JP5939163B2 - Radiography equipment - Google Patents

Description

  The present invention relates to a radiographic apparatus that obtains a radiographic image, and more particularly to a technique for removing scattered radiation using scattered radiation removing means.

  This type of radiation imaging apparatus is used in medical radiographic image diagnosis apparatuses, industrial radiographic image inspection apparatuses, and the like. As an industrial radiographic image inspection apparatus, there are a nondestructive inspection apparatus, an X-ray inspection apparatus for inspecting an integrated circuit of a substrate, a solder joint, and the like. Examples of the medical radiological image diagnostic apparatus include a medical X-ray fluoroscopic apparatus that performs X-ray fluoroscopic imaging using a subject as a human body. In the following, X-rays will be described as an example of radiation, and a medical X-ray fluoroscopic apparatus will be described as an example of a radiation imaging apparatus.

  In the case of a medical X-ray fluoroscopic apparatus, scattered X-rays are generated when X-rays pass through a subject that is a human body. The image quality deteriorates due to the scattered X-rays from the subject. Therefore, in order to reduce deterioration in image quality due to scattered X-rays, a grid (scattered radiation removing means) configured by alternately arranging grid foils that absorb scattered radiation and intermediate layers that transmit radiation is used. The grid foil is formed of a material that absorbs radiation represented by X-rays such as lead, and the intermediate layer is an intermediate material that transmits radiation represented by X-rays such as aluminum, organic materials, and graphite. (Spacer). However, when X-rays pass through the intermediate layer, X-rays other than scattered X-rays (direct X-rays) are also absorbed by the intermediate substance (spacer). Therefore, an air grid that reliably transmits X-rays other than scattered X-rays (direct X-rays) by using an intermediate layer as a gap has recently been used as a grid.

  By the way, in the portion where X-rays are directly blocked by the grid foil, the foil shadows by the grid foil are reflected and superimposed on the X-ray image (captured image) as a fine lattice pattern. In recent years, flat panel X-ray detectors (FPDs) have been used as X-ray detectors, and the use of FPD has improved the spatial resolution and sensitivity of captured images, and their use is rapidly increasing. On the other hand, as the spatial resolution and sensitivity of the X-ray detector improve, the foil shadow becomes clearer and interferes with image diagnosis. Therefore, in order to remove this, a technique of removing a foil shadow caused by the grid foil by image processing using frequency conversion is used (for example, see Patent Document 1).

  On the other hand, there is a synchronous grid in which a grid foil is arranged so that a foil shadow is reflected at an integral multiple of a distance (pixel pitch) between pixels of a captured image (see, for example, Patent Document 2). The above-described air grid is used as the synchronous grid. When an air grid is used as the synchronous grid, as described above, spacers such as aluminum, organic substances, and graphite are not used. Therefore, direct X-ray detection efficiency can be improved. On the other hand, the grid foil is not supported by the spacer for manufacturing and structural reasons, and a slight distortion may occur in the foil shadow due to a slight distortion in the linear grid foil. Therefore, the above-described grid foil shadow removal method using frequency conversion cannot sufficiently remove the foil shadow of the grid pattern.

  Therefore, a method for removing the foil shadow on the synchronous grid has been proposed (see, for example, Patent Document 3). In the foil shadow removal method disclosed in Patent Document 3, an X-ray image without a subject (also referred to as an “air image”) without placing a subject in advance on an X-ray fluoroscopic apparatus equipped with a synchronous grid. And an image representing a direct ray transmission coefficient (“direct ray transmittance” in Patent Document 3) and a scattered ray transmission coefficient based on these images. Calculate the image. Next, an X-ray image of a desired subject (simply abbreviated as “subject image”) is taken.

  Then, solve the simultaneous equations composed of the direct ray transmission coefficient image, scattered ray transmission coefficient image and individual pixel values of the subject image, and calculate the direct X-ray image and scattered X-ray image of the subject. To do. The direct X-ray image here represents the direct X-ray image immediately before passing through the subject and entering the synchronous grid, and therefore an image from which the foil shadow has been removed can be obtained.

  However, for example, in a C-arm X-ray fluoroscopic apparatus having an X-ray tube held at one end and an FPD held at the other end, the C-arm rotates or moves depending on the weight of the X-ray tube or FPD. As a result, a slight deflection or the like occurs in the C-arm. Due to this bending or the like, the position of the X-ray tube focal point with respect to the FPD is slightly moved (about 2 mm at the maximum).

  The image of the direct ray transmission coefficient and the image of the scattered ray transmission coefficient in which the X-ray tube focus is moved slightly change with respect to the image of the direct ray transmission coefficient and the image of the scattered ray transmission coefficient prepared in advance. As a result, when solving the simultaneous equations composed of the pixel values of the image of the direct ray transmission coefficient and the image of the scattered ray transmission coefficient measured in advance and the subject image with the X-ray tube focus moved, the grid foil shadow is obtained. Not only can it be removed well, but artifacts also appear. Therefore, the foil shadow removal method in Patent Document 3 cannot be applied to an X-ray fluoroscopic apparatus in which the position of the X-ray tube focal point with respect to the FPD changes.

On the other hand, a technique has been proposed that can remove a foil shadow even when the position of the X-ray tube focal point changes with respect to the FPD (see, for example, Patent Document 4). The procedure according to Patent Document 4 is as follows (1) to (4).
That is, (1) The X-ray tube focal point is moved little by little along the grid foil arrangement direction (direction perpendicular to the direction in which the grid foil extends) on a plane parallel to the FPD detection surface. Thus, a plurality of air images (X-ray images without a subject) are captured in advance. (2) After photographing the subject image, the subject row data is selected from one row in the subject image, and the same number of air row data from a plurality of air images in the row corresponding to the selected subject row data. Create each. (3) A correlation value related to the grid foil shadow is calculated from one subject row data and a plurality of air row data. (4) The air image with the maximum correlation value is selected, and the above-described grid foil shadow removal calculation is performed.

  The correlation values calculated in steps (1) to (4) above are the same when the X-ray tube focal point where the subject row data is imaged and the X-ray tube focal point where the air row data is imaged. And the smaller the deviation of the X-ray tube focal point position, the smaller. Therefore, by comparing the correlation values, it is possible to select air line data captured at the same position or the closest position, that is, a corresponding air image.

  By using this method, even when a small positional deviation (about 2 mm at the maximum) of the X-ray tube focus with respect to the FPD that cannot be controlled at the time of imaging of the subject occurs, as in a C-arm X-ray fluoroscopic apparatus. If an air image of each X-ray tube focal point moved in advance at a plurality of appropriate intervals is captured and held, the optimum air image can be selected in real time after the subject is imaged. It is possible to carry out foil shadow removal without any problem.

  Note that an image of a direct ray transmission coefficient of the subject from the subject image and the air image, that is, the direct X-ray transmittance (“CP” in one pixel when the direct X-ray dose in the absence of the grid is set to “1.0” A method for obtaining a value is also proposed (for example, see Patent Document 5).

JP 2000-83951 A JP 2002-257939 A JP 2009-172184 A JP 2011-101686 A JP 2010-213902 A

However, in the case of Patent Document 4 described above, there are the following restrictions.
That is, there is a restriction that the positional relationship between the FPD and the grid is the same as when shooting an air image and when shooting a subject. That is, at the time of subject imaging, the positional relationship between the FPD and the grid at the time of air image capturing must be maintained. On the other hand, in an apparatus that involves rotation or movement of an X-ray tube or an F-arm that holds an FPD, such as a C-arm X-ray fluoroscopic apparatus, the position of the grid may shift with respect to the FPD.

  When the grid position is shifted with respect to the FPD, the profile of the grid foil shadow is as shown in the lower part of FIG. 7 even if the X-ray focal point positions are the same between the air image capturing and the object capturing. Change. Therefore, there is no correlation between the X-ray focal position and the row data (grid foil shadow profile). As a result, in the method of Patent Document 4 on the premise that the row data (grid foil shadow profile) is the same if the X-ray focal point positions are the same at the time of air imaging and subject imaging, The optimal air image cannot be selected.

Therefore, in order to apply the method according to Patent Document 4, not only the positional deviation of the X-ray tube focus with respect to the FPD but also the positional deviation of the grid with respect to the FPD must be considered. Specifically, when N _A is the number of X-ray tube focal position misalignment patterns with respect to the FPD and N _B is the number of grid misalignment patterns with respect to the FPD, N _A × N _B number of airs Images must be taken in advance. For this reason, an increase in the number of times of taking an air image and an increase in a memory area for holding the air image are inevitable. Thus, there exists a subject that the number of patterns increases according to the position shift of the grid with respect to FPD.

  The present invention has been made in view of such circumstances, and even if a positional deviation of the scattered radiation removing means with respect to the radiation detecting means occurs, the scattered radiation is prepared without preparing the number of patterns corresponding to the positional deviation. And it aims at providing the radiography apparatus which can remove a grid foil.

As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
That is, in Patent Document 4, grid foil shadow patterns (when grid foil shadows are taken) when an air image is photographed (when photographing is performed without the subject interposed) and when the subject is photographed (when photographing is performed with the subject interposed). The profile was compared with each other to calculate the correlation value. As shown in the equations of paragraph numbers “0064” to “0066”, the sum of squares of the pixel values for all grid foils was obtained as the correlation value. Yes. When such a sum of squares is obtained as a correlation value, when the position of the grid (scattered radiation removing means) shifts and changes with respect to the FPD (radiation detection means), the degree of correlation in each grid foil is not known.

  So, I changed my mind and focused on each grid foil. Specifically, the absorption ratio of direct radiation in each grid foil at the time of air image capturing (when imaged without the subject interposed) and when the subject is imaged (when imaged with the subject interposed), respectively. And a distance function based on these absorption ratios is calculated. This distance function is a parameter (physical quantity) indicating the degree of correlation between the radiographic image without the subject and the radiographic image of the subject and the distance of the grid foil from the ideal value. Based on this distance function, if the selection is made from the absorption ratio of the direct radiation of each grid foil in a plurality of radiation images without an object, the positional deviation of the grid (scattered radiation removal means) relative to the FPD (radiation detection means) occurs. However, it is possible to select the absorption ratio only with the number of conventional patterns without preparing the number of patterns according to the positional deviation, and based on the selected absorption ratio and the radiation image of the subject, the scattered radiation and the grid foil I got the knowledge that can be removed.

The present invention based on such knowledge has the following configuration.
That is, the radiation imaging apparatus according to the present invention is a radiation imaging apparatus that obtains a radiation image, and is configured by arranging a radiation source that irradiates radiation and a radiation detection element that detects the irradiated radiation vertically and horizontally. Scattered radiation removing means provided on the detection side of the detection means and the radiation detection means, and configured to arrange grid foil that absorbs scattered radiation in parallel in at least one of the vertical and horizontal directions of the radiation detection element; An image generation unit configured to generate a radiographic image based on a radiation detection signal detected by the radiation detection unit, and the radiation imaging apparatus further includes a subject between the radiation source and the radiation detection unit. Direct radiation of each grid foil in a plurality of non-subject radiation images based on radiation detection signals detected without a subject. A first absorption ratio calculating means for calculating a yield ratio as a first absorption ratio; and a radiation detection signal detected in a state of the subject with the subject interposed between the radiation source and the radiation detecting means. A second absorption ratio calculating means for calculating the absorption ratio of the direct radiation of each grid foil in the radiation image of the subject based on the second absorption ratio; the first absorption ratio calculated by the first absorption ratio calculating means; and Based on the second absorption ratio calculated by the second absorption ratio calculating means, the degree of correlation between the radiographic image without the subject and the radiographic image of the subject is shown, and the distance of the grid foil from the ideal value is shown. A distance function calculating unit that calculates a distance function to be indicated, and the first absorption ratio that minimizes the distance function calculated by the distance function calculating unit is selected from a plurality of first absorption ratios Based on the selection means, the first absorption ratio selected by the selection means, and the radiographic image of the subject, a foil shadow by the grid foil of the radiographic image of the subject is removed, and the direct radiation of the subject is And image processing means for performing image processing for calculating a component and acquiring a radiation image from which the foil shadow has been removed.

  [Operation / Effect] According to the radiation imaging apparatus of the present invention, the first absorption ratio calculating means is detected in the absence of the subject without interposing the subject between the radiation source and the radiation detecting means. The direct radiation absorption ratio of each grid foil in a plurality of non-subject radiation images based on the radiation detection signal is calculated as the first absorption ratio. On the other hand, the second absorption ratio calculation means includes each subject in the radiation image of the subject based on the radiation detection signal detected with the subject interposed between the radiation source and the radiation detection means. The direct radiation absorption ratio is calculated as the second absorption ratio.

  Then, the distance function calculating means calculates a distance function based on the first absorption ratio calculated by the first absorption ratio calculating means and the second absorption ratio calculated by the second absorption ratio calculating means. As described above, this distance function is a parameter indicating the degree of correlation between the radiographic image without the subject and the radiographic image of the subject and the distance of the grid foil from the ideal value.

  The selection means selects the first absorption ratio that minimizes the distance function calculated by the distance function calculation means from the plurality of first absorption ratios. That is, as the correlation between the radiographic image without the subject and the radiographic image of the subject increases, the distance approaches the ideal value and the distance function approaches “0”. Therefore, the first absorption ratio that minimizes the distance function is selected as the optimal first absorption ratio. Then, based on the first absorption ratio selected by the selection means and the radiographic image of the subject, the image processing means removes the foil shadow by the grid foil of the radiographic image of the subject, and direct radiation of the subject Image processing is performed to calculate a component and acquire a radiation image from which a foil shadow is removed.

  As described above, by selecting from a plurality of first absorption ratios based on the distance function, even if a positional deviation of the scattered radiation removing unit relative to the radiation detecting unit occurs, the number of patterns corresponding to the positional deviation is prepared. Scattered radiation and grid foil can be removed.

  In the radiographic apparatus according to the present invention, the first absorption ratio calculating means, the second absorption ratio calculating means, and the distance function calculating means are specifically calculated as follows. That is, the first absorption ratio calculating means calculates a direct radiation absorption ratio of each grid foil in each pixel of a plurality of radiation images without a subject, and among a plurality of radiation image pixels without a subject, The pixel that is least affected by the shadow of each grid foil is regarded as one unit, and the sum of the absorption ratios of each grid foil is calculated for each unit pixel. Almost the same as the first absorption ratio calculation means, the second absorption ratio calculation means calculates the direct radiation absorption ratio of each grid foil in each pixel of the subject radiation image, and among the pixels of the subject radiation image. The sum of the absorption ratios of the respective grid foils is calculated for each unit pixel, with the unit between the pixels that are least affected by the foil shadow caused by each grid foil. The distance function calculating means is based on the sum of the absorption ratios of the grid foils calculated by the first absorption ratio calculating means and the sum of the absorption ratios of the grid foils calculated by the second absorption ratio calculating means. Is calculated.

  In the radiographic apparatus according to these inventions, it is preferable to include a change rate calculation means as described below. That is, the absorption ratio of the direct radiation of each grid foil in the plurality of radiation images without the subject calculated by the first absorption ratio calculation means, and each of the radiation images of the subject calculated by the second absorption ratio calculation means Based on the direct radiation absorption ratio of the grid foil, the change rate calculation means calculates the change rate of the radiation image of the subject with respect to the radiation image without the subject. The rate of change is ideally “1”. Since the distance function calculating means calculates the distance function based on the change rate calculated by the change rate calculating means, the change rate is “1” as the correlation between the radiographic image without the subject and the radiographic image of the subject increases. As the distance approaches, the distance approaches the ideal value, and the distance function approaches “0”. In this way, it is possible to calculate the distance function using the change rate.

  In particular, when calculating the distance function using the rate of change, when using scattered radiation removing means configured by arranging grid foils in parallel in only one of the longitudinal and lateral directions of the radiation detection element (for example, the row direction) Useful for. In this structure for removing scattered radiation, the grid foil does not cross in the direction in which the grid foil extends (the grid foil extends in the column direction when the grid foil is arranged in parallel to the row direction). Statistical error (ie noise) occurs in the direction of Therefore, in order to reduce the influence of image noise, the addition value calculation means adds or averages the change rates calculated by the change rate calculation means along the direction in which the grid foil extends. An arithmetic average (also referred to as “addition average”) is performed, and the addition or an addition value by the arithmetic average is calculated. The distance function calculation means calculates the distance function based on the addition value calculated by the addition value calculation means, thereby reducing the influence of image noise.

  The radiographic apparatus according to these inventions described above includes first absorption ratio storage means for writing and storing the first absorption ratio calculated by the first absorption ratio calculation means, and stored in the first absorption ratio storage means. It is preferable to read out and use the first absorption ratio for various calculations. If a radiographic image without a subject is written and stored, each time the first absorption ratio is calculated by the first absorption ratio calculating means, the radiographic image without the subject must be read and the first absorption ratio calculated. However, there is an effect that the number of calculations can be reduced by calculating, writing and storing the first absorption ratio in advance. In addition, in the memory area for writing and storing the radiation image without the subject, the size of all pixels is necessary, but in the case of the memory area for writing and storing the first absorption ratio, the size of only the grid foil There is also an effect that the size of the memory area of the first absorption ratio storage means can be reduced.

  According to the radiation imaging apparatus according to the present invention, the absorption ratio of direct radiation in each grid foil between the time when imaging is performed without the subject and the time when imaging is performed with the subject is calculated. A distance function based on the ratio is calculated. This distance function is a parameter indicating the degree of correlation between the radiographic image without the subject and the radiographic image of the subject and the distance of the grid foil from the ideal value. If the absorption ratio (first absorption ratio) of the direct radiation of each grid foil in the radiation image without an object having the smallest distance function is selected from the plurality of first absorption ratios, even if the scattered radiation removing means for the radiation detecting means is selected. Even if the positional deviation occurs, the absorption ratio can be selected only by the conventional number of patterns without preparing the number of patterns according to the positional deviation, and the scattered radiation and the grid foil can be removed.

1 is a schematic configuration diagram and a block diagram of an X-ray imaging apparatus according to an embodiment. It is a schematic diagram of the detection surface of a flat panel X-ray detector (FPD). It is the schematic of a grid. It is a block diagram of a specific image generation / processing unit according to the embodiment. It is a flowchart of a series of image processing. It is the schematic which showed typically an example of the positional relationship of a peak pixel coordinate and foil shadow coordinate, CP value (direct X-ray transmittance), and foil shadow amount. It is the schematic which showed typically the change of the grid foil shadow by the position shift | offset | difference of FPD and a grid, and the profile of a direct X-ray transmittance.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic configuration diagram and a block diagram of an X-ray imaging apparatus according to an embodiment, FIG. 2 is a schematic diagram of a detection surface of a flat panel X-ray detector (FPD), and FIG. FIG. In this embodiment, X-rays will be described as an example of radiation, and a C-arm provided with a C-arm for implementation in, for example, an apparatus (CVS: cardiovascular systems) used for cardiovascular diagnosis as a radiographic apparatus. An explanation will be given by taking an X-ray fluoroscopic apparatus as an example. Further, as an example of the scattered radiation removing means, an air grid having an intermediate layer as a gap and a focusing grid in which a grid foil is arranged along a ray connecting the focal point of the X-ray tube will be described. This air grid is also a synchronous grid in which grid foils are arranged so that a foil shadow is reflected at an integral multiple of a distance (pixel pitch) between pixels of a captured image.

  As shown in FIG. 1, the X-ray imaging apparatus according to this embodiment includes a top plate 1 on which a subject M is placed, an X-ray tube 2 that irradiates X-rays, and a flat that detects the irradiated X-rays. A panel type X-ray detector (hereinafter abbreviated as “FPD”) 3 and a grid foil 4a (see FIG. 3 etc.) provided on the detection side of the FPD 3 and absorbing scattered X-rays d (see FIG. 2) and a grid 4 arranged in parallel in at least one of the vertical and horizontal directions (row direction in this embodiment). The X-ray tube 2 corresponds to the radiation source in the present invention, the flat panel X-ray detector (FPD) 3 corresponds to the radiation detection means in the present invention, and the grid 4 corresponds to the scattered radiation removal means in the present invention. Equivalent to.

In addition, the X-ray imaging apparatus includes a C-arm 5 that holds the X-ray tube 2 at one end and holds the FPD 3 together with the grid 4 at the other end. In FIG. 1, the C-arm 5 is formed in a curved shape (curved in a C shape) in the body axis direction of the subject M. The C arm 5 is held by the C arm 5 by rotating around the axis of the rotation center axis orthogonal to the body axis of the subject M along the C arm 5 itself (arm feed θ _{S in} FIG. 1). The X-ray tube 2, the FPD 3 and the grid 4 can also rotate in the same direction. Further, the C-arm 5 rotates around the body axis of the subject M (arm rotation θ _{R in} FIG. 1), so that the X-ray tube 2, the FPD 3 and the grid 4 can also rotate in the same direction. It is.

  Specifically, the C arm 5 is held on a base 6 fixedly arranged on the floor surface via a support column 7 and an arm holding unit 8. The support column 7 can rotate around the axis of the vertical axis with respect to the base 6, and by this rotation, the X-ray tube 2, the FPD 3 and the grid 4 rotate in the same direction together with the C-arm 5 held by the support column 7. Is possible. Further, by holding the arm holding unit 8 with respect to the column 7 so as to be rotatable around the axis of the body axis of the subject M, the C-arm 5 held by the arm holding unit 8 together with the X-ray tube 2, the FPD 3 and The grid 4 can also rotate in the same direction. Further, the C arm 5 is held by the arm holding unit 8 so as to be rotatable around the axis of the rotation center axis orthogonal to the body axis of the subject M, whereby the X-ray tube 2, the FPD 3 and the grid together with the C arm 5. 4 can also rotate in the same direction.

  Further, the FPD 3 is moved closer to and away from the X-ray irradiation axis connecting the X-ray tube 2 and the FPD 3, or moved closer to and away from the focusing line direction (grid foil arrangement direction) perpendicular to the irradiation axis. Configure. The SID is changed for each Zf by approaching / separating along the irradiation axis. The SID is a distance (SID: Source Image Distance) from the focal position in the perpendicular direction to the FPD 3 when a perpendicular is drawn from the focal position of the X-ray tube 2 to the FPD 3. Even under the condition that the positional relationship between the X-ray tube 2, the FPD 3 and the grid 4 is constant, the C arm 5 is slightly bent due to the rotation and movement of the C arm 5 due to the weight of the X-ray tube 2 and the FPD 3. May occur, and the positional relationship between the X-ray tube 2, the FPD 3 and the grid 4 may be shifted.

  Furthermore, the X-ray imaging apparatus generates an X-ray image based on an X-ray detection signal detected by the FPD 3 and performs various image processing, an object image, an air image, A memory unit 12 for writing and storing data such as an absorption ratio, a change rate, an added value, a distance function, an X-ray image from which foil shadows are removed, an input unit 13 for inputting data and commands, and image generation / processing The display part 14 which displays the image obtained in the part 11 and the controller 15 which controls these collectively are provided. In addition, a high voltage generating unit for generating a high voltage and supplying a tube current and a tube voltage to the X-ray tube 2 is provided. However, since it is not a feature portion or a configuration related to the feature portion of the present invention, Omitted.

  The memory unit 12 writes and stores the data obtained by the image generation / processing unit 11 via the controller 15, reads out the data as necessary, and stores these data on the display unit 14 via the controller 15. Send in and display. The memory unit 12 includes a storage medium represented by ROM (Read-only Memory), RAM (Random-Access Memory), a hard disk, and the like. In particular, in this embodiment, the memory unit 12 includes a first absorption ratio memory area 12a (see FIG. 4) and a second image memory area 12b (see FIG. 4) which will be described later.

  The input unit 13 sends data and commands input by the operator to the controller 15. The input unit 13 includes a pointing device represented by a mouse, a keyboard, a joystick, a trackball, a touch panel, and the like. The display unit 14 includes a monitor.

  The image generation / processing unit 11 and the controller 15 described above are configured by a central processing unit (CPU) or the like. Data obtained by the image generation / processing unit 11 is written to the memory unit 12 via the controller 15 and stored or sent to the display unit 14 for display. A specific configuration of the image generation / processing unit 11 will be described later in detail.

  As shown in FIG. 2, the FPD 3 is configured by arranging a plurality of X-ray detection elements d sensitive to X-rays in a two-dimensional matrix (vertical and horizontal) on the detection surface. The X-ray detection element d detects X-rays by converting X-rays transmitted through the subject M into X-ray detection signals (electrical signals), temporarily storing them, and reading the stored X-ray detection signals. To do. The X-ray detection signals respectively detected by the respective X-ray detection elements d are converted into pixel values corresponding to the X-ray detection signals, and the pixel values are assigned to the pixels respectively corresponding to the positions of the X-ray detection elements d. Thus, the image generation unit 21 (see FIG. 4) of the image generation / processing unit 11 generates an X-ray image.

  As shown in FIG. 3, the grid 4 is configured by alternately arranging grid foils 4 a that absorb scattered X-rays and intermediate layers 4 b that transmit X-rays. A grid cover 4c covering the grid foil 4a and the intermediate layer 4b sandwiches the grid foil 4a and the intermediate layer 4b from the X-ray incident surface and the opposite surface. In order to clarify the illustration of the grid foil 4a, the grid cover 4c is illustrated by a two-dot chain line, and the illustration of other configurations of the grid 4 (such as a mechanism for supporting the grid foil 4a) is omitted. The grid foil 4a corresponds to the grid foil in the present invention.

  Further, as shown in FIG. 3, the grids 4 are arranged by arranging the respective grid foils 4 a in parallel to the detection surface of the FPD 3. In the present embodiment, the intermediate layer 4b is a gap, and the grid 4 is also an air grid. The grid foil 4a is not particularly limited as long as it is a substance that absorbs radiation represented by X-rays such as lead. In the present embodiment, the grid foil 4a is arranged along a ray connecting the focal point of the X-ray tube 2 (see FIG. 1). However, in FIG. Parallel arrangement.

  As shown in FIG. 3, when each pixel size is ΔX, the grid foil 4a is arranged so that a foil shadow is reflected in synchronization with each pixel. The grid foil 4a is arranged so that the foil shadow is reflected in synchronization with a plurality of (for example, four) pixels. Therefore, when the grid foil 4a absorbs X-rays, a foil shadow is generated in the FPD 3 and the foil shadow is reflected in the X-ray image. However, the grid foil 4a is reflected so that the foil shadow is reflected in synchronization with each pixel. Be placed. As described above, the grid 4 is a synchronous grid in which the grid foil 4a is arranged so that the foil shadow is reflected at an integral multiple (for example, four times) of the distance (pixel pitch) between the pixels.

  In FIG. 3, the grid foil 4a extends in the y direction (column direction) by arranging the grid foils 4a in parallel in the x direction (row direction). Further, since the X-ray detection elements d (see FIG. 2) are arranged in parallel in the x and y directions in the vertical and horizontal directions, the grid foil 4a is arranged in parallel in the x direction (row direction) of the X-ray detection elements d. Will be placed.

  Next, the flow of the image generation / processing unit and a series of image processing will be described with reference to FIGS. 4 is a block diagram of a specific image generating / processing unit according to the embodiment, FIG. 5 is a flowchart of a series of image processing, and FIG. 6 is a diagram showing the positional relationship between the peak pixel coordinates and the foil shadow coordinates. FIG. 7 is a schematic view schematically showing an example, a CP value (direct X-ray transmittance) and a foil shadow amount, and FIG. 7 shows changes in the profile of the grid foil shadow and the direct X-ray transmittance due to the FPD / grid misalignment. It is the schematic which showed typically.

  As shown in FIG. 4, the image generation / processing unit 11 includes an image generation unit 21, a first absorption ratio calculation unit 22, a second absorption ratio calculation unit 23, a change rate calculation unit 24, an addition value calculation unit 25, and a distance function. A calculation unit 26, a selection unit 27, and an image processing unit 28 are provided. The memory unit 12 has a first absorption ratio memory area 12a and a second image memory area 12b. The image generation unit 21 corresponds to the image generation unit in the present invention, the first absorption ratio calculation unit 22 corresponds to the first absorption ratio calculation unit in the present invention, and the second absorption ratio calculation unit 23 in the present invention. The change rate calculator 24 corresponds to the second absorption ratio calculator, the change rate calculator 24 corresponds to the change rate calculator in the present invention, the added value calculator 25 corresponds to the added value calculator in the present invention, and the distance function calculator 26 corresponds to the distance function calculation means in the present invention, the selection unit 27 corresponds to the selection means in the present invention, the image processing unit 28 corresponds to the image processing means in the present invention, and the first absorption ratio memory area 12a corresponds to the first absorption ratio storage means in this invention.

  Here, terms for describing the present embodiment are defined in this specification. The pixel that is least affected by the foil shadow by the grid foil 4a (see FIG. 3) is the peak pixel, and this pixel is located in the middle part between the foil shadow and the foil shadow adjacent thereto, and is as far as possible from the foil shadow. Pixel. The x coordinate of this peak pixel is taken as the peak pixel coordinate. Conversely, a pixel that is most susceptible to the influence of the foil shadow by the grid foil 4a is a foil shadow pixel, and this pixel is located on the foil shadow and is as close as possible to the foil shadow. The x-coordinate of this foil shadow pixel is defined as the foil shadow coordinate. As described above, the direct X-ray transmittance in one pixel when the direct X-ray dose without a grid is “1.0” is defined as a CP value. Further, the absorption ratio of the direct X-rays of the grid foil 4a in one pixel is defined as a foil shadow amount. The pixel having no grid foil shadow is “0.0”, and the pixel having the grid foil shadow is “1.0-CP”. Value ". Further, the sum of the foil shadow amounts of the pixels between the target peak pixel coordinates when attention is paid between one peak pixel coordinate in one row in the pixel row is defined as the foil shadow integrated value. However, since the peak pixel is as far as possible from the foil shadow, even if there is distortion, the foil shadow is not reflected on the peak pixel, and the integrated value of the foil shadow is the amount of foil shadow on the peak pixel. Not included.

Moreover, the character for demonstrating a present Example is defined in this specification. The subject when x is the x coordinate of a certain pixel, y is the y coordinate of a certain pixel, g is the grid foil 4a, p is the peak pixel in one row, and the subject is obtained under certain predetermined conditions. The X-ray image without air (air image) is the a-th image, the total number of pixels (total number of pixels) of the horizontal size of the captured image (size in the x direction) is X _SIZE, and the vertical size of the captured image (size in the y direction) ) _{Is set} to Y _SIZE , the total number of grid foils 4a is set to _NG , the total number of peak pixel columns is set to N _P (where N _P = _NG + 1), The number of sheets is N _A. Then, the _{0 ≦ x <X SIZE, 0} ≦ y <Y SIZE, 0 ≦ g <N G, 0 ≦ p <N P, 0 ≦ a <N A.

In the case of this embodiment, the number of patterns of positional deviation of the X-ray tube 2 focus the N _A for FPD3 as described above. At this time, while changing each Zf the SID (see Figure 1), and respectively capturing air image in the pattern number of the positional deviation of the X-ray tube 2 focus for FPD3 per each SID, total N _A piece of Acquire an air image.

  Also, A (x, y, a) is the pixel value of the air image at the time of the a-th sheet (hereinafter, simply referred to as “air image”), and I (x, y) is the pixel value of the subject image (hereinafter, “air image”) , Simply referred to as “subject image”), and A_CP (x, y, a) as an image in which CP values calculated from the air image are assigned to pixels (hereinafter simply referred to as “CP value image”), Let I_CP (x, y, a) be a CP value image calculated from the subject image.

  As shown in FIG. 4, the image generation unit 21 generates an X-ray image based on the X-ray detection signal detected by the FPD 3 (see FIGS. 1 to 3). Specifically, a plurality of non-subjects based on the X-ray detection signal detected without the subject between the X-ray tube 2 (see FIG. 1) and the FPD 3 are detected. The image generation unit 21 generates an X-ray image (that is, an air image A (x, y, a)). On the other hand, the subject M (see FIG. 1) is interposed between the X-ray tube 2 and the FPD 3, and an X-ray image of the subject M based on an X-ray detection signal detected in a certain state of the subject M ( That is, the image generation unit 21 generates the subject image I (x, y)).

  In this way, the image generation unit 21 generates a plurality of air images A (x, y, a) and a subject image I (x, y). The subject image I (x, y) is written and stored in the second image memory area 12b of the memory unit 12 via the controller 15 (not shown in FIG. 4). Prior to the time of subject imaging, air image capturing for generating a plurality of air images A (x, y, a) is performed, and then a subject image I (x, y) is generated to generate a second image. Subject imaging stored in the image memory area 12b is performed.

  The first absorption ratio calculation unit 22 calculates the direct X-ray absorption ratio of each grid foil 4a in the air image A (x, y, a). That is, the absorption ratio of the direct X-ray of each grid foil 4a in the air image A (x, y, a) is “0.0” in the pixel having the grid foil shadow as described above, and the pixel having no grid foil shadow. Since “1.0−CP value”, the first absorption ratio calculation unit 22 directly determines the X-ray absorption ratio based on the CP value image A_CP (x, y, a) calculated from the air image. The CP value image A_CP (x, y, a) calculated from the air image is expressed by the following equation (1).

A_CP (x, y, a)
= A (x, y, a) / Spline {A (x, y, a)} (1) where Spline {A (x, y, a)} is an air image A in the x-axis direction ( This is a spline interpolation of x, y, a). However, unlike the following equations (4) and (5) described later, in the above equation (1), the scattered X-rays are not generated because the subject does not intervene, so the air image is scattered X-rays S {I. (X, y), A (x, y, a)} are not included.

(Step S1) Calculate A_SHsum (g, y, a) Further, as shown in Step S1 of the flowchart of FIG. 5, the first absorption ratio calculation unit 22 includes a plurality of air images A (x, y, a). ), The sum of the absorption ratios of each grid foil 4a is calculated for each unit pixel, with the unit between the pixels that are least affected by the shadow of the grid foil 4a (that is, between the peak pixels) as one unit. To do. In the present specification, an image is generated by determining the sum of the absorption ratios for each foil shadow of the grid foil 4a and assigning it to the pixels. The image relating to the sum of the generated absorption ratios is a foil shadow integrated value image calculated from the air image, and A_SHsum (g, y, a) is a foil shadow integrated value image calculated from the air image. The foil shadow integrated value image A_SHsum (g, y, a) corresponds to the first absorption ratio in the present invention.

Here, let P _A (p, a) be the peak pixel coordinates of the air image at the time of the a-th sheet. Peak pixel coordinate of the air image, the number of air image content (N _A street) as there, p, expressed by two variables function of a.

  The foil shadow integrated value image A_SHsum (g, y, a) is calculated from each air image A (x, y, a) using the following equation (2).

  Here, w in the above equation (3) is the number of pixels between the foil shadow pixel in one row of the pixel row and the foil shadow pixel adjacent to the foil shadow pixel. A peak pixel is also included in between.

  The CP value image A_CP (g, y, a) calculated from the air image is also the direct X-ray transmittance, and the maximum value of the direct X-ray dose (that is, the direct X-ray dose when there is no foil shadow) is “1.0” It is also a normalized parameter. Therefore, the number of pixels w which is the first term on the right side of the equation (2) is a parameter obtained by normalizing the total dose between the foil shadow pixels in one row and the foil shadow pixels adjacent thereto. Thus, one pixel when there is no foil shadow directly corresponds to the maximum value “1.0” of the X-ray dose. On the other hand, the second term on the right side of the equation (2) indicates that the CP value image A_CP (g, y, g, y, y) is a pixel between a foil shadow pixel in one row and a foil shadow pixel adjacent thereto. Since this is the sum of a), the foil shadow normalized to the sum of the absorption ratios of the grid foils 4a between the unit pixels is obtained by subtracting the second term from the first term on the right side of the equation (2). The integrated value image A_SHsum (g, y, a) can be obtained.

  In this way, the first absorption ratio calculation unit 22 calculates the direct X-ray absorption ratio of each grid foil 4a in the air image A (x, y, a), and each grid foil 4a has a unit-pixel interval. The foil shadow integrated value image A_SHsum (g, y, a) normalized to the sum of the absorption ratios is calculated. The foil shadow integrated value image A_SHsum (g, y, a) is written and stored in the first absorption ratio memory area 12a of the memory unit 12 via the controller 15 (not shown in FIG. 4).

  It should be noted that air image capturing is performed each time immediately before subject imaging, and the foil shadow integrated value image A_SHsum (g, y, a) is calculated each time, and the foil shadow integrated value image A_SHsum (g, y, a) is calculated. It is not necessary to perform various calculations described later using each time. The air image was photographed in the past, the foil shadow integrated value image A_SHsum (g, y, a) was calculated, written and stored in the first absorption ratio memory area 12a, and stored in the first absorption ratio memory area 12a It is more preferable to perform various calculations described later using the foil shadow integrated value image A_SHsum (g, y, a) in common. That is, as shown in FIG. 4, the first absorption ratio memory area 12a is provided, and the foil shadow integrated value image A_SHsum (g, y, a) is calculated in advance, and is written and stored in the first absorption ratio memory area 12a. The number of operations can be reduced.

  The foil shadow integrated value image A_SHsum (g, y, a) calculated by the first absorption ratio calculation unit 22 and stored in the first absorption ratio memory area 12a is read from the first absorption ratio memory area 12a, and the second This is sent to the absorption ratio calculation unit 23, the change rate calculation unit 24 and the selection unit 27.

  On the other hand, the subject image I (x, y) stored in the second image memory area 12b is read, and the second absorption ratio calculation unit 23 directly applies each grid foil 4a in the subject image I (x, y). X-ray absorption ratio is calculated. That is, the direct X-ray absorption ratio of each grid foil 4a in the subject image I (x, y) is “0.0” in the pixel having the grid foil shadow as described above, and in the pixel having no grid foil shadow. Since it is “1.0−CP value”, the second absorption ratio calculation unit 23 directly determines the X-ray absorption ratio based on the CP value image I_CP (x, y, a) calculated from the subject image. The CP value image I_CP (x, y, a) calculated from the subject image is expressed by the following equation (4).

I_CP (x, y, a)
= G (x, y, a) / Spline {G (x, y, a)} (4) Formula G (x, y, a)
= I (x, y) -S {I (x, y), A (x, y, a)} (5) Here, in the above expressions (4) and (5), the subject M is Since the scattered X-rays are generated by intervening, the subject image includes components of the scattered X-rays S {I (x, y), A (x, y, a)}. Therefore, an image obtained by removing components of scattered X-rays S {I (x, y), A (x, y, a)} from the subject image I (x, y) as expressed by the above equation (5) is represented by G. (X, y, a). Spline {G (x, y, a)} is a spline interpolation of G (x, y, a) in the x-axis direction. The scattered X-rays S {I (x, y), A (x, y, a)} are obtained from the above-described foil shadow integrated value image A_SHsum (g, y, a).

  Accordingly, the second absorption ratio calculation unit 23 adds the foil shadow integrated value image stored in the first absorption ratio memory area 12a in addition to the subject image I (x, y) stored in the second image memory area 12b. A_SHsum (g, y, a) is also read out, and the scattered X-ray S {I (x, y), based on the subject image I (x, y) and the foil shadow integrated value image A_SHsum (g, y, a), A (x, y, a)} is calculated. Then, using the scattered X-rays S {I (x, y), A (x, y, a)} as well, the direct X-ray absorption ratio of each grid foil 4a in the subject image I (x, y). Is calculated. As described above, the second absorption ratio calculation unit 23 reads and uses the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) stored in the first absorption ratio memory area 12a, which will be described later. An operation for calculating the second absorption ratio (foil shadow integrated value image I_SHsum (g, y, a)) is performed.

(Step S2) Calculate I_SHsum (g, y, a) Further, as shown in step S2 of the flowchart of FIG. 5, the second absorption ratio calculation unit 23 calculates the pixel of the subject image I (x, y). Among these, between pixels that are most unaffected by the foil shadow by each grid foil 4a (that is, between peak pixels) as one unit, the sum of the absorption ratios of each grid foil 4a is calculated for each unit pixel. In the present specification, an image is generated by determining the sum of the absorption ratios for each foil shadow of the grid foil 4a and assigning it to the pixels. The image relating to the sum of the generated absorption ratios is a foil shadow integrated value image calculated from the subject image, and I_SHsum (g, y, a) is a foil shadow integrated value image calculated from the subject image. The foil shadow integrated value image I_SHsum (g, y, a) corresponds to the second absorption ratio in the present invention.

Here, P _I (p) is the peak pixel coordinate of the subject image. Unlike the peak pixel coordinates of the air image, since the subject image is data for only one sheet, the peak pixel coordinates of the subject image are represented by a single variable function of only p.

  The foil shadow integrated value image I_SHsum (g, y, a) is calculated from the subject image I (x, y) using the following equation (6).

  Here, w in the equation (7) is the number of pixels between the foil shadow pixel in one row and the foil shadow pixel adjacent thereto as described in the w in the equation (3). There is a peak pixel between the foil shadow pixel and the adjacent foil shadow pixel.

  As described in the air image, the CP value image I_SHsum (g, y, a) calculated from the subject image is also the direct X-ray transmittance, and the maximum value of the direct X-ray dose (that is, the direct X when there is no foil shadow). (Dose) is also a parameter normalized to “1.0”. Therefore, the number of pixels w, which is the first term on the right side of the equation (6), is a parameter obtained by normalizing the total dose between the foil shadow pixels in one row of the pixel rows and the foil shadow pixels adjacent thereto. Thus, one pixel when there is no foil shadow directly corresponds to the maximum value “1.0” of the X-ray dose. On the other hand, the second term on the right side of the above equation (6) indicates that the CP value image I_SHsum (g, y, g, y) at a pixel between the foil shadow pixel in one row of the pixel row and the foil shadow pixel adjacent thereto. Since this is the sum of a), the foil shadow normalized to the sum of the absorption ratios of the respective grid foils 4a between the unit pixels is obtained by subtracting the second term from the first term on the right side of the above equation (6). The integrated value image I_SHsum (g, y, a) can be obtained.

  In this way, the second absorption ratio calculation unit 23 calculates the direct X-ray absorption ratio of each grid foil 4a in the subject image I (x, y), and the absorption of each grid foil 4a for each unit pixel. The foil shadow integrated value image I_SHsum (g, y, a) normalized to the sum of the ratios is calculated. The foil shadow integrated value image I_SHsum (g, y, a) calculated by the second absorption ratio calculation unit 23 is sent to the change rate calculation unit 24. Actually, the foil shadow integrated value image I_SHsum (g, y, a) calculated by the second absorption ratio calculation unit 23 is written and stored in the memory unit 12 via the controller 15 (not shown in FIG. 4). When calculating the rate of change by the rate-of-change calculating unit 24 described later, the foil shadow integrated value image I_SHsum (g, y, a) stored in the memory unit 12 is read and used.

Peak pixel coordinate _P A (p, a) of the air image, in FIG. 6 p [a], and p [a + 1], is in FIG. 7 _{p n,} and _{p n + 1.} As shown in the upper left part of FIG. 6, when the total number of grid foils 4a (see FIG. 3) is _NG , the foil numbers are assigned “0”, “1”, “2”,. The right end becomes “ _NG- 1”. The middle row of FIG. 6 is an enlarged pixel row in the air image in the upper right row of FIG. As shown in the middle part of FIG. 6, the pixel coordinates are assigned “0”, “1”, “2”,... In order from the left end, the foil shadow coordinate of the foil number a is g [a], and the foil number (a The peak pixel coordinate between the foil shadow coordinate g [a-1] of -1) and the foil shadow coordinate g [a] of the foil number a is p [a], and the foil shadow coordinate g [a] of the foil number a is ] And the peak pixel coordinate between the foil shadow coordinate g [a + 1] of the foil number (a + 1) is p [a + 1]. The lower half of FIG. 6 is a further enlarged view of the foil shadow pixel of foil number “N _X −1” in the middle stage of FIG.

  As shown in the lower part of FIG. 6, when the pixel width is t and u and v are X-ray transmission widths, the CP value is CP value = (u + v) / t, and the foil shadow amount is foil shadow amount = {t− (U + v)} t = 1.0−CP value. In FIG. 6, for convenience of illustration, a state in which a foil shadow having a width of {t− (u + v)} is reflected in the foil shadow pixel is illustrated, but the pixel is configured by a pixel having a pixel width t which is the minimum width. Therefore, the actual pixel is not reflected with a foil shadow having a pixel width of less than t, and the pixel value in which X-rays are absorbed by the grid foil is uniformly output to the foil shadow pixel for one pixel. Please keep in mind.

7 is the profile of the grid foil shadow and the direct X-ray transmittance when the FPD / grid position is not misaligned, and the lower left of FIG. 7 is the profile of the grid foil shadow and direct X-ray transmittance when there is no misalignment of the FPD / grid. Assume that the steps are FPD / grid misalignment, and the lower right in FIG. 7 is the profile of the grid foil shadow and direct X-ray transmittance when the FPD / grid misalignment is present. As shown in the lower left and lower right of FIG. 7, pixel coordinates between the peak pixel coordinate _pn and the peak pixel coordinate _{pn + 1} are a, b, and c in order from the left. As shown in FIG. 7, it is assumed that a foil shadow is reflected in synchronization with four pixels.

At this time, when there is no misalignment of the FPD / grid, the grid foil shadow is reflected only in the pixel corresponding to the pixel coordinate b at the center between the peak pixel coordinate _pn and the peak pixel coordinate _{pn + 1,} and directly X The profile of the line transmittance drops at only the pixel corresponding to the pixel coordinate b, but remains “1.0” at the other pixel coordinates corresponding to the pixel coordinate a and the pixel coordinate c. On the other hand, when there is an FPD / grid misalignment, the grid foil shadow is reflected across both the pixel coordinates b and the pixel coordinates c between the peak pixel coordinates _pn and the peak pixel coordinates _{pn + 1} . The direct X-ray transmittance profile remains “1.0” at the pixel corresponding to the pixel coordinate a, but falls at the pixel corresponding to the pixel coordinate b or the pixel coordinate c.

As described above, as described in the section “Problems to be solved by the invention”, when the position of the grid relative to the FPD is shifted, the profile of the grid foil shadow changes as shown in the lower part of FIG. On the other hand, when attention is paid to the peak pixels (p _n , p _{n + 1} ) that are least affected by the foil shadow caused by the grid foil, the sum of the direct X-ray dose (foil shadow amount) absorbed by the grid foil (in the lower part of FIG. 7). It can be seen that the gray area does not change even when the grid position is deviated from the FPD. Specifically, when there is no positional displacement of the FPD / grid, the sum of the foil shadow amounts (that is, the sum of the absorption ratios) in the lower left of FIG. 7 is the area of a triangle surrounded by a, b, and c. , When the FPD / grid is misaligned, the sum of the foil shadow amounts (that is, the sum of the absorption ratios) in the lower right of FIG. 7 is the area of a rectangle surrounded by a, b, c, _{pn + 1} . The area does not change.

  For the above reasons, among the pixels of each image, between the pixels that are least affected by the foil shadow by each grid foil (between peak pixels) as one unit, the absorption ratio of each grid foil for each unit pixel By taking the correlation by calculating the sum and calculating a distance function described later, the positional relationship between the FPD and the grid is the same as that at the time of air image capturing and at the time of subject image capturing. In this embodiment, as described in steps S1 and S2 of the flowchart of FIG. 5, the foil shadow integrated value image A_SHsum (g, y, g) normalized to the sum of the absorption ratios of the grid foils 4a for each unit pixel. a) and the foil shadow integrated value image I_SHsum (g, y, a) are respectively calculated.

Returning to FIG. 4 and FIG.
As shown in FIG. 4, the change rate calculation unit 24 is calculated by the first absorption ratio calculation unit 22, stored in the first absorption ratio memory area 12 a, and read out a plurality of air images A (x, y, The direct X-ray absorption ratio of each grid foil 4a (see FIG. 3) in a) and the direct X-ray of each grid foil 4a in the subject image I (x, y) calculated by the second absorption ratio calculation unit 23 Based on the absorption ratio of the line, the rate of change of the subject image with respect to the air image is calculated.

(Step S3) R (g, y, a) in the present embodiment for calculating a, as shown in step S3 in the flowchart of FIG. 5, each of the _{N A} sheet calculated from the air image foil shadow integrated value image A_SHsum ( The rate of change is calculated based on g, y, a) and the foil shadow integrated value image I_SHsum (g, y, a) calculated from the subject image. When R (g, y, a) is a change rate, the foil shadow integrated value image A_SHsum (g, y, a) and the foil shadow integrated value image I_SHsum (g) normalized to the sum of the absorption ratios of the grid foils 4a. , Y, a), the rate of change R (g, y, a) is calculated using the following equation (8).

R (g, y, a)
= I_SHsum (g, y, a) / A_SHsum (g, y, a) (8) Equation As in the above equation (8), the foil shadow integrated value image I_SHsum (g, y, a) calculated from the subject image ) Is divided by the foil shadow integrated value image I_SHsum (g, y, a) calculated from the respective air images, to obtain the rate of change R (g, y, a).

  In this way, the change rate calculation unit 24 calculates the change rate R (g, y, a) of the subject image with respect to the air image. The change rate R (g, y, a) calculated by the change rate calculation unit 24 is sent to the addition value calculation unit 25. Actually, the change rate R (g, y, a) calculated by the change rate calculation unit 24 is written and stored in the memory unit 12 via the controller 15 (not shown in FIG. 4), and an addition value described later is stored. When calculating the addition value by the calculation unit 25, the change rate R (g, y, a) stored in the memory unit 12 is read and used. In this way, the change rate calculation unit 24 reads and uses the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) stored in the first absorption ratio memory area 12a. An operation for calculating g, y, a) is performed.

  The addition value calculation unit 25 uses the change rate R (g, y, a) calculated by the change rate calculation unit 24 in the direction in which the grid foil 4a extends (in this embodiment, the column in the y direction in FIG. 3). Addition along (direction), or an arithmetic average (addition average) of adding and averaging. And the addition value by the said addition or the said arithmetic mean (addition average) is calculated. In this embodiment, the center pixel coordinate with respect to the y direction is set to “0”, and 64 pixels in the upper and lower parts corresponding to one line (total of 129 pixels including the pixel corresponding to the center pixel coordinate “0”) are averaged. Is applied to the rate of change R (g, y, a) to calculate the average rate of change of the CP integrated value. When AveR (g, a) is an average rate of change, the average rate of change AveR (g, a) is calculated using the following equation (9).

  Here, the targety in the above equation (9) is the target y coordinate and the central pixel coordinate “0”. Note that the target y coordinate is not limited to the center pixel coordinate “0”, and the lower pixel coordinate may be the target y coordinate, or the upper pixel coordinate may be the target y coordinate. Further, the average change rate AveR (g, a) is not limited to the addition value (addition average value) for one line, and except for the upper end and the lower end, the upper and lower pixels are centered on the target y coordinate as the addition average range. Also good. Moreover, although the addition average range is 64 pixels above and below in the above equation (9), the specific range of the addition average range is not particularly limited.

  The vertical addition averaging process has an effect of reducing the influence of image noise when calculating a distance function described later. Therefore, if the addition average range is adjusted according to the noise level and the calculation range of the foil shadow integrated value, the CP value, and the CP integrated value is limited according to the addition average range, the processing speed can be increased.

  In this way, the addition value calculation unit 25 calculates the average rate of change AveR (g, a) as an addition value based on the addition or the arithmetic mean (addition average). The average change rate AveR (g, a) calculated by the addition value calculation unit 25 is sent to the distance function calculation unit 26. Actually, the average rate of change AveR (g, a) calculated by the addition value calculation unit 25 is written and stored in the memory unit 12 via the controller 15 (not shown in FIG. 4), and a distance function calculation described later is performed. When calculating the distance function by the unit 26, the average rate of change AveR (g, a) stored in the memory unit 12 is read and used.

  The distance function calculation unit 26 calculates a distance function based on the average change rate AveR (g, a) that is the addition value calculated by the addition value calculation unit 25. The average rate of change AveR (g, a) is ideally “1.0”. Therefore, when the distance from the ideal value is calculated for each grid foil 4a, the average value is the distance D, and the distance function is D (c), the distance function D (c) is calculated using the following equation (10). To do.

Here, the denominator of the formula (9) is the total number _NG of the grid foils 4a, and the average value for each foil is obtained by dividing by the total number _NG of the grid foils 4a.

  In this manner, the distance function calculation unit 26 calculates the distance function D (c) indicating the degree of correlation between the air image and the subject image and indicating the distance of the grid foil 4a from the ideal value. The distance function D (c) calculated by the distance function calculation unit 26 is sent to the selection unit 27. Actually, the distance function D (c) calculated by the distance function calculation unit 26 is written and stored in the memory unit 12 via the controller 15 (not shown in FIG. 4), and is stored in the first by the selection unit 27 described later. When selecting the absorption ratio, the distance function D (c) stored in the memory unit 12 is read and used.

(Step S4) Search for the minimum value of the distance function D (c) As shown in Step S4 of the flowchart of FIG. 5, the foil shadow integrated value image A_SHsum (g, y, stored in the first absorption ratio memory area 12a a) is read, and the selection unit 27 calculates the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) in which the distance function D (c) calculated by the distance function calculation unit 26 is the smallest. A plurality of foil shadow integrated value images A_SHsum (g, y, a) are selected. That is, the range of the distance D is 0 ≦ D, and the distance D approaches “0” as the correlation between the CP integrated values of the air image and the subject image increases. Therefore, the first absorption ratio that minimizes the distance function D (c) is selected as the optimal first absorption ratio. As shown in the flowchart of FIG. 5, Optimum when to be selected (but, 0 ≦ optimum _{<N A)} and, to a first absorption ratio selected A_SHsum (g, y, optimum) and.

  In this way, the selection unit 27 sets the first absorption ratio with the smallest distance function D (c) calculated by the distance function calculation unit 26 as the optimum foil shadow integrated value image A_SHsum (g, y, optimum). A plurality of foil shadow integrated value images A_SHsum (g, y, a) are selected. The foil shadow integrated value image A_SHsum (g, y, optimum) selected by the selection unit 27 is sent to the image processing unit 28. Actually, the foil shadow integrated value image A_SHsum (g, y, optimum) selected by the selection unit 27 is written and stored in the memory unit 12 via the controller 15 (not shown in FIG. 4), and an image described later is stored. When image processing is performed by the processing unit 28, the foil shadow integrated value image A_SHsum (g, y, optimum) stored in the memory unit 12 is read and used. The foil shadow integrated value image A_SHsum (g, y, optimum) selected by the selection unit 27 may be written and stored in the first absorption ratio memory area 12a, but the foil shadow integrated value image A_SHsum before selection is stored. In order to distinguish from (g, y, a), it is preferable to write and store the selected foil shadow integrated value image A_SHsum (g, y, optimum) in another area. In this manner, the selection unit 27 reads out and uses the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) stored in the first absorption ratio memory area 12a and uses the distance function D (c). Is selected as the optimum foil shadow integrated value image A_SHsum (g, y, optimum).

  The foil shadow integrated value image A_SHsum (g, y, optimum) selected by the selection unit 27 and stored in the memory unit 12 is read, and the subject image I (x, y) stored in the second image memory area 12b is read out. Is read. Then, the image processing unit 28, based on the foil shadow integrated value image A_SHsum (g, y, optimum) and the subject image I (x, y) selected by the selection unit 27, the subject image I (x, The image processing is performed to obtain the X-ray image from which the foil shadow is removed by calculating the direct X-ray component of the subject M by removing the foil shadow by the grid foil 4a of y).

  An image processing method for acquiring an X-ray image from which the foil shadow is removed based on the foil shadow integrated value image A_SHsum (g, y, optimum) and the subject image I (x, y) is not particularly limited. For example, an optimized CP value is calculated based on the foil shadow integrated value image A_SHsum (g, y, optimum), and the optimized CP value is divided from the subject image I (x, y). Thus, an X-ray image from which the foil shadow has been removed may be acquired. In this case, since the first absorption ratio that minimizes the distance function D (c) is selected as the optimum foil shadow integrated value image A_SHsum (g, y, optimum), the object image I (x, y) Each foil shadow pixel corresponds to each foil shadow pixel of the foil shadow integrated value image A_SHsum (g, y, optimum), and only the foil shadow pixel can be simply removed by dividing the simple pixel value. There is also an effect of being able to.

  In this way, the image processing unit 28 writes and stores the X-ray image from which the foil shadow has been removed in the memory unit 12 via the controller 15 (not shown in FIG. 4). If necessary, the X-ray image stored in the memory unit 12 may be read out and displayed on the display unit 14 (see FIG. 1), or may be printed out on a printing unit such as a printer. Also good.

  According to the X-ray imaging apparatus according to the present embodiment, the first absorption ratio calculation unit 22 detects X-rays detected without the subject between the X-ray tube 2 and the FPD 3 and without the subject. The direct X-ray absorption ratio of each grid foil 4a in a plurality of X-ray images (air images) without a subject based on the detection signal is calculated as the first absorption ratio. On the other hand, the second absorption ratio calculation unit 23 interposes the subject M between the X-ray tube 2 and the FPD 3 and detects the X of the subject M based on the X-ray detection signal detected in a state where the subject M is present. The direct X-ray absorption ratio of each grid foil 4a in the line image (subject image) is calculated as the second absorption ratio.

  Then, based on the first absorption ratio calculated by the first absorption ratio calculation unit 22 and the second absorption ratio calculated by the second absorption ratio calculation unit 23, the distance function calculation unit 26 calculates a distance function. As described above, the distance function is a parameter indicating the degree of correlation between the air image and the subject image and the distance of the grid foil 4a from the ideal value.

  The selection unit 27 selects a first absorption ratio that minimizes the distance function calculated by the distance function calculation unit 26 from a plurality of first absorption ratios. That is, the greater the correlation between the air image and the subject image, the closer the distance is to the ideal value, and the distance function approaches “0”. Therefore, the first absorption ratio that minimizes the distance function is selected as the optimal first absorption ratio. Then, based on the first absorption ratio and the subject image selected by the selection unit 27, the image processing unit 28 removes the shadow of the subject image by the grid foil 4a and directly X-rays the subject M. Is calculated, and image processing for obtaining an X-ray image from which the foil shadow is removed is performed.

As described above, if a plurality of first absorption ratios are selected based on the distance function, even if the grid 4 is misaligned with respect to the FPD 3, scattered X-rays are prepared without preparing the number of patterns corresponding to the misalignment. And the grid foil 4a can be removed. In the case of the present embodiment, only the N _A number of air images need to be taken in advance, the number of times of taking air images can be reduced, and the memory area for holding the air images can be reduced. There is also an effect of being able to.

  In the present embodiment, the first absorption ratio calculation unit 22, the second absorption ratio calculation unit 23, and the distance function calculation 26 are specifically calculated as described above. That is, the first absorption ratio calculation unit 22 calculates the direct X-ray absorption ratio of each grid foil 4a in each pixel of the plurality of air images, and among the pixels of the plurality of air images, each grid foil 4a A pixel unit that is most unaffected by the foil shadow (that is, between peak pixels) is taken as one unit, and the sum of the absorption ratios of each grid foil 4a for each unit pixel (in this embodiment, normalized to the sum of the absorption ratios). The foil shadow integrated value image A_SHsum (g, y, a)) is calculated. Almost the same as the first absorption ratio calculation unit 22, the second absorption ratio calculation unit 23 calculates the direct X-ray absorption ratio of each grid foil 4a in each pixel of the subject image, and among the subject pixels, A pixel unit (between peak pixels) that is least affected by the foil shadow caused by the grid foil 4a is regarded as one unit, and the sum of the absorption ratios of the grid foils 4a between the unit pixels (in this embodiment, the sum of the absorption ratios). A normalized foil shadow integrated value image I_SHsum (g, y, a)) is calculated. The distance function calculation unit 26 includes the sum of the absorption ratios of the respective grid foils 4a calculated by the first absorption ratio calculation unit 22 (foil shadow integrated value image A_SHsum (g, y, a)) and the second absorption ratio calculation unit. The distance function is calculated based on the sum of the absorption ratios of the respective grid foils 4a calculated in step 23 (foil shadow integrated value image I_SHsum (g, y, a)).

  In the present embodiment, it is preferable to include a change rate calculation unit 24. That is, the direct X-ray absorption ratio of each grid foil 4a in the plurality of air images calculated by the first absorption ratio calculation unit 22 (in this embodiment, the foil shadow integrated value image A_SHsum normalized to the sum of the absorption ratios) (G, y, a)), and the direct X-ray absorption ratio of each grid foil 4a in the subject image calculated by the second absorption ratio calculation unit 23 (normalized to the sum of the absorption ratios in this embodiment) Based on the foil shadow integrated value image I_SHsum (g, y, a)), the change rate calculation unit 24 calculates the change rate of the subject image with respect to the air image. The rate of change is ideally “1”. The distance function calculation unit 26 calculates the distance function based on the change rate calculated by the change rate calculation unit 24. Therefore, the greater the correlation between the air image and the subject image, the closer the change rate approaches “1” and the distance. Approaches the ideal value and the distance function approaches “0”. In this way, it is possible to calculate the distance function using the change rate.

  In particular, when the distance function is calculated using the rate of change, as in this embodiment, as shown in FIG. 3, at least one of the vertical and horizontal directions of the X-ray detection element d (see FIG. 2) ( In this embodiment, it is useful when the grid 4 configured by arranging the grid foils 4a in parallel in the row direction) is used. In the grid 4 having this structure, the grid foil 4a extends in the direction in which the grid foil 4a extends (when the grid foil 4a is arranged in parallel to the row x direction, the direction in which the grid foil 4a extends is the column direction y). A statistical error (that is, noise) occurs in the extending direction without the foils 4a intersecting. Therefore, in order to reduce the influence of image noise, the addition value calculation unit 25 adds or adds the change rate calculated by the change rate calculation unit 24 along the direction in which the grid foil 4a extends. Then, an arithmetic average (addition average) is averaged, and an addition value by the addition or the arithmetic average (addition average) is calculated. In the present embodiment, the averaging process of 64 pixels above and below corresponding to one line (total of 129 pixels including the pixel corresponding to the central pixel coordinate “0”) is performed for the change rate R (g, y, a). The average change rate AveR (g, a) is calculated. The distance function calculation unit 25 calculates the distance function based on the addition value (average rate of change AveR (g, a)) calculated by the addition value calculation unit 25, so that the effect of noise of the image is reduced. is there.

In the present embodiment, as shown in FIG. 4, the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) calculated by the first absorption ratio calculation unit 22 is written and stored. A ratio memory area 12a is provided, and the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) stored in the first absorption ratio memory area 12a is read out and used for various calculations (this embodiment) Then, it is preferable to perform calculation by the second absorption ratio calculation unit 23, the change rate calculation unit 24, and the selection unit 27). If an air image is written and stored, each time the first absorption ratio calculation unit 22 calculates the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)), the air image is read out. The first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) must be calculated, but the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, a)) is calculated in advance. Thus, the number of operations can be reduced by writing and storing. Further, although the memory area for writing and storing air images are necessary size of all the pixels _{(X SIZE × Y SIZE × N} A), the first absorption ratio (foil shadow integrated value image A_SHsum (g, y, If the memory area for writing and storing a)) can be reduced to a size of only grid foil _{(N G × Y sIZE × N} a), the memory area of the first absorbent ratio memory area 12a There is also an effect that the size can be reduced.

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

  (1) In the above-described embodiment, the X-ray is taken as an example of the radiation, but the present invention may be applied to radiation other than the X-ray (for example, γ-ray).

  (2) In the above-described embodiments, the X-ray imaging apparatus is a C-arm X-ray fluoroscopic imaging apparatus including a C-arm for implementation in a CVS apparatus, but is not limited thereto. For example, it may have a structure in which a subject (in this case, the subject to be examined is a subject) is transported on a belt and photographed, as in a non-destructive inspection apparatus used for industrial use, etc. It may be a structure such as an X-ray CT apparatus used in the present invention.

  (3) In the above-described embodiment, the air grid is adopted as the scattered radiation removing means, but the present invention is not limited to this. In addition to the air gap, a grid made of an intermediate material (spacer) that transmits radiation represented by X-rays such as aluminum or an organic material may be used. Moreover, the cross grid comprised by arranging grid foil in parallel with both the vertical and horizontal directions (x, y direction) of a radiation detection element may be sufficient.

  (4) In the above-described embodiment, the focusing grid is used, but the present invention can also be applied to scattered radiation removing means in which grid foils are arranged in parallel.

  (5) In the above-described embodiment, the synchronous grid in which the foil foil is arranged so that the foil shadow is reflected at an integer multiple of the distance (pixel pitch) between the pixels of the captured image has been described. May be. In the case of a grid other than an air grid, the present invention may be applied to a grid having a structure in which a plurality of grid foils are arranged in parallel on one pixel. However, if a plurality of grid foils are evenly arranged over the entire region in one pixel, the whole becomes a foil shadow pixel, no peak pixel exists, and the present invention cannot be applied. When arranging a plurality of grid foils in one pixel, it is assumed that the grid foil is not arranged over one pixel or more so that a peak pixel exists in the vicinity of the grid foil.

  (6) In the above-described embodiment, the absorption ratio of the direct radiation of each grid foil in the radiation image (air image) without the subject, and the absorption of the direct radiation of each grid foil in the radiation image (subject image) of the subject. When calculating the distance function based on the ratio, it is calculated using the rate of change, but the rate of change is not necessarily used. For example, each absorption ratio may be normalized to obtain a difference value, and a distance function using the difference value as a variable may be calculated.

  (7) In the above-described embodiment, the distance function is calculated based on the addition value obtained by addition or addition averaging along the direction in which the grid foil extends using the grid 4 shown in FIG. 3 instead of the cross grid. However, it is not limited to this. When noise generated in the extending direction is small, or when a cross grid is used, the distance function may be calculated using the rate of change R (g, y, a) before addition.

2 ... X-ray tube 3 ... Flat panel X-ray detector (FPD)
DESCRIPTION OF SYMBOLS 4 ... Grid 4a ... Grid foil 12a ... 1st absorption ratio memory area 21 ... Image generation part 22 ... 1st absorption ratio calculation part 23 ... 2nd absorption ratio calculation part 24 ... Change rate calculation part 25 ... Addition value calculation part 26 ... Distance function calculation unit 27 ... selection unit 28 ... image processing unit A (x, y, a) ... air image I (x, y) ... subject image A_SHsum (g, y, a), I_SHsum (g, y, a ) ... Foil shadow integrated value image R (g, y, a) ... Rate of change AveR (g, a) ... Average rate of change D (c) ... Distance function M ... Subject

Claims (5)

A radiographic apparatus for obtaining radiographic images,
A radiation source that emits radiation;
Radiation detecting means configured to arrange radiation detecting elements for detecting irradiated radiation vertically and horizontally; and
Scattered radiation removing means provided on the detection side of the radiation detecting means and configured to arrange a grid foil that absorbs scattered radiation in parallel in at least one of the longitudinal and lateral directions of the radiation detecting element;
An image generation means for generating a radiation image based on a radiation detection signal detected by the radiation detection means,
Furthermore, the radiation imaging apparatus includes:
Direct radiation of each grid foil in a plurality of non-subject radiation images based on a radiation detection signal detected without the subject interposed between the radiation source and the radiation detection means. First absorption ratio calculating means for calculating the absorption ratio as the first absorption ratio;
A direct radiation absorption ratio of each grid foil in a radiographic image of a subject based on a radiation detection signal detected with the subject interposed between the radiation source and the radiation detection means A second absorption ratio calculating means for calculating as two absorption ratios;
Based on the first absorption ratio calculated by the first absorption ratio calculation means and the second absorption ratio calculated by the second absorption ratio calculation means, the radiation image without the subject and the radiation of the subject A distance function calculating means for calculating a distance function indicating a distance of the grid foil from the ideal value, and indicating a degree of correlation of the image;
Selecting means for selecting the first absorption ratio from which the distance function calculated by the distance function calculating means is the smallest from a plurality of first absorption ratios;
Based on the first absorption ratio selected by the selection means and the radiographic image of the subject, the shadow of the radiographic image of the subject is removed by the grid foil, and the direct radiation component of the subject is calculated. And an image processing means for performing image processing for obtaining a radiation image from which the foil shadow has been removed. The radiographic apparatus according to claim 1,
The first absorption ratio calculating means includes
Calculate the absorption ratio of direct radiation of each grid foil in each pixel of a plurality of radiation images without the subject,
Among the plurality of pixels of the radiographic image without the subject, the pixel that is least affected by the foil shadow by each grid foil is taken as one unit, and the sum of the absorption ratio of each grid foil is calculated for each unit pixel. Calculate
The second absorption ratio calculating means includes
Calculating an absorption ratio of direct radiation of each grid foil in each pixel of the radiation image of the subject;
Among the pixels of the radiographic image of the subject, a unit between the pixels that are least affected by the foil shadow by each grid foil is calculated as a unit, and the sum of the absorption ratios of each grid foil is calculated for each unit pixel,
The distance function calculation means is based on the sum of the absorption ratios of the grid foils calculated by the first absorption ratio calculation means and the sum of the absorption ratios of the grid foils calculated by the second absorption ratio calculation means. And calculating the distance function. The radiographic apparatus according to claim 1 or 2,
The absorption ratio of the direct radiation of each grid foil in the plurality of radiation images without the subject calculated by the first absorption ratio calculating means, and the radiation image of the subject calculated by the second absorption ratio calculating means A rate-of-change calculating means for calculating a rate of change of the radiation image of the subject with respect to the radiation image without the subject, based on the absorption ratio of the direct radiation of each grid foil in
The radiation imaging apparatus, wherein the distance function calculating unit calculates the distance function based on the change rate calculated by the change rate calculating unit. The radiographic apparatus according to claim 3,
The scattered radiation removing means is configured by arranging the grid foils in parallel with only one of the vertical and horizontal directions of the radiation detection element,
Furthermore, the radiation imaging apparatus includes:
The change rate calculated by the change rate calculation means is added along the direction in which the grid foil extends, or an arithmetic average of adding and averaging is performed, and the addition or an added value by the arithmetic average is performed. An added value calculating means for calculating
The radiographic apparatus characterized in that the distance function calculating means calculates the distance function based on the added value calculated by the added value calculating means. In the radiography apparatus in any one of Claims 1-4,
First absorption ratio storage means for writing and storing the first absorption ratio calculated by the first absorption ratio calculation means;
A radiographic apparatus characterized by performing various calculations by reading out and using the first absorption ratio stored in the first absorption ratio storage means.

Images