JP4504740B2 - Computed tomography equipment - Google Patents

Description

  The present invention relates to a computed tomography apparatus using a TR system that combines translation and rotation.

  A TR-type computed tomography apparatus (hereinafter referred to as CT) is well-known, for example, “CT Scanner” (Non-patent Document 1) edited by Yoshinori Iwai. FIG. 15 is a conceptual diagram showing a conventional TR system.

  A fan-shaped X-ray beam 102 having a fan angle θ 0 from the X-ray tube 101 is detected by an N-channel X-ray detector 103. θ0 is set to θ0 = 180 ° / K, where K is a natural number.

  First, when the subject 104 on the rotary table 105 is translated (t) and transmission data of the subject 104 is collected at a constant pitch during this time, the transmission data of parallel beams is obtained for each channel. After completion of one translation, the subject 104 is rotated stepwise with respect to the rotation center C by the fan angle θ0 and translated in the reverse direction. By repeating this and completing the K translations, parallel transmission data for 180 ° with respect to the subject reference X-ray path azimuth ψ is obtained at an angle pitch of θ0 / N. Using these data, a cross-sectional image of the subject is usually reconstructed by filtered back projection.

  FIG. 16 is a conceptual diagram showing a conventional TR reconfiguration. The outline of the reconstruction is as follows. First, logarithmically converted parallel transmission data is set to P (ψ, t), and a high-frequency emphasis filter is applied in the t direction, and then backprojected to virtual lattice points along the X-ray beam 102. By doing so, a cross-sectional image is obtained. At this time, it is necessary to perform back projection so that the rotation center projection position tc (hereinafter referred to as the rotation center tc) on the transmission data P (ψ, t) matches the rotation center C.

  On the other hand, apart from the TR method, an RR method CT that scans only by rotation is known. In this RR method CT, a cone beam RR method CT is recently used to collect data on cone beam X-rays using a detector having a two-dimensional resolution and obtain a three-dimensional image (multiple cross-sectional images) by one rotation scan. Is made. This is known, for example, in Japanese Patent Laid-Open No. 2001-330568 (Patent Document 1).

This cone beam reconstruction is usually performed by the method described in Feldkamp, Davis and Kress 1984 (Non-patent Document 2). This method is a kind of filtered back projection method (FBP (Filtered Back Projection) method), and is three-dimensional back projection. One reconstruction by this method is described in detail in US Pat.
JP 2001-330568 PR. Yoshinori Iwai edited by “CT Scanner” Corona Co., Ltd. February 20, 1979 First edition LAFeldkamp, LCDavis and J.W.Kress, Practical cone-beam algorithm, J.Opt.Soc.Am.A / Vol.1, / No.6 / June, P612-619,1984

  However, the above configuration has the following problems.

  In the prior art, there is a problem that a three-dimensional image of a large subject that does not fit in the X-ray beam cannot be obtained by a single scan.

  The present invention has been made for the above-described circumstances, and an object thereof is to provide a computed tomography apparatus capable of obtaining a large three-dimensional image of a subject using cone beam TR type CT.

A computed tomography apparatus according to an embodiment of the present invention includes a radiation source, a radiation detector that detects a radiation beam transmitted through the subject, a rotating unit that provides relative rotation between the subject and the radiation beam, and translation means for providing relative translation in the direction perpendicular to the axis of the rotation to the object and said radiation beam, said parallel TR scan repeating steps rotation by said rotating means and translated by the translation means alternately A computed tomography apparatus that obtains a cross-sectional image of the subject from transmission data of the subject detected at the plurality of parallel movement positions by the radiation detector for each movement , wherein the resolution of the radiation detector is defined The TR scan is performed so that the azimuth overlaps beyond π with respect to the radiation path azimuth ψ of the subject reference along the rotation of the radiation path to be rotated. A scan control means for canceling, and a weight W (1) that gradually shifts from 0 to 1 at the front end of the radiation path azimuth ψ, 1 to 0 at the rear end, and W (ψ) + W (ψ + π ) = 1. reconstructing means for obtaining the cross-sectional image using the transmission data multiplied by ψ) ;
It is characterized by having.

Therefore, according to the present invention, the TR scan is performed so that the azimuth overlaps beyond π with respect to the direction ψ along the rotation of the radiation path defined by the resolution of the radiation detector, and 0 to 1 at the front end of the relative rotation azimuth ψ. In addition, a cross-sectional image is obtained using transmission data obtained by gradually shifting from 1 to 0 at the rear end and multiplied by weight W (ψ) where W (ψ) + W (ψ + π ) = 1. Therefore, it is possible to provide a computed tomography apparatus that obtains a three-dimensional image of a large subject using cone beam TR CT.

  By using the present invention, it is possible to provide a computed tomography apparatus that obtains a three-dimensional image of a large subject using cone-beam TR CT.

  Embodiments of the present invention will be described below with reference to the drawings.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.

  1 and 2 are schematic views showing the configuration according to the first embodiment of the present invention. The X-ray tube 1 and the X-ray detector 3 are arranged to face each other, and the cone-shaped X-ray beam 2 is measured by the X-ray detector 3 having a two-dimensional resolution. The X-ray tube 1 uses a microfocus X-ray tube whose focal point F of the generated X-ray beam 2 is several μm, and the X-ray detector 3 is an X-ray flat panel detector (a scintillator bonded to a two-dimensional semiconductor optical sensor). FPD) is used.

  Further, a data processing unit 19 that processes transmission data from the X-ray detector 3, a display unit 20 that displays processing results and the like, and a mechanism control unit (not shown) that controls the mechanism unit 12 according to commands from the data processing unit 19. ), And an X-ray control unit (not shown) for controlling the X-ray tube 1.

  The data processing unit 19 and the display unit 20 are ordinary computers and store software or the like for reconstructing a three-dimensional image (cross-sectional image). The X-ray detector 3 obtains two-dimensional transmission data I (m, n) through vertical and horizontal channels m and n. The subject 4 is placed on the rotary table 11, and the rotary table 11 is rotated along the imaging surface 14 (of the cross-sectional image) by the mechanism unit 12 and along the imaging surface 14 together with the rotation shaft 13. It is moved across the X-ray beam 2. The mechanism unit 12 can further move the rotary table 11 up and down, and can align the imaging region of the subject with the imaging surface 14.

  In the figure, F is the X-ray focal point, C is the center of rotation, θmax is the spread angle of the X-ray beam 2 to be measured along the imaging surface 14, and B is the scan region. The scan region B is defined as a cylindrical region in which the diameter can be arbitrarily set around the rotation axis 13 and the rotation axis direction is included in the X-ray beam 2. The scan area B is an area where sufficient data that can be reconstructed without difficulty is collected. Further, FCD is the distance from F to C's t movement locus (that is, the minimum distance between F and C), and FDD is the distance from F to the X-ray detector 3.

  Next, the operation in the first embodiment will be described.

  First, the operator places the subject on the rotary table 11 and aligns the imaging region of the subject 4 with the rotation center C. Subsequently, the diameter of the scan area B is input, for example, in mm. When the scan is started, the data processing unit 19 controls the TR scan. In this scan, similarly to the known TR scan, t movement and step rotation of θ0 with respect to the rotating shaft 13 are repeated. Here, for all t movements, the diameter of the scan region B set arbitrarily is set to the X-ray beam 2. Is completely traversed, and the scan is completed after K times of t movement.

The data processing unit 19 collects transmission data of the subject 104 that is the output of the detector 3 at a constant pitch during the t movement. Here, the step rotation angle θ0 and the translation count K are given in advance by the equation:
θmax = 2 ・ atan {N / 2 ・ △ ξ / FDD} (1)
θ0 = θmax−2 · β, (θ0> 0) (β ≧ 0) (2)
(K-1) · θ0 + θmax = π + 2 · α, (α> O) (3)
Set to be. Here, N is the number of channels in the n direction, and Δξ is the channel pitch.

  If the subject reference X-ray path orientation along rotation is ψ, data in the range of (K−1) · θ0 + θmax is collected for ψ in this scan.

Next, duplicate data of 2 · α is collected at both ends of the data range of ψ, and duplicate data of 2 · β is collected between translations. ψ is determined by the channel n and the translation number k, (1 to k). ψ is, for example, an equation:
θ = atan {(n−nc) · Δξ / FDD} (4)
ψ = θ + (k−1) · θO + θmax / 2 (5)
Determined by. Here, nc is the center channel. θ corresponds to the arrangement angle of the channel n in the X-ray beam. In the first embodiment, there is no overlap between translations (β = O).

  Next, the data processing unit 19 reconstructs a three-dimensional image of the scan area B from the transmission data.

  3 and 4 are schematic views showing cone beam TR reconstruction. xyz is a coordinate fixed to the subject, and the rotation axis 13 is the z-axis. An outline of reconstruction will be described with reference to the drawings.

  If the vertical and horizontal positions of the X-ray detector 3 are m, n, the t movement position is t, and the translation number is k, the transmission data can be described by m, n, t, k. First, the detected intensity I (m, n, t, k), which is transmission data proportional to the X-ray intensity, is logarithmically converted into projection data P (m, n, t, k) (S1). Next, a high frequency enhancement filter is applied to the projection data P in the t direction (S2). This filter is the same | ω | filter as normal CT.

  Next, a weight W (ψ) is multiplied to the projection data P (S3). Here, ψ is determined by n and k. As shown in FIG. 5, W (ψ) is a weight having an inclined portion that gradually changes linearly from 0 to 1 in the overlapping section 2 · α (or lower) between the front end and the rear end. This weight may be smoothly (gradually) shifted from 0 to 1 at the front end of ψ, and from 1 to 0 at the rear end, and W (ψ) + W (ψ + π) = 1. A curved line such as W ′ (ψ) may be used. The weighting may be performed before the filtering.

  By the weighting described above, artifacts due to data seams at both ends of the data range can be reduced, and the cone beam TR reconstruction can be completed.

  Next, after weighting as described above, the subject is directed toward the focal point F with P (m, t) described by the vertical position m and t moving position of the X-ray detector 3 as a set of plane data. 3D backprojection is performed on a virtual 3D lattice representing 4 (S4) (FIG. 4).

  Since the focus F at this time moves by t (focal point F locus 33), when viewed from the direction of the rotation axis 13, the back projection direction is parallel and forms an angle ψ with respect to the y axis. Back projection is to add the data of the corresponding point Gp on the mt plane 34 to all the lattice points G. Since Gp generally does not coincide with the data point, interpolation calculation is performed (by using four neighboring points). When this back projection is performed for all n, k (π + 2 · α of ψ), a three-dimensional image of the scan region B of the subject 4 is obtained.

  According to the above, the data seams at both ends of the data range are smoothly connected, and artifacts due to the seams are reduced, thereby enabling cone beam TR reconstruction.

  Next, FIGS. 6 and 7 are schematic views showing a state in which the X-ray paths do not coincide at the data seam in the cone beam TR scan.

  The above-described back projection can be reconstructed if it is performed within the range of π (see FIG. 6). However, when the data of π is simply back projected, except for on the imaging surface 14 in the case of cone beam TR scan. In principle, the reverse paths do not match at the data seam, so that there is a discrepancy in the transmission data and a step is generated. This creates a linear artifact.

  In the first embodiment of the present invention, by applying the weight W (ψ), the seam is smoothly connected and the step is eliminated, so that the cone beam TR reconstruction can be completed. In addition to the above-mentioned principle seam step, there is a step caused by imperfect measurement. That is, this step is also caused by the fact that data P (ψ, t) and P (ψ + π, t) collected at different times and states are adjacent to each other at this joint. Specifically, the difference in detector channel n, characteristic drift state, t movement state (forward / return), mechanism error, X-ray drift, subject movement (including mounting play), scattering The effect of X-rays causes a step. By performing the above-described weighting, the level difference caused by these measurements can be eliminated at the same time.

  In addition, since a two-dimensional detector is used as in the conventional cone beam RR type CT, not only a three-dimensional image but also a transmission image can be easily obtained.

  Next, as a modification of the first embodiment, the following configuration is also possible. The cone beam TR reconstruction can be used as it is for a normal fan beam TR scan. That is, only the data on the imaging surface 14 is collected, and only the imaging surface 14 may be reconstructed in the same manner. Thereby, even in a normal TR scan, a step caused by imperfect measurement can be eliminated by weighting, and an image can be improved in quality.

  Although omitted in the description of the TR scan reconstruction of the first embodiment described above, normally, projection data P (m, n, t, k) is a number that is bundled or thinned out in the n direction before back projection. Squeeze out. By doing so, the number of back projections for ψ can be reduced, and reconstruction can be performed at high speed.

  Here, the reason for bundling in the n direction before back projection is that if there is no bundling or thinning, the number of directions in ψ will be excessive in the TR system. Further, when the setting of the reconstruction three-dimensional lattice pitch is made coarse, the directions of m, n, and t may be bundled or thinned out accordingly. Bundling and thinning may be performed at any stage before back projection. Further, bundling and thinning may be performed in the X-ray detector 3.

  In the first embodiment, the TR scan is a half scan for obtaining a three-dimensional image from transmission data in the π + 2 · α direction. However, a full scan of 2 · π + 2 · α or a double full scan of 4 · π + 2 · α is used. It can be easily understood that the above may be used. (These scans are known in Japanese Patent Application Laid-Open No. 11-108857.) That is, in this case, the back projection may be performed after applying the same weight at 2 · α of the front end and the rear end of the data range. . In the case of double full scan, after weighting, P (m, t) that is a natural number multiple of 2 · π is averaged to obtain P (m, t) of 2 · π, and then backprojected The same is true.

  In the case of the above-described full scan or double full scan, there is no principle seam level difference, but the level difference caused by imperfect measurement can be eliminated by weighting, and the image can be of high quality. It can be easily understood that i is an arbitrary natural number and can be similarly reconstructed by scanning i · π + 2 · α.

  Further, in the first embodiment, it is possible to scan a plurality of times while changing the ascending / descending position of the subject and connect the obtained three-dimensional images in the z direction to obtain a wide three-dimensional image in the z direction. In this case, the three-dimensional images can be overlapped and the position error can be adjusted by fitting, or the overlapping portions can be averaged with weights to create a three-dimensional image that is smoothly connected.

(Second Embodiment)
The configuration of the second embodiment is the same as that of the first embodiment shown in FIG.

  Next, in the second embodiment, first, scanning is performed in the same manner as in the first embodiment, but β> 0 is set, and duplicate data of 2 · β is collected between translations.

The reconstruction in the second embodiment is obtained by adding a smoothing process for a translation joint to the reconstruction in the first embodiment shown in FIGS. 3 and 4, and this smoothing is performed on the projection data P (ψ, m, t). This is performed by multiplying the weight for each translation rate and multiplying by W _k (ψ). The weighting may be performed before or after the weighting W (ψ) (S3) or before the filtering (S2).

8, 9, and 10 show projection data P and weight W _k (ψ) for each translation rate. Here, k is a translation number. W _k (ψ) is a weight that linearly changes from 1 to 0 between overlapping portions of translation and between 2 and β (or less) (see FIG. 9). This weight W _k (ψ) should be smoothly (gradually) shifted from 1 to 0 at the overlapping portion, and W _k (ψ) + W _{k + 1} (ψ) = 1, and the weight W shown in FIG. _A curved line such as _k ′ (ψ) may be used.

  According to the above, in addition to the effects of the first embodiment, three-dimensional backprojection is performed after smoothing the step in the ψ direction generated at the overlapping portion (translation seam) of the projection data P (ψ, m, t). Artifacts due to translation seams can be reduced. That is, this step is caused by the fact that data collected at different times and states are adjacent at this seam. Specifically, the difference in detector channel n, characteristic drift state, t movement state (forward / return), mechanism error, X-ray drift, subject movement (including mounting play), scattering The effect of X-rays causes a step.

Next, as a modification of the second embodiment, the following configuration is also possible. After multiplying the weight W _k (ψ) of the second embodiment described above, back projection may be performed after data combination. In the data combination, the data of overlapping portions are added to form one data P (ψ, m, t), so that the back projection process can be performed only once. Moreover, the smoothing process of 2nd Embodiment is not restricted to this.

  FIGS. 11 to 13 are schematic diagrams illustrating another embodiment of the smoothing process.

A step ΔP in the projection data P at the joint between the translation k and k + 1
ΔP = Pa−Pb (6)
Suppose there was.

In this case, the function λ (ψ) is
λ (ψ): {Function where λ (0) = 1 and smoothly decreases with increasing ψ to become O (converge)},
For example,
λ (ψ) = exp (−ψ / Δψ), (Δψ: constant)
Etc., as a correction function,
λa (ψ) = − ΔP / 2 · λ (ψa−ψ), (ψ ≦ ψa) (7)
λb (ψ) = ΔP / 2 · λ (ψ−ψa), (ψ ≧ ψa) (8)
Is added to the projection data P. Here, ψa is the ψ of the joint, and ΔP has different values depending on m, t, and k.

  The seam can be smoothed by this correction function addition. Note that when ΔP is obtained, if it is obtained after applying a high-cut filter in the t direction or the m direction, smoothing can be performed while removing the influence of noise. Further, the smoothing processing may be performed at any stage before logarithmic conversion (S1) or after weight W (ψ) multiplication (S3) in the flowchart of FIG.

  According to the above, the step ΔP can be obtained from the duplicated data, the translation joint can be smoothed so as to eliminate this ΔP, and artifacts can be reduced. Furthermore, when this smoothing is employed, there is an advantage that β can be reduced. That is, smoothing is possible if there is even one overlap. In the second embodiment, the same modifications as in the first embodiment are possible.

(Third embodiment)
The configuration of the third embodiment is the same as the configuration of the first embodiment shown in FIG. 1 except that an FCD and FDD variable mechanism is added to the mechanism unit 12.

  The FCD and FDD can be changed via a mechanism control unit (not shown) by inputting a change instruction to the data processing unit 19.

  First, the operator places the subject on the rotary table 11 and sets FCD and FDD according to the size of the subject. As a method of selecting FCD and FDD, increasing the enlargement ratio (FDD / FCD) is preferable because the resolution is improved, but the thickness of the scan region B is reduced.

  In addition, when the magnification is the same, an image with less noise can be obtained by reducing the FCD as much as possible, but interference between the subject and the mechanism occurs. Actually, considering this, the geometric setting is performed according to the subject and the examination purpose.

  Next, the imaging region of the subject 4 is aligned with the rotation center C, and the diameter of the scan region B is input, for example, in mm. When scanning is started, the data processing unit 19 controls TR scanning. The data processing unit 19 obtains θmax using equation (1), and automatically determines α, β, θ0, and K so as to satisfy equations (2) and (3).

  For example, α, β, θ0, and K can be determined by the following procedure.

  First, with βmin as a constant, θ0 is obtained by θO = θmax−2 · βmin, and this is rounded down to determine θO.

  Next, with αmin as a constant, K, which is the smallest natural number that satisfies (K−1) · θ0 + θmax> π + 2 · αmin, is determined.

  Further, α and β are obtained using equations (2) and (3).

  Next, scanning and reconstruction are performed in the same manner as in the first or second embodiment to obtain a three-dimensional image.

  As described above, in addition to the effects of the first embodiment and the second embodiment, FCD, FDD, etc. can be made variable, so that optimum geometric conditions such as an enlargement ratio can be set according to the subject, and high quality 3D images can be obtained. Further, since the step rotation angle θO and the t movement count K are automatically set in accordance with the change in θmax when the FDD is changed, the FDD can be easily changed.

  As a modification of the third embodiment, when α, β, θ0, and K are determined in the second embodiment, constants αmin and βmin are set, for example (not given by angles), for the channel of the X-ray detector. You may make it determine on the basis of a number. For example, αmin and βmin may be determined by 8 percent, 3 percent position, etc. of the total number of channels in the n direction, respectively. Further, when determining α, β, θ0, K, it is not uniquely determined but is arbitrary. In other words, there is an arbitrary manner in how the surplus angle {(K-1) · θO + θmax−π} generated by K being an integer is distributed to α and β, and the deformation can be performed innumerably. .

  In the embodiment described above, a microfocus X-ray tube is used as the X-ray tube 1, but the cone beam TR type CT of the present invention is not limited to this, and other X-ray tubes may be used. Furthermore, in the above-described embodiment, X-rays are used as radiation. However, the cone beam TR CT of the present invention is not limited to this, and it is obvious that other transmissive radiation can be established.

  Furthermore, in the present invention, an X-ray flat panel detector is used as the two-dimensional X-ray detector, but any two-dimensional detector may be used. In addition, the present invention can be applied to other mechanism systems as long as the mechanism operation is relatively equivalent. For example, although the rotation is performed on the subject side, the X-ray tube and the X-ray detector may be rotated integrally. Similarly, in the t movement, the X-ray tube and the X-ray detector may be moved together.

  For example, FIG. 14 shows one form using different mechanisms. The rotation mechanism 56 is supported from the floor 50 by a support portion (not shown). There is a t-movement mechanism 57 on this rotation, an x-shift mechanism 58 on the t-movement, and an X-ray tube 51 and an X-ray detector 53 are arranged on the x-shift. t is the direction perpendicular to the page. The subject 54 is supported from the floor 50 by the XYZ mechanism 55 and can move in three directions with respect to the X-ray beam 52. Although details are not described here, a three-dimensional image can be obtained with the same operation as in the first embodiment even in such a mechanism configuration.

  Further, the CT according to the present invention can enable the CT of the cone beam TR type regardless of its application.

The schematic diagram (plan view) which showed the structure which concerns on 1st Embodiment of this invention. The schematic diagram (front view) which showed the structure which concerns on 1st Embodiment of this invention. The schematic diagram which showed cone beam TR reconstruction which concerns on 1st Embodiment of this invention. The schematic diagram which showed cone beam TR reconstruction which concerns on 1st Embodiment of this invention. The schematic diagram which showed weight W ((psi)) which concerns on 1st Embodiment of this invention. FIG. 3 is a schematic diagram (plan view) showing a state in which X-ray paths do not match at the data seam according to the first embodiment of the present invention. The schematic diagram (front view) which shows the state in which an X-ray path does not correspond in the data seam which concerns on 1st Embodiment of this invention. The schematic diagram which showed the projection data P for every translation which concerns on 2nd Embodiment of this invention. The schematic diagram which showed weight _Wk ((psi)) for every translation which concerns on 2nd Embodiment of this invention. The schematic diagram which showed weight _Wk '((psi)) for every translation which concerns on 2nd Embodiment of this invention. The schematic diagram which showed another form of the smoothing process which concerns on the modification of 2nd Embodiment of this invention. The schematic diagram which showed another form of the smoothing process which concerns on the modification of 2nd Embodiment of this invention. The schematic diagram which showed another form of the smoothing process which concerns on the modification of 2nd Embodiment of this invention. The schematic diagram which showed one form using the different mechanism which concerns on embodiment of this invention. The conceptual diagram which showed the conventional TR system. The conceptual diagram which showed the conventional TR system reconstruction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 2 ... X-ray beam, 3 ... X-ray detector, 4 ... Subject, 11 ... Rotary table, 12 ... Mechanism part, 13 ... Rotary axis, 14 ... Imaging surface, 19 ... Data processing part, 20 ... Display section

Claims (5)

A radiation source, a radiation detector that detects a radiation beam that has passed through the subject, a rotation means that provides relative rotation between the subject and the radiation beam, and an axis of rotation of the subject and the radiation beam. and translation means for providing relative translation direction perpendicular to each of the parallel movement of the TR scan repeating steps rotation by said rotating means and translated by the translation means alternately a plurality of said parallel by said radiation detector A computed tomography apparatus for obtaining a cross-sectional image of the subject from transmission data of the subject detected at a moving position,
Scan control means for performing TR scan so that the azimuth overlaps beyond π with respect to the radiation path azimuth ψ of the subject reference along the rotation by the rotation means of the radiation path defined by the resolution of the radiation detector;
The transmission data which is gradually shifted from 0 to 1 at the front end of the radiation path azimuth ψ and from 1 to 0 at the rear end and is multiplied by a weight W (ψ) where W (ψ) + W (ψ + π ) = 1. Reconstructing means for obtaining the cross-sectional image using
A computer tomography apparatus characterized by comprising: The computed tomography apparatus according to claim 1,
The reconstruction means smoothes a step in the radiation path azimuth direction of the transmission data at the joint between the translations generated with respect to the radiation path azimuth ψ for each translation , and obtains the cross-sectional image. A computed tomography apparatus. The computed tomography apparatus according to claim 1 or 2,
The radiation beam has a cone shape, the radiation detector has a two-dimensional resolution, and the reconstruction means sets the rotational axial position of the radiation path defined by the two-dimensional resolution as m and the parallel movement position as t. When the transmission data is described using the radiation path azimuth ψ, the axial position m of the rotation, and the translation position t, a high frequency enhancement filter is applied to the translation position t direction, and then the axis of rotation A computer tomography characterized in that three-dimensional backprojection is performed on a virtual three-dimensional lattice representing a subject toward the focal point of the radiation beam as a set of plane data described by a directional position m and the translation position t. Shooting device. The computed tomography apparatus according to any one of claims 1 to 3,
A computed tomography apparatus further comprising a minimum distance changing unit that changes a minimum distance between the focal point of the radiation beam and the rotation axis in accordance with a size of a subject. The computed tomography apparatus according to any one of claims 1 to 4,
Distance changing means for changing a distance between the focal point of the radiation beam and the radiation detector;
A computed tomography apparatus further comprising: a scan control unit that automatically changes an angle of the step rotation according to a distance changed by the distance changing unit.

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