JP5601909B2 - X-ray imaging apparatus and X-ray imaging method using the same - Google Patents

Description

  The present invention relates to an X-ray imaging apparatus for observing the internal structure of a subject with high sensitivity using the phase of X-rays.

  Since X-rays have high penetrating power, they are widely used in medical image diagnosis, non-destructive inspection, security check and the like as a probe for seeing through the inside of an object. The contrast of an X-ray fluoroscopic image depends on the difference in X-ray attenuation rate, and an object that strongly absorbs X-rays is rendered as an X-ray shadow. X-ray absorption ability becomes stronger as more elements with larger atomic numbers are included. Conversely, it can be pointed out that a substance composed of an element with a small atomic number is difficult to contrast, and this is also a principle defect of an X-ray fluoroscopic image. Therefore, sufficient sensitivity cannot be obtained with respect to biological soft tissue or soft material.

  On the other hand, it is known that the use of X-ray phase information realizes a sensitivity increase of up to about three digits compared to a general conventional X-ray fluoroscopic image. Since it can be applied to the observation of substances (light body soft tissues, organic materials, etc.) made of light elements that do not absorb X-rays very much, their practical use is expected.

  Research on high-sensitivity imaging using this X-ray phase information is a field that has been around for about 15 years, but since an advanced X-ray source is usually required, its practical use is not practical. Not progressing. That is, X-ray optical systems that use monochromatic plane wave X-rays have been studied as the mainstream, and it is therefore premised on the use of an extremely high-brightness X-ray source.

  In order to obtain a monochromatic plane wave, it is necessary to select only a specific spectral component traveling in a specific direction from the originally obtained X-ray. Therefore, in order to ensure the intensity necessary for imaging, the original X-ray is required to have sufficient brightness to compensate for the loss due to sorting. A single crystal such as silicon is used as an optical element for such selection, but at the same time it must be essentially assumed to use a huge synchrotron radiation facility. It is an obstacle.

  If a phase-based imaging method that functions with a wide-band cone beam is realized, an apparatus using a compact X-ray source other than synchrotron radiation can be expected. As a candidate for such an imaging method, an X-ray phase imaging method using an X-ray Talbot interferometer is expected (see Patent Documents 1 and 2 below). In this method, since an X-ray lattice is used instead of a single crystal, imaging using multi-colored divergent beam X-rays is possible.

  However, in the phase-based imaging method, a certain degree of spatial coherence is required for X-rays. For this purpose, the size of the X-ray generation source must be small to some extent. Then, even if it is a compact X-ray source, an existing X-ray source that can be used in this method is substantially a microfocus X-ray source. This does not apply to normal focus X-ray sources.

  In a microfocus X-ray source, X-rays are generated by irradiating an electron beam onto a minute region of a target. When it is desired to generate a large amount of X-rays, it is necessary to irradiate many electrons. However, there is a problem of thermal load on the target, and the upper limit of the actual X-ray power is determined. As a result, except for the case where a long exposure time is allowed, there is a problem that the power of the X-ray obtained by the microfocus X-ray source is insufficient for application to X-ray imaging.

  In the conventional X-ray Talbot-Lau interferometer, a normal focus X-ray source is used to avoid the problem of insufficient intensity. The configuration of the X-ray Talbot-Lau interferometer is obtained by adding a multi-slit to the configuration of the X-ray Talbot interferometer (see Patent Documents 3 to 4 and Non-Patent Document 1 below). That is, in this technique, an X-ray is generated by irradiating an electron beam to a relatively large area of the target, and the generated X-ray is partially transmitted through a multi slit. As a result, it is possible to realize a radiation source in which narrow and linear virtual X-ray sources are arranged at a predetermined pitch. Such a radiation source having a plurality of micro lines may be referred to as a micro multi-line radiation source.

  The X-ray Talbot-Lau interferometer can be said to be a technology in which the X-ray source in the X-ray Talbot interferometer is realized by a normal focus X-ray generator and a multi-slit. It should be said.

  By the way, in each of the conventional X-ray Talbot interferometers (including the Talbot-Lau interferometer), the second grating 4 is disposed in front of the X-ray image detector 5 as shown in FIG. ing. FIG. 1 shows a Talbot-Lau interferometer using an X-ray generator 1 and a multi-slit 2. The X-ray generation unit 1 and the multi slit 2 constitute an X-ray source.

  The second grating 4 forms a moire interference fringe between the self-image formed by the X-rays diffracted by the first grating 3 and the second grating 4. The X-ray image detector 5 can detect this moire interference fringe.

  When a subject (that is, a sample) is placed between the image detector 5 and the multi-slit 2, the phase of the X-ray is changed by the subject, and the moire interference fringes are also deformed. By detecting and analyzing this deformation, it is possible to acquire information on the subject.

  By the way, the second grid 4 arranged in front of the image detector 5 substantially corresponds to the size of the field of view necessary for photographing, and therefore has the largest area among the grids used in the imaging apparatus. Further, the second grating 4 is generally an absorption grating made of a heavy metal such as gold and having a high aspect ratio, and high technology is required for its production. Therefore, it is not easy to increase the field of view of the photographing apparatus, and the cost for realizing it is enormous.

International Publication WO2004 / 058070 U.S. Pat. No. 5,821,629 JP 2008-145111 A JP 2009-240378 A

Donath et al., J. Appl. Phys. 106, 054703 (2009)

  The present invention has been made in view of the above circumstances. One object of the present invention is to provide a technique capable of omitting the second grating which has been conventionally required.

  The present invention can be expressed as an invention described in the following items.

(Item 1)
An X-ray source, a grating, and an X-ray image detector;
The X-ray source is configured to irradiate a required amount of X-rays toward the grating,
The grating is configured to form a self-image of the grating at a position separated from the grating by diffracting the X-rays irradiated toward the grating.
The X-ray image detector is configured to detect the X-ray diffracted by the grating,
And the said X-ray image detector is arrange | positioned in the position where the said self-image is formed, or its vicinity,
Furthermore, the X-ray imaging device characterized by satisfying the following conditions 1 and 2:
(Condition 1)
here,
R ₁ : distance from the X-ray source to the grating;
R ₂ : distance from the lattice to the position where the self-image is formed;
λ: center wavelength of the X-ray;
R: R ₁ + R ₂ ;
d: period of the lattice;
p: a parameter related to the distance from the lattice.
(Condition 2)
here,
D: spatial resolution in the X-ray image detector;
d ₁ is the period of the self-image.

  In an X-ray imaging apparatus that satisfies the above-described conditions, a self-image of a grating can be detected by an image detector without using a second grating as in the prior art. In the present invention, “satisfying the condition” does not need to have a mathematically exact meaning. The further away from the correct conditions, the more blurred the image will be. However, if the blur that occurs under a certain condition is a level that does not impede practical use, adopting the condition is substantially included in the concept of “condition”.

(Item 2)
The X-ray source includes an X-ray generation unit and a multi-slit,
The X-ray imaging apparatus according to Item 1, wherein the X-ray generation unit is configured to irradiate the X-rays toward the lattice through the multi-slit.

(Item 3)
The X-ray source includes an X-ray generator,
The X-ray imaging apparatus according to Item 1, wherein the X-ray generation unit is configured to directly irradiate the grating with X-rays having spatial coherence.

(Item 4)
It also has a translation mechanism,
Item 1 is characterized in that the translation mechanism is configured to translate one of the grating and the multi-slit relative to the other in a direction substantially parallel to a diffraction vector in the grating. The X-ray imaging apparatus of any one of -3.

(Item 5)
An X-ray imaging method using the X-ray imaging apparatus according to any one of Items 1 to 3,
(1) placing a subject between the X-ray source and the X-ray image detector;
(2) irradiating X-rays from the X-ray source toward the grating;
(3) a step in which the X-ray detector detects the X-rays diffracted by the grating to form a self-image;

  According to the present invention, it is possible to omit the second grating, which has been conventionally required. By this. According to the present invention, it is possible to provide a technique that has a simple structure, facilitates a wide field of view, and enables cost reduction.

(Configuration of X-ray imaging apparatus in the first embodiment)
The configuration of the X-ray imaging apparatus according to the first embodiment of the present invention will be described below with reference to the drawings.

  The X-ray imaging apparatus according to the present embodiment includes an X-ray source 10, a grating 20, and an X-ray image detector 30.

The X-ray source 10 is configured to irradiate a required amount of X-rays toward the grating 20. More specifically, the X-ray source 10 of this embodiment includes an X-ray generation unit 101 and a multi slit 102. The X-ray generation unit 101 is configured to irradiate X-rays toward the grating 20 through the multi slit 102. In this example, a transmissive absorption grating having a period of d _s is used as the multi slit 102. The members constituting the slits in the multi slit 102 are extended in a direction perpendicular to the paper surface in FIG. The multi-slit 102 can be the same as that used in the conventional Talbot-Lau interferometer, and thus further description thereof will be omitted.

  The grating 20 is configured to form a self-image of the grating 20 at a position away from the grating 20 by diffracting X-rays irradiated toward the grating 20. The grating 20 may be a phase grating or an absorption grating. In FIG. 2, members (so-called “lines”) constituting the diffraction pattern in the grating 20 are extended in a direction perpendicular to the paper surface. The grating 20 can be configured in the same manner as the first grating in a conventional Talbot interferometer (for example, see Patent Document 1) or a Talbot-Lau interferometer (for example, see Patent Document 4). Description is omitted.

  The X-ray image detector 30 is configured to detect X-rays diffracted by the grating 20. Furthermore, the X-ray image detector 30 is disposed at or near the position where the self-image is formed.

  Furthermore, each element of the X-ray imaging apparatus of this embodiment is arrange | positioned so that the following conditions 1 and 2 may be satisfy | filled.

(Condition 1)
here,
R ₁ : distance from the X-ray source to the grating;
R ₂ : distance from the lattice to the position where the self-image is formed;
λ: center wavelength of the X-ray;
R: R ₁ + R ₂ ;
d: period of the lattice;
p: a parameter related to the distance from the lattice.

(Condition 2)
here,
D: spatial resolution in the image detector;
d ₁ is the period of the self-image.

  Hereinafter, the meaning of the above-described conditions will be described.

For X-ray Talbot interferometers (including Talbot-Lau interferometers), the ideal conditions for grid placement are
Given in. R ₁ is the distance from the X-ray source to the grating (see FIG. 1), and R ₂ is the distance from the grating to the position where the self-image is formed. Conventionally, another grating (second grating) has been arranged at the position of the self-image. In the case of an X-ray Talbot-Lau interferometer, a multi-slit is placed at the position of the X-ray source.

Now, assuming that the overall length of the device is constant (ie R ₁ + R ₂ ≡R = constant), the solution that R ₂ satisfies is:
Is obtained. Also,
It is. In other words, it can be pointed out that there are two ways to select R ₁ and R ₂ , as shown in FIG. According to the above equation, that is, R ₁ and R ₂ may be interchanged. The X-ray phase imaging technique using the conventional X-ray Talbot interferometer and X-ray Talbot-low interferometer is considered to correspond to FIG. That is, here, a fine self-image is generated by the lattice. The self image and the second grating (not shown in FIG. 3A) arranged at the position of the self image cause moire interference. Therefore, the state in which the self-image is deformed by the subject is detected as a change in moire interference between the self-image and the second grating.

  On the other hand, the principle of the apparatus of the present embodiment corresponds to the situation shown in FIG. At this time, the self-image of the lattice is greatly enlarged. Therefore, by making the pitch of the self-image larger than the resolution of the image detector, it is possible to directly take an image with the image detector without using the second grating as in the prior art. That is, the second lattice can be omitted.

(Multi slit period)
In this embodiment (see FIG. 2), a multi-slit 102 is arranged near the common X-ray generation unit 101, are arranged grating 20 from the multi-slit 102 a certain distance R ₁ away. Both have a stripe-like structure pattern and are arranged so that the stripes are substantially parallel to each other.

The multi-slit period is d _s and the self-image period is d ₁
When
Need to be met. from here,
Can be found. m is an integer of 1 or 2 determined by the type of the lattice 20. From these relationships, design conditions for the multi slit period d _s can be derived.

In the present embodiment, since the self-image having the period d ₁ is directly detected by the image detector 30, the spatial resolution D of the image detector 30 is
It is necessary to satisfy. This equation (8) is the minimum condition, and d ₁ is preferably larger than D. Conventional X-ray Talbot interferometers and X-ray Talbot-low interferometers are methods for imaging moire interference fringes by using an amplitude grating (second grating) having a period d ₁ and D> d _1. . Therefore, the object of the present embodiment cannot be achieved simply by omitting the absorption grating (second grating) in the conventional X-ray Talbot interferometer or X-ray Talbot-low interferometer.

From equations (6) and (7),
Or
So, equation (8) is
Or
It may be expressed as

  Note that p is a half integer if the grating 20 is a π / 2 phase grating, and an integer if the grating 20 is an absorption grating. At this time, m = 1. When the grating is a π phase grating, p is a multiple of 1/8, and m = 2 is set to reflect the effect that the period of the self-image is halved. If the lattice is an absorption lattice, p is an integer and m = 1. λ is the wavelength of X-rays. However, there is no problem in practical use as long as the above expression is almost satisfied. As for the wavelength of X-rays, imaging is possible even when X-rays having a wavelength slightly different from λ are irradiated, and X-rays containing various spectral components (this is rather normal for general X-ray sources) Even if the image is irradiated, a contrast generation effect based on the principle described in the present embodiment can be obtained although there is a certain degree of image quality degradation.

  As described above, the absorption grating (second grating) in front of the image detector 30 used in the conventional X-ray Talbot interferometer and the X-ray Talbot-low interferometer has the largest area among the gratings used. Is required. Furthermore, this absorption grating is made of a heavy metal such as gold and requires a stripe structure with a high aspect ratio. For this reason, extremely high technology is required for manufacturing this absorption grating. The fact that this absorption grating can be omitted according to the present embodiment is a tremendous advantage in apparatus configuration. In this embodiment, the multi-slit also requires a high aspect ratio structure, but since this is close to the X-ray source 10, the area thereof may be relatively small and is smaller than the absorption grating (second grating). Easy to manufacture.

Note that the period d _s of the multi-slit 102 generally needs to be smaller than the period of the multi-slit used in the conventional X-ray Talbot-Lau interferometer. However, many techniques have been developed for microfabrication in a limited narrow area, and the production of the multi-slit in this embodiment is relatively easy.

The X-ray imaging method of the present embodiment can be implemented by the following steps using the above-described apparatus.
(1) A step of placing a subject between the X-ray source 10 and the X-ray image detector 30. That is, the place where the subject is arranged may be between the multi slit 102 and the grating 20 or between the grating 20 and the X-ray image detector 30.
(2) A step of irradiating X-rays from the X-ray source 10 toward the grating 20.
(3) A step in which the X-ray detector 30 detects X-rays diffracted by the grating 20 to form a self-image.

  In the present embodiment, since the above-described conditions 1 and 2 are satisfied, a self-image of the grating 20 can be acquired by the X-ray detector 30. Then, by analyzing the change in the self-image, the state or change of the subject can be detected.

(Experimental example)
FIG. 4 shows an image detected by the image detector 30 using the configuration of the present embodiment described above. The experimental conditions are as follows.
d _s = 5.3 μm;
d = 5.157 μm;
λ = 0.071 nm;
X-ray generator: The Mo target is irradiated with an excitation beam. The generated X-rays were reflected by the graphite plate, and only the Kα characteristic X-rays were taken out and irradiated toward the grating 20 through the multi slit 102.

In this experimental example, the interval R ₁ between the multi slit 102 and the grating 20 was set to 192 mm. That is, p = 0.5 and m = 1. The distance R ₂ from the grid 20 to the image detector 30 was about 6.9 m. The sample (polymer sphere) was placed at a position 2.4 m from the image detector.

  As a result, as shown in FIG. 4, the self-image of the grating 20 can be acquired in the image detector 30 in the same shape. That is, a stripe pattern with a period of about 0.2 mm was observed. Further, it can be observed that the stripe pattern is deformed by the polymer sphere arranged as the subject (sample).

  In order to obtain a phase differential image based on a sample from an image as shown in FIG. 4, a fringe scanning method and a Fourier transform method can be applied. In the fringe scanning method, a phase differential image is calculated from a plurality of images obtained by translating the grating 20 in a direction approximately along the diffraction vector (vertical direction in FIG. 2) at a predetermined pitch. The specific calculation method is the same as that described in Japanese Patent No. 4445397, and is therefore omitted. The Fourier transform method is also described in M. Takeda, H. Ina, and S. Kobayashi, J. Opt. Soc. Am. 72, 156 (1982). Instead of translation of the grating 20, the multi slit 102 may be translated in a direction parallel to the diffraction vector of the grating 20. In short, it is sufficient that the grating 20 or the multi slit 102 can relatively translate in a direction substantially parallel to the diffraction vector (that is, to the extent that a desired image quality can be obtained). Of course, it is also possible to translate both the grating 20 and the multi-slit 102 together. Translation of the lattice 20 or the multi slit 102 can be realized by an appropriate translation mechanism (not shown). As such a translation mechanism, the same translation mechanism as that of the prior art can be used, and a detailed description of the translation mechanism is omitted.

In addition, the contrast of an image observed due to X-ray scattering by the sample may be reduced. By imaging this, novel information can be obtained. This is the case with an X-ray Talbot-Lau interferometer.
Has been reported. This is called a visibility image. That is, a phase differential image, an absorption image, and a visibility image are obtained from a single measurement. Also in the image obtained in this experimental example, in the case of the fringe scanning method, the above-described image can be obtained by performing the calculation described in the above F. Pfeiffer paper. Also in the case of the Fourier transform method, each image can be obtained from 2 | a ₁ | / | a ₀ | by setting the _0th and 1st order components to be a ₀ and a ₁ , respectively. .

  FIG. 5 shows an image obtained by the fringe scanning method. This image is a differential phase image generated from nine images obtained by translating the grating 20 by 1/9 period in the period direction. Although a fringe pattern (a trace of the self-image) remains due to an experimental error, a contrast due to a polymer sphere as an object and a void structure therein was detected.

  Note that a three-dimensional image (phase CT image) can be reconstructed by repeating such measurement in a plurality of projection directions by rotating the sample. Details thereof may be the same as those in the prior art, and detailed description thereof is omitted.

Furthermore, since d _s and d were set to a size of slightly over 5 μm in the above-described experimental example, the overall size was about 7 m. To change this to a more compact one, d _s and d can be reduced, which can be achieved relatively easily.

(Second Embodiment)
Next, an X-ray imaging apparatus according to a second embodiment of the present invention will be described with reference to FIG. In the description of the second embodiment, the same reference numerals are used for elements that are basically the same as those in the first embodiment, thereby avoiding complicated description.

  In the first embodiment described above, the X-ray source has a multi-slit as in the case of the Talbot-Lau interferometer. On the other hand, in the second embodiment, as in the case of the Talbot interferometer disclosed in Patent Document 1 or the like, the X-ray source 210 including the X-ray generator 2101 that generates X-rays from a sufficiently narrow region is provided. Using. Therefore, the X-ray generation unit 2101 of this embodiment is configured to directly irradiate the grating 20 with X-rays having spatial coherence.

  Other configurations and advantages of the second embodiment are the same as those of the first embodiment described above, and thus detailed description thereof is omitted.

  Note that the description of the embodiment and the examples is merely an example, and does not indicate a configuration essential to 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.

It is explanatory drawing for demonstrating the schematic structure of the conventional X-ray imaging device. It is explanatory drawing for demonstrating the schematic structure of the X-ray imaging device which concerns on 1st Embodiment of this invention. FIG. 1A is an explanatory diagram for explaining a self-image formation state in a conventional X-ray imaging apparatus. FIG. 5B is an explanatory diagram for explaining a self-image formation state in the X-ray imaging apparatus of the first embodiment. It is a figure which shows the X-ray image acquired by the experiment example of this invention. It is a figure which shows the differential phase image obtained by the experiment example of this invention. It is explanatory drawing for demonstrating the schematic structure of the X-ray imaging device which concerns on 2nd Embodiment of this invention.

10.210 X-ray source 101-2101 X-ray generator 102 Multi-slit 20 Grating 30 X-ray image detector

Claims (5)

An X-ray source, a grating, and an X-ray image detector;
The X-ray source is configured to irradiate a required amount of X-rays toward the grating,
The grating is configured to form a self-image of the grating at a position separated from the grating by diffracting the X-rays irradiated toward the grating.
The X-ray image detector is configured to detect the X-ray diffracted by the grating,
And the said X-ray image detector is arrange | positioned in the position where the said self-image is formed, or its vicinity,
Furthermore, the X-ray imaging device characterized by satisfying the following conditions 1 and 2:
(Condition 1)
here,
R ₁ : distance from the X-ray source to the grating;
R ₂ : distance from the lattice to the position where the self-image is formed;
λ: center wavelength of the X-ray;
R: R ₁ + R ₂ ;
d: period of the lattice;
p: a parameter related to the distance from the lattice.
(Condition 2)
here,
D: spatial resolution in the X-ray image detector;
d ₁ is the period of the self-image. The X-ray source includes an X-ray generation unit and a multi-slit,
The X-ray imaging apparatus according to claim 1, wherein the X-ray generation unit is configured to irradiate the X-rays toward the grating through the multi slit. The X-ray source includes an X-ray generator,
The X-ray imaging apparatus according to claim 1, wherein the X-ray generation unit is configured to directly irradiate the grating with X-rays having spatial coherence. It also has a translation mechanism,
The translation mechanism is configured to translate one of the grating and the multi-slit relative to the other in a direction substantially parallel to a diffraction vector in the grating. 2. The X-ray imaging apparatus according to 2. An X-ray imaging method using the X-ray imaging apparatus according to claim 1,
(1) placing a subject between the X-ray source and the X-ray image detector;
(2) irradiating X-rays from the X-ray source toward the grating;
(3) A step in which the X-ray image detector detects the X-rays that are diffracted by the grating to form a self-image.