X-ray imaging system and imaging method

The present invention relates to an X-ray imaging apparatus that utilizes the phase of X-rays.

In the visible light region, a Talbot interferometer is known in which a diffraction grating causes a Talbot effect and another diffraction grating is used in combination to generate moire fringes. This makes it possible to visualize the wave surface of visible light, that is, to generate image contrast using the phase. On the other hand, X-ray imaging devices using phase have been researched and developed in the X-ray region, and for example, the documents described in JP-A-10-248833 are known. However, this technique has the inconvenience that the device configuration is complicated or a huge X-ray source called synchrotron radiation is required.

The present invention has been made in view of the above circumstances. An object of the present invention is to provide an apparatus capable of X-ray imaging using the phase of X-rays with a simple structure. The X-ray imaging apparatus of the present invention includes first and second diffraction gratings and an X-ray image detector, and the first diffraction grating is Talbot by X-rays applied to the first diffraction grating. The second diffraction grating is configured to diffract the X-rays diffracted by the first diffraction grating, and the X-ray image detector is configured to produce the effect. It is configured to detect the X-ray diffracted by the diffraction grating of. By diffracting the X-rays diffracted by the first diffraction grating, the second diffraction grating is in front of the first diffraction grating or between the first diffraction grating and the second diffraction grating. Image contrast due to the phase change of X-rays due to the placed subject can be formed. The X-ray image detector can detect the X-ray that causes this image contrast. The first and second diffraction gratings can be transmission type. The X-ray imaging apparatus may further include an X-ray source. This X-ray source irradiates the X-ray image detector with X-rays through the first diffraction grating and the second diffraction grating. The X-ray imaging apparatus may be capable of arranging a subject between the X-ray source and the first diffraction grating. The ratio of the distance from the X-ray source to the first diffraction grating and the period in the first diffraction grating, and the ratio of the distance from the X-ray source to the second diffraction grating to the period in the second diffraction grating. Can be set approximately equal to. The first diffraction grating may be configured to apply phase modulation of about 80 ° to 100 ° to the irradiated X-rays. The periods in the first and second diffraction gratings may be substantially equal to each other. Moire fringes may be formed on the X-ray image detected by the X-ray image detector. The first or second diffraction grating may be relatively rotatable so that the spacing between the moire fringes can be adjusted. The first and second diffraction gratings each include a diffraction member that diffracts the X-ray, the diffraction grating is extended in at least one direction, and the first or second diffraction grating is It may be movable along these diffraction grating planes and along the direction intersecting the diffraction member. The X-ray imaging method of the present invention uses an X-ray source, first and second diffraction gratings, and an X-ray image detector, and further includes the following steps. (1) A step of arranging a subject between the X-ray source and the first diffraction grating or between the first diffraction grating and the second diffraction grating; (2) A step of irradiating X-rays from the X-ray source toward the first diffraction grating; (3) A step in which the second diffraction grating is irradiated with the X-rays that have been diffracted by the first diffraction grating to produce the Talbot effect; (4) A step in which the X-rays diffracted by the first diffraction grating are diffracted by the second diffraction grating; (5) A step in which the X-ray image detector detects the X-rays diffracted by the second diffraction grating.

FIG. 1 is an explanatory diagram showing a schematic configuration of an X-ray imaging apparatus according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of the first diffraction grating. FIG. 3 is a cross-sectional view of the second diffraction grating. FIG. 4 is a side view of the X-ray imaging apparatus of FIG. FIG. 5 is a diagram showing the experimental results of the X-ray Talbot effect in Experimental Example 1, and is a diagram when the distance from the diffraction grating to the measurement surface is 32 cm. FIG. 6 is a diagram showing the experimental results of the X-ray Talbot effect in Experimental Example 1, and is a diagram when the distance from the diffraction grating to the measurement surface is 64 cm. FIG. 7 is a diagram showing the experimental results of the X-ray Talbot effect in Experimental Example 1, and is a diagram when the distance from the diffraction grating to the measurement surface is 96 cm. FIG. 8 is a diagram showing the experimental results of the X-ray Talbot interferometer in Experimental Example 2. FIG. 9 is a diagram showing a phase shift differential image obtained in Experimental Example 3. FIG. 10 is a stereoscopic image obtained by tomography in Experimental Example 3.

The X-ray imaging apparatus according to the embodiment of the present invention will be described below. This imaging device mainly includes an X-ray source 1, a first diffraction grating 2, a second diffraction grating 3, and an X-ray image detector 4 (see FIG. 1). It is assumed that the X-ray source 1 has "spatial coherence sufficient to generate a Talbot effect when the first diffraction grating 2 is irradiated with X-rays". For example, "the size of the X-ray emission point (that is, the opening diameter of the X-ray source) is about 30 microns, and the spatial coherence at a position of about 5 meters or more from the X-ray source" corresponds to this. The conditions under which the Talbot interferometer is composed of the X-ray source 1, the diffraction grating 2 and the diffraction grating 3 will be described later. The first diffraction grating 2 includes a substrate 21 and a plurality of diffraction members 22 attached to the substrate 21 (see FIG. 2). The substrate 21 is, for example, glass. Each of the plurality of diffraction members 22 has a linear shape extending in one direction (the thickness direction of the paper surface in FIG. 2). The distance between the plurality of diffraction members 22 (that is, the period of the diffraction grating) d (see FIG. 2) is constant (that is, the distance between the diffraction members 22 is equal) in this embodiment. As the material of the plurality of diffraction members 22, for example, gold can be used. The diffractive member 22 preferably constitutes a so-called phase type diffraction grating that applies phase modulation of about 80 ° to 100 ° (ideally 90 °) to the irradiated X-rays. That is, the diffractive member 22 changes the phase velocity of the X-rays irradiated to that portion. X-rays do not necessarily have to be monochromatic, and may have an energy width (that is, a wavelength spectrum width) within a range satisfying the above conditions. Like the first diffraction grating 2, the second diffraction grating 3 includes a substrate 31 and a diffraction member 32 (see FIG. 3). The second diffraction grating 3 is configured to form image contrast by diffracting the X-rays diffracted by the first diffraction grating 2. The second diffraction grating 3 is preferably an amplitude type diffraction grating in which the diffraction member 32 is thicker, but it can be configured in the same manner as the first diffraction grating 2. The X-ray image detector 4 detects X-rays that have generated image contrast. Since such a detector 4 may be the same as that used in the conventional X-ray imaging apparatus, detailed description thereof will be omitted. Next, the conditions under which the first and second diffraction gratings 2 and 3 constitute the Talbot interferometer will be described. First, the coherent distance 1 is as follows.<img file="JP4445397B2_D0001.tif" /> here, λ: X-ray wavelength (usually the center wavelength), a: Opening diameter of the X-ray source in the direction almost orthogonal to the diffractive member, L: Distance from the X-ray source to the first grating (see Figure 4), Z₁: Distance from the first grating to the second grating, Z₂: Distance from the second grating to the detector Is. Further, the distance Z between the first diffraction grating 2 and the second diffraction grating 3₁Assuming that the first diffraction grating 2 is a phase type diffraction grating, the following conditions must be substantially satisfied.<img file="JP4445397B2_D0002.tif" /> However, m is an integer and d is the period of the above-mentioned lattice member. Here, assuming that 1 = αd, it is desirable that α in the equation (1) is α&gt; 5 (m + 1/2). Next, the operation of the X-ray imaging apparatus of this embodiment will be described. This imaging device is used with the subject 10 (see FIGS. 1 and 4) placed between the X-ray source 1 and the first diffraction grating 2. Then, X-rays are emitted from the X-ray source 1 toward the first diffraction grating 2. Then, the irradiated X-rays pass through the first diffraction grating 2. At this time, the Talbot effect is generated in the first diffraction grating 2. Here, the Talbot effect is to form a self-image of the diffraction grating at the distance given by the equation (2) in the case of the phase type diffraction grating when the plane wave passes through the diffraction grating. In this case, since there is a phase shift of the X-rays due to the subject 10, the wave plane of the X-rays incident on the first diffraction grating 2 is distorted. Therefore, the self-image of the first diffraction grating 2 is deformed depending on it. Subsequently, the X-ray passes through the second diffraction grating 3. As a result, an image contrast can be generated for X-rays by superimposing the self-image of the deformed first diffraction grating 2 and the second diffraction grating 3. The image contrast is generally moire fringes and can be detected by the X-ray image detector 4. The generated moire fringes are modulated by subject 10. The amount of modulation is proportional to the angle at which the X-ray is bent due to the refraction effect of the subject 10. Therefore, by analyzing the moire fringes detected by the X-ray image detector 4, the subject 10 and its internal structure can be detected. The diffraction members of the first or second diffraction gratings 2 and 3 are arranged so as to be rotated by a relatively small angle θ around a virtual axis passing through the X-ray source and the X-ray image detector. And. The interval between the generated moire fringes changes depending on the size of θ. If there is no subject 10, the moiré fringe spacing is given by d / θ. Where d is the period of the diffraction grating. If a mechanism for changing the minute angle θ (for example, a mechanism for rotating one of the first diffraction grating 2 and the second diffraction grating 3 relative to the other) is provided, moire fringes are preferable for observation. Can be adjusted. Further, if the minute angle θ is adjusted to be almost zero, moire fringes do not appear except in the portion corresponding to the subject 10 (that is, in the non-modulated portion). As a result, in the obtained X-ray image, only the contrast due to the subject 10 appears. Further, in the above example, the case where the subject 10 is located between the X-ray source 1 and the diffraction grating 2 has been described, but there is a case where the subject 10 is located between the diffraction grating 2 and the diffraction grating 3. However, the self-image of the diffraction grating 2 generated at the position of the diffraction grating 3 is deformed by the subject 10. Therefore, even in this case, the X-ray image detector 4 can detect the moire fringes (image contrast) modulated by the subject 10 in essentially the same manner as in the above example. That is, in the apparatus of the present embodiment, imaging when the subject 10 is located between the diffraction grating 2 and the diffraction grating 3 is also effective. The apparatus of the present embodiment has an advantage that X-ray imaging can be realized with a simple configuration for a subject that is difficult to observe by a general method that relies on contrast generation depending on the magnitude of X-ray absorption. .. (Experimental example 1) An experimental example 1 in which the Talbot effect is generated by irradiating the diffraction grating with X-rays is shown. (Experimental conditions) X-rays used: Synchrotron radiation with a wavelength of 0.1 nm. Diffraction grating: A gold pattern with a thickness of about 1 micron formed on a glass plate with a thickness of 150 microns with a period of d = 8 microns. The conditions under which the Talbot effect occurs in a phased diffraction grating are<img file="JP4445397B2_D0003.tif" />Therefore, first, when the distance Z from the first diffraction grating 2 to the detection surface is 32 cm, a self-image of the diffraction grating should be formed corresponding to the case of m = 0. Therefore, a high-resolution X-ray image detector was placed at a distance Z of 32 cm, and the image was recorded. The result is shown in Fig. 5. Similarly, when the distance Z is 64 cm, the self-image becomes unclear because it deviates most from the above conditions (Fig. 6). When the distance Z was 96 cm, the self-image of the diffraction grating was formed again, corresponding to the case of m = 1 (Fig. 7). This showed the occurrence of the Talbot effect. In Experimental Example 1, a high-resolution X-ray image detector was used because the purpose was to directly observe the self-image, but when using it as a Talbot interferometer, it is sufficient if moire fringes can be observed, so it is not always high-resolution. No X-ray image detector is required. (Experimental example 2) Here, Experimental Example 2 of this embodiment using the X-ray Talbot effect is shown. (Experimental conditions) X-rays used: Synchrotron radiation with a wavelength of 0.1 nm. Diffraction grating 2: A gold pattern with a thickness of about 1 micron formed on a glass plate with a thickness of 150 microns with a period of d = 8 microns. Diffraction grating 3: A gold pattern with a thickness of about 8 microns formed on a glass plate with a thickness of 150 microns with a period of d = 8 microns. Spacing Z of diffraction gratings 2 and 3₁Was 32 cm at which the Talbot effect due to the diffraction grating 2 appeared. A plastic ball having a diameter of about 1 mm was used as the subject 10. The subject 10 was placed between the X-ray source 1 and the first diffraction grating 2 and immediately before the first diffraction grating 2. In this state, the X-rays emitted from the X-ray source were detected by the X-ray image detector 4 arranged immediately after the diffraction grating 3. As a result, the image shown in FIG. 8 could be obtained. In this experiment, since the adjustment was made so that θ0 °, moire fringes were hardly generated, and only the contrast corresponding to the phase shift due to the plastic sphere as the subject appeared. In this experimental example, the sphere and the bubbles existing inside it are clearly captured. In Experimental Example 2, the first and second diffraction gratings are of the transmission type, but they may be of the reflection type. However, since the amount of X-ray reflection is generally small, the transmission type is more efficient. Further, in Experimental Example 2, the periods in the first and second diffraction gratings 2 and 3 were made uniform and the same, which is the distance Z shown in FIG.₁This is because it was possible to assume that the distance L to the X-ray source was sufficiently large. In general, the ratio of the distance from the X-ray source 1 to the first diffraction grating 2 to the period in the first diffraction grating is the distance from the X-ray source 1 to the second diffraction grating 3 and the second. It is desirable that the ratio to the period in the diffraction grating 3 is approximately equal. Further, in Experimental Example 2, the first diffraction grating has a configuration in which a phase difference is given to the irradiated X-rays (phase type diffraction grating), but a configuration in which an intensity difference is given to the X-rays. (Amplitude type diffraction grating) may be used. In this case, the diffractive member may be configured to absorb X-rays. Even in this case, the Talbot interferometer can be configured by the above-mentioned principle. Further, in Experimental Example 2, the first and second diffraction gratings 2 and 3 have a flat plate shape, but may have a spherical shape. In this case, it is preferable to use a spherical surface with the radiation source as the center of curvature. Further, in the above embodiment, either the subject or the imaging system (radioactive source, each diffraction grating, and detector) is rotated to acquire images in a plurality of projection directions, and the images are subjected to tomography processing. Therefore, the subject and its internal structure can be observed three-dimensionally. In this case, unlike the conventional tomography, a three-dimensional image is formed by the refractive index distribution, and it is possible to depict a structure that is difficult to depict with the sensitivity of the conventional tomography. (Experimental example 3) Here, an experimental example of tomography using the apparatus according to the present embodiment is shown. (Experimental conditions) The experimental conditions of Experimental Example 3 are the same as those of Experimental Example 2. Tomography using this embodiment requires the following three steps. Step 1 is a "distribution image of the angle at which X-rays are bent by the refraction effect of the subject 10" (hereinafter "phase") from the X-ray image detected by the X-ray image detector 4 (hereinafter referred to as "moire fringe image"). It is a conversion to (called a shift differential image). Step 2 is to acquire an image (hereinafter referred to as "phase shift image") representing the phase shift itself by integrating the phase shift differential image. Step 3 is to reconstruct the stereoscopic image by tomography using the phase shift images obtained in a plurality of projection directions. The fringe scanning method is used in step 1. In this method, one of the diffraction gratings 2 or 3 is moved in translation relative to the other. The translational direction is a direction substantially parallel to the plane of the moving diffraction grating and substantially perpendicular to the diffraction member. Therefore, when tomography is performed by the apparatus of the present embodiment, it is preferable that the apparatus of the present embodiment further includes a moving mechanism for moving the first diffraction grating 2 or the second diffraction grating 3. The moire fringes move with the translational movement of the diffraction grating, and when the translation distance reaches one cycle of the diffraction grating, the moire fringe image returns to the original state. In the fringe scanning method, changes in a moire fringe image are recorded while being translated by an integer of one cycle, and the phase shift differential image ψ (x, y) is obtained by arithmetically processing them. (x, y) are coordinates indicating the position of the pixel. The moire fringe image I (x, y) is generally obtained, where the translational movement amount is ξ.<img file="JP4445397B2_D0004.tif" />Given in. Here A_k(k = 0,1, ...) Is a constant determined by the shape of the diffraction grating. Δ (x, y) represents the contribution of contrast that occurs independently of the subject due to diffraction grating distortion, fabrication error, and placement error. d is the period of the diffraction grating to be translated, Z₁Is the distance between the diffraction grating 2 and the diffraction grating 3. Now, suppose that M moire fringe images are acquired while changing ξ in step d / M (M: integer). If the term k&gt; N is small enough to be ignored in equation (3), then M should be chosen to satisfy M&gt; N + 1.<img file="JP4445397B2_D0005.tif" />Is satisfied. arg [] means the extraction of the argument. Ip (x, y) is the value of equation (3) when ξ = pd / M. d and Z₁Is known, and Δ (x, y) can be obtained in advance by performing the same measurement when there is no subject (that is, ψ (x, y) = 0). Therefore, ψ (x, y) can be obtained from the above. FIG. 9 shows a phase shift differential image ψ (x, y) obtained from an image obtained by translating the second diffraction grating 3 at M = 5. For the subject 10, a plastic sphere having a diameter of about 1 mm (same as the example in FIG. 8) was used. In FIG. 9, the phase shift differential image including the bubbles contained inside the plastic sphere is well depicted. What is the phase shift image Φ (x, y) and the phase shift differential image ψ (x, y)?<img file="JP4445397B2_D0006.tif" />Is related with. Here, x corresponds to the direction in which the diffraction grating is translated by the fringe scanning method. From this, the phase shift image Φ (x, y) is given by integrating ψ (x, y) along the x-axis. This is step 2. In the phase shift image Φ (x, y), the refractive index distribution of the subject is n (x, y, z).<img file="JP4445397B2_D0007.tif" />Given in. Here, the z-axis is the direction in which X-rays travel. Tomography is a technique for reconstructing a three-dimensional image of a subject from a projected image, which is a two-dimensional image, when it can be obtained from a plurality of projection directions. Since the phase shift image Φ (x, y) corresponds to the projection image of 1-n (x, y, z), if the phase shift distribution image can be obtained from multiple projection directions, n (x, y, z) can be obtained. The stereoscopic image shown is reconstructed (step 3). Note that steps 2 and 3 may be performed at the same time. FIG. 10 shows the result of reconstructing a stereoscopic image of the plastic sphere of FIG. 9 using the above procedure. The example of FIG. 10 is the result of rotating the plastic sphere by 0.72 °, acquiring 250 images corresponding to FIG. 9, and reconstructing using these images. In order to show the inside of the plastic sphere, some parts are excluded by computer processing. It should be noted that such an imaging method is not meaningless unless the process proceeds to step 3, and the image (raw image) directly obtained by the X-ray image detector 4 of the above-described embodiment, the phase shift differential image φ. Both (x, y) and the phase shift image Φ (x, y) can be sufficiently used depending on the purpose of imaging. Further, in the above-described embodiment, the structure in which the diffraction member is attached to the substrate is shown as the first and second diffraction gratings, but the structure is not limited to this. As the structure of the diffraction grating, for example, the diffraction members may be attached to both sides of the flat plate to form the first and second diffraction members. Further, a diffraction grating may be formed by alternately laminating a large number of two types of films or foils having different refractive indexes (or absorptivity) and cutting them in the thickness direction of the film or foil. Further, in the above-described embodiment, the configuration includes the X-ray source 1, but the device may not have the X-ray source 1. In this case, it may be combined with an X-ray source at the time of use. It should be noted that the description of the above-described embodiments and examples is merely an example, and does not indicate an essential configuration for the present invention. The configuration of each part is not limited to the above as long as the gist of the present invention can be achieved. For example, the components in each of the above-described embodiments may exist as functional elements, and as a device or component, they may be integrated with other elements, and one element is realized by a plurality of components. It may have been done.

Possibility of industrial use

According to the present invention, it is possible to provide an apparatus capable of X-ray imaging using the phase of X-rays with a simple structure.