JPH105210A - X-ray CT apparatus and its misalignment correction method - Google Patents

Abstract

(57) [Summary] Cone beam or multi-slice X-ray CT
In the apparatus, the focal point shift of the X-ray source is corrected to obtain a reconstructed image without artifacts. SOLUTION: Reconstruction parameters 31a, 31b, ... and / or beam position information 33a, 33b corresponding to a plurality of focal positions (a, b, ...) of an X-ray tube 3 relative to a main X-ray detector 7 are provided. ,... Are stored in the storage device 17 in advance. Before or during the scan, the data processing device 15 converts the X-rays to the main X-ray detector 7 from the profile detection beam detected by the profile detector 9.
The relative focus position of the ray tube 3 is calculated. Reconstruction parameters 31 and / or beam position information 33 corresponding to the focal position closest to the detected and calculated relative focal position
Is read from the storage device 17 and is subjected to reconfiguration by the reconfiguration device 25.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus used for imaging a tomographic plane of a subject or a method of correcting misalignment thereof, and more particularly, to a direction around a rotation axis of a system (a channel direction) and a method thereof. The present invention relates to a multi-slice CT apparatus or a cone-beam CT apparatus in which X-ray detector elements are arranged two-dimensionally with respect to a direction of a rotation axis (slice direction) perpendicular to the above, and a misalignment correction method thereof.

[0002]

2. Description of the Related Art A fan beam X-ray CT apparatus is conventionally known. This means that a fan (fan) -shaped X-ray beam is emitted from an X-ray source, and 100 X-rays are arranged in a line around a rotation axis.
This is a CT apparatus that collects data of X-rays that have passed through a subject with an X-ray detector of about 0 channel. Normally, data is collected about 1000 times per rotation while rotating around the subject (1
Data collection is referred to as one view), and an image is reconstructed based on the data.

[0003] As a reconstruction method in such a fan beam X-ray CT apparatus, the fan beam projection data is converted into parallel beam projection data by rearranging and interpolating the fan beam projection data. A fan-para conversion method of backprojecting parallel beam projection data to obtain a desired tomographic image, and backprojecting fan beam projection data once to a reference axis (centering axis) and backprojecting again for each pixel row of a reconstructed image Two types of methods have been devised and adopted.

[0004] In any of the methods, high-speed processing is performed using dedicated hardware. However, these reconstruction methods naturally have parameters in the slice thickness direction (the body axis direction of the subject). Instead, the reconstruction is completed by processing in each plane.

A helical scan method is known as a method for shortening the data collection time. The basic form of the fan beam X-ray CT apparatus is that an X-ray
The principle was that the line focus was rotated, and the subject was sent in the body axis direction by the slice thickness every rotation, but recently, there has been a demand for scanning a wider range of the subject in the body axis direction in a short time. In contrast, a helical scanning method in which the subject is scanned while relatively moving the subject in the body axis direction (the X-ray focal point draws a spiral trajectory instead of a circular orbit with respect to the subject) was devised and put into practical use. ing.

In the helical scan method, since the position of the focal point in the body axis direction shifts for each focal angle, a strong artifact (false image) occurs if the fan beam is reconstructed as it is. As a countermeasure, a method is used in which fan data is reconstructed by interpolating from two data of the equivalent rotational phase sandwiching the reconstruction plane, converting and creating projection data of the corresponding reconstruction plane of the rotational phase. ing. The interpolation method includes a 360 ° interpolation method using projection data for two rotations across the reconstruction plane, and a reconstruction method using projection data for one rotation across the reconstruction plane and using the opposite beam. Two types of counter beam interpolation method (180 ° interpolation method) for converting and creating the projection data of a plane have been devised and used properly according to the purpose.

In the reconstruction method in the helical scan, the parameters in the body axis direction are used in the interpolation of the projection beam. However, in the back projection, the same as the basic form of the fan beam X-ray CT apparatus, the slice thickness direction (coverage) is used. It does not have parameters for the direction of the body axis of the specimen), and the processing is performed in the same plane.

In general, in the conventional single-slice CT apparatus and the fan beam CT apparatus, the X-ray tube alignment adjustment in the channel direction is performed by measuring the position of the focal point in the channel direction and making the tube coincide with the designed position. A fine adjustment mechanism that moves the ball is used. However, a positional error (misalignment) of about several tens of microns ultimately remains due to a limit of mechanically moving the tube and a limit of processing and assembly accuracy of the tube. In such a state,
Because the design X-ray path is different from the actual X-ray path,
There is a problem that artifacts occur in the reconstruction (especially in the high resolution mode) as it is. Therefore, the reconstruction parameters are corrected using the misalignment amount finally left due to the limit at the time of alignment adjustment, and the actual X
Correction has been made to match the line path. Next, focus movement in a conventional fan beam X-ray CT apparatus and its countermeasures will be described. Due to the principle of the fan beam X-ray CT system,
It is assumed that the focal point is rotating in the same plane, and the position of the fan beam incident on the X-ray detector needs to be stable. Even in the helical scan method, although a spiral trajectory is relatively formed with respect to the subject, it is actually a combination of the rotation of the focal point and the linear movement of the subject in the body axis direction.

However, in practice, due to the limitation of the position adjustment (alignment) of the X-ray tube, the X-ray detector, and the like, the deviation of the focal position (focal point) with respect to the center plane of the X-ray detector and the optical system (such as a pre-collimator). It is known that the focal position moves in the slice thickness direction depending on the tube structure, such as the misalignment of the target cannot be reduced to 0, or the thermal expansion due to the temperature rise of the target when X-rays are generated.

[0010] The deviation of the focal position can be suppressed to 0.1 mm or less, and the influence of the current X-ray CT apparatus is within an allowable range and does not cause a serious problem.

On the other hand, the focus movement is a problem that cannot be ignored, and the focus movement of a rotating anode type X-ray tube as shown in FIG. 16 will be described as an example. Usually, the X-ray tube accelerates and focuses the thermoelectrons emitted from the filament (cathode) 101 by a high voltage applied between the cathode and the anode, and collides the high-speed electrons with the target (anode) 103. As a result, X-rays are generated.

The conversion efficiency from electric energy to X-ray energy depends on the acceleration voltage and the atomic number (Z) of the target material.
For example, if the tube voltage is 100 kV and the target material is tungsten (Z = 74), the X-ray conversion efficiency is only 0.74%, and the remaining 99% or more is
Converted to heat.

For this reason, the target is made of a material having a high melting point and a high atomic number, but a rotating anode structure is employed in order to effectively reduce the heat generation per unit area of the target. That is, target 1
03 is formed in a disk shape, and a rotor 111 is fixed to a rotating shaft 105 connected to the center thereof. The rotor 111 is driven to rotate by a stator (not shown) that generates a rotating magnetic field from outside the glass bulb 113.

In such a rotating anode structure, since it is difficult to directly cool the anode, most of the heat generated in the target 103 is emitted from the target 103 to the surroundings as radiant energy. Is released through the rotating shaft 105 by heat conduction. When the X-ray irradiation from the rotating anode X-ray tube is repeated, the amount of heat accumulated in the target 103 in the X-ray tube increases, and the temperature of the rotating shaft 105 and the like increases. As the temperature rises, the rotary shaft 105 and the like undergo thermal expansion, and the X-ray focus shifts to the design focus position 121.
It moves in the slice direction as shown in a position 123 (open circle) where the focal point has moved from (black circle) (in FIG. 16, the thermal expansion is exaggerated).

In the X-ray CT apparatus, the X-ray beam shape is adjusted by an optical system such as the slit 115, so that when the focal point shifts, the beam position incident on the X-ray detector (not shown) changes from a solid arrow to a broken line. Cause fluctuation as shown by the arrow. When the beam position changes, the output data of the X-ray detector fluctuates due to the sensitivity distribution (slice thickness direction) on the X-ray detector side. If the sensitivity distribution varies for each element of the X-ray detector, finally It causes artifacts on the image.

As a countermeasure against the artifact caused by the focal point movement, a method of reducing the focal point movement, a method of keeping the position of the beam incident on the X-ray detector even after the focal point movement, and a method of reducing the variation in sensitivity distribution of the X-ray detector. In addition to the above, a method of correcting a change in sensitivity of the X-ray detector has been devised.

The details of these correction methods are described in JP-A-6-169914 (published on June 21, 1994).
Japanese Patent Application No. 4-325159, filing date December 1992
4) and JP-A-6-269443 (publication date: September 27, 1994, Japanese Patent Application No. 5-57645, application date: March 18, 1993). This correction method divides the focus movement range into several parts, collects and stores X-ray detector sensitivity correction data in advance for each small range, and stores X-ray detector sensitivity correction data corresponding to the focus position at the time of data collection of the subject. This is a method of correcting the X-ray detector sensitivity using the line detector sensitivity correction data.

An X-ray radiating a cone-shaped X-ray beam
An X-ray detector element (M channels × N) is arranged on a cylindrical surface such that an N-ray source and a plurality of fan beam X-ray detector rows are stacked in the Z-axis (rotation axis, body axis of the subject) direction. 2. Description of the Related Art A cone beam (multi-slice) X-ray CT apparatus including a two-dimensional X-ray detector in which rows are arranged is known.

A typical reconstruction (Feldkamp reconstruction) method in this cone beam X-ray CT apparatus is disclosed in the following literature. “Practical cone-beam algorithm”
LA Feldkamp, LC Davis, and JWKress, J. Opt. S
oc. Am. A / Vol.1, No.6, pp.612-619 / June 1984. This reconstruction method is a fan-beam (in a two-dimensional plane) reconstruction algorithm that is a mathematically exact reconstruction method. [Filtered-Back
projection (filtered backprojection method)] in the Z-axis direction, and is an approximate three-dimensional reconstruction algorithm obtained for a conventional scan (static scan) using a cone beam.
It comprises the following steps (a) to (c).

(A) Projection data weighting step The projection data is multiplied by a term dependent on the Z coordinate and a cos term.

(B) Convolution calculation step The convolution calculation of the data of step (a) and the same reconstruction function as the fan beam is performed.

(C) BackProjection (backprojection) step The data of step (b) is backprojected onto the path where the X-rays have passed (from the focal point to the channel of the X-ray detector).
That is, from the focal point, a straight line passing through the voxel to be backprojected is X
Calculate the point of intersection with the line detector plane, it creates data to be backprojected from the data of step (b) around that point interpolation or the like, back projection by weighting it in FCD / FVD ^2. Backprojection is performed over 360 °.

Next, the fact that the fan beam reconstruction method is very similar to the cone beam reconstruction method (Feldkamp reconstruction method), but the back projection method is significantly different will be described. 2
In the three-dimensional fan beam reconstruction, back projection is performed from the data of the one-dimensionally arranged X-ray detectors for all the pixels (pixels) in the reconstruction plane, while the cone beam (F
eldkamp) In reconstruction, a point at which a straight line connecting the focal point and the voxel to be reconstructed intersects the two-dimensional X-ray detector plane is determined, and the data of the X-ray detector located at this intersection is converted into a voxel on the straight line. Backproject.

Therefore, when reconstructing a certain plane, data of a specific X-ray detector channel of a specific X-ray detector row is back-projected to only some voxels of the reconstruction plane. Since it is necessary to select data (X-ray detector row and X-ray detector channel) to be back-projected for each voxel, a three-dimensional surface of the X-ray detector and a straight line focusing on the reconstructed voxel must be selected. Positional relationship becomes important. In addition, when considering an X-ray detector array having the same Z coordinate and a straight line focusing on the X-ray detector element, the voxel is similar to the X-ray detector surface centered on the focal point (the cylindrical detector). Concentric circles)
Due to the arrangement on the top, the calculation of this positional relationship becomes very complicated.

Next, a method for reconstructing a helical scan in a cone beam X-ray CT apparatus (Feldkamp-He
lic.) will be described. Spiral helical scan around the subject using a two-dimensional planar detector
A method of performing reconstruction by a three-dimensional reconstruction method to which the ldkamp reconstruction method is applied is disclosed in the following document.

(A) "3D helical scan CT using cone beam projection", Hiroyuki Kudo, Tohoku University, Tsuneo Saito, University of Tsukuba, Transactions of IEICE DII Vol. J74-D
-II, No. 8, pp. 1108-1114, August 1991.

(B) “Cone beam CT”, Japanese Patent Application No. Hei 7-
No. 169963 (filed on July 5, 1995).

Next, a description will be given of a fan beam assumed multi-slice reconstruction in a cone beam X-ray CT apparatus. Although cone beam data is collected, various methods have been proposed for performing fan beam reconstruction by regarding cone beam data as fan beam data without performing cone beam reconstruction.

In particular, an interpolation method in helical scanning is disclosed in Japanese Patent Application No. Hei 6-212046 (filing date: September 5, 1994).
Date, priority date September 6, 1993, Japanese Patent Application No. 5-220918
And Japanese Patent Application No. 6-93296) and Japanese Patent Application No. 7-33.
No. 7123 (filing date: December 25, 95).

The method described in Japanese Patent Application No. 7-337123 is
This is referred to as a filter interpolation method, and includes a procedure 1 for obtaining a plurality of pieces of interpolation data in which the slice position is slightly shifted with respect to an arbitrary phase, and a procedure 2 for obtaining the data of a target phase by weighting and adding the interpolation data. You. The weighted addition of the data interpolated by shifting the slice position slightly increases the execution slice thickness, but the effect of switching the X-ray detector row is reduced in inverse proportion to the number of interpolation data.
Reduces artifacts and improves image quality.

Next, a cone beam (multi-slice) X
A description will be given of a dose change accompanying a focus shift in the X-ray CT apparatus and its countermeasures. Cone beam (multi-slice) X
In the X-ray CT apparatus as well, it is expected that the displacement of the focal position and the phenomenon of the focal point shift will occur similarly to the fan beam X-ray CT apparatus.

The countermeasures described in the section of the fan beam X-ray CT apparatus including the correction method can be considered for the focal point movement. This correction method aims at correcting “focus movement → beam position fluctuation → occurrence of artifacts due to variation in sensitivity distribution of the X-ray detector”. However, in a cone beam (multi-slice) X-ray CT apparatus, When the beam position fluctuates, there is a problem that the output of the detection element in each column changes even if the sensitivity distribution in the slice direction of the X-ray detector does not fluctuate. The details of the correction method as a countermeasure for this are disclosed in Japanese Patent Application No. 6-302405 (filing date: December 6, 1994).
This is a method in which a profile detector that monitors the dose for each X-ray detector row is provided and the dose change for each main detector in each row is corrected.

As described above, regarding the accuracy of the focal position, in the fan beam X-ray CT apparatus, there are misalignment of the focal point (not a temporal change but an adjustment deviation) and a temporal movement due to heat. There is no problem because the deviation can be suppressed to an acceptable level, and the latter is solved by taking some measures including correction. On the other hand, cone beam (multi-slice) X-ray C
It has been described that also in the T apparatus, there is already a method of correcting the output fluctuation of the X-ray detector with respect to the temporal focus movement.

Further, in a multi-slice CT system, a data processing method called a segment bundling method is known. This method is a method in which data collected by a plurality of detection elements arranged in the slice thickness direction is added, or bundled by interpolation or the like to obtain data corresponding to a thick slice and then processed.
No. 7, JP-A-4-224736 and the like.

Further, as for the size in the slice direction of the detection elements arranged in the slice thickness direction, not only those having the same size are arranged but also, for example, Japanese Patent Application Laid-Open No. 6-16991.
As disclosed in Japanese Patent Laid-Open No. 2 (1999) -2002, an arrangement in which elements of different sizes are arranged has been proposed.

[0036]

However, in the conventional single-slice CT apparatus and multi-slice CT apparatus, the purpose of correcting misalignment in the channel direction is to correct the misalignment remaining at the time of tube alignment, such as thermal expansion. It does not assume misalignment that changes with time, such as focus shift due to the above, or misalignment due to an assembly error of each detection element, and has a problem that artifacts occur due to these.

Further, a cone beam (multi-slice) X
The line CT apparatus has the following problems related to the focal position.

First, X-ray CT for cone beam reconstruction
In the apparatus, the projection data back-projected to one pixel (voxel) in a certain reconstruction plane needs to be projection data of a point where a straight line connecting the focal point and the voxel intersects the two-dimensional X-ray detector. It is. By design, a plane that passes through the center of the slice direction of the two-dimensional X-ray detector and is orthogonal to the rotation axis is defined as a mid-plane (Mid-Plane), and it is assumed that a focus is on that cross-section. If the focal position is deviated from the center cross section for some reason (this is relatively the same in terms of reconstruction even if the X-ray detector is attached to the designed position. The case where the focus is deviated based on the line detector will be described.)

The output data at the point on the two-dimensional X-ray detector assumed from the design X-ray path (whether obtained directly or
Alternatively, the output data at the point obtained by interpolating the output data of the detection elements around the point) is actually projection data of a completely different X-ray path. That is, considering a target reconstruction plane, data to be back-projected to another voxel may be projected back to another voxel. If the projection data is back-projected to a completely different position, it can be easily estimated that it will cause artifacts.

This problem occurs not only when the focus is moved but also when the X-ray detector is attached to the system, such as a misalignment (misalignment parallel to the pre-collimator), which is constant in the channel direction. In addition, since this problem is different from the variation in the sensitivity distribution in the slice direction of the X-ray detector and the change in output due to the fluctuation of the beam on the X-ray detector, there is no effect of the correction method shown in the prior art.

Also in a multi-slice X-ray CT apparatus for performing fan beam deemed reconstruction, if the focal point moves in the slice thickness direction, the Z coordinate of the collected data deviates from the design value. There arises a problem that the position of the image is displaced and an error occurs in filter interpolation in the helical mode described above.

The above is a problem associated with a relative positional shift between the focal point and the X-ray detector due to misalignment when mounting the entire X-ray detector, misalignment of the position, and movement over time. , And, apart from these,
Problems also exist within the X-ray detector. Due to the structure of the X-ray detector, assembling all elements integrally has a significant problem in terms of cost and the like, and it is common to assemble by dividing into several blocks having a plurality of elements.

When the X-ray detectors are finally arranged, it is inevitable that the alignment in the slice direction will be out of order for each block or each channel. Or different for each channel. Focus shift, X
Even if there is no misalignment in mounting the line detector, the Z coordinate of the collected data deviates from the design value. This problem also occurs not only inside the X-ray detector but also in misalignment in the slice direction, which changes in the channel direction, such as twisting with the pre-collimator when the X-ray detector is attached to the system.

In a multi-slice CT system, in an actual system, not only the misalignment in the slice direction but also
Misalignment in the channel direction may also remain.

For this reason, when performing the data processing of the segment bundling method, it is expected that the misalignment amount in the channel direction of each of a plurality of detection elements that supply the original data to be bundled is arranged in the slice thickness direction. There is a new problem that the amount of misalignment of the bundled data differs depending on the position of the original data.

In view of the above problems, it is an object of the present invention to provide a cone beam CT apparatus or a multi-slice CT apparatus, in which misalignment or X-ray detection occurs between channels or blocks in the assembly of an X-ray detector. Eliminating artifacts that occur in principle during image reconstruction due to misalignment in the slice direction that changes in the channel direction caused by misalignment (torsion of the entire X-ray detector with respect to the precollimator) when the entire device is mounted on the system. To obtain a good reconstructed image.

It is another object of the present invention to provide a system for mounting an entire X-ray detector on a system (misalignment parallel to a pre-collimator) or misalignment of a focus (temporal position fluctuation (focal movement) and position adjustment). The objective of the present invention is to obtain a good reconstructed image by removing artifacts that occur in principle at the time of image reconstruction due to misalignment due to the accuracy limit of the image.

[0048]

In order to solve the above problems, the present invention has the following arrangement. That is, the invention according to claim 1 includes an X-ray source that irradiates an X-ray beam toward a subject, and a plurality of channels each of which is opposed to each other across a space where the X-ray source and the subject are arranged. Comprising an X-ray detector having at least a reconstruction parameter and / or beam position information about information obtained by scanning at least the X-ray source in the channel arrangement direction around a space where the subject is arranged. A focus position detecting means for detecting a relative focus position of the X-ray source with respect to the X-ray detector in a channel direction with respect to the X-ray detector before or during the scan; Correction means for changing the reconstruction parameter and / or the beam position information according to the relative focal position. .

According to a second aspect of the present invention, there is provided an X-ray source for irradiating an X-ray beam toward a subject, and an X-ray source opposed to the X-ray source and a space in which the subject is disposed. An X-ray detector comprising a plurality of X-ray detection segments having a large number of channels, wherein the X-ray source is obtained by scanning at least the X-ray source around the space in which the subject is arranged in the channel arrangement direction. X-ray CT apparatus for reconstructing the information obtained by using reconstruction parameters and / or beam position information, in the slice direction and / or the channel direction of the X-ray source with respect to the X-ray detector before or during the scan. Focus position detecting means for detecting a relative focus position, and the reconstruction parameter and / or the beam position information according to the detected relative focus position. A correction means for changing the X-ray CT apparatus according to subject matter in that it comprises.

According to the first or second aspect of the present invention, before the X-ray CT apparatus is used, the relative focus position of the X-ray source with respect to the X-ray detector is detected, and this detection is performed. By changing the reconstruction parameters and / or beam position information according to the result, according to the misalignment in units of division of the X-ray detector or the misalignment when attaching the entire X-ray detector to the X-ray CT apparatus, The reconstruction parameters and / or beam position information can be corrected,
A reconstructed image without artifacts can be obtained.

Further, immediately before or during scanning of the subject, the relative focus position of the X-ray source with respect to the X-ray detector is detected, and the reconstruction parameters and / or beam position information are changed according to the detection result. This makes it possible to perform image reconstruction using reconstruction parameters and / or beam position information corresponding to the focal position measured in real time, eliminates the influence of misalignment that changes over time such as focal point movement, and reduces artifacts. A reconstructed image having no image can be obtained.

According to a third aspect of the present invention, in the X-ray CT apparatus according to the first or second aspect, each of the X-ray sources corresponds to a plurality of relative focal positions with respect to the X-ray detector. A storage unit that stores the reconstruction parameters and / or the beam position information in advance, wherein the correction unit is configured to store the reconstruction position corresponding to the focal position closest to the detected relative focal position. The gist is to use parameters and / or the beam position information.

According to the third aspect of the present invention, reconstruction parameters and / or beam position information are previously stored for a plurality of misalignment amounts at a pitch of a misalignment amount such that artifacts can be ignored.
Since the stored reconstruction parameters and / or beam position information can be selected and used according to the detection result of the focus position detected immediately before or during the scan, the update calculation of the reconstruction parameters is not required, and the error No extra processing time is required for alignment correction,
There is no increase in the delay time until the reconstructed image is output.

According to a fourth aspect of the present invention, there is provided an X-ray source for irradiating an X-ray beam toward a subject, and the X-ray source and the X-ray source which are opposed to each other with a space in which the subject is placed. An X-ray detector comprising a plurality of X-ray detection segments having a large number of channels, wherein the X-ray source is obtained by scanning at least the X-ray source around the space in which the subject is arranged in the channel arrangement direction. A misalignment correction method of an X-ray CT apparatus for reconstructing information obtained by using reconstruction parameters and / or beam position information, wherein a slice direction of an X-ray source with respect to the X-ray detector before or during the scan And / or a position detection step of detecting a relative focus position in the channel direction,
A parameter changing step of changing reconstruction parameters and / or beam position information according to the detected relative focus position, and reconstructing a tomographic image using the changed reconstruction parameters and / or beam position information And a method of correcting misalignment of the X-ray CT apparatus.

According to a fifth aspect of the present invention, in the method of correcting a misalignment of an X-ray CT apparatus according to the fourth aspect, the parameter changing step is performed in interpolation for obtaining data to be back-projected onto an arbitrary voxel. The gist of the present invention is to change the combination of the X-ray detector segment from which the X-ray detection data is collected and the weight of the data according to the X-ray path based on the detected relative focus position.

According to a sixth aspect of the present invention, in the method of correcting a misalignment of an X-ray CT apparatus according to the fourth aspect, the parameter changing step includes the steps of: collecting the X-ray detection data; The data interpolated using the combination with the data weight is moved from the position of the voxel to be back-projected calculated based on the design focus position to the X-ray path based on the detected relative focus position. The gist of this is

According to a seventh aspect of the present invention, in the misalignment correction method for an X-ray CT apparatus according to the fourth aspect, the parameter changing step includes the step of obtaining slice position information of data obtained in each segment of the detector. Is changed to the slice direction position information of the intersection of the X-ray path and the rotation center axis based on the detected relative focus position.

According to the present invention, the relative focus position of the X-ray source with respect to the X-ray detector is detected immediately before or during the scan of the subject, and according to the detection result. By changing the reconstruction parameters and / or the beam position information, the image reconstruction can be performed using the reconstruction parameters and / or the beam position information corresponding to the focal position measured in real time, and the influence of the misalignment can be obtained. And a reconstructed image without artifacts can be obtained.

According to the eighth aspect of the present invention, there is provided an X-ray source for irradiating an X-ray beam toward a subject, and the X-ray source and the X-ray source opposed to each other with a space in which the subject is disposed. An X-ray detector comprising a plurality of X-ray detection segments having a large number of channels, wherein the X-ray source is obtained by scanning at least the X-ray source around the space in which the subject is arranged in the channel arrangement direction. Information
A method for correcting misalignment of an X-ray CT apparatus, wherein a plurality of data obtained from detection elements belonging to different segments are bundled and used as reconstruction as representative data, wherein the X-ray detector is provided before or during the scan. A position detection step of detecting a misalignment amount for each element, and a weighted average value or a simple average value obtained by weighting the detected misalignment amounts according to the sizes of the elements in the plurality of slice directions to be bundled. A misalignment amount, a parameter changing step of changing a reconstruction parameter and / or beam position information according to the misalignment amount of the representative data, and a tomography using the changed reconstruction parameter and / or beam position information. A reconstruction process for reconstructing an image. It is a misalignment correction method of T devices.

According to the eighth aspect of the present invention, even when data obtained from a plurality of elements having different amounts of misalignment are bundled and used, a weighted average or a simple average according to the size of the elements is performed. Thereby, appropriate misalignment correction can be performed.

[0061]

Next, an embodiment of the X-ray CT apparatus according to the present invention will be described in detail with reference to the drawings.
FIG. 1 is a system configuration diagram showing an X-ray CT apparatus 1 according to the present invention. As shown in the figure, the X-ray CT apparatus 1
X-ray tube 3, which is a X-ray beam generator, slit 5,
A main X-ray detector 7 in which detection elements are arranged in a dimension, a profile detector 9, and a data collection device 11 for a main X-ray detector
, A profile detector data collection device 13, a data processing device 15, and a plurality of focal positions (a, b,...)
A plurality of reconstruction parameters 31a, 3 respectively corresponding to
, A plurality of beam position information 33a, 33b,... Respectively corresponding to a plurality of focal positions, a high voltage generator 19, and a host controller 2.
1, a display device 23, a reconfiguration device 25, an auxiliary storage device 27, and a bus 29 connecting these devices.

In FIG. 1, the host controller 21
Is a central control device that controls the operation of the entire X-ray CT apparatus 1. The high voltage generator 19 supplies a high voltage to the X-ray tube 3 according to a control signal from the host controller 21 to generate X-rays. The generated X-rays pass through the subject P, and the energy thereof is converted into a current by the main X-ray detector 7 arranged opposite to the X-ray tube 3.

At this time, the X-ray tube 3, the slit 5, the main X-ray detector 7, and the profile detector 9 are integrally rotated around the subject P by a gantry rotating device (not shown). The rotating device of the gantry is controlled by a control signal from the host controller 21. The signals detected as currents by the main X-ray detector 7 and the profile detector 9 are as follows:
The data is amplified by the data detector 11 for the main detector and the data collector 13 for the profile detector, converted into a voltage signal, A / D converted, and transferred to the data processor 15 as digital signal projection data.

The data processing device 15 performs dark current correction and the like for each element of the X-ray detector for each projection data, performs logarithmic conversion, and stores the data in the storage device 17.

The projection data stored in the storage device 17 is transferred to the image reconstruction device 25. The tomographic image imaged by the image reconstruction device 25 is transferred to the display device 23 and displayed. Further, the collected projection data or the imaged tomographic image is transferred to the auxiliary storage device 27 and stored.

To detect the movement of the focal point of the X-ray tube 3, the slit 5 is provided with an X-ray beam X for the profile detector 9.
A gap for passing P is provided. The focus position calculation function built in the profile detector 9 for detecting the X-ray beam XP, the data acquisition device 13 for the profile detector, and the data processing device 15 makes the X-ray tube 3 relative to the main X-ray detector 7. This constitutes a focus position means for detecting a proper focus position.

An example of the profile detector 9 and the profile detector data collection device 13 will be described. As shown in FIG. 2, the profile detector 9 includes two detection elements A and B of an X-ray detector,
As shown in FIG. 3, each element is constituted by a scintillator 41 and a photodiode 43 provided therebelow. The scintillator 41 emits light according to the incident X-ray intensity, and the light emission of the scintillator 41 is converted into a current by the photodiode 43. The current is amplified by the profile detector data collection device 13, converted into a voltage signal, and converted into a digital signal after A / D conversion.

After correcting these digital voltage signals so as to be proportional to the incident X-ray energy,
The ratio is calculated by the data processing device 15. The calculated digital voltage signal indicates the ratio of the X-ray irradiation area on the element A and the element B of the profile detector 9. FIG.
As shown in (a), by combining the slit 5 and the profile detector 9, it is possible to detect the amount of movement of the X-ray focus in the slice direction.

FIG. 5 shows the flow of the correction process. The two modes using the two-dimensional X-ray detector are two types of acquisition methods, a static mode fixed to a bed and a helical mode, and two types of reconstruction methods, a cone beam reconstruction method and a fan beam deemed reconstruction method, totaling four. It is assumed that there are different modes. (It is not necessary to provide all modes. For example, when there is only one mode, the mode is fixed.) First, a scan mode is selected from the above four modes according to an instruction from the operator (step S10).
After selecting the mode, scanning is started according to the instruction of the operator (step S20). When the scan is started, X-ray irradiation from the X-ray tube 3 starts, and the main X-ray detector 7 and the main X-ray detector data collection device 11 collect projection data of the subject P (step S30). In parallel with the above, the focus position is measured by the profile detector 9 and the profile detector data collection device (step S40), and the focus position is calculated by the data processing device 15 (step S50).

Based on the obtained focal position, the storage device 1
, The reconstruction parameter closest to the actual focal position is selected from the reconstruction parameters 31a, 31b,... Respectively corresponding to the plurality of focal positions and sent to the reconstruction device 25. Position information 33a, 33 respectively corresponding to a plurality of focal positions stored in
The beam position information closest to the actual focal position is selected from b,... and sent to the reconstruction device 25 to correct the misalignment (step S60).

Next, the procedure proceeds to image reconstruction (step S70) and image display (step S80) using the collected projection data and the corrected reconstruction parameters. In step S60, the correction of the reconstruction parameter may be considered as a part of the reconstruction, and the reconstruction device 25 may have its function in the block diagram of FIG.

Next, a correction method will be described for each scan mode. First, correction in cone beam reconstruction will be described. The static mode and the helical mode will be described without distinction.

FIG. 6 shows the design geometry of the cone beam CT apparatus. The rotation axis is set on the vertical axis, and the horizontal axis is set on a plane parallel to the slice plane. The focus is on the left side,
It is a cross section in the horizontal direction at the phase where the X-ray detector is on the right side.
The X-ray detector will be described as having 16 elements. As described above, in the cone beam reconstruction, a point at which a straight line connecting the focal point and the voxel to be reconstructed intersects the two-dimensional X-ray detector plane is obtained, and the point is back-projected on the straight line. Considering voxel a and voxel b in the reconstruction plane shown in FIG.
o. 12, No. The data of the beam A (for example, No. 12 and No. 13) which is incident almost at the center of the X-ray detector No. 13
From the data of the X-ray detector described above. ) Is projected back to voxel a, 11 X-ray detectors (N)
o. Data of the beam B incident on the laser beam (approximately 10)
No. It is obtained by interpolation from the data of the X-ray detectors No. 10 and No. 11. ) Is back-projected onto voxel b.

Here, if the relative position of the focal point with respect to the X-ray detector changes for some reason such as thermal focal point movement (hereinafter referred to as focal point movement), the state shown in FIG. 7 is obtained. As data of the beam A in FIG. 6 (indicated by a dotted line in FIG. 7), for example, No. 12. No. The data obtained by interpolation from the data of the X-ray detector No. 13 is actually the one that has moved the focal point and entered the X-ray detector through the position of the beam A shown in FIG. (The same applies to beam B). In other words, the data that has passed through voxel a is considered to have passed through voxel a, and is back-projected. (Similarly for voxel b). The basis of reconstruction is to project back onto the voxels on the path through which the beam has passed, and it is easily analogized that artifacts occur in the state of FIG.

Therefore, in the present invention, this deviation is corrected by the method shown in FIGS.

In FIG. 8, beam A is assumed as data to be back-projected onto voxel a in accordance with the actual focal position. 12, No. Assuming the beam A ′ instead of the data obtained by interpolation from the data of the X-ray detector No. 13, for example, 14, No. Data obtained by interpolation from the data of the 15 X-ray detectors is selected. Voxel b
Similarly, No. 10, No. Not data obtained by interpolation from the data of the X-ray detector No. 11, but data of the beam B '(for example, data obtained by interpolating the data of the X-ray detectors No. 10 and No. 11) Is back-projected. As a result, data that has actually passed through the relevant voxel is back-projected onto the voxel, and the original performance of cone beam reconstruction can be exhibited.

Here, a simple realizing method of FIG. 8 will be described. The basis of the method of FIG. 8 is to change the combination of interpolation (detection element combination and weight) for obtaining data to be back-projected to the corresponding voxel according to the actual focal position. In order to change the combination of interpolation for each of the continuously changing focal positions, it is necessary to calculate and find the combination of interpolation each time. Therefore, first, the range of the expected focus movement is divided by a width in which an artifact due to misalignment is allowed, and one interpolation combination is set as a representative of the range. At the time of actual data collection, a method of selecting and interpolating a combination of interpolation at a position closest to the measured focal position is employed. By preliminarily setting and storing interpolation combinations, extra time is not required for calculating the interpolation combinations.

FIG. 9 shows a method of back-projecting the beam A data at the actual focal position onto the actually passed voxel a '. Assuming a voxel a 'where the straight line connecting the actual focal position and the position on the X-ray detector where the relevant beam is incident intersects the reconstruction plane, the design should project back to voxel a. Is back-projected to the voxel a '. As a result, data that has actually passed through the relevant voxel is back-projected onto the voxel, and the original performance of cone beam reconstruction can be exhibited.

The method shown in FIG. 8 changes the data to be back-projected onto the voxel in accordance with the movement of the focal point based on the voxel position. On the other hand, the method shown in FIG. 9 detects the data detected by the detector. In this method, the voxel position at which data or data interpolated from the data is to be back-projected is changed in accordance with the focal point movement based on the data. As a result, the method of FIG. 8 and the method of FIG. 9 are the same, and the effect obtained is the same regardless of which method is selected according to the configuration of the reconstruction apparatus.

Second, a mode using the fan beam deemed reconstruction method will be described. FIG. 10 shows the design geometry. The rotation axis is set on the vertical axis, and the horizontal axis is set parallel to the slice plane. A lateral cross section at a phase with the focus on the left and the X-ray detector on the right. Here, the X-ray detector is described as having eight elements. As described above, the method is such that the cone beam data is regarded as the fan beam data and the fan beam is reconstructed. In this method, the data obtained by the X-ray detector No. 6 is regarded as fan beam data of the plane of the hatched portion and reconstructed. For example, No. The position at which the linear beam A connecting the center of the detection element 6 and the focal position intersects the rotation center axis is the center of the reconstruction plane, which is the coordinate of the image in the slice thickness direction (referred to as the Z axis). Become.

Nview: view number Zview: Z coordinate of a plane including a focal point in design at an arbitrary view (constant in static mode, changes according to helical pitch in helical mode) nseg: X-ray detector seg Number Zseg: Any seg. Of the X-ray detector. FCD: distance from focal point to rotation axis (Z-axis) FDD: distance from focal point to X-ray detection surface Given the following, the beam at (nview, nseg) The Z coordinate of Z: (nview, nseg) is

Z (nview, nseg) = Zview + Zseg × FCD / FDD (1) Equation (1) is obtained. In the static mode, these coordinates take the Z coordinates of the image as they are, and in the helical mode, they serve as the Z coordinates of the respective collected data and serve as parameters for determining the weight of interpolation and the like.

Here, if the relative position of the focal point with respect to the X-ray detector changes for some reason such as thermal focal point movement (hereinafter referred to as focal point movement), the state shown in FIG. 11 is obtained. For example, no. The linear beam A connecting the center of the detection element 6 and the actual focal position changes as shown in FIG. 11, and the position crossing the rotation center axis (the center of the reconstruction plane) is the design position (thin net). Multiplied).

Here, assuming that Δf: the moving amount of the focal point, the Z coordinate of the beam in the focal point moving state at (nview, nseg): Z ′ (nview, nseg) is:

Z (nview, nseg) = Zview + (Zseg−Δf) × FCD / FDD + Δf = Zview + Zseg × FCD / FDD + Δf × (1-FCD / FDD) = Z (nview, nseg) + Δf × (1-FCD) / FDD) (2) Equation (2) is obtained. As the Z-coordinate of the image or the Z-coordinate of the acquired data, Z '(nview, nse) based on the actually measured focal position instead of the design value Z (nview, nseg)
By using g), the influence of misalignment due to relative fluctuation of the focal position can be suppressed.

In the cone beam reconstruction, complicated correction processing is required to adjust the combination of interpolation, but in the case of the fan beam deemed reconstruction method, it is sufficient to correct the Z-coordinate of the acquired data because the correction is sufficient. The misalignment can be corrected with simple processing.

Next, a method of correcting misalignment in the slice direction, which changes in the channel direction, will be described.

In this case, it is necessary to first measure the displacement of the arrangement of each detection element with respect to the system. The element arrangement inside the X-ray detector can be measured in the manufacturing process. For example, there is a method of arranging an X-ray film behind the X-ray detector, irradiating the X-ray from the front, and measuring a displacement of each detection element from the shadow of the X-ray detector. The measurement may be performed by replacing the X-ray film with another X-ray detector. The detection element data measured here is managed together with the X-ray detector body, and when the X-ray detector is attached to the system, the X-ray detector element array data is input to the system and used for correction.

Next, when the X-ray detector is attached to the system, it is measured whether or not the pre-collimator is twisted. The precollimator profile is measured at the elements at both ends of the X-ray detector in the channel direction,
The precollimator and the X-ray detector center line (for example, all channels) are obtained from the difference between the profiles at both ends and the array data of the corresponding detector elements at the corresponding ends in the detector array data previously measured and input to the system. And the torsion of the average).

Next, based on these data, each seg. Each time, the array position (Z coordinate) for the system is calculated. The Z coordinate (Zseg (n
ch, nseg)) can be used in place of Zseg described above to define the beam position of the data obtained by each element.

The subsequent specific correction may be basically equivalent to the correction of misalignment in the slice direction, which is constant in the channel direction as described above. In the case of cone beam reconstruction, adjust with a combination that interpolates according to the beam position,
In the case of a multi-slice helical assuming a fan beam, a filter interpolation weight is calculated for each detection element. Due to the structure of the X-ray detector, if the variation among elements is small and varies in blocks, a sufficient effect can be obtained by calculating in units of blocks instead of calculating for each element.

However, in the case of the fan beam assumed multi-slice static mode, a new method is also proposed. As shown in FIG. 12A, consider a case where an X-ray detector is configured using six detector blocks each having four channels and a total of 32 elements from 1 to 8 in the segment direction. When each detector block is incorporated in the system, it is assumed that a deviation in the Z direction occurs for each block. In this system, multiple
g. Irradiate the beams A and B over the data and bundle the data to create an image of two thick slices. If the data is added without considering the misalignment in the Z direction, the beam is processed as an uneven beam as shown in FIG.

Therefore, the present embodiment employs a method of weighting and adding the data of the elements irradiated with the beam based on the array data. Block No. that is aligned 1, No. 4, no. 6, seg. Corresponding to beam A and beam B, respectively. No. 3, No. 4
And seg. No. 5, no. 6 are added. On the other hand, in other blocks, each beam is received (seg. Can be determined from the detection element array data). Are obtained by weighting and adding the above data. As a result, instead of FIG.
The beam of (a) can be reproduced.

As described above, even when the misalignment in the slice direction changes in the channel direction, the effect can be suppressed as in the case of the above-described misalignment constant in the channel direction.

In the above embodiment, the misalignment in the slice direction has been described. However, the present invention can be applied to the misalignment in the channel direction.
That is, in the profile detector shown in FIG. 4A, the array of the detection elements is extended two-dimensionally, and the X-ray beam for profile detection having the intensity distribution also in the channel direction is used, so that the focus in the slice direction is obtained. Not only the movement but also the focus movement in the channel direction can be detected.

As shown in FIG. 4B, for example, an X-ray beam for profile detection formed into a square is directed toward a profile detector in which four detection elements A, B, C, and D are arranged in a square. Irradiate. The fact that the focus movement in the slice direction is detected based on the signal ratio of the detection elements A and B is the same as that in FIG. 4A, but in addition, the focus movement in the channel direction is detected by the detection elements A and C. It can be detected by the signal ratio (or the signal ratio of B and D).

The method of collecting data on the arrangement error of the detection elements has been described by using an X-ray film or the like in the manufacturing process. However, this method also collects not only the slice direction but also the position information in the channel direction. Thus, information necessary for correcting misalignment in the channel direction can be obtained.

Next, in a multi-slice system,
An embodiment of misalignment correction in a case where data of a plurality of X-ray detection elements is bundled will be described. First, FIG.
As shown in the figure, when the sizes of the detection elements in the slice direction are equal, an average value of the amount of misalignment of the respective elements to be bundled is obtained, and the average value is regarded as the amount of misalignment of the bundled data, and Change the reconstruction parameters corresponding to the data.

In the case of FIG. 13, data of the segments 1 to 4 having the same size of the detecting elements are bundled. That is, if the misalignment amounts of the elements belonging to the respective segments 1, 2, 3, and 4 are δ1, δ2, δ3, and δ4, the misalignment amount of the bundled data is (δ1 + δ2 + δ3 + δ
4) / 4.

Next, as shown in FIG. 14, when the sizes of the detection elements in the slice direction are not equal, a weighted average value (weighted average value) of the misalignment amount according to the size of each element to be bundled. The weighted average value is regarded as the amount of misalignment of the bundled data, and the reconstruction parameter corresponding to the data is changed.

That is, each segment 1, 2,.
The sizes of the elements belonging to 3, 4 are L1, L2, L3, L4
And the amounts of misalignment are δ1, δ2, δ
3, δ4, the misalignment amount of the bundled data is (δ1L1 + δ2L2 + δ3L3 + δ4L4) /
(L1 + L2 + L3 + L4).

As shown in FIG. 15, as a simple method, when the sizes of the detection elements in the slice direction are not equal, the largest element (L
The reconstruction parameter corresponding to the data may be changed by regarding the misalignment amount δ1 of 1) as the misalignment amount of the bundled data.

The preferred embodiments have been described above, but they do not limit the present invention. The essence of the concept of the present invention is that the focus position detected by the focus position detecting means is not actually related to the X-ray detector regardless of the generation of the X-ray CT scanner, the type of scan method, the type of scan mode, and the difference in the interpolation method. The object is to process data for reconstruction by changing reconstruction parameters such as a combination of interpolation, beam position information, and the like according to the relative position.

[0102]

As described above, according to the present invention, the relative focus position of the X-ray source with respect to the X-ray detector is detected by the position detecting means before or during the scan, and the detected relative position is determined. Since reconstruction is performed by correcting reconstruction parameters and / or beam position information in accordance with a proper focal position, even if a permanent focal position shift or a temporal focus shift occurs in the X-ray CT apparatus, good artifacts are less generated. There is an effect that a reconstructed image can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a cone beam X-ray CT apparatus according to the present invention.

FIG. 2 is an explanatory diagram of the principle of focus position detection by a focus position detection means.

FIG. 3 is a block diagram illustrating a configuration of a focus position detection unit.

FIG. 4 is an explanatory diagram of an operation of a focus position detecting means.

FIG. 5 is a flowchart illustrating a procedure of focus movement correction in the cone beam X-ray CT apparatus according to the present invention.

FIG. 6 is a cross-sectional view of a principal part explaining a design geometry of the cone beam X-ray CT apparatus.

FIG. 7 is a partial cross-sectional view illustrating a state before correction when a misalignment such as a focal point shift occurs in a cone beam X-ray CT apparatus.

FIG. 8 is a diagram illustrating a first correction method in a state where misalignment such as a focus shift has occurred in the X-ray CT apparatus (cone beam) according to the present invention.

FIG. 9 is a view for explaining a second correction method in a state where misalignment such as a focal point shift has occurred in the X-ray CT apparatus (cone beam) according to the present invention.

FIG. 10 is a cross-sectional view of a principal part for explaining the design geometry of a multi-slice X-ray CT apparatus that performs fan beam deemed reconstruction.

FIG. 11: Fan beam deemed multi-slice X-ray CT
FIG. 4 is a cross-sectional view of a main part for explaining a positional shift of a beam when the apparatus moves a focal point.

FIG. 12 is an X-ray detector development diagram for explaining a beam position shift when there is variation between blocks.

FIG. 13 is a diagram illustrating the amount of misalignment in the case where X-ray detection data of a plurality of segments of the same size are used in a bundle.

FIG. 14 is a diagram illustrating the amount of misalignment in the case where X-ray detection data of a plurality of segments of different sizes are used in a bundle.

FIG. 15 is a diagram illustrating a misalignment amount (simple definition) in a case where X-ray detection data of a plurality of segments of different sizes are used in a bundle.

FIG. 16 is a view for explaining the focal point movement of the rotating anode X-ray tube.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... X-ray CT apparatus, 3 ... X-ray tube, 5 ... Slit, 7 ...
Main X-ray detector, 9: Profile detector, 11: Data collection device for main X-ray detector, 13: Data collection device for profile detector, 15: Data processing device, 17: Storage device, 19: High voltage generator, DESCRIPTION OF SYMBOLS 21 ... Host controller, 23 ... Display device, 25 ... Reconstruction device, 27 ... Auxiliary storage device, 29 ... Bus, 31 ... Reconstruction parameter, 33 ...
Beam position information.

Claims (8)

[Claims] An X-ray for irradiating an X-ray beam toward a subject
A source, and an X-ray detector having a plurality of channels, each of which is opposed to each other across a space in which the X-ray source and the subject are arranged, and at least around a space in which the subject is arranged. An X-ray CT apparatus for reconstructing information obtained by a scan in which the X-ray source is rotated in the channel arrangement direction using a reconstruction parameter and / or beam position information, wherein the X-ray is scanned before or during the scan. A focus position detecting means for detecting a relative focus position of the X-ray source in a channel direction with respect to a detector; and changing the reconstruction parameter and / or the beam position information according to the detected relative focus position. An X-ray CT apparatus comprising: a correction unit. 2. An X-ray for irradiating an X-ray beam toward a subject
A source, and an X-ray detector composed of a plurality of X-ray detection segments each having a large number of channels disposed opposite to each other with a space in which the X-ray source and the subject are arranged, wherein the subject is An X-ray CT apparatus configured to reconstruct information obtained by scanning at least the X-ray source around the space to be arranged in the channel arrangement direction using reconstruction parameters and / or beam position information. Focus position detecting means for detecting a relative focus position of the X-ray source in a slice direction and / or a channel direction with respect to the X-ray detector before or during scanning, and in accordance with the detected relative focus position, An X-ray CT apparatus comprising: a correction unit that changes a reconstruction parameter and / or the beam position information. 3. The apparatus according to claim 1, further comprising a storage unit configured to store the reconstruction parameter and / or the beam position information in advance in accordance with a plurality of relative focal positions of the X-ray source with respect to the X-ray detector. The correction means uses the reconstruction parameters and / or the beam position information stored according to a focal position closest to the detected relative focal position. X-ray CT apparatus. 4. An X-ray for irradiating an X-ray beam toward a subject
A source, and an X-ray detector composed of a plurality of X-ray detection segments each having a large number of channels disposed opposite to each other with a space in which the X-ray source and the subject are arranged, wherein the subject is Misalignment correction of an X-ray CT apparatus for reconstructing at least the information obtained by scanning the X-ray source around the space to be arranged in the channel arrangement direction using reconstruction parameters and / or beam position information A method for detecting a relative focus position of an X-ray source in a slice direction and / or a channel direction with respect to the X-ray detector before or during the scan; and the detected relative focus. A parameter changing step of changing a reconstruction parameter and / or beam position information according to a position, and the changed reconstruction parameter Misalignment correction method of X-ray CT apparatus characterized by comprising: a reconstruction step of reconstructing a tomographic image by using the Bimatawa beam position information. 5. The parameter changing step includes the step of detecting a combination of the X-ray detector segment that has collected X-ray detection data in interpolation for obtaining data to be back-projected onto an arbitrary voxel and a weight of the data. 5. The method according to claim 4, wherein the change is performed according to an X-ray path based on the relative focus position. 6. The parameter changing step is a step of calculating data interpolated using a combination of the X-ray detector segment from which the X-ray detection data has been collected and the weight of the data, based on a design focal position. 5. The method according to claim 4, wherein the position of the voxel to be back-projected is moved on an X-ray path based on the detected relative focal position. 7. The parameter changing step includes the step of using slice position information of data obtained in each segment of the detector based on the detected relative focal position.
5. The method for correcting misalignment of an X-ray CT apparatus according to claim 4, wherein the information is changed to slice direction position information of an intersection between the line path and the rotation center axis. 8. An X-ray for irradiating an X-ray beam toward a subject
A source, and an X-ray detector composed of a plurality of X-ray detection segments each having a large number of channels disposed opposite to each other with a space in which the X-ray source and the subject are arranged, wherein the subject is For information obtained by scanning at least the X-ray source in the channel arrangement direction around the space where the X-ray source is arranged, a plurality of data obtained from the detection elements belonging to different segments are bundled and reconstructed as representative data. A misalignment correction method for an X-ray CT apparatus used for: a position detection step of detecting a misalignment amount of each element of the X-ray detector before or during the scan; and the detected misalignment amount. Is a weighted average or a simple average weighted by the size of the elements in the plurality of slice directions to be bundled. Using the changed reconstruction parameters and / or beam position information, and changing the reconstruction parameters and / or beam position information according to the misalignment amount of the representative data. A reconstruction process for reconstructing a tomographic image by using a method for correcting misalignment of an X-ray CT apparatus.