JP4537090B2 - Tomosynthesis equipment - Google Patents

Description

  The present invention generally relates to a tomosynthesis apparatus (or a laminograph), and more particularly to a circular tomosynthesis apparatus that changes the direction of irradiation of a subject along a cone.

  Tomosynthesis is a method of taking a large number of transmission images from different directions with respect to a subject using radiation and processing the images as digital data to obtain a three-dimensional image inside the subject. is there.

  In particular, a device that changes the irradiation direction of a subject along a cone is called a circular tomosynthesis device. This circular tomosynthesis apparatus is also called a tilted CT (computer tomograph).

  In recent years, circular tomosynthesis apparatuses for inspecting printed circuit boards and the like have been developed (see, for example, Patent Document 1). FIG. 7 is a conceptual diagram showing an outline of a circular tomosynthesis apparatus.

  This apparatus has a structure for rotating the subject 106. The principle of this apparatus is that an X-ray detector 102 detects an X-ray 102 generated from an X-ray source 101 and images a subject 106 to obtain a digital transmission image. The subject 106 is rotated on the sample table 104. Here, the rotating shaft 105 and the X-ray 102 intersect not at a right angle but at an intersection angle θ.

  The apparatus captures a large number of transmission images over one rotation (referred to as scanning) while changing the rotational position of the subject 106, and digitally processes them to reconstruct a three-dimensional image of the subject 106.

  In this apparatus, the crossing angle θ can be changed by inclining the inclined frame 107. Further, in this apparatus, it is possible to change the enlargement ratio by moving the X-ray source 101 and the X-ray detector 103 closer to or away from the subject 106, and obtain a geometric condition according to the subject 106. Can do.

  In this circular tomosynthesis apparatus (inclined CT), an apparatus having a specification in which the crossing angle θ is 90 ° is called a cone beam CT. In this cone beam CT, the method for reconstructing a three-dimensional image of the subject 106 is basically the same.

  By the way, as a reconstruction processing method of a three-dimensional image in tomosynthesis, a reconstruction method is known in which a frequency filtering process for increasing the spatial resolution and reducing false images is added (see, for example, Patent Document 2). ). This document 2 describes a reconstruction method of | ω | filtering (omega absolute value filtering).

In addition, a reconstruction method for tomosynthesis using the CT reconstruction method is known (see, for example, Patent Documents 3 and 4). Furthermore, tomosynthesis filtering is also known in other documents (see, for example, Non-Patent Documents 1 and 2).
JP 2003-260049 A JP 2002-267622 A JP 2000-350721 A JP 2003-344316 A P. Edholm et al., A new radiographic method for reproducing a selected slice of varying thickness, Acta Radiologica, Vol.21, Fase.4, pp433-442, (1980) H. Knutsson et al., Ectomography-a new Radiographic Reconstruction method-I, IEEE Trans, on Biomedical Eng. Vol. BME-27, No. 11, pp640-648 (Nor. 1980)

  The conventional circular tomosynthesis apparatus as shown in FIG. 7 has a high degree of correspondence with the subject because the crossing angle θ, the position of the X-ray source and the X-ray detector are variable, and the following problems occur.

  That is, every time the geometry is changed, the rotation axis on the transmission image is out of order, and calibration is required. In order to correctly reconstruct a three-dimensional image, it is necessary to accurately know the position of the rotation axis on the transmission image. For this reason, after setting a geometric condition while viewing a transmission image of the subject, it is necessary to perform calibration by replacing it with a pin-shaped calibration phantom.

  However, after calibration, it is very difficult to return the subject to the exact same position at high magnification. As described above, the conventional apparatus has to be replaced every time the geometric condition is changed, which causes a very troublesome problem.

  Further, even in the case of a geometrically fixed device, there is a problem that if the enlargement ratio is high, a deviation is likely to occur with time, and calibration must be performed frequently.

  An object of the present invention is to provide a tomosynthesis apparatus capable of obtaining a rotation axis in a circular tomosynthesis apparatus without replacing a subject on a calibration phantom.

As a means for solving the above-mentioned problems, the invention according to claim 1 is directed to a circular tomosynthesis apparatus, in which a plurality of two-dimensional transmission images obtained by a two-dimensional radiation detector during one rotation scanning of tomosynthesis. converts each projection image corresponding to the decaying exponential, by utilizing the two-dimensional adder projection image of the plurality of projection images by adding one rotation is paired Shi symmetrical to the rotation axis of tomosynthesis, the addition projection A tomosynthesis apparatus having data processing means for calculating the inclination and intercept of a rotation axis on an image.

The invention according to claim 2 is the invention according to claim 1, while varying the slope and intercept of the virtual axis on said summing projected image, calculating the deviation from the symmetry of the summing projected image with respect to the virtual axis and deviations are those having a data processing means for calculating the slope and intercept of the imaginary axis at a minimum as the slope and intercept of the rotary shaft.

The inventions according to claims 1 and 2 have the following principle. That is, when the subject is regarded as a multipoint distribution of the linear absorption coefficient μ, the projection image P is expressed as a sum “Σ _point μpoint” of one projection μpoint. Here, one rotation sum "sigma _{R P,} where R denotes a rotation" of P if it is expressed as, the _{"Σ R P = Σ R Σ point} μpoint = Σ point Σ R μpoint ".

1 rotation sum Σ _R μpoint one point of the projection Myupoint is because symmetric to the rotation axis, sigma _{R P} is symmetrical to the rotation axis. By utilizing this, the rotation axis can be obtained by calculating the symmetry axis of the summation projection image of one rotation addition. Thus, the rotation axis can be obtained from the tomosynthesis scan data of the subject itself.

Furthermore, the invention according to claim 3 is the invention according to claim 1 or claim 2, wherein the mounting table of the subject in which the reference body is arranged at a position that does not interfere with the subject, the mounting table and the rotation shaft. A means for relative movement in a direction orthogonal to the rotation axis; and a tilt and intercept of the rotation axis are calculated by the data processing means from a transmission image of the reference body obtained during one rotation scan of tomosynthesis. It is the structure which can calibrate the said rotating shaft, without taking down from the mounting table mentioned above.

  With such a configuration, it is possible to place the reference body in the vicinity of the rotation axis instead of the subject using the moving means, and calculate the rotation axis from the transmission image of the reference body. After the rotation axis is calculated, the movement is returned and the subject is imaged, so that the rotation axis can be calibrated without taking the subject off the mounting table even in the case of a subject whose rotation axis is difficult to obtain (for example, a homogeneous plate). .

Further, the invention according to claim 4 is an apparatus for forming a cone beam CT in the circular tomosynthesis apparatus, wherein the rotation axis intersects the radiation beam at approximately 90 ° .

In addition, the calculation of the rotation center of claims 1 to 3 may be performed with an intersection angle of 90 °, which indicates that the tomosynthesis apparatus referred to in claims 1 to 3 includes a cone beam CT.

  According to the present invention, it is possible to provide a circular tomosynthesis apparatus or a cone beam CT that can determine the rotation axis without replacing the subject on the calibration phantom.

(Configuration of the first embodiment)
FIG. 1 is a schematic configuration diagram of a circular tomosynthesis apparatus according to the first embodiment of the present invention.

  This apparatus includes an X-ray tube 1, an X-ray detector 3, a sample table (mounting table) 5 on which a subject 4 is placed, a rotation / lifting mechanism 6, a data processing unit 7, and a display unit 8. Have

  As the X-ray tube 1, a microfocus X-ray tube having a focal point F of generated X-rays of several μm is used. As the X-ray detector 3, an X-ray flat panel detector (FPD) in which a scintillator is bonded to a two-dimensional semiconductor optical sensor is used. The X-ray tube 1 and the X-ray detector 3 are arranged to face each other.

  The subject 4 is placed on the sample table 5 and rotated around the rotation axis 9 in the X-ray beam 2 by the rotation / elevation mechanism 6 and is moved up and down in the direction of the rotation axis.

  Here, F is the X-ray focal point, C is the center of rotation, and D is the detection center. The center line FD of the X-ray beam 2 and the rotation axis 9 intersect at an angle α, and α is a value of 90 ° or less. The F position and the D position can be moved by a mechanism (not shown), and α and the magnification of photographing can be changed.

  In addition, an xy mechanism that moves the sample table 5 in the horizontal xy direction on rotation is omitted in the figure. The subject 4 can be moved horizontally (and moved up and down) without shifting the rotating shaft 9 by the xy mechanism (and the rotating and raising / lowering mechanism 6), and the part to be observed can be placed in the field of view.

  The data processing unit 7 and the display unit 8 are components of a normal computer, and include elements such as a CPU, a memory, a disk, a keyboard, and an interface. The data processing unit 7 functions as software functional blocks, such as a rotation axis obtaining unit 11 that obtains a rotation axis position on the image from a transmission image, a reconstruction unit 12 that creates a cross-sectional image, and a scanning control unit (not shown). ) Etc.

(Operation of the first embodiment)
Hereinafter, the operation of the first embodiment will be described with reference to FIGS.

  The operation of the first embodiment is to obtain a three-dimensional image of the subject 4 from the transmission image of the subject 4 obtained at a constant angular interval during the tomosynthesis scan in which the subject 4 is rotated once. . Then, the rotational axis 9 (position) on the transmission image necessary for the reconstruction of the three-dimensional image is obtained from the transmission image itself obtained by the scan and the reconstruction is performed.

  First, with reference to FIG. 2, the principle of rotation axis finding (rotation axis calculation) by the rotation axis finding unit 11 will be described.

  FIG. 2 is an X-ray geometric diagram for explaining the principle. FIG. 2A is a front view, and FIG. 2B is a plan view.

  Here, a plane determined by the rotation axis 9 and the focal point F is a center plane 14. On the detection surface 15 of the X-ray detector 3, orthogonal coordinates ξ and η are determined with the detection center D as a starting point, as shown in FIG. Further, the orthogonal coordinates x ′, y ′, z ′ are determined from the rotation center C as a starting point.

  One point A in the subject 4 draws a circle 16 around the rotation axis 9 during 2π rotation. At this time, the projection point Ap of the point A onto the detection surface 15 draws a certain closed drawing figure 17. The drawing figure 17 is intuitively understood to be symmetric with respect to the η axis. If the rotation is constant speed, it can be seen that the drawing speed of Ap is also a target.

  Considering mathematically, if the x ′, y ′, z ′ coordinates of the point A are (r · cos ψ, r · sin ψ, z), the ξ and η coordinates of the projection point Ap are expressed by the following equations (1), (2 ).

ξ = FDD · r · cos ψ / (FCD-r · sin α · sin ψ-z · cos α) (1)
η = FDD · (r · cos α · sin φ−z · sin α) / (FCD−r · sin α · sin φ 1 z · cos α) (2)
From these expressions (1) and (2), it can be seen that the drawing figure 17 drawn by Ap is a figure that is close to an ellipse but slightly different. Further, from these equations (1) and (2), it can be seen that there is a relationship as shown by the following equations (3) and (4).

ξ (ψ) = − ξ (π−ψ) (3)
η (ψ) = η (π−φ) (4)
This indicates that when the point A is rotated at a constant speed, the drawing figure 17 by the Ap point and the drawing speed are symmetrical with respect to the η axis, that is, the rotation axis 9 (projection thereof). Therefore, when there is a minute object at point A, the one-rotation integration of the projection becomes a figure having a shape and density symmetrical to the η axis. This is true for every point in the subject 4.

Next, the transmission image I obtained by the X-ray detector 3 is an X-ray intensity distribution. For this transmission image I, when I0 is expressed as a transmission image when the subject 4 is not present (not limited to this, any logarithm proportional to the pixel sensitivity may be used), the logarithm as shown in the following formula (5) When subjected to conversion, attenuation exponent image P is obtained (e- ^p attenuation).

P = −LN (I / I0) (5)
P corresponds to the line integral of the linear absorption coefficient μ along the X-ray path, and is called a projected image of μ. That is, it can be expressed by the following formula (6).

P = ∫μdt (6)
Here, when capturing the object 4 a number point distribution of mu, summing Σ _point μpoint next projection Myupoint of the projected image P is 1 point, the sum sigma _{R P} of one revolution addition of P is the following formula (7) As shown. “R” means one rotation.

_{Σ R P = Σ R Σ point} μpoint = Σ point Σ R μpoint ... (7)
Sum Σ _R μpoint by one revolution addition of one point of the projection Myupoint is, since symmetric η axis sigma _{R P} is symmetrically η axis.

  As described above, it has been proved that “the total image obtained by adding one rotation of the projection image P is symmetric with respect to the rotation axis” as the principle of finding the rotation axis. Here, it should be noted that this principle holds for the projection image P that is an image of the attenuation index, not for the intensity distribution image I.

  Next, a specific procedure for finding the rotation axis will be described with reference to FIGS. FIG. 3 is a flowchart for calculating the rotation axis. FIG. 4 is a diagram showing an image and a rotation axis (η) on the image.

  In finding the rotation axis, a transmission image I of the subject 4 obtained at a constant angular interval during one rotation is used. The coordinates representing the transmission image I are g and h.

  First, as shown in FIG. 3, a total projection image Pm is calculated by one rotation addition (step S1). Specifically, Pm is calculated from the following formula (8) using the obtained transmission image I and air image I0.

_{Pm = Σ R P = Σ R} LN (I0 / I) ... (8)
Pm is obtained with coordinates g and h.

  Next, a virtual rotation axis (ε, gc) is set (step S2). That is, the intercept gc between the inclination ε of the rotation axis and the g axis is set. Furthermore, the symmetry deviation of Pm is calculated (step S3). The symmetry deviation is calculated by the following formula (9), for example.

Symmetry deviation = Σpoint B [| Pm (point B) −Pm (symmetric position of point B) |] / number of points (9)
As shown in FIG. 4 (A), the absolute value of the difference between Pm at B and B ′ is added while changing B, and the average is obtained by dividing by the score. In general, since B ′ does not come to the center of the pixel, interpolation calculation is required. The deviation is not limited to this, and may be a standard deviation. The deviation value obtained here is stored together with ε and gc.

  The above processing is repeated while changing ε and gc (step S4). That is, a two-dimensional search process is executed.

  Then, from the set of the obtained deviation values, ε, gc, ε, gc having the smallest deviation value is obtained as the rotation axis (step S5). As shown in FIG. 4B, by determining gc and ε, the rotation axis 9 (position on the image) can be obtained. When the intersection of the rotation axis 9 (η axis) and the g axis is set as the detection center D, and the orthogonal coordinates ξ and η are set with the detection center D as the starting point, the rotation axis reference coordinates are obtained.

  In the flow chart of FIG. 3, the details of the search method are not described in the two-dimensional search process of ε and gc represented by steps S2 to S4, but the simplest method is a method for calculating a grid having a constant pitch. That is, first, an approximate minimum position is obtained with a large cell, and then the area around that point is searched with a fine cell, and the cell is gradually made finer. There is also a method of automatically determining and searching for a direction in which the target value decreases.

  Next, the three-dimensional image of the subject 4 is reconstructed using the projection image Pm (g, h) obtained in step S1, and the reconstruction method is known in the above-mentioned prior art documents. Since there is, explanation is omitted. The reconstruction unit 12 reconstructs a three-dimensional image of the subject 4 based on the relationship between coordinates (g, h) and coordinates (ξ, η).

(Effects of the first embodiment)
According to the first embodiment, the rotation axis can be obtained from the transmission image of the subject 4 itself. Therefore, before the subject 4 is scanned after the geometric setting is completed, it is not necessary to carry out rotation axis calibration by replacing it with a pin-shaped phantom, for example.

(Modification of the first embodiment)
In the first embodiment, the rotation axis can be obtained even when the crossing angle α is 90 °.

  As described above, the circular tomosynthesis apparatus having an intersection angle of 90 ° corresponds to a cone beam CT (Computer Tomograph). That is, the rotation axis finding method according to the first embodiment can be applied to the cone beam CT.

  As a modification of the present embodiment, for example, a mechanism that rotates the subject 4 around the subject 4 by integrating the X-ray tube 1 and the X-ray detector 3 with respect to the same rotation axis 9 without rotating the subject 4. May be adopted.

  Further, the obtained rotation axis position can be displayed on the display unit 8 so as to overlap the transmission image. This makes it easy to align the site of interest with the center of rotation when changing the region of the subject and continuing scanning without changing the geometry.

  In addition, in finding the rotation axis, the fact that FDD, FCD, and α are variable is not directly related, and the rotation axis 9 can be obtained effectively even when it is not variable.

  Furthermore, the X-ray detector 3 used for finding the rotation axis is not limited to a flat panel detector, and other types may be used. Further, instead of X-rays, an apparatus that uses other transmissive radiation such as γ-rays may be used.

  Furthermore, the mechanism may be calibrated in advance, ε and gc may be predicted from FDD, FCD, and α, and the center may be obtained based on this prediction. Thereby, since the center search area can be narrowed, the calculation time can be shortened.

  In addition, the tomosynthesis apparatus regarding this embodiment can perform a rotation axis | shaft search, without being limited to the application field (industrial use, medical use, etc.).

(Second Embodiment)
FIG. 5 is a diagram showing a schematic configuration relating to the second embodiment.

  The apparatus according to the second embodiment is obtained by adding a reference body 20 to the apparatus according to the first embodiment (see FIG. 1). FIG. 5 is a view for explaining photographing of the reference body 20.

  The reference body 20 has a structure in which a tungsten wire 21 is molded with plastic. The reference body 20 is attached to the edge of the sample table 5 so as not to interfere with the subject 4. The tungsten wire 21 is attached so as to be substantially parallel to the rotating shaft 9.

(Operation of Second Embodiment)
In the apparatus of the first embodiment described above, depending on the subject 4, the rotation axis may not be obtained from the transmission image of the subject 4 itself. Specifically, when the subject 4 is a nearly homogeneous flat plate or the like, changes that occur in the transmission image are reduced, and it may be difficult to find the rotation axis.

  In such a case, the apparatus of the second embodiment sets the tungsten wire 21 to be near the rotation center C by moving the sample table 5 by xy as shown in FIG. Then, as in the case of the subject 4, the reference body 20 is imaged at a constant angular interval during one rotation. Next, as in the first embodiment, the rotation axis is obtained from the obtained transmission image. Then, using this rotation axis, a three-dimensional image is reconstructed from the data of the imaged subject. Alternatively, the specimen table 5 may be returned to the original position by xy movement, and imaging of the subject 4 may be executed again.

(Effect of 2nd Embodiment)
In the apparatus according to the second embodiment, calibration can be performed by moving the sample table 5 and placing the reference body 20 in the field of view. Therefore, before scanning the subject 4 after finishing the geometric setting, for example, it is placed on a pin-shaped phantom. In other words, it is not necessary to perform rotation axis calibration.

(Modification of the second embodiment)
As a modification of the second embodiment, a modification similar to the modification of the first embodiment described above is possible. Furthermore, the following modifications are possible.

  That is, since the rotation axis is obtained in the same manner as in the first embodiment, the reference body 20 may not be linear and may not be parallel to the rotation axis. In short, if the transmission image of the subject 4 with good contrast can be obtained, the rotation axis can be obtained accurately.

  In order to move the reference body 20 to the vicinity of the rotation center C, the elevation in the first embodiment may be used. Furthermore, various attachment positions of the reference body 20 are possible, and the inside or the back surface of the sample table 5 may be used. Moreover, you may attach the mechanism made to project from a sample table.

  Further, the xy movement and elevation with respect to the sample table 5 may be relative to the rotation axis (rotation center C), and the X-ray tube 1 and the X-ray detector 3 may be moved. Further, instead of the tungsten wire 21, other materials may be used as long as the member is a material having a strong X-ray absorption.

  6A and 6B are summed projection images obtained by one rotation addition of the transmission image (or projection image) of the tungsten wire 21 according to this modification.

  Since the rotation axis is obtained by using the thin tungsten wire 21 substantially parallel to the rotation axis for the reference body 20, it is not always necessary to obtain the same central axis as in the first embodiment, but a simplified method. Is possible.

  6A and 6B, it can be seen that the rotation axis η can be obtained by simply obtaining the average of the line positions along the g direction. That is, for example, the rotational axis η can be obtained by obtaining the center of gravity of the profile of each line at each height. Here, the number of shots during one rotation can also be reduced. As shown in FIG. 6B, in the extreme case, if there are two transmitted images at intervals of 180 °, the rotation axis can be obtained. However, if the degree of coincidence between the tungsten wire 2 and the rotation axis is poor, an error occurs with a small number. In addition, in the above-described rotation center finding using the tungsten wire 2, it is not always necessary to obtain the total projection image, and it can be seen that the rotation center can be obtained simply by obtaining the pin positions on each projection image and averaging them. At this time, an image before logarithmic conversion can also be used.

  Note that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. In addition, the embodiments may be appropriately combined as much as possible, and in that case, combined effects can be obtained. Furthermore, the above embodiments include inventions at various stages, and various inventions can be extracted by appropriately combining a plurality of disclosed constituent elements. For example, when an invention is extracted by omitting some constituent elements from all the constituent elements shown in the embodiment, when the extracted invention is implemented, the omitted part is appropriately supplemented by a well-known common technique. It is what is said.

The figure which shows schematic structure regarding the 1st Embodiment of this invention. The X-ray geometric diagram for demonstrating the principle of a rotating shaft search regarding 1st Embodiment. The flowchart for demonstrating the procedure of the rotating shaft search regarding 1st Embodiment. The figure which shows the image regarding 1st Embodiment, and the rotating shaft ((eta)) on an image. The figure which shows schematic structure regarding 2nd Embodiment. The figure regarding the modification of 2nd Embodiment. The conceptual diagram for demonstrating the outline of the conventional circular tomosynthesis apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 3 ... X-ray detector, 4 ... Subject, 5 ... Sample table (mounting table),
6 ... Rotation / lifting mechanism, 7 ... Data processing unit, 8 ... Display unit, 11 ... Rotary shaft rescue unit,
12 ... reconstructed part, 20 ... reference body, 21 ... tungsten wire.

Claims (4)

In circular tomosynthesis equipment,
Converting each of a plurality of two-dimensional transmission images obtained by the two-dimensional radiation detector during scanning of one rotation of tomosynthesis projection images corresponding to the decaying exponential, two dimensions of this plurality of projection images by adding one rotation of utilizing the fact summing projected image is a pair Shi symmetrical to the axis of rotation of tomosynthesis, tomosynthesis apparatus characterized by having a data processing means for calculating the slope and intercept of the rotation axis on the summing projected image. Wherein the data processing means, while changing the inclination and intercept of the virtual axis on said summing projected image, the deviation and deviation calculated from symmetry of the summing projected image with respect to the virtual axis of the imaginary axis is minimized tomosynthesis apparatus according to claim 1, characterized in that it is configured to calculate the slope and intercept as the slope and intercept of the rotary shaft. A subject mounting table in which a reference body is arranged at a position not interfering with the subject;
Means for relatively moving the mounting table and the rotating shaft in a direction perpendicular to the rotating shaft;
The data processing means includes
3. The tomosynthesis apparatus according to claim 1, wherein the tomosynthesis apparatus is configured to calculate an inclination and an intercept of the rotation axis from a transmission image of the reference body obtained during one rotation scanning of tomosynthesis. . The tomosynthesis apparatus according to any one of claims 1 to 3, wherein the rotation axis intersects the radiation beam at approximately 90 ° to constitute a cone beam CT .

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