This luminance change amount C2 tends to be large for pixels with a high pixel value in the original image P0 and small for pixels with a low pixel value in the original image P0. For example, it is assumed that there is a target pixel having a pixel value of 1000 and a target pixel having a pixel value of 10. When both of the luminance change amounts are 2, the luminance change amounts are the same value. However, the weight change is 2/1000 = 0.002 and 2/10 = 0.2 for the brightness change amount 2 when the pixel value is 1000 and the brightness change amount 2 when the pixel value is 10. . For this reason, the luminance change amount of the high luminance pixel and the low luminance pixel can be compared on the same basis.

  The normalization processing unit 62 obtains a luminance change amount C2 for each pixel (luminance change amount C1) constituting the image P3, and generates an intermediate map P4 in which the luminance change amounts C2 are two-dimensionally arranged. The intermediate map P4 is a flatness map in the middle of image processing, and this is not final.

  The smoothing processing unit 63 performs addition averaging of the luminance change amount for each region obtained by dividing the intermediate map P4 with a plurality of pixels, and smoothes the addition average value by applying it to each pixel in the divided region. As shown in FIG. 13, for example, when the intermediate map P4 is composed of 64 × 64 pixels, the smoothing processing unit 63 performs addition averaging, for example, with 16 × 16 pixels, and reduces the result to one pixel. Then, a reduced map P5 reduced by 4 × 4 pixels is acquired. Then, the flatness map P6 that has been smoothed is obtained by returning (enlarging) the 4 × 4 pixel reduced map P5 to the original size of 64 × 64 pixels. Note that when returning from one pixel of the reduced map P5 to the size of 16 × 16 pixels of the flatness map P6, all the luminance change amounts of 16 × 16 pixels of the flatness map P6 are the same as those of one pixel of the reduced map P5. A brightness change amount is given. The reduced map P5 is a flatness map in the middle of image processing, and this is not final.

<Configuration of output selector>
The output selection unit 31 outputs a wideband moire pattern removed image in the flat region of the flatness map P6, and outputs a narrowband moire pattern removed image in a region other than the flat region of the flatness map P6. That is, as shown in FIG. 14, the output selection unit 31 selects whether to output either one of the narrowband moire pattern removed image P1 and the wideband moire pattern removed image P2 based on the flatness map P6. To do.

  If the luminance change amount of the flatness map P6 is less than the threshold value, it is identified as a flat region, and if it is greater than or equal to the threshold value, it is identified as a region other than the flat region. Since each pixel (16 × 16 pixels) in the divided area obtained by averaging by 16 × 16 pixels by the smoothing processing unit 63 has the same luminance change amount, the output selection unit is in units of the 16 × 16 pixel area. 31 is selected and output. As shown in FIG. 14, in the flat region (symbol f) of the flatness map P6, a broadband moiré pattern-removed image is output. On the other hand, a narrowband moire pattern-removed image is output in an area other than the flat area of the flatness map P6.

  Next, with reference to FIG. 15, the operation of the X-ray diagnostic apparatus 1 of the present embodiment will be described with reference to a flowchart.

[Step S01] X-ray Imaging In the X-ray diagnostic apparatus 1, X-rays are emitted from the X-ray tube 3 toward the subject M (see FIG. 1). The irradiated X-rays pass through the subject M, and the scattered X-rays are removed by the grid 5 and enter the X-ray detector 4. The X-ray detector 4 outputs an X-ray detection signal corresponding to the X-ray intensity distribution. The A / D converter 8 converts the analog X-ray detection signal output from the X-ray detector 4 into a digital signal. An image (original image) is acquired based on the digitally converted X-ray detection signal.

[Step S02] Peak Frequency Detection The frequency characteristic generation unit 21 converts the frequency of the original image P0 (see FIG. 2). The peak frequency detector 23 detects the peak frequency Pk1 of the first harmonic of the moire pattern mp0 of the grid 5 reflected in the original image P0 from the frequency characteristics (see FIG. 3).

[Step S03] Narrowband Moire Pattern Removal Processing The narrowband moire pattern removal unit 25 removes the narrowband moire pattern mp1 from the original image P0 based on the narrowband filter 40, and acquires the narrowband moire pattern removed image P1. . The narrowband filter 40 includes a frequency band 201a formed in a preset width between a peak frequency Pk1 and a frequency fr1 lower than the peak frequency Pk1 (see FIG. 5A). The narrow band filter 40 has a frequency band 201 a narrower than the wide band filter 50. By forming the narrow band filter 40 relatively narrow, it is possible to suppress the removal of patterns other than the moire pattern mp0 reflected in the original image P0. Therefore, it is possible to suppress deterioration in image quality in a structure region such as a bone. Note that the moiré pattern mp0a remains in the narrow-band moiré pattern removed image P1.

[Step S04] Broadband Moire Pattern Removal Processing The wideband moire pattern removal unit 27 removes the wideband moire pattern mp2 from the narrowband moire pattern removed image P1 based on the wideband filter 50, and acquires the wideband moire pattern removed image P2. The wideband filter 50 has a wider frequency band 202a than the frequency band 201a of the narrowband filter 40 (see FIG. 5B). Thereby, the moire pattern mp0 can be more completely removed in the flat region in the narrow-band moire pattern removal image P1.

[Step S11] Calculation of luminance change amount The luminance change amount calculation unit 61 subtracts the addition average value A of the target pixel and its surrounding pixels from the pixel value B of the target pixel in the narrowband moire pattern removal image P1. The flatness map (luminance change amount C1) P3 is acquired. That is, as shown in FIG. 12, the luminance change amount calculating unit 61 sets the average value of 21 × 21 pixel values including the target pixel as A, sets the pixel value of the target pixel as B, and sets the luminance change amount C1 = | B−A | is calculated. By performing this process for each pixel, an image P3, which is a kind of flatness map, is acquired as a new image.

[Step S12] Normalization Processing The normalization processing unit 62 performs normalization processing by dividing the luminance change amount C1 of each pixel of the image P3 by the pixel value of the narrowband moire pattern-removed image at the corresponding position, and performs an intermediate map. P4 is generated. That is, the luminance change amount C1 is divided by the pixel value B of the target pixel used in the calculation of the luminance change amount C1. The luminance change amount C2 after the normalization process is calculated by C2 = | B−A | / B.

[Step S13] Smoothing Processing The smoothing processing unit 63 averages the luminance change amount for each region obtained by dividing the intermediate map P4 by a plurality of pixels (for example, 16 × 16 pixels), and divides the average value. Smoothing is applied to each pixel (16 × 16 pixels) in the area. The flatness map P6 is acquired by this flattening process. In other words, as shown in FIG. 13, the smoothing processing unit 63 performs an averaging process with 16 × 16 pixels, for example, and reduces it to one pixel. Then, when returning to the original size, the luminance change amount obtained by adding and averaging all the pixels is given to the original size of 16 × 16 pixels.

[Step S05] Output Selection As shown in FIG. 14, the output selection unit 31 outputs a wideband moire pattern removal image P2 in the flat region f of the flatness map P6, and structures other than the flat region f of the flatness map P6. In the area, the narrowband moire pattern removed image P1 is output. In this way, an output selection image P7 that is selectively output by the flatness map P6 and that combines the narrowband moire pattern removed image P1 and the wideband moire pattern removed image P2 is acquired. In other words, the output selection unit 31 selectively outputs a structure region in which the visibility is not deteriorated and a flat region f that does not depend on the visibility. Further, the remaining moire pattern mp0a is not conspicuous in the structure region, but is conspicuous in the flat region f.

  Further, the smoothing processing unit 63 smoothes the intermediate map P4 in units of regions divided by a plurality of pixels. That is, output selection by the output selection unit 31 is not performed in units of pixels, but is performed in units of arbitrary regions by smoothing. When the output selection of the image by the output selection unit 31 is performed in units of pixels, if the flat region f is scattered in a region other than the flat region f, the visibility of the selectively output image may be deteriorated. However, it is possible to suppress deterioration in the visibility of the image by performing smoothing in units of regions divided by a plurality of pixels.

[Step S06] Display / Save The output selection image P7 selected for output in step S05 is subjected to other necessary processing in the image processing section 9. The output selection image P <b> 7 is displayed on the display unit 11 and stored in the storage unit 13.

  As described above, according to the embodiment of the present invention, the frequency band 201 a of the narrow band filter 40 is narrowed, and the frequency band 202 a of the wide band filter 50 is wider than that of the narrow band filter 40. As a result, the narrow-band moire pattern removal image P1 can suppress deterioration in visibility in the structure region other than the flat region f, and the wide-band moire pattern removal image P2 more completely displays the moire pattern mp0 in the flat region f. Can be removed. Further, the flatness calculation unit 29 calculates the flatness in the narrowband moire pattern removed image P1 and acquires the flatness map P6. Since the moiré pattern mp0 is once removed from the narrow-band moire pattern removed image P1, the flatness map P6 can be acquired with higher accuracy than the original image P0. Then, based on the flatness map P6, the output selection unit 31 outputs the wideband moire pattern removal image P2 in the flat region f, and outputs the narrowband moire pattern removal image P1 in the region other than the flat region f. Yes. Therefore, in the selectively output image, the moire pattern mp0 in the flat region f is more completely removed, and the deterioration of the visibility in the structure region other than the flat region f is suppressed. Therefore, the visibility in the image after removing the moire pattern mp0 from the grid 5 can be improved.

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

  (1) In each embodiment described above, the wideband filter 50 is a high-pass filter, but may be a bandpass filter. As shown in FIG. 16A, in the wideband filter 50a, bandpass filters are formed at the peak frequency Pk1 of the first harmonic and the peak frequency Pk2 of the second harmonic, respectively. The widths of the frequency bands 202a to 202d constituting the wide band filter 50a are formed to be wider than the frequency bands 201a to 201d (FIG. 5A) of the narrow band filter 40, respectively.

  Note that, as shown in FIG. 16B, the wideband filter 50a of the bandpass filter is formed by offsetting a frequency (reference frequency) 204 serving as a reference of the bandpass filter with respect to the peak frequencies Pk1 and Pk2. May be. That is, the reference frequency 204 is determined based on the peak frequencies Pk1 and Pk2. The reference frequency 204 includes peak frequencies Pk1 and Pk2 without offset and a frequency offset 205 with respect to the peak frequencies Pk1 and Pk2.

  (2) In the above-described embodiment and modification (1), the wideband moire pattern removing unit 27 removes the wideband moire pattern mp2 from the narrowband moire pattern removed image P1 to obtain the wideband moire pattern removed image P2. . However, as shown in FIG. 17, the wideband moire pattern removal unit 27 may remove the wideband moire pattern mp2 from the original image P0 to obtain the wideband moire pattern removed image P2. For example, as shown in FIG. 5B, the present invention can be applied when the wide band filter 50 includes all the narrow band filters 40.

  (3) In the above-described embodiments and modifications, the narrowband / broadband moire pattern removal units 25 and 27 extract the narrowband / broadband moire patterns mp1 and mp2, respectively, using the FIR filter. However, it is not limited to this. For example, in the frequency characteristics shown in FIG. 3, the narrowband / wideband moire pattern images P1, P2 obtained by extracting the narrowband / wideband moire patterns mp1, mp2 by extracting the regions D1, D2 to a predetermined width and performing inverse Fourier transform. May be obtained. The narrowband / wideband moire pattern removal units 25 and 27 perform narrowband / wideband moire pattern removal by performing inverse Fourier transform on the frequency characteristics regions D1 and D2 of FIG. 3 that have been removed to a preset width. Images P1 and P2 may be acquired.

  (4) In the above-described embodiments and modifications, the narrowband / wideband moire pattern removal units 25 and 27 include the vertical LPF processing units 43 and 53, but may be omitted.

  (5) In the above-described embodiments and modifications, the broadband moire pattern removing unit 27 uses the peak frequency Pk1 obtained from the original image P0, but is not limited thereto. For example, the peak frequency obtained from the narrowband moire pattern removed image P1 may be used.

  (4) In the above-described embodiments and modifications, the image processing unit 9 includes a control unit configured by a CPU or the like for executing a program, and a storage medium such as a ROM or RAM that stores the program or the like. And a storage unit. The storage unit may store a program for the operations of steps S01 to S06 and S11 to S13, and the program may be executed by the control unit. In this case, operations necessary for this program are input through the input unit 12, and the output selection image P <b> 7 is displayed on the display unit 11.

  (5) In the above-described embodiments and modifications, the operation is not limited to the image processing unit 9, and the operation of each of steps S01 to S06 and S11 to S13 is stored in the storage unit 13, and the main control unit 10 may be executed. In this case, operations necessary for this program are input through the input unit 12, and the output selection image P <b> 7 is displayed on the display unit 11. The operation program may be executed on a personal computer connected to the X-ray diagnostic apparatus 1 in a network system such as a LAN.

DESCRIPTION OF SYMBOLS 1 ... X-ray diagnostic apparatus 3 ... X-ray tube 4 ... X-ray detector 5 ... Grid 9 ... Image processing part 23 ... Peak frequency detection part 25 ... Narrow band moire pattern removal part 27 ... Broadband moire pattern removal part 29 ... Flatness Calculation unit 31 ... Output selection unit 40 ... Narrow band filter 50, 50a ... Wide band filter 61 ... Luminance change calculation unit 62 ... Normalization processing unit 63 ... Smoothing processing unit P0 ... Original image P1 ... Narrow band moire pattern removal image P2 ... Wideband moire pattern removal image mp0 ... Moire pattern mp1 ... Narrowband moire pattern mp2 ... Wideband moire pattern Pk1 ... Peak frequency of the first harmonic 201a to 201d ... Frequency band 204 ... Reference frequency fr1 to fr4 ... Frequency

Claims (5)

An X-ray irradiation unit that irradiates the subject with X-rays;
An X-ray detector for detecting X-rays transmitted through the subject;
A grid disposed on the X-ray incident side of the X-ray detector and removing scattered X-rays;
A peak frequency detector for detecting a peak frequency of the first harmonic of the moire pattern of the grid reflected in the original image output by the X-ray detector;
Based on a narrowband filter including a frequency band formed in a preset width between a reference frequency determined based on the peak frequency and a frequency lower than the reference frequency, a narrowband moiré is obtained from the original image. A narrowband moire pattern removal unit that removes the pattern and obtains a narrowband moire pattern removal image;
A wideband moire pattern-removed image is obtained by removing a wideband moire pattern from either the original image or the narrowband moire pattern-removed image based on a wideband filter having a wider frequency band than the narrowband filter. A broadband moiré pattern removal unit to be acquired;
A flatness calculation unit that calculates flatness in the narrowband moire pattern removed image and obtains a flatness map;
An output selection unit that outputs the broadband moiré pattern removal image in a flat region of the flatness map, and outputs the narrowband moiré pattern removal image in a region other than the flat region of the flatness map;
An X-ray diagnostic apparatus comprising: The X-ray diagnostic apparatus according to claim 1,
X-ray diagnosis characterized in that the flatness calculation unit includes a luminance change amount calculation unit that calculates a luminance change amount between a target pixel to be processed and surrounding pixels and obtains a flatness map. apparatus. The X-ray diagnostic apparatus according to claim 2,
The X-ray diagnostic apparatus, wherein the luminance change amount calculation unit obtains a flatness map by subtracting an average value of the target pixel and surrounding pixels from a pixel value of the target pixel. The X-ray diagnostic apparatus according to claim 2 or 3,
The flatness calculation unit includes a normalization processing unit that performs normalization processing by dividing a luminance change amount of each pixel of the flatness map by a pixel value of the corresponding narrowband moire pattern removed image. A characteristic X-ray diagnostic apparatus. In the X-ray diagnostic apparatus according to any one of claims 2 to 4,
The flatness calculation unit performs an average of luminance change amounts for each region obtained by dividing the flatness map by a plurality of pixels, and applies the averaged value to each pixel in the divided region for smoothing. An X-ray diagnostic apparatus comprising a processing unit.

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