WO2016038667A1

WO2016038667A1 - X-ray imaging method and device

Info

Publication number: WO2016038667A1
Application number: PCT/JP2014/073737
Authority: WO
Inventors: 明男米山; 馬場　理香
Original assignee: 株式会社日立製作所
Priority date: 2014-09-09
Filing date: 2014-09-09
Publication date: 2016-03-17

Abstract

For each of at least three types of X-ray radiation with different average energies, the average intensity I₀ (En, X, Y) of a background region of an object and the intensity I (En, X, Y) of each pixel in a transmission image of X-rays transmitted through the same region of the object are obtained. For each average energy, the measured absorption coefficient μ of the object is determined by dividing each intensity I (En, X, Y) by the average intensity I₀ (En, X, Y) at the same average energy, and calculating the minus natural logarithm (-ln(x)). One of the at least three types of X-ray radiation is set as a standard energy E₀, and a relative measured absorption coefficient μ' is calculated by dividing, by the measured absorption coefficient μ corresponding to the standard energy E₀, a measured absorption coefficient μ corresponding to another average energy. Then, the theoretical composite absorption coefficient μ_Ic of each expected element is calculated for each average energy on the basis of the theoretical absorption coefficient μ_I and content σ_i of the element. The content σ_i with which the sum of squares of the differences from the relative measured absorption coefficients μ' calculated for each average energy is smallest is calculated by repeated computation.

Description

X-ray imaging method and apparatus

The present invention relates to a method and apparatus for imaging an object using X-rays.

The X-ray imaging method is a method for observing the structure inside the subject in a non-destructive manner by utilizing the high X-ray transmittance, and is widely used in various fields ranging from product inspections to airport security checks. Further, there is X-ray CT (Computed Tomography) that uses transmission images from a plurality of different angles acquired by rotating a subject relative to an X-ray source to calculate a cross-sectional image of the subject. X-ray CT is an indispensable technique for medical diagnosis because it allows non-destructive observation of the internal structure of a subject in three dimensions.

The X-ray imaging method detects the change in signal intensity that occurs when X-rays pass through the subject using the principle that the density of the X-ray absorption increases as the density of the image increases. Is forming. That is, in the X-ray imaging method, since the spatial density distribution in the subject can be measured, an area having a large density change (such as a boundary area (shape) or an internal structure having a different composition) can be visualized with high definition.

JP 2010-509180 A

Problems to be solved by the invention

However, the conventional X-ray imaging method detects only the spatial density distribution in the subject, and information such as what elements the subject is composed of (ie, information on constituent elements (composition)). ) Is not expected to obtain. For this reason, in order to identify the elements contained in the subject, it is necessary to combine with other methods using fluorescent X-rays or absorption edges, but the fluorescence detector has no spatial resolution (only intensity detection) ), It is necessary to limit the region where fluorescent X-rays are generated by narrowing down X-rays, and a large sample cannot be measured at one time. In addition, in the method using the absorption edge, the number of elements that can be detected is limited to one, and it is not possible to detect all the elements contained in a composite material or compound composed of a plurality of elements.

In addition, in recent years, certain elements have been emphasized based on transmission images acquired using X-rays of two different energies generated by changing the voltage applied to the X-ray tube as an X-ray source. A dual energy X-ray imaging method has been developed (see Patent Document 1). However, even with this imaging method, there is a problem that, for example, it is only possible to acquire an image in which iodine or bones contained in a contrast medium are emphasized, and it is not possible to acquire information on a constituent element (composition) of a subject. The inventor discovered that.

In order to solve the above-described problems, the present application adopts, for example, the technical configuration described in the claims. In addition, although the present specification includes a plurality of means for solving the above-described problems, as an example thereof, a technique is adopted in which a processing procedure shown below is executed by an arithmetic device such as a computer.
(1) First processing for obtaining the average intensity I ₀ (En, X, Y) in the background area of the subject for each of three or more types of X-rays having different average energies
(2) Second processing for irradiating the same region of the subject with the X-rays having different average energies and obtaining the intensity I (En, X, Y) of each pixel in the transmission image for each X-ray
(3) Each intensity I (En, X, Y) acquired in the second process is divided by the average intensity I ₀ (En, X, Y) acquired for X-rays of the same average energy, and further natural A third process for calculating the measured absorption coefficient μ of the subject for X-rays having respective average energies by calculating the logarithm-(minus) (-ln (x))
(4) 3 sets one of the mean energy of the kinds of X-rays to the reference energy E _0, by measuring the absorption coefficient obtained μ for the reference energy E _0, separately for X-rays having other average energy The fourth process of calculating the relative measured absorption coefficient μ ′ by dividing the measured absorption coefficient μ obtained by
(5) A fifth method of calculating a theoretical composite absorption coefficient μ _Ic theoretically synthesized from each theoretical absorption coefficient μ _I and each content rate σ _{i of} an element expected to be included in the subject for each average energy processing
(6) Comparing the theoretically synthesized absorption coefficient μ _Ic calculated for each average energy and the relative measured absorption coefficient μ ′, and including each element contained in the subject so that the sum of squares of the difference is minimized. A sixth process for determining the rate σ by repeated calculation

Further, as another example of the means for solving the above-mentioned problem included in the present specification, each of three or more types of X-rays having different average energies is irradiated from a plurality of angles to the same region of the subject. Intensity I (En, X, Y) of each pixel of the cross-sectional image calculated from the transmission image acquired for the X-ray having the same average energy transmitted through the same region (corresponding to the processing (2) described above) This is a technique for causing a computer to execute the above-described processes (1), (3), (5) and (6). However, in the process (6), “measured absorption coefficient μ ′” is used instead of “relative measured absorption coefficient μ ′”.

According to the present invention, it is possible to accurately detect information about constituent elements of a subject even when the element contained in the subject is unknown or the number of elements contained in the subject is larger than the number of projection images. Problems, configurations, and effects other than those described above will become apparent from the following description of embodiments.

The flowchart explaining an example of the X-ray imaging procedure to propose. The figure which shows the example which calculated the element contained in a to-be-photographed object (metal foil) using the proposal method. The figure explaining the change (difference) which arises in the linear absorption coefficient of each element when the energy of X-ray is changed from 10 keV to 20 keV. The figure explaining the relationship between the absorption edge (K edge) and energy of each element. 1 is a diagram illustrating a schematic configuration of an X-ray imaging apparatus according to Embodiment 1. FIG. 3 is a flowchart for explaining measurement processing in the first embodiment. The figure which shows the structural example of the crystal spectrometer used for taking out the X-ray which has specific energy. The figure which shows the structural example of the rotation revolver type energy change mechanism used for taking out the X-ray which has specific energy. 9 is a flowchart for explaining an initial value setting method according to the second embodiment. FIG. 10 is a diagram illustrating an example of measurement of the transmittance of a subject by the method of the third embodiment. 10 is a flowchart for explaining an initial value setting method according to the third embodiment. FIG. 6 is a diagram illustrating a schematic configuration of an X-ray imaging apparatus according to Embodiment 4. 10 is a flowchart for explaining measurement processing in the fourth embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiment of the present invention is not limited to the embodiments described later, and various modifications are possible within the scope of the technical idea.

(basic way of thinking)
First, a method for accurately detecting information about constituent elements of a subject using an image (transmission image) obtained by imaging an X-ray that has passed through the subject will be described. The technique proposed in this specification observes a subject using three or more types of X-rays having different energies, and repeats calculation for a transmission image (measurement information) captured for each X-ray, whereby all the subjects included in the subject are observed. It is characterized by acquiring information on elements. This will be specifically described below.

When the intensity of incident X-rays is I _o , the intensity of transmitted X-rays is I, the linear absorption coefficient of the subject is μ, and the thickness is t, generally, the following relationship is established between these variables.
μt = -Ln (I / _Io ) (1)

If the subject is composed of a plurality of elements, and the linear absorption coefficient of each element is μ _i and the content rate (percentage) is σ _i , the expression (1) is given by the following expression.
Σ _i σ _i μ _i t = −Ln (I / I _o ) (2)

In equation (2), if the X-ray energy is known, the linear absorption coefficient μ _{i of} each element is also known. However, in addition to the thickness t, since the formula is one for a plurality of unknowns (i.e. content sigma _i), (2) can not be uniquely determined the content of sigma _i of each element from the equation. When the subject includes n elements, theoretically, n + 1 different formulas (2) including the thickness t are required to uniquely determine the content σ _i of each element. Therefore, the X-ray energy is changed, the same measurement is performed for a plurality of X-rays having different energies, and n + 1 equations (2), that is, I (E) / I _o (E) are obtained from each transmission image. get.

Here, if the energy of the X-ray is E1, E2, E3,..., En, the content rate σ _i and the thickness t are invariant with respect to the energy. (3) given by the following n relational expressions is obtained.
Σσ _i × μ _i (E1) × t = −Ln (I (E1) / Io (E1))
Σσ _i × μ _i (E2) × t = −Ln (I (E2) / Io (E2))
Σσ _i × μ _i (E3) × t = −Ln (I (E3) / Io (E3))
...
Σσ _i × μ _i (En) × t = −Ln (I (En) / Io (En)) (3)

For example, when the content σ _i of three types of elements contained in the subject is unknown, when measurement is performed with four types of X-rays having different energies, assuming that the reciprocal of the thickness t is −a, Equation (3) is It is represented by
σ ₁ × μ ₁ (E1) + σ ₂ × μ ₂ (E1) + σ ₃ × μ ₃ (E1) = a Ln (I (E1) / I _o (E1))
σ ₁ × μ ₁ (E2) + σ ₂ × μ ₂ (E2) + σ ₃ × μ ₃ (E2) = a Ln (I (E2) / I _o (E2))
σ ₁ × μ ₁ (E3) + σ ₂ × μ ₂ (E3) + σ ₃ × μ ₃ (E3) = a Ln (I (E3) / I _o (E3))
σ ₁ × μ ₁ (E4) + σ ₂ × μ ₂ (E4) + σ ₃ × μ ₃ (E4) = a Ln (I (E4) / I _o (E4))
... (4) formula

In this case, obtaining the three content rates σ ₁ , σ ₂ , and σ ₃ corresponds to solving simultaneous linear equations represented by the following matrix.

(5) Since there is one (4) known information for the unknown 3 plus one, the content of each element can be uniquely determined by Gaussian elimination or the like. As described above, if the elements contained in the subject are limited and the data of the absorption coefficient acquired by X-rays of different energy is larger than the number of elements contained in the subject, it is included in the subject. It is possible to quantitatively detect the amount of the element.

On the other hand, if it is unknown what kind of element is contained in the subject, the number of energy scans is limited, so that the number of unknowns is basically larger than the number of data obtained by measurement. For this reason, theoretically, the content rate cannot be uniquely determined by the above method.

Therefore, the inventor calculates the theoretical synthetic absorption coefficient μ _I of each element using the contents σ _i of each element (all elements if completely unknown) that are likely to be contained in the subject as variables, and calculates the theoretical synthesis. We propose a method to determine what elements are contained in the subject by repeating the comparison between the absorption coefficient μ _I and the measured absorption coefficient μ calculated from the measured values. FIG. 1 shows an outline of an X-ray imaging procedure executed in the X-ray imaging apparatus.

・ Step S1
The X-ray imaging apparatus acquires the average intensity I ₀ (En, X, Y) in the background area of the subject for each X-ray having different average energies.

・ Step S2
The X-ray imaging apparatus irradiates the same region of the subject with the X-rays having different average energies, and obtains the intensity I (En, X, Y) of each pixel in the transmission image for each X-ray having different average energies. . Steps S1 and S2 may be executed in time sequence every time X-rays having different average energies are set, or after step S1 is executed for all X-rays having different average energies, the average energies differ. Step S2 may be executed for all X-rays.

・ Step S3
The X-ray imaging apparatus divides each intensity I (En, X, Y) acquired in step S2 by the average intensity I ₀ (En, X, Y) acquired using X-rays having the same average energy. Further, by calculating-(minus) (-ln (x)) of the natural logarithm, the measured absorption coefficient μ of the subject with respect to the X-ray having each average energy is obtained. This process may be executed after all the processes of steps S1 and S2 are completed, or in parallel with the processes of steps S1 and S2, the average intensity of step S1 and the process of step S2 for one average energy. The processing may be started when both of the intensities are acquired.

・ Step S4
The X-ray imaging apparatus sets the average energy of one of the three or more types of X-rays as the reference energy E ₀ , and has the other average energy according to the measured absorption coefficient μ determined for the reference energy E _0. The relative measured absorption coefficient μ ′ is calculated by dividing the individually determined measured absorption coefficient μ for the line. This process is executed for the purpose of removing the unknown variable a (the reciprocal of the object thickness t) from the relational expression described above. By executing step S4, the only unknown variable remaining in the relational expression is the element content σ _i expected to be included in the subject.

・ Step S5
The X-ray imaging apparatus calculates, for each average energy, a theoretical synthetic absorption coefficient μ _Ic that is theoretically synthesized from each theoretical absorption coefficient μ _Ii and each content rate σ _{i of} an element expected to be included in the subject. .

・ Step S6
The X-ray imaging apparatus compares the theoretical synthetic absorption coefficient μ _Ic calculated for each average energy and the relative measured absorption coefficient μ ′, and each of the objects included in the subject so that the sum of squares of the difference is minimized. The element content σ _{i is obtained} by repeated calculation.

Here, the process of step S6 can be reduced to a non-linear optimization problem. Specific calculation methods include various methods such as a direct search method, a gradient method, and a Gauss-Newton method that have been developed as an optimization method. Can be used.

For example, in the simplex method, which is a direct search method, calculation is performed as follows. Since the content rate σ _i of each element is completely independent, it can be considered as one point p on the n-dimensional space. Here, no (n + 1) points on a line p _1, p _2, ..., taking the p _{n + 1,} to form an n + 1 prismatic virtually. In this method, the residual sum of squares (hereinafter referred to as “error E”) with the measured absorption coefficient (transmittance) is calculated at each point thus formed, and each point is determined based on a pre-defined law. The point is deformed or moved to approach the optimal solution. The deformation or movement here is generally performed as follows.
(1) The error E (pi) at each of n + 1 points p is calculated and arranged in descending order.
(2) Using the n points p _i excluding the point p ₀ where E is maximum, the center of gravity p _g is obtained as p _g = (Σp _i ) / n.
(3) An error E is calculated for each of the following points p on a straight line connecting the points p ₀ and p _g in the virtual space.
(3-1) p _r = p ₁ +2 (p _g -p ₁ )
(3-2) p _e = p ₁ +3 (p _g -p ₁ )
(3-3) p _cr = p ₁ +1.5 (p _g -p ₁ )
(3-4) p _cw = p ₁ +0.5 (p _g -p ₁ )
(4) Of the error E obtained in the above (3), the point p first smaller than E (p ₀ ) is exchanged with the point p ₀ .
(5) The above (1) to (4) are repeated until the error E becomes smaller than a preset value or the average distance between the points p becomes smaller than a certain value.

With the above calculation method, the point p (that is, the content ratio σ _{i of} each element) at which the error E is minimized can be obtained. Figure 2 shows the measurement of X-rays with energy of 8, 10, 15, 20, 30, 40, and 50 keV for metal foils composed of aluminum, iron, copper, molybdenum, and tin. It is the result of calculating (simulating) the content σ _i of each metal by calculating the absorption coefficient (transmittance) μ and repeating the calculation based on the optimization method. In addition, as elements to be included in repeated calculations, calculation is performed including titanium, cobalt, and nickel having relatively close atomic numbers in addition to the above metals. In the repeated calculation, the above simplex method is used, and the content ratio σ _i of each element is set to the same ratio as an initial value. From this result, it can be seen that the assumed content rate σ _i is almost calculated.

However, since the above iterative calculation finds a convergent solution in n dimensions, depending on the selection of the initial value and the calculation method, the converged solution may be a minimum value and may not be the correct minimum value. In order to obtain the correct minimum value, it is extremely important to set the initial value to the correct content rate σ _i and start the calculation.

As a method for giving a desirable initial value, for example, there is a method focusing on the linear absorption coefficient μ _i of each element. It is known that the linear absorption coefficient μ _i of each element has a unique value for each energy of X-rays. For this reason, the amount of change in the linear absorption coefficient μ _i between specific energy is also characteristic. FIG. 3 is a result of calculating a change (difference) that occurs when the X-ray energy is changed from 10 keV to 20 keV with respect to the linear absorption coefficient μ _i of each element. FIG. 3 shows that the amount of change in the linear absorption coefficient μ _i has a unique value for each element. That is, the contained element can be estimated to some extent from the change to energy.

Therefore, the inventor obtains a change from data acquired (measured) with each energy of X-ray, compares the obtained change with a change obtained by theoretical calculation, and further tends to be similar. Based on the assumption that the subject contains the largest number of elements indicating the above, a method is proposed that employs an initial value with a higher content of elements that show similar tendencies and elements in the vicinity thereof. In the calculation process employing this method, compared to a method not employing this method, the initial value generally approaches a correct solution, and the content rate σ _i can be obtained more accurately.

As another method for giving a desirable initial value, for example, there is a method focusing on the absorption edge. As shown in FIG. 4, each element has an energy called an absorption edge, and the linear absorption coefficient μ _i changes abruptly. When the energy interval to be measured is narrow, projection images are obtained before and after such an absorption edge, so that the linear absorption coefficient μ _i changes abruptly for a specific element. Therefore, the projection images acquired for each energy are arranged in order of energy, and it is determined whether or not the transmittance with respect to the energy changes abruptly in each pixel, and the element contained in the subject based on the energy in which the abrupt change is recognized We propose a method for setting an initial value with a large content σ _i of this element. In the calculation process employing this method, compared to a method not employing this method, the initial value generally approaches a correct solution, and the content rate σ _i can be obtained more accurately.

In the above-described example, the projection image (two-dimensional image) is described. However, the subject is rotated relative to the X-ray source, and the cross-sectional image is obtained nondestructively from the acquired projection images at the respective rotation angles. In X-ray CT, it is possible to three-dimensionally acquire information on the elements of a subject by performing similar processing on images (cross-sectional images) obtained with different X-ray energies.

As described above, the theoretical synthetic absorption coefficient μ _Ic that minimizes the residual from the relative measured absorption coefficient μ ′ acquired by irradiating the same region of the subject with three or more types of X-rays having different average energies. By the optimization calculation for repeatedly calculating the content ratio σ _i of each element contained in the subject whose element is unknown, it becomes possible to obtain the content ratio σ _{i of the} element.

(Example)
Below, the Example of the X-ray imaging method and apparatus based on the above-mentioned view is described in order.

(Example 1)
FIG. 5 illustrates a schematic configuration of the X-ray imaging apparatus according to the first embodiment. The X-ray imaging apparatus shown in FIG. 5 includes an X-ray source 1, a subject 2, a subject holder 3, a subject positioning mechanism 4, an X-ray image detector 5, a control unit 6, an estimated content rate calculation unit 7, and a display device 8. is doing. The X-ray beam emitted from the X-ray source 1 is held by the subject holder 3 and irradiated to the subject 2 positioned by the subject positioning mechanism 4. The X-ray beam transmitted through the subject 2 is detected by the X-ray image detector 5. The detection result of the X-ray image detector 5 is given to the control unit 6.

The control unit 6 executes setting of irradiation conditions (energy change, etc.) of the X-ray source 1 and imaging processing of a projected image (background image and transmission image) under each energy according to a measurement procedure described later. The estimated content rate calculation unit 7 receives images captured by three or more types of X-rays having different average energies and executes the above-described repetitive calculation to obtain the content rate of the elements contained in the subject. Here, the processing operations of the control unit 6 and the estimated content rate calculation unit 7 are realized, for example, through execution of a program by a computer (computer). The program is stored in a storage device (not shown). However, part or all of the processing operations executed by the control unit 6 and the estimated content rate calculation unit 7 may be realized as an ASIC or a dedicated calculation module.

The display device 8 displays an image (element map) representing the calculated spatial distribution of each element on the display screen based on the calculation result of the estimated content calculation unit 7. In FIG. 5, the display device 8 is connected to the estimated content rate calculation unit 7, but the printing device as the output device is replaced with the display device 8 or together with the display device 8 in the estimated content rate calculation unit 7. It may be connected. The printing apparatus prints the element map described above. Further, instead of the display device 8 or together with the display device 8, a communication device may be connected so that transmission to an external device is possible.

Subsequently, the intensity information measurement operation under the control of the control unit 6 will be described with reference to FIG.

Step S11
The control unit 6 transmits a drive signal to a drive unit (x-axis direction drive device, y-axis direction drive device, z-axis direction drive device) (not shown) of the subject positioning mechanism 4, and the subject 2 is an X-ray beam. A coordinate (for example, X1) located at the center is set. If necessary, a coordinate Y1 on the y axis and a coordinate Z1 on the z axis are also set.

・ Step S12
The controller 6 moves the subject 2 to a position X0 where the subject 2 is completely retracted from the X-ray beam. The completely retracted position means a position where the X-ray beam does not pass through the subject 2.

Step S13
The control unit 6 sets the irradiation condition of the X-ray source 1. In the case of this embodiment, the control unit 6 selects one of n setting conditions (n is 3 or more) set in advance. The irradiation condition corresponds to any of the energy E1 to En. The setting order here may be determined in advance or one of the setting conditions that are not used may be selected each time.

・ Step S14
The control unit 6 controls the X-ray source 1 to irradiate the X-ray beam, and the intensity I _o is detected by the X-ray image detector 5. The X-ray beam in this step does not pass through the subject 2. Therefore, the X-ray image detector 5 acquires a background image of the subject 2 (a projection image that is the background of the subject 2).

・ Step S15
The control unit 6 transmits a drive signal to a drive unit (an x-axis direction drive device, a y-axis direction drive device, and a z-axis direction drive device) (not shown) of the subject positioning mechanism 4 and changes the coordinates from X0 to coordinates X1. change. Thereby, the subject 2 is positioned at the center of the X-ray beam.

・ Step S16
The control unit 6 controls the X-ray source 1 to irradiate the X-ray beam, and detects the intensity I by the X-ray image detector 5. The X-ray beam in this step passes through the subject 2. Therefore, the X-ray image detector 5 acquires a transmission image of the subject 2.

Step S17
The controller 6 determines whether or not the measurement (imaging) of the background image and the transmission image has been completed for all irradiation conditions set in advance. When the irradiation conditions that are not used for measurement (imaging) remain, the control unit 6 returns to step S12. This operation is repeated for all irradiation conditions.

In the processing procedure shown in FIG. 6, a background image and a transmission image are acquired for one energy at a time, but after acquiring a background image for all the energy as in steps S <b> 1 and S <b> 2 of FIG. 1 described above, You may acquire a transmission image about all the energy anew.

Next, a method for changing X-ray energy will be described. When an X-ray tube generally used in a laboratory or the like is used as the X-ray source 1, the average energy of the X-ray is changed only by changing the voltage (tube voltage) applied to the X-ray tube. Can do. Therefore, in this case, in step S13 described above, the tube voltage may be changed to a preset voltage, and each projection image (background image and transmission image) may be captured.

In addition, as the X-rays used for capturing the projected image, X-rays generated by radiated light or inverse Compton scattering can be used. However, the energy of these X-rays cannot be easily changed by changing the conditions of the source. Therefore, it is necessary to newly provide a mechanism capable of changing the energy distribution of the generated X-rays between the X-ray source and the subject. For example, a crystal spectrometer that can extract only X-rays having specific energy using X-ray diffraction is installed. FIG. 7 shows an example of a double crystal spectrometer.

The two-crystal spectrometer here is composed of a rotary stage 71 and two

single crystal plates

72 and 73 disposed on the upper surface thereof, and the two

single crystal plates

72 and 73 are parallel to each other so as to sandwich the rotation axis. Is arranged. When using a double crystal spectrometer, in step S13 described above, the rotation stage 71 is rotated by a predetermined angle around the rotation axis, and the incident angle of the white X-rays with respect to the single crystal plate 72 is changed. A monochromatic X-ray having a corresponding energy can be output toward the subject.

Here, in the

single crystal plates

72 and 73, Bragg's diffraction condition = 2λ = 2d sin θ (where d is the lattice spacing of the crystal related to diffraction, λ is the X-ray wavelength, and θ is the X-ray wavelength relative to the crystal. Only monochromatic X-rays satisfying (incident angle) are transmitted through the double crystal spectrometer. Therefore, X-rays having an extremely narrow energy bandwidth can be obtained. When a double crystal spectrometer is used, it is possible to use monochromatic X-rays with higher purity than when an X-ray tube is used, and the element content can be determined more accurately.

In addition, as a method for changing the energy of X-rays irradiated to the subject, a mechanism capable of inserting a plurality of metal plates having different thicknesses and types between the X-ray source and the subject may be provided. FIG. 8 shows an example of a rotating revolver type energy changing mechanism (also referred to as a metal foil rotating mechanism) in which a plurality of metal plates having different thicknesses and types are arranged along the circumferential direction of the disc. In the energy changing mechanism shown in FIG. 8, the type and thickness of the metal film 82 through which X-rays pass are changed in a very short time by rotating the disk 81 (that is, the X-ray energy is changed). Can do. As a result, even when the change of the subject with time occurs in a short time, it is possible to capture the projected image of substantially the same shot with X-rays having different energy. It is of course possible to combine the double crystal spectrometer illustrated in FIG. 7 and the rotary revolver type energy conversion mechanism illustrated in FIG. 8 with a normal X-ray tube, and at the same time, by optimizing the tube voltage, The energy of X-rays can be changed efficiently.

By the way, the method for obtaining the content rate of the element contained in the subject from the plurality of projection images acquired with the X-rays of different energy described above is divided into the following cases (1) and (2). It is good to handle.
(1) When the number of contained elements is less than the number of projected images
(2) When there are more projections (including unknown cases)

In the former case, it can result in simultaneous linear equations for obtaining the aforementioned (5) solution from the equation (content sigma _i), can be determined uniquely content sigma _i of each element due to Gaussian elimination. On the other hand, in the latter case, the calculation for determining the content rate σ _i is reduced to a nonlinear optimization problem, and the difference between the theoretical synthetic absorption coefficient σ _Ic calculated by calculation at each energy and the relative measured absorption coefficient μ ′ obtained by measurement is calculated. The content rate σ _i can be obtained by repeated calculation of optimization so that the sum of squares is minimized.

At present, optimization methods can be roughly classified into the following three types.
(1) Direct search method
(2) Gradient method
(3) Inverse matrix operation method

Here, the direct search method searches for a direction in which the square sum of the difference between the theoretical synthetic absorption coefficient μ _Ic calculated from the expected content and the relative measured absorption coefficient μ ′ obtained by measurement becomes smaller, This is a method for obtaining the most probable solution (content ratio) by repeated calculation. This method is intuitive and has the advantage of being able to monitor the iteration process, but has the feature of slow convergence. In the latter two methods, the differential coefficient of the function related to the content rate is calculated in each process of the iterative calculation, and a condition (content rate) that minimizes the sum of squares of the difference in the direction in which the deviation becomes smaller is searched. Is the method. Although the convergence speed of these methods is faster than the convergence speed of the direct search method, it is less affected by the initial value and often converges to a minimum point.

For this reason, if the following processes are performed as calculation which calculates a content rate, an exact solution (content rate) can be calculated | required.
(1) List the elements considered to be contained in the subject.
(2) The content ratio of each element is set to the same ratio, and the content ratio is obtained by a direct search method.
(3) Using the content obtained in (2) above as the initial content, the more accurate content is determined by the gradient method or the inverse matrix calculation method.

The X-ray imaging apparatus according to the present embodiment performs the above-described calculation processing for each pixel (pixel) of a plurality of projection images acquired for X-rays having different energies, and obtains the content rate for each element for each pixel. Thereafter, a spatial distribution image of the selected element is displayed on the display device 8 in accordance with an operator instruction. In addition, if it is possible to display a composite image of the projected image and element content ratio image at the selected energy, the operator can better understand what structure of the element is distributed. It can be easy.

(Example 2)
When the number of elements contained in the subject is larger than the number of projection images or when the number of elements is completely unknown, the X-ray imaging apparatus described in the first embodiment results in a nonlinear optimization problem, and the optimum value (Content) is obtained by repeated calculation. However, in the case of Example 1 in which the initial content of each element is treated as the same, the direct search method may converge to a local minimum value (minimum value) and may not be able to accurately determine the content. . Therefore, in this example, a method for improving the reliability of the obtained solution by setting the content rate predicted from the measured absorption coefficient as the initial content rate will be described.

As described with reference to FIG. 3, the linear absorption coefficient μ _i of each element has a unique value for each energy of the X-ray, and the amount of change in the linear absorption coefficient μ _i between two specific energies is also determined. It will have a unique value. Therefore, the X-ray imaging apparatus according to the present embodiment sets the initial value of the content rate by the method shown in FIG. First, the estimated content rate calculation unit 7 calculates the amount of change in the linear absorption coefficient between the two energies for each pixel of the transmission image acquired for each energy (step S21). Next, the estimated content calculation unit 7 theoretically calculates the amount of change in the linear absorption coefficient between the same two energies (step S22). Thereafter, the estimated content rate calculation unit 7 compares the amount of change of the linear absorption coefficient calculated in step 21 with the theoretical amount of change calculated in step S22, and predicts the element contained in the subject (step S23). Furthermore, the estimated content rate calculation unit 7 sets initial values so that the content rates of the element predicted in step S23 and the elements in the vicinity thereof are increased (step S24).

The initial value obtained by the above processing is generally close to the correct solution as compared with the first embodiment. Therefore, the estimated content rate calculation unit 7 using the initial value can determine the elements included in the subject and their contents more accurately than in the first embodiment.

(Example 3)
In the case of Example 2 described above, the initial value of the content rate was determined based on a comparison between the change amount of the linear absorption coefficient μ _i measured between two energies and the theoretical change amount calculated theoretically. However, the initial value can be determined by other methods. This is a technique that uses a change at the absorption edge when an X-ray having a small energy step width and high monochromaticity such as radiated light is used as an X-ray source.

FIG. 10 shows the transmittance calculated for the metal foil assumed in FIG. 2 with a step width of 1 keV when the energy is between 7 and 35 keV. In FIG. 10, it can be seen that the transmittance changes abruptly in the vicinity of 9, 20, and 29 keV. It can be seen from FIG. 4 that the transmittance changes suddenly at these energies from Cu, CuMo, and Sn. Therefore, the estimated content calculation unit 7 in this example calculates the initial content values of these elements. Set higher than other elements. FIG. 11 shows processing (steps S31 to S33) executed in the estimated content rate calculation unit 7.

Even with the method of this example, it is possible to obtain a more accurate content rate as in Example 2. Incidentally, in step S33, how much the element content determined in step S32 is set may be determined from information on the change in transmittance. For example, in the case of FIG. 10, since the magnitude of the change of Mo and Sn is about 1/4 with respect to the magnitude of the change of Cu, it is sufficient to set the ratio of 4: 1: 1 and other elements to about 0.5.

As described above, in this embodiment, since an initial value in which the content rate of an element that is highly likely to be contained can be increased, the estimated content rate calculation unit 7 calculates the element contained in the subject and its content. As compared with the first embodiment, it can be obtained more accurately.

Example 4
In the X-ray imaging apparatus according to the above-described embodiment, only a projection image of a subject can be acquired, and when different elements are multiplexed in the X-ray transmission direction, an accurate content rate of the element can be obtained. was difficult. In this embodiment, in order to solve the above problem, information related to elements is acquired from a plurality of cross-sectional images acquired at different energies in combination with an X-ray CT (Computed Tomography) method.

The X-ray CT method is a method of obtaining a cross-sectional image of a subject by rotating a subject relative to an X-ray source and executing a calculation called reconstruction from a plurality of projection images acquired from different projection angles. Since the obtained cross-sectional image represents the distribution of the linear absorption coefficient inside the subject, when the content ratio of each element at each pixel position is σ _i and the linear absorption coefficient of each element is μ _i , the following expression is obtained.
μ = Σ _i μ _i σ _i (6)

(6) The formula (6) is completely the same as the formula (2) except that the thickness t is removed. Therefore, an element at each pixel position in the cross-sectional image can be obtained from a cross-sectional image acquired with different energy in the same manner as the projected image by the same iterative calculation. However, in the case of the present embodiment, it is not necessary to perform step S4 (FIG. 1) for removing information on the thickness t. Therefore, in step S6 (FIG. 1), a predetermined process is performed on “measured absorption coefficient μ ′” instead of “relative measured absorption coefficient μ ′”.

FIG. 12 shows a configuration example of the X-ray imaging apparatus according to the present embodiment. In FIG. 12, parts corresponding to those in FIG. The difference from FIG. 5 is that the X-ray imaging apparatus shown in FIG. 12 has a new sample rotation mechanism 9 that can rotate the sample with respect to the X-ray source, and the subject positioning mechanism 4 In addition, a control unit 160 that also controls the rotation of the sample rotation mechanism 9 and an estimated content rate calculation unit 170 that can calculate the content rate for each pixel of the cross-sectional image are used. The sample rotation mechanism 9 may be of any drive system as long as it is a mechanism that is rotationally driven in one or both directions by a drive mechanism (for example, a motor) (not shown). As shown in FIG. 12, the subject positioning mechanism 4 is placed on the upper surface of the sample rotating mechanism 9.

The X-ray imaging procedure in the present embodiment is the same as that shown in FIG. 1 except that the X-ray image acquired for each energy is a cross-sectional image, and the element estimation process for each pixel is also executed for each pixel of the cross-sectional image. It is the same. Therefore, hereinafter, a measurement procedure for obtaining a cross-sectional image will be described with reference to FIG. The measurement procedure is executed through the control unit 160.

The control unit 160 performs measurement operations in the following order.

Step S41
The control unit 160 uses the subject positioning mechanism 4 to set coordinates (for example, X1) at which the subject 2 is positioned at the center of the X-ray beam. If necessary, a coordinate Y1 on the y axis and a coordinate Z1 on the z axis are also set.

Step S42
The controller 160 moves the subject 2 to the position X0 where the subject 2 is completely retracted from the X-ray beam.

Step S43
The controller 160 sets the irradiation conditions for the X-ray source 1. As in the case of the first embodiment, for example, one of n preset conditions (n is 3 or more) is selected. The irradiation condition corresponds to any of the energy E1 to En. The setting order here may be determined in advance or one of the setting conditions that are not used may be selected each time.

Step S44
The control unit 160 controls the X-ray source 1 to irradiate the X-ray beam, and detects the intensity I _o with the X-ray image detector 5. The X-ray beam in this step does not pass through the subject 2. Therefore, the X-ray image detector 5 acquires a background image of the subject 2 (a projection image that is the background of the subject 2).

Step S45
The control unit 160 transmits a drive signal to a drive unit (x-axis direction drive device, y-axis direction drive device, z-axis direction drive device) (not shown) of the subject positioning mechanism 4 and changes the coordinates from X0 to coordinates X1. change. Thereby, the subject 2 is positioned at the center of the X-ray beam.

Step S46
The control unit 160 controls the X-ray source 1 to irradiate the X-ray beam, and the intensity I is detected by the X-ray image detector 5. The X-ray beam in this step passes through the subject 2. Therefore, the X-ray image detector 5 acquires a transmission image of the subject 2 at the current rotation position of the sample rotation mechanism 9.

Steps S47 and S48
The control unit 160 rotates the sample rotation mechanism 9 to rotate the subject 2 with respect to the X-ray source 1 by a preset angle. Thereafter, the control unit 160 determines whether or not the position after rotation reaches a preset rotation angle, and returns to step S46 while a negative result is obtained. That is, until reaching a preset rotation angle, the X-ray image detector 5 acquires a transmission image obtained by X-ray beams incident from different angles on the same coordinates of the subject 2.

Step S49
The control unit 160 that has obtained a positive result in step S48 moves the subject 2 to the position X0 completely retracted from the X-ray beam.

・ Step S50
The control unit 160 acquires a background image again for the retracted position. In step S44 and step S50, an X-ray background image having the same energy is acquired because there is a change in the energy of the X-ray output from the X-ray source 1 between the start of rotation of the subject 2 and the end of rotation. It is for confirming. If a change is detected, the intensity of the transmission image acquired for each rotation angle is corrected based on the intensity of the X-ray background image acquired in steps S44 and S50. For example, the intensity is corrected by linear interpolation.

Step S51
The controller 160 determines whether or not the measurement (imaging) of the background image and the transmission image has been completed for all irradiation conditions set in advance. If irradiation conditions that are not used for measurement (imaging) remain, the control unit 160 returns to step S43. This operation is repeated for all irradiation conditions. Through the above processing, transmission images with different angles and different energy are acquired for each coordinate of the subject 2.

As described above, when all the preset projection images (background image and transmission image) are obtained (when a set of a plurality of transmission images obtained for each energy (a set of CT data) is obtained), the estimated inclusion The rate calculation unit 170 calculates a cross-sectional image of the subject 2 for each energy by a normal reconstruction process (filtered back projection method or the like). Thereafter, the estimated content rate calculation unit 170 performs the same process as described above for each pixel of each cross-sectional image, and calculates the content rate of each element included in each pixel. In the case of the present embodiment, since the term relating to the thickness of the subject is not included, if the number of elements is n, the content rate can be uniquely determined if there are n cross-sectional images.

As described above, according to the present embodiment, an image representing the three-dimensional distribution of the content of each element contained in the subject is accurately obtained by repeated calculation from a plurality of cross-sectional images acquired with X-rays having different energies. Can be sought.

(Other examples)
The present invention is not limited to the configuration of the embodiment described above, and includes various modifications. For example, in the above-described embodiments, some of the embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, and the present invention does not necessarily have all the configurations described. Further, a part of a certain embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of a certain embodiment. It is also possible to add other configurations to the configuration of each embodiment, replace a partial configuration of each embodiment with another configuration, or delete a partial configuration of each embodiment.

In addition, each of the above-described configurations, functions, processing units, processing means, and the like may be partly or entirely realized as, for example, an integrated circuit or other hardware. Each of the above-described configurations, functions, and the like may be realized by a processor interpreting and executing a program that realizes each function. That is, each configuration may be realized by software. In this case, information such as programs, tables, and files for realizing each function can be stored in a storage device such as a memory, a hard disk, an SSD (Solid State Drive), or a storage medium such as an IC card, an SD card, or a DVD. .

Also, the control lines and information lines indicate what is considered necessary for explanation, and do not represent all control lines and information lines necessary for the product. In practice, it can be considered that almost all components are connected to each other.

Reference Signs List 1 X-ray source 2 Subject 3 Subject holder 4 Subject positioning mechanism 5 X-ray image detector 6 Control unit 7 Estimated content rate calculation unit 8 Display device 9 Subject rotation mechanism 71 Rotation stage 72 Single crystal plate 73 Single crystal plate 81 Disc 82 Metal film 160 Control unit 170 Estimated content rate calculation unit

Claims

A first process for obtaining an average intensity I 0 (En, X, Y) in the background area of the subject for each of three or more types of X-rays having different average energies;
A second process of irradiating the same region of the subject with the X-rays having different average energies and obtaining the intensity I (En, X, Y) of each pixel in the transmission image for each X-ray;
Each intensity I (En, X, Y) acquired in the second process is divided by the average intensity I 0 (En, X, Y) acquired for the X-rays having the same average energy, and further the natural logarithm. A third process of calculating a measured absorption coefficient μ of an object for X-rays having each average energy by calculating − (minus) (−ln (x));
One average energy of the three or more kinds of the X-ray was set to the reference energy E 0, by measuring the absorption coefficient obtained μ for the reference energy E 0, determined separately for X-rays having other average energy A fourth process of calculating the relative measured absorption coefficient μ ′ by dividing the measured measured absorption coefficient μ;
A fifth process for calculating a theoretical synthetic absorption coefficient μ Ic theoretically synthesized from each theoretical absorption coefficient μ I and each content rate σ i of an element expected to be included in the subject for each average energy;
The theoretical synthetic absorption coefficient μ Ic calculated for each average energy is compared with the relative measured absorption coefficient μ ′, and the content ratio σ i of each element contained in the subject is minimized so that the sum of squares of the difference is minimized. An X-ray imaging method, characterized by causing a computer to execute a sixth process for repeatedly calculating.
The X-ray imaging method according to claim 1,
The fifth process includes
A process for obtaining a change in the measured absorption coefficient μ calculated for a transmission image acquired for two X-rays among the three or more types of X-rays having different average energies;
A process of calculating a change in the theoretical absorption coefficient μ I corresponding to the two X-rays;
Comparing the change in the measured absorption coefficient μ and the change in the theoretical absorption coefficient μ I to obtain an element showing the same change;
An X-ray imaging method, further comprising: a process of providing, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging method according to claim 1,
The fifth process includes
A process for determining an average energy at which the plurality of measured absorption coefficients μ determined for each average energy in the third process change abruptly;
A process for obtaining an element having an average energy at which the measured absorption coefficient μ changes abruptly at the absorption edge;
An X-ray imaging method, further comprising: a process of providing, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging method according to claim 1,
After the execution of the first process for all of the three or more types of X-rays having different average energies, the second process is performed for all of the three or more types of X-rays having different average energies. X-ray imaging method.
The X-ray imaging method according to claim 1,
After the first process and the second process are performed on one X-ray among the three or more types of X-rays having different average energies, the first process and the first process on another X-ray are performed. 2. An X-ray imaging method, wherein the process 2 is repeatedly executed.
A first process for obtaining an average intensity I 0 (En, X, Y) in the background area of the subject for each of three or more types of X-rays having different average energies;
The X-rays having different average energies are irradiated to the same region of the subject from a plurality of directions to obtain cross-sectional images for each X-ray, and the intensity I (En of each pixel in the cross-sectional image for each X-ray. , X, Y)
Each intensity I (En, X, Y) acquired in the second process is divided by an average intensity I 0 (En, X, Y) acquired using X-rays having the same average energy, and further natural A third process for calculating the measured absorption coefficient μ of the subject with respect to X-rays having respective average energies by calculating the logarithm-(minus) (-ln (x));
A fourth process of calculating a theoretical synthetic absorption coefficient μ Ic theoretically synthesized from each theoretical absorption coefficient μ I and each content rate σ i of an element expected to be included in the subject, for each average energy;
Compare the theoretically synthesized absorption coefficient μ Ic calculated for each average energy and the measured absorption coefficient μ ′, and repeatedly calculate the content σ i of each element in the subject so that the sum of squares of the difference is minimized. An X-ray imaging method characterized by causing a computer to execute a fifth process obtained by the following.
The X-ray imaging method according to claim 6,
The fourth process includes
A process for obtaining a change in the measured absorption coefficient μ calculated for a transmission image acquired for two X-rays among the three or more types of X-rays having different average energies;
A process of calculating a change in the theoretical absorption coefficient μ I corresponding to the two X-rays;
Comparing the change in the measured absorption coefficient μ and the change in the theoretical absorption coefficient μ I to obtain an element showing the same change;
An X-ray imaging method, further comprising: a process of providing, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging method according to claim 6,
The fourth process includes
A process for determining an average energy at which the plurality of measured absorption coefficients μ determined for each average energy in the third process change abruptly;
A process for obtaining an element having an average energy at which the measured absorption coefficient μ changes abruptly at the absorption edge;
An X-ray imaging method, further comprising: a process of providing, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging method according to claim 6,
For each of the X-rays, the first process is executed before the X-ray irradiation to the subject and after the irradiation from all of the plurality of directions is completed.
A subject holder for placing a subject, a subject positioning mechanism for adjusting the position of the subject holder and positioning the subject with respect to X-rays;
An X-ray image detector for detecting the intensity of the X-ray;
A control unit for controlling the subject holder and the subject positioning mechanism;
First processing for obtaining the average intensity I 0 (En, X, Y) in the background area of the subject for each of three or more types of X-rays having different average energies, and the X-rays having different average energies in the same area of the subject , And a second process for obtaining the intensity I (En, X, Y) of each pixel in the transmission image for each X-ray, and the respective intensity I (En, X, Y, acquired in the second process) Y) is divided by the average intensity I 0 (En, X, Y) acquired using X-rays having the same average energy, and a third measurement absorption coefficient μ of the subject for the X-rays having each average energy is obtained. For the X-rays having other average energies by setting the average energy of one of the three or more types of X-rays to the reference energy E 0 and measuring the absorption coefficient μ determined for the reference energy E 0 Relative measurement absorption coefficient by dividing the measured absorption coefficient μ obtained individually A fourth process for calculating several μ ′, and a theoretical synthetic absorption coefficient μ Ic theoretically synthesized from each theoretical absorption coefficient μ I and each content rate σ i of the element expected to be included in the subject, The fifth process to calculate for each average energy is compared with the theoretically synthesized absorption coefficient μ Ic calculated for each average energy and the relative measured absorption coefficient μ ′, and the sum of squares of the differences is minimized. An X-ray imaging apparatus comprising: an estimated content rate calculation unit that executes a sixth process of repeatedly calculating the content rate σ i of each element contained in the subject.
The X-ray imaging apparatus according to claim 10,
In the fifth process, the estimated content rate calculation unit,
A process for obtaining a change in the measured absorption coefficient μ calculated for a transmission image acquired for two X-rays among the three or more types of X-rays having different average energies;
A process of calculating a change in the theoretical absorption coefficient μ I corresponding to the two X-rays;
Comparing the change in the measured absorption coefficient μ and the change in the theoretical absorption coefficient μ I to obtain an element showing the same change;
An X-ray imaging apparatus further comprising: a process of giving, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging apparatus according to claim 10,
In the fifth process, the estimated content rate calculation unit,
A process for determining an average energy at which the plurality of measured absorption coefficients μ determined for each average energy in the third process change abruptly;
A process for obtaining an element having an average energy at which the measured absorption coefficient μ changes abruptly at the absorption edge;
An X-ray imaging apparatus further comprising: a process of giving, as an initial value, a content rate σ i containing the element obtained by the process as a main component.
The X-ray imaging apparatus according to claim 10,
An X-ray imaging apparatus further comprising an X-ray tube that outputs three or more types of X-rays having different average energies.
The X-ray imaging apparatus according to claim 10,
An X-ray imaging apparatus, further comprising: a crystal spectrometer that outputs three or more types of X-rays having different average energies by selectively extracting monochromatic X-rays from white X-rays.
The X-ray imaging apparatus according to claim 10,
An X-ray imaging apparatus further comprising a rotary revolver type energy conversion mechanism that outputs three or more types of X-rays having different average energies by selectively extracting monochromatic X-rays from white X-rays.