JP5753503B2 - Image processing apparatus and method - Google Patents

Description

  The present invention relates to an image processing apparatus and method for performing image processing for suppressing a periodic pattern included in an image.

  2. Description of the Related Art Conventionally, in the medical field and the like, various types of radiation detectors that record a radiation image related to a subject by irradiation with radiation that has passed through the subject have been proposed and put into practical use. As such a radiation detector, for example, there is a radiation detector using amorphous selenium that generates charges by irradiation of radiation. In an imaging apparatus using this radiation detector, lead and the like that do not transmit radiation and aluminum and wood that are easily transmitted are alternately arranged at a predetermined pitch between the radiation source that emits radiation and the radiation detector. A scattered radiation removal grid (hereinafter simply referred to as a grid) is provided, and the scattered component of radiation is removed by this grid.

  However, when a radiographic image of a subject is captured using a grid, periodic patterns such as periodic stripes and moire due to the grid are generated as noise in the acquired radiographic image. For this reason, by calculating the frequency characteristics of the periodic pattern included in the radiation image and performing processing for suppressing the frequency component of the periodic pattern on the radiation image, it is possible to prevent image quality deterioration due to the periodic pattern. Various proposals have been made. For example, in Patent Document 1, a plurality of linear regions are set in a radiographic image, and a frequency spectrum is obtained as a frequency characteristic by performing frequency analysis such as Fourier transform on the image signal of the linear region, A method for detecting a spatial frequency having a peak response in the frequency spectrum as a frequency characteristic of a periodic pattern has been proposed.

  By the way, in order to observe the affected area of the subject in more detail using the radiographic image, the X-ray tube is moved to irradiate the subject with X-rays from different directions, and the acquired multiple radiographic images are added. Thus, tomosynthesis imaging for obtaining a tomographic image in which a desired tomographic plane is emphasized is known. Also, when performing tomosynthesis imaging, there is a method of moving the grid so that the moiré appearance position is different from each other in the majority of the series of radiographic images in order to make the periodic pattern caused by the grid inconspicuous. It has been proposed (see Patent Document 1).

  In addition, as a technique for suppressing the periodic pattern of a plurality of radiographic images acquired by continuous imaging such as tomosynthesis imaging, an image component caused by a grid is created from one radiographic image among the plurality of radiographic images, and acquired later. There has also been proposed a method for removing the image component from the radiographic image (see Patent Document 2). By using the method described in Patent Document 2, it is not necessary to perform frequency analysis on all the radiographic images, so even if a plurality of radiographic images are acquired, it is caused by the grid included in the radiographic images. The periodic pattern to be performed can be efficiently suppressed.

JP 2009-219556 A JP 2002-330343 A

  However, when a plurality of radiographic images are acquired using a grid, the position at which the periodic pattern included in each radiographic image appears may be slightly different for each radiographic image. For example, when tomosynthesis imaging is performed, since at least one of the position of the X-ray tube and the position of the detector changes for each imaging, the position of the periodic pattern included in each radiographic image is slightly shifted. Here, the technique described in Patent Literature 2 uses only the image component of the periodic pattern created from one of the plurality of radiation images to suppress the periodic pattern of the other radiation image. Yes. For this reason, although the amount of calculation can be reduced, if the position at which the periodic pattern appears in one radiographic image other than the one radiographic image in which the image component of the periodic pattern is created shifts, In an image, a periodic pattern cannot be suppressed with high accuracy.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to accurately and efficiently suppress a periodic pattern caused by a grid when acquiring a plurality of radiation images by imaging using a grid. And

The image processing apparatus according to the present invention results from a grid by performing frequency analysis on one radiographic image among a plurality of radiographic images acquired by continuous imaging using a grid that removes radiation scattering components. A frequency analysis means for acquiring a frequency characteristic of the periodic pattern;
And a suppression unit that suppresses the periodic pattern of at least one radiographic image other than the radiographic image and the radiographic image based on the frequency characteristics.

  Here, acquisition of the radiation image may be performed by using a radiation detector, and a part of the radiation energy is accumulated by irradiation with radiation, and then accumulated by irradiation with excitation light such as visible light or laser light. It may be carried out by using a stimulable phosphor sheet using a stimulable phosphor that emits stimulated emission light according to the emitted radiation energy. When a stimulable phosphor sheet is used, radiation image information is once stored and recorded by irradiating the stimulable phosphor sheet with radiation that has passed through the subject. Radiation images are acquired by generating emission light and photoelectrically converting the stimulated emission light.

  In addition to the tomosynthesis imaging described above, for example, the plurality of radiographic images is for performing energy subtraction processing using two radiographic images obtained by irradiating the subject with radiation of two types of energy having different energies. It can be acquired by energy subtraction imaging, long imaging in which an X-ray tube and a radiation detector are simultaneously translated with respect to a subject to acquire a long radiographic image, moving image imaging, and the like.

  Moreover, the grid may be composed of a plurality of plates provided along the main direction or the sub direction of the radiation detector, for example, as long as it removes the radiation scattering component. It may be composed of a plurality of plates provided to be inclined with respect to the main direction and the sub-direction of the radiation detector.

  A periodic pattern means noise having a periodic pattern included in a radiographic image. For example, it means periodic stripes or moire included in a radiographic image acquired by photographing a subject using a grid.

  In the image processing apparatus according to the present invention, the suppression means creates a filter that extracts frequency components corresponding to the respective periodic patterns of the plurality of radiation images based on the frequency characteristics, and the plurality of radiation images are obtained by the filter. A plurality of filtered radiographic images are obtained by performing a filtering process on each of the radiographic images, and one radiographic image and one radiographic image are obtained by subtracting the plurality of filtered radiographic images from the plurality of radiographic images. It is good also as a means to suppress the periodic pattern of at least 1 other radiographic image other than.

  In the image processing apparatus according to the present invention, the continuous shooting may be tomosynthesis shooting.

The image processing method according to the present invention results from a grid by performing frequency analysis on one radiographic image among a plurality of radiographic images acquired by continuous imaging using a grid that removes radiation scattering components. Get the frequency characteristics of the periodic pattern,
Based on the frequency characteristics, the periodic pattern of one radiographic image and at least one radiographic image other than the one radiographic image is suppressed.

  According to the present invention, frequency analysis is performed on one radiographic image among a plurality of radiographic images acquired by continuous imaging using a grid, and frequency characteristics of a periodic pattern resulting from the grid are acquired. Based on the acquired frequency characteristics, the periodic pattern of at least one radiographic image other than the radiographic image and the radiographic image is suppressed. For this reason, since it is not necessary to acquire frequency characteristics for individual images, it is possible to reduce the amount of calculation for frequency analysis, and as a result, it is possible to efficiently generate periodic patterns included in a plurality of radiation images. Can be suppressed. On the other hand, when continuous imaging is performed using a grid, the position at which the periodic pattern included in each radiographic image appears may be slightly different for each radiographic image. According to the present invention, for each radiation image, a spatial filter for extracting a periodic pattern can be individually created based on the acquired frequency characteristics to suppress the periodic pattern. A periodic pattern caused by the grid included in the image can be accurately suppressed.

1 is a schematic block diagram showing the configuration of a radiological image diagnostic system to which an image processing apparatus according to an embodiment of the present invention is applied. Illustration for explaining tomosynthesis shooting Schematic block diagram showing the configuration of the periodic pattern suppression processing unit Figure showing a small area for frequency analysis Diagram showing an example of frequency spectrum Diagram for explaining peak frequency The figure for demonstrating the suppression process of a periodic pattern Diagram showing frequency spectrum of processed radiographic image A flowchart showing processing performed in the present embodiment

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic block diagram showing a configuration of a radiological image diagnostic system to which an image processing apparatus according to an embodiment of the present invention is applied. As shown in FIG. 1, the radiological image diagnostic system 1 is for performing tomosynthesis imaging, and includes an X-ray tube 2 and a radiation detector 3. The X-ray tube 2 moves along a straight line or an arc by the moving mechanism 4 and irradiates the subject S on the imaging table top 5 with X-rays at a plurality of positions on the moving path. In the present embodiment, the X-ray tube 2 is moved in the direction of arrow A along a straight line.

  Further, a collimator (irradiation field stop) 6 is connected to the X-ray tube 2 so that the operator can set an X-ray range (irradiation range) irradiated to the subject S. Note that when the irradiation range is set using the collimator 6, visible light is irradiated to the subject S via the collimator 6 instead of the X-rays. The visible light is emitted from an irradiation field lamp (not shown) provided in the collimator 6. Thereby, the operator can set the X-ray irradiation range by adjusting the range of visible light irradiated to the subject S using the collimator 6.

  In order to detect X-rays transmitted through the subject S, the radiation detector 3 is disposed so as to face the X-ray tube 2 with the imaging table top plate 5 on which the subject S is placed interposed therebetween. The radiation detector 3 accumulates radiation image information including radiation transmitted through the subject S as an electrostatic latent image, and detects the radiation transmittance distribution as a radiation image by reading the accumulated electrostatic latent image. . The radiation detector 3 is not limited in its configuration as long as it detects radiation and outputs it as image information. For example, the radiation detector 3 may be a TFT solid detector or a light readout solid detector. Also good.

  The radiation detector 3 is moved along a straight line or an arc as necessary by the moving mechanism 7 and detects X-rays transmitted through the subject S at a plurality of positions on the moving path. In the present embodiment, the radiation detector 3 is moved in the direction of arrow B along a straight line. In some cases, imaging is performed by moving only the X-ray tube 2 without moving the radiation detector 3.

  Here, the radiological image diagnosis system 1 is configured such that the grid 8 is detachable between the subject S and the radiation detector 3, so that both imaging with a grid and imaging without a grid are possible. Yes. In the case of shooting with a grid, various types of grids (grid ratio, grid pattern, etc.) can be used. In the grid 8, lead that absorbs radiation and aluminum that transmits radiation are alternately arranged at a pitch of about 4 lines / mm, for example. Further, the lead is installed with a slight change in inclination according to the position so that the radiation passes through the aluminum and enters the radiation detector 3. The grid 8 is moved by the moving mechanism 7 together with the radiation detector 3.

  The radiological image diagnosis system 1 includes an image processing device 10. The image processing apparatus 10 is a computer having a high-definition liquid crystal display that displays images and the like, a keyboard and a mouse that accept input from a user, and a main body that includes a CPU, a memory, a hard disk, a communication interface, and the like. A function that reconstructs multiple radiographic images acquired by tomosynthesis imaging to generate tomographic images, acquires frequency characteristics of periodic patterns due to grids from radiographic images, and further suppresses periodic patterns Have. FIG. 1 is a schematic block diagram showing the configuration of the image processing apparatus 10.

  As illustrated in FIG. 1, the image processing apparatus 10 includes an image acquisition unit 11 and a reconstruction unit 12. The image acquisition unit 10 moves the X-ray tube 2 along a straight line, irradiates the subject S with a plurality of source positions by the movement of the X-ray tube 2, and detects the X-rays transmitted through the subject S by radiation detection. A plurality of radiation images at a plurality of moving radiation source positions are detected by the detector 3. Specifically, as shown in FIG. 2, a plurality of radiation images are acquired by moving the X-ray tube 2 in the direction of arrow A and irradiating the subject S with radiation from a plurality of different imaging directions. To do. In FIG. 2, the radiation detector 3 and the grid 8 are not moved for explanation.

  The reconstruction unit 12 reconstructs a plurality of processed radiographic images that have been subjected to processing for suppressing a periodic pattern as will be described later, thereby generating a tomographic image in which a desired cross section of the subject S is emphasized. Specifically, the reconstruction unit 12 reconstructs these processed radiographic images using a backprojection method such as a simple backprojection method or a filtered backprojection method, and generates a tomographic image.

  The radiological image diagnosis system 1 includes an operation unit 13, a display unit 14, and a storage unit 15. The operation unit 13 includes a keyboard, a mouse, or a touch panel type input device, and receives an operation on the system 1 by an operator. It also accepts input of various information such as imaging conditions and information correction instructions necessary for performing tomosynthesis imaging.

  The display unit 14 is a display device such as a liquid crystal monitor, and displays a radiation image acquired by the image acquisition unit 11 and a tomographic image reconstructed by the reconstruction unit 12 as well as messages necessary for operation. The display unit 14 may include a speaker that outputs sound.

  The storage unit 15 includes a hard disk that stores radiation images acquired by imaging, a ROM that stores various parameters for setting imaging conditions necessary for operating the system 1, a RAM that serves as a work area, and the like.

  The radiological image diagnosis system 1 also includes a periodic pattern suppression processing unit 16. FIG. 3 is a schematic block diagram showing the configuration of the periodic pattern suppression processing unit 16. As shown in FIG. 3, the periodic pattern suppression processing unit 16 includes a frequency analysis unit 30 and a suppression unit 32.

  The frequency analysis unit 30 performs frequency analysis on one radiographic image (hereinafter, referred to as a reference radiographic image B1) among a plurality (n) of radiographic images acquired by the image acquisition unit 11, and performs a reference radiographic image B1. The frequency characteristic C0 of the periodic pattern resulting from the grid 8 is acquired. Here, the periodic pattern suppression processing unit 16 may sequentially process the acquired radiographic images while performing imaging, or may perform the processing after acquiring a plurality of radiographic images. In the former case, the radiation image acquired by the first imaging is used as the reference radiation image B1. In the latter case, a predetermined radiographic image such as a radiographic image acquired by the first imaging, a radiographic image acquired by the final imaging, or a radiographic image acquired by imaging at the intermediate position is used as the reference radiographic image B1.

  FIG. 4 is a diagram for explaining the frequency analysis. As illustrated in FIG. 4, the frequency analysis unit 30 sets a 3 × 9 rectangular small region A10 having a long side in the x direction on the reference radiation image B1. Here, the small region A10 includes nine line-shaped regions having a length of 1024 pixels in the x direction in FIG. 4 at intervals of three pixels. And the frequency analysis part 30 performs a Fourier transform with respect to the image signal of each linear area | region in the small area | region A10, and calculates a frequency spectrum. Then, the nine frequency spectra calculated in the small region A10 are averaged, the averaged frequency spectrum calculated for the 3 × 9 small region A10 is further averaged, and the frequency in the x direction of the reference radiation image B1 Calculate the spectrum. Note that frequency analysis using the small region A10 having a long side in the x direction is referred to as frequency analysis in the x direction.

  Similarly, a plurality of small regions are set for the y direction, and the frequency spectrum for the y direction is calculated. Note that frequency analysis using a small region having a long side in the y direction is referred to as frequency analysis in the y direction.

  FIG. 5 is a diagram illustrating an example of a frequency spectrum. The frequency spectrum shown in FIG. 5 gradually decreases as the spatial frequency changes from a low frequency to a high frequency, and has a peak at a certain spatial frequency. The spatial frequency having a peak in the frequency spectrum calculated in this way matches the spatial frequency of the periodic pattern caused by the grid. In addition, when the pitch of the grid 8 is the x direction at the time of photographing, the peak frequency of the periodic pattern due to the grid appears in the frequency spectrum calculated by the frequency analysis in the x direction, but was calculated by the frequency analysis in the y direction. It will not appear in the frequency spectrum. Conversely, when the pitch of the grid 8 is in the y direction at the time of shooting, the peak frequency of the periodic pattern due to the grid appears in the frequency spectrum calculated by the frequency analysis in the y direction, but is calculated by the frequency analysis in the x direction. It will not appear in the frequency spectrum.

  The frequency analysis unit 30 acquires a frequency component that becomes a peak in the calculated frequency spectrum as the frequency characteristic C0 of the periodic pattern. In addition to the frequency characteristic C0, information on the direction in which the frequency analysis in which the peak frequency appears is also acquired. For example, as shown in FIG. 4, when frequency analysis in the x direction is performed, if a peak frequency appears in the frequency spectrum, information on the “x direction” is displayed, and frequency analysis in the y direction is performed on the frequency spectrum. When the peak frequency appears, information in the “y direction” is acquired.

  When the grid pitch direction is inclined with respect to the radiation image, a peak appears in the wave number spectrum obtained by both the frequency analysis in the x direction and the frequency analysis in the y direction. Thus, when a peak appears in the frequency spectrum obtained by both the frequency analysis in the x direction and the frequency analysis in the y direction, the frequency analysis unit 30 acquires information on both the “x direction” and the “y direction”. To do.

  The suppression unit 32, based on the frequency characteristic C0 acquired by the frequency analysis unit 30 and the information on the direction in which the frequency analysis is performed, a plurality of radiation images Bi (i = 2) other than the reference radiation image B1 and the reference radiation image B1. A filter that extracts only the frequency component of the periodic pattern is created for each of the .about.n), and a filtering process is performed on each of the reference radiation image B1 and the other radiation images Bi with the created filter. The frequency characteristic of the radiographic image obtained by this filtering process includes only frequency components corresponding to the periodic pattern as shown in FIG. Then, the processed radiation image BSj (j = 1 to n) is acquired by subtracting the filtered radiation image from the reference radiation image B1 and the other radiation images Bi.

  The periodic pattern suppression process for each of the reference radiation image B1 and the radiation image B2 of the other radiation images Bi will be described with reference to FIG. As shown in FIG. 7, the reference radiation image B1 and the radiation image B2 include a periodic pattern due to the grid, and the positions (that is, the phase of the periodic pattern) are the reference radiation image B1 and the other radiation images B2. Is shifted. On the other hand, the frequency characteristics of the periodic patterns included in the reference radiation image B1 and the other radiation images B2 are the same. In FIG. 7, since the periodic patterns are arranged in the x direction, the frequency analysis unit 30 acquires information in the x direction.

  For this reason, when a filter is created and filtered for each of the reference radiation image B1 and the radiation image B2 based on the frequency characteristic C0 acquired by the frequency analysis unit 30, it corresponds to a periodic pattern arranged in the x direction. Radiation images M1 and M2 including only frequency components to be acquired are acquired. In the radiographic images M1 and M2, the periodic patterns appear at the same positions in the reference radiographic image B1 and the radiographic image B2, respectively. Therefore, the processed radiographic images BS1 and BS2 obtained by subtracting the radiographic images M1 and M2 from the reference radiographic image B1 and the radiographic image B2, respectively, have the periodic patterns removed. As a result, the frequency spectrum of the processed radiation image BSj does not have a peak in the frequency component of the periodic pattern caused by the grid, as shown in FIG.

  The radiological image diagnosis system 1 includes a control unit 17. The control unit 17 controls each unit of the system 1 according to an instruction from the operation unit 13.

  Next, processing performed in the present embodiment will be described. FIG. 9 is a flowchart showing processing performed in the present embodiment. First, in response to an instruction from the operation unit 13, the image acquisition unit 11 performs tomosynthesis imaging, acquires a plurality of radiation images, and stores them in the storage unit 15 (step ST1). Then, in the periodic pattern suppression processing unit 16, the frequency analysis unit 30 performs frequency analysis on the reference radiation image B1 among the plurality of radiation images, and acquires the frequency characteristic C0 of the periodic pattern in the reference radiation image B1. (Step ST2).

  And the suppression part 32 produces the filter which removes the frequency component of the periodic pattern resulting from a grid based on the acquired frequency characteristic C0 about each of reference | standard radiographic image B1 and other radiographic image Bi (step ST3). ) A process for suppressing the periodic pattern is performed on each of the reference radiation image B1 and the other radiation image Bi with the created filter to obtain a processed radiation image BSj (step ST4), and the process is terminated.

  Thus, in the present embodiment, the frequency analysis is performed on one reference radiation image B1 among a plurality of radiation images acquired by continuous imaging such as tomosynthesis imaging, and the periodic pattern resulting from the grid Frequency characteristic C0 is acquired, and the periodic pattern is suppressed based on the acquired frequency characteristic C0 with respect to the reference radiation image B1 and the other radiation images Bi. For this reason, since it is not necessary to acquire the frequency characteristic C0 for each image, the amount of calculation for the frequency analysis can be reduced, and as a result, the periodic patterns included in a plurality of radiation images can be efficiently used. It can be well suppressed. In addition, for each radiographic image, a filter that extracts a periodic pattern can be created based on the acquired frequency characteristic C0 to suppress the periodic pattern. The resulting periodic pattern can be accurately suppressed.

  In the above embodiment, the image processing apparatus according to the present invention is applied to the radiographic image diagnosis system 1 that performs tomosynthesis imaging. However, any system that acquires a plurality of radiographic images by continuous imaging may be used. It can also be applied to the system. For example, in the case of performing energy subtraction imaging for performing energy subtraction processing using two radiation images obtained by irradiating a subject with radiation of two types of energy having different energies, the two radiations are continuously provided. An image is acquired. Therefore, the frequency analysis unit 30 performs frequency analysis on one of the two radiographic images to obtain the frequency characteristic C0, and based on this, the periodic pattern of the two radiographic images is obtained. If the suppression process is performed, it is possible to reduce the amount of calculation of frequency analysis for other radiographic images.

  In addition, a plurality of radiographic images can be continuously acquired even by long imaging, moving image shooting, etc., in which an X-ray tube and a radiation detector are simultaneously translated with respect to a subject to acquire a long radiographic image. Can do. For this reason, also by suppressing the periodic pattern of a plurality of radiographic images based on the frequency characteristic C0 acquired by the frequency analysis unit 30 in the same manner as described above for the radiographic images acquired by the long imaging and the video imaging. The amount of calculation for frequency analysis can be reduced.

  Here, in the case of acquiring a plurality of radiographic images by moving image capturing, the periodic pattern of the first frame and subsequent frames is suppressed based on the frequency characteristic C0 acquired by frequency analysis for the first frame. However, for example, the frequency analysis unit 30 may perform frequency analysis for each predetermined number of frames to obtain the frequency characteristic C0, and may be used to suppress a periodic pattern of the predetermined number of frames. The predetermined number may be 2. In this case, the frequency characteristic C0 is acquired every two frames, and based on this, the periodic pattern of the two frames is suppressed.

  Moreover, in the said embodiment, although the process which suppresses a periodic pattern is performed with respect to all the other several radiographic images Bi, a periodic pattern is suppressed only with respect to at least one required radiographic image. Processing may be performed.

  In the above embodiment, the radiation detector 3 is used to acquire a radiation image of the subject S. However, a part of the radiation energy is accumulated by irradiation of radiation, and then excitation of visible light, laser light, or the like is performed. A radiation image may be acquired by using a stimulable phosphor sheet that uses a stimulable phosphor that emits photostimulated luminescence according to the stored radiation energy. When a stimulable phosphor sheet is used, radiation image information is temporarily accumulated and recorded by irradiating the stimulable phosphor sheet with radiation that has passed through the subject. A radiographic image is acquired by generating exhausted light and photoelectrically converting the stimulated light.

DESCRIPTION OF SYMBOLS 1 Radiographic image diagnosis system 2 X-ray tube 3 Radiation detector 8 Grid 10 Image processing apparatus 16 Periodic pattern suppression process part 30 Frequency analysis part 32 Suppression part

Claims (4)

By performing frequency analysis on one radiographic image among a plurality of radiographic images acquired by continuous imaging using a grid that removes radiation scattering components, the frequency characteristics of the periodic pattern caused by the grid are obtained. Frequency analysis means to obtain;
An image processing apparatus comprising: a suppression unit configured to suppress the periodic pattern of the one radiation image and at least one other radiation image other than the one radiation image based on the frequency characteristic.   The suppression means creates a filter that extracts a frequency component corresponding to each of the periodic patterns of the plurality of radiographic images based on the frequency characteristics, and each of the plurality of radiographic images by the filter. By performing filtering processing, a plurality of filtered radiographic images are obtained, and by subtracting the plurality of filtered radiographic images from the plurality of radiographic images, the one radiographic image and the one radiographic image other than the one radiographic image The image processing apparatus according to claim 1, wherein the image processing apparatus suppresses the periodic pattern of at least one other radiographic image.   The image processing apparatus according to claim 1, wherein the continuous photographing is tomosynthesis photographing. By performing frequency analysis on one radiographic image among a plurality of radiographic images acquired by continuous imaging using a grid that removes radiation scattering components, the frequency characteristics of the periodic pattern caused by the grid are obtained. Acquired,
An image processing method for suppressing the periodic pattern of the one radiographic image and at least one radiographic image other than the one radiographic image based on the frequency characteristic.

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