JP2010115354A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus capable of correcting the variation of an X-ray beam position even when an X-ray beam is interrupted by a subject or the like. <P>SOLUTION: Shift detectors 204a and 204b for detecting the shift of an X-ray beam irradiation position are provided between an X-ray source 201 and a collimator 203, and a part of the channel of the X-ray detector 205 is utilized as a shift detection channel 205a. Then, when measurement of X-ray transmission data is started, a controller 202 selects either one of the output data of the shift detection channel 205a and the output data of the shift detectors 204a and 204b, calculates the shift position of the X-ray beam on the basis of the selected output data, and corrects the X-ray beam irradiation position by controlling the position of the collimator 203 or the like on the basis of the calculated shift position. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an X-ray CT apparatus that corrects a positional shift (shift) at the center of an X-ray beam.

  The X-ray source of the X-ray CT apparatus emits X-rays by irradiating an electron beam from the cathode side to the target anode surface. Although the electron beam is accelerated by a high voltage applied between the cathode and the anode, most of the energy of the accelerated electron beam is converted into heat, so that the anode is heated. As a result, the anode surface is thermally expanded, and the position of the X-ray generation point fluctuates. In recent years, the rotation of the gantry has increased, and the position of the X-ray beam irradiation destination (X-ray detector) fluctuates due to an external force such as centrifugal force accompanying the high-speed rotation, causing fluctuations in the X-ray beam position. Yes. Such fluctuations in the X-ray beam position become measurement errors in the transmitted X-ray data acquired by the X-ray detector and cause artifacts on the tomographic image obtained after reconstruction. Hereinafter, the fluctuation of the X-ray beam position is referred to as X-ray beam shift.

Therefore, a part of the X-ray detector for detecting X-rays transmitted through the subject is used to detect the beam irradiation position in the body axis direction, and the X-ray detector and the X-ray beam are detected. A device that moves the collimator in the body axis direction so that the position becomes the optimum position has been developed (Patent Document 1).
In addition to the X-ray detector, a dedicated shift detector for detecting the fluctuation of the X-ray beam is provided in the vicinity of the X-ray source, and the X-ray beam is shifted (fluctuated) from the output value of the shift detector. A device that controls the position of the X-ray source when it is shifted by more than an allowable value has been developed (Patent Document 2).

JP 2002-143144 A JP 2005-143812 A

However, since the method disclosed in Patent Document 1 uses a part of the channel of the X-ray detector for detecting the shift position, X-rays incident on the channel are blocked by the subject, the bed, or the like. In such a case, there is a problem that the shift position cannot be detected.
Further, in the technique disclosed in Patent Document 2, when a problem occurs in the shift detector and the X-ray detector is not properly irradiated with the X-ray beam, there is no way to detect this, and the inspection itself is not performed. There was a problem of becoming invalid.

  The present invention has been made in view of the above problems, and an X-ray CT apparatus capable of correcting variation (shift) of the X-ray beam position even when the X-ray beam is blocked by a subject or the like. The purpose is to provide.

  In order to achieve the above-described object, the present invention provides a shift detector provided between an X-ray source and a collimator, which detects a shift from the irradiation center of an X-ray beam emitted from the X-ray source, and a subject. An X-ray CT apparatus comprising: an X-ray detector side shift detection unit for detecting the shift, which is assigned to a part of a detection element group of an X-ray detector for detecting an X-ray beam transmitted through the X-ray detector And selecting means for selecting any one of the output data of the shift detector or the output data of the X-ray detector side shift detecting section, and any one of the outputs selected by the selecting means. Shift position calculating means for calculating the shift position of the X-ray beam based on the data; shift position correcting means for correcting the irradiation position of the X-ray beam based on the shift position calculated by the shift position calculating means; It is an X-ray CT apparatus, characterized in that it comprises.

  The selection means selects the output data of the X-ray detector side shift detector when the output data of the X-ray detector side shift detector is equal to or greater than a predetermined threshold value, and the X-ray detector side When the output data of the shift detector is smaller than a predetermined threshold, it is desirable to select the output data of the shift detector.

  The shift detector is provided at both ends of the spread angle of the X-ray beam in the channel direction, and detects X-ray irradiation dose in each region divided at least in the slice direction as output data, and the shift detector When the output data of the shift detector is selected by the selection means, the position calculating means uses the output data in each region of each shift detector provided at both ends to generate an X-ray beam. It is desirable to calculate the shift position.

  According to the present invention, it is possible to provide an X-ray CT apparatus capable of correcting positional fluctuations of an X-ray beam even when the X-ray beam is blocked by a subject or the like.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, the configuration of the X-ray CT apparatus 1 of the present embodiment will be described.
FIG. 1 is a hardware configuration diagram showing the overall configuration of the X-ray CT apparatus 1.
FIG. 2 is a diagram illustrating the arrangement of the X-ray source 201, the collimator 203, the shift detectors 204a and 204b, and the X-ray detector 205.

  As shown in FIG. 1, the X-ray CT apparatus 1 includes a scanner 2, a bed 3, and an operation console 4. The X-ray CT apparatus 1 obtains transmission X-ray data after passing through the subject 6 by carrying the subject 6 fixed on the bed 3 into the opening of the scanner 2 and scanning it.

  The operation console 4 includes a display device 7, an operation device 8, a system control device 401, an image calculation device 402, a storage device 404, and a communication interface. The operation console 4 is connected to the scanner 2 and executes various operations and processes related to the setting and shooting of the scanner 2 or image reconstruction and display. The operation console 4 is connected to the network 9 via the communication interface 406 and communicates with other devices such as the DICOM server 10 connected to the network 9.

The display device 7 includes a display device such as a liquid crystal panel and a CRT monitor, and a logic circuit for executing display processing in cooperation with the display device. The display device 7 displays a reconstructed image and a scanogram image output from the image calculation device 402 and various information handled by the system control device 401.
The operation device 8 includes, for example, an input device such as a keyboard, a mouse, and a numeric keypad, and various switch buttons. The operation device 8 outputs various instructions and information input by the operator to the system control device 401. The operator interactively operates the X-ray CT apparatus 1 using the display device 7 and the operation device 8.

  The system control device 401 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and controls the entire X-ray CT apparatus 1.

The image calculation device 402 acquires transmission X-ray data collected by the data collection device 206 in the scanner 2 under the control of the system control device 401, and reconstructs a scanogram image or a tomographic image.
The storage device 404 is configured by a hard disk or the like. Under the control of the system control device 401, an image created by the image arithmetic device 402, shooting conditions input from the operation device 8, other data, A program for executing the process is stored.

  The scanner 2 includes an X-ray source 201, a control device 202, a collimator 203, an X-ray detector 205, a data acquisition device 206, a rotating plate 207, and a shift detector 204.

  The X-ray source 201 is controlled by the control device 202 and irradiates the subject 6 with an X-ray beam continuously or intermittently. The X-ray beam emitted from the X-ray source 201 is a fan beam having a predetermined angle spread in the channel direction (gantry rotation direction) and the slice direction (body axis direction).

  The control device 202 includes a CPU, a ROM, a RAM, and the like, and controls the X-ray source 201, the collimator 203, the X-ray detector 205, the shift detectors 204a and 204b, the rotating plate 207, and the like of the scanner 2. The control device 202 detects and corrects an X-ray beam shift position, which will be described later, when the subject 6 is imaged (measured) by the scanner 2 (see FIGS. 3 and 5).

  The collimator 203 shapes the X-ray fan beam radiated from the X-ray source 201 into a predetermined slice thickness, and includes a plurality of blades 203a and 203b as shown in FIG. The plurality of blades 203 a and 203 b are provided so as to block part or all of the X-ray irradiation path, and the position of the blades 203 a and 203 b of the collimator 203 is controlled by the control device 202.

  In the present embodiment, the collimator 203 functions as a shift position correction mechanism. That is, based on the X-ray beam shift position correction signal input from the control device 202, the positions of the blades 203a and 203b of the collimator 203 are moved in the slice direction (body axis direction), and the X-ray beam irradiation position. Is moved to the optimum position.

  As shown in FIGS. 1 and 2, the X-ray detector 205 is disposed so as to face the X-ray source 201 through the subject 6. In the X-ray detector 205, for example, a plurality of detection elements each composed of a scintillator and a photodiode are two-dimensionally arranged in the channel direction and the slice direction. Specifically, for example, about 1 to 1000 detection elements are arranged in the channel direction and about 1 to 1000 detection elements are arranged in the column direction, and the plurality of detection elements as a whole is a cylindrical surface or a broken line in the channel direction. An X-ray incident surface curved in a shape is formed. Each detection element detects an X-ray dose (transmitted X-ray data) transmitted through the subject 6 and outputs the detected transmitted X-ray data to the data collection device 206.

  In addition, one or a plurality of channels on one end side or both end sides of the detection elements of the X-ray detector 205 are assigned for X-ray beam position shift detection. In the present embodiment, a channel on one end side is set as a shift detection channel, and a reference numeral 205a is given. Note that the shift detection channel 205a of the X-ray detector 205 is preferably assigned to a position where the X-ray irradiation path is not blocked by the shift detectors 204a and 204b (see FIGS. 3 and 4).

  The data collection device 206 collects transmission X-ray data detected by the individual detection elements of the X-ray detector 205 and outputs it to the image calculation device 402 of the operation console 4.

The shift detectors 204a and 204b detect a shift from the irradiation center of the X-ray beam irradiated from the X-ray source 201.
In the present embodiment, as shown in FIG. 2, the two shift detectors 204a and 204b detect the both ends of the spread angle in the channel direction of the X-ray fan beam emitted from the X-ray source 201, respectively. Is provided. Note that the shift detector 204a (or 204b) may be provided only on one end side as in the conventional case.

  Further, as shown in FIG. 3, each of the shift detectors 204a and 204b has an X-ray fan beam by a shift detector support mechanism 204c fixed between the X-ray beam irradiation port of the X-ray source 201 and the collimator 203. It is attached so as to reach the spreading end of the channel direction. The position approaching the X-ray fan beam is divided into two equal parts in the slice direction, and a detection element is provided in each of the two divided areas. Each shift detector 204a, 204b outputs the detected value of the X-ray dose in each region to the control device 202 as output data. In this embodiment, as an example, the shift detector 204a is divided into areas A and B in the slice direction, and output data in the areas A and B are Va and Vb, respectively. Similarly, the shift detector 204b is divided into C and D regions in the slice direction, and the output data in the regions C and D are set to Vc and Vd, respectively (see FIG. 4B).

Next, with reference to FIG. 3 and FIG. 4, functions regarding detection and correction of the shift of the X-ray irradiation position of the control device 202 will be described.
FIG. 3 is a functional block diagram relating to detection and correction control of the X-ray beam shift position in the X-ray CT apparatus 1.
4A and 4B are diagrams for explaining detection and correction of the shift position of the X-ray beam. FIG. 4A illustrates a case where the shift detector 204a is provided on one side, and FIG. The case where it is provided on both sides is shown.

  As shown in FIG. 3, the control device 202 includes an X-ray detector output processing unit 11, a shift detector output processing unit 12, an output selection control unit 13, a shift position calculation unit 14, and a shift position correction unit 15. Yes.

  The X-ray detector output processing unit 11 processes output data from the shift detection channel 205 a provided in the X-ray detector 205. Specifically, in the example shown in FIGS. 3 and 4, a part of the channel 1ch side of the X-ray detector 205 is used as the shift detection channel 205a. The X-ray detector output processing unit 11 detects the sum Vs of detection values of the detection elements in the region S from 1 slice to M / 2 slice of the shift detection channel 205a and the detection from the region T from M / 2 slice to M slice. The sum Vt of the detection values of the elements is calculated and output to the output selection control unit 13.

  The shift detector output processing unit 12 processes output data from the shift detector 204a and the shift detector 204b. Specifically, when the shift detector 204a is provided on one side as shown in FIG. 4A, each output data (Va, Vb) in each of the areas A and B divided in the slice direction is controlled. Input to the shift detector output processing unit 12 of the apparatus 202. The shift detector output processing unit 12 calculates an absolute value Vd1 of the difference between the output data (Va, Vb) in the regions A and B, and sends the absolute value Vd1 to the output selection control unit 13 together with the output data Va and Vb. Vd1 is expressed by the following formula (1).

          Vd1 = | Va−Vb | (1)

  As shown in FIG. 4B, when shift detectors 204a and 204b are provided on both sides, the output data Va of region A, the output data Vb of region B, the output data Vc of region C, and The output data Vd of the region D is input to the shift detection processing unit 12 of the control device 202. The shift detector output processing unit 12 calculates the sum (Va + Vb) of the output data Va of the region A and the output data Vb of the region B from the input output data, and the output data Vc of the region C and the output data of the region D. The sum (Vc + Vd) with Vd is calculated and sent to the output selection control unit 13.

The output selection control unit 13 determines whether one of the output data of the X-ray detector shift detection channel 205a or the output data of the shift detectors 204a and 204b is used for detection of the shift position. Specifically, the output selection control unit 13 selects the output of the X-ray detector shift detection channel 205a when the output data of the X-ray detector output processing unit 11 is larger than a predetermined threshold, and detects the X-rays. When the output data of the detector output processing unit 11 is equal to or less than a predetermined threshold, the outputs of the shift detectors 204a and 204b are selected.
Here, as the output data referred to in the determination of the output selection, any of the sum, average value, minimum value, etc. of the output data Vs and Vt of the X-ray detector shift detection channel 205a may be used.

  The shift position calculation unit 14 selects one of the output data (output data (Vs, Vt) of the shift detection channel device of the X-ray detector 205, or the shift detectors 204a and 204b) selected by the output selection control unit 13. Using the output data (Va + Vb, Vc + Vd), the shift position of the X-ray irradiation position is calculated, and a correction signal indicating a correction direction corresponding to the shift position is sent to the shift position correction unit 15.

  Specifically, when the output selection control unit 13 selects the output of the shift detection channel 205a of the X-ray detector 205, the output data (Vs, Vt) of each region S, T of the shift detection channel 205a. Corresponds to the shift amount from the center. Since the amount of X-ray irradiation is maximum at the center and attenuates toward the outside, the output data (Vs, Vt) indicates which region of the X-ray detector 205 the irradiation center is shifted to. ) Value. For example, when the center of the X-ray beam is shifted to the region S side of the X-ray detector 205 (Vs> Vt), a correction signal is sent to the shift position correction unit 15 so that Vs = Vt. To do. In this case, the shift position correction unit 15 shifts the collimator to the region T side until Vs = Vt.

  Further, when the shift detectors 204a and 204b are selected by the output selection control unit 13, the shift position calculation unit 14 determines the shift detector output processing unit based on the output data of the shift detectors 204a and 204b. 12 is compared with which value is larger among the sums (Va + Vc) and (Vb + Vd) of the output data of each region calculated in step 12, and based on the comparison result, which region is shifted to is calculated. Then, the correction direction and the correction amount are determined so that Va + Vc = Vb + Vd, and a correction signal is sent to the shift position correction unit 15.

  For example, when the X-ray beam is shifted to the position indicated by the alternate long and short dash line in FIG. 4B, the output data of each region of the shift detectors 204a and 204b is (Va + Vc)> (Vb + Vd). . In this case, the shift position calculation unit 14 sends the correction signal to the shift position correction unit 15 with the direction of the regions B and D having a small sum of output data as the correction direction.

  The shift position correction unit 15 corrects the shift position until the center of the X-ray beam coincides with the center of the X-ray detector 205.

  The shift position correction unit 15 controls the position of the collimator 203 according to the correction signal input from the shift position calculation unit 14, and X-ray irradiation is performed until the center of the X-ray detector 205 matches the center of the X-ray beam. Correct the position.

  Hereinafter, with reference to FIG. 5 and FIG. 6, operations related to X-ray beam shift detection and correction in the X-ray CT apparatus 1 will be described.

FIG. 5 is a flowchart for explaining the flow of X-ray beam shift detection and correction processing.
FIG. 6 is a diagram for explaining the shift position detection operation during one rotation.

  As shown in FIG. 5, in the X-ray CT apparatus 1, when imaging (measurement) is started, X-ray beam shift detection and correction processing is started (step S1). In step S1, the control device 202 outputs the output data (Va, Vb, Vc, Vd) from each region A, B, C, D of the shift detectors 204a, 204b and the shift detection channel 205a of the X-ray detector 205. Output data (output data from each detection element for M slices) is acquired.

  The control device 202 processes each acquired output data by the X-ray detector output processing unit 11 and the shift detector output processing unit 12. That is, the X-ray detector output processing unit 11 calculates the output data Vs and Vt in the regions S and T from the acquired output data from each detection element. Further, the shift detector output processing unit 12 calculates the sum (Va + Vc) and (Vb + Vd) based on the output data of each region of each shift detector 204a, 204b.

In step S <b> 2, the control device 202 determines whether the output data from the shift detection channel 205 a of the X-ray detector 205 is equal to or greater than a predetermined threshold by the output selection control unit 13. (Step S2; true), the shift detection channel 205a of the X-ray detector 205 is selected (Step S3).
If the output data from the shift detection channel 205a is smaller than the predetermined threshold (step S2; false), the shift detectors 204a and 204b are selected (step S4).

  When the shift detection channel 205a of the X-ray detector 205 is selected (step S3), the control device 202 causes the shift position calculation unit 14 to select one of the output data (Vs, Vt) of the shift detection channel 205a. It is detected that the position of the X-ray beam is shifted to a region indicating the larger one. And the control apparatus 202 determines a correction direction so that it may become Vs = Vt, and sends a correction signal to the shift position correction | amendment part 15 (step S5).

  On the other hand, when the shift detectors 204a and 204b are selected (step S4), the control device 202 uses the sum data (Va + Vc, Vb + Vd) calculated by the shift detector output processing unit 12 to determine which slice. Detect if the direction is shifted. That is, it is detected that the X-ray beam is shifted in the direction indicating a larger value in the sum of output data (Va + Vc, Vb + Vd). Then, a correction signal is sent to the shift position correction unit 15 such that (Va + Vc) = (Vb + Vd) (step S5).

The control device 202 controls the collimator 203 and the like according to the correction signal transmitted in step S5, and corrects the position of the X-ray beam (step S6). Note that the movement amount of the collimator 203 is set in increments of a predetermined width that is set in advance, and is repeated until the control device 202 does not detect the shift of the X-ray beam.
That is, the control device 202 repeats the processing of Steps S1 to S6 and performs control so that the X-ray irradiation center always coincides with the center of the X-ray detector 205.

In the shift position detection and correction processing shown in FIG. 5, which of the shift detectors 204a and 204b or the X-ray detector 205 is selected during one gantry rotation will be described with reference to FIG.
As shown in FIG. 6A, an X-ray source 201, shift detectors 204a and 204b, and an X-ray detector 205 are arranged at positions facing each other through the subject 6. A channel at one end of the X-ray detector 205 is assigned to the shift detection channel 205a.

  When the measurement is started, the X-ray source 201, the shift detectors 204a and 204b, and the X-ray detector 205 rotate on the rotation trajectory. At the position shown in FIG. 6B, the X-ray incident on the shift detection channel 205a of the X-ray detector 205 is not blocked by the subject 6 or the like, so the output of the shift detection channel 205a of the X-ray detector 205 is When selected (operation ON), the operations of the shift detectors 204a and 204b are turned off.

At the positions shown in FIGS. 6C and 6D, the X-rays incident on the shift detection channel 205a of the X-ray detector are blocked by the subject 6 or the like, so the outputs of the shift detectors 204a and 204b are selected ( Operation ON), the operation of the shift detection channel 205a of the X-ray detector 205 is turned OFF.
In this way, during the measurement, the control device 202 switches which output of the shift detection channel 205a of the X-ray detector 205 or the shift detectors 204a and 204b is selected.

  FIG. 6E is a diagram showing which detector is selected during one rotation. When the X-ray source 201 is on the rotation start of the white area in the figure, the outputs of the shift detectors 204a and 204b are selected, and when the X-ray source 201 is on the rotation orbit of the shaded area in the figure, The output of the shift detection channel 205a of the line detector 205 is selected.

  As described above, the X-ray CT apparatus 1 according to the present embodiment has the shift detector 204a for detecting the shift (shift) of the irradiation position of the X-ray beam between the X-ray source 201 and the collimator 203. 204b, and a part of the channel of the X-ray detector 205 is used as the shift detection channel 205a. Then, the control device 202 selects either the output data of the shift detectors 204a and 204b or the output data of the shift detection channel 205a, and calculates the shift position of the X-ray beam based on the selected output data. Then, based on the calculated shift position, the position of the collimator 203 or the like is controlled to correct the X-ray beam irradiation position.

  As a result, the X-ray shift is detected by the shift detectors 204a and 204b even when the X-ray is not incident on the shift detection channel 205a provided in the X-ray detector 205 due to being blocked by the subject 6 or the bed 3 or the like. It is possible to always correct the shift position during measurement.

  In addition, when X-rays are incident on the shift detection channel 205a of the X-ray detector 205, the output data from the shift detection channel 205a is preferentially selected and the shift position is corrected. On the X-ray detector 205 side, which is the transmission X-ray data collection position, the X-ray positional deviation can be grasped and corrected more accurately.

  In addition, since both ends of the spread of the X-ray beam in the channel direction are detected by the plurality of shift detectors 204a and 204b, the X-ray is larger than the X-ray detector 205 on one end side in the channel direction. Even for a shift that shifts the position, the shift position can be detected more accurately, and the correction accuracy is improved.

  In the above-described embodiment, the output selection control unit 13 of the control device 202 selects the output of the shift detection channel 205a when the output data of the shift detection channel 205a of the X-ray detector 205 is larger than a predetermined threshold value. However, the present invention is not limited to this. For example, when the X-ray source 201 reaches a predetermined position, the shift detection channel 205a of the X-ray detector 205 is selected, and the outputs of the shift detectors 204a and 204b are selected at other positions. Also good.

  In addition, although an example using the collimator 203 has been described as means for correcting the shift position, the invention is not limited to this, and the X-ray detector 205 is moved or the X-ray source 201 (X-ray tube) is used. Alternatively, a method may be used in which the X-ray beam is bent in a predetermined direction by generating a magnetic field or by generating a magnetic field.

  Moreover, although the example which detects the shift of a slice direction was shown, you may make it detect the shift to a channel direction (rotation direction). Also in this case, the shift in the channel direction can be detected from the difference in the values of the output data Va, Vb, Vc, Vd of the areas A, B, C, D of the plurality of shift detectors 204a, 204b.

  In addition, it is obvious that those skilled in the art can come up with various changes and modifications within the scope of the technical idea disclosed in the present application, and these naturally belong to the technical scope of the present invention. It is understood.

Hardware configuration diagram showing the overall configuration of the X-ray CT apparatus 1 The figure explaining arrangement | positioning of the X-ray source 201, the collimator 203, the shift detectors 204a and 204b, and the X-ray detector 205. Functional block diagram relating to detection and correction control of the X-ray beam shift position in the X-ray CT apparatus 1 The figure explaining the detection and correction | amendment of the shift position of a X-ray beam Flowchart for explaining the flow of detection and correction processing of the X-ray beam shift position The figure explaining shift position detection operation in one rotation

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... X-ray CT apparatus 2 ... Scanner 3 ... Sleeper 4 ... Operation console 201 ... X-ray source 202 ... Control apparatus 203 ... Collimator 204a, 204b ... shift detector 205 ... X-ray detector 205a ... shift detection channel (X-ray detector side shift detector)
6 ... Subject

Claims (3)

A shift detector that is provided between the X-ray source and the collimator and detects a shift from the irradiation center of the X-ray beam emitted from the X-ray source;
An X-ray CT comprising: an X-ray detector side shift detection unit for detecting the shift, which is assigned to a part of a detection element group of an X-ray detector for detecting an X-ray beam transmitted through a subject. A device,
Selection means for selecting either the output data of the shift detector or the output data of the shift detector on the X-ray detector side,
Shift position calculating means for calculating the shift position of the X-ray beam based on any one of the output data selected by the selecting means;
Shift position correcting means for correcting the irradiation position of the X-ray beam based on the shift position calculated by the shift position calculating means;
An X-ray CT apparatus comprising:   The selection means selects the output data of the X-ray detector side shift detection unit when the output data of the X-ray detector side shift detection unit is equal to or greater than a predetermined threshold value, and detects the X-ray detector side shift detection. The X-ray CT apparatus according to claim 1, wherein the output data of the shift detector is selected when the output data of the unit is smaller than a predetermined threshold value. The shift detector is provided at both ends of the spread angle of the X-ray beam in the channel direction, and detects X-ray irradiation dose in each region divided at least in the slice direction as output data,
The shift position calculation means, when the output data of the shift detector is selected by the selection means, uses each output data in each region of each shift detector provided at the both end portions, The X-ray CT apparatus according to claim 1, wherein a beam shift position is calculated.

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