JP2013042788A - Radiographic apparatus and unwrapping processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiographic apparatus in which imaging of a soft tissue is prevented from being inhibited due to an unwrapping error.SOLUTION: An X-ray image detector detects an X-ray which is emitted from an X-ray source and passes through first and second gratings, and generate image data. A phase-differential image generating section generates a phase-differential image on the basis of the image data. An OK/NG region detecting section detects an NG region where an unwrapping error easily occurs in the phase-differential image, and detects the other regions as OK regions. An unwrapping processing section sets a starting point SP in an OK region, applies unwrapping processing to adjacent pixels while using the starting point SP as a reference point, and then, sequentially performs unwrapping processing for the pixels in the OK regions while changing the reference point for the pixels subjected to the unwrapping processing.

Description

  The present invention relates to a radiation imaging apparatus that detects an image based on a phase change of radiation by a subject and an unwrap 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).

  This unwrapping process is also performed in the field of optical measurement or the like (see, for example, Patent Document 3). In general, unwrap processing is performed in order along a predetermined route from a starting point at a predetermined position in the image. 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 JP 2008-082869 A

  However, when the subject includes a high-absorber having a high X-ray absorption capacity such as a bone part, the high-absorber decreases the intensity and amplitude of the intensity modulation signal by greatly attenuating the X-ray. Therefore, 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.

  As shown in FIG. 14, when a starting point is set in a bone region that is a high-absorber and unwrap processing is performed along a path extending in one direction from the starting point, an unwrapping error is likely to occur in the bone region. Once an unwrap error occurs, an error value (a value corresponding to the value range of the function) is accumulated in the path after the location where the unwrap error has occurred. As a result, streaky noise along the path direction of the unwrapping process is generated in the phase differential image after the unwrapping process, and this noise overlaps a part of the cartilage part that is a soft tissue. There is a problem in that imaging of the soft tissue of the heart, which is a region, is hindered.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a radiographic apparatus and an unwrap processing method that can prevent the inhibition of soft tissue imaging due to an unwrap error.

  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 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, and an unwrapping error in the phase differential image. After detecting an NG region that is likely to occur, an OK / NG region detection unit that detects other regions as an OK region, and a starting point is set in the OK region, and an adjacent pixel is unwrapped using this starting point as a reference point An unwrap processing unit that sequentially unwraps the pixels in the OK region while changing the reference point to the pixels on which the unwrap processing has been performed.

  In pre-imaging performed in a state in which the subject is not arranged, the image processing apparatus includes an offset image storage unit that stores the phase differential image generated by the phase differential image generation unit as an offset image, and the unwrap processing unit adds the offset image to the offset image. In contrast, it is preferable to perform the unwrapping process. In this case, it is preferable to include an offset processing unit that subtracts the offset image after the unwrap processing from the phase differential image after the unwrap processing.

  It is preferable that the unwrap processing unit simultaneously unwraps adjacent pixels that are not subjected to unwrap processing in the OK region among adjacent pixels adjacent to each other in four directions orthogonal to the reference point.

  The unwrap processing unit preferably unwraps adjacent pixels which are adjacent to the reference point in one direction and are not subjected to unwrap processing in the OK region one pixel at a time.

  The unwrap processing unit preferably performs the unwrap processing by setting the starting point in each OK region when there are a plurality of OK regions.

  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 OK / NG area detection unit detects an NG area based on one or a combination of average intensity, amplitude, and visibility of an intensity modulation signal representing intensity change of a pixel value.

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

  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 may be an absorption grating or a phase grating, and may generate the first periodic pattern image 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 unwrap processing method of the present invention detects an NG region in which an unwrap error is likely to occur in a phase differential image expressed by a value wrapped in a predetermined range, and detects other regions as OK regions, and the OK region A starting point is set inside, the adjacent pixels are unwrapped using this starting point as a reference point, and then the pixels in the OK region are unwrapped in order while changing the reference point to the unwrapped pixel. To do.

  According to the present invention, an NG region in which an unwrapping error is likely to occur is detected in a phase differential image expressed by a value wrapped in a predetermined range, the other region is set as an OK region, and a starting point is set in the OK region. Then, after unwrapping adjacent pixels using this starting point as a reference point, the pixels in the OK region are unwrapped in order while changing the reference point to the unwrapped pixel, so that unwrapping errors are unlikely to occur. Inhibition of imaging of soft tissue due to.

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 the 1st explanatory view explaining an unwrap processing method. It is the 2nd explanatory view explaining an unwrap processing method. It is the 3rd explanatory view explaining an unwrap processing method. 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. It is explanatory drawing explaining the 1st modification of an unwrap processing method. It is explanatory drawing explaining the 2nd modification of an unwrap processing method. 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 the conventional unwrap process.

  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 X-ray wavelength is λ and the refractive index distribution of the subject H is n (x, z).

  The refraction angle φ (x) is related to the phase shift distribution Φ (x) by 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 that is convolved (wrapped) in the range of 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 convoluted 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. Note that an image represented by data obtained by multiplying or adding a phase shift amount ψ (x) by a constant may be a phase differential image. Hereinafter, it is assumed that the phase differential image is convoluted in a range of −α / 2 to + α / 2.

  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, and a phase contrast image generation unit 45. . The phase differential image generation unit 40 uses the image data for M sheets stored in the memory 14 as a result of performing the fringe scanning in the main photographing or the pre-photographing, and based on the above equation (8) or the above equation (9). A phase differential image is generated by performing an operation.

  The phase differential image generated by the phase differential image generation unit 40 during 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 the offset image is input again from the phase differential image generation unit 40, the offset image storage unit 41 deletes the stored offset image and then stores the input offset image.

The OK / NG area detection unit 42 detects an area where an unwrapping error is likely to occur in the phase differential image (hereinafter referred to as an NG area) based on the M pieces of image data stored in the memory 14, and other than the NG area. This area is detected as an 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 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 . Further, when the NG area is scattered and cannot be obtained as a gathering area having a certain size, the size of the NG area may be adjusted by changing the threshold value.

  The unwrap processing unit 43 performs unwrap processing on the phase differential image input from the phase differential image generation unit 40 only for the OK region other than the NG region. Further, the unwrap processing unit 43 performs an unwrap process on the offset image stored in the offset image storage unit 41 only for the OK region.

  The offset processing unit 44 performs offset correction by subtracting the offset image after unwrapping from the phase differential image after unwrapping. The phase contrast image generation unit 45 generates a phase contrast image representing the phase shift distribution by integrating the phase differential image after the offset correction along the x direction. The phase differential image after 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 in more detail with reference to FIGS. In these figures, the phase differential image is represented as an image of 10 × 6 pixels for the sake of simplicity of explanation.

  As shown in FIG. 6A, first, the starting point SP for starting the unwrapping process is set in the OK region of the phase differential image. The starting point SP is set as a reference pixel, and the reference pixel is set in the x direction (left and right) and y. Unwrap processing is performed on adjacent pixels adjacent in the four directions (up and down). If there are a plurality of OK regions in the phase differential image, the starting point SP is set in each OK region and unwrap processing is performed in the same manner.

  In this unwrap processing, a difference in pixel value is calculated between the reference pixel and an adjacent pixel adjacent to the reference pixel, and the difference value Δψ is within a predetermined range (for example, −α / 2 ≦ Δψ ≦ + α / 2). Otherwise, this adjacent pixel is determined as a discontinuous point, and the pixel value of the adjacent pixel is shifted in units of α so that the difference value Δψ is within the predetermined range.

  When the unwrapping process of the adjacent pixels adjacent to the starting point SP is performed, as shown in FIG. 6B, each pixel subjected to the unwrapping process in FIG. Unwrap processing of adjacent pixels adjacent to the pixel is performed. Dashed arrows indicate unwrap processing paths in the previous step. A solid line arrow indicates an unwrap processing path in the current step.

  In the current step, since there are a plurality of reference points, an adjacent pixel adjacent in the x direction from one reference point and an adjacent pixel adjacent in the y direction from another reference point become the same pixel, and the target of unwrap processing overlaps. Therefore, it is necessary to prioritize either direction. In this embodiment, priority is given to the x direction. In addition, when an adjacent pixel adjacent to the reference point is not a pixel in the OK area (a pixel in the NG area), the unwrapping process is not performed on the pixel.

  Thereafter, as shown in FIGS. 6C, 6D, 7A to 7D, 8A, and 8B, the unwrapped pixels are sequentially set as reference pixels. It is set, and an unwrapping process of adjacent pixels adjacent to each reference pixel is performed. When the unwrapping process is completed for all the pixels in each OK region, a series of unwrapping processes by the unwrapping processing unit 43 is finished.

  In the unwrap processing, the above-mentioned shift in units of α with respect to the pixel value of the discontinuous point may be performed every time a discontinuous point is detected. The shift amount for eliminating the continuous points may be stored, and the shift may be performed for each discontinuous point after all the discontinuous points in the OK region are detected.

  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, a standby state for photographing instructions is entered (step S22). When an imaging instruction is given using the operation unit 18a (YES in step S22), the second grating 22 is translated by a predetermined scanning pitch by the scanning mechanism 23, and at each scanning position k by the X-ray source 11. X-ray irradiation 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 M image data 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, the OK / NG area detection unit 42 detects the NG area and the OK area based on the image data stored in the memory 14 (step S33). Based on the detection result, the unwrap processing unit 43 performs the above-described unwrap processing on the OK regions of the phase differential image generated in step S32 and the offset image stored in the offset image storage unit 41. (Step S34). Thereafter, the offset processing unit 44 performs offset correction for subtracting the unwrapped offset image from the unwrapped phase differential image (step S35).

  The phase contrast image generation unit 45 integrates the phase differential image after the offset correction to generate a phase contrast image (step S36), and the phase differential image and the phase contrast image after the offset correction are stored in the image recording unit 16. After the recording, the image is displayed on the monitor 18b (step S37).

  As described above, in the unwrapping process in step S34, an NG region where an unwrapping error is likely to occur is detected, and the unwrapping process is performed only for the OK region other than this NG region. A differential 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.

  In the above embodiment, the starting point SP for starting the unwrapping process is set near the center of the OK region, but the starting point SP may be set at any pixel in the OK region. For example, as shown in FIG. 11, the starting point SP may be set at the corners (ends in the x direction and the y direction) of the OK region. The unwrap processing method is the same as that in the above embodiment, and as shown in FIGS. 11A to 11D, adjacent pixels are sequentially unwrapped from the starting point SP. Since the steps in FIG. 11D are the same, description thereof is omitted.

  Further, in the above embodiment, among the adjacent pixels adjacent to the reference pixel in the four directions of the x direction (left and right) and the y direction (up and down), the adjacent pixels that are not subjected to the unwrapping process in the OK region. Although the unwrap processing is performed at the same time, adjacent pixels adjacent to one direction in the x direction or the y direction may be sequentially unwrapped one by one.

  For example, as shown in FIG. 12A, first, the starting point SP is set at the corner (the end in the x direction and the y direction) of the OK region, and the adjacent pixels adjacent in the x direction are unwrapped. Next, the unwrapped adjacent pixel is used as a reference pixel, and the adjacent pixel adjacent in the x direction is unwrapped. By repeating this, as shown in FIG. 12B, when reaching the end of the OK region in the x direction, the adjacent pixels adjacent in the y direction from the starting point SP are unwrapped, and as shown in FIG. Unwrap processing is sequentially performed in the x direction from the unwrapped adjacent pixels. And when an adjacent pixel is a pixel in a NG area | region, the unwrap process of this pixel is not performed. As shown in FIG. 12D, the pixels in the same row in the OK region are unwrapped using the pixels in the upper row that have already been unwrapped as reference pixels. Hereinafter, similarly, the unwrapping process can be performed for all the pixels in the OK region.

  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 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 predetermined 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 is a noise in which discontinuous points remain. Therefore, as shown in FIG. 13, an NG region image replacement unit 50 may be provided in the image processing unit 15.

  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.

  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). 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. In this case, the pitch p _{0 of the} multi-slit needs to satisfy the following formula (10). In this case, 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, 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 of 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 20 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 (17)

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;
Based on the image data, a phase differential image generation unit that generates a phase differential image expressed by a value wrapped in a predetermined range;
An OK / NG region detecting unit that detects an NG region in which an unwrapping error is likely to occur in the phase differential image, and detects other regions as OK regions;
Unwrapping is performed by setting a starting point in the OK region, unwrapping adjacent pixels using this starting point as a reference point, and then unwrapping the pixels in the OK region in order while changing the reference point to the unwrapped pixel. A processing unit;
A radiation imaging apparatus comprising:   In pre-imaging performed in a state in which the subject is not arranged, the image processing apparatus includes an offset image storage unit that stores the phase differential image generated by the phase differential image generation unit as an offset image, and the unwrap processing unit adds the offset image to the offset image. The radiation imaging apparatus according to claim 1, wherein the unwrap processing is also performed.   The radiation imaging apparatus according to claim 2, further comprising an offset processing unit that subtracts the offset image after the unwrap processing from the phase differential image after the unwrap processing.   The unwrap processing unit simultaneously unwraps adjacent pixels that are not subjected to unwrap processing in the OK region among adjacent pixels adjacent to each other in four directions orthogonal to a reference point. The radiation imaging apparatus according to any one of claims 1 to 3.   The unwrap processing unit unwraps adjacent pixels that are adjacent to a reference point in one direction and that are not unwrapped in the OK region one by one. The radiation imaging apparatus according to any one of the above.   6. The unwrap processing unit according to claim 1, wherein the unwrap processing unit performs the unwrap processing by setting the starting point in each OK region when there are a plurality of the OK regions. 6. Radiography equipment. 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 detects the second periodic pattern image to generate image data. 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 radiographic apparatus according to claim 7, wherein the phase differential image generation unit generates a phase differential image based on a plurality of image data generated by the radiological image detector.   The radiation imaging apparatus according to claim 8, wherein the scanning mechanism moves the first grating or the second grating in a direction orthogonal to the grating line.   The radiation imaging apparatus according to claim 8, 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 7, wherein the phase differential image generation unit generates the phase differential image based on single image data obtained by the radiation detector.   9. 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. 11. The radiographic apparatus according to any one of 11 to 11.   13. An NG region image replacement unit that generates any one of an absorption image, a differential image of an absorption image, and a small-angle scattered image and replaces the NG region of the phase differential image. The radiation imaging apparatus according to item 1.   The said 1st grating | lattice is an absorption-type grating | lattice, and produces | generates a said 1st periodic pattern image by projecting the incident radiation geometrically optically, The any one of Claim 7 to 13 characterized by the above-mentioned. The radiation imaging apparatus described.   The first grating is an absorption grating or a phase grating, and generates the first periodic pattern image by causing a Talbot effect to incident radiation. The radiographic apparatus according to the item.   The radiation imaging apparatus according to claim 1, further comprising a multi-slit that partially shields radiation emitted from the radiation source and disperses the focal point.   In the phase differential image expressed by a value wrapped in a predetermined range, an NG region where an unwrap error is likely to occur is detected, the other region is detected as an OK region, and a starting point is set in the OK region. An unwrap processing method, comprising: unwrapping adjacent pixels using a starting point as a reference point, and sequentially unwrapping pixels in the OK region while changing the reference point to a pixel on which unwrap processing has been performed.

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