JP2013063098A - Radiographic apparatus and image processing method - Google Patents

Abstract

The present invention eliminates the need for complicated route setting processing for unwrapping processing and easily prevents unwrapping errors.
An X-ray image detector detects X-rays emitted from an X-ray source and passed through first and second gratings to generate image data. The NG area detection unit detects an NG area where an unwrap error is likely to occur based on the image data 50. The noise reduction processing unit reduces the noise by performing a moving average process on the NG area of the image data 50. The phase differential image generation unit generates a phase differential image based on the image data after the noise reduction processing. The unwrap processing unit performs unwrap processing on the phase differential image.
[Selection] Figure 6

Description

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

  Radiation, such as X-rays, has a characteristic that it is absorbed and attenuated 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.

  In a general X-ray imaging apparatus, an object is placed between an X-ray source that emits X-rays and an X-ray image detector that detects X-rays, and X-rays transmitted through the object are imaged. Do. In this case, X-rays emitted from the X-ray source toward the X-ray image detector are absorbed and attenuated when passing through the subject, and then enter the X-ray image detector. As a result, an image based on an X-ray intensity change by the subject is detected by the X-ray image detector.

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

  Against this background, research on X-ray phase imaging that obtains 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 X-ray phase change based on the fact that the phase change of the X-ray incident on the subject is larger than the intensity change. A high-contrast image can be obtained. As one type of X-ray phase imaging, an X-ray imaging apparatus that detects an X-ray phase change by configuring an X-ray Talbot interferometer using two diffraction gratings and an X-ray image detector is known. (For example, refer to Patent Document 1).

  In this X-ray imaging apparatus, the first diffraction grating is disposed behind the subject as viewed from the X-ray source, and the second diffraction grating is disposed at a position separated from the first diffraction grating by the Talbot distance. An X-ray image detector is arranged behind. The Talbot distance is the distance at which the X-rays that have passed through the first diffraction grating form a self-image (striped image) of the first diffraction grating due to the Talbot effect, and the grating pitch of the first diffraction grating and the X-ray wavelength Depends on and. This self-image is modulated by refraction caused by the phase change of X-rays in the subject. By detecting this modulation amount, the phase change of the X-ray is imaged.

  A fringe scanning method is known as a method for detecting the modulation amount. In the fringe scanning method, the second diffraction grating is scanned with respect to the first diffraction grating in a direction parallel to the plane of the first diffraction grating and perpendicular to the grating line direction of the first diffraction grating. X-rays are radiated from the X-ray source at each scanning position while being translated (scanned) at a pitch, and the X-ray image passing through the subject and the first and second diffraction gratings is imaged by the X-ray image detector. Is the method. Calculating a phase shift amount (phase difference from an initial position when no subject exists) for a signal (intensity modulation signal) representing a change in the pixel value of each pixel obtained by the X-ray image detector with respect to the scanning. Thus, an image related to the modulation amount is obtained. This image is an image reflecting the refractive index of the subject, and corresponds to the differential amount of the X-ray phase change (phase shift), and is called a phase differential image.

As shown in Patent Document 1, the phase shift amount is calculated using a function (arg [...]) for extracting a complex argument and an arctangent function (tan ^-1 [...]). . For this reason, the phase differential image is expressed by a value convolved (wrapped) in the range of the function (−π to + π or −π / 2 to + π / 2). In the phase differential image wrapped in this way, discontinuous points may occur at locations where the upper limit of the range changes to the lower limit or at locations where the lower limit changes to the upper limit. It is known to perform the unwrapping process (see, for example, Patent Document 2).

  The unwrap process is performed in order along a predetermined path from a predetermined position in the image as a starting point. When the discontinuous point is detected in the path, a value corresponding to the range of the function is uniformly added to or subtracted from data after the discontinuous point. Thereby, discontinuous points are eliminated and data is continuous.

WO2004 / 058070 JP 2011-045655 A

  However, when the subject includes a high-absorber having a high X-ray absorption capacity such as a bone part, the high-absorber greatly attenuates the X-ray and decreases the intensity and amplitude of the intensity modulation signal. The calculation accuracy of the phase shift amount is lowered. Thereby, an unwrapping error is likely to occur in the high-absorber region of the phase differential image. In this unwrapping error, discontinuity occurs at a place that is not a discontinuous point and is considered as a discontinuous point. There are cases where the unwrapping process is not performed because it is not regarded as a discontinuous point.

  For example, when there is a bone region on the path to be unwrapped, an unwrap error is likely to occur in the bone region, and an error value (a value corresponding to the range of the above function) in the path after the location where the unwrap error has occurred Is accumulated. As a result, streak noise along the path direction of the unwrap process is generated in the phase differential image after the unwrap process. There is a problem that this noise overlaps a part of the cartilage region (soft tissue region) around the bone region, and obstructs the imaging of the soft tissue of the heart, which is the region of interest in X-ray phase imaging.

  Therefore, a method is conceivable in which a region (NG region) where an unwrap error is likely to occur is determined from the phase differential image, and an unwrap processing path is set only in other regions (OK regions) while avoiding the NG region. . However, in this method, it is indispensable to set the optimum unwrap processing path one by one in accordance with the shape of the OK area, and there is a problem that a complicated path setting process is required.

  The present invention has been made in view of the above problems, and a radiation imaging apparatus and an image processing method capable of easily preventing an unwrapping error without requiring a complicated route setting process for the unwrapping process. The purpose is to provide.

  In order to achieve the above object, a radiation imaging apparatus of the present invention includes a radiation detector that detects radiation emitted from a radiation source and transmitted through a subject to generate image data, and the radiation source and the radiation detector. A NG region detecting unit that detects an NG region in which an unwrap error is likely to occur based on the image data, and a noise reduction processing unit that performs noise reduction processing on the NG region of the image data A phase differential image generation unit that generates a phase differential image expressed by a value wrapped in a predetermined range based on the image data after the noise reduction process, and an unwrap process that performs an unwrap process on the phase differential image And a section.

  The noise reduction process is preferably a smoothing process for smoothing the NG region of the image data. The smoothing process is preferably a moving average process in which each pixel value in the NG area is a target pixel, and the pixel value of the target pixel is replaced with an average value of pixel values of a predetermined area including the target pixel. .

  An offset image storage unit that stores the offset image as an offset image; and an offset processing unit that subtracts the offset image from the phase differential image after the unwrap processing, wherein the offset image is pre-photographed in a state where the subject is not disposed. Preferably, the phase differential image generation unit generates a phase differential image based on image data generated by the radiation detector, and the unwrap processing unit performs unwrap processing.

  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. It is preferable that the radiographic image detector generates the image data by detecting the second periodic pattern image.

  The grating unit 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, and the radiological image detector includes the scanning mechanism at each scanning position. Preferably, the second periodic pattern image is detected to generate image data, and the phase differential image generation unit generates a phase differential image based on a plurality of image data generated by the radiation image detector. In this case, it is preferable that the scanning mechanism moves the first grating or the second grating in a direction orthogonal to the grating line. The scanning mechanism preferably moves the first grating or the second grating in a direction inclined with respect to the grating line.

  It is also preferable that the phase differential image generation unit generates the phase differential image based on single image data obtained by the radiation detector.

  It is preferable that the NG area detecting unit detects an NG area based on one or a combination of average intensity, amplitude, and visibility of an intensity modulation signal representing an intensity change of a pixel value.

  Preferably, the first grating is an absorption grating, and the first periodic pattern image is generated by geometrically optically projecting incident radiation. The first grating is preferably an absorption grating or a phase grating, and the first periodic pattern image is preferably generated by causing a Talbot effect to incident radiation.

  It is preferable to provide a multi slit that partially blocks the radiation emitted from the radiation source and disperses the focal point.

  The image processing method of the present invention detects an NG region in which an unwrapping error is likely to occur based on image data obtained by imaging a subject with a grating portion disposed between a radiation source and a radiation detector. An NG region detection step, a noise reduction processing step for performing noise reduction processing on the NG region of the image data, and a phase expressed by a value wrapped in a predetermined range based on the image data after the noise reduction processing A phase differential image generation step for generating a differential image, and an unwrap processing step for applying an unwrap process to the phase differential image are provided.

  According to the present invention, an NG region where an unwrap error is likely to occur is detected, a noise reduction process is performed on the NG region of the image data, a phase differential image is generated, and the unwrap process is performed. A complicated route setting process is unnecessary, and no unwrapping error occurs regardless of the unwrapping process along any route. Thus, according to the present invention, unwrapping errors can be easily prevented.

It is a block diagram which shows the structure of an X-ray imaging apparatus. It is a schematic diagram which shows the structure of a 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 explanatory drawing explaining a moving average process. It is explanatory drawing explaining an unwrap process. It is a flowchart explaining the effect | action of the X-ray imaging apparatus at the time of pre imaging | photography. It is a flowchart explaining the effect | action of the X-ray imaging apparatus at the time of this imaging | photography.

  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. The X-ray source 11 includes, for example, a rotary anode type X-ray tube and a collimator that limits the X-ray irradiation field, and emits X-rays toward the subject H based on the control of the imaging control unit 17. To do.

  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 with respect to the z direction that 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, for example, a flat panel detector using a semiconductor circuit, and is arranged behind the second grating 22 so that the detection surface 13a is orthogonal to the z direction.

  The 1st grating | lattice 21 is an absorption type grating | lattice provided with the some X-ray absorption part 21a and X-ray transmission part 21b extended | stretched in the y direction which is one direction in the grating plane orthogonal to az direction. The X-ray absorption part 21a and the X-ray transmission part 21b are alternately arranged in the x direction orthogonal to the z direction and the y direction, and form a striped pattern. The second grating 22 is an absorption type grating having a plurality of X-ray absorbing portions 22a and X-ray transmitting portions 22b that are extended in the y direction and arranged alternately in the x direction, like the first grating 21. is there. The X-ray absorbing portions 21a and 22a are formed of a material having X-ray absorption properties such as gold (Au) and platinum (Pt). The X-ray transmissive portions 21b and 22b are formed of a material having X-ray permeability such as silicon (Si) or resin or a gap.

  The first grating 21 partially passes X-rays emitted from the X-ray source 11 to generate a first periodic pattern image (hereinafter referred to as G1 image). The second grating 22 partially transmits the G1 image generated by the first grating 21 to generate a second periodic pattern image (hereinafter referred to as G2 image). When the subject H is not arranged, the G1 image substantially matches the lattice pattern of the second lattice 22.

  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 translates the second grating 22 in the x direction, and sequentially changes the relative position of the second grating 22 with respect to the first grating 21. 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 in order to execute a fringe scanning described later. The memory 14 collectively stores image data obtained by the X-ray image detector 13 at each scanning position of fringe scanning.

  The console 18 includes an operation unit 18a and a monitor 18b. The operation unit 18a includes a keyboard, a mouse, and the like, and sets operation conditions such as setting of imaging conditions such as tube voltage, tube current, and irradiation time of the X-ray source 11, mode selection for main imaging or pre-imaging, and imaging execution instruction. Is possible. 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. As will be described in detail later, the pre-photographing is used to acquire a background component caused by a manufacturing error or an arrangement error of the first and second gratings 21 and 22 as an offset image.

  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.

  In FIG. 2, an X-ray image detector 13 includes a pixel electrode 31 that collects charges generated in a semiconductor film (not shown) by incident X-rays, and a TFT (Thin for reading charges collected by the pixel electrode 31). A plurality of pixel portions 30 having a film transistor (32) are arranged in a two-dimensional manner. The semiconductor film is made of amorphous selenium, for example.

  The X-ray image detector 13 includes a gate scanning line 33, a scanning circuit 34, a signal line 35, and a readout circuit 36. The gate scanning line 33 is provided for each row of the pixel unit 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 pixel unit 30. The readout circuit 36 reads out electric charges from the pixel unit 30 through the signal lines 35, converts them into image data, and outputs them. The detailed layer configuration of each pixel unit 30 is the same as the layer configuration described in JP-A-2002-26300, for example.

  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 unit 30 via 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. The corrected image data is stored in the memory 14.

  The X-ray image detector 13 is not limited to the direct conversion type in which incident X-rays are directly converted into electric charges with a semiconductor film, and the incident X-rays are visible with a scintillator such as cesium iodide (CsI) or gadolinium oxysulfide (GOS). It may be an indirect conversion type that converts light into light and converts visible light into electric charge with a photodiode. Further, the X-ray image detector 13 may be configured by combining a scintillator and a CMOS sensor.

  In FIG. 3, the X-rays emitted from the X-ray source 11 are cone beams having the X-ray focal point 11a as a light emission point. The 1st grating | lattice 21 is comprised so that the Talbot effect may not arise and the X-rays which passed X-ray transmissive part 21b may be projected geometrically. Specifically, the width of the X-ray transmission part 21b in the x direction is set to a value sufficiently larger than the peak wavelength of X-rays irradiated from the X-ray source 11, and most of the X-rays are diffracted by the X-ray transmission part 21b. It is realized by not doing. When tungsten is used as the rotating anode of the X-ray source 11 and the tube voltage is 50 kV, the peak wavelength of the X-ray is about 0.4 mm. In this case, the width of the X-ray transmission part 21b may be about 1 to 10 μm.

  Thus, the G1 image is always a self-image of the first grating 21 regardless of the distance from the first grating 21 downstream in the z direction. The G1 image is enlarged in proportion to the distance from the X-ray focal point 11a to the downstream in the z direction.

As described above, the grating pitch p ₂ of the second grating 22 is set so that the grating pattern of the second grating 22 matches the G1 image at the position of the second grating 22. Specifically, the grating pitch _{p 2} of the second grating 22, the distance _{L 1} between the grating pitch _p 1, X-ray focal point 11a and the first grating 21 of the first grating 21, the first grating 21 and the distance L ₂ between the second grating 22, is set following equation (1) so as to satisfy substantially.

  The G1 image is modulated by being refracted by a phase change in the X-ray at the subject H. This modulation amount reflects the X-ray refraction angle φ (x) of the subject H. The figure illustrates an X-ray path that is refracted in accordance with a phase shift distribution Φ (x) representing a phase change of the X-ray in the subject H. Reference numeral X1 indicates a path along which the X-ray goes straight when the subject H does not exist, and reference numeral X2 indicates an X-ray path refracted by the subject H.

  The phase shift distribution Φ (x) is expressed by the following equation (2), where λ is the wavelength of the X-ray and n (x, z) is the refractive index distribution in the xz plane of the subject H.

  The refraction angle φ (x) is in the relationship of the phase shift distribution Φ (x) and the following equation (3).

  At the position of the second grating 22, the X-ray is displaced in the x direction by an amount corresponding to the refraction angle φ (x). This displacement amount Δx is approximately expressed by the following equation (4) based on the fact that the X-ray refraction angle φ (x) is very small.

  Thus, the displacement amount Δx is proportional to the differential value of the phase shift distribution Φ (x). Therefore, by detecting the displacement amount Δx by fringe scanning, which will be described later, a differential value of the phase shift distribution Φ (x) is obtained, and a phase differential image is generated.

In the fringe scanning, a value obtained by dividing the grating pitch p ₂ into M pieces (p ₂ / M) is used as a scanning pitch, and the second grating 22 is translated by the scanning mechanism 23 at this scanning pitch. Each time the image is translated, X-rays are emitted from the X-ray source 11 and a G2 image is captured by the X-ray image detector 13. M is an integer greater than or equal to 3, for example, it is preferable that M = 5.

If the above equation (1) is not satisfied slightly, or if rotation around the z direction or slight inclination with respect to the xy plane occurs between the first grating 21 and the second grating 22, G2 Moire fringes appear in the image. The moire fringes are moved along with the translational movement of the second grating 22, the moving distance in the x direction matches the original moiré fringe reaches the grating pitch p _2. By confirming the movement of the moire fringes, the translational movement amount of the second grating 22 can be verified.

M pixel values are obtained for each pixel unit 30 of the X-ray image detector 13 by the fringe scanning. As shown in FIG. 4, the M pixel values I _k periodically change with respect to the scanning position k of the second grating 22. The scanning position k is each position for each scanning pitch (p ₂ / M) when the second grating 22 is translated by one period. A signal indicating a change in the pixel value I _k with respect to the scanning position k is referred to as an intensity modulation signal.

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

Therefore, for each pixel unit 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.

  Next, a method for calculating the phase shift amount ψ (x) will be described. The intensity modulation signal is generally expressed by the following formula (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, the following expression (7) is established.

  When the above equation (7) is applied to the above equation (6), the phase shift amount ψ (x) is expressed by the following equation (8).

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

  Since the declination angle of the complex number ranges from −π to + π, when the phase shift amount ψ (x) is calculated based on the above equation (8), the phase shift amount ψ (x) is − Take a value wrapped in the range from π to + π. On the other hand, the arc tangent function usually has a range of −π / 2 to + π / 2. Therefore, when the phase shift amount ψ (x) is calculated based on the above equation (9), The phase shift amount ψ (x) takes a value wrapped in a range of −π / 2 to + π / 2. In the above formula (9), by determining the denominator and the sign of the numerator in the arctangent function, the value range can be changed from −π to + π, and therefore the phase shift amount ψ ( It is also possible to calculate x).

  In the present embodiment, data obtained by calculating the phase shift amount ψ (x) for each pixel unit 30 is referred to as a phase differential image. Data obtained by multiplying or adding a constant to the phase shift amount ψ (x) may be used as the phase differential image. Hereinafter, it is assumed that the pixel values of the phase differential image are wrapped in a predetermined value range having a width α (for example, a range from 0 to α).

  In FIG. 5, the image processing unit 15 includes an NG region detection unit 40, a noise reduction processing unit 41, a phase differential image generation unit 42, an unwrap processing unit 43, an offset image storage unit 44, an offset processing unit 45, and a phase contrast image generation. Part 46 is provided. The image processing unit 15 includes M image data (main image data) 50 stored in the memory 14 by main shooting and M image data (pre-image data) stored in the memory 14 by pre-shooting. 51 are input.

Based on the main image data 50, the NG region detection unit 40 detects a region where an unwrap error is likely to occur when a phase differential image is generated (hereinafter referred to as an NG region). Specifically, NG region detection unit 40, based on the image data 50, for each pixel unit 30 generates the aforementioned intensity-modulated signal, the mean strength A ₀ is lower region than a predetermined threshold value of the intensity modulated signal, the amplitude An area where A ₁ is lower than a predetermined threshold or an area where visibility A ₁ / A ₀ is lower than a predetermined threshold is detected as an NG area.

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 . If the NG areas are scattered and cannot be obtained as a collective area having a certain size, the size of the NG area may be adjusted by changing the threshold value.

The noise reduction processing unit 41 reduces noise by performing moving average processing on the NG area detected by the NG area detection unit 40 for each of the M main image data 50. Specifically, as illustrated in FIG. 6, the noise reduction processing unit 41 sets each pixel in the NG area of each main image data 50 as the target pixel Q _{x, y,} and the pixel value of each target pixel Q _{x, y} . the target pixel _{Q x,} and the pixel values of _y, the target pixel _{Q x,} 8 contiguous pixels adjacent around the _{y Q x-1, y-} 1, Q x, y-1, Q x + 1, y- ₁ , Q _{x−1, y} , Q _{x + 1, y} , Q _{x−1, y + 1} , Q _{x, y + 1} , Q _{x + 1, y + 1} The moving average process is performed. Thereby, pixel values in the NG area of each main image data 50 are smoothed, and noise is reduced.

In this embodiment, the average value is calculated using nine pixels including the target pixel Q _{x, y} , but the number of pixels for calculating the average value is arbitrary. However, when the number of pixels is large, the resolution of the NG area of the phase differential image generated based on the main image data 50 is unnecessarily lowered. On the other hand, when the number of pixels is small, noise is not sufficiently reduced, and an unwrapping error may occur in unwrapping processing described later. For this reason, it is preferable that the number of pixels for calculating the average value in the moving average process is appropriately set in consideration of the balance between the resolution and the degree of occurrence of the unwrap error.

  The phase differential image generation unit 42 uses the M pieces of main image data 50 for which noise reduction processing has been performed on the NG region by the noise reduction processing unit 41, and is based on the above equation (8) or the above equation (9). A first phase differential image is generated by performing an operation. On the other hand, at the time of pre-photographing, M pieces of pre-image data 51 are directly input to the phase differential image generation unit 42, and the phase differential image generation unit 42 performs calculation based on the above equation (8) or the above equation (9). By doing so, a second phase differential image is generated.

  The unwrap processing unit 43 performs unwrapping processing on the first and second phase differential images generated by the phase differential image generation unit 42, respectively. The unwrap processing unit 43 causes the offset image storage unit 44 to store an image obtained by performing the unwrap processing on the second phase differential image at the time of pre-photographing. Note that when a new pre-shooting is performed and a new offset image is input, the offset image storage unit 44 deletes the already stored offset image and stores the newly input offset image. . Further, the unwrap processing unit 43 inputs the first phase differential image that has been subjected to the unwrap process to the offset processing unit 45 as an unwrapped phase differential image during the main photographing.

  The offset processing unit 45 performs offset correction by subtracting the offset image stored in the offset image storage unit 44 from the unwrapped phase differential image input from the unwrap processing unit 43. The phase contrast image generation unit 46 integrates the unwrapped phase differential image that has been offset-corrected by the offset processing unit 45 along the x direction to generate a phase contrast image representing a phase shift distribution. The phase differential image and the phase contrast image after the offset correction are recorded in the image recording unit 16.

Next, the unwrap processing method by the unwrap processing unit 43 will be specifically described with reference to FIG. The unwrap processing unit 43 sets the starting point groups SP _{1 to} SP _n along one side of the end portion of the first phase differential image 60 and linearly extends in a direction orthogonal to the arrangement direction of the starting point groups SP _{1 to} SP _n. Routes UR _{1 to} UR _n are set. The unwrap processing unit 43 unwraps the starting point groups SP _{1 to} SP _n and then performs unwrap processing along each straight path UR _{1 to} UR _n .

This unwrap processing detects discontinuous points where the amount of change in the pixel values of adjacent pixels on the unwrap path is a predetermined amount (for example, α / 2) or more, and exists on the unwrap path after the detected discontinuity point. This is a process of adding or subtracting the width α in which the pixel value is wrapped to the pixel value of the pixel to be processed. Thereby, the change of the pixel value on the unwrap path is almost continuous. In the NG region of the first differential phase image 60, since the noise reduction processing unit 41 is reduced at the stage of the main image data 50, the noise of the pixel value is small and the occurrence of an unwrapping error is prevented. The unwrap processing unit 43 performs the same unwrap processing on the second phase differential image as described above. In the present embodiment, the arrangement direction of the start point groups SP _{1 to} SP _n is the x direction, but may be the y direction.

  Hereinafter, 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 S10), it is determined whether or not the selected shooting mode is pre-shooting (step S11). If it is pre-photographing (YES in step S11), a shooting instruction standby state is set (step S12). When an imaging instruction is given using the operation unit 18a (YES in step S12), the second grating 22 is translated by a predetermined scanning pitch by the scanning mechanism 23, and the X-ray source 11 at each scanning position k. X-ray irradiation and detection of the G2 image by the X-ray image detector 13 are performed (step S13). As a result of the fringe scanning, M pieces of pre-image data 51 are generated and stored in the memory 14.

  M pieces of pre-image data 51 are read out to the image processing unit 15. In the image processing unit 15, the phase differential image generation unit 42 generates a second phase differential image based on the M pre-image data 51 (step S14). The second phase differential image is unwrapped by the unwrap processing unit 43 (step S15), and the second phase differential image after the unwrapping process is stored in the offset image storage unit 44 as an offset image (step S16). ). 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 S10 (NO in step S11), the imaging instruction standby state is set (step S20). When a shooting instruction is issued using the operation unit 18a (YES in step S20), stripe scanning is performed (step S21), and M main image data 50 is stored in the memory 14.

  The M main image data 50 is read by the image processing unit 15. In the image processing unit 15, the NG region is detected by the NG region detection unit 40 (step S22), and the noise reduction processing unit 41 applies a moving average process to the NG region of each main image data 50 to reduce noise. (Step S23). Thereafter, the phase differential image generation unit 42 generates a first phase differential image based on the M main image data 50 subjected to the noise reduction process (step S24).

  The first phase differential image is subjected to unwrap processing by the unwrap processing unit 43 (step S25), and the first phase differential image after the unwrap processing is input to the offset processing unit 45 as an unwrapped phase differential image. The offset processing unit 45 performs offset correction by subtracting the offset image stored in the offset image storage unit 44 from the unwrapped phase differential image (step S26).

  Then, the phase contrast image generation unit 46 integrates the phase differential image after the offset correction to generate a phase contrast image (step S27), and the phase differential image and the phase contrast image after the offset correction are stored in the image recording unit 16. Is recorded on the monitor 18b, and then displayed on the monitor 18b (step S28).

  As described above, since the NG area where an unwrap error is likely to occur is detected from the main image data 50 at the time of the main photographing, and the first phase differential image is generated after performing noise reduction processing on the NG area, the first phase differential image is generated. Generation of an unwrapping error is prevented at the time of unwrapping the phase differential image 1. For this reason, the soft tissue region (see FIG. 7) located around the NG region (bone region or the like) is not hindered from being imaged by streak noise due to unwrapping errors as in the past, and visibility is improved. Is good.

  In the above embodiment, as illustrated in FIG. 7, the unwrap processing unit 43 performs the unwrap process on the entire first and second phase differential images including the NG region. The starting point may be set in the OK area, and the unwrap process may be performed only for the OK area.

  Moreover, in the said embodiment, although the noise reduction process part 41 is reducing the noise in an NG area | region by the moving average process which is a linear filter process, it smoothes the pixel value in an NG area | region, and reduces noise. Other smoothing processing methods can be used as long as the processing can be performed. For example, as the noise reduction process, a known nonlinear filter process such as a median filter process or a mode value filter process may be used. The median filter process is a process of replacing the pixel value of the target pixel with the median value of the pixel values in a predetermined area including the target pixel. The mode value filter process is a process of replacing the pixel value of the target pixel with the mode value of the pixel value in the predetermined area including the target pixel. Furthermore, noise reduction processing can be performed by combining linear processing and non-linear processing. For example, noise reduction processing is performed by weighting the filter matrix (linear processing) and threshold determination (nonlinear processing).

  In the above embodiment, the noise reduction processing unit 41 changes the target pixel one pixel at a time in the NG region, and sets the pixel value of each target pixel to the average value, median value, and maximum value of the pixel values of the region including the target pixel. Although a method of replacing with a frequent value or the like is used, the NG area is divided into a plurality of areas, and the pixel value in each of the divided areas is changed to the average value, median value, mode value, etc. of the pixel values in the area. The noise may be reduced by replacing all of them.

  In the above embodiment, the noise reduction processing unit 41 detects the NG region based on the average intensity, amplitude, or visibility of the intensity modulation signal. However, the method for detecting the NG region is not limited to this, and the intensity modulation is not limited to this. A region where the variation in the average intensity of the signal between the pixels (that is, the variation between the pixels of the absorption image) or the variation between the pixels of the phase differential image is larger than a predetermined value may be detected as the NG region. In addition, it is preferable that the dispersion | variation between the pixels of this phase differential image is a dispersion | variation in the direction (x direction) orthogonal to the lattice line of the 1st and 2nd grating | lattices 21 and 22. FIG.

  In addition, an absolute value is taken for each pixel of the phase differential image, and an edge portion of the superabsorbent region can be detected by detecting a portion where the absolute value exceeds a predetermined value. The region may be detected as an NG region.

  Alternatively, an area 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 may be detected 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. You may combine the above detection criteria suitably.

  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.

  Even in the case where an NG region due to the above-described element missing or pixel defect is detected, an unwrapping error is prevented by the noise reduction processing by the noise reduction processing unit 41. Since the NG region caused by such a missing or pixel defect can be detected also in the pre-image data 51, the NG region detection and noise reduction processing by the NG region detection unit 40 is also performed on the pre-image data 51. It is also preferable that the second differential phase image is generated by the differential phase image generation unit 42 after the noise reduction processing by the unit 41 is performed.

  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.

  Moreover, in the said embodiment, although the 2nd grating | lattice 22 is moved to the direction (x direction) orthogonal to a grating | lattice line at the time of fringe scanning, as filed as Japanese Patent Application No. 2011-097090 by this applicant. The second grating 22 may be moved in a direction inclined with respect to the grid line (a direction not orthogonal to the x direction and the y direction in the xy plane). This moving direction may be any direction as long as it is within the xy plane and other than 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 grating lines, the stroke (movement distance) required for one period of the fringe scanning is increased, 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 .

Moreover, in the said embodiment, the X-rays inject | emitted from the X-ray source 11 are made to inject into the 1st grating | lattice 21, and although the X-ray source 11 is a single focus, immediately after the emission side of the X-ray source 11 The X focus may be dispersed by providing a multi slit (radiation source lattice) described in WO 2006/131235. As a result, a high-power X-ray source can be used, and the X-ray dose is improved, so that the image quality of the phase differential image is improved. The grid lines of the multi slit are along the y direction. In this case, the pitch p _{0 of the} multi-slit needs to satisfy the following formula (10). Here, the distance L ₁ represents the distance from the multi slit to the first grating 21.

  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 the multi-slit may be used so as to enhance the spatial coherence of X-rays.

  When the Talbot effect is generated in the first grating 21, it is preferable that the first grating 21 is a phase grating instead of the absorption grating. The phase type grating is configured by replacing the X-ray absorption part of the absorption type grating with an X-ray phase forming part. The X-ray phase forming part is formed of a material having a predetermined refractive index difference with respect to the adjacent X-ray transmitting part. As this phase type grating, there are a type in which incident X-rays are transmitted with π / 2 phase modulation, and a type of grating in which incident X-rays are transmitted with π phase modulation.

When the Talbot effect is generated in the first grating 21, the self-image (G1 image) of the first grating 21 is generated at a position away from the first grating 21 by the Talbot distance Z _m downstream in the z direction. the distance _{L 2} from the first grid 21 to the second grid 22 is required to be Talbot distance _{Z m.}

Talbot distance Z _m is dependent on the beam shape of the structure and the X-ray of the first grating 21. An absorption grating first grating 21, when X-rays emitted from the X-ray source 11 is a cone beam shape, Talbot distance Z _m is represented by the following formula (11). Here, m is a positive integer.

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 expressed by the following formula (12). Here, m is 0 or a positive integer.

In addition, when the first grating 21 is a phase-type grating that imparts π phase modulation to X-rays and the X-rays emitted from the X-ray source 11 have a cone beam shape, the Talbot distance Z _m is as follows. It is represented by Formula (13). Here, m is 0 or a positive integer.

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 following formula (14). Here, m is a positive integer.

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 Is represented by the following formula (15). Here, m is 0 or a positive integer.

When the first grating 21 is a phase type grating that imparts 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 is It is represented by Formula (16). Here, m is 0 or a positive integer.

  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 Application Laid-Open No. 2009-133823, the second grating 22 can be omitted and only the first grating 21 can be used. 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.

  Further, there is a method of omitting the scanning mechanism 23 and 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. As this method, there is a pixel division method filed as Japanese Patent Application No. 2010-256241 by the present applicant. In this pixel division method, 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. A 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. A phase differential image is generated in the same procedure as the above-described fringe scanning method, assuming that the images are based on a plurality of different G2 images by fringe scanning. 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. This is a method for generating a phase differential image 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.

DESCRIPTION OF SYMBOLS 10 X-ray imaging apparatus 12 Lattice part 13 X-ray image detector 21 1st grating | lattice 21a X-ray absorption part 21b X-ray transmission part 22 2nd grating | lattice 22a X-ray absorption part 22b X-ray transmission part 30 Pixel part 31 Pixel electrode 33 Gate scanning line 35 Signal line

Claims (14)

A radiation detector that detects radiation emitted from the radiation source and transmitted through the subject to generate image data; and
A grating portion disposed between the radiation source and the radiation detector;
An NG region detection unit that detects an NG region in which an unwrap error is likely to occur based on the image data;
A noise reduction processing unit that performs noise reduction processing on the NG region of the image data;
Based on the image data after the noise reduction processing, a phase differential image generation unit that generates a phase differential image expressed by a value wrapped in a predetermined range;
An unwrap processing unit that performs unwrap processing on the phase differential image;
A radiation imaging apparatus comprising:   The radiographic apparatus according to claim 1, wherein the noise reduction process is a smoothing process for smoothing the NG area of the image data.   The smoothing process is a moving average process in which each pixel value in the NG area is a target pixel, and the pixel value of the target pixel is replaced with an average value of pixel values of a predetermined area including the target pixel. The radiation imaging apparatus according to claim 2. An offset image storage unit for storing as an offset image, and an offset processing unit for subtracting the offset image from the phase differential image after the unwrap processing,
The offset differential image is generated when the phase differential image generation unit generates a phase differential image based on image data generated by the radiation detector during pre-imaging performed in a state where the subject is not disposed. The radiation imaging apparatus according to claim 1, wherein the processing unit performs unwrapping processing. 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,
5. The radiation imaging apparatus according to claim 1, wherein the radiation image detector generates the image data by detecting the second periodic pattern image. 6. The grating unit 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 to generate image data;
The radiation imaging apparatus according to claim 5, wherein the phase differential image generation unit generates a phase differential image based on a plurality of image data generated by the radiation image detector.   The radiation imaging apparatus according to claim 6, wherein the scanning mechanism moves the first grating or the second grating in a direction orthogonal to the grating lines.   The radiographic apparatus according to claim 6, wherein the scanning mechanism moves the first grating or the second grating in a direction inclined with respect to a grating line.   The radiation imaging apparatus according to claim 5, wherein the phase differential image generation unit generates the phase differential image based on single image data obtained by the radiation detector.   10. The NG region detection unit detects an NG region based on one or a combination of average intensity, amplitude, and visibility of an intensity modulation signal that represents a change in intensity of a pixel value. The radiation imaging apparatus according to any one of the above.   The said 1st grating | lattice is an absorption type grating | lattice, The said 1st periodic pattern image is produced | generated by projecting the incident radiation geometrically optically, The any one of Claim 5 to 10 characterized by the above-mentioned. The radiation imaging apparatus described.   11. The first periodic pattern image is generated by generating a Talbot effect on incident radiation and generating the first periodic pattern image, wherein the first grating is an absorption grating or a phase grating. The radiographic apparatus according to the item.   13. The radiographic apparatus according to claim 1, further comprising a multi-slit that partially blocks the radiation emitted from the radiation source and disperses the focal point. An NG region detection step for detecting an NG region in which an unwrapping error is likely to occur, based on image data obtained by imaging a subject with a grating portion disposed between a radiation source and a radiation detector;
A noise reduction processing step of applying noise reduction processing to the NG region of the image data;
Based on the image data after the noise reduction processing, a phase differential image generation step for generating a phase differential image expressed by a value wrapped in a predetermined range;
An unwrapping step for unwrapping the phase differential image;
An image processing method comprising: