JP2013094253A - X-ray ct apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus capable of properly applying exposure reduction scanning in which X-ray output is made smaller only for the predetermined view angle than in usual scanning to a high-radiation-sensitive portion even if a scanning gantry is tilted.SOLUTION: The X-ray CT apparatus includes: a first setting means for setting a range RD0 enveloping the desired portion 40a of a subject and in at least the body axis direction of the subject; a second setting means for setting a tilting angle α of the scanning gantry; and a third setting means for setting the conditions of exposure reduction scanning so that the desired portion 40a can be enveloped by a region DT of exposure reduction scanning when the scanning gantry is tilted at the tilting angle α, on the basis of the range RD0 set by the first setting means and the tilting angle α set by the second setting means.

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus and a program, and more specifically, an X-ray CT apparatus suitable for execution of a scan capable of reducing exposure of a highly radiation-sensitive part and a program for the same. About.

  In imaging using an X-ray CT apparatus, the X-ray tube outputs normal X-ray output only when the X-ray tube is in a predetermined view angle range approaching a radiation-sensitive part near the body surface of the subject. Scans that are smaller than level (hereinafter referred to as exposure reduction scans) are known (see, for example, Patent Document 1, Abstract). According to this exposure reduction scan, it is possible to reduce the exposure dose to a highly radiation sensitive part while maintaining the image quality of the reconstructed image at a certain level or higher.

JP 2004-321587 A

  Conventionally, it has been assumed that the exposure reduction scan is performed without tilting the scanning gantry. In this case, the scan plane is perpendicular to the body axis of the subject. For this reason, the range in the body axis direction in which the site to be subjected to exposure reduction is included may be set as the exposure reduction scan range as it is.

  However, there are actually cases where it is desired to perform exposure reduction scanning by tilting the scanning gantry. For example, when the imaging part is the head, tilt scan is performed so that the metal pieces of dental treatment traces that cause artifacts do not enter the scan area while reducing exposure of the eye, which is a highly radiation-sensitive part. There are cases where you want to do. In this case, since the scan surface is inclined, the range in the body axis direction set as the exposure reduction scan range does not match the exposure reduction scan region. For this reason, the settings related to the range of conventional exposure reduction scan cannot be applied as they are, or the portion that is subject to exposure reduction protrudes from the exposure reduction scan area, and appropriate exposure reduction scan can be performed. Can not.

  Under such circumstances, there is a demand for a technique capable of performing appropriate exposure reduction scanning even when the scanning gantry is tilted.

The invention of the first aspect
The X-ray output level (level) when the X-ray source is located in the first view angle range is higher than that when the X-ray source is located in a second view angle range different from the first view angle range. An X-ray CT apparatus that performs a scan to be reduced,
First setting means for setting at least a range in the body axis direction of the subject including a desired part of the subject by defining a boundary perpendicular to the body axis direction;
Second setting means for setting a tilt angle of the scanning gantry;
The area of the scan when the scanning gantry is tilted at the tilt angle based on the first range set by the first setting means and the tilt angle set by the second setting means An X-ray CT apparatus comprising: a third setting unit that calculates and sets a range in the body axis direction for performing the scan so that the desired part is included in the body.

The invention of the second aspect is
The third setting means comprises:
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, and is included in the first range in a cylindrical imaging field of view and includes a predetermined part including the desired part The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is one scan surface at the start and end of the scan,
The scan is performed such that the inclined surface that is in contact with the other end of the region of interest along the body axis direction is the other scan surface at the start and end of the scan. An X-ray CT apparatus according to the first aspect for calculating a range in the body axis direction is provided.

The invention of the third aspect is
The third setting means comprises:
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, and is included in the first range in a cylindrical imaging field of view and includes a predetermined part including the desired part The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is placed on the other end side along the body axis direction of the region of interest ((the imaging field of view tilted by the tilt angle). (Diameter) / 4) × Sin (the tilt angle) is a surface moved by a distance equal to or smaller than one of the scanning surfaces at the start and end of the scan,
The scan is performed such that the inclined surface that is in contact with the other end of the region of interest along the body axis direction is the other scan surface at the start and end of the scan. An X-ray CT apparatus according to the first aspect for calculating a range in the body axis direction is provided.

The invention of the fourth aspect is
The third setting means comprises:
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, which is inside the first range in a cylindrical imaging field of view and includes a predetermined part. The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is set to the other end side along the body axis direction of the region of interest ((the imaging field inclined by the tilt angle). ) / 4) × Sin (the tilt angle) is a surface moved by a distance equal to or smaller than one of the scan surfaces at the start and end of the scan,
The inclined surface that is in contact with the other end is on the one end side by a distance of ((diameter of the imaging field inclined by the tilt angle) / 4) × Sin (the tilt angle) or less. Provided is the X-ray CT apparatus according to the first aspect, which calculates the range in the body axis direction for performing the scan so that the moved surface becomes the other scan surface at the start and end of the scan. .

The invention of the fifth aspect is
The third setting means comprises:
A surface obtained by inclining a surface perpendicular to the body axis direction by the tilt angle, and a contour in the body axis direction of the body surface on the side where the desired portion exists, and one end of the first range The plane passing through the intersection with the plane that is perpendicular to the body axis direction is one scan plane at the start and end of the scan,
The inclined surface, the surface passing through the intersection of the contour and the surface passing through the other end of the first range and perpendicular to the body axis direction is the other at the start and end of the scan An X-ray CT apparatus according to the first aspect of the present invention that calculates a range in the body axis direction for performing the scanning so as to be a scanning plane of the first aspect.

The invention of the sixth aspect is
The first range is a range in the body axis direction when the scan is performed without tilting the scanning gantry from a plane perpendicular to the body axis. Any one of the first to fifth aspects. An X-ray CT apparatus according to one aspect is provided.

The invention of the seventh aspect
Further comprising a specifying means for specifying a deviation between the body axis of the subject and an axis passing through the rotation center of the scan and parallel to the body axis;
The X-ray CT apparatus according to the first aspect, wherein the third setting means calculates a range in the body axis direction for performing the scan based on the deviation specified by the specifying means.

The invention of the eighth aspect
The X-ray CT apparatus according to any one of the first to seventh aspects is provided, wherein the desired part is an eye.

The invention of the ninth aspect is
A computer constituting the X-ray CT apparatus;
First setting means for setting at least a range in the body axis direction of the subject including a desired part of the subject by defining a boundary perpendicular to the body axis direction;
Second setting means for setting a tilt angle of the scanning gantry;
When the X-ray source is positioned in the first view angle range based on the first range set by the first setting means and the tilt angle set by the second setting means The scanning gantry is tilted at the tilt angle, in which the X-ray output level is smaller than when the X-ray output level is located in a second view angle range different from the first view angle range. A program for functioning as third setting means for calculating and setting the range in the body axis direction for performing the scan so that the desired part is included in the scan region.

  Here, the “view angle” defines the rotational angle position of the X-ray tube or the X-ray focal point, and is also called a gantry angle.

  The “scan area” means an area irradiated with X-rays by the scan.

The “axis passing through the center of rotation and parallel to the body axis” is the central axis of the cavity of the scanning gantry and is also called an iso-center.

  According to the invention of the above aspect, it is possible to estimate the location of a part that is subject to exposure reduction from the set range in the body axis direction, and to set a scan region in which the scan plane is tilted so as to include the location. Even if the scanning gantry is tilted, an appropriate scan can be performed.

1 is a diagram schematically showing a configuration of an X-ray CT apparatus according to an embodiment of the invention. It is a functional block (block) figure of the part which concerns on execution of the scan in the X-ray CT apparatus by this embodiment. It is a figure which shows the example of a setting of CT scan range. It is a figure which shows the example of a setting of an exposure reduction scanning range. It is a figure which shows an example of the relationship between an imaging | photography site | part and an X-ray output reduction rate. It is a figure for demonstrating the 1st adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 2nd adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 3rd adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 4th adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 5th adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 6th adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 7th adjustment method of an exposure reduction scanning range. It is a figure for demonstrating the 8th adjustment method of an exposure reduction scanning range. It is a flowchart (flowchart) which shows the flow of operation | movement in the X-ray CT apparatus by this embodiment.

  Embodiments of the invention will be described below.

  FIG. 1 is a diagram schematically showing the configuration of the X-ray CT apparatus according to the present embodiment. The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 includes an input device 2 that receives input from an operator, a central processing unit 3 that performs control of each unit for imaging a subject, data processing for generating an image, and the like, and scanning. A data collection buffer (buffer) 5 that collects data acquired by the gantry 20, a monitor 6 that displays images, and a storage device 7 that stores programs and data (data) are provided.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and conveyed to the opening of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10. Here, the body axis direction of the subject 40, that is, the horizontal linear movement direction of the cradle 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

  The scanning gantry 20 includes a rotating unit 15 and a main body 20a that rotatably supports the rotating unit 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller (controller) 22 that controls the X-ray tube 21, and a collimator (collimate) an X-ray beam (beam) 81 generated from the X-ray tube 21. collimator) 23, an X-ray detector 24 for detecting an X-ray beam 81 transmitted through the subject 40, and a data acquisition system (DAS; Data Acquisition System) that converts the output of the X-ray detector 24 into projection data and collects it. ) 25 and a rotation unit controller 26 for controlling the X-ray controller 22, the collimator 23, and the DAS 25. The main body 20 a includes a controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating part 15 and the main body part 20 a are electrically connected via a slip ring 30.

FIG. 2 is a functional block diagram of a portion related to execution of scanning in the X-ray CT apparatus according to the present embodiment. As shown in FIG. 2, the X-ray CT apparatus according to the present embodiment includes an imaging region selection unit 101, a subject posture selection unit 102, a scout data acquisition unit 103, a CT scan range setting unit 104, and an exposure reduction scan. Range setting unit (first setting unit) 105, X-ray output setting unit 106, tilt angle setting unit (second setting unit) 107, subject height position specifying unit (specifying unit) 108, exposure reduction scan parameter ( scan parameter) setting unit (third setting means) 109, CT scan data (scan
data) acquisition unit 110, image reconstruction unit 111, and image display control unit 112.

  The imaging part selection unit 101 selects the imaging part B of the subject 40 from a plurality of candidates according to the operation of the operator. Examples of the imaging region candidates include “head”, “neck”, “chest”, and “abdomen”. Note that the imaging region selection unit 101 may select an imaging region in accordance with the analysis result of the previously acquired scout data and information such as the read scan protocol.

  The subject posture selection unit 102 selects the posture C of the subject 40 from a plurality of candidates according to the operation of the operator. Examples of posture candidates include “back-up”, “prone-down”, “right sideways”, and “left sideways”, and these are 0 degrees and 180 degrees as the view angles at which the front of the subject faces, respectively. Corresponding to degrees, 90 degrees and 270 degrees. Note that the subject posture selection unit 102 may select the posture of the subject based on the analysis result of the scout data PS acquired in advance and information such as the read scan protocol.

  The scout data acquisition unit 103 controls the imaging table 10 and the scanning gantry 20 to perform a scout scan of the subject 40 and acquire the scout data PS. The scout scan is a preliminary scan performed before the main scan. As the scout scan, for example, an X-ray beam having a lower dose than the main scan is applied to the subject 40 while moving the subject or the scanning gantry in the z direction while keeping the view angle of the X-ray tube (X-ray focal point) constant. Scanning that irradiates and collects projection data is an example. Further, for example, a helical scan using an X-ray beam having a dose lower than that of the main scan can be given.

  The CT scan range setting unit 104 sets the CT scan range in the main scan according to the operation of the operator. The CT scan range is a range in the z direction in which an X-ray CT scan is performed. Note that the CT scan range setting unit 104 may set the CT scan range based on other information such as the analysis result of the scout data PS.

  FIG. 3 shows an example of setting the CT scan range. For example, as shown in FIG. 3, on the scout image 41 created and displayed based on the scout data PS, the operator designates the range in the z direction graphically. The CT scan range setting unit 104 sets the designated range as the CT scan range RC.

  The exposure reduction scan range setting unit 105 sets the exposure reduction scan range in the main scan according to the operation of the operator. The exposure reduction scan range is a range in the z direction in which the exposure reduction scan is performed, and is set by defining a boundary perpendicular to the z direction. In the exposure reduction scan, the X-ray output of the X-ray tube 21 is reduced from the first level L1 which is the normal level to the lower second level L2 only while the X-ray focal point 21f is located within a predetermined view angle range. This is a scan for collecting projection data.

  FIG. 4 shows an example of setting the exposure reduction scan range. For example, as shown in FIG. 4, on the scout image 41 created and displayed based on the scout data PS, the operator graphically designates the range in the z direction within the CT scan range RC. The exposure reduction scan range setting unit 105 sets the designated range as the exposure reduction scan range RD0.

  In the present embodiment, the exposure reduction scan range setting unit 105 sets, as the exposure reduction scan range RD0, a range in the z direction in which the exposure reduction scan is performed when scanning is performed without tilting the scanning gantry 20. Incidentally, the setting of the exposure reduction scan range is equivalent to setting a range in the z direction that includes a portion that is a target of exposure reduction from the viewpoint of the operator.

  In the present embodiment, the exposure reduction scan range RD0 is defined as the range in the z direction of the X-ray focal point 21f when the view angle is 0 degree (+ y direction). That is, during the CT scan, the exposure reduction scan is performed when the X-ray focal point 21f is positioned within the set exposure reduction scan range RD0 at the view angle of 0 degree. Note that the exposure reduction scan range setting unit 105 may set the exposure reduction scan range RD0 based on other information such as the analysis result of the scout data PS.

  The X-ray output setting unit 106 sets the X-ray output of the X-ray tube in the main scan, that is, the tube voltage and the tube current, according to the operation of the operator. Note that the X-ray output setting unit 106 includes an automatic exposure mechanism, and based on a noise index and scout data designated by the operator, the position in the z direction of the subject 40 and the X-ray tube 21. A modulation curve of the X-ray output to be changed according to the rotation angle position may be determined.

  The tilt angle setting unit 107 sets the tilt angle α in the z direction of the scanning gantry 20 according to the operation of the operator.

  The subject height position specifying unit 108 specifies the height position of the subject 40, that is, the position in the y direction. For example, the height position of the body axis of the subject 40 when the isocenter IC is used as a reference, that is, the deviation in the height direction between the isocenter IC and the body axis is specified based on the analysis result of the scout data. As the analysis of the scout data, for example, a method of matching with a template in which the position of the body axis is defined, a method of searching the position of the top of the head or the center of gravity as the position of the body axis, or the like can be used. Further, for example, the operator specifies the position of the body axis of the subject 40 graphically on the scout image, and obtains the distance from the isocenter IC to the specified position, thereby specifying the deviation.

  The exposure reduction scan parameter setting unit 109 sets the X-ray output reduction rate k in the exposure reduction scan. The X-ray output reduction rate k is a tube current reduction rate under a constant tube voltage. The X-ray output reduction rate k is a reduction rate when the X-ray output is reduced from the output A1 at the first level L1 to the output at the second level L2. That is, the X-ray output of the second level L2 is (1-k) · A1. In this embodiment, the X-ray output reduction rate k is set according to the set imaging region B with reference to the table shown in FIG. When an X-ray output modulation curve is obtained by the automatic exposure mechanism, the X-ray output along the modulation curve becomes the above-described first level L1. Further, the exposure reduction scan parameter setting unit 108 may set a desired reduction rate according to the operation of the operator.

  Further, the exposure reduction scan parameter setting unit 109 sets an exposure reduction view angle range. The exposure reduction view angle range is an X-ray focal view angle range that reduces the X-ray output from the first level L1 output A1 to the second level L2 output. That is, the X-ray output is reduced from the first level L1 to the second level L2 while the X-ray focal point is located within the exposure reduction view angle range. In the present embodiment, the central angular position of the exposure reduction view angle range is set as the angular position in the front direction of the subject taking the set posture. Then, referring to the table shown in FIG. 5, the angle width of the exposure reduction view angle range is set according to the set imaging region.

  Further, the exposure reduction scan parameter setting unit 109 is based on the set posture, the tilt angle, the specified height position (deviation), etc., when the tilt angle is other than 0 degrees, that is, when the scanning gantry 20 is tilted. To adjust the previously set exposure reduction scan range. That is, when scanning is performed with the scanning gantry 20 tilted, the exposure reduction scan range is adjusted so that the portion that is the subject of exposure reduction is included in the exposure reduction scan region. The method for adjusting the exposure reduction scan range will be described in detail later.

  The CT scan data acquisition unit 109 controls the imaging table 10 and the scanning gantry 20 to perform a CT scan with respect to the set CT scan range RC as a main scan, and acquires CT scan data PC. At this time, an exposure reduction scan is performed based on the set exposure reduction scan parameter. The CT scan data is also referred to as projection data.

  The image reconstruction unit 110 performs an image reconstruction process such as a filter back projection process based on the CT scan data PC to reconstruct a tomographic image at each position in the z direction in the CT scan range RC.

  The image display control unit 111 controls the monitor 6 to display an image based on the scout data PS, a reconstructed image, and other information on the screen of the monitor 6.

  A method for adjusting the exposure reduction scan range will now be described.

(First adjustment method)
FIG. 6 is a diagram for explaining the first adjustment method.

  Here, as shown in FIG. 6, the imaging region B is “head”, the posture C is “backward”, the CT scan range RC is substantially the entire range of the head, and the initial exposure reduction scan range RD0 is the subject 40. A range in the z direction including the eye 40a and a tilt angle α is assumed to be a predetermined angle of 15 degrees or less. The exposure reduction view angle range AR is assumed to be an angle range in which the center angle position is an angle position of a view angle of 0 degrees (+ y direction) and the angle width is approximately 100 degrees.

  As shown in FIG. 6, first, a region of interest SV that encloses the eye 40a that is the subject of exposure reduction is defined. This region of interest SV is, for example, included in the exposure reduction scan range RD0 (inside the exposure reduction scan range RD0) in the columnar SFOV (imaging field of view) space FV having the isocenter IC as a column axis, and the subject. A region included in a predetermined view angle range including 40 eyes (inside the predetermined view angle range) can be used. The region of interest SV may be an overlapping region between the region of the exposure reduction scan range RD0 and the region of the predetermined view angle range in the SFOV space FV (the same applies to other adjustment methods described later).

  Next, a first slope K1 that is a plane perpendicular to the z direction, that is, a slope in which the xy plane is tilted in the z direction by the tilt angle α and is in contact with one end along the z direction of the region of interest SV is defined. Further, a second inclined surface K2 that is an inclined surface in which the xy plane is inclined in the z direction by the tilt angle α and is in contact with the other end along the z direction of the region of interest SV is defined.

  The first slope K1 is one scan surface at the start and end of the exposure reduction scan, and the second slope K2 is the other scan surface at the start and end of the exposure reduction scan. Adjust the exposure reduction scan range.

  For example, as shown in FIG. 6, the position in the z direction representing one end on the side where the scanning gantry 20 is tilted (the −z direction side in this example) among the both ends of the initial exposure reduction scan range RD0 is the first position. A position in the z direction representing Z1 and the other end is taken as a second position Z2. Also, let D be the major axis of the inclined elliptical SFOV (imaging field of view). Furthermore, a position where the scanning gantry 20 is moved to the tilted side by (D / 2) × Sinα from the first position Z1 is defined as a third position Z3. A range in the z direction from the third position Z3 to the second position Z2 is set as a new exposure reduction scan range RD1.

  Thus, during the scan, the exposure reduction scan is performed when the z-direction position of the X-ray focal point 21f when the view angle is 0 degrees is located in the new exposure reduction scan range RD1. .

  According to such a first adjustment method, even when scanning is performed with the tilt angle α, the portion that is the subject of exposure reduction can be reliably included in the exposure reduction scan region DT.

  Note that the first adjustment method can be applied in the same manner even when the posture of the subject is “downside down”.

(Second adjustment method)
FIG. 7 is a diagram for explaining the second adjustment method.

  Various settings and prescribed conditions are the same as those in the first adjustment method.

  In the second adjustment method, a slope obtained by moving the first slope K1 by a predetermined distance equal to or less than (D / 4) × Sinα to the side opposite to the tilted side of the scanning gantry 20 is defined as a third slope K3. The third slope K3 becomes one of the scan surfaces at the start and end of the exposure reduction scan, and the second slope K2 becomes the other scan surface at the start and end of the exposure reduction scan. In addition, the exposure reduction scan range is adjusted.

  For example, as shown in FIG. 7, a position where the scanning gantry 20 is moved to the tilted side by (D / 4) × Sinα from the first position Z1 is set as a fourth position Z4. A range in the z direction from the fourth position Z4 to the second position Z2 is defined as a new exposure reduction scan range RD1.

  Usually, the site to be subjected to exposure reduction is a site with high radiation sensitivity. The site having high radiation sensitivity is, for example, the eye (lens), the thyroid gland, the mammary gland, the genital organ, and the like, and these are near the body surface on the front side of the subject 40. Therefore, in consideration of the size of the subject 40 and the positional relationship with the SFOV space FV, the site that is subject to exposure reduction is a location that is approximately (D / 2) away from the isocenter IC in the y direction along the scan plane. There are many cases located in. In other words, when the subject 40 is in the “back-to-back” or “prone-down” posture, a portion of the region of interest SV near the outer periphery of the isocenter IC or SFOV is subject to exposure reduction. Almost does not exist. Therefore, the exposure reduction scan range is set so that at least a part of such a portion is excluded from the exposure reduction scan region.

  According to such a second adjustment method, the exposure reduction scan region DT is included more than necessary while the portion that is the subject of exposure reduction is reliably included in the exposure reduction scan region DT when scanning at the tilt angle α. You can prevent it from spreading. As a result, it is possible to suppress deterioration of the image quality of the reconstructed image while reliably reducing the exposure of the target part.

  Note that the second adjustment method can be similarly applied to the case where the posture of the subject is “downside down”.

(Third adjustment method)
FIG. 8 is a diagram for explaining the third adjustment method.

  Various settings and prescribed conditions are the same as those in the first adjustment method.

  In the third adjustment method, a slope obtained by moving the second slope K2 to the tilted side of the scanning gantry 20 by a predetermined distance equal to or less than (D / 4) × Sinα is defined as a fourth slope K4. The fourth slope K4 becomes one of the scan surfaces at the start and end of the exposure reduction scan, and the first slope K1 becomes the other scan surface at the start and end of the exposure reduction scan. In addition, the exposure reduction scan range is adjusted.

  For example, as shown in FIG. 8, a position where the scanning gantry 20 is moved to the tilted side by (D / 4) × Sinα from the second position Z2 is defined as a fifth position Z5. The z-direction range from the first position Z1 to the fifth position Z5 is defined as a new exposure reduction scan z-direction range RD1.

  According to such a third adjustment method, similarly to the second adjustment method, the portion that is the subject of exposure reduction is reliably included in the exposure reduction scan region DT at the tilt angle α, while reducing exposure. It is possible to prevent the scan area DT from being unnecessarily widened. As a result, it is possible to suppress deterioration of the image quality of the reconstructed image while reliably reducing the exposure of the target part.

  It should be noted that the third adjustment method can be similarly applied to the case where the subject posture is “depression”.

(Fourth adjustment method)
FIG. 9 is a diagram for explaining the fourth adjustment method.

  Various settings and prescribed conditions are the same as those in the first adjustment method.

  In the fourth adjustment method, the third slope K3 serves as one scan surface at the start and end of the exposure reduction scan, and the fourth slope K4 serves as the other scan at the start and end of the exposure reduction scan. The exposure reduction scan range is adjusted so as to be a plane.

  For example, as shown in FIG. 9, a range in the z direction from the fourth position Z4 to the fifth position Z5 is set as a new exposure reduction scan range RD1.

  As described above, in the portion of the region of interest SV that is near the outer periphery of the isocenter IC or SFOV and near both ends in the z direction, there is almost no portion that is subject to exposure reduction. Therefore, the exposure reduction scan z-direction range is set so that many of these portions are excluded from the exposure reduction scan region.

  According to the fourth adjustment method, the exposure reduction scan region DT is included more than necessary while the portion that is the subject of exposure reduction is reliably included in the exposure reduction scan region DT when scanning at the tilt angle α. You can prevent it from spreading. As a result, it is possible to further suppress the deterioration of the image quality of the reconstructed image while reliably reducing the exposure of the target portion.

  Note that the fourth adjustment method can also be applied in the same manner when the subject posture is “downside down”.

(Fifth adjustment method)
FIG. 10 is a diagram for explaining the fifth adjustment method.

  Various settings and prescribed conditions are the same as those in the first adjustment method.

  In the fifth adjustment method, the contour in the z direction of the body surface of the subject on the side (in this example, on the + y direction side) on which the site to be subjected to exposure reduction exists is defined as a contour F. In addition, a point on the contour F, which is a position in the z direction that represents one end of the initially set exposure reduction scan range RD0, is defined as a first contour point E1. In addition, a point on the contour E, which is the point at the z-direction position of the other end of the initially set exposure reduction scan range RD0, is set as a second contour point E2.

  Furthermore, a surface obtained by inclining the xy plane by the tilt angle α in the z direction and passing through the first contour point E1 is defined as a fifth inclined surface K5. Also, a surface that is inclined in the z direction by a tilt angle α and that is perpendicular to the z direction and that passes through the second contour point E2 is defined as a sixth inclined surface K6. The fifth slope K5 becomes one of the scan planes at the start and end of the exposure reduction scan, and the sixth slope K6 becomes the other scan plane at the start and end of the exposure reduction scan. In addition, the exposure reduction scan range is adjusted.

  For example, as shown in FIG. 10, the distance from the isocenter IC to the first contour point E1 is a first distance a. The distance from the isocenter IC to the second contour point E2 is a second distance b. Furthermore, a position where the scanning gantry 20 is moved to the tilted side by ((D / 2) × Cos α−a) × Tan α from the first position Z1 is defined as a sixth position Z6. Further, a position where the scanning gantry 20 is moved to the tilted side by the amount of ((D / 2) × Cos α−b) × Tan α from the second position Z2 is set as a seventh position Z7. The z-direction range from the sixth position Z6 to the seventh position Z7 is set as a new exposure reduction scan range RD1.

  The first distance a and the second distance b can be obtained from the analysis result of the scout data PS, for example. Further, for example, the operator may graphically designate the first and second contour points E1 and E2 with reference to the scout image, and may obtain them based on this.

  According to such a fifth adjustment method, it is possible to accurately capture the position of the body surface of the subject 40 and estimate the position of the part that is the target of exposure reduction more accurately. While ensuring, the margin of the exposure reduction scan area can be further narrowed.

  Note that the fifth adjustment method can be similarly applied to the case where the posture of the subject is “downside down”.

(Sixth adjustment method)
FIG. 11 is a diagram for explaining the sixth adjustment method.

  Here, it is assumed that the posture of the subject is “left sideways”. Other various settings and specified conditions are the same as those in the first adjustment method.

  First, as in the case of the first adjustment method, a region of interest SV that includes the eye 40a that is the subject of exposure reduction is defined. The region of interest SV is, for example, a predetermined view angle range that is included in the exposure reduction scan range RD0 in the columnar SFOV space FV with the isocenter IC as a column axis and includes both eyes 40a of the subject 40. Can be included in the area. Here, the predetermined view angle range is a range with an angular width of 60 degrees centering on the left lateral direction of the subject 40, that is, the direction where the view angle is 270 degrees.

  Next, a seventh inclined surface K7 that is an inclined surface in which the xy plane is inclined in the z direction by the tilt angle α and is in contact with one end along the z direction of the region of interest SV2 is defined. Also, an eighth inclined surface K8 that is an inclined surface in which the xy plane is inclined in the z direction by the tilt angle α and is in contact with the other end of the region of interest SV2 along the z direction is defined.

  Then, the seventh slope K7 becomes one of the scan surfaces at the start and end of the exposure reduction scan, and the eighth slope K8 becomes the other scan surface at the start and end of the exposure reduction scan. Adjust the exposure reduction scan range.

  For example, as shown in FIG. 11, a position where the scanning gantry 20 is moved to the tilted side by ((D / 4) × 3 × Sinα) from the first position Z1 is defined as an eighth position Z8. . Further, a position where the scanning gantry 20 is moved to the tilted side by ((D / 4) × Sinα) from the second position Z2 is defined as a ninth position Z9. The z-direction range from the eighth position Z8 to the ninth position Z9 is set as a new exposure reduction scan range RD1.

  According to such a sixth adjustment method, when scanning with the tilt angle α, even if the posture of the subject is horizontal, the portion to be subjected to exposure reduction is surely included in the exposure reduction scan region DT. Can be included.

  Note that the sixth adjustment method can be similarly applied to the case where the posture of the subject is “right sideways”.

(Seventh adjustment method)
FIG. 12 is a diagram for explaining the seventh adjustment method.

  Various settings and prescribed conditions are the same as in the sixth adjustment method.

  In the seventh adjustment method, the contour in the y direction of the body surface of the subject on the + z direction side is defined as a contour G. In addition, a point on the contour G, that is, a point on the −y direction side at a position in the z direction corresponding to the end on the side where the scanning gantry 20 is tilted, of both ends of the initially set exposure reduction scan range RD0, The third contour upper point E3. Further, a point on the contour G, which is an end on the other side of the initially set exposure reduction scan range RD0, and + y at a position in the z direction corresponding to the end opposite to the side on which the scanning gantry 20 is tilted. The point on the direction side is set as a fourth contour point E4.

  Furthermore, a surface obtained by inclining the xy plane in the z direction by the tilt angle α and passing through the third contour point E3 is defined as a ninth inclined surface K9. Also, a surface that is inclined in the z direction by a tilt angle α and that is perpendicular to the z direction and that passes through the fourth contour point E4 is defined as a tenth slope K10.

  The ninth inclined surface K9 becomes one scan surface at the start and end of the exposure reduction scan, and the tenth inclined surface K10 becomes the other scan surface at the start and end of the exposure reduction scan. In addition, the exposure reduction scan range is adjusted.

  For example, as shown in FIG. 12, the distance from the isocenter IC to the third contour point E3 is a third distance c. The distance from the isocenter IC to the fourth contour point E4 is a fourth distance d. Further, a position where the scanning gantry 20 is moved to the tilted side by ((D / 2) × Cos α + c) × Tan α from the first position Z1 is defined as a tenth position Z10. Further, a position where the scanning gantry 20 is moved to the tilted side by the amount of ((D / 2) × Cos α−d) × Tan α from the second position Z2 is defined as an eleventh position Z11. A range in the z direction from the tenth position Z10 to the eleventh position Z11 is set as a new exposure reduction scan range RD1.

  Note that the third distance c and the fourth distance d can be obtained from the analysis result of the scout data PS, for example. Further, for example, the operator may graphically designate the third and fourth contour points E3 and E4 with reference to the scout image, and obtain them based on this.

  According to such a seventh adjustment method, even when the posture of the subject is set sideways, the position of the body to be subject to exposure reduction can be determined by accurately capturing the position of the body surface of the subject 40. It is possible to accurately estimate, and the margin of the exposure reduction scan region can be further narrowed while further reducing the exposure of the target.

  Note that the seventh adjustment method can be applied in the same manner even when the posture of the subject is “right lateral direction”.

(Eighth adjustment method)
FIG. 13 is a diagram for explaining an eighth adjustment method.

  Various settings and prescribed conditions are the same as those in the first adjustment method.

  If the body axis of the subject 40 is deviated from the isocenter IC, there is a possibility that a portion that is subject to exposure reduction protrudes from the exposure reduction scan region DT. Therefore, in the eighth adjustment method, in the first to seventh adjustment methods, the deviation between the body axis of the subject 40 and the isocenter IC in the height direction of the subject 40, that is, the y direction is specified, and the deviation is determined. Adjust the exposure reduction scan range in consideration.

  For example, as shown in FIG. 13, when the body axis AZ of the subject 40 is shifted from the isocenter IC by the distance f in the −y direction (lower by the distance f), one of the first to seventh adjustment methods is used. The obtained exposure reduction scanning range RD1 is moved by a distance f × Tanα to the side where the scanning gantry 20 is tilted (RD1 ′).

  Further, for example, when the body axis of the subject 40 is shifted from the isocenter IC by the distance f in the + y direction (higher by the distance f), the exposure reduction scan range RD1 obtained by any of the first to seventh adjustment methods. Is moved by a distance f × Tanα to the side opposite to the side where the scanning gantry 20 is tilted.

  According to the eighth adjustment method, even when the height position of the body axis of the subject 40 is deviated from the isocenter IC, the position of the part to be subjected to exposure reduction is accurately estimated. , Within the exposure reduction scan area.

  Note that the eighth adjustment method can also be applied in the same manner when the subject posture is “down-facing”, “left lateral orientation”, or “right lateral orientation”.

  The operation of the X-ray CT apparatus according to the present embodiment will now be described.

  FIG. 14 is a flowchart showing an operation flow in the X-ray CT apparatus according to the present embodiment.

  In step S <b> 1, the imaging region selection unit 101 selects an imaging region of the subject 40 according to the operation of the operator.

  In step S2, the subject posture selection unit 102 selects the posture C of the subject 40 according to the operation of the operator.

  In step S3, the scout data acquisition unit 103 sets a predetermined range in the z direction including the imaging part B as the scout scanning range RC according to the imaging part B selected in step S1. Further, the scout data acquisition unit 103 controls the imaging table 10 and the scanning gantry 20, performs a scout scan for the set scout scan range RC, and acquires the scout data PS.

  In step S4, the image display control unit 112 displays a scout image of the subject 40 on the screen of the monitor 6 based on the scout data PS acquired in step S3.

  In step S5, the operator designates a desired range in the z direction on the scout image. The CT scan range setting unit 104 sets the designated range as the CT scan range RC.

  In step S <b> 6, the operator designates a range in the z direction including a highly radiation-sensitive tissue present in the CT scan range RC on the displayed scout image 41. The exposure reduction scan range setting unit 105 sets the designated range in the z direction as the exposure reduction scan range RD0.

In step S7, the operator selects a desired tube voltage and a noise index value (noise) of the reconstructed image.
index). The X-ray output setting unit 106 sets the input tube voltage as the tube voltage V in the main scan. Further, the X-ray output setting unit 106 sets the tube current A1 of the first level L1 corresponding to the normal tube current in the main scan by the automatic exposure mechanism. Based on the acquired scout data PS and the input noise index value NI, the automatic exposure mechanism changes the tube current A (Z) or X-ray that changes according to the coordinate Z in the z direction of the X-ray tube 21. The tube current A (Z, θ) that changes in accordance with the z-direction coordinate Z of the tube 21 and the view angle θ is obtained. Therefore, the X-ray output setting unit 106 sets the tube current A (Z) or the tube current A (Z, θ) as the tube current A1 of the first level L1.

  In step S8, the operator designates a desired angle. The tilt angle setting unit 107 sets the designated angle as the tilt angle α of the scanning gantry 20.

  In step S9, the subject height position specifying unit 108 specifies the height position of the body axis of the subject 40 based on the isocenter according to the analysis result of the scout data PS or the operation of the operator. .

  In step S10, the exposure reduction scan parameter setting unit 109 performs the exposure reduction scan based on the set imaging region B, posture C, tilt angle α, the specified height position (deviation) of the subject, and the like. Set the parameters. That is, the exposure reduction scan parameter setting unit 109 sets the X-ray output reduction rate k and the exposure reduction view angle range AR in the exposure reduction scan, and sets the previously set exposure reduction scan range RD0 from the first to the first. Adjust using any of the adjustment methods in 8 and set again.

  In step S11, the CT scan data acquisition unit 110 controls the imaging table 10 and the scanning gantry 20, performs a CT scan for the set CT scan range RC as a main scan, and acquires CT scan data PC. At this time, an exposure reduction scan is performed based on the set exposure reduction scan parameter. Here, although an axial scan is assumed as the CT scan, a helical scan may be used.

  In step S12, the image reconstruction unit 111 performs an image reconstruction process such as a filter back projection process based on the CT scan data PC acquired in step S11, and a tomogram at each position in the z direction in the CT scan range RC. Reconstruct the image.

  In step S13, the image display control unit 112 displays the tomographic image reconstructed in step S12 on the screen of the monitor 6.

  The embodiment of the invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Rotating part 20 Scanning gantry 21 X-ray tube 22 X-ray controller 23 Collimator 24 X-ray detector 25 Data collection device 26 Rotation unit controller 29 Control controller 30 Slip ring 40 Subject 40a Eye 41 Scout image 81 X-ray beam 100 X-ray CT apparatus 101 Imaging region selection unit (first setting means)
DESCRIPTION OF SYMBOLS 102 Subject attitude | position selection part 103 Scout data acquisition part 104 CT scan range setting part 105 Exposure reduction scan range setting part 106 X-ray output setting part 107 Tilt angle setting part (2nd setting means)
108 Subject height position specifying unit (specifying means)
109 Exposure reduction scan parameter setting unit (third setting means)
110 CT scan data acquisition unit 111 Image reconstruction unit 112 Image display control unit

Claims (9)

Scan in which the X-ray output level when the X-ray source is located in the first view angle range is made smaller than when the X-ray source is located in a second view angle range different from the first view angle range An X-ray CT apparatus for performing
First setting means for setting at least a range in the body axis direction of the subject including a desired part of the subject by defining a boundary perpendicular to the body axis direction;
Second setting means for setting a tilt angle of the scanning gantry;
The area of the scan when the scanning gantry is tilted at the tilt angle based on the first range set by the first setting means and the tilt angle set by the second setting means An X-ray CT apparatus comprising: a third setting unit that calculates and sets a range in the body axis direction for performing the scan so that the desired part is included in the body. The third setting means includes
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, and is included in the first range in a cylindrical imaging field of view and includes a predetermined part including the desired part The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is one scan surface at the start and end of the scan,
The scan is performed such that the inclined surface that is in contact with the other end of the region of interest along the body axis direction is the other scan surface at the start and end of the scan. The X-ray CT apparatus according to claim 1, wherein a range in a body axis direction is calculated. The third setting means includes
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, and is included in the first range in a cylindrical imaging field of view and includes a predetermined part including the desired part The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is placed on the other end side along the body axis direction of the region of interest ((the imaging field of view tilted by the tilt angle). (Diameter) / 4) × Sin (the tilt angle) is a surface moved by a distance equal to or smaller than one of the scanning surfaces at the start and end of the scan,
The scan is performed such that the inclined surface that is in contact with the other end of the region of interest along the body axis direction is the other scan surface at the start and end of the scan. The X-ray CT apparatus according to claim 1, wherein a range in a body axis direction is calculated. The third setting means includes
A plane obtained by inclining a plane perpendicular to the body axis direction by the tilt angle, which is inside the first range in a cylindrical imaging field of view and includes a predetermined part. The surface in contact with one end along the body axis direction of the region of interest included in the view angle range is set to the other end side along the body axis direction of the region of interest ((the imaging field inclined by the tilt angle). ) / 4) × Sin (the tilt angle) is a surface moved by a distance equal to or smaller than one of the scan surfaces at the start and end of the scan,
The inclined surface that is in contact with the other end is on the one end side by a distance of ((diameter of the imaging field inclined by the tilt angle) / 4) × Sin (the tilt angle) or less. The X-ray CT apparatus according to claim 1, wherein the range in the body axis direction for performing the scan is calculated so that the moved surface becomes the other scan surface at the start and end of the scan. The third setting means includes
A surface obtained by inclining a surface perpendicular to the body axis direction by the tilt angle, and a contour in the body axis direction of the body surface on the side where the desired portion exists, and one end of the first range The plane passing through the intersection with the plane that is perpendicular to the body axis direction is one scan plane at the start and end of the scan,
The inclined surface, the surface passing through the intersection of the contour and the surface passing through the other end of the first range and perpendicular to the body axis direction is the other at the start and end of the scan The X-ray CT apparatus according to claim 1, wherein a range in the body axis direction in which the scan is performed is calculated so as to be a scan plane.   6. The first range according to claim 1, wherein the first range is a range in the body axis direction when the scan is performed without tilting the scanning gantry from a plane perpendicular to the body axis. 6. X-ray CT system. Further comprising a specifying means for specifying a deviation between the body axis of the subject and an axis passing through the rotation center of the scan and parallel to the body axis;
The X-ray CT apparatus according to claim 1, wherein the third setting unit calculates a range in the body axis direction in which the scan is performed based on a deviation specified by the specifying unit.   The X-ray CT apparatus according to claim 1, wherein the desired part is an eye. A computer constituting the X-ray CT apparatus;
First setting means for setting at least a range in the body axis direction of the subject including a desired part of the subject by defining a boundary perpendicular to the body axis direction;
Second setting means for setting a tilt angle of the scanning gantry;
When the X-ray source is positioned in the first view angle range based on the first range set by the first setting means and the tilt angle set by the second setting means The scanning gantry is tilted at the tilt angle, in which the X-ray output level is smaller than when the X-ray output level is located in a second view angle range different from the first view angle range. A program for functioning as third setting means for calculating and setting the range in the body axis direction for performing the scan so that the desired part is included in the scan region.

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