WO2013065502A1 - Radiographic imaging method and device - Google Patents

Abstract

A phase differential image, generated by pre-imaging without a subject, is stored as an offset image. NG areas, where unwrap errors readily occur, are detected in a phase differential image generated by imaging having the subject in place, and the remaining areas are considered to be OK areas. The OK areas, only, of the phase differential image and the offset image are unwrapped. Offset compensation is performed, whereby the offset image after unwrapping is deducted from the phase differential image after unwrapping. Inclination correction, whereby noise that is residual in the OK areas and has an almost constant inclination is removed, and level different correction, whereby level differences formed between the OK areas are removed, are performed on the phase differential image after offset compensation.

Description

Radiography method and apparatus

The present invention relates to a radiation imaging method and apparatus for detecting an image based on a phase change of radiation.

Radiation, such as X-rays, has a characteristic of decaying depending on the weight (atomic number) of the elements constituting the substance and the density and thickness of the substance. Focusing on this characteristic, X-rays are used as a probe for seeing through the inside of a subject in fields such as medical diagnosis and nondestructive inspection.

A general X-ray imaging apparatus includes an X-ray source that emits X-rays and an X-ray image detector that detects X-rays. Shoot. In this case, the X-rays emitted from the X-ray source are absorbed when passing through the subject, and enter the X-ray image detector in a state where the intensity is attenuated. As a result, an image representing an X-ray intensity change by the subject is detected by the X-ray image detector.

Since the X-ray absorption ability is lower with an element having a smaller atomic number, there is a problem that a change in X-ray intensity is small and a sufficient contrast cannot be obtained in an image in a soft body tissue or soft material. For example, most of the components of the cartilage part constituting the joint of the human body and the joint fluid in the vicinity thereof are water, and the difference in X-ray absorption capacity between the two is small, so that it is difficult to obtain contrast.

Against the background of such problems, research on X-ray phase imaging for obtaining an image based on the phase change of the X-ray by the subject instead of the change in the intensity of the X-ray by the subject has been actively conducted in recent years. X-ray phase imaging is a method of imaging the phase change of X-rays, focusing on the fact that the phase change of X-rays incident on the subject is larger than the intensity change. Can also obtain a high-contrast image.

In order to enable such X-ray phase imaging, an X-ray imaging apparatus is proposed in which first and second gratings are arranged in parallel at a predetermined interval between an X-ray source and an X-ray image detector. (See, for example, WO 2004/058070 (US2005 / 0286680A1)).

In this X-ray imaging apparatus, the first periodic pattern image is generated when the X-ray source passes through the first grating, and the second grating partially shields the first periodic pattern image. Two periodic pattern images are generated. The X-ray image detector detects the second periodic pattern image and generates image data. The subject is disposed, for example, between the X-ray source and the first grating, and the subject undergoes a phase change in the X-ray, thereby modulating the first periodic pattern image. By detecting this modulation amount through the second periodic pattern image, the X-ray phase change can be imaged.

The fringe scanning method is used to detect the modulation amount. This is because the second grating is intermittently moved relative to the first grating in a direction parallel to the plane of the first grating and perpendicular to the grating line direction of the first grating, and during each stop thereof Image data is generated by photographing. Based on the obtained plurality of image data, an intensity modulation signal representing an intensity change accompanying the movement of the second lattice is generated for each pixel. For each pixel, the phase shift amount of the intensity modulation signal (the phase shift amount from the case where the subject does not exist) is calculated, and the phase shift amount is imaged to obtain an image representing the modulation amount. Since this image represents the differential amount of the phase change (phase shift) of the X-rays by the subject, it is called a phase differential image.

Since the phase shift amount of the intensity modulation signal is calculated using a function (arg [...]) for extracting a complex argument, or an arctangent function (tan ^-1 [...]), the phase differential image is calculated It is expressed by a value convolved (wrapped) in the range of the function used in (−π to + π or −π / 2 to + π / 2). In the phase differential image wrapped in this way, jumps (discontinuous points) corresponding to the above-described range occur at locations where the upper limit of the range changes from the lower limit or locations where the lower limit changes to the upper limit. For this reason, the wrapped phase differential image is subjected to unwrap processing for eliminating the discontinuity and making it continuous (see, for example, Japanese Patent Application Laid-Open No. 2011-045655).

Generally, unwrap processing is performed in order along a predetermined path from a starting point at a predetermined position in the image. Specifically, when a discontinuous point is detected in the path, the discontinuous point is determined by uniformly adding or subtracting a value corresponding to the range of the function to data on the path after the discontinuous point. It is made continuous without any change (see, for example, Japanese Patent Application Laid-Open No. 2008-082869).

In X-ray phase imaging, noise unevenness may appear in the phase differential image due to lattice distortion or scanning error. For this reason, in WO2004 / 058070, offset correction is performed to remove noise unevenness from the phase differential image. In this offset correction, a phase differential image is acquired in advance without a subject, stored as an offset image, and the offset image is subtracted from the phase differential image acquired with the subject placed. It is processing.

However, when the subject includes a high-absorber having a high X-ray absorption capability such as a bone part, the high-absorber has a large amount of X-ray attenuation, and the intensity and amplitude of the intensity modulation signal are reduced. The calculation accuracy of the phase shift amount decreases. As a result, there is a problem that an unwrapping error is likely to occur in the region of the phase differential image corresponding to the high absorber. In this unwrapping error, there are a case where an unwrapping process is performed because it is erroneously determined as a discontinuous point, and a case where an unwrapping process is not performed because it is erroneously determined that it is not a discontinuous point.

As shown in FIG. 19, in the wrapped phase differential image, when the origin is set in the region of the bone that is a high-absorber, and the unwrap processing is performed along the path that goes downward from the origin, the bone Unwrapping errors are likely to occur in the area. When an unwrap error occurs, an error value (a value corresponding to the range of the above function) is sequentially added to or subtracted from the path after that point. As a result, the phase differential image after the unwrap process has a difference in the path direction of the unwrap process. A streak of noise along the line occurs. This streak noise overlaps with a soft tissue (cartilage portion) that is a target portion of X-ray phase imaging, and obstructs imaging of the soft tissue.

In order to reduce the unwrapping error, a method of detecting a region (NG region) where an unwrapping error is likely to occur from the phase differential image and unwrapping only the other region (OK region) can be considered. However, when the phase differential image is divided into NG regions and there are a plurality of OK regions in the phase differential image, if each OK region is individually unwrapped, as shown in FIG. There is no correlation in pixel values between regions. For this reason, in order to remove the noise unevenness described above, subtracting the offset image that has been unwrapped as a whole as shown in FIG. 20B from this phase differential image, as shown in FIG. In the phase differential image (difference image) after the offset processing, a step is generated in the pixel value between the OK regions. Furthermore, when there is a difference in the noise unevenness gradient between the phase differential image of FIG. 20A and the offset image of FIG. 20B, the phase differential image after the offset processing shown in FIG. The tilt will remain.

It is an object of the present invention to provide a radiography method and apparatus capable of reducing unwrapping errors and reducing data step and inclination between OK regions in a phase differential image after offset correction.

In order to achieve the above object, a radiation imaging apparatus of the present invention includes a radiation source, a radiation detector, a grating unit, a phase differential image generation unit, an offset image storage unit, an OK / NG region detection unit, An unwrap processing unit, an offset processing unit, an inclination correction processing unit, and a step correction processing unit are provided. The radiation source emits radiation. The radiation detector detects radiation transmitted through the subject and generates image data. The grating portion is disposed between the radiation source and the radiation detector. The phase differential image generation unit generates a phase differential image in a state where the phase differential value is wrapped in a predetermined range based on the image data. The offset image storage unit stores, as an offset image, the phase differential image generated by the phase differential image generation unit without placing the subject. The OK / NG region detection unit detects an NG region in which an unwrap error is likely to occur from the phase differential image generated by the phase differential image generation unit in a state where the subject is arranged, and sets other regions as OK regions. An area in the offset image corresponding to this OK area is defined as an OK area. The unwrap processing unit unwraps only the OK region for each of the phase differential image and the offset image. The offset processing unit subtracts the offset image subjected to the unwrap process from the phase differential image subjected to the unwrap process. The inclination correction processing unit performs an inclination correction process for removing noise having a substantially constant inclination that remains in the OK region of the phase differential image subjected to the offset correction. The step correction processing unit performs a step correction process for removing a step formed between the OK regions.

It is preferable that a determination unit that determines whether or not there are a plurality of OK regions is further provided, and that the inclination correction processing unit and the step correction processing unit perform each process only when there are a plurality of OK regions.

The inclination correction processing unit obtains a correction function by fitting a change in the pixel value of the OK region in one direction with a linear form or a polynomial for the phase differential image subjected to the offset correction, and obtains the correction function in the one direction It is preferable to create a two-dimensional tilt correction image by extending in a direction orthogonal to the direction and remove noise based on the tilt correction image.

The inclination correction processing unit collectively fits changes in pixel values in one direction of adjacent OK areas in a linear form or polynomial form when there are a plurality of OK areas and the inclinations of the correction functions of adjacent OK areas are close. Thus, it is preferable to obtain a correction function and use this correction function as a correction function for adjacent OK regions.

The inclination correction processing unit fits the first and second correction functions by fitting the change of the pixel value in two directions with a linear form or a polynomial in the OK region existing in the phase differential image subjected to the offset correction. Then, a two-dimensional tilt correction image may be created based on the first and second correction functions, and noise may be removed based on the tilt correction image.

The step correction processing unit calculates an average value of the pixel values for at least a part of each OK region in the phase differential image after the tilt correction processing, and each OK region has the same average value in each OK region. It is preferable to add or subtract a certain value from the pixel value for each region.

The unwrap processing unit sets a starting point along a through line that penetrates the OK region in one direction, and performs unwrap processing between the starting points and unwrap processing along a straight path perpendicular to the through line from each starting point. preferable. In this case, it is preferable that the unwrap processing unit further performs unwrap processing on the pixels in the OK region remaining behind the NG region as viewed from the starting point. The unwrap processing unit preferably determines the setting direction of the starting point so that the number of times of unwrap processing for the pixels remaining behind the NG region is reduced.

It is preferable that the unwrap processing unit sets the starting point along any one side of the quadrilateral phase differential image.

The grating unit generates a second periodic pattern image by partially shielding the first periodic pattern image and a first grating that generates a first periodic pattern image by passing radiation from a radiation source. It is preferable to have the second lattice. In this case, the radiation image detector detects the second periodic pattern image and generates image data.

The phase differential image generation unit preferably generates a phase differential image based on single image data obtained by the radiation detector.

It is preferable that the grating unit further includes a scanning mechanism that moves the first grating or the second grating at a predetermined scanning pitch and sequentially sets the plurality of scanning positions. In this case, the radiation image detector detects the second periodic pattern image at each scanning position and generates image data. The phase differential image generation unit generates a phase differential image based on a plurality of image data generated by the radiation image detector at a plurality of scanning positions.

The scanning mechanism preferably moves the first grating or the second grating in a direction orthogonal to or inclined with respect to the grating line.

The OK / NG area detecting unit detects an NG area based on one or a combination of an average intensity, an amplitude, and a visibility of an intensity modulation signal representing an intensity change of a pixel value of image data with respect to a plurality of scanning positions. Is preferred.

The OK / NG area detection unit generates an absorption image based on the average intensity of the intensity modulation signal indicating the intensity change of the pixel value of the image data for a plurality of scanning positions, and performs an absorption differentiation by performing a differentiation process on the absorption image. An image may be generated and an NG region may be detected based on the absorption differential image.

The step correction processing unit sets an OK region in the absorption differential image, extracts a pixel region having substantially the same pixel value from each OK region, and outputs each OK for the phase differential image subjected to the tilt correction processing. It is preferable to add or subtract a constant value to each pixel value for each OK region so that the pixel values of the pixel regions in the region are the same.

It is preferable to further include an NG region image replacement unit that generates any one of the absorption image, the absorption differential image, and the small angle scattered image and replaces the NG region of the phase differential image.

The radiation imaging method of the present invention includes a pre-imaging process, a main imaging process, an OK / NG area detection process, an unwrap process process, an offset process process, an inclination correction process process, and a step correction process process. In the pre-imaging process, in a state in which the subject is not arranged, the radiation emitted from the radiation source and detected through the grating portion is generated to generate image data. Based on this image data, the phase differential value is within a predetermined range. A wrapped phase differential image is generated and stored as an offset image. In this imaging process, in a state in which the subject is arranged, the radiation emitted from the radiation source and passed through the subject and the lattice part is detected to generate image data. Based on this image data, the phase differential value is a predetermined value. A phase differential image that is wrapped in a range is generated. In the OK / NG area detection step, an NG area where an unwrapping error is likely to occur is detected from the phase differential image, and the other area is set as an OK area, and the area in the offset image corresponding to this OK area is set as the OK area. And In the unwrap processing step, only the OK region is unwrapped for each of the phase differential image and the offset image. In the offset processing step, offset correction is performed by subtracting the offset image that has been unwrapped from the phase differential image that has been unwrapped. In the tilt correction processing step, noise having a substantially constant tilt remaining in the OK region of the phase differential image subjected to offset correction is removed. In the step correction processing step, the step formed between the OK regions is removed.

In the present invention, only the OK region is unwrapped for each of the phase differential image and the offset image, and then offset correction is performed to subtract the unwrapped offset image from the unwrapped phase differential image. Since the inclination correction process for removing noise having a substantially constant inclination remaining in the OK region of the phase differential image after the offset correction and the step correction process for removing the step formed between the OK regions are performed. In addition to reducing unwrapping errors, it is possible to reduce the level difference and inclination of data between OK regions in the phase differential image after offset correction.

It is a block diagram which shows an X-ray imaging apparatus. It is a schematic diagram which shows an X-ray image detector. It is explanatory drawing explaining the structure of the 1st and 2nd grating | lattice. It is a graph which shows an intensity | strength modulation signal. It is a block diagram which shows the structure of an image process part. It is a flowchart explaining the flow of the unwrap processing method. It is a figure explaining the starting method of an unwrap process, and the setting method of a path | route. It is explanatory drawing explaining an unwrap process. It is a figure explaining the setting method of the starting point and path | route when a some OK area | region exists. It is explanatory drawing explaining the preparation method of an inclination correction image. It is a flowchart explaining pre imaging | photography. It is a flowchart explaining this imaging | photography. It is explanatory drawing explaining offset correction. It is explanatory drawing explaining another preparation method of an inclination correction image. It is a figure explaining the example which performs an unwrap process in order for every pixel from one starting point. It is a block diagram which shows the structure of the image process part provided with the NG area | region image replacement part. It is explanatory drawing explaining another form of a level | step difference correction process. It is explanatory drawing explaining the structure of the X-ray imaging apparatus which has a multi slit. It is explanatory drawing explaining the conventional unwrap process. It is explanatory drawing explaining the conventional offset correction.

In FIG. 1, an X-ray imaging apparatus 10 includes an X-ray source 11, a grating unit 12, an X-ray image detector 13, a memory 14, an image processing unit 15, an image recording unit 16, an imaging control unit 17, a console 18, and a system. A control unit 19 is provided. As is well known, the X-ray source 11 includes a rotary anode type X-ray tube (not shown) and a collimator (not shown) for limiting the X-ray irradiation field, and is controlled by the imaging control unit 17. Based on the above, X-rays are emitted toward the subject H.

The grating unit 12 includes a first grating 21, a second grating 22, and a scanning mechanism 23. The first and second gratings 21 and 22 are disposed to face the X-ray source 11 in the Z direction, which is the X-ray irradiation direction. A space is provided between the X-ray source 11 and the first grating 21 so that the subject H can be arranged. The X-ray image detector 13 is a flat panel detector using a semiconductor circuit, and is disposed close to the back of the second grating 22. The detection surface 13a of the X-ray image detector 13 exists on the XY plane orthogonal to the Z direction.

The first lattice 21 has a lattice plane on the XY plane, and a plurality of X-ray absorption portions 21a and X-ray transmission portions 21b extending in the Y direction (lattice direction) are formed on the lattice plane. . The X-ray absorption parts 21a and the X-ray transmission parts 21b are alternately arranged along the X direction to form a striped pattern. Similar to the first grating 21, the second grating 22 includes a plurality of X-ray absorption parts 22 a and X-ray transmission parts 22 b that extend in the Y direction and are alternately arranged along the X direction. The X-ray absorbing portions 21a and 22a are formed of a metal having X-ray absorption properties such as gold (Au) and platinum (Pt). The X-ray transmissive portions 21b and 22b are formed of an X-ray transmissive material such as silicon (Si) or resin, or a gap.

The first grating 21 partially passes the X-rays emitted from the X-ray source 11 to generate a first periodic pattern image (hereinafter referred to as a G1 image). This G1 image substantially coincides with the lattice pattern of the second lattice 22 at the position of the second lattice 22. The second grating 22 partially shields the G1 image generated by the first grating 21 to generate a second periodic pattern image (hereinafter referred to as G2 image).

The X-ray image detector 13 detects the G2 image and generates image data. The memory 14 temporarily stores the image data read from the X-ray image detector 13. The image processing unit 15 generates a phase differential image based on the image data stored in the memory 14, and generates a phase contrast image based on the phase differential image. The image recording unit 16 records a phase differential image and a phase contrast image.

The scanning mechanism 23 intermittently moves the second grating 22 in the X direction, and changes the position (scanning position) of the second grating 22 with respect to the first grating 21 in a stepwise manner. The drive unit of the scanning mechanism 23 is configured by a piezoelectric actuator or an electrostatic actuator, and is driven based on the control of the imaging control unit 17 at the time of stripe scanning described later. The memory 14 stores image data obtained by the X-ray image detector 13 at each scanning position of the second grating 22 with respect to the first grating 21.

The console 18 includes an operation unit 18a and a monitor 18b. The operation unit 18a is configured by a keyboard, a mouse, and the like, and sets imaging conditions such as tube voltage, tube current, and irradiation time of the X-ray source 11, selection of an imaging mode (main imaging or pre-imaging), imaging execution instruction, and the like. The operation input can be performed. The main imaging is an imaging mode performed with the subject H placed between the X-ray source 11 and the first grating 21. Pre-imaging is an imaging mode performed without placing the subject H between the X-ray source 11 and the first grating 21. Pre-photographing is performed in order to acquire a background component (offset image) caused by a manufacturing error or an arrangement error of the first and second gratings 21 and 22.

The monitor 18b displays photographing information such as photographing conditions, and a phase differential image and a phase contrast image recorded in the image recording unit 16. The system control unit 19 comprehensively controls each unit according to a signal input from the operation unit 18a.

2, the X-ray image detector 13 includes a plurality of pixels 30 arranged two-dimensionally, a gate scanning line 33, a scanning circuit 34, a signal line 35, and a readout circuit 36. As is well known, the pixel 30 includes a pixel electrode 31 for collecting charges generated in a semiconductor film such as amorphous selenium (a-Se) by incident X-rays, and a TFT (for reading the charges collected by the pixel electrode 31). Thin Film Transistor) 32.

The gate scanning line 33 is provided for each row of the pixels 30. The scanning circuit 34 applies a scanning signal for turning on / off the TFT 32 to each gate scanning line 33. The signal line 35 is provided for each column of the pixels 30. The readout circuit 36 reads out electric charges from the pixels 30 through the signal lines 35, converts them into image data, and outputs them. The detailed layer configuration of each pixel 30 is the same as the layer configuration described in Japanese Patent Laid-Open No. 2002-26300.

As is well known, the readout circuit 36 includes an integration amplifier, an A / D converter, a correction circuit (none of which is shown), and the like. The integrating amplifier integrates the charges output from each pixel 30 through the signal line 35 to generate an image signal. The A / D converter converts the image signal generated by the integrating amplifier into digital image data. The correction circuit performs dark current correction, gain correction, linearity correction, and the like on the image data, and inputs the corrected image data to the memory 14.

The X-ray image detector 13 is not limited to a direct conversion type that directly converts incident X-rays into electric charges, but converts incident X-rays into visible light with a scintillator such as cesium iodide (CsI) or gadolinium oxysulfide (GOS). Alternatively, an indirect conversion type in which visible light is converted into electric charge by a photodiode may be used. The X-ray image detector 13 is not limited to a radiographic image detector based on a TFT panel, and a radiographic image detector based on a solid-state imaging device such as a CCD sensor or a CMOS sensor can also be used. .

In FIG. 3, X-rays irradiated from the X-ray source 11 are cone beams having the X-ray focal point 11a as a light emitting point. The first grating 21 is configured to project the X-rays that have passed through the X-ray transmission part 21b substantially geometrically. Specifically, the width of the X-ray transmission part 21b in the X direction is set to a value sufficiently larger than the effective wavelength of X-rays radiated from the X-ray source 11, and straightness is achieved without diffracting most of the X-rays. It is realized by letting it pass while keeping. For example, when tungsten is used as the rotating anode of the X-ray source 11 and the tube voltage is 50 kV, the effective wavelength of X-rays is about 0.4 mm. In this case, the width of the X-ray transmission part 21b may be about 1 to 10 μm. The same applies to the second grating 22.

The G1 image generated by the first grating 21 expands in proportion to the distance from the X-ray focal point 11a. The grating pitch p ₂ of the second grating 22 is determined so as to coincide with the periodic pattern of the G1 image at the position of the second grating 22. Specifically, the grating pitch p ₂ of the second grating 22 is the grating pitch of the first grating 21, p ₁ , the distance L ₁ between the X-ray focal point 11 a and the first grating 21, the first grating 21. If the distance L ₂ between the grid 21 and second grid 22 is set to equation (1) so as to satisfy substantially. Hereinafter, the coordinates in the X, Y, and Z directions are x, y, and z.

The G1 image is modulated by the phase change in the X-ray caused by the subject H. The modulation amount reflects the X-ray refraction angle φ (x) of the subject H. FIG. 3 illustrates an X-ray path emitted from the X-ray focal point 11a. Reference numeral X1 indicates a path along which the X-ray goes straight when the subject H does not exist. X-rays traveling along the path X 1 pass through the first and second gratings 21 and 22 and enter the X-ray image detector 13. Reference numeral X2 indicates an X-ray path refracted by the subject H when the subject H exists. X-rays traveling along the path X <b> 2 pass through the first grating 21 and are then absorbed by the X-ray absorption unit 22 a of the second grating 22.

X-rays are refracted according to the phase shift distribution Φ (x) representing the amount of X-ray phase change by the subject H. This phase shift distribution Φ (x) is expressed by Equation (2), where λ is the wavelength of the X-ray and n (x, z) is the refractive index distribution of the subject H. Here, the y-coordinate is omitted for simplification of description.

This phase shift distribution Φ (x) is in the relationship of the refraction angle φ (x) of X-rays and the equation (3).

The amount of displacement Δx in the X direction at the position of the second grating 22 between the X-ray traveling along the path X1 and the X-ray traveling along the path X2 is based on the fact that the refraction angle φ (x) of the X-ray is very small. Approximately expressed by equation (4).

Thus, it can be seen that the displacement Δx is proportional to the differential value of the phase shift distribution Φ (x). This displacement amount Δx can be detected by a fringe scanning method, and as a result, a phase differential image is obtained.

In the present embodiment, a value obtained by dividing the grating pitch p ₂ into M pieces (p ₂ / M) is set as a scanning pitch, and the scanning mechanism 23 intermittently moves the second grating 22 in the X direction at this scanning pitch. By doing so, fringe scanning is performed. M is an integer greater than or equal to 3, for example, it is preferable that M = 5. During each stop of the second grating 22, X-rays are emitted from the X-ray source 11 and a G2 image is detected by the X-ray image detector 13. By this fringe scanning, M pieces of image data are obtained, and M pixel values are obtained for each pixel 30 of the X-ray image detector 13.

As shown in FIG. 4, the M pixel values I _k periodically change with respect to the scanning position k (k = 0, 1, 2,..., M−1) of the second grating 22. . The scanning position k is a position that is discrete in the X direction by a scanning pitch (p ₂ / M). Hereinafter, a signal representing a change in the pixel value I _k with respect to the scanning position k is referred to as an intensity modulation signal.

The broken line in the figure shows an intensity modulation signal obtained by pre-imaging (a state where the subject H is not arranged). On the other hand, the solid line indicates the intensity modulation signal in which the phase shift amount ψ (x) is generated by the subject H in the main imaging (the state where the subject H is arranged). This phase shift amount ψ (x) is in the relationship of the displacement amount Δx and the equation (5).

Therefore, for each pixel 30, a phase differential image is obtained by obtaining the phase shift amount ψ (x) of the intensity modulation signal based on the M pixel values I _k obtained by the fringe scanning.

In the case where the expression (1) is not satisfied slightly, or when the rotation around the Z direction or the inclination with respect to the XY plane is slightly generated between the first grating 21 and the second grating 22, Moire fringes occur in the G2 image. The moire fringes are moved by the movement of the second grating 22, to match the original moire fringes when the movement distance in the X direction to reach the grating pitch p _2. By detecting the movement amount of the moire fringes, the actual movement amount of the second grating 22 can be detected with high accuracy.

Next, a method for calculating the phase shift amount ψ (x) will be described. The intensity modulation signal is generally expressed by Equation (6).

Here, A ₀ represents the average intensity of the incident X-ray, A _n represents the amplitude of the intensity-modulated signal. “N” is a positive integer and “i” is an imaginary unit. As shown in FIG. 4, when the intensity modulation signal draws a sine wave, n = 1.

In the present embodiment, since the scanning pitch (p ₂ / M) is constant, Expression (7) is satisfied.

When equation (7) is applied to equation (6), phase shift amount ψ (x) is represented by equation (8).

Here, arg [...] is a function that extracts the argument of a complex number. Further, the phase shift amount ψ (x) can also be expressed as an equation (9) using an arctangent function.

Since the deviation angle of the complex number ranges from −π to + π, when the phase shift amount ψ (x) is calculated based on the equation (8), the phase shift amount ψ (x) is −π Take a value that is convolved (wrapped) in the range from to + π. On the other hand, since the range of the arc tangent function is usually in the range of −π / 2 to + π / 2, when the phase shift amount ψ (x) is calculated based on the equation (9), the phase shift The deviation amount ψ (x) takes a value convolved in the range of −π / 2 to + π / 2. In Equation (9), the range of the arc tangent function can be expanded from −π to + π by determining the denominator and the sign of the numerator in the arc tangent function. Therefore, the phase shift amount ψ (x) can be calculated in the range of −π to + π based on the equation (9).

In this embodiment, an image represented by a value (phase differential value) obtained by calculating the phase shift amount ψ (x) for each pixel 30 is referred to as a phase differential image. Note that an image represented by a value obtained by multiplying or adding a constant to the phase shift amount ψ (x) may be a phase differential image.

In FIG. 5, the image processing unit 15 includes a phase differential image generation unit 40, an offset image storage unit 41, an OK / NG region detection unit 42, an unwrap processing unit 43, an offset processing unit 44, a division determination unit 45, and an inclination correction processing unit. 46, a level difference correction processing unit 47, and a phase contrast image generation unit 48. The phase differential image generation unit 40 uses the image data for M sheets acquired by the fringe scanning in the main shooting or the pre-shooting and stored in the memory 14, and performs the calculation based on the formula (8) or the formula (9). By doing so, a phase differential image is generated.

The phase differential image generated by the phase differential image generation unit 40 at the time of pre-photographing is stored in the offset image storage unit 41 as an offset image. The phase differential image generated by the phase differential image generation unit 40 during the main photographing is input to the unwrap processing unit 43. In addition, when an offset image is newly input from the phase differential image generation unit 40, the offset image storage unit 41 deletes the stored offset image and then stores the newly input offset image.

The OK / NG area detection unit 42 detects an area (hereinafter referred to as an NG area) in which an unwrapping error is likely to occur in the phase differential image based on the M image data stored in the memory 14 at the time of actual photographing. The area other than is set as the OK area, and the area in the offset image corresponding to the OK area is set as the OK area. OK / NG area detection unit 42, for each pixel unit 30, the average intensity _{A 0} is lower than the threshold region of the intensity modulated signal, domain amplitude _{A 1} is lower than the threshold value or visibility _A 1 / _{A 0} is lower than the threshold region, Is detected as an NG region.

This NG region corresponds to a high-absorber region included in the subject H (when the subject H is a human body, a bone portion having a high X-ray absorption capability). This is based on the fact that the average intensity A ₀ , the amplitude A ₁ , or the visibility A ₁ / A ₀ decreases due to the X-rays being absorbed by the high absorber. The NG region may be detected by combining two or more of the average intensity A ₀ , the amplitude A ₁ , and the visibility A ₁ / A ₀ . In addition, when the NG regions are scattered and cannot be obtained as an aggregate region having a certain size, the size of the detected NG region may be adjusted by changing the threshold value.

The unwrap processing unit 43 unwraps only the OK region with respect to the phase differential image input from the phase differential image generation unit 40. The unwrap processing unit 43 unwraps only the OK region with respect to the offset image stored in the offset image storage unit 41.

The offset processing unit 44 performs offset correction by subtracting the offset image after the unwrapping process from the phase differential image after the unwrapping process. Specifically, the pixel value is subtracted for each corresponding pixel 30. The division determination unit 45 determines whether the OK region detected by the OK / NG region detection unit 42 is divided by the NG region (that is, there are a plurality of OK regions).

When the division determination unit 45 determines that there are a plurality of OK regions, the inclination correction processing unit 46 and the step correction processing unit 47 operate. The inclination correction processing unit 46 removes noise having a substantially constant inclination remaining in each OK region of the phase differential image (difference image) after offset correction by correction. The step correction processing unit 47 removes the step formed between the OK regions remaining in the phase differential image on which the tilt correction processing has been performed by correction.

On the other hand, when the division determination unit 45 determines that only one OK region exists, the inclination correction processing unit 46 and the step correction processing unit 47 do not operate, and thus the phase subjected to the offset correction by the offset processing unit 44 is performed. The differential image is directly input to the phase contrast image generation unit 48. This is because when there is only one OK area, there is no problem that a data level difference occurs between the OK areas, and the inclination hardly remains.

The phase contrast image generation unit 48 integrates the phase differential image with the level difference corrected or the phase differential image directly input from the offset processing unit 44 along the X direction, thereby expressing the phase contrast representing the phase shift distribution. Generate an image. Then, the phase differential image (difference image) after the step correction or the offset correction and the phase contrast image are recorded in the image recording unit 16.

Next, the unwrap processing method by the unwrap processing unit 43 will be described with reference to FIGS. FIG. 7 shows the phase differential image as an image of 10 × 7 pixels for the sake of simplicity of explanation. In this phase differential image, the NG area detected by the OK / NG area detection unit 42 is shown. The OK area is an area other than the NG area.

First, the starting point for starting the unwrapping process is set for each row or column of the phase differential image (step S10). In this step, a through line that passes only through the OK region and penetrates the phase differential image in the X direction or the Y direction is searched, and a starting point is set along one of the through lines. In FIG. 7, since there are penetrating lines in each of the X direction and the Y direction, the penetrating lines along the shorter Y direction are given priority, and starting points P0 to P6 are set in one of them. Here, the starting points P0 to P6 are set along the X direction end (short side) of the phase differential image.

After setting the starting points P0 to P6 in this way, linear straight paths R0 to R6 extending in the X direction from the starting points P0 to P6 are set, and the unwrapping process is executed along each of the linear paths R0 to R6. (Step S11). These straight paths R0 to R6 are not set in the NG area. Therefore, behind the NG area viewed from the starting points P0 to P6, pixels that belong to the same OK area as the starting points P0 to P6 but do not have the straight paths R0 to R6 set remain.

Specifically, in step S11, first, unwrap processing is performed in order along the straight line route R0 from the starting point P0, and when the unwrap processing of the straight line route R0 ends, the unwrapping processing of the starting point P1 is performed with reference to the starting point P0. Thereafter, the unwrapping process is sequentially performed from the starting point P1 along the straight path R1. Then, the unwrapping process is not performed on the pixels remaining behind the NG area in the same row as the straight line R1, and the unwrapping process of the starting point P2 is performed with the starting point P1 as a reference. Thereafter, the unwrapping process is performed in the same procedure, and when the unwrapping process for the straight line route R6 is finished, the step S11 is finished.

Thereafter, a wraparound path is set for the pixels remaining behind the NG area, and a wraparound process is performed for performing an unwrap process along the wraparound path (step S12). Specifically, in this step, the wraparound path WR0 is set for pixels remaining in the same row as the straight line route R1, and the wraparound path WR1 is set for pixels remaining in the same row as the straight line route R5. The wraparound path WR0 is unwrapped from the pixel on the adjacent straight path R0. The wraparound path WR1 is subjected to unwrap processing starting from a pixel on the adjacent straight path R6.

As shown in FIG. 8, the unwrapping process on each path sequentially detects and detects discontinuous points DP that change from the upper limit to the lower limit of the function range of the function of Equation (8) or Equation (9), or from the lower limit to the upper limit. In this process, the data after the discontinuous point DP is uniformly added or subtracted with a value corresponding to this range to eliminate the discontinuous point DP and to make the data continuous.

As shown in FIG. 9, the phase differential image may be divided by the NG region, and there may be a plurality of OK regions. In this case, steps S10 to S12 are individually executed for each OK area. Specifically, the phase differential image of FIG. 9 includes first and second OK regions. In the first OK region, starting points P0a to P6a are set on the penetrating line along the end in the X direction, and straight paths R0a to R6a are set in the Y direction from the starting points P0a to P6a. Then, an unwrap process along each straight path R0a to R6a and an unwrap process between the start points of the respective start points P0a to P6a are performed.

Similarly, in the second OK area, starting points P0b to P6b are set on the penetrating line along the end in the X direction, and straight paths R0b to R6b are set in the Y direction from the starting points P0b to P6b. Then, an unwrap process along each of the straight paths R0b to R6b and an unwrap process between the start points of the start points P0b to P6b are performed. In addition, when it is necessary to set a wraparound path in the first and second OK regions, the setting is appropriately performed, and the unwrap process is performed along the set wraparound path.

The unwrap processing unit 43 sets the starting point, the straight path, and the wraparound path for the offset image as well as the phase differential image, and performs unwrap processing according to the same procedure.

Next, the tilt correction and level difference correction methods will be described. The inclination correction processing unit 46 creates an inclination correction image representing noise having a substantially constant inclination that remains in each OK region with respect to the phase differential image subjected to the offset processing by the offset processing unit 44, and the created inclination correction image Is subtracted from each OK region of the phase differential image.

Specifically, as shown in FIG. 10, the inclination correction processing unit 46 fits a change in the pixel value ψ ′ along the line Lx penetrating the OK region in the X direction by linearly fitting the correction function (ψ '= Ax + b, a, and b are parameters) and are expanded in the Y direction to create an inclination-corrected image. The expansion in the Y direction is performed, for example, by applying the correction function to each row along the X direction. In addition to the correction function (first correction function), a correction function (second correction function) is similarly obtained along the line Ly penetrating the OK region in the Y direction, and the first and second correction functions are obtained. An inclination correction image may be created based on the correction function. The inclination of the noise described above depends on the direction of moire fringes generated in the G2 image, and is likely to occur in the X direction and the Y direction. Therefore, by creating an inclination correction image based on the first and second correction functions, a more accurate inclination correction image reflecting the inclination in the X direction and the Y direction can be obtained.

The tilt correction processing unit 46 performs the above process for each OK region, and generates a tilt correction image for each OK region.

The step correction processing unit 47 calculates the average value of the pixel values for at least a part of each OK region in the phase differential image after the tilt correction processing by the tilt correction processing unit 46, and the average value is the same in each OK region. A constant value is added to or subtracted from the pixel value for each OK region. The step correction processing unit 47 is not limited to a part of each OK region, and may calculate an average value of all pixel values included in each OK region. In this case, since all pixel values in the OK region are averaged, the noise is leveled and the influence of the local trend can be kept low. Further, in order to calculate the average value of the pixel values for a part of each OK region, the step correction processing unit 47 calculates the average value of the pixel values for the central region (for example, a rectangular region) among the OK regions. Or a partial area along one of the four sides of the phase differential image in each OK area (for example, the NG area divides the phase differential image in the Y direction, and the OK area is X If they are separated in the direction, the average value of the pixel values may be calculated for a region along one of the two sides facing the Y direction of the phase differential image.

Next, the operation of the X-ray imaging apparatus 10 will be described with reference to the flowcharts shown in FIGS. When the shooting mode is selected using the operation unit 18a (step S20), it is determined whether or not the selected shooting mode is pre-shooting (step S21). If it is pre-photographing (YES in step S21), a standby state for photographing instructions is set (step S22).

When an imaging instruction is given using the operation unit 18a (YES in step S22), the X-ray source 11 X is scanned at each scanning position k while the second grating 22 is moved by a predetermined scanning pitch by the scanning mechanism 23. Radiation and detection of the G2 image by the X-ray image detector 13 are performed (step S23). As a result of the fringe scanning, M pieces of image data are generated and stored in the memory 14.

Thereafter, the image data for M sheets stored in the memory 14 is read by the image processing unit 15. In the image processing unit 15, a phase differential image is generated by the phase differential image generation unit 40 (step S24). This phase differential image is stored in the offset image storage unit 41 as an offset image (step S25). The pre-photographing operation ends here. Note that this pre-imaging may be performed at least once in a state in which the subject H is not disposed when the X-ray imaging apparatus 10 is started up, and need not be performed every time before the main imaging.

Next, when the subject H is arranged and the main imaging is selected by selecting the imaging mode in step S20 (NO in step S21), the imaging instruction standby state is set (step S30). When a photographing instruction is given using the operation unit 18a (YES in step S30), the same stripe scanning as in step S23 is performed (step S31), and M pieces of image data are stored in the memory 14. Thereafter, similarly, the phase differential image is generated by the phase differential image generation unit 40 (step S32).

Then, based on the image data stored in the memory 14, the NG area and the OK area are detected by the OK / NG area detecting unit 42 (step S33). When the NG area and the OK area are detected, the unwrap processing unit 43 unwraps only the OK area of the phase differential image generated in step S32 (step S34). Similarly, only the OK region of the offset image stored in the offset image storage unit 41 is unwrapped by the unwrap processing unit 43 (step S35).

When the unwrap process is completed, the offset processing unit 44 performs offset correction by subtracting the offset image after the unwrap process from the phase differential image after the unwrap process (step S36). Then, the division determination unit 45 determines whether or not the OK area is divided (a plurality of OK areas exist) (step S37).

When there are a plurality of OK areas (YES in step S38), the inclination correction processing unit 46 creates an inclination correction image for each OK area of the phase differential image after the offset process, and each inclination correction image is converted into the phase differential image. By subtracting from each OK area of the image, the inclination is removed (step S39). Then, the step correction processing unit 47 calculates the average value of the pixel values for at least a part of each OK region of the phase differential image after the tilt correction processing, and the average value is the same in each OK region. A fixed value is added to or subtracted from the pixel value for each OK region (step S40). Thereby, the level | step difference between OK area | regions is eliminated.

On the other hand, when there is one OK region (NO in step S38), the processing by the inclination correction processing unit 46 and the step correction processing unit 47 is not performed, and the phase differential image after the step correction is directly sent to the phase contrast image generation unit 48. Entered.

The phase contrast image generation unit 48 integrates the phase differential image after the step correction or the phase differential image directly input from the offset processing unit 44, thereby generating a phase contrast image (step S41). Alternatively, the phase differential image and the phase contrast image after the offset correction are recorded in the image recording unit 16, and then displayed on the monitor 18b (step S42).

Note that the determination process by the division determination unit 45 may be performed at any time after the determination of the OK / NG region and before the inclination correction process is performed.

As described above, the unwrap processing unit 43 performs the unwrap process only on the OK region other than the NG region where the unwrap error is likely to occur on the phase differential image, thereby preventing the occurrence of the unwrap error and reducing the phase differential with less noise. An image is obtained. Since soft tissue (cartilage portion or the like), which is a region of interest in X-ray phase imaging, exists outside the NG region, it is prevented that imaging of the soft tissue is inhibited by noise due to unwrapping errors.

Further, since the unwrap processing unit 43 performs the same unwrap processing on the offset image, as shown in FIGS. 13A and 13B, the noise unevenness included in the phase differential image is It is almost the same as noise unevenness. In the phase differential image (difference image) after the offset correction, as shown in FIG. 13C, a slight inclination remains in each OK region and a slight step S remains between the OK regions. This inclination is removed by the inclination correction processing unit 46. The step S is removed by the step correction processing unit 47. Note that the soft tissue that is the region of interest is represented by data at a higher frequency than the tilt correction image created by the tilt correction processing unit 46, and therefore is not removed by the tilt correction processing.

In the above embodiment, when there are a plurality of OK areas, the inclination correction processing unit 46 obtains a correction function for each OK area as shown in FIG. 14A. It is determined whether or not the slope of the correction function is close between the areas. If the slope is close, as shown in FIG. 14B, the pixel values in the OK area are arranged so that the pixel values are substantially aligned on a straight line. The correction function may be obtained by shifting and fitting pixel value changes in a plurality of OK regions in a linear format. The shift of the pixel value may be performed based on, for example, an average value of pixel values in each OK region and a difference between the average values. As a result, the correction function can be obtained with higher accuracy.

In the above embodiment, the inclination correction processing unit 46 is not limited to the linear format, and may obtain a correction function by performing fitting using a polynomial.

Further, in the above embodiment, the unwrap processing unit 43 unwraps the starting point of the straight line route and the starting point of the next straight line route each time the straight line route is unwrapped among the plurality of straight line routes R0 to R6. However, before unwrapping the straight paths R0 to R6, the unwrap processing is first performed between the starting points of the starting points P0 to P6, and the straight paths R0 to R6 are started from the starting points P0 to P6 after the unwrapping process. Each may be unwrapped.

Conversely, after unwrapping each of the straight paths R0 to R6, unwrapping is performed between the starting points of the respective starting points P0 to P6, and the data of each of the straight paths R0 to R6 after the unwrapping is obtained as each starting point after the unwrapping process. You may shift according to the data of P0-P6.

Moreover, in the said embodiment, when the unwrap processing part 43 has a penetration line in each of the X direction and the Y direction, the starting point is set along the Y direction in preference to the Y direction. The starting point may be set along the X direction with priority on the X direction.

Further, based on the shape of the NG area, it may be determined whether the starting point is set along the Y direction or the X direction. For example, the setting direction of the starting point is determined so that the number of wraparound processes performed on the pixels remaining behind the NG area is reduced. For example, when the starting point is set along the Y direction as shown in FIG. 7, the wrapping process is required for two lines along the X direction (lines including the starting points P1 and P5). The number of times is “2”. On the other hand, when the starting point is set along the X direction, the wraparound process is required for six lines along the Y direction, so the number of wraparound processes is “6”. Therefore, when the NG area has the shape shown in FIG. 7, the Y direction in which the number of wraparound processes is reduced is determined as the starting direction setting direction.

Moreover, in the said embodiment, in the unwrap process by the unwrap process part 43, the starting point is set along the X direction end in a OK area | region, or a Y direction end, However, A starting point does not necessarily follow an edge part in an OK area | region. It is not necessary to set.

In the above embodiment, the unwrap processing unit 43 sets the starting point for each column or each row so that the unwrap processing is performed for each column or each row of the phase differential image. It is also possible to set one starting point (starting point) for each pixel and sequentially unwrap the pixels adjacent from this starting point.

For example, as shown in FIG. 15A, a start point P0 is set in the OK region, and pixels adjacent in the X direction and the Y direction from this start point P0 are unwrapped. Next, as shown in FIG. 15B, the pixels adjacent in the X direction and the Y direction are unwrapped from each unwrapped pixel. At this time, if the adjacent pixel is a pixel belonging to the NG area, it is not considered and the unwrapping process is not performed. Further, when the adjacent pixel is an adjacent pixel of another pixel that has been unwrapped, priority is given to one of them. Here, the adjacent pixel in the X direction has priority over the adjacent pixel in the Y direction. Thereafter, the unwrapping process may be advanced in the same manner.

In the above-described embodiment, the OK / NG region detection unit 42 detects an NG region in which an unwrapping error is likely to occur based on the average intensity, amplitude, or visibility of the intensity modulation signal. Is not limited to this, and an area where variation between pixels of the average intensity of the intensity modulation signal (that is, dispersion between pixels of the absorption image) or dispersion between pixels of the phase differential image is larger than a predetermined value is detected as an NG area. May be. In addition, it is preferable that the dispersion | variation between the pixels of this phase differential image is a dispersion | variation to the direction (X direction) orthogonal to the lattice line of the 1st and 2nd grating | lattices 21 and 22. FIG.

Further, an absolute value is taken for each pixel of the phase differential image, and an edge portion of the superabsorbent region is detected by detecting a portion where the absolute value exceeds a predetermined value, and a region surrounded by the edge portion is determined as an NG region You may detect as. However, since the phase differential image used for the detection of the NG region here is the one before the unwrap processing by the unwrap processing unit 43, the discontinuous point DP shown in FIG. 8 exists, and the edge portion of the high absorber region It is possible to inhibit detection. Even if the absorption image is used instead of the phase differential image, the NG region cannot be detected with high accuracy because the signal of the absorption image has a gradual change at the edge portion of the high absorber region.

For this reason, it is preferable to detect the NG region based on the absorption differential image obtained by differentiating the absorption image in one direction. In this case, the OK / NG region detection unit 42 generates an absorption image by calculating the average value of the intensity modulation signals corresponding to each pixel unit 30 based on the M pieces of image data obtained during the main photographing. Then, an absorption differential image is generated by differentiating the generated absorption image. The differential processing of the absorption image is performed, for example, by calculating a difference value between pixels in the X direction. Then, the OK / NG region detection unit 42 performs differentiation processing or the like on the absorption differential image, and detects the NG region by detecting a peak protruding in the positive or negative direction. In the absorption differential image, the signal changes sharply at the edge portion of the high-absorber region, and no wrap occurs (no discontinuous point exists), so that the NG region can be accurately detected.

Note that the OK / NG region detection unit 42 may also refer to the absorption image in addition to the absorption differential image, and detect a region having a large absorption amount surrounded by a peak generated in the absorption differential image as an NG region.

Further, the OK / NG area detection unit 42 generates an absorption image based on M image data obtained at the time of pre-shooting, stores this as a corrected image, and stores M images at the time of main shooting. An absorption differential image may be generated after the correction image is subtracted or divided from the absorption image generated based on the image data. Thereby, since the noise component not related to the subject H is removed from the absorption differential image, the NG region is detected with higher accuracy.

Further, the OK / NG region detection unit 42 may detect a region where the average intensity or the maximum intensity of the intensity modulation signal is larger than a predetermined value and the intensity modulation signal is saturated as an NG area. This saturation of the intensity modulation signal is likely to occur in a pixel region (elementary region) that is directly transmitted to the X-ray image detector 13 through the first and second gratings 21 and 22 without passing through the subject H. When the intensity modulation signal is saturated, the phase shift amount ψ (x) cannot be obtained accurately, and this unaccompanied region is also a region where unwrapping errors are likely to occur. The OK / NG area detecting unit 42 may detect the NG area by combining the above detection criteria.

Furthermore, when a defect occurs in the X-ray image detector 13, the first grating 21, or the second grating 22, or dust or the like adheres, the pixel value of the specific pixel unit 30 is always high, or May be lower. The region where such a pixel defect occurs is a region where an unwrapping error is likely to occur because the average intensity, amplitude, or visibility of the intensity modulation signal indicates an abnormal value. Such a pixel defect region can also be detected as an NG region by appropriately combining the above detection criteria.

In the above embodiment, since the unwrapping process is not performed in the NG area, the NG area of the phase differential image that is finally recorded in the image recording unit 16 and displayed on the monitor 18b has a discontinuous point remaining. Since there is a possibility that the image has a large noise, an NG region image replacement unit 50 may be provided in the image processing unit 15 as shown in FIG. Here, the same reference numerals are given to the same components as those in the above embodiment, and the description thereof is omitted.

The NG region image replacement unit 50 generates an absorption image, a differential image of the absorption image, or a small-angle scattered image based on the M image data stored in the memory 14 at the time of the main photographing, and corresponds to the NG region of the image. The portion to be replaced is inserted into the NG area of the phase differential image after offset correction. Similarly, the NG area of the phase contrast image may be replaced. The absorption image is generated by imaging the average intensity of the intensity modulation signal. The differential image of the absorption image is generated by differentiating the absorption image in a predetermined direction (for example, the X direction). The small angle scattered image is generated by imaging the amplitude of the intensity modulation signal. Furthermore, when replacing the NG area of the phase differential image or the phase contrast image with each of the above images, the NG area is replaced after multiplying each image by an appropriate coefficient so that the balance between the OK area and the contrast is improved. May be.

Further, in the above embodiment, the step correction processing unit 47 performs the step correction based on making the average value of the pixel values the same for at least a part of each OK region, but using an absorption differential image. The step may be corrected. Since the absorption differential image does not cause wrapping, there is almost no step between OK regions, and it is suitable as reference data for step correction.

Specifically, the level difference correction processing unit 47 generates the absorption differential image 60 as described above, and the OK / NG region detection unit 42 detects the OK in the absorption differential image 60 as shown in FIG. A region is set, and pixel regions D1 and D2 each including one or a plurality of pixels having substantially the same pixel value are extracted from each OK region. Then, a constant value is added to each pixel value for each OK region so that the pixel values of the pixel regions D1, D2 of each OK region are the same in the phase differential image after the tilt correction processing by the tilt correction processing unit 46. Or subtract. In addition, when each pixel area | region D1, D2 is made into the area | region which consists of a some pixel, let the average value of this some pixel value etc. be the pixel value of each pixel area | region D1, D2.

Further, it is preferable that the level difference correction processing unit 47 sets the pixel areas D1 and D2 to areas where noise is less than a predetermined threshold in each OK area. In this way, the pixel areas D1 and D2 are set as a blank area other than the area where the subject H (soft tissue or flesh) exists in each OK area. In the blank area, there is little noise and the change in pixel value is almost constant, so that a step between the OK areas can be obtained remarkably.

In this embodiment as well, an absorption differential image may be generated after subtracting or dividing a correction image generated by pre-imaging from an absorption image generated by actual imaging.

In the above-described embodiment, the subject H is disposed between the X-ray source 11 and the first grating 21, but the subject H is disposed between the first grating 21 and the second grating 22. You may arrange.

In the above-described embodiment, the second grating 22 is moved in the direction (X direction) perpendicular to the grid lines during fringe scanning. However, the second grid 22 is inclined with respect to the grid lines (XY plane). May be moved in a direction not orthogonal to the X direction and the Y direction. In this case, the scanning position k may be set based on the X-direction component of the movement of the second grating 22. By moving the second grating 22 in a direction inclined with respect to the grid line, the stroke (movement distance) required for scanning for one period of the fringe scanning becomes longer, and there is an advantage that the movement accuracy is improved.

Moreover, in the said embodiment, although the 2nd grating | lattice 22 is moved at the time of fringe scanning, it replaces with the 2nd grating | lattice 22, and the 1st grating | lattice 21 is moved to the direction orthogonal to the grid line, or the direction which inclines. May be.

In the first embodiment, the X-ray source 11 that emits cone-beam X-rays emitted from the X-ray source 11 is used. However, an X-ray source that emits parallel-beam X-rays is used. Is also possible. In this case, instead of the above equation (1), the first and second gratings 21 and 22 may be configured so as to substantially satisfy p ₂ = p ₁ .

In the above embodiment, the X-rays emitted from the X-ray source 11 are incident on the first grating 21 and the X-ray source 11 has a single focal point. However, as shown in FIG. The X focus is dispersed by providing a multi-slit (source grating) 70 described in WO2006 / 131235 etc. immediately after the emission side of the source 11 (between the X-ray source 11 and the first grating 21). May be used. The grid lines of the multi slit 70 are parallel to the Y direction. As a result, it becomes possible to use a high-power X-ray source and the X-ray dose is improved, so that the image quality of the phase differential image is improved. In this case, the pitch p ₀ of the multi slit 70 needs to satisfy the following expression (10). Here, the distance L ₁ represents the distance from the multi slit 70 to the first grating 21.

When the multi slit 70 is provided in this way, the position of the multi slit 70 becomes the position of the X-ray focal point, and therefore the distance L _{1 in the} above embodiment is replaced with the distance L ₀ .

In addition, when the multi slit 70 is provided, the first and second stripes are scanned in addition to moving the first grating 21 or the second grating 22 while the multi slit 70 is fixed. It is possible to perform fringe scanning by moving the multi slit 70 while the gratings 21 and 22 are fixed. In this case, the multi slit 70 may be intermittently moved in the X direction using a value (p ₀ / M) obtained by dividing the pitch p ₀ of the multi slit 70 by M as described above. As a result, the scanning position k of the multi slit 70 with respect to the first and second gratings 21 and 22 is changed in order of k = 0, 1, 2,..., M−1.

Moreover, in the said embodiment, although the 1st grating | lattice 21 is comprised so that incident X-ray may be projected geometrically optically, as known in WO2004 / 058070 etc., the 1st grating | lattice 21 is comprised. May be configured to generate the Talbot effect. In order to generate the Talbot effect in the first grating 21, a small-focus X-ray light source or a multi-slit 70 may be used so as to enhance the spatial coherence of X-rays.

If the Talbot effect in the first grating 21 occurs, the self-image of the first grating 21 (G1 image) occurs at a distance in the Z direction by the Talbot distance Z _m from the first grating 21. Therefore, it is necessary to distance L ₂ from the first grid 21 to the second grid 22 and Talbot distance Z _m. In this case, the first grating 21 can be a phase-type grating.

Talbot distance Z _m is dependent on the beam shape of the structure and the X-ray of the first grating 21. For example, the first grating 21 are absorption type grating, X-rays emitted from the X-ray source 11 in the case of a cone beam shape, Talbot distance Z _m is represented by the following formula (11). Here, “m” is a positive integer. In this case, the grating pitches p ₁ and p ₂ are set so as to substantially satisfy the above expression (1) (however, when the multi-slit 70 is used, the distance L ₁ is replaced with the distance L ₀ ). .

Further, when the first grating 21 is a phase-type grating that applies phase modulation of π / 2 to the X-ray, and the X-ray emitted from the X-ray source 11 has a cone beam shape, the Talbot distance Z _m is And represented by equation (12). Here, “m” is “0” or a positive integer. In this case, the grating pitches p ₁ and p ₂ are set so as to substantially satisfy the expression (1) (however, when the multi slit 70 is used, the distance L ₁ is replaced with the distance L ₀ ).

In addition, when the first grating 21 is a phase-type grating that gives a phase modulation of π to the X-ray, and the X-ray emitted from the X-ray source 11 has a cone beam shape, the Talbot distance Z _m is expressed by the equation It is represented by (13). Here, “m” is “0” or a positive integer. In this case, since the pattern period of the G1 image is ½ times the grating period of the first grating 21, the grating pitches p ₁ and p ₂ are set so as to substantially satisfy Expression (14) ( However, when using a multi-slit 70, the distance _{L 1} is replaced by a distance _{L 0).}

The first grating 21 is absorption grating, if X-rays emitted from the X-ray source 11 is a parallel beam shape, Talbot distance Z _m is represented by the formula (15). Here, “m” is a positive integer. In this case, the lattice pitches p ₁ and p ₂ are set so as to substantially satisfy the relationship of p ₂ = p ₁ .

Further, when the first grating 21 is a phase-type grating that applies phase modulation of π / 2 to X-rays, and the X-rays emitted from the X-ray source 11 are parallel beams, the Talbot distance Z _m is And represented by equation (16). Here, “m” is “0” or a positive integer. In this case, the lattice pitches p ₁ and p ₂ are set so as to substantially satisfy the relationship of p ₂ = p ₁ .

When the first grating 21 is a phase-type grating that applies π phase modulation to X-rays, and the X-rays emitted from the X-ray source 11 are parallel beams, the Talbot distance Z _m It is represented by (17). Here, “m” is “0” or a positive integer. In this case, since the pattern period of the G1 image is ½ times the grating period of the first grating 21, the grating pitches p ₁ and p ₂ almost satisfy the relationship of p ₂ = p _1/2. Set to

In the above embodiment, the grating portion 12 is provided with the two gratings of the first and second gratings 21 and 22. However, the second grating 22 may be omitted and only the first grating 21 may be used. Is possible.

For example, by using an X-ray image detector described in Japanese Patent Laid-Open No. 2009-133823, the second grating 22 can be omitted and only the first grating 21 can be provided. This X-ray image detector is a direct conversion type X-ray image detector including a conversion layer that converts X-rays into electric charges and a charge collection electrode that collects electric charges converted in the conversion layer. The charge collection electrode includes a plurality of linear electrode groups. One linear electrode group is obtained by electrically connecting linear electrodes arranged at a constant period, and is arranged so that the phases thereof are different from those of other linear electrode groups. This linear electrode group functions as the second grating 22, and the presence of a plurality of linear electrode groups allows detection of a plurality of G2 images having different phases in one imaging. Therefore, in this configuration, the scanning mechanism 23 can be omitted.

In addition, a pixel division method for generating a phase differential image based on single image data obtained by the X-ray image detector 13 via the first and second gratings 21 and 22 without the scanning mechanism 23 is provided. It has been proposed by the applicant (currently published as WO2012 / 056724). The first grating 21 and the second grating 22 are slightly rotated around the Z direction, and moire fringes having a period in the Y direction are generated in the G2 image. The single image data obtained by the X-ray image detector 13 is divided into groups of pixel rows (pixels arranged in the X direction) having different phases from each other with respect to the moire fringes, and the plurality of divided image data are divided into fringes. Assuming that it is based on a plurality of G2 images that are different from each other by scanning, a phase differential image is generated in the same procedure as in the fringe scanning method. In this pixel division method, the intensity modulation signal described above is expressed as a change in intensity of pixel values for one cycle of moire fringes generated in single image data.

Further, similarly to the pixel division method, the scanning mechanism 23 is omitted, and the phase differential image is obtained based on the single image data obtained by the X-ray image detector 13 via the first and second gratings 21 and 22. As a generation method, a Fourier transform method described in WO2010 / 050484 is known. This Fourier transform method obtains a Fourier spectrum by performing a Fourier transform on the single image data, separates a spectrum corresponding to a carrier frequency (a spectrum carrying phase information) from the Fourier spectrum, and then reverses the spectrum. In this method, a phase differential image is generated by performing Fourier transform. In this Fourier transform method, the intensity modulation signal described above is expressed as a change in intensity of pixel values for one cycle of moire fringes generated in a single image data, as in the case of the pixel division method.

The present invention can be applied to an industrial radiography apparatus and the like in addition to a radiography apparatus for medical diagnosis. In addition to X-rays, gamma rays or the like can be used as radiation.

Claims (19)

1. A radiation source that emits radiation; and
   A radiation detector that detects the radiation transmitted through the subject and generates image data;
   A grating portion disposed between the radiation source and the radiation detector;
   Based on the image data, a phase differential image generation unit that generates a phase differential image in a state where the phase differential value is wrapped within a predetermined range;
   An offset image storage unit that stores the phase differential image generated by the phase differential image generation unit in a state where the subject is not disposed as an offset image;
   From the phase differential image generated by the phase differential image generation unit in a state where the subject is arranged, an NG region in which an unwrap error is likely to occur is detected, and other regions are set as OK regions and An OK / NG area detecting unit that sets an area in the corresponding offset image as an OK area;
   An unwrap processing unit that unwraps only the OK region for each of the phase differential image and the offset image;
   An offset processing unit for performing offset correction for subtracting the offset image on which the unwrap processing has been performed from the phase differential image on which the unwrap processing has been performed;
   An inclination correction processing unit that performs an inclination correction process for removing noise having a substantially constant inclination that remains in the OK region among the phase differential images subjected to the offset correction;
   A step correction processing unit for performing a step correction process for removing a step formed between the OK regions;
   A radiation imaging apparatus comprising:
2. A determination unit that determines whether there are a plurality of the OK regions;
   The radiation imaging apparatus according to claim 1, wherein the inclination correction processing unit and the step correction processing unit perform the processes only when there are a plurality of the OK regions.
3. The inclination correction processing unit obtains a correction function for the phase differential image subjected to the offset correction by fitting a change in the pixel value of the OK region in one direction with a linear form or a polynomial, and calculates the correction function. The radiography according to claim 2, wherein a two-dimensional tilt correction image is created by extending in a direction orthogonal to the one direction, and the noise is removed based on the tilt correction image. apparatus.
4. The inclination correction processing unit is configured to change a pixel value of the adjacent OK region in one direction in a linear form or a polynomial when there are a plurality of the OK regions and the inclination of the correction function of the adjacent OK regions is close. 4. The radiographic apparatus according to claim 3, wherein a correction function is obtained by fitting together in step (1), and the correction function is used as a correction function for the adjacent OK region.
5. The inclination correction processing unit performs first and second fitting by fitting a change in pixel value in two directions with a linear form or a polynomial in the OK region existing in the phase differential image subjected to the offset correction. The correction function is obtained, a two-dimensional tilt correction image is created based on the first and second correction functions, and the noise is removed based on the tilt correction image. The radiation imaging apparatus described in 1.
6. The step correction processing unit calculates an average value of pixel values for at least a part of each OK region in the phase differential image after the tilt correction processing, and the average value is the same in each OK region. The radiation imaging apparatus according to claim 2, wherein a constant value is added to or subtracted from the pixel value for each OK region.
7. The unwrap processing unit sets a starting point along a penetrating line that penetrates the OK region in one direction, and unwraps between the starting points and unwraps along a straight path perpendicular to the penetrating line from the starting point. The radiation imaging apparatus according to claim 1, wherein the radiation imaging apparatus performs processing.
8. The radiation imaging apparatus according to claim 7, wherein the unwrap processing unit further performs unwrap processing for pixels in the OK region remaining behind the NG region as viewed from the starting point.
9. 9. The radiation imaging apparatus according to claim 8, wherein the unwrap processing unit determines the setting direction of the starting point so that the number of times of unwrap processing for pixels remaining behind the NG region is reduced. .
10. The radiation imaging apparatus according to claim 7, wherein the unwrap processing unit sets the starting point along any one side of the quadrilateral phase differential image.
11. The grating unit partially shields the first periodic pattern image by passing the radiation from the radiation source to generate the first periodic pattern image, and displays the second periodic pattern image. A second grid to generate,
    The radiation imaging apparatus according to claim 1, wherein the radiation image detector generates the image data by detecting the second periodic pattern image.
12. The radiation imaging apparatus according to claim 11, wherein the phase differential image generation unit generates the phase differential image based on single image data obtained by the radiation detector.
13. The grating unit further includes a scanning mechanism that moves the first grating or the second grating at a predetermined scanning pitch and sequentially sets a plurality of scanning positions.
    The radiation image detector detects the second periodic pattern image at each scanning position and generates the image data;
    The phase differential image generation unit generates the phase differential image based on a plurality of image data generated by the radiation image detector at the plurality of scanning positions. Radiography equipment.
14. The radiation imaging apparatus according to claim 13, wherein the scanning mechanism moves the first grating or the second grating in a direction perpendicular to or inclined with respect to the grating line.
15. The OK / NG area detection unit is configured to generate the NG area based on one or a combination of an average intensity, an amplitude, and a visibility of an intensity modulation signal representing an intensity change of a pixel value of the image data with respect to the plurality of scanning positions. The radiation imaging apparatus according to claim 13, wherein:
16. The OK / NG region detection unit generates an absorption image based on an average intensity of an intensity modulation signal representing an intensity change of a pixel value of the image data with respect to the plurality of scanning positions, and performs a differentiation process on the absorption image. The radiation imaging apparatus according to claim 13, wherein an absorption differential image is generated by the method and the NG region is detected based on the absorption differential image.
17. The step correction processing unit sets the OK region in the absorption differential image, extracts a pixel region having substantially the same pixel value from each OK region, and performs the phase differentiation in which the tilt correction processing has been performed. 17. The image according to claim 16, wherein a constant value is added to or subtracted from each pixel value for each OK region so that the pixel values of the pixel regions of the OK regions are the same for the image. Radiography equipment.
18. The NG region image replacement unit that generates any one of an absorption image, an absorption differential image, and a small-angle scattered image and replaces the NG region of the phase differential image, further comprising: Radiography equipment.
19. In a state in which the subject is not arranged, the radiation emitted from the radiation source and detected through the lattice portion is generated to generate image data. Based on this image data, the phase differential value is wrapped within a predetermined range. A pre-shooting step of generating a phase differential image and storing this as an offset image;
    In a state in which the subject is arranged, the radiation emitted from the radiation source and passed through the subject and the lattice unit is detected to generate image data. Based on the image data, the phase differential value is within a predetermined range. A main photographing process for generating a phase differential image in a wrapped state;
    An NG area in which an unwrap error is likely to occur is detected from the phase differential image, and an area other than this is set as an OK area, and an area in the offset image corresponding to the OK area is set as an OK area. A detection process;
    For each of the phase differential image and the offset image, an unwrap processing step of unwrapping only the OK region;
    An offset processing step for performing offset correction for subtracting the offset image on which the unwrap processing has been performed from the phase differential image on which the unwrap processing has been performed;
    An inclination correction processing step of removing noise having a substantially constant inclination remaining in the OK region of the phase differential image subjected to the offset correction;
    A step correction processing step of removing a step formed between the OK regions;
    A radiation imaging method comprising:

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