Method and apparatus for calculating elastic modulus

Abstract

The present invention enables a modulus of elasticity of a viscoelastic body to be calculated with a high spatial resolution and with a relatively short measuring time. In the present invention, an X-ray projected image is detected by a detecting unit 24 while a subject 1 is caused to vibrate, while a relative positional relationship between a first diffraction grating 211 and a second diffraction grating 212 is maintained. Next, the modulus of elasticity of the subject 1 is calculated on the basis of an amount of displacement of a wave due to vibrations in the X-ray projected image detected by the detecting unit 24.

Description

Technical Field

[0001] The present invention relates to a technique for calculating the elastic modulus of a subject (i.e., a sample).

Background Art

[0002] Elastography is known as a technique for non-invasively imaging the hardness distribution of viscoelastic materials. By examining the hardness distribution of biological tissues, it can be used for the diagnosis and location of cancer and arteriosclerosis.

[0003] Elastography is roughly divided into static elastography (see, for example, Patent Document 1 below) and dynamic elastography (see, for example, Non-Patent Document 1 below). Static elastography determines the elastic modulus of an object based on the deformation caused by pressing on a viscoelastic material. In static elastography, it is necessary to construct a model of the viscoelastic material in order to estimate the elastic modulus from the amount of deformation, and there is a problem that it is difficult to construct an accurate model.

[0004] Dynamic elastography vibrates a viscoelastic material, acquires the propagation velocity distribution of shear waves in the viscoelastic material as a displacement map, and determines the elastic modulus (complex elastic modulus) of the viscoelastic material. In Non-Patent Document 1 below, MRI is used to acquire the displacement map. In addition, techniques for acquiring a displacement map using ultrasonic waves have also been proposed. However, techniques using MRI or ultrasonic waves have a problem of low spatial resolution of the elastic modulus distribution. If the spatial resolution is low, it may be difficult to detect small lesions, or the boundary between the lesion area and the normal area may become unclear. In order to facilitate diagnosis, it is desirable to have as high a spatial resolution as possible.

[0005] Therefore, the present inventors proposed a method (dynamic X-ray elastography) for acquiring a displacement map of a viscoelastic material using an X-ray projection image in Non-Patent Document 2 below. According to this method, since X-rays are used, higher spatial resolution can be expected than with MRI or ultrasonic waves.

Prior Art Documents

[0006] [Patent Document 1] International Publication No. WO2016 / 176044 [Non-patent literature]

[0007] [Non-Patent Document 1] R. Muthupillai, DJ Lomas, PJ Rossman, JF Greenleaf, A. Manduca, and RL Ehman, Science 269, 1854 (1995). [Non-Patent Document 2] Kamezawa et al., Applied Physics Express 13, 042004 (2020) [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In dynamic X-ray elastography, depicting the internal structure is essential for obtaining a displacement map when a viscoelastic material is vibrated. To further improve the sensitivity of depicting the internal structure, and consequently the spatial resolution, one method is to use a phase-contrast X-ray optical system, such as a Talbot interferometer, and acquire an X-ray phase image using the fringe scanning method. However, this method requires moving the grating for fringe scanning, which increases the time required for imaging. Furthermore, increasing the number of scans may increase the amount of X-ray exposure, and there is also a higher possibility that the subject will move significantly during imaging. Another method is to calculate the X-ray phase image from the X-ray projection image using the Fourier transform method without performing fringe scanning, but this method has the problem of lower spatial resolution compared to the fringe scanning method.

[0009] As a result of research conducted to resolve the aforementioned problems, the inventors have found that the elastic modulus distribution of a viscoelastic material can be determined with high spatial resolution by using the projection image in a phase-contrast X-ray optical system, without performing the fringe scanning method.

[0010] This invention is based on this finding. The main objective of this invention is to provide a technique that can calculate the elastic modulus of a viscoelastic material with high spatial resolution and in a relatively short measurement time. [Means for solving the problem]

[0011] This invention can be expressed as the invention described in the following items.

[0012] (Item 1) It uses a phase-contrast X-ray optical system capable of detecting the refraction or scattering of X-rays by the subject. The phase-contrast X-ray optical system comprises a grating, a radiation source for irradiating the grating and the subject with X-rays, and a detection unit for detecting the X-rays that have passed through the grating and the subject, pixel by pixel. The lattice portion comprises a first diffraction grating and a second diffraction grating arranged parallel to the self-image of the first diffraction grating. Furthermore, the process includes the step of detecting the X-ray projection image using the detection unit while maintaining the relative positional relationship between the first diffraction grating and the second diffraction grating and vibrating the subject, A step of calculating the elastic modulus of the subject based on the amount of wave displacement due to the vibration in the X-ray projection image detected by the detection unit. A method for calculating the elastic modulus, comprising the characteristics of the elastic modulus.

[0013] (Item 2) The subject is biological tissue. The method for calculating the modulus of elasticity as described in item 1.

[0014] (Item 3) A phase contrast X-ray optical system capable of detecting X-ray refraction or scattering by a subject, a vibrating unit that vibrates the subject, and a processing unit. The phase contrast X-ray optical system includes a grating unit, a radiation source for irradiating the grating unit and the subject with X-rays, and a detection unit that detects the X-rays that have passed through the grating unit and the subject for each pixel. The grating unit includes a first diffraction grating and a second diffraction grating arranged so as to be parallel to the self-image of the first diffraction grating. Furthermore, the relative positional relationship between the first diffraction grating and the second diffraction grating is maintained. The vibrating unit is configured to vibrate the subject by applying vibration to the subject. The processing unit is configured to calculate the elastic modulus of the subject based on the amount of displacement of the wave due to the vibration in the projection image of the X-rays detected by the detection unit. Elastic modulus calculation device.

Advantages of the Invention

[0015] According to the present invention, since a phase contrast X-ray optical system is used, the elastic modulus of a viscoelastic body can be calculated with high spatial resolution. Moreover, since the elastic modulus of the subject is calculated using the X-ray projection image obtained by imaging the subject while maintaining the relative positional relationship between the first diffraction grating and the second diffraction grating, the measurement time can be shortened as compared with the case of performing the fringe scanning method.

Brief Description of the Drawings

[0016] [Figure 1] It is an explanatory diagram showing a schematic configuration of an elastic modulus calculation device according to an embodiment of the present invention. [Figure 2] It is a block diagram for explaining the configuration of the processing unit in the device of FIG. 1. [Figure 3] It is an explanatory diagram for explaining the procedure of an elastic modulus calculation method using the device of FIG. 1. [Figure 4] It is an explanatory diagram for explaining the positional relationship of the grating members in the device of FIG. 1. [Figure 5] This graph illustrates the relationship between the relative displacement of the G1 grid's self-image and the G2 grid, and the received light intensity. The horizontal axis represents the relative displacement of the G1 grid 211 relative to the G2 grid 212 (more precisely, its self-image) divided by the period of the G2 grid 212 (χ), and the vertical axis represents the received light intensity (I(x,y)) at a specific position (x,y) at the pixel. [Figure 6] Figure (a) is an axial cross-sectional view, Figure (b) is a coronal cross-sectional view, and Figure (c) is a sagittal cross-sectional view, as shown in the example of the method for calculating the elastic modulus. [Figure 7] This figure shows the displacement map within the coronal cross-section when the sample is agarose with a concentration of 0.9%, with figure (a) showing the vertical displacement and figure (b) showing the horizontal displacement. [Figure 8] This figure shows the displacement map within the coronal cross-section when the sample is agarose with a concentration of 0.7%, with figure (a) showing the vertical displacement and figure (b) showing the horizontal displacement. [Figure 9] Figure (a) is the storage modulus map calculated using the longitudinal displacement within the coronal cross-section of the sample in Figure 7, and Figure (b) is the storage modulus map calculated using the longitudinal displacement within the coronal cross-section of the sample in Figure 8. [Figure 10] Figure (a) is the storage modulus map calculated using the lateral displacement within the coronal cross-section of the sample in Figure 7, and Figure (b) is the storage modulus map calculated using the lateral displacement within the coronal cross-section of the sample in Figure 8. [Modes for carrying out the invention]

[0017] (Configuration of this embodiment) Hereinafter, an elastic modulus calculation device according to one embodiment of the present invention will be described with reference to the attached drawings. As a prerequisite for the explanation, the subject matter will first be described.

[0018] (subject) Subject 1 is a viscoelastic body through which vibrations propagate upon excitation. For example, Subject 1 is biological tissue. Here, biological tissue may be tissue separated from the body, or it may not be separated, as in the case of mammography. Subject 1 may also be an industrial product. In this specification, a viscoelastic body includes materials with low viscosity and properties close to those of an elastic body.

[0019] (Equilibrium Modulus Calculation Device) The elastic modulus calculation device of this embodiment (hereinafter sometimes simply referred to as "device") has as its basic configuration a phase contrast X-ray optical system 2 capable of detecting the refraction or scattering of X-rays by the subject 1, a vibration unit 3 that vibrates the subject 1, and a processing unit 4.

[0020] (Phase-contrast X-ray optical system) The phase contrast X-ray optical system 2 includes a grating 21, a radiation source 22 for irradiating the grating 21 and the subject 1 with X-rays, a shutter (chopper) 23, and a detection unit 24 for detecting the X-rays that have passed through the grating 21 and the subject 1, pixel by pixel.

[0021] The grating section 21 consists of a G1 grating (first diffraction grating) 211 and a G2 grating (second diffraction grating) 212 arranged parallel to the self-image of the G1 grating 211. The basic configuration of the G1 grating 211 and the G2 grating 212 can be the same as that of a conventional Talbot interferometer. However, in this embodiment, imaging is performed while the relative positional relationship between the G1 grating 211 and the G2 grating 212 is maintained (that is, while the relative positional relationship between the self-image of the G1 grating 211 and the G2 grating 212 is maintained). This point will be described later.

[0022] The radiation source 22 is configured to irradiate the grating section 21 with X-rays. In this embodiment, the radiation source 22 is configured to generate X-rays with sufficient spatial coherence for the G1 grating 211 of the grating section 21 to produce a clear self-image. For example, a synchrotron radiation source or a microfocus X-ray source can be used as the radiation source 22. In this example, an example in which a microfocus X-ray source is used as the radiation source 22 will be described. However, it is also possible to use X-rays with low spatial coherence, in which case a G0 grating (not shown) is placed between the radiation source 22 and the G1 grating 211. An X-ray optical system with such an arrangement is called a Talbot-Lowe interferometer. The G0 grating is an absorption grating that generates multiple equivalent coherent point sources by transmitting X-rays from a radiation source that generates non-coherent X-rays. In other words, the G0 grating can be said to be essentially part of the radiation source. When using a G0 grating, it becomes possible to use a high-intensity white X-ray source.

[0023] The shutter (chopper) 23 periodically irradiates the grating 21 with X-rays at the same frequency as the vibration frequency applied to the subject 1 from the vibrating unit 3. Specifically, a disk with slits formed on it is rotated, and the X-ray irradiation frequency can be adjusted by the rotation frequency of the slits. Furthermore, the phase of the X-ray irradiation period can also be adjusted by shifting the phase of the rotation period of the slits. However, if the time resolution of the detection unit 24 is sufficiently high, it is possible to omit the shutter (chopper) 23 and have the detection unit 24 perform the same function as the shutter (chopper) 23.

[0024] The detection unit 24 can acquire an X-ray intensity distribution image (i.e., the X-ray intensity value for each pixel) using multiple pixels (not shown) arranged in a two-dimensional plane. The intensity distribution image acquired by the detection unit 24 is sent to the processing unit 4.

[0025] The detection unit 24 may be an indirect imaging image detector that images via a scintillator (not shown) that converts X-rays to visible light, or a direct imaging image detector that directly images X-ray photons. Since direct imaging image detectors generally have a spatial resolution about the same as the pixel size, it is preferable to arrange the pixels so that the arrangement is parallel to the G2 grating 212 and the pixel size is an integer multiple of the period of the G2 grating 212. Alternatively, by using a direct imaging image detector with a pixel size that is an integer fraction of the period of the G2 grating 212 and arranging the pixels so that the arrangement is parallel to the self-image of the G1 grating 211, or by using a direct or indirect imaging image detector with a spatial resolution that can sufficiently resolve the G2 grating, the G2 grating can be omitted because it is possible to achieve substantially the same function as the G2 grating 212 through image processing. In this case, the detection unit 24 corresponds to the second diffraction grating in the present invention.

[0026] Other than the above, the configuration of the phase-contrast X-ray optical system 2 can be the same as that of a conventional Talbot interferometer (including the case of a Talbot-Raw interferometer), so further detailed explanation is omitted.

[0027] (Vibrating part) The vibration unit 3 is configured to vibrate the subject 1 in order to vibrate the subject 1. Specifically, the vibration unit 3 in this embodiment consists of a vibration table 31 that supports the subject 1, a hose 32 connected to the vibration table 31, a speaker 33 connected to the hose 32, an amplifier 34 that drives the speaker 33, and a control computer 36 that sends a drive signal to the amplifier 34 via a data acquisition module (DAQ) 35. The vibration unit 3 drives the speaker 33 at a predetermined frequency based on the drive signal from the control computer 36, and transmits elastic waves to the vibration table 31 via the hose 32, thereby vibrating the subject 1. However, unlike MRI, in X-ray imaging, it is also possible to use a vibration table made of metal, and for example, the subject 1 can be directly vibrated using a voice coil.

[0028] (processing) The processing unit 4 is configured to calculate the elastic modulus of the subject 1 based on the amount of wave displacement due to vibration in the X-ray projection image detected by the detection unit 24. The processing unit 4 in this embodiment consists of a displacement amount calculation unit 41 that calculates the amount of wave displacement due to vibration in the subject 1, and an elastic modulus calculation unit 42 that calculates the elastic modulus of the subject 1 based on this displacement amount (see Figure 2). The specific operation of the processing unit 4 will be described later.

[0029] (Method for calculating the modulus of elasticity according to this embodiment) Next, the method for calculating the elastic modulus using the aforementioned apparatus will be explained with further reference to Figures 3 to 5.

[0030] (Step SA-1 in Figure 3) First, we establish the relative positional relationship between the G1 grid 211 and the G2 grid 212. To illustrate this point, Figure 4 schematically shows a state in which the self-image of the G1 grid 211 and the grid members of the G2 grid 212 overlap. For ease of viewing, the vertical positions are slightly shifted. Here, if the G1 grid 211 (or G2 grid 212) is shifted in the grid periodic direction (either left or right in Figure 4), the intensity of the X-rays passing through the grid section 21 (intensity for each pixel) changes periodically depending on the degree of overlap between the self-image of the G1 grid 211 and the G2 grid 212 (see Figure 5). The horizontal axis in Figure 5 is χ, which is the amount obtained by dividing the relative displacement of the G1 grid 211 (more precisely, its self-image) with respect to the G2 grid 212 by the period of the G2 grid 212. As shown in Figure 5, the X-ray intensity I(x,y) when focusing on a single pixel at position (x,y) changes approximately sinusoidally (or approximately triangularly if spatial coherence is very high). Generally, the χ dependence I(χ) of this X-ray intensity I(x,y) can be described as follows (especially in the case of a plane-wave Talbot interferometer).

[0031] TIFF0007868864000001.tif12168

[0032] Here, A: Amplitude ω: Angular frequency χ: The relative displacement of the G1 grid 211 with respect to the G2 grid 212 (more precisely, its self-image), divided by the period of the G2 grid 212. φ: Initial phase B: Average value That is the case.

[0033] Then, at the position where the average intensity B is obtained (indicated by the symbol P in Figure 5), the sensitivity to X-ray refraction by subject 1 is maximized. Therefore, it is preferable to set the relative positional relationship between the G1 grating 211 and the G2 grating 212 so that it is as close as possible to or near this average intensity position. This position can be obtained in advance by calculation or experimentally. However, if I(x,y) is I max or I min Any position other than the one where this occurs can be expected to have some degree of sensitivity.

[0034] (Steps SA-2 and SA-3 in Figure 3) Next, the vibration unit 3 vibrates the subject 1. In parallel with this, the shutter (chopper) 23 is driven in synchronization with the frequency of this vibration, and the detection unit 24 acquires an X-ray intensity distribution image (X-ray projection image). By fixing the vibration frequency of the shutter (chopper) 23 and shifting the phase little by little, the wave caused by the vibration of the subject 1 can be resolved. The resolution of this wave itself can be the same as in Non-Patent Documents 1 and 2 described above, so further detailed explanation is omitted.

[0035] (Steps SA-4 and SA-5 in Figure 3) The X-ray projection image obtained in step SA-3 has contrast (structural contrast) due to the vibration of the subject 1. The displacement calculation unit 41 of the processing unit 4 calculates the wave displacement in the subject 1 based on the displacement of this structural contrast. Then, the elastic modulus calculation unit 42 of the processing unit 4 calculates the elastic modulus (e.g., complex elastic modulus) of the subject 1 based on this displacement.

[0036] The complex shear modulus G of an object (sample) can be determined as follows.

[0037] TIFF0007868864000002.tif34168

[0038] Here TIFF0007868864000003.tif50168

[0039] The method for calculating the complex shear modulus is the same as that described in Non-Patent Document 2 above, so a detailed explanation will be omitted.

[0040] According to the method of this embodiment, since the subject 1 is resolved using the phase-contrast X-ray optical system 2, a displacement map with high spatial resolution (distribution of displacement amount for each pixel) can be obtained, and thereby the elastic modulus of the viscoelastic body can be calculated with high spatial resolution. The subject 1 in this embodiment may be a material with G'' ≈ 0 (i.e., a material close to an elastic body), and as described above, in this specification, such materials are also included in the concept of viscoelastic bodies.

[0041] Furthermore, in the method of this embodiment, the elastic modulus of the subject 1 is calculated using an X-ray projection image obtained by imaging the subject 1 while maintaining the relative positional relationship between the G1 grid 211 and the G2 grid 212. When performing the fringe scanning method, time is required for the grid to move, but in this embodiment, such grid movement time is unnecessary, so the measurement time can be shortened compared to when performing the fringe scanning method.

[0042] Importantly, the method of this embodiment can obtain spatial resolution equivalent to that of the fringe scanning method. This point will be explained further. When the fringe scanning method is performed, a quantitative X-ray phase image can be obtained. Quantitative means that absorption images, differential phase images, and scattering images can be generated depending on the cause of the contrast generation. However, according to the inventors' findings, in order to generate a displacement map of the subject (i.e., calculate the amount of displacement), it is sufficient to capture some structural contrast in the X-ray projection image, even if the cause cannot be identified. This structural contrast is convolved with contrasts due to various factors (scattering, edge effects, etc.), but the elastic modulus can be calculated by capturing the displacement of the structural contrast itself without quantifying it. Moreover, the spatial resolution of the structural contrast in the X-ray projection image obtained by the phase contrast X-ray optical system 2 is equivalent to that of the X-ray phase image obtained by the fringe scanning method. Therefore, according to this embodiment, the elastic modulus of the subject 1 can be obtained with the same high spatial resolution as the fringe scanning method.

[0043] In the X-ray projection image of this embodiment, the structural contrast of the sample is integrated along the path of the X-rays. Therefore, if the homogeneity of the subject 1 is low in the thickness direction, it is preferable to take measures such as forming the subject 1 thinly. Alternatively, by reconstructing X-ray projection images from multiple directions to obtain a CT image, three-dimensional structural contrast can be obtained even for a non-homogeneous subject 1. In this case, the elastic modulus can be calculated in any direction (for example, the x, y, z directions in three-dimensional space). Thus, a CT image obtained from an X-ray projection image is also included in the concept of an X-ray projection image in this invention. Here, when performing CT in this embodiment, it is preferable to orient the lines of each diffraction grating perpendicular to the rotation axis of the sample (i.e., subject 1) in the CT. In the case of a normal X-ray diffraction grating interferometer, the rotation axis of the CT is arranged parallel to the lines of the diffraction grating. However, if this is done, the differential phase signal and scattering (so-called dark-field) signal may change depending on the projection direction, so it may not be possible to obtain the "integral value" necessary for CT reconstruction.

[0044] (Examples) Next, an example of calculating the elastic modulus using the method of this embodiment described above will be explained as an example, with further reference to Figures 6 to 10. In this example, cylindrical agarose in which minute alumina markers are dispersed was used as the subject (sample).

[0045] In this embodiment, the CT reconstructed image obtained from the X-ray projection image detected by the detection unit 24 using the filter-corrected back projection method (FBP method) is used as the X-ray projection image in step SA-3. The reconstructed results are shown in Figures 6(a) to (c). Figure 7 shows the displacement map within the coronal cross-section when the agarose concentration is 0.9%, and Figure 8 shows the displacement map within the coronal cross-section when the agarose concentration is 0.7%. The direction of displacement is indicated in the figures.

[0046] Figure 9 shows the storage modulus map (storage modulus for each pixel) calculated from the vertical displacement. Figure 9(a) corresponds to the sample in Figure 7, and Figure 9(b) corresponds to the sample in Figure 8. Figure 10 shows the storage modulus map calculated from the horizontal displacement. Figure 10(a) corresponds to the sample in Figure 7, and Figure 10(b) corresponds to the sample in Figure 8.

[0047] The above-described embodiments are merely examples and do not represent configurations essential to the present invention. The configuration of each part is not limited to those described above, as long as it can achieve the spirit of the present invention. [Explanation of symbols]

[0048] 1. Subject 2 Phase-contrast X-ray optical system 21 Lattice section 211 G1 grating (first diffraction grating) 212 G2 grating (second diffraction grating) 22 Source 23 Shutter (Chop) 24 Detection unit 3. Vibrating part 31. Vibration Table 32 hoses 33 speakers 34 Amplifier 35 Data Acquisition Module 36 Control Computer 4 Processing Unit 41 Displacement Amount Calculation Unit 42 Elastic Modulus Calculation Unit

Claims

1. It uses a phase-contrast X-ray optical system capable of detecting the refraction or scattering of X-rays by the subject. The phase-contrast X-ray optical system comprises a grating, a radiation source for irradiating the grating and the subject with X-rays, and a detection unit that images each pixel of the X-rays that have passed through the grating and the subject, either via a scintillator or by direct imaging. The lattice portion comprises a first diffraction grating and a second diffraction grating arranged parallel to the self-image of the first diffraction grating. Furthermore, while maintaining the relative positional relationship between the first diffraction grating and the second diffraction grating, and while vibrating the subject, the detection unit acquires an X-ray intensity value for each pixel without performing fringe scanning by the source grating, thereby capturing an X-ray projection image which is an X-ray intensity distribution image consisting of the acquired X-ray intensity values. A step of calculating the elastic modulus of the subject based on the amount of wave displacement due to the vibration in the X-ray projection image detected by the detection unit. It has, The amount of wave displacement due to the vibration is calculated based on the amount of contrast displacement in the X-ray projection image that is displaced by the vibration. Method for calculating the modulus of elasticity.

2. The relative positional relationship between the first diffraction grating and the second diffraction grating is set such that the X-ray intensity of the pixel, which changes due to the relative positional relationship between the first diffraction grating and the second diffraction grating, becomes equal to or near the average value of the X-ray intensity. The method for calculating the modulus of elasticity according to claim 1.

3. The aforementioned projection image is an image other than an X-ray phase image. The method for calculating the modulus of elasticity according to claim 1 or 2.

4. It comprises a phase-contrast X-ray optical system capable of detecting the refraction or scattering of X-rays by a subject, a vibration unit that vibrates the subject, and a processing unit. The phase-contrast X-ray optical system comprises a grating, a radiation source for irradiating the grating and the subject with X-rays, and a detection unit that images each pixel of the X-rays that have passed through the grating and the subject, either via a scintillator or by direct imaging. The lattice portion comprises a first diffraction grating and a second diffraction grating arranged parallel to the self-image of the first diffraction grating. Furthermore, the relative positional relationship between the first diffraction grating and the second diffraction grating is maintained. The vibrating unit is configured to vibrate the subject in order to excite the subject. The processing unit is configured to calculate the elastic modulus of the subject based on the amount of wave displacement due to the vibration in the X-ray projection image captured by the detection unit, wherein the X-ray projection image is an X-ray intensity distribution image consisting of X-ray intensity values ​​acquired for each pixel by the detection unit, and the amount of wave displacement due to the vibration is calculated based on the amount of contrast displacement due to the vibration in the X-ray projection image. An elastic modulus calculation device (excluding configurations in which the phase-contrast X-ray optical system performs fringe scanning using a source grating).

5. The relative positional relationship between the first diffraction grating and the second diffraction grating is set such that the X-ray intensity of the pixel, which changes due to the relative positional relationship between the first diffraction grating and the second diffraction grating, becomes equal to or near the average value of the X-ray intensity. The elastic modulus calculation device according to claim 4.

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