JP2012228361A - Radiographic apparatus - Google Patents

Abstract

It is possible to obtain a clear phase image while suppressing the number of exposures.
First and second gratings 21 and 22 are disposed opposite to each other between an X-ray source 11 and an X-ray image detector 20. The moving mechanism 23 moves the second grating 22 to a first position and a second position separated by a distance corresponding to half the grating pitch in a direction orthogonal to the grating lines. The image averaging unit 14a uses the first image data generated by the X-ray image detector 20 at the first position and the second image data generated by the X-ray image detector 20 at the second position. An absorption image is generated by averaging. The subtraction processing unit 14b generates a display image by subtracting the absorption image from the first image data.
[Selection] Figure 1

Description

  The present invention relates to a radiation imaging apparatus that performs phase imaging using a grating.

  Radiation, such as X-rays, has a characteristic that it attenuates depending on the atomic number of the elements constituting the substance, and the density and thickness of the substance, so it is used as a probe for seeing through the inside of the subject. Yes. X-ray imaging is widely used in fields such as medical diagnosis and non-destructive inspection.

  In a general X-ray imaging apparatus, a subject is placed between an X-ray source that emits X-rays and an X-ray image detector that detects X-rays, and a transmission image of the subject is taken. In this case, the X-rays emitted from the X-ray source toward the X-ray image detector correspond to the difference in the characteristics (atomic number, density, thickness) of the substances existing on the path to the X-ray image detector. After receiving a certain amount of attenuation (absorption), it enters the pixel of the X-ray image detector. As a result, an X-ray absorption image of the subject is detected and imaged by the X-ray image detector. As an X-ray image detector, in addition to a combination of an X-ray intensifying screen and a film and a stimulable phosphor, a flat panel detector (FPD) using a semiconductor circuit is widely used.

  However, since the X-ray absorption ability is lower as a substance composed of an element having a smaller atomic number, there is a problem that a soft tissue or soft material of a living body cannot obtain a sufficient contrast (contrast) in an X-ray absorption image. 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.

  In recent years, the development of X-ray phase imaging that obtains a phase contrast image based on a phase change of X-rays due to a difference in the refractive index of the subject instead of a change in X-ray intensity due to a difference in the X-ray absorption ability of the subject has been advanced. It has been. In X-ray phase imaging based on this phase change, a high-contrast image can be acquired even for a weakly absorbing object with low X-ray absorption capability.

  As an X-ray imaging apparatus capable of such X-ray phase imaging, first and second gratings are arranged in parallel at a predetermined interval between an X-ray source and an X-ray image detector. An X-ray imaging apparatus has been proposed that acquires a phase contrast image by imaging an X-ray image generated by passing a first and second grating from a source with an X-ray image detector (for example, a patent) References 1 and 2).

  In the X-ray imaging apparatuses described in Patent Documents 1 and 2, the second grating is relatively translated relative to the first grating by a predetermined amount smaller than the grating pitch in a direction substantially perpendicular to the grating direction. However, each time translation is performed, imaging is performed to generate image data, and the amount of X-ray phase change caused by interaction with the subject is detected based on a plurality of image data obtained as a result of a series of imaging. Then, a fringe scanning method for generating a phase differential image is performed. A phase contrast image of the subject can be acquired based on the phase differential image.

  In Patent Document 1, in addition to the phase differential image and the phase contrast image, an image based on a single image data obtained by a single photographing while the first and second gratings are fixed may be used as the display image. It is described. Similarly, Patent Document 2 describes that an image based on the single image data subjected to offset correction or the like is used as a display image.

WO2004 / 058070 WO2008 / 102654

  However, as described in Patent Documents 1 and 2, using an image based on a single image data as a display image is useful for reducing the number of exposures, but a portion corresponding to the edge of the subject. The trade-off is that it is unclear and has poor visibility.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a radiation imaging apparatus that can obtain a clear phase image while suppressing the number of exposures.

  In order to achieve the above object, a radiation imaging apparatus according to the present invention includes a first grating that generates a first periodic pattern image by passing radiation emitted from a radiation source, and the first periodic pattern image. A second grating that generates a second periodic pattern image by partially shielding, a radiation image detector that detects the second periodic pattern image and generates image data, and an absorption image of the subject An absorption image generation means for generating, and a subtraction processing unit that subtracts the absorption image from the image data to obtain a display image.

  The absorption image generating means moves the first grating or the second grating to a first position and a second position separated by a distance corresponding to half of the grating pitch in a direction orthogonal to the grating line. An average of the moving mechanism, the first image data generated by the radiation image detector at the first position, and the second image data generated by the radiation image detector at the second position. And an image averaging unit for generating an absorption image.

  The absorption image generation means includes a retracting device for retracting the first and second gratings from between the radiation source and the radiation image detector, and the first and second gratings to the radiation source and the radiation source. And a control unit that generates image data by the radiological image detector in a state of being retracted from the radiological image detector.

  The radiation imaging apparatus of the present invention includes a correction image storage unit that stores, as a correction image, image data generated by the radiation image detector in a state in which no subject is disposed, and correction that subtracts the correction image from the display image A processing unit.

  The first grating is an absorption grating and generates the first periodic pattern image by geometrically optically projecting incident radiation onto the second grating.

  The first grating may be an absorption grating or a phase grating, and may generate a Talbot effect on incident radiation to generate the first periodic pattern image.

  The radiation imaging apparatus of the present invention includes a multi-slit that partially blocks the radiation emitted from the radiation source and disperses the focal point.

  According to the present invention, the absorption image is subtracted from the single image data obtained by photographing through the first and second gratings to obtain a display image. A clear phase image can be obtained.

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 a schematic side view which shows the structure of the 1st and 2nd grating | lattice. It is a graph which illustrates change of a pixel value to the amount of movement of the 2nd lattice. It is a figure which shows the evacuation apparatus used for the X-ray imaging apparatus of 2nd Embodiment. It is a block diagram which shows the structure of the image process part used for the X-ray imaging apparatus of 3rd Embodiment.

(First embodiment)
In FIG. 1, an X-ray imaging apparatus 10 includes an X-ray source 11, an imaging unit 12, a memory 13, an image processing unit 14, an image recording unit 15, an imaging control unit 16, a console 17, and a system control unit 18. . The X-ray source 11 includes, for example, a rotary anode type X-ray tube and a collimator that limits an X-ray irradiation field, and emits X-rays toward the subject H.

  The imaging unit 12 includes an X-ray image detector 20, first and second gratings 21 and 22, and a moving mechanism 23. The first and second gratings 21 and 22 are absorption gratings, and 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 20 is a flat panel detector using a semiconductor circuit, for example, and is disposed behind the second grating 22 so that the detection surface 20a is orthogonal to the z direction.

  The first grating 21 includes a plurality of X-ray absorption parts 21a and X-ray transmission parts 21b that are extended in the y direction, which is one direction in the grating plane orthogonal to the z direction. The X-ray absorption part 21a and the X-ray transmission part 21b are alternately arranged along the x direction orthogonal to the z direction and the y direction, forming a striped pattern. Similar to the first grating 21, the second grating 22 includes a plurality of X-ray absorbing parts 22a and X-ray transmitting parts 22b that are extended in the y direction and are alternately arranged along the x direction. The X-ray absorption parts 21a and 22a are made of a metal having X-ray absorption such as gold (Au) or platinum (Pt). The X-ray transmission portions 21b and 22b are made of a material having X-ray transmission properties such as silicon (Si) or resin.

  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). The G1 image substantially matches the lattice pattern of the second lattice 22.

  The moving mechanism 23 includes an actuator such as a piezoelectric element, translates the second grating 22 in the x direction, and changes the relative position of the second grating 22 with respect to the first grating 21. Specifically, the moving mechanism 23 moves the second grating 22 between the first position and a second position shifted from the first position by a distance corresponding to half the grating pitch in the x direction. Move.

  The X-ray image detector 20 detects image G2 generated when the second grating 22 is at the first position and when the second grating 22 is at the second position, and generates image data. Hereinafter, the image data generated when the second grid 22 is at the first position is the first image data, and the image data generated when the second grid 22 is at the second position is the second image. It is called data.

  The memory 13 temporarily stores the first and second image data read from the X-ray image detector 20. The image processing unit 14 includes an image averaging unit 14a and a subtraction processing unit 14b. The image averaging unit 14a generates an absorption image by averaging the first image data and the second image data for each pixel. The subtraction processing unit 14b generates a display image by subtracting the absorption image for each pixel from the first image data. The image recording unit 15 records a display image. The imaging control unit 16 controls the X-ray source 11 and the imaging unit 12.

  The console 17 includes an operation unit 17a that enables operations such as setting of shooting conditions and a shooting execution instruction, and a monitor 17b that displays shooting information, display images, and the like. The system control unit 18 comprehensively controls each unit in accordance with a signal input from the operation unit 17a.

  In FIG. 2, an X-ray image detector 20 collects charges generated in a semiconductor film such as amorphous selenium (a-Se) by incident X-rays, and reads out the charges collected by the pixel electrodes 31. A plurality of pixels 30 each having a thin film transistor (TFT) 32 are two-dimensionally arranged. The X-ray image detector 20 includes a gate scanning line 33 provided for each row of the pixels 30, a scanning circuit 34 that applies a scanning signal for turning on and off the TFT 32 to each gate scanning line 33, A signal line 35 is provided for each column, and a readout circuit 36 that reads out charges from the pixels 30 via the signal lines 35, converts the charges into image data, and outputs the image data. The detailed layer configuration of each pixel 30 is the same as the layer configuration described in JP-A-2002-26300.

    The readout circuit 36 includes an integration amplifier, an A / D converter, a correction circuit (none of which is shown), and the like. The integrating amplifier integrates the charges output from each pixel 30 through the signal line 35 to generate an image signal. The A / D converter converts the image signal generated by the integrating amplifier into digital image data. The correction circuit performs dark current correction, gain correction, linearity correction, and the like on the image data, and inputs the corrected image data to the memory 13.

  The X-ray image detector 20 is not limited to a direct conversion type that directly converts incident X-rays into electric charges, but converts incident X-rays into visible light with a scintillator such as cesium iodide (CsI) or gadolinium oxysulfide (GOS). Alternatively, an indirect conversion type in which visible light is converted into electric charge by a photodiode may be used. Further, a CMOS sensor or the like can be used instead of the TFT.

  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 first and second gratings 21 and 22 are configured to project geometrically optically the X-rays that have passed through the X-ray transmission parts 21b and 22b. Specifically, the width of the X-ray transmission parts 21b and 22b in the x direction is set to a value sufficiently larger than the peak wavelength of X-rays emitted from the X-ray source 11. As a result, the first and second gratings 21 and 22 allow most of the X-rays to pass through without being diffracted while maintaining straightness. For example, 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 X-direction width of the X-ray transmission portions 21b and 22b may be about 1 to 10 μm.

The G1 image generated by the first grating 21 is enlarged in proportion to the distance from the X-ray focal point 11a. In the present embodiment, the grating pitch p ₂ of the second grating 22 is determined so as to coincide with the periodic pattern of the G1 image at the position of the second grating 22. That is, the grating pitch p ₂ of the second grating 22 is p _{1 as} the grating pitch of the first grating 21, the distance L ₁ between the X-ray focal point 11 a and the first grating 21, When the distance L ₂ between the second grating 22 and the second grating 22 is determined, it is determined so as to substantially satisfy the following expression (1).

  When the subject H is disposed between the X-ray source 11 and the first grating 21, the G2 image is modulated by the subject H. In the figure, X-rays 40 that are emitted from the X-ray source 11 and pass through the first grating 21 without being refracted by the subject H are shielded by the second grating 22 and are refracted by the subject H to be refracted. The first and second gratings 21 and 22 are arranged so that the X-rays that have passed through one grating 21 pass through the second grating 22 and enter the X-ray image detector 20. This position is the first position described above.

  The X-ray refraction angle φ of the subject H is expressed by the following equation (2) when the phase shift distribution of the subject H is Φ (x). Therefore, the image based on the first image data is It is an image reflecting the phase information of the specimen H. Here, the y-coordinate is omitted for simplification of description.

Moving mechanism 23, a second grid 22, the x-direction from a first position shown in FIG 3 by p _2/2 distance to translation. The position after this translational movement is the second position described above. In the second position, X-rays 40 that have been emitted from the X-ray source 11 and have passed through the first grating 21 without being refracted by the subject H pass through the second grating 22 and are therefore X-ray image detectors 20. Then, the X-rays refracted by the subject H and passed through the first grating 21 are shielded by the second grating 22.

When the second grating 22 is translated in the x direction by a distance corresponding to the grating pitch p ₂ with respect to the first grating 21, as illustrated in FIG. It changes so as to draw a sine wave with the grating pitch p ₂ as one period with respect to the moving distance of the grating 22. Since the first and second positions are in a positional relationship separated by a half cycle, each pixel value becomes the average value of the sine wave by averaging the first and second pixel data. An image having the average value as a pixel is the above-described absorption image. In the present embodiment, the moving mechanism 23 and the image averaging unit 14a constitute an absorption image generating unit.

  Next, the operation of the X-ray imaging apparatus 10 configured as described above will be described. When the subject H is disposed between the X-ray source 11 and the first grating 21 and an imaging instruction is input from the operation unit 17a, the second grating 22 is disposed at the first position by the moving mechanism 23. X-ray irradiation is performed by the X-ray source 11, and the G2 image is detected by the X-ray image detector 20 to generate first image data. Next, the second grating 22 is moved to the second position by the moving mechanism 23, X-ray irradiation is performed in the same manner, G2 image detection is performed, and second image data is generated. The first and second image data are stored in the memory 13, respectively.

  Thereafter, the image processing unit 14 reads the first and second image data stored in the memory 13. In the image processing unit 14, the image averaging unit 14a averages the first and second image data for each pixel to generate an absorption image. In the subtraction processing unit 14b, the display image is generated by subtracting the absorption image for each pixel from the first image data. By subtraction of the absorption image, the display image has a clear portion corresponding to the edge of the subject H, and the visibility is improved.

  The display image is recorded in the image recording unit 15 and then input to the console 17 and displayed on the monitor 17b.

  In the first embodiment, the subject H is arranged between the X-ray source 11 and the first grating 21, but the subject H is arranged with the first grating 21 and the second grating 22. You may arrange | position between.

In the first embodiment, the moving mechanism 23 causes the second grating 22 to be shifted from the first position shown in FIG. 3 by a half period of the grating pitch p ₂ from the first position. However, the first and second positions are not limited to the above positions, and may be any positions as long as they are shifted by a half cycle.

In the first embodiment, although by moving the second grating 22 by the movement mechanism 23, instead of the second grating 22, a half period of the grating pitch p ₁ in the same manner of the first grating 21 You may make it move to the 1st and 2nd position separated by a minute.

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

  In the first embodiment, the display image is obtained by subtracting the absorption image from the first image data. Further, the absorption image is added after multiplying the display image by an appropriate correction coefficient. The display image may be used as a display image. By doing so, it is possible to improve the visibility by aligning the contrast of both the edge portion and the absorption portion of the subject.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. In the first embodiment, either one of the first and second gratings 21 and 22 is moved to the first and second positions separated by a half period of the grating pitch, and the image obtained at each position is obtained. An absorption image is generated by averaging the data. However, in the X-ray imaging apparatus of the second embodiment, instead of the moving mechanism 23, as shown in FIG. A retracting device 50 for retracting 22 from between the X-ray source 11 and the X-ray image detector 20 is provided.

  In the present embodiment, X-ray irradiation is performed with the first and second gratings 21 and 22 retracted, and the image data generated by the X-ray image detector 20 is subtracted from the first image data as an absorption image. To do. In the present embodiment, the evacuation device 50 and the system control unit 18 constitute an absorption image generation unit. Other configurations and operations are the same as those in the first embodiment.

(Third embodiment)
Next, a third embodiment of the present invention will be described. In the first embodiment, a series of imaging is performed in a state where the subject H is disposed between the X-ray source 11 and the first grating 21, and a display image is generated. The display image may be uneven due to noise due to manufacturing errors or placement errors of the grids 21 and 22 of No. 2, and thus imaging (pre-imaging) is performed without arranging the subject H. The image data may be corrected as a corrected image.

  The X-ray imaging apparatus according to the third embodiment includes a main imaging mode in which imaging is performed by placing the subject H by the operation unit 17a of the console 17, and a pre-imaging mode in which imaging is performed without placing the subject H. Switching is possible.

  In FIG. 6, the image processing unit of the present embodiment includes an image averaging unit 60, a subtraction processing unit 61, a corrected image storage unit 62, and a correction processing unit 63. The image averaging unit 60 and the subtraction processing unit 61 are the same as the image averaging unit 14a and the subtraction processing unit 14b of the first embodiment. The corrected image storage unit 62 stores image data generated by the X-ray image detector 20 by performing X-ray irradiation in the pre-imaging mode as a corrected image.

  In the main shooting mode, shooting is performed as in the first embodiment, and first and second image data are generated. The image averaging unit 60 generates an absorption image from the first and second image data, and the subtraction processing unit 61 subtracts the absorption image generated by the image averaging unit 60 from the first image data.

  Then, the correction processing unit 63 subtracts the correction image stored in the correction image storage unit 62 from the image after the subtraction processing by the subtraction processing unit 61. In the present embodiment, the image after the correction processing by the correction processing unit 63 is the display image. An absorption image may be acquired by the method of the second embodiment. Other configurations and operations are the same as those in the first embodiment.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described. In the first embodiment, the X-ray source 11 has a single focal point. However, in the X-ray imaging apparatus of the fourth embodiment, immediately after the emission side of the X-ray source 11, it is described in WO2006 / 131235. A multi-slit (source grid) is provided to disperse the focal point. As a result, a high-power X-ray source can be used, and the X-ray dose is improved, so that the image quality is improved. In this case, the pitch p _{0 of the} multi-slit needs to satisfy the following formula (3). In the present embodiment, the distance L ₁ is a distance from the multi slit to the first grating 21. Other configurations and operations are the same as those in the first embodiment.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described. In the first embodiment, the first grating 21 is geometrically optically projected without diffracting incident X-rays. However, in the X-ray imaging apparatus of the fifth embodiment, Japanese Patent Application Laid-Open No. 2008-200361. As described in Gazette and the like, the first lattice 21 has a Talbot effect. In order to generate the Talbot effect, a small-focus X-ray light source or an X-ray source having a multi-slit as described above may be used so as to enhance the spatial coherence of X-rays.

If the Talbot effect occurs, the self-image of the first grating 21 (G1 image) results from the first grating 21 in a position apart Talbot distance Z _m downstream. Therefore, in the present embodiment, it is necessary to set the distance L ₂ from the first grid 21 to the second grating 22 on the Talbot distance Z _m. Other configurations and operations are the same as those in the first embodiment.

An absorption grating first grating 21, when X-rays emitted from the X-ray source 11 is a cone beam, Talbot distance Z _m is represented by the following formula (4). Here, m is a positive integer.

Further, when the first grating 21 is a phase-type grating that gives a phase modulation of π / 2 and the X-ray emitted from the X-ray source 11 is a cone beam, the Talbot distance Z _m is expressed by the following formula ( 5). Here, m is 0 or a positive integer.

Further, when the first grating 21 is a phase type grating that gives a phase modulation of π and the X-ray emitted from the X-ray source 11 is a cone beam, the Talbot distance Z _m is expressed by the following equation (6). It is represented by 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 collimated beam, Talbot distance Z _m is represented by the following formula (7). Here, m is a positive integer.

Further, when the first grating 21 is a phase type grating that gives a phase modulation of π / 2, and the X-ray emitted from the X-ray source 11 is a parallel beam, the Talbot distance Z _m is expressed by the following formula ( 8). Here, m is 0 or a positive integer.

When the first grating 21 is a phase-type grating that gives a phase modulation of π, and the X-ray emitted from the X-ray source 11 is a parallel beam, the Talbot distance Z _m is expressed by the following equation (9). It is represented by Here, m is 0 or a positive integer.

  The present invention is not limited to a radiographic apparatus for medical diagnosis, but can be applied to other radiographic apparatuses for industrial use. Further, the radiation is not limited to X-rays, and gamma rays can also be used.

DESCRIPTION OF SYMBOLS 10 X-ray imaging apparatus 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 31 Pixel electrode 33 Gate scanning line 35 signal lines

Claims (7)

A first grating for generating a first periodic pattern image by passing radiation emitted from a radiation source;
A second grating that generates a second periodic pattern image by partially shielding the first periodic pattern image;
A radiation image detector for detecting the second periodic pattern image and generating image data;
An absorption image generating means for generating an absorption image of the subject;
A subtraction processing unit that subtracts the absorption image from the image data to obtain a display image;
A radiation imaging apparatus comprising: The absorption image generating means moves the first grating or the second grating to a first position and a second position separated by a distance corresponding to half of the grating pitch in a direction orthogonal to the grating line. A moving mechanism
Absorption image by averaging the first image data generated by the radiation image detector at the first position and the second image data generated by the radiation image detector at the second position An image averaging unit for generating
The radiation imaging apparatus according to claim 1, comprising: The absorption image generating means includes a retracting device for retracting the first and second gratings from between the radiation source and the radiation image detector;
A controller that generates image data by the radiation image detector in a state in which the first and second gratings are retracted from between the radiation source and the radiation image detector;
The radiation imaging apparatus according to claim 1, comprising: A corrected image storage unit that stores image data generated by the radiation image detector in a state where no subject is disposed as a corrected image;
A correction processing unit that subtracts the corrected image from the display image;
The radiation imaging apparatus according to claim 1, further comprising:   The first periodic pattern image is generated by geometrically optically projecting incident radiation onto the second grating, wherein the first grating is an absorption grating. 4. The radiographic apparatus according to any one of 4 above.   5. The first grating according to claim 1, wherein the first grating is an absorption grating or a phase grating, and generates a Talbot effect on incident radiation to generate the first periodic pattern image. The radiation imaging apparatus described in 1.   The radiation imaging 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.

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