JP5068604B2 - Radiation CT system - Google Patents

Description

The present invention relates to radiation CT (Computed).
In particular, the present invention relates to a radiation CT apparatus that adjusts the dose of radiation applied to a subject according to the irradiation position in the body axis direction of the subject.

  In general, in order to obtain a high-quality tomographic image in a radiation CT apparatus, the dose of transmitted radiation that passes through the subject and is detected by the detector must be large. In order to obtain it, the dose of radiation applied to the subject must be large.

  However, it is not desirable to increase the dose of radiation irradiated to the subject with the primary focus only on improving the image quality, resulting in an increase in the dose of the subject.

  Therefore, conventionally, in radiation CT imaging, the radiation absorption dose of the subject in the body axis direction of the subject is obtained in advance, and the radiation dose to be irradiated on the subject portion according to the radiation absorption dose of the subject portion to be scanned is determined. A technique for adjusting is known. For example, a subject is irradiated with a predetermined dose of X-rays from a predetermined direction, a transmission X-ray dose measurement result at that time is obtained, and the tube current of the X-ray tube at the time of scanning is controlled based on the result (Patent Document 1). , 2 etc.).

According to such a technique, for each position in the body axis direction of the subject, the noise level (noise) of the image generated corresponding to the position is determined.
level) can reach only a predetermined target level. That is, according to this method, unnecessary radiation dose is reduced while maintaining the image quality of each generated image substantially constant regardless of individual differences of the subjects, for example, the size, shape, type, etc. of the subjects. Thus, the exposure dose of the subject can be reduced.
JP 2007-000407 A Japanese Patent Laid-Open No. 2003-070779

By the way, the above-described method for adjusting the dose of radiation applied to the subject according to the absorbed dose in the body axis direction of the subject is a multi-slice (multi-slice).
The present invention can be applied to scanning using a slice type radiation CT apparatus. In this case, in order to keep the image quality of all the images generated corresponding to the respective positions in the body axis direction of the subject at a certain level (level) or higher, the radiation is simultaneously applied with respect to the dose of radiation applied to the subject. In any part of the subject in the irradiated region, it is necessary to guarantee a dose for making the image quality above a certain level. In other words, the radiation dose to the subject is set to the largest dose among the “sufficient dose necessary to obtain a certain noise level” corresponding to each subject portion in the region where radiation is simultaneously irradiated. It is necessary to match.

For this reason, when the above method is applied to a scan using a multi-slice type radiation CT apparatus, the time required for imaging and the exposure dose to the subject are traded off (trade).
off). If you set the range to which the radiation is simultaneously irradiated, the imaging time will be shortened because the range that can be scanned simultaneously will increase, but the range of change of the “sufficient dose necessary to obtain a certain noise level” corresponding to that range will be Increased dose increases unnecessary exposure. On the other hand, if the range where radiation is simultaneously irradiated is set to be small, the amount of change in the “sufficient and sufficient dose to obtain a certain noise level” corresponding to that range will be small, so unnecessary dose will decrease. Since the range that can be scanned simultaneously becomes smaller, the shooting time becomes longer.

  In view of the above circumstances, an object of the present invention is to provide a radiation CT apparatus in which a balance between an imaging time and an exposure dose to a subject is appropriate.

  In a first aspect, the present invention provides an irradiation unit that irradiates a subject with radiation, a detection unit that is disposed opposite to the irradiation unit with the subject interposed therebetween, detects the radiation, the irradiation unit, A collimator provided between the subject and capable of adjusting the width of the subject in the body axis direction of an aperture through which the radiation passes, and can be rotated around the body axis of the subject A rotating unit supported by the moving unit, a moving unit for moving the rotating unit relative to the subject in the body axis direction, and scanning the subject to obtain projection data (data) of the subject. In a radiation CT apparatus comprising: a control unit that controls a rotating unit and the moving unit; and an image generation unit that generates an image of the subject based on the projection data. A first irradiation dose distribution determining means for determining a first irradiation dose distribution representing a dose of radiation to be irradiated to a portion corresponding to the position; and the subject to which the radiation passing through the aperture is irradiated simultaneously. The radiation in the body axis direction with respect to the subject is irradiated so that the maximum dose of radiation on the first irradiation radiation dose distribution corresponding to the region in the body axis direction is irradiated to each position in the region. A second irradiation dose distribution determining means for determining a second irradiation dose distribution representing a relationship between a position and a dose of the radiation to be set when the irradiation unit is positioned at the position; The body axis direction with respect to the subject so that the distribution of the exposure dose with respect to the body axis direction of the subject becomes a desired distribution when scanned by irradiating the subject with radiation according to the irradiation dose distribution An aperture width distribution determining means for determining an aperture width distribution representing a relationship between the position of the aperture and the width of the aperture to be set when the irradiation unit is positioned at the position, and the control means includes the aperture width The rotating unit and the moving unit are controlled to scan the subject by adjusting the width of the aperture and the dose of the radiation according to the second irradiation radiation dose distribution based on the distribution and the aperture width distribution. A radiation CT apparatus is provided.

  In a second aspect, the present invention provides the first radiation dose distribution determining means in the subject radiation absorption information that reflects at least a change in the radiation absorbed dose of the subject with respect to the body axis direction of the subject. The radiation CT apparatus according to the first aspect for determining the first irradiation radiation dose distribution is provided.

  In a third aspect, the present invention provides the radiation CT apparatus according to the second aspect, wherein the subject radiation absorption information is basic projection data obtained by previously scanning the subject with the rotating unit. .

  In a fourth aspect, the present invention provides the third aspect, wherein the basic projection data is projection data obtained by scanning the subject along the body axis direction with the rotational position of the rotating unit fixed. The radiation CT apparatus of the viewpoint is provided.

  In a fifth aspect, the present invention provides the fourth aspect from the second aspect, wherein the first irradiation radiation dose distribution determining means determines the first irradiation radiation dose distribution based on the sensitivity of the imaging region. A radiation CT apparatus according to any one of the aspects is provided.

  In a sixth aspect, the present invention provides the first embodiment of the present invention, wherein the first irradiation radiation dose distribution determining means generates an image noise level corresponding to each position in the body axis direction of the subject generated by the image generating means. There is provided a radiation CT apparatus according to any one of the first to fifth aspects, wherein the first irradiation radiation dose distribution is determined so as to reach a predetermined target level.

  In a seventh aspect, the present invention provides the aperture width distribution determining means, wherein the aperture width distribution is determined based on a change in radiation dose in each local region of the first irradiation radiation dose distribution. A radiation CT apparatus according to any one of the sixth aspects from a viewpoint is provided.

  In an eighth aspect, the present invention relates to the second condition under a predetermined condition in which the width of the aperture in the body axis direction is fixed and under a predetermined condition in which the width of the aperture in the body axis direction is fixed. An exposure dose distribution representing a relationship between the position of the subject in the body axis direction and the exposure dose of the subject portion corresponding to the position when the subject is scanned by irradiating radiation according to the irradiation dose distribution. The exposure dose distribution generating means for generating the aperture when the width of the aperture is fixed to a plurality of different widths, wherein the aperture width distribution determining means is a plurality of exposures generated by the exposure dose distribution generating means. The seventh aspect for determining the aperture width distribution based on the relationship between the width of the aperture and the exposure dose of the subject, obtained from the dose distribution To provide a radiation CT apparatus.

  In a ninth aspect, the present invention relates to a first exposure dose distribution obtained when the exposure dose distribution generation unit fixes the width of the aperture to a first width, and the width of the aperture. And a second exposure dose distribution when the second exposure dose distribution is fixed to a second width larger than the first width, and the aperture width distribution determining means has the first exposure dose distribution in which the positions in the body axis direction correspond to each other. The radiation CT apparatus according to the eighth aspect, wherein the aperture width distribution is determined based on the magnitude of the ratio between the exposure dose in the exposure dose and the exposure dose in the second exposure dose distribution.

  According to a tenth aspect, in the present invention, the aperture width distribution determining means has a ratio of the exposure dose in the second exposure dose distribution to the exposure dose in the first exposure dose distribution equal to or greater than a predetermined threshold value. The radiation CT apparatus according to the ninth aspect, wherein the aperture width distribution is determined so that the position of the aperture to be set in the body axis direction is the first width.

  In an eleventh aspect, according to the present invention, the aperture width distribution determining means is configured so that a width in the body axis direction of the aperture to be set is set at a position where the ratio of the exposure dose is less than the predetermined threshold value. The radiation CT apparatus according to the ninth aspect or the tenth aspect for determining the aperture width distribution so as to be the second width.

In a twelfth aspect, the present invention provides the control unit, wherein the subject is subjected to a helical scan.
The relative moving speed of the rotating unit according to the width of the aperture is controlled so that the helical pitch is constant or less than a predetermined value during the helical scan. The radiation CT apparatus according to any one of the first to eleventh aspects, wherein the moving means is controlled to adjust the distance.

  In a thirteenth aspect, the present invention provides the first aspect in which the irradiation unit includes an X-ray tube, and the control unit adjusts the radiation dose by adjusting a tube current of the X-ray tube. To the radiation CT apparatus according to any one of the twelfth aspects.

  Here, the helical pitch is the amount of movement of the rotation unit relative to the subject while the rotation unit makes one rotation with respect to the irradiation width of the radiation in the body axis direction in the image reconstruction space. Mean percentage.

  The noise level can be considered to indicate, for example, how much noise is included in the image or how much the SN ratio is.

  As a method for determining the first radiation dose distribution, for example, methods described in Japanese Patent Application Laid-Open Nos. 2003-070779 and 2001-218761 can be applied.

  Sensitivity means the extent to which it is adversely affected by radiation exposure.

  According to the radiation CT apparatus of the present invention, the distribution of the exposure dose with respect to the body axis direction of the subject when the aperture width distribution determining unit scans the subject by irradiating the subject with radiation according to the second irradiation dose distribution. The aperture width distribution is determined so that the desired distribution is obtained. Therefore, in areas where the dose of radiation to be irradiated varies greatly, the aperture width should be reduced to give priority to suppression of unnecessary exposure, and radiation to be irradiated. In a region where the dose change is small, priority can be given to improving the scanning efficiency by increasing the width of the aperture, and it is possible to realize a radiation CT apparatus with an appropriate balance between the imaging time and the exposure dose to the subject. It becomes possible.

(First embodiment)
FIG. 1 is a block diagram showing an overall configuration of an X-ray CT apparatus (radiation CT apparatus) 1 according to an embodiment of the present invention.

  The X-ray CT apparatus 1 is configured as a CT apparatus that collects projection data of a subject from a plurality of view directions by so-called helical scanning, and performs image reconstruction based on the projection data. The present invention can also be applied to a non-helical scan X-ray CT apparatus.

  The X-ray CT apparatus 1 includes a scanning gantry 2, an operation console 3, and an imaging table (moving means) 4.

The scanning gantry 2 detects an X-ray tube (irradiation unit) 20 that irradiates X-rays, a collimator 22 that shapes the X-rays irradiated from the X-ray tube 20, and X-rays irradiated from the X-ray tube 20. An X-ray detector (detection unit) 23 that outputs an electrical signal corresponding to the detected X-ray dose, a data collection unit 24 that collects projection data based on the electrical signal output from the X-ray detector 23, and an X-ray An X-ray tube controller (controller) 25 that drives and controls the tube 20, and a collimator controller (collimator) that drives and controls the collimator 22
controller) 26.

  In addition, the scanning gantry 2 includes an X-ray tube 20 and an X-ray detector 23, and includes a rotation unit (rotation unit) 27 that rotates integrally therewith, and a rotation controller 28 that drives and controls the rotation unit 27. ing. The scanning gantry 2 includes a bore 29 that is a hollow portion into which a subject is carried, and an X-ray tube 20 and an X-ray detector 23 are disposed to face each other with the bore 29 interposed therebetween.

  The operation console 3 converts the projection data collected by the data collection unit 24 based on signals from the input device 31 that outputs a signal according to the input operation of the operator and various devices such as the input device 31 and the scanning gantry 2. A central processing unit 30 that executes various processes such as an image reconstruction process based on it, a display device 32 that displays a CT image or the like reconstructed by the central processing unit 30, and a program ( program), data, and an X-ray CT image.

The imaging table 4 includes a cradle 41 on which a subject is placed and taken in and out of the bore 29 of the scanning gantry 2. The cradle 41 is, for example, a servo motor (servo) (not shown) built in the imaging table 4.
and the servo motor is controlled based on a control signal from the central processing unit 30 via a servo amplifier (not shown).

  FIG. 2 is a schematic diagram showing an imaging state by the X-ray CT apparatus 1. In the present embodiment, the body axis direction of the subject is described as the z-axis direction, the channel direction of the X-ray detector is defined as the x-axis direction, and the direction perpendicular to the x-axis and the z-axis is described as the y-axis direction.

  As shown in FIG. 2, the X-ray tube 20 and the X-ray detector 23 rotate around the body axis (z axis) of the subject H when the rotating unit 27 rotates. On the other hand, the cradle 41 conveys the subject H in the z-axis direction. As a result, the X-ray tube 20 and the X-ray detector 23 relatively move around the subject H in a spiral.

  FIG. 3 is a perspective view showing details of the collimator 22 and the X-ray detector 23.

  As shown in FIG. 3, the X-ray tube 20 and the X-ray detector 23 are disposed to face each other with the bore 29 interposed therebetween. The collimator 22 is located between the X-ray tube 20 and the X-ray detector 23 and in the vicinity of the X-ray tube 20.

  The collimator 22 includes a plurality of collimator plates 221 that define the aperture ap, a plurality of shafts (not shown) fixed to the plurality of collimators 221, and a plurality of motors (not shown) that respectively drive the plurality of shafts. And. Here, a pair of collimator plates 221 extending in the x-axis direction are arranged in parallel in the z-axis direction, and a pair of collimator plates 221 extending in the z-axis direction are further arranged in parallel in the x-axis direction. . Therefore, by moving the pair of collimator plates 221 arranged in parallel in the z-axis direction in the z-axis direction, the width of the aperture ap in the z-axis direction can be adjusted, and the aperture ap can be adjusted in parallel in the x-axis direction. By moving the pair of collimator plates 221 arranged in the x-axis direction, the width of the aperture ap in the x-axis direction can be adjusted. Each collimator plate 221 moves when a shaft is driven by a motor. The motor is controlled by a collimator controller 26.

  The collimator plate 221 is made of a material capable of blocking X-rays, for example, lead or tungsten.

  The collimator 22 is merely an example, and any collimator 22 may be used as long as the mechanism can adjust at least the width of the aperture ap in the z-axis direction.

  The X-ray detector 23 is a so-called multi-row detector. That is, a plurality of detection elements 231 are arranged in the channel direction (a direction orthogonal to the z-axis), and the detection elements 231 are also arranged in the z-axis direction. The number in the channel direction and the number of columns in the z-axis direction may be set as appropriate. For example, the number in the channel direction is 1024, and the number of columns in the z-axis direction is 4 to 128 columns.

  The detection element 231 includes a scintillator and a photoelectric conversion element such as a photodiode, and can output an electrical signal corresponding to the incident X-ray dose. Therefore, the data collection unit 24 collects information on the X-ray dose incident on the plurality of detection elements 231 and outputs the collected information to the central processing unit 30.

  The central processing unit 30 of the operation console 3 includes a control unit (control unit) 30a, a subject X-ray absorption information acquisition unit 30b, and a first tube current curve determination unit (first irradiation radiation dose distribution determination unit) 30c. An exposure curve generation section (exposure dose distribution generation means) 30d, an aperture width curve determination section (aperture width distribution determination means) 30e, and a second tube current curve determination section (second irradiation radiation dose distribution determination means). 30f and an image generation unit (image generation means) 30g. These units constituting the central processing unit 30 are constructed, for example, by the central processing unit 30 executing a program recorded in the storage device 33 or the like.

  The control unit 30a controls the rotation unit 27, the imaging table 4, and the like so as to scan the subject H and obtain projection data of the subject H. More specifically, the control unit 30a includes the X-ray tube controller 25, the collimator controller 26, the rotation controller 28, and the servo amplifier built in the imaging table 4, and the X-ray tube 20, the collimator 22, the rotation unit 27, The cradle 41 is driven and controlled.

Here, the control unit 30a scans the subject H with a scout scan (scout).
These units are controlled so as to be scanned. The scout scan is performed by, for example, scanning the subject along the body axis direction while fixing the rotation position of the rotation unit 27 of the scanning gantry 2.
In addition, the control unit 30a performs a main scan on the subject H according to the aperture width curve determined by the aperture width curve determination unit 30e and the second tube current curve determined by the second tube current curve determination unit 30f. Control each of these parts.

  Further, the control unit 30a changes the width of the aperture ap of the collimator 22 in the z-axis direction during scanning (hereinafter also simply referred to as the aperture width) according to the aperture width so that the helical pitch becomes constant. The cradle 41 of the imaging table 4 is controlled to adjust the transport speed of the subject H.

  The X-ray absorption information acquisition unit 30b acquires X-ray absorption information reflecting changes in the X-ray absorption dose of the subject with respect to the z-axis direction.

  Here, the X-ray absorption information acquisition unit 30b acquires basic projection data obtained by performing a scout scan of the subject H in advance by the rotation unit 27 as X-ray absorption information.

  Since the basic projection data is data obtained by detecting X-rays that are partially absorbed by the subject H and transmitted through the X-rays irradiated to the subject H, the X-ray absorbed dose of the subject H is Reflected accurately.

  The X-ray absorption information acquisition unit 30b is based on projection data obtained by a low-dose helical scan, an axial scan, or the like performed immediately before the main scan, or projection data obtained by the main scan at the time of past imaging. You may acquire as projection data.

  In addition, the X-ray absorption information acquisition unit 30b includes a visible image representing a three-dimensional shape of the subject H obtained by imaging the subject H with a visible light camera, the sex, height, weight, age, race of the subject. Based on such information, basic projection data may be estimated and acquired. However, the X-ray absorbed dose of the subject H is perpendicular to the organ type of the subject portion corresponding to the target position in the z-axis direction of the subject H and the z-axis, assuming that the dose of X-rays to be irradiated is constant. Depends on the cross-sectional area in different directions. For example, the X-ray absorbed dose is small because the cross-sectional area is relatively small at the neck position of the subject H, and the X-ray absorbed dose is large because the cross-sectional area is relatively large at the body portion of the subject H. Even in the same torso, the X-ray absorbed dose is small because there is a lung containing a lot of air at the chest position, and the X-ray absorbed dose is large at positions other than the chest. Therefore, as X-ray absorption information, it can be said that projection data obtained by actual scanning is more suitable than information estimated from a visible image or the like of the subject H.

FIG. 8 is a diagram illustrating an example of a graph obtained based on the basic projection data. This graph is a graph obtained based on basic projection data when the subject H is scout scanned while irradiating X-rays in the y-axis direction. The horizontal axis is the channel of the X-ray detector 23, and the vertical axis. Is a logarithmic form ratio of the X-ray dose I ₀ irradiated to the scout scan position of interest and the X-ray dose I transmitted through the scout scan position. The area S (hereinafter referred to as the transmission area) of the region surrounded by the curve and the horizontal axis of this graph is determined according to the X-ray dose absorbed by the subject H. Therefore, a change in the X-ray absorbed dose in the z-axis direction of the subject H is grasped by obtaining a transmission area curve A representing the change in the transmission area S in the z-axis direction of the subject H based on the basic projection data. be able to.

  The first tube current curve determination unit 30c is related to the position of the subject H in the z-axis direction and the tube current of the X-ray tube 20 to be set when the subject portion corresponding to the position is irradiated with X-rays. A first tube current curve I1 representing is determined. The tube current of the X-ray tube 20 to be set is the position of the subject H in the z-axis direction when the subject H is scanned with the tube voltage of the X-ray tube 20 and the rotation speed of the rotating unit 27 constant. In order to set the noise level of the image generated corresponding to the above to a certain target level, the X-ray dose that needs to be irradiated to the subject portion corresponding to each of these positions is defined. That is, the first tube current curve I1 irradiates the subject portion corresponding to each position in order to align the image quality level of each image generated corresponding to each position of the subject H in the z-axis direction. It represents the tube current of the X-ray tube 20 that defines the X-ray dose to be performed.

  Here, the first tube current curve determination unit 30c determines the first tube current curve I1 based on the basic projection data acquired by the X-ray absorption information acquisition unit 30b. The noise level of the image generated corresponding to the position of interest of the subject H in the z-axis direction is basically applied to the X-ray absorbed dose of the subject portion corresponding to the position of interest and the subject portion. It depends on the ratio of the X-ray to the tube current of the X-ray tube 20. Therefore, the first tube current curve I1 is basically a curve close to the transmission area curve A. However, depending on the age, sex, etc. of the subject H, there may be an organ whose X-ray dose to be irradiated should be smaller than usual. In such a case, the tube current depends on the position of the organ. In some cases, the curve is adjusted to be a considerably different curve from the transmission area curve A.

  The exposure curve generation unit 30d scans the subject H under a predetermined condition based on the first tube current curve I1, and each subject position in the z-axis direction of the subject H and the subject portion corresponding to each position. An exposure curve that represents the relationship with the exposure dose is estimated and generated. Note that the exposure dose is proportional to the product of the tube current of the X-ray tube 20 and the X-ray irradiation time, assuming that the tube voltage of the X-ray tube 20 when irradiating X-rays is constant. Here, the following conditions are considered as the predetermined conditions.

  The tube voltage of the X-ray tube 20 is made constant, the aperture width of the collimator 22 is fixed to a predetermined width, and a helical scan is performed at a predetermined helical pitch while rotating the rotating unit 27 at a predetermined rotational speed. During the helical scan, the maximum tube current among the tube currents corresponding to the respective positions in the region on the first tube current curve I1 with respect to the region irradiated with the X-rays simultaneously through the aperture of the collimator 22 The tube current of the X-ray tube 20 is adjusted according to the position of the X-ray tube 20 so that the X-rays are irradiated.

  That is, the exposure curve corresponds to a case where the subject H is helically scanned with the aperture width of the collimator 22 fixed, and corresponds to the position of the subject corresponding to each position of the subject H in the z-axis direction. This shows how much the subject portion corresponding to each of these positions is exposed when X-rays with a dose such that the noise of the generated image is below a certain level are irradiated.

  The exposure curve generation unit 30d generates such an exposure curve when the aperture width of the collimator 22 is fixed to a plurality of different widths. Here, the exposure curve generator 30d fixes the first exposure curve D1 when the aperture width of the collimator 22 is fixed to the first width d1, and the aperture width is fixed to the second width d2 (d2> d1). Then, a second exposure curve D2 is generated.

  The aperture width curve determination unit 30e causes the exposure dose distribution in the z-axis direction of the subject H to be a desired distribution when the subject H is scanned by irradiating the subject H with X-rays according to a second current curve described later. Then, an aperture width curve Apt representing the relationship between the position of the subject H in the z-axis direction and the aperture width of the collimator 22 to be set when the X-ray tube 20 is positioned at that position is determined. In addition, the aperture width curve determination unit 30e determines the aperture width curve Apt based on the change in the tube current in each local region of the first tube current curve I1.

  For example, the aperture width curve determination unit 30e calculates the aperture width curve Apt based on the relationship between the aperture width of the collimator 22 and the exposure dose of the subject H obtained from the plurality of exposure curves generated by the exposure curve generation unit 30d. decide. More specifically, for example, the aperture width curve determination unit 30e is based on the ratio of the exposure dose in the first exposure curve D1 and the exposure dose in the second exposure curve D2 corresponding to the same position of the subject H. Then, the aperture width curve Apt is determined.

  Here, the aperture width curve determination unit 30e should set the position where the ratio Dr of the exposure dose in the second exposure curve D2 to the exposure dose in the first exposure curve D1 is equal to or greater than the predetermined threshold th. An aperture in which the aperture width of the collimator 22 is a first width d1 and the aperture width of the collimator 22 to be set is a second width d2 at a position where the ratio Dr of the exposure dose is less than a predetermined threshold th. The width curve Apt is determined. That is, the exposure doses of the subject portions corresponding to the respective positions in the z-axis direction are compared between when the aperture width of the collimator 22 is fixed to the first width d1 and when the aperture width is fixed to the second width d2. . In a position where the difference is large, priority is given to the suppression of the exposure dose, and the relatively small first width d1 is adopted as the aperture width of the collimator 22, and in a position where the difference is small, priority is given to higher scanning efficiency. Then, a relatively large second width d2 is employed as the aperture width of the collimator 22.

  The second tube current curve determination unit 30f includes a position in the z-axis direction relative to the subject H when scanning the subject H under a predetermined condition according to the collimator width curve Apt determined by the collimator width curve determination unit 30e. A second tube current curve I2 representing the relationship with the tube current to be set when the X-ray tube 20 is located at that position is determined. Here, the following conditions are considered as the predetermined conditions.

  The aperture width of the collimator 22 is set to a predetermined width according to the aperture width curve Apt in accordance with the position of the X-ray tube 20, and helical scanning is performed at a predetermined helical pitch while rotating the rotating unit 27 at a predetermined rotational speed. During the helical scan, the maximum tube current among the tube currents corresponding to the respective positions in the region on the first tube current curve I1 with respect to the region irradiated with the X-rays simultaneously through the aperture of the collimator 22 Is adjusted according to the position of the X-ray tube 20. That is, when the second tube current curve determination unit 30f scans while changing the aperture width of the collimator 22 in accordance with the aperture width curve Apt, the first tube current curve I1 is obtained for any subject portion to be scanned. The second tube current curve I2 is determined so that X-rays having a tube current equal to or greater than the tube current corresponding to the subject portion defined by (2) are irradiated.

  The image generation unit 30g generates an image of the subject by image reconstruction based on data from the data collection unit 24.

  From now on, the work flow in the X-ray CT apparatus 1 in the first embodiment will be described. Here, a helical scan is performed as the main scan. Further, the position in the z-axis direction where the subject H exists is set to z0 to nz.

  FIG. 4 is a diagram illustrating a workflow in the X-ray CT apparatus 1 according to the first embodiment. FIG. 5 shows a transmission area curve A, a first tube current curve I1, a first exposure curve D1, a second exposure curve D2, and a collimator width curve Apt in the first embodiment. FIG. 6 is a diagram illustrating an example of a correspondence relationship between the second tube current curve I2 and the transport speed Vt of the cradle 41.

  In the X-ray CT apparatus 1, first, a scout scan is performed in order to acquire basic projection data for obtaining the transmission area curve A of the subject H (step S1). In the scout scan, for example, the cradle 41 on which the subject H is placed is transported into the bore 29 without rotating the rotating unit 27 of the scanning gantry 2, and relatively few X-rays are emitted from the subject H by the X-ray tube 20. , And X-rays are detected by the detector 23 to obtain projection data in the front direction of the subject H. The scout scan is performed with each unit being controlled by the control unit 30a of the central processing unit 30.

  In step S2, based on the basic projection data obtained in step S1, a transmission area curve A representing the relationship between the position of the subject H in the z-axis direction and the X-ray absorbed dose of the subject portion corresponding to that position is obtained. To be acquired.

  For example, as shown in FIG. 5, the transmission area curve A acquired in step S <b> 2 has a value corresponding to a position belonging to the periphery of the head and neck of the subject among the respective positions in the z-axis direction of the subject H. The curve changes relatively large. The transmission area curve A is performed by analyzing the basic projection data obtained in step S1 by the first tube current curve determination unit 30c of the central processing unit 30.

  In step S3, based on the transmission area curve A acquired in step S2, in order to set the noise of the image generated corresponding to each position of the subject H in the z-axis direction to a certain level, each of these positions And a first tube current curve I1 representing the relationship between the tube current of the X-ray tube 20 to be set when X-rays are irradiated to the subject portion corresponding to each position. For example, an algorithm for calculating the optimum value of the tube current according to the transmission area S is prepared in advance, and the optimum value of the tube current is calculated for each position in the z-axis direction using the algorithm. Alternatively, a numerical table such as a lookup table in which optimum tube current values corresponding to various transmission areas S are registered in the storage device 33 in advance, and the optimum tube current value is determined by referring to the table.

  The first tube current curve I1 determined in step S3 is a curve close to the transmission area curve A acquired in step S2, for example, as shown in FIG. The first tube current curve I1 is determined based on the transmission area curve A acquired in step S2 by the first tube current curve determination unit 30c of the central processing unit 30 based on information such as the age and sex of the subject. It is determined with appropriate consideration.

  In step S4, the position of the subject H in the z-axis direction when the subject H is scanned under a predetermined condition based on the first tube current curve I1 determined in step S3 and the subject corresponding to the position. An exposure curve representing the relationship with the exposure dose of the part is estimated and generated. As described above, the predetermined condition is that the aperture width of the collimator 22 is fixed to a predetermined width, and a helical scan is performed at a predetermined helical pitch while rotating the rotating unit 27 at a predetermined rotational speed. X-ray with the maximum tube current among the tube currents corresponding to each position in the region on the first tube current curve I1 is irradiated to the region irradiated with X-rays through the 22 apertures simultaneously. As described above, the tube current of the X-ray tube 20 is adjusted according to the position of the X-ray tube 20 with respect to the subject H. Here, the first width d1 is fixed when the aperture width of the collimator 22 is fixed to the first width d1, for example, the width in the z-axis direction irradiated with X-rays is 20 [mm] in the image reconstruction space. The exposure curve D1 and the second aperture when the aperture width is fixed to a second width d2, for example, such that the width in the z-axis direction irradiated with X-rays is 40 mm in the image reconstruction space. An exposure curve D2 is generated. Note that the rotational speed of the rotating unit 27 and the conditions of the helical pitch when estimating the exposure curve are set to the same conditions as in the main scan performed in step S7.

Note that the exposure curve need not be an accurate representation of the exposure dose itself, but may be an index value depending on the exposure dose. As an index value representing the exposure dose, the effective dose (Effective dose)
Dose), CTDI (Computed Tomography Dose Index), DLP (Dose Length product), etc. are known, and a predetermined index value is calculated using or applying them. For example, assuming that the subject H is to be scanned for the actual time, the tube current time of the X-ray tube 20 in the time zone in which the subject portion corresponding to the target position in the z-axis direction of the subject H is irradiated with X-rays. A change is obtained, and an integral value obtained by integrating the tube current along the time axis is calculated as an index value depending on the exposure dose. An exposure curve is generated by calculating and plotting such an index value while changing the target position to a plurality of different positions.

  The first exposure curve D1 and the second exposure curve D2 generated in step S4 are, for example, as shown in FIG. 5, the head and neck of the subject among the positions in the z-axis direction of the subject H. A first exposure curve D1 having a relatively large change in exposure dose corresponding to a position belonging to the periphery, and a second exposure curve that is generally more gradual than the first exposure curve, and whose exposure dose is always greater than that of the first exposure curve. Exposure curve D2. The exposure curve is generated by the exposure curve generation unit 30d of the central processing unit 30 based on the first tube current curve I1 determined in step S3.

  In step S5, based on the first exposure curve D1 and the second exposure curve D2 generated in step S4, the position in the z-axis direction with respect to the subject H and the X-ray tube 20 are positioned at that position. An aperture width curve Apt representing the relationship with the aperture width of the collimator 22 to be set is determined. The aperture width curve Apt is determined according to the following procedure, for example.

Is initialized by setting the variable z representing the position of the z-axis direction with respect to the subject H in z ₀ (step S51).

  Next, a ratio Dr (z) (= D2 (z) / D1 (z)) of the exposure dose at the position z in the second exposure curve D2 to the exposure dose at the position z in the first exposure curve D1 is calculated. (Step S52).

  Then, it is determined whether or not the ratio Dr (z) is a predetermined threshold value th, for example, 1.2 or more (step S53). If it is determined that the ratio Dr (z) is greater than or equal to the threshold th, the aperture width Apt (z) of the collimator 22 at that position z is set to the first width d1 (step S54). ). On the other hand, when it is determined that the ratio Dr (z) is less than the threshold value th, the aperture width Apt (z) of the collimator 22 at the position z is set to the second width d2 (step S55). .

  Then, it is determined whether or not the variable z is zn (step S56). Here, when it is determined that the variable z is zn, it is determined that the determination of the aperture width at all positions is completed, and the generation process of the aperture width curve Apt ends. On the other hand, if it is determined that the variable z is not zn, it is determined that the setting of the aperture width at all positions is not yet completed, the variable z is incremented (step S57), and the process returns to step S52.

  In this manner, the aperture width of the collimator to be set at all the positions z is determined, and the aperture width curve Apt is generated.

  For example, as shown in FIG. 5, the exposure dose ratio Dr (z) is equal to or greater than the threshold th at positions z0 to za corresponding to the periphery of the head and neck of the subject. Also, the position za to zn corresponding to the periphery of the leg or the like is less than the threshold value th. In addition, as shown in FIG. 5E, the aperture width curve Apt at this time has the aperture widths Apt (z0) to Apt (za) at the positions z0 to za as the first width d1, and the positions za to zn. Aperture widths Apt (za) to Apt (zn) in FIG. 4 are curves (broken lines) having the second width d2.

  The collimator width curve is determined by the collimator width curve determination unit 30e of the central processing unit 30.

  In step S6, the first tube current curve I1 determined in step S2 is used to scan the subject H under a predetermined condition according to the aperture width curve Apt generated in step S5. A second tube current curve I2 representing the relationship between the position in the z-axis direction and the tube current to be set when the X-ray tube 20 is located at that position is generated. The second tube current curve I2 is generated according to the following procedure.

  First, the variable z representing the position in the z-axis direction with respect to the subject H is set to z0 and initialized (step S61).

  Next, when the X-ray tube 20 is positioned at the position z and the width to be set when the X-ray tube 20 is positioned at the position, which is represented by the aperture width curve Apt, is set as the aperture width of the collimator 22 The region irradiated simultaneously through the aperture of the collimator 22 is specified. Then, in the first tube current curve I1, the tube current corresponding to each position zj to zk in the z-axis direction in the specified region is specified, and the maximum tube current among the specified tube currents is The tube current I2 (z) to be set when the X-ray tube 20 is located at the position z is determined (step S62).

  Then, it is determined whether or not the variable z is zn (step S63). Here, when it is determined that the variable z is zn, it is determined that the determination of the tube current at all positions is completed, and the determination process of the second tube current curve I2 is ended. On the other hand, if it is determined that the variable z is not zn, it is determined that the determination of the tube current at all positions is not yet completed, the variable z is incremented (step S64), and step S62 is performed. Return to.

  In this way, the tube current to be set at all positions z is determined, and the second tube current curve I2 is determined.

  For example, as shown in FIG. 5, the second tube current curve I2 is a curve obtained by broadening the first tube current curve I1 as a whole. If a relatively large second width d2 is set as the aperture width of the collimator 22 at positions z0 to za, the first tube current curve I1 becomes a broad curve in this range.

  The determination of the second tube current curve I2 is performed by the second tube current curve determination unit 30f of the central processing unit 30.

  Here, each curve is obtained on the assumption that the position of the X-ray tube 20 is changed at positions z0 to zn where the subject exists, but the position of the X-ray tube 20 is set to the positions z0 to zn. A case of changing in a wider range including zn or a narrower range corresponding to a part of the positions z0 to zn may be assumed. Even in such a case, each curve can be obtained by the same method as described above.

  In step S7, the main scan is performed based on the aperture width curve Apt and the second tube current curve I2 determined in steps S5 and S6. That is, the central processing unit 30 drives the rotating unit 27 and the cradle 41. When the X-ray tube 20 reaches a position where the subject can be irradiated with X-rays, the central processing unit 30 emits X-rays from the X-ray tube 20 and collects data. Data from the unit 24 is recorded in the storage device 33. At this time, the collimator controller 26 controls the collimator 22 so as to change the aperture width of the collimator 22 in accordance with the aperture width curve Apt based on the control signal from the control unit 30a. The X-ray tube controller 25 controls the X-ray tube 20 so as to change the tube current of the X-ray tube 20 according to the second tube current curve I2 based on the control signal from the control unit 30a. Further, the control unit 30a controls the imaging table 4 so as to adjust the transport speed Vt of the cradle 41 so that the helical pitch becomes constant according to the set aperture width. For example, as shown in FIG. 5, the transport speed Vt of the cradle 41 is a high-speed V1 at positions z0 to za where the first width d1 is set as the aperture width, and is larger than the first width d1 as the aperture width. The positions z0 to za where the second width d2 is set are represented by a curve that is a low speed V2. Note that X-ray detection from the X-ray tube 20 is performed in a plurality of view directions while the X-ray tube 20 makes one round of the subject.

  In step S8, an image is reconstructed based on the projection data obtained by the scan, and a tomographic image of the subject is generated. The generated image or image data is appropriately output to an output device such as the display device 32, the storage device 33, or a printer (not shown). The generation of the tomographic image of the subject is performed by the image generation unit 30g of the central processing unit 30.

  According to the present embodiment described above, the exposure width of the subject H in the z-axis direction when the aperture width curve determination unit 30e scans the subject H with X-rays according to the second tube current curve I2 is scanned. Since the aperture width curve Apt is determined so that the distribution becomes a desired distribution, in a region where the change in the X-ray dose to be irradiated is large, priority is given to the suppression of unnecessary exposure by reducing the aperture width. In an area where the change in X-ray dose to be performed is small, priority can be given to improving the scanning efficiency by increasing the aperture width, and the X-ray CT apparatus has an appropriate balance between the imaging time and the exposure dose to the subject. Can be realized.

  In addition, according to the present embodiment, an exposure curve is generated when the aperture width of the collimator 22 is fixed to two different widths, and the aperture width and the subject obtained from a plurality of generated exposure curves are obtained. Since the aperture width is determined based on the relationship with the exposure dose, it is possible to intuitively understand how the scan time and exposure dose change due to changes in the aperture width. Can be easily set to a desired balance.

  In addition, according to the present embodiment, the aperture width is switched between two predetermined widths according to the position z, so that the aperture width can be changed with relatively simple control. For a device that already has a width switching control mechanism, the control mechanism can be used as it is, and is easy to implement.

(Second Embodiment)
FIG. 6 is a diagram illustrating a correspondence relationship between the curves in the second embodiment. Note that the X-ray CT apparatus of the second embodiment has the same configuration as that of the first embodiment. The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  In the second embodiment, the aperture width curve determination method by the collimator width curve determination unit 30e is different from the first embodiment. In the second embodiment, the aperture width Apt (z) of the collimator 22 at the position z is set so as to continuously change in accordance with the dose ratio Dr (z).

  A workflow in the X-ray CT apparatus 1 in the second embodiment will be described.

  FIG. 6 is a diagram illustrating a workflow in the X-ray CT apparatus according to the second embodiment. FIG. 7 shows a transmission area curve A, a first tube current curve I1, a first exposure curve D1, a second exposure curve D2, and a collimator width curve Apt in the second embodiment. FIG. 6 is a diagram illustrating an example of a correspondence relationship between the second tube current curve I2 and the transport speed Vt of the cradle 41.

  In the workflow in the second embodiment, steps S11 to S14 correspond to steps S1 to S4 in the first embodiment, and steps S16 to S18 correspond to steps S6 to S8 in the first embodiment. It corresponds. For this reason, description thereof will be omitted, and only step S15 will be described here. The transmission area curve A, the first tube current curve I1, the first exposure curve D1, the second exposure curve D2, and the exposure dose ratio Dr obtained in Steps S11 to S14 are respectively Similar to the corresponding curves in one embodiment, but are also shown in FIG. 7 for convenience.

  In step S15, the aperture width curve Opt is determined based on the first exposure curve D1 and the second exposure curve D2 generated in step S4. The aperture width curve Apt is determined according to the following procedure, for example.

  First, a variable z representing a position in the z-axis direction is set to z0 and initialized (step S151). Next, the ratio of the exposure dose at the position z in the second exposure curve D2 to the exposure dose at the position z in the first exposure curve D1 = Dr (z) (= D2 (z) / D1 (z)). Calculate (step S152). Then, a weight w is set according to the following equation (1) (step S153).

When DL> Dr (z), w = 0
When DU ≧ Dr (z) ≧ DL, w = (Dr (z) −DL) / (DU−DL)
When Dr (z)> DU, w = 1 (1)

  Here, DL is a first reference value (eg, 1.1), and DU is a second reference value (eg, 1.3).

That is, when DU ≧ Dr (z) ≧ DL, the weight w becomes closer to 0 as the ratio Dr (z) is closer to the first reference value DL, and the ratio Dr (z) is closer to the second reference value DU. The value of the weight w is set so that the weight w becomes closer to 1. The weight w is set to 0 when DL> Dr (z), and the weight w is set to 1 when Dr (z)> DU.
Further, the aperture width Apt (z) of the collimator 22 at the position z is set according to the following equation (2) (step S154).

      Apt (z) = w × d1 + (1−w) × d2 (2)

  That is, the aperture width Apt (z) is set such that the closer the weight w is to 1, the closer the aperture width is to d1, and the closer the weight w is to 0, the closer the aperture width is to d2.

  Then, it is determined whether or not the variable z is zn (step S155). Here, when it is determined that the variable z is zn, it is determined that the determination of the aperture width at all positions is completed, and the generation process of the aperture width curve Apt ends. On the other hand, when it is determined that the variable z is not zn, it is determined that the setting of the aperture width at all positions is not yet completed, the variable z is incremented (step S156), and the process returns to step S152.

  In this way, the aperture width of the collimator to be set at all the positions z is determined, and the aperture width curve Apt is determined. For example, as shown in FIG. 7, the aperture width curve Apt is a curve obtained by multiplying the exposure dose ratio Dr by a negative coefficient. In other words, the aperture width is set to be smaller as the ratio Dr is larger and larger as the ratio Dr is smaller, and the change according to the position z is continuous.

  The second tube current curve I2 determined in step S16 is, for example, as shown in FIG. 7, compared with the second tube current curve in the first embodiment, more first tube current curve. The curve is close to I1.

  Further, the transport speed Vt of the cradle 41 at this time is a curve similar to the aperture width curve Apt as shown in FIG. 7 in order to keep the helical pitch substantially constant. That is, the conveyance speed Vt is set so as to increase as the aperture width increases and decreases as the aperture width decreases, and the change according to the position z is continuous.

  According to the present embodiment described above, the aperture width Apt (z) at the position z is set as a value that continuously changes in accordance with the above-mentioned exposure dose ratio Dr (z). Scanning can be performed while always maintaining the optimal balance.

  In addition, this invention is not limited to the above embodiment, You may implement in a various aspect.

  In the above embodiment, the second tube current curve is a curve in which the tube current does not change according to the view direction. However, each of the plurality of view directions, e.g. Based on a plurality of basic projection data obtained by projection, the cross-sectional shape of the subject H is obtained by ellipse approximation or the like, and a tube current corresponding to the thickness of the subject H for each view direction is obtained as a second tube current curve. You may obtain | require the curve which changes.

  Moreover, in the said embodiment, although the aperture width was set based on the exposure dose curve produced | generated from the 1st tube current curve, the change of the X-ray absorbed dose with respect to the z-axis direction of the subject H was reflected. The aperture width can be set using various indexes based on the X-ray absorption information. For example, the aperture width may be set so that the aperture width is switched between the change point where the transmission area changes greatly on the transmission area curve A or the change point where the tube current changes greatly on the first tube current curve. In addition, for example, based on X-ray absorption information such as basic projection data and a visible image of the subject H, a position of a change point where the X-ray absorbed dose of the subject H changes greatly, for example, a position corresponding to between the neck and shoulder The aperture width may be switched between large and small with the specified position as a boundary, or the aperture width may be continuously changed based on the specified position.

  In the above embodiment, the first tube current curve is determined based on the basic projection data, but may be determined based on the sensitivity of the imaging region. For example, it is generally known that the sensitivity of the thyroid gland is large. Therefore, when the imaging region includes the neck of a human body, the first tube current curve is set so that the tube current for the position around the neck is smaller than usual. You may decide.

1 is a block diagram showing a configuration of an X-ray CT apparatus according to a first embodiment of the present invention. It is the schematic which shows the imaging | photography state by the X-ray CT apparatus in 1st Embodiment. It is a perspective view which shows the detail of the collimator and detector of the X-ray CT apparatus in 1st Embodiment. It is a flowchart which shows the operation | movement procedure in the X-ray CT apparatus of 1st Embodiment. It is a figure which shows the example of each curves, such as an aperture width curve, in 1st Embodiment. It is a flowchart which shows the operation | movement procedure in the X-ray CT apparatus of 2nd Embodiment. It is a figure which shows the example of each curves, such as an aperture width curve, in 2nd Embodiment. It is a figure which shows an example of the graph obtained based on basic projection data.

Explanation of symbols

1 X-ray CT system (radiation CT system)
2 Scanning gantry 3 Operation console 4 Imaging table (moving means)
20 X-ray tube (irradiation part)
21 X-ray tube moving part 22 Collimator (collimator)
23 X-ray detector (detector)
231 Detection element 24 Data collection unit 25 X-ray tube controller 26 Collimator controller 27 Rotating unit
28 Rotation controller 29 Bore 30 Central processing unit 30a Control unit (control means)
30b X-ray absorption information acquisition unit 30c first tube current curve determination unit (first irradiation dose distribution determination means)
30d exposure curve generator (exposure dose distribution generator)
30e Aperture width curve determining section (aperture width distribution determining means)
30f 2nd tube current curve determination part (2nd irradiation radiation dose distribution determination means)
30g image generation unit (image generation means)
31 Input device 32 Display device 33 Storage device

Claims (13)

An irradiation unit that irradiates a subject with radiation; a detection unit that is disposed opposite to the irradiation unit with the subject interposed therebetween; and that is provided between the irradiation unit and the subject. A collimator capable of adjusting the width of the subject in the body axis direction of the aperture through which the radiation passes, and a rotating unit supported rotatably around the body axis of the subject;
Moving means for moving the rotating unit relative to the subject in the body axis direction;
Control means for controlling the rotating unit and the moving means to scan the subject and obtain projection data of the subject;
In a radiation CT apparatus comprising: an image generation unit that generates an image of the subject based on the projection data;
First irradiation radiation dose distribution determining means for determining a first irradiation radiation dose distribution representing a dose of radiation to be irradiated to a portion corresponding to each position in the body axis direction of the subject;
The radiation of the maximum dose among the doses on the first irradiation dose distribution corresponding to the region in the body axis direction of the subject to which the radiation passing through the aperture is simultaneously irradiated is each position in the region. A second irradiation radiation dose distribution representing a relationship between a position in the body axis direction relative to the subject and the radiation dose to be set when the irradiation unit is positioned at the position. A second irradiation radiation dose distribution determining means;
The body axis for the subject so that the exposure dose distribution in the body axis direction of the subject becomes a desired distribution when the subject is scanned by irradiating radiation according to the second radiation dose distribution. Aperture width distribution determining means for determining an aperture width distribution representing a relationship between a position in a direction and the width of the aperture to be set when the irradiation unit is positioned at the position;
The control means scans the subject by adjusting the width of the aperture and the dose of the radiation according to the aperture width distribution and the second irradiation dose distribution based on the aperture width distribution. A radiation CT apparatus for controlling a rotating part and the moving means.   The first irradiation radiation dose distribution determining means is configured to determine the first irradiation radiation dose based on subject radiation absorption information that reflects at least a change in the radiation absorption dose of the subject with respect to the body axis direction of the subject. The radiation CT apparatus according to claim 1, wherein distribution is determined.   The radiation CT apparatus according to claim 2, wherein the subject radiation absorption information is basic projection data obtained by scanning the subject in advance by the rotating unit.   The radiation CT apparatus according to claim 3, wherein the basic projection data is projection data obtained by scanning the subject along the body axis direction while fixing a rotation position of the rotation unit.   5. The radiation CT apparatus according to claim 2, wherein the first irradiation radiation dose distribution determining unit determines the first irradiation radiation dose distribution based on sensitivity of an imaging region. .   The first irradiation radiation dose distribution determining unit is configured to cause the noise level of the image corresponding to each position in the body axis direction of the subject generated by the image generating unit to be a predetermined target level. The radiation CT apparatus according to any one of claims 1 to 5, wherein a radiation dose distribution is determined.   The aperture width distribution determining unit determines the aperture width distribution based on a change in radiation dose in each local region of the first irradiation radiation dose distribution. Radiation CT system. The subject in the body axis direction when the subject is scanned by irradiating the subject with radiation according to the second irradiation dose distribution under a predetermined condition in which the width of the aperture in the body axis direction is fixed. An exposure dose distribution generating unit configured to generate an exposure dose distribution representing a relationship between a position and an exposure dose of a subject portion corresponding to the position when the width of the aperture is fixed to a plurality of different widths. ,
The aperture width distribution determining means obtains the aperture width distribution based on a relationship between the width of the aperture and the exposure dose of the subject obtained from a plurality of exposure dose distributions generated by the exposure dose distribution generation means. The radiation CT apparatus according to claim 7 to be determined. The exposure dose distribution generating means fixes the first exposure dose distribution when the width of the aperture is fixed to the first width, and the width of the aperture is fixed to a second width larger than the first width. And a second exposure dose distribution for
The aperture width distribution determining means is based on the magnitude of the ratio between the exposure dose in the first exposure dose distribution and the exposure dose in the second exposure dose distribution, the positions in the body axis direction corresponding to each other. The radiation CT apparatus according to claim 8, wherein the aperture width distribution is determined.   The aperture width distribution determining means is configured to determine the aperture to be set at a position where a ratio of the exposure dose in the second exposure dose distribution to the exposure dose in the first exposure dose distribution is a predetermined threshold value or more. The radiation CT apparatus according to claim 9, wherein the aperture width distribution is determined so that a width in the body axis direction becomes the first width.   The aperture width distribution determining means is configured so that the width in the body axis direction of the aperture to be set is the second width at a position where the ratio of the exposure dose is less than the predetermined threshold value. The radiation CT apparatus according to claim 9 or 10, wherein an aperture width distribution is determined.   The control unit controls the rotating unit and the moving unit to helically scan the subject, and the rotation according to the width of the aperture so that a helical pitch is constant or less than a predetermined value during the helical scan. The radiation CT apparatus according to claim 1, wherein the moving unit is controlled to adjust a relative moving speed of the unit. The irradiation unit has an X-ray tube,
The radiation CT apparatus according to any one of claims 1 to 12, wherein the control unit adjusts the dose of the radiation by adjusting a tube current of the X-ray tube.

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