Energy spectrum CT experimental device and method based on white light X-ray

Abstract

The present invention relates to an energy spectrum computed tomography (CT) experimental device and method based on white light X-rays. The device comprises multiple crystals, multiple detectors, and a sample stage. The multiple crystals are arranged sequentially along a first axis in the X-direction. The sample stage deviates from the first axis in the Y-direction. The sample stage is used to support the sample and rotate it about the Z-direction. Each detector corresponds to a crystal. Each crystal can diffract white light X-rays incident along the X-direction to produce diffracted X-rays. The diffracted X-rays from each crystal pass through the sample. Each detector is located in the optical path of the diffracted X-rays from the corresponding crystal and downstream of the sample to receive the diffracted X-rays from the corresponding crystal that have passed through the sample. The energy spectrum CT experimental device and method based on white light X-rays of the present invention can simultaneously detect the three-dimensional spatial distribution of multiple elements and has the characteristics of a wide energy spectrum detection range and high time-space resolution.

Description

Technical Field

[0001] The present invention relates to the technical field of X-ray experiments, and more particularly to an energy spectrum CT experimental device and method based on white light X-rays. Background Art

[0002] Compared to traditional X-ray absorption contrast imaging, X-ray spectral imaging can image specific elements. The basic principle is that the target element in the sample system has a significant absorption difference between X-rays with energies slightly above and slightly below the absorption edge, while other elements have a very small absorption difference between these two X-ray energies. Therefore, by subtracting the X-ray projections of the two energies, the two-dimensional spatial distribution of the target element in the projection direction can be obtained. The X-ray spectral CT experimental method, which combines X-ray spectral imaging with CT (computed tomography) scanning technology, can obtain the three-dimensional spatial distribution of the target element.

[0003] Traditional X-ray spectral CT experimental methods are mainly divided into two technical routes: spectral CT based on monochromatic X-rays and spectral CT based on white light X-rays. The former first adjusts optical components such as the monochromator to screen out monochromatic X-rays with photon energy slightly lower than the absorption edge of the target element to perform a CT scan on the sample. Then, monochromatic X-rays with photon energy slightly higher than the absorption edge of the target element are screened out to perform a second CT scan on the same sample. Because this method does not require modifications to existing high-resolution micro-CT systems, it has the highest spatial resolution. However, two CT scans are required for each target element. Therefore, when multiple target elements are present in the system at the same time, a large number of CT scans will be required. On the one hand, the time cost of CT scanning increases exponentially, and on the other hand, the sample system must be a static system with no changes, which poses a huge challenge to in situ experimental research.

[0004] The energy spectrum CT technology route based on white light X-rays uses white light X-rays as a light source, and can achieve energy spectrum CT imaging through a single CT scan. This technology route replaces the traditional imaging area array detector with a photon counting detector with energy resolution capability. By setting multiple electronic thresholds, it can identify the energy information of incident X-ray photons and correspond them to different energy zones for accumulation, thereby counting the X-ray energy zones with a wide energy spectrum distribution and directly obtaining imaging results in different energy zones. This experimental method can achieve multi-element energy spectrum CT scanning through a single CT scan, but due to the limitations of detector technology development, the single pixel size of existing energy resolution area array detectors is on the order of tens to hundreds of microns, so the spatial resolution of energy spectrum CT is not high. In addition, this type of detector is very expensive and easily damaged.

[0005] In summary, the existing X-ray energy spectrum CT experimental method cannot take into account both temporal resolution and spatial resolution in multi-target element sample systems. Summary of the Invention

[0006] The purpose of the present invention is to provide an energy spectrum CT experimental device and method based on white light X-rays to simultaneously detect the three-dimensional spatial distribution of multiple elements, with the characteristics of wide energy spectrum detection range and high time-space resolution.

[0007] Based on the above objectives, on one hand, the present invention provides an energy spectrum CT experimental device based on white light X-rays, comprising multiple crystals, multiple detectors and a sample stage, wherein the multiple crystals are arranged in sequence on a first axis along the X direction, the sample stage deviates from the first axis along the Y direction, the sample stage is used to support the sample and rotate the sample around the Z direction, each detector corresponds to each crystal one by one, each crystal can diffract white light X-rays incident along the X direction to generate diffracted X-rays, the diffracted X-rays of each crystal pass through the sample, and each detector is located in the optical path of the diffracted X-rays of the corresponding crystal and downstream of the sample to receive the diffracted X-rays of the corresponding crystal that pass through the sample.

[0008] Furthermore, each crystal is divided into at least one group, each group includes two crystals, the two crystals in each group and the two detectors corresponding to the two crystals form a detection system, and each detection system is used to detect one element of the sample.

[0009] Furthermore, the wavelengths of the diffracted X-rays of the two crystals of each detection system are wavelengths corresponding to the front and rear energies of the absorption edge of the element corresponding to the detection system.

[0010] Furthermore, each crystal is placed on a slide, and the slide is used to move the crystal along the X direction and rotate around the Z direction.

[0011] Furthermore, the crystal is a diamond crystal.

[0012] Furthermore, the detector is an indirect X-ray detector.

[0013] Another aspect of the present invention provides a white light X-ray based spectral CT experimental method, which comprises the following steps:

[0014] S100: Provide a white light X-ray based energy spectrum CT experimental device according to any one of claims 2 to 6;

[0015] S200: for each crystal, moving the crystal along the X-axis to a preset position of the crystal, and rotating the crystal around the Z-axis to adjust the incident angle of the crystal to a preset angle of the crystal;

[0016] S300: transmitting white light X-rays along a first axis and sequentially passing through each crystal, so that each crystal generates diffracted X-rays, and the diffracted X-rays from each crystal pass through the sample and are projected onto a detector corresponding to the crystal, so that the detector detects a projection image of the sample;

[0017] S400: Using the sample stage to gradually rotate the sample according to a preset step angle until it is rotated 180 degrees, and after each rotation of the preset step angle, each detector detects a projection image of the sample;

[0018] S500: performing CT reconstruction on multiple projection images detected by each detector when the sample is rotated at different angles to obtain slice data of the internal three-dimensional structure of the sample detected by the detector;

[0019] S600: For each detection system, obtain the three-dimensional spatial distribution of elements corresponding to the detection system according to slice data of the three-dimensional structure inside the sample detected by two detectors of the detection system.

[0020] Furthermore, in step S200, the preset angle of each crystal is set to be calculated in advance based on the crystal plane index of the crystal and the wavelength of the diffracted X-ray of the crystal.

[0021] Furthermore, in step S200, the preset position of each crystal is set to be calculated in advance based on the preset angle of the crystal and the Y-direction distance between the sample and the first axis.

[0022] Furthermore, in step S600, the grayscale values ​​of the slice data of the three-dimensional structure inside the sample detected by the two detectors of each detection system at the same height position are subtracted to obtain the three-dimensional spatial distribution of the corresponding element of the detection system.

[0023] The white-light X-ray-based energy spectrum CT experimental device and method of the present invention use synchrotron white-light X-rays with a wide energy spectrum range as the light source, which can cover the absorption edge range of more elements. The three-dimensional spatial distribution of multiple target elements can be simultaneously detected through a single CT scan, making operation simpler. By moving the crystal along the X-direction and rotating it around the Z-direction, the type of elements detected by the detection system where the crystal is located can be adjusted, providing greater flexibility. The detector does not need to be a photon counting detector with energy resolution, but can use an indirect detector with a smaller pixel size, thereby having a higher spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the structure of a white light X-ray based energy spectrum CT experimental device according to an embodiment of the present invention;

[0025] Figure 2 for Figure 1 A top view of

[0026] Figure 3 for Figure 1 Schematic diagram of the structure of the second detection system of the white light X-ray based energy spectrum CT experimental device;

[0027] Figure 4 for Figure 3 A top view of

[0028] Figure 5 Flowchart of a white light X-ray based energy spectrum CT experimental method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are given below in conjunction with the accompanying drawings and described in detail.

[0030] like Figure 1 and Figure 2As shown, an embodiment of the present invention provides an energy spectrum CT experimental device based on white light X-rays (i.e., X-rays with an energy bandwidth of 1 keV or more), including multiple crystals, multiple detectors, and a sample stage. The multiple crystals are arranged in sequence on a first axis L along the X direction, and the sample stage is offset from the first axis L along the Y direction. The sample stage is used to support a sample 301 and rotate the sample 301 about the Z direction. Each detector corresponds to each crystal one by one. Each crystal can diffract the white light X-rays to generate diffracted X-rays. Each crystal can rotate about the Z direction. For example, the crystal can perform Laue diffraction (transmission diffraction) or Bragg diffraction (reflection diffraction), and the diffracted X-rays from each crystal are irradiated on the sample 301. Each detector is located on the optical path of the diffracted X-rays from the corresponding crystal and downstream of the sample 301, so that the diffracted X-rays after passing through the sample 301 are projected onto the detector, so as to obtain a projection image of the sample 301 under the white light X-rays.For example, there may be eight crystals, including a first crystal 101, a second crystal 102, a third crystal 103, a fourth crystal 104, a fifth crystal 105, a sixth crystal 106, a seventh crystal 107, and an eighth crystal 108; correspondingly, there may also be eight detectors, including a first detector 201, a second detector 202, a third detector 203, a fourth detector 204, a fifth detector 205, a sixth detector 206, a seventh detector 207, and an eighth detector 208. The first to eighth crystals 101-108 are sequentially arranged along the X direction on the first axis L, and the first crystal 1 01 corresponds to the first detector 201, the second crystal 102 corresponds to the second detector 202, the third crystal 103 corresponds to the third detector 203, the fourth crystal 104 corresponds to the fourth detector 204, the fifth transistor 105 corresponds to the fifth detector 205, the sixth transistor 106 corresponds to the sixth detector 206, the seventh transistor 107 corresponds to the seventh detector 207, and the eighth transistor 108 corresponds to the eighth detector 208. White light X-rays are transmitted along the first axis L, first irradiated on the first crystal 101, and generate transmission diffraction to obtain diffracted X-rays. The first crystal 1 The diffracted X-rays generated by the crystal 01 are irradiated on the sample 301 and projected onto the first detector 201 after penetrating the sample 301. At the same time, the white light X-rays will also pass through the first crystal 101 and obtain transmitted X-rays. The transmitted X-rays continue to irradiate the second crystal 102 and produce transmission diffraction to obtain diffracted X-rays. The diffracted X-rays of the second crystal 102 are irradiated on the sample 301 and projected onto the second detector 202 after penetrating the sample 301. The transmitted X-rays pass through the second crystal 101 and continue to irradiate the third crystal 103, and produce transmission diffraction to obtain diffracted X-rays. Similarly, the fourth crystal 104, the fifth crystal 105, the sixth crystal 106, the seventh crystal 107, and the eighth crystal 108 can all sequentially generate transmission diffraction or Bragg diffraction to obtain diffracted X-rays. After penetrating the sample 301, the diffracted X-rays from the fourth to eighth crystals 104-108 are projected onto the corresponding fourth to eighth detectors 204-208, respectively. Each detector can receive the diffracted X-rays from the corresponding crystal. The diffracted X-rays contain structural information of the sample 301, and therefore the imaging projections detected by the detectors also contain structural information of the sample 301. Figure 1 Among the eight crystals shown, the first to fourth crystals 101 - 104 all generate transmission diffraction, and the fifth to eighth crystals 105 - 108 generate Bragg diffraction.

[0031] like Figure 2As shown, assuming that the distance between the sample 301 and the first axis L along the Y direction is h, the intersection points of the first to eighth crystals 101-108 with the first axis L are A, B, C, D, E, F, G, and H respectively. A perpendicular line is drawn from the sample 301 along the Y direction so that it intersects the first axis L, and the intersection point is recorded as O. The incident angle between the transmitted X-ray incident on the fifth crystal 105 and the fifth crystal 105 is θ. According to the diffraction formula 2dsinθ=nλ, θ is determined by the interplanar spacing d of the fifth crystal 105, the diffraction order n, and the X-ray wavelength λ that meets the diffraction condition; for a specified The crystal face indices d and n of the fifth crystal 105 can be determined. Combined with the desired diffracted X-rays of a specified wavelength λ, the angle θ of the fifth crystal 105 can be determined. Therefore, the angle between the first axis L and the diffracted X-rays of the fifth crystal 105 is π-θ. The trigonometric relationship yields: EO = h / tan(π-θ). Therefore, once the position of the sample 301 is determined, the position of the fifth crystal 105 can also be determined. Similarly, the values ​​of AO, BO, CO, DO, FO, GO, and HO can also be determined, and thus the positions of the remaining crystals. In other words, once the position of the sample 301, as well as the crystal face indices and wavelength of the diffracted X-rays of each crystal, are determined, the position of that crystal relative to the sample 301 can be determined, allowing the diffracted X-rays from each crystal to pass through the sample 301. The detector corresponding to each crystal is placed downstream of the sample 301 and in the optical path of the diffracted X-rays from that crystal, thereby detecting the spatial distribution of the intensity of the diffracted X-rays from that crystal.

[0032] In some embodiments, each crystal can be divided into at least one group, each group including two crystals, and the two crystals in each group and the corresponding two detectors form a detection system, and each detection system is used to detect one element of the sample 301. For example, the first crystal 101, the second crystal 102, the first detector 201, and the second detector 202 can form a first detection system, the third crystal 103, the fourth crystal 104, the third detector 203, and the fourth detector 204 can form a second detection system, the fifth crystal 105, the sixth crystal 106, the fifth detector 205, and the sixth detector 206 can form a third detection system, the seventh crystal 107, the eighth crystal 108, the seventh detector 207, and the eighth detector 208 can form a fourth detection system, the first detection system is used to detect element A of the sample 301, the second detection system is used to detect element B of the sample 301, the third detection system is used to detect element C of the sample 301, and the fourth detection system is used to detect element D of the sample 301.

[0033] like Figure 3 and Figure 4As shown, assuming that the energies before and after the absorption edge of element B are Q1 and Q2 respectively, Q1 corresponds to an X-ray wavelength of λ1, and Q2 corresponds to an X-ray wavelength of λ2. In order for the second detection system to detect element B, it is necessary to make one of the third crystal 103 and the fourth crystal 104 generate diffracted X-rays with a wavelength of λ1, and the other generate diffracted X-rays with a wavelength of λ2. For example, the third crystal 103 generates diffracted X-rays with a wavelength of λ1, and the fourth crystal 104 generates diffracted X-rays with a wavelength of λ2. The crystal plane indices of the third crystal 103 and the fourth crystal 104 can be obtained in advance, so the incident angle θ1 between the fourth crystal 104 and the X-ray and the incident angle θ2 of the third crystal 103 and the fourth crystal 104 can be calculated respectively. 03 and the incident angle θ2 between the X-ray and the Y-axis, the Y-direction spacing h between the sample 301 and the first axis L can also be measured in advance, thereby obtaining the values ​​of CO and DO, and then moving the third crystal 103 and the fourth crystal 104 to the corresponding positions respectively, so that the second detection system can detect the B element of the sample 301; similarly, the positions of other crystals can be calculated based on the front and rear energies of the absorption edges of elements A, C and D, and each crystal is moved to its corresponding position and rotated around the Z axis to adjust its incident angle to the pre-calculated incident angle, and each detector is moved to a position where it can receive the diffracted X-rays of the corresponding crystal, so that each detection system can detect its corresponding element.

[0034] In some embodiments, each crystal is set on a slide, which is used to move the crystal along the X direction and rotate around the Z direction, so as to adjust the position of the crystal on the first axis L and the incident angle of the crystal. In this way, when other elements of the sample 301 need to be detected, it is only necessary to calculate the position and incident angle of the crystal in advance, and then place the crystal and the detector according to the calculated position and incident angle.

[0035] In order to obtain the three-dimensional spatial distribution of sample 301, sample 301 can be scanned by CT, that is, the sample stage is used to rotate sample 301 step by step according to a preset step angle until it rotates 180 degrees. After each rotation of the preset step angle, each detector detects a projection image, so that each detector can detect a set of projection image sequences. This set of projection image sequences includes the two-dimensional structural information of sample 301 at different angles. CT reconstruction is performed on each set of projection image sequences to obtain slice data of the three-dimensional structure inside sample 301. The two detectors of the detection system can obtain two sets of projection image sequences. Therefore, two slice data can be obtained through CT reconstruction. By subtracting the grayscale values ​​at the same height position of the two slices, the three-dimensional spatial distribution of the elements detected by the detection system can be obtained. For example, the third detector 203 and fourth detector 204 of the second detection system detect a set of projection image sequences M1 and M2, respectively. By importing M1 and M2 into CT reconstruction software, slice data R1 and R2 of the sample's internal three-dimensional structure can be reconstructed. The grayscale values ​​of slices R1 and R2 at the same height are subtracted. Regions without element B in R1 and R2 are unaffected by the absorption edge, resulting in a small difference. However, regions with denser element B distribution experience a large difference due to the strong difference in absorption coefficients before and after the absorption edge. This allows the three-dimensional spatial distribution of element B to be determined. Similarly, by operating other detection systems according to the above method, the three-dimensional spatial distribution of elements A, C, and D can be obtained.

[0036] In some embodiments, the crystal can be a diamond crystal, which has a lower absorption rate for white light X-rays than common diffraction crystals such as single crystal silicon. Thus, when multiple crystals are arranged in sequence, the upstream crystal will not significantly affect the incident light of the downstream crystal. Furthermore, diamond crystals have a higher thermal conductivity and are less susceptible to radiation damage under high-flux synchrotron X-ray radiation than common diffraction crystals such as single crystal silicon. The detector can be a high-resolution imaging detector, which is an indirect X-ray detector. It converts X-rays into visible light using a scintillation crystal and combines a high-resolution visible light camera with the scintillation crystal into a single unit using fiber coupling or lens coupling to achieve high-resolution imaging detection of X-rays. The spatial resolution can reach micrometers to submicrometers depending on the magnification of the coupling system. For example, the detector can be a Hamamatsu C12849-111U imaging detector, rather than an energy-resolved photon counting detector. This results in a smaller pixel size, higher spatial resolution, and lower cost. The incident white light X-rays are synchrotron white light X-rays with a wide energy spectrum, which can cover the absorption edge range of more elements.

[0037] It is understandable that although the above description is based on 8 crystals and 8 detectors as an example, the energy spectrum CT experimental device of the present invention is not limited to this. The number of crystals can be set as needed, for example, 2, 4, 6 or more. In order to achieve the detection of multiple different elements, the number of crystals needs to be set to an even number and at least 4. Every two crystals can detect one element. The number of crystals can be adjusted accordingly according to the number of elements required by the detector.

[0038] like Figure 5 As shown, an embodiment of the present invention further provides an energy spectrum CT experimental method based on white light X-rays, which includes the following steps:

[0039] S100: providing a white light X-ray based spectral CT experimental device as described in the above embodiment;

[0040] S200: for each crystal, moving the crystal along the X-axis to a preset position of the crystal, and rotating the crystal around the Z-axis to adjust the incident angle of the crystal to a preset angle of the crystal;

[0041] S300: Transmitting white light X-rays along the first axis L and sequentially passing through each crystal, so that each crystal generates diffracted X-rays. The diffracted X-rays from each crystal pass through the sample 301 and are projected onto a detector corresponding to the crystal, so that the detector detects a projection of the sample 301.

[0042] S400: Using the sample stage, the sample 301 is rotated stepwise by a preset step angle until it is rotated 180 degrees. After each rotation by the preset step angle (e.g., 5 degrees), each detector detects a projection image of the sample 301.

[0043] S500: performing CT reconstruction on multiple projection images detected by each detector when the sample 301 is rotated at different angles to obtain slice data of the internal three-dimensional structure of the sample detected by the detector;

[0044] S600: For each detection system, obtain the three-dimensional spatial distribution of elements corresponding to the detection system according to slice data of the three-dimensional structure inside the sample detected by two detectors of the detection system.

[0045] In step S200, the preset angle of each crystal can be pre-calculated based on the crystal plane index of the crystal and the wavelength of the diffracted X-ray of the crystal. The preset position of each crystal can be pre-calculated based on the preset angle of the crystal and the Y-direction distance between the sample 301 and the first axis L. The wavelength of the diffracted X-ray of the crystal is predetermined by the absorption edge energy of the element (i.e., the target element) that the detection system where the crystal is located wishes to detect. For the specific calculation method, please refer to the description in the device embodiment and will not be repeated here.

[0046] In step S500 , the grayscale values ​​of the slice data of the three-dimensional structure inside the sample detected by the two detectors of each detection system at the same height position may be subtracted to obtain the three-dimensional spatial distribution of the corresponding element.

[0047] The white-light X-ray-based energy spectrum CT experimental device and method of the embodiments of the present invention use synchrotron white-light X-rays with a wide energy spectrum range as a light source, which can cover the absorption edge range of more elements. The three-dimensional spatial distribution of multiple target elements can be simultaneously detected through a single CT scan, making operation simpler. By moving the crystal along the X-direction and rotating it around the Z-direction, the type of elements detected by the detection system where the crystal is located can be adjusted, which provides greater flexibility. The detector does not need to use a photon counting detector with energy resolution, but can use an indirect detector with a smaller pixel size, thereby having a higher spatial resolution.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications are possible. In other words, any simple, equivalent changes and modifications made in accordance with the claims and description of the present invention are within the scope of protection of the patent claims. Anything not fully described in this invention constitutes conventional technology.

Claims

1. A white light X-ray based spectral CT experimental device, characterized in that: The device comprises a plurality of crystals, a plurality of detectors and a sample stage, wherein the plurality of crystals are sequentially arranged on a first axis along the X direction, the sample stage deviates from the first axis along the Y direction, the sample stage is used to support the sample and rotate the sample around the Z direction, each detector corresponds to each crystal one by one, each crystal can diffract white light X-rays incident along the X direction to generate diffracted X-rays, the diffracted X-rays of each crystal pass through the sample, and each detector is located on the optical path of the diffracted X-rays of the corresponding crystal and downstream of the sample to receive the diffracted X-rays of the corresponding crystal that pass through the sample.

2. The white light X-ray based spectral CT experimental device according to claim 1, characterized in that: Each crystal is divided into at least one group, each group includes two crystals, the two crystals in each group and the two detectors corresponding to the two crystals form a detection system, and each detection system is used to detect one element of the sample.

3. The white light X-ray based spectral CT experimental device according to claim 2, characterized in that: The wavelengths of the diffracted X-rays from the two crystals of each detection system are wavelengths corresponding to the front and rear energies of the absorption edge of the element corresponding to the detection system.

4. The white light X-ray based spectral CT experimental device according to claim 1, characterized in that: Each crystal is placed on a slide, and the slide is used to move the crystal along the X direction and rotate around the Z direction.

5. The white light X-ray based spectral CT experimental device according to claim 1, characterized in that: The crystal is a diamond crystal.

6. The white light X-ray based spectral CT experimental device according to claim 1, characterized in that: The detector is an indirect X-ray detector.

7. A white light X-ray based spectral CT experimental method, characterized in that: The following steps are involved: S100: Provide a white light X-ray based energy spectrum CT experimental device according to any one of claims 2 to 6; S200: for each crystal, moving the crystal along the X-axis to a preset position of the crystal, and rotating the crystal around the Z-axis to adjust the incident angle of the crystal to a preset angle of the crystal; S300: transmitting white light X-rays along a first axis and sequentially passing through each crystal, so that each crystal generates diffracted X-rays, and the diffracted X-rays from each crystal pass through the sample and are projected onto a detector corresponding to the crystal, so that the detector detects a projection image of the sample; S400: Using the sample stage to gradually rotate the sample according to a preset step angle until it is rotated 180 degrees, and after each rotation of the preset step angle, each detector detects a projection image of the sample; S500: performing CT reconstruction on multiple projection images detected by each detector when the sample is rotated at different angles to obtain slice data of the internal three-dimensional structure of the sample detected by the detector; S600: For each detection system, obtain the three-dimensional spatial distribution of elements corresponding to the detection system according to slice data of the three-dimensional structure inside the sample detected by two detectors of the detection system.

8. The white light X-ray based spectral CT experimental method according to claim 7, characterized in that: In step S200, the preset angle of each crystal is calculated in advance based on the crystal plane index of the crystal and the wavelength of the diffracted X-ray of the crystal.

9. The white light X-ray based spectral CT experimental method according to claim 8, characterized in that: In step S200 , the preset position of each crystal is set to be calculated in advance based on the preset angle of the crystal and the Y-direction distance between the sample and the first axis.

10. The white light X-ray based spectral CT experimental method according to claim 7, characterized in that: In step S600, the grayscale values ​​of the slice data of the three-dimensional structure inside the sample detected by the two detectors of each detection system at the same height position are subtracted to obtain the three-dimensional spatial distribution of the corresponding element of the detection system.

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