JP5536440B2 - X-ray CT system - Google Patents

Description

The present invention relates to an X-ray CT (Computed Tomography) apparatus, and more particularly to a helical scan.
The present invention relates to an X-ray CT apparatus that performs scanning.

  In recent years, in the X-ray CT apparatus, exposure has been reduced by improving the detection efficiency of the X-ray detector and improving the algorithm of image reconstruction processing. In addition, the imaging speed has been increased due to an increase in coverage due to an increase in the number of detector rows of the X-ray detector and an increase in the rotational speed of the scanning gantry. Accompanying the low exposure and high-speed imaging of such X-ray CT apparatuses, in the medical field, for the purpose of improving examination accuracy, the scan range is set wider than before and imaging is performed in one machine by helical scanning. A lot is happening.

  On the other hand, there has been proposed a technique for reducing exposure to highly sensitive organs existing near the body surface of a subject when performing a helical scan. For example, there has been proposed a method of modulating the X-ray output with respect to the scan position so that the X-ray output when scanning a site including such an organ, mainly the tube current becomes smaller than the other sites. Also, for example, there is a method of controlling the X-ray irradiation angle with respect to the scan position, that is, the rotation angle of the X-ray tube so that X-rays are not irradiated at a short distance from the body surface close to such an organ (see, for example, Patent Document 1). Proposed.

JP 2007-20604 A

  By the way, when the rotation speed of the scanning gantry increases, the control of the X-ray output cannot catch up and the modulation becomes difficult. Further, when the coverage is expanded, scanning is performed with a wide X-ray beam width in the slice direction. In this case, when the X-ray output is modulated, the X-ray output is not originally required to be reduced. However, there are problems such as a small X-ray output. Therefore, as a technique for reducing the exposure to highly sensitive organs, the technique for controlling the rotation angle of the X-ray tube is easier to use.

  Further, a plurality of organs having high radiation sensitivity exist in the body axis direction of the subject, such as the lens, the thyroid gland, the mammary gland, and the genital organs. Therefore, when the scan range becomes wide, a plurality of organs with high radiation sensitivity may be included in the scan range.

  Therefore, in the future, there will be more opportunities to perform helical scans on scan ranges that include multiple radiation-sensitive organs. At that time, using a method to control the rotation angle of the X-ray tube, It is required to reduce exposure. In this case, since the position and size of the organs whose exposure should be reduced are different, in order to reduce the exposure efficiently, the trajectory of the X-ray tube and the X-ray beam width in the slice direction for these organs are finely adjusted. It is necessary to determine scanning conditions.

  However, the calculation for obtaining such a scanning condition is very complicated, and the operation is not easy.

  Under these circumstances, it is possible to support the specification of scanning conditions for efficiently reducing exposure to multiple organs using a method that controls the rotation angle of the X-ray tube when performing helical scanning. An X-ray CT apparatus is desired.

  An invention according to a first aspect includes an X-ray tube, and an X-ray CT apparatus that performs a helical scan by rotating the X-ray tube around the subject and moving it relative to the body axis of the subject. A setting means for setting a scan range and first and second regions each containing different organs included in the scan range based on the position in the body axis direction; and a helical scan When performed, the rotation angle of the X-ray tube at a predetermined position in the body axis direction in the first region belongs to the first angle range, and the body axis direction in the second region is Candidate combinations of a helical pitch, an X-ray beam width in the body axis direction, and a start angle of the X-ray tube such that the rotation angle of the X-ray tube at a predetermined position belongs to the second angle range Generating means for generating There is provided an X-ray CT apparatus comprising: selection means for selecting one of the selected combination candidates; and scanning means for performing a helical scan in accordance with a scan condition including the selected combination.

  Here, the first angle range is an angle range to which the rotation angle of the X-ray tube should belong in order to reduce exposure of organs included in the first region, for example, one rotation of the X-ray tube The angle range in which the distance between the organ included in the first region and the X-ray tube is equal to or greater than a predetermined value in the minute angle range.

  The second angle range is an angle range to which the rotation angle of the X-ray tube should belong in order to reduce exposure of organs included in the second region. For example, the second angle range corresponds to one rotation of the X-ray tube. The angle range in which the distance between the organ included in the second region and the X-ray tube is a predetermined value or more can be set.

  According to a second aspect of the present invention, the generating means performs a helical scan from a scan start position in the set scan range according to the candidate conditions for the helical pitch and the X-ray beam width in the body axis direction. The X-ray CT apparatus according to the first aspect of the present invention that searches for the start angle of the X-ray tube when it is performed and generates the combination candidates.

  The invention of a third aspect is the first aspect or the first aspect, wherein the predetermined position in the body axis direction in the first region and the predetermined position in the body axis direction in the second region are central positions. An X-ray CT apparatus according to two aspects is provided.

  According to a fourth aspect of the present invention, when a helical scan is performed on the generated combination candidate according to the combination condition, the rotation angle of the X-ray tube in the first region is the first angle. The ratio of the region including the position in the body axis direction that belongs to the range, and the body axis in which the rotation angle of the X-ray tube belongs to the second angle range in the second region The X-ray CT apparatus according to the third aspect is further provided with calculation means for calculating the ratio of the region including the position in the direction.

  In the invention of the fifth aspect, when a helical scan is performed among the generated combination candidates, the rotation angle of the X-ray tube at both end positions of the first region is within the first angle range. Determination means for determining whether or not a combination candidate is included that belongs to and the rotation angle of the X-ray tube at both end positions of the second region belongs to the second angle range; In the case of being denied, when a helical scan is performed on the generated combination candidate according to the conditions of the combination, the rotation angle of the X-ray tube in the first region is within the first angle range. The ratio of the region including the position in the body axis direction to belong to, and the rotation angle of the X-ray tube in the second region in the body axis direction out of the second region. The percentage of the area containing the location Further comprising a calculation means for output to provide an X-ray CT apparatus of the third aspect.

  The sixth aspect of the invention is the X of the fifth aspect, further comprising display means for displaying the generated combination candidate and the calculated ratio in association with the combination condition. A line CT apparatus is provided.

  According to a seventh aspect of the invention, the display means displays the generated combination candidates in order according to a predetermined rule based on the magnitude of the calculated ratio with respect to the condition of the combination. An X-ray CT apparatus according to the sixth aspect is provided.

  According to an eighth aspect of the invention, there is provided the X-ray CT apparatus according to the sixth aspect or the seventh aspect, wherein the selection means selects a combination designated by an operator from the displayed combination candidates. provide.

  According to a ninth aspect of the present invention, in any one of the fourth to seventh aspects, the selecting means selects one of the combination candidates based on the calculated ratio. An X-ray CT apparatus according to one aspect is provided.

  The tenth aspect of the invention is that the predetermined position in the body axis direction in the first region and the predetermined position in the body axis direction in the second region are both end positions. An X-ray CT apparatus according to two aspects is provided.

  The eleventh aspect of the invention is based on the first aspect, wherein the generation unit generates the combination candidates based on a helical pitch designated by an operator and / or an X-ray beam width in the body axis direction. An X-ray CT apparatus according to any one of the tenth aspects is provided.

  In a twelfth aspect of the invention, the first and second angle ranges are an angle range obtained by excluding a part of the angle range from the angle range for one rotation, and the part of the angle range has a vertical direction. An X-ray CT apparatus according to any one of the first to eleventh aspects, which is a predetermined angle range from π / 2 to π radians at the center, is provided.

  Since the X-ray CT apparatus of the invention of the above aspect includes the generation unit, a scan condition that is considered to have an effect of reducing exposure of organs included in the first and second regions may be used as a candidate. In addition, when performing a helical scan, it is possible to support specification of scan conditions for efficiently reducing exposure to a plurality of organs by using a method of controlling the rotation angle of the X-ray tube.

1 is a diagram schematically showing an X-ray CT apparatus according to a first embodiment. It is a figure which shows roughly the structural example of the X-ray tube corresponding to a wide coverage type (wide coverage type) by this embodiment, an X-ray detector, and a collimator. It is a functional block diagram functionally showing the composition of the X-ray CT apparatus of a first embodiment. It is a figure for demonstrating an X-ray tube angle. It is a figure for demonstrating the reference position of a X-ray tube. It is a figure which shows the positional relationship of a scan start position and an attention position. It is a figure which shows the mutual positional relationship of a scan start position, a 1st attention area, and a 2nd attention area. It is a flowchart (flowchart) which shows the flow of the scanning condition setting process in the X-ray CT apparatus by 1st embodiment. It is a figure which shows a mode that a scanning range and an exposure reduction area | region are set on a setting screen. It is a figure which shows a mode that a regulation angle range is set on a setting screen. It is a flowchart which shows the scanning condition candidate production | generation process by 1st embodiment. It is a flowchart which shows the scanning condition candidate narrowing-down process by 1st embodiment. It is an example of a list display of the 2nd scanning condition candidate. It is a flowchart which shows the scanning condition setting process by 2nd embodiment. It is a flowchart which shows the scanning condition candidate production | generation process by 2nd embodiment. It is a flowchart which shows the scanning condition setting process by 3rd embodiment. It is a flowchart which shows the scanning condition candidate production | generation process by 3rd embodiment.

  Embodiments of the invention will be described below.

(First embodiment)
FIG. 1 is a diagram schematically showing the X-ray CT apparatus of the first 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 an input from the operator, a central processing unit 3 that controls each unit for scanning the subject 40 and performs various data processing, and data acquired by the scanning gantry 20. A data collection buffer (buffer) 5 for collecting data, a monitor 6 for displaying images, and a storage device 7 for storing programs and data are provided.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and conveyed to the opening B 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 support unit 16 that rotatably supports the rotating unit 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller 22 that controls the X-ray tube 21, and a collimator 23 that collimates and shapes the X-ray 81 generated from the X-ray tube 21. An X-ray detector 24 that detects X-rays 81 irradiated from the X-ray tube 21 and transmitted through the subject 40, and a data acquisition device (DAS;) that converts and collects the output of the X-ray detector 24 into projection data. Data Acquisition System) 25 and a rotating unit controller 26 for controlling the X-ray controller 22, the collimator 23, and the DAS 25 are mounted. The X-ray detector 24 is a multi detector in which a plurality of detector arrays are arranged. The support unit 16 includes a control 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 support part 16 are electrically connected via a slip ring 30.

  As the X-ray CT apparatus 100 of the present embodiment, a so-called wide coverage type is assumed. The coverage means a range that can be scanned at one time on the rotation center axis IC, and is geometrical depending on the maximum value of the X-ray beam width d in the z direction on the rotation center axis IC and the entire width of the X-ray detector 24. Determined scientifically. Although there is no strict definition of “wide coverage”, here, for example, a coverage of 40 mm or more can be considered. As the coverage increases, the number of X-ray beam width d options that can be set as scan conditions increases, and the calculation for obtaining the scan conditions suitable for exposure reduction becomes complicated. For this reason, the greater the coverage, the greater the meaning of supporting the specification of scan conditions for efficiently reducing exposure to a plurality of areas. In the X-ray CT apparatus 100 of this embodiment, the coverage is set to about 160 [mm].

  FIG. 2 is a diagram schematically illustrating a configuration example of an X-ray tube, an X-ray detector, and a collimator compatible with the wide coverage type according to the present embodiment, and FIG. b) shows a second configuration example. In the first configuration example, as shown in FIG. 2A, a single focus type X-ray tube 21A is used. In the case of this first configuration example, the X-ray beam width in the z direction on the rotation center axis IC is controlled by controlling the opening in the z direction of the collimator 23 with the X-ray 81 from the single focal point 21f as before. Adjust d. On the other hand, in the second configuration example, as shown in FIG. 2B, a bifocal type X-ray tube 21B is used. The bifocal type X-ray tube 21B has a first focal point 21fa and a second focal point 22fb at positions separated by a predetermined distance Df in the z direction. The collimator 23 detects the X-rays from the first collimator 23a for adjusting the X-ray beam width of the X-rays 81a irradiated from the first focal point 21fa to the X-ray detector 24 and the second focal point 21fb. And a second collimator 23b for adjusting the X-ray beam width of the X-ray 81b irradiated to the device 24. In the case of this second configuration example, the X-ray beam widths of the X-rays 81a and 81b are adjusted by controlling the z-direction openings of the first and second collimators 23a and 23b, and the X-ray tube 21B is adjusted. The X-ray 81a and the X-ray 81b are alternately switched and irradiated during scanning to adjust the effective X-ray beam width d in the z direction on the rotation center axis IC.

  FIG. 3 is a functional block diagram functionally showing the configuration of the X-ray CT apparatus of the first embodiment. In this figure, the main part of the operation console is functionally represented.

  As shown in FIG. 3, the operation console 1 in the X-ray CT apparatus 100 of the first embodiment includes a scan control unit 60, an image generation unit 61, a display control unit 62, a scan range setting unit 63, and an exposure reduction region setting unit 64. , A specified angle range setting unit 65, a pitch / beam width temporary setting unit 66, a scan condition candidate generation unit 67, an appropriate region ratio calculation unit 68, and a scan condition selection unit 69.

  The scan control unit 60 controls the imaging table 10 and the scanning gantry 20 to perform various scans. For example, the scan control unit 60 performs a scout scan SS on the subject 40 or performs a helical scan HS that is the main scan according to the scan condition selected by the scan condition selection unit 72. The helical scan HS is a scan in which the X-ray tube 21 is rotated around the subject 40 and relatively linearly moved in the z direction that is the body axis direction of the subject 40.

  Here, the rotation angle (hereinafter referred to as the X-ray tube angle) θ of the X-ray tube 21 is a vertically upward straight line extending from the rotation center IC and the X-ray tube 21 from the rotation center IC as shown in FIG. An angle formed by a straight line extending in the direction of the focal point 21f (in the case of a bifocal type X-ray tube, for example, an intermediate position between the two focal points) is 0 [rad] when the X-ray tube 21 is located directly above. ] (0 °). Further, the reference position Zt in the z direction of the X-ray tube 21 is set to the position in the z direction of the focal point 21f of the X-ray tube 21 or the center position 24c in the z direction of the X-ray detector 24 as shown in FIG. Therefore, the scan start position and the scan end position in the helical scan are the reference position Zt of the X-ray tube 21 when the helical scan HS is started and the reference position Zt of the X-ray tube 21 when the helical scan HS is ended. .

  The image generation unit 61 generates various images based on the data obtained by the scanning gantry 20. For example, the image generation unit 61 generates a scout image GS of the subject 40 based on the scout data PS obtained by performing the scout scan SS, or based on the projection data PH obtained by performing the main scan HS. Then, a tomographic image GH of the subject 40 is generated.

  The display control unit 62 displays various information including images and character information on the screen of the monitor 6. For example, the display control unit 62 displays a scout image GS, a tomographic image GH, scan condition candidates, and the like as necessary.

  The scan range setting unit 63 sets a scan range in the z direction of the subject 40, that is, a scan start position Zs and a scan end position Zf, based on an input from the operator. For example, the operator designates a desired range to be scanned on the scout image GS in the setting screen displayed on the monitor 6 by using the GUI. The scan range setting unit 63 sets a scan range based on the designated range. Note that the scan range setting unit 63 may automatically set the scan range based on other information.

  The exposure reduction region setting unit 64 sets different first and second exposure reduction regions R1, R2 in the z direction of the subject 40 based on the input from the operator. The exposure reduction region is a region including an organ whose exposure should be reduced within the scan range. Usually, an area including an organ with high radiation sensitivity is set as the exposure reduction area. For example, the operator designates an area including the mammary gland and an area including the thyroid gland on the scout image GS in the setting screen displayed on the monitor 6 using the GUI. The exposure reduction area setting unit 64 sets the exposure reduction area based on the designated area. Note that the exposure reduction region setting unit 64 may automatically set the exposure reduction region based on other information such as the result of image analysis on the scout image GS.

  The specified angle range setting unit 65 sets the first specified angle range α1 for the first exposure reduction region R1 and the second specified angle range α2 for the second exposure reduction region R2. The specified angle range is an angle range that the X-ray tube 21 wants to be positioned when passing through the exposure reduction region in order to achieve exposure reduction for a radiation sensitive organ included in the exposure reduction region during the helical scan HS. That is, the angle range to which the X-ray tube angle θ should belong. Usually, the specified angle range is an angle range excluding a part of the angle range 2π [rad] (360 °) for one rotation of the X-ray tube 21 on the body surface side close to a highly sensitive organ. It is. In general, since the subject 40 is placed on the cradle 12 in a supine or prone position, a highly radiation-sensitive organ has a predetermined angular range that extends in the horizontal direction, that is, the x direction, centering on the vertical direction, that is, the y direction. Will be located within. In this case, the specified angle range is an angle range obtained by excluding such a predetermined angle range from the angle range for one rotation. Specifically, assuming that the subject 40 is placed on the back and the subject of exposure reduction is the mammary gland, for example, the remaining lower half rotation π [rad] (180 °) excluding the upper half rotation. If the target of exposure reduction is the thyroid gland, for example, the remaining 3π / 2 [rad] (270 °) excluding the upper center π / 2 [rad] (90 °), and the exposure reduction target In the case of the crystalline lens, for example, the remaining 4π / 3 [rad] (240 °) excluding the upper center 2π / 3 [rad] (120 °) is set as the specified angle range. In this example, the specified angle range setting unit 65 sets the specified angle range based on an input from the operator. The operator specifies a desired angle range from a plurality of preset angle ranges, and the selected specified angle range setting unit 65 sets the specified angle range as the specified angle range. You may do it.

  The pitch / beam width temporary setting unit 66 temporarily sets a desired helical pitch and X-ray beam width input by the operator. The temporarily set helical pitch pa and X-ray beam width da are used as one of the parameters constituting the scan condition candidate when the scan condition candidate generation unit 67 generates the scan condition candidate. The input of the desired helical pitch pa and X-ray beam width da can be omitted. Incidentally, in this embodiment, the helical pitch p is set to a pitch based on the IEC standard pitch, that is, the entire width of the X-ray detector 24 in the z direction. For example, when the distance of the relative linear movement during one rotation of the X-ray tube 21 is equal to the full width of the X-ray detector 24 in the z direction, the helical pitch p is 1.

  The scan condition candidate generation unit 67 includes the set scan start position Zs, the first and second exposure reduction regions R1, R2, the first and second specified angle ranges α1, α2, the temporarily set helical pitch pa, Scan condition candidates are generated based on the X-ray beam width da.

  Here, before explaining a method for generating scan condition candidates, optimization of scan conditions will be briefly described.

  First, in the helical scan, a condition is considered in which the X-ray tube angle at the position of interest in the z direction belongs to a predetermined specified angle range.

  FIG. 6 is a diagram illustrating a positional relationship between the scan start position Zs and the target position Z1. As shown in FIG. 6, the scan start position is Zs, the target position is Z1, the helical pitch is p, the rotation speed of the X-ray tube 21 is s, and the X-ray beam width in the z direction on the rotation center axis IC is d. Then, the movement amount Vz per unit time can be obtained by Equation 1.

  The time Tz1 required for the X-ray tube 21 to move from the scan start position Zs to the target position Z1 can be obtained from Equation 2.

  Further, the rotation angle amount Vθ per unit time can be obtained by Expression 3.

  Then, assuming that the X-ray tube start angle is θ0, the X-ray tube angle θ1 at the target position Z1 can be obtained by Equation 4.

  Therefore, the condition that the X-ray tube angle θ1 at the target position Z1 belongs to the predetermined prescribed angle range can be expressed by Equation 5.

  Spec0 and Spec1 are values that respectively define the start angle and the end angle of the specified angle range when the angle range 2π [rad] for one rotation is 0 to 1.

  Next, in the helical scan, consider a condition in which the X-ray tube angles in the two regions of interest in the z direction belong to a predetermined angle range.

  FIG. 7 is a diagram illustrating the positional relationship between the scan start position Zs, the first region of interest R1, and the second region of interest R2. As shown in FIG. 7, the scan start position is Zs, the center position of the first region of interest R1 is Z1, its both end positions are Z1 ± ΔZ1, the center position of the second region of interest R2 is Z2, and its both end positions are Z2. ± ΔZ2. Similarly, the helical pitch is p, the rotational speed of the X-ray tube 21 is s, and the X-ray beam width in the z direction on the rotation center axis IC is d. Then, the condition that the X-ray tube angle at the center position Z1 belongs to the predetermined first specified angle range can be expressed by Equation 6.

  Spec10 and Spec11 are values that respectively define the start angle and the end angle of the first specified angle range when the angle range 2π [rad] for one rotation is 0 to 1, and n is It is an integer.

  Further, the condition that the X-ray tube angle at both end positions Z1 ± ΔZ1 belongs to the first specified angle range can be expressed by Expression 7 and Expression 8.

  Similarly, the condition that the X-ray tube angle at the center position Z2 belongs to the predetermined second specified angle range can be expressed by Equation 9.

  Spec20 and Spec21 are values that respectively define the start angle and the end angle of the second specified angle range when the angle range 2π [rad] for one rotation is 0 to 1, and m is It is an integer different from n.

  Further, the condition that the X-ray tube angle at both end positions Z2 ± ΔZ2 belongs to the second specified angle range can be expressed by Expressions 10 and 11.

  That is, if a combination of [Zs, p, d, θ0] that satisfies both of the expressions 6 and 9 is obtained, the X-ray tube angle at least at the center position of the first region of interest R1 is the first. This means that the scanning conditions are determined such that the X-ray tube angle at least at the center position of the second region of interest R2 belongs to the second specified angle range. Alternatively, if a combination of [Zs, p, d, θ0] that satisfies all the conditions of Equation 7, Equation 8, Equation 10, and Equation 11 is obtained, X at all z-direction positions of the first region of interest R1 is obtained. Scan conditions are determined such that the tube angle belongs to the first specified angle range, and the X-ray tube angles at all z-direction positions of the second region of interest R2 belong to the second specified angle range. .

  Further, in the mathematical expression representing the above condition, if three parameters of the four parameters Zs, p, d, and θ0 are fixed, the X-ray tube angle at a predetermined position in the z direction in the region of interest can be obtained. The allowable range of the remaining parameters for belonging to the predetermined specified angle range can be obtained.

  Therefore, the scan condition candidate generating unit 67 applies the above formula and uses the following generation method.

  First, a plurality of combinations [pk, di] (k = 1, 2,..., Np, i = 1, 2,..., Nd) are prepared for the helical pitch p and the X-ray beam width d. At this time, when the helical pitch pa and the X-ray beam width da are temporarily set, one or both of the helical pitch p and the X-ray beam width d are fixed to the temporarily set values pa and da, A combination is prepared, for example, using values pa and da with an appropriate gap between them.

  Next, for each combination [pk, di] of the prepared helical pitch p and X-ray beam width d, the first exposure reduction is performed when the helical scan HS is performed from the scan start position Zs according to the conditions of this combination. The X-ray tube angle θ at the center position Z1 of the region R1 specifies the angle range βki1 of the X-ray tube start angle θ0 belonging to the first specified angle range α1, and X at the center position Z2 of the second exposure reduction region R2 An angle range βki2 of the X-ray tube start angle θ0 such that the tube angle θ belongs to the second specified angle range α2 is specified. The X-ray tube start angle θ0 is the X-ray tube angle θ when the helical scan HS is started at the scan start position Zs. For each combination [pk, di], a predetermined number of X-ray tube start angles θ0kij (j = 1, 2,..., Nki) belonging to the angle range βki3 where the angle ranges βki1 and βki2 overlap are prepared. . Thereby, a plurality of scan condition candidates [Zs, pk, di, θ0kij] are generated.

  Next, further narrowing down of the plurality of scan condition candidates is attempted. When a helical scan HS is performed for each of a plurality of scan condition candidates [Zs, pk, di, θ0kij] according to the scan condition candidates, the X-ray tube angle at both end positions Z1 ± ΔZ1 of the first exposure reduction region R1 The X-ray tube start angle θ0 angle range γki11, γki12 such that θ belongs to the first specified angle range α1 within one rotation is specified, and X at the two end positions Z2 ± ΔZ2 of the second exposure reduction region R2 The angle ranges γki21 and γki22 of the X-ray tube start angle θ0 are specified such that the tube angle θ belongs to the second specified angle range α2 within one rotation. For each scan condition candidate [Zs, pk, di, θ0kij], there is an angle range γki3 in which the angle ranges γki11, γki12, γki21, γki22 overlap, and the X-ray tube start angle θ0kij is in this angle range γki3. The scan condition candidate that belongs is specified. The identified scan condition candidate is registered as the first scan condition candidate C1. In the first scan condition candidate C1 registered in this manner, the X-ray tube angles θ at all the z-direction positions of the first exposure reduction region R1 belong to the first specified angle range α1, and the second exposure Since the X-ray tube angles θ at all the z-direction positions in the reduction region R2 belong to the second specified angle range α2, this is the most ideal scan condition in terms of exposure reduction. On the other hand, when such a first scan condition candidate C1 cannot be specified, all the generated scan condition candidates [Zs, pk, di, θ0kij] are registered as second scan condition candidates C2. In the second scan condition candidate C2 registered in this way, the X-ray tube angle θ at least at the center position of the first exposure reduction region R1 belongs to the first specified angle range α1, and the second exposure reduction region Since the X-ray tube angle θ at least at the center position of R2 belongs to the second specified angle range α2, the scanning condition is close to ideal from the viewpoint of reducing exposure.

  When the second scan condition candidate C2 is obtained, the appropriate area ratio calculation unit 68 performs helical scan according to the scan condition candidate for each scan condition candidate [Zs, pk, di, θ0kij] included in the candidate C2. First and second appropriate area ratios ηki1 and ηki2 when HS is performed are calculated. The first appropriate area ratio ηki1 is the ratio of the appropriate area Aki1 in which the X-ray tube angle θ belongs to the first specified angle range α1 in the first exposure reduction area R1. The second appropriate area ratio ηki2 is a ratio of the appropriate area Aki2 in which the X-ray tube angle θ belongs to the second specified angle range α2 in the second exposure reduction area R2. The larger the ratio of the first and second appropriate areas, the closer to the ideal scan condition candidate from the viewpoint of exposure reduction.

  The display control unit 62 displays the first scan condition candidate C1 or the second scan condition candidate C2 as a list on the monitor 6. However, in the case of the second scan condition candidate C2, each scan condition candidate included in the candidate C2 and the first and second appropriate area ratios ηki1 and ηki2 calculated for the scan condition candidate are obtained. A list is displayed on the monitor 6 in association with each other. Thereby, it is possible to quantitatively grasp which scan condition candidate has how much exposure reduction effect. Further, the scan condition candidates are displayed in order according to a predetermined rule based on the magnitudes of the first and second appropriate area ratios ηki1, ηki2. For example, the order of the sum or average of the first and second appropriate area ratios ηki1, ηki2 is large, the order of the maximum values of the first and second appropriate area ratios ηki1, ηki2 is large, or the first and second ratios ηki1 , Ηki2 are both greater than or equal to a predetermined value and are arranged in the order of their variances. As a result, it is possible to deal with the case where it is desired to place importance on the total amount of exposure reduction as the scanning condition, the case where it is desired to place importance on the amount of exposure reduction for a specific organ, or the case where it is desired to place importance on the balance of the amount of exposure reduction for each organ.

  The scan condition selection unit 70 selects one scan condition candidate from the first scan condition candidate C1 or the second scan condition candidate C2, and sets it as the scan condition used for the helical scan HS of the main scan. For example, the operator designates a desired scan condition candidate from the first or second scan condition candidates C1 / C2 displayed as a list on the monitor 6. The scan condition selection unit 72 selects and sets the designated scan condition candidate as a scan condition used for the main scan. When the second scan condition candidate C2 is obtained, the scan condition selection unit 70 selects one of the candidates C2 based on the magnitudes of the first and second appropriate area ratios ηki1 and ηki2. Scan condition candidates may be selected and set as scan conditions used for the main scan. For example, the first or second appropriate area ratios ηki1, ηki2 have the largest sum or average, the first and second appropriate area ratios ηki1, ηki2 have the largest maximum values, or the first and second The appropriate region ratios ηki1 and ηki2 are selected to be both equal to or larger than a predetermined value and have the smallest variance.

  The flow of scan condition setting processing in the X-ray CT apparatus according to the first embodiment will be described below.

  FIG. 8 is a flowchart showing a flow of scan condition setting processing in the X-ray CT apparatus according to the first embodiment.

  In step S1, a scout image GS is acquired and displayed on the screen of the monitor 6. Specifically, a scout scan SS is performed on the subject 40 to obtain scout data PS. When the scout data PS is obtained, a scout image GS is generated based on the scout data PS. Then, a setting screen including the scout image GS is displayed on the screen of the monitor 6.

  In step S2, a scan range and an exposure reduction area are set.

  FIG. 9 is a diagram illustrating how the scan range and the exposure reduction region are set on the setting screen. For example, as shown in FIG. 9, the operator specifies that the range to be scanned is sandwiched between the first straight line Ls and the second straight line Lf on the scout image GS in the setting screen SC displayed on the monitor 6. To do. Accordingly, the position corresponding to the first straight line Ls is set as the scan start position Zs, and the position corresponding to the second straight line Lf is set as the scan end position Zf. Further, as shown in FIG. 9, the operator designates a region including a highly radiation-sensitive organ on the scout image GS so as to surround it with a frame. For example, a region including the thyroid gland KS is specified by being surrounded by the first frame F1, and a region including the mammary gland NS is specified by being surrounded by the second frame F2. As a result, the z-direction region of the first frame F1 is set as the first exposure reduction region R1, and the z-direction region of the second frame F2 is set as the second exposure reduction region R2. Here, as shown in FIG. 9, the scan start position Zs is Z = 0 [cm], the scan end position Zf is Z = 45 [cm], and the first exposure reduction region R1 including the thyroid gland is Z = 5. The second exposure reduction region R2 including the mammary gland is assumed to be Z = 25 to 35 [cm].

  In step S3, a specified angle range is set.

  FIG. 10 is a diagram illustrating a state in which the specified angle range is set on the setting screen. For example, as shown in FIG. 10, the operator displays a pie chart (graph) displayed in association with the first and second frames F1, F2 on the scout image GS in the setting screen SC displayed on the monitor 6. ) Use E1 and E2 to specify a predetermined angle range to be set. Thereby, the designated angle range is set as the specified angle range. Here, as the first specified angle range α1 for the first exposure reduction region R1 including the thyroid gland, the remaining 3π / 2 [rad] (270 °) excluding the upper center π / 2 [rad] (90 °). ) Minutes, that is, α1 = π / 4 to 7π / 4 [rad]. Further, as the second specified angle range α2 for the second exposure reduction region R2 including the mammary gland, π [rad] (180 °) of the remaining lower half rotation excluding the upper half rotation, that is, α2 = π / 2. -3π / 2 [rad] shall be set.

  In step S4, a helical pitch and an X-ray beam width are temporarily set. Here, it is assumed that the operator inputs a desired helical pitch and temporarily sets the input helical pitch pa. The helical pitch pa is, for example, pa = 1.

  In step S5, scan condition candidates are generated.

  FIG. 11 is a flowchart showing the scan condition candidate generation process in step S5.

  In step S501, d = d1, d2,..., Dn are prepared as the X-ray beam width d. Here, d1 = 10 [cm] and d2 = 15 [cm] are prepared.

  In step S502, the parameter i is initialized to i = 1.

  In step S503, the condition [Zs, pa, di] based on the scan start position Zs and the combination of the helical pitch pa and the X-ray beam width di is selected as the processing target condition.

  In step S504, an angle range βi1 of X0 in which the X-ray tube angle θ belongs to the first specified angle range α1 is specified at the center Z1 of the first exposure reduction region R1.

  In step S505, the angle range βi2 in which the X-ray tube angle θ belongs to the second prescribed angle range α2 at the center Z2 of the second exposure reduction region R2 is specified.

  In step S506, a predetermined number of X-ray tube start angles θ0ij are prepared in the angle range βi3 where the angle ranges βi1 and βi2 overlap. Here, θ0ij is prepared in increments of π / 8. As a result, scan condition candidates [Zs, pa, di, θ0ij] are generated.

  In step S507, it is determined whether i = n. If i = n, the process proceeds to step S508, i is incremented by 1, and the process returns to step S503. If i = n, the process ends.

  In step S6, the scan condition candidates are narrowed down.

  FIG. 12 is a flowchart showing the scan condition candidate narrowing-down process in step S6.

  In step S601, i is initialized to i = 1.

  In step S602, the condition [Zs, pa, di] based on the scan start position Zs and the combination of the helical pitch pa and the X-ray beam width di is selected as a processing target condition.

  In step S603, the angle ranges γi11 and γi12 of the X-ray tube angle θ belonging to the prescribed range α1 are specified at both ends Z1 ± ΔZ1 of the first exposure reduction region R1.

  In step S604, the angle ranges γi21 and γi22 of the X-ray tube angle θ belonging to the prescribed range α2 are specified at both ends Z2 ± ΔZ2 of the second exposure reduction region R2.

  In step S605, an angle range γi3 in which the angle ranges γi11, γi12, γi21, and γi22 overlap is specified.

In step S606, j is initialized and j = 1.
In step S607, it is determined whether or not the angle range γi3 exists. If the angle range γi3 exists, it is further determined whether or not the X-ray tube start angle θ0ij belongs to the angle range γi3. If the X-ray tube start angle θ0ij belongs to the angle range γi3, the process proceeds to step S608, and if the angle range γi3 does not exist or if the X-ray tube start angle θ0ij does not belong to the angle range γi3, the process jumps to step S609. .

  In step S608, the scan condition candidate [Zs, pa, di, θ0ij] is registered in the first scan condition candidate C1.

  In step S609, it is determined whether j = ni. If j = ni is not satisfied, the process proceeds to step S610, j is incremented by 1, and the process returns to step S607. If j = ni, the process proceeds to step S611.

  In step S611, it is determined whether i = n. If i = n, the process proceeds to step S612, i is incremented by 1, and the process returns to step S602. If i = n, the process ends.

  In step S7, it is determined whether the first scan condition candidate C1 is registered. If not registered, all the generated scan condition candidates are registered as second scan condition candidates C2, and the process proceeds to step S8. If registered, the process proceeds to step S11.

  In step S8, for each scan condition candidate [Zs, pa, di, θ0ij] included in the second scan condition candidate C2, the X-ray tube angle θ is the first specified angle in the first exposure reduction region R1. The first appropriate region Ai1 belonging to the range α1 is specified, and the second appropriate region Ai2 whose X-ray tube angle θ belongs to the second specified angle range α2 is specified in the second exposure reduction region R2.

  In step S9, for each scan condition candidate [Zs, pa, di, θ0ij] included in the second scan condition candidate C2, the first appropriate area Ai1 occupies the first exposure reduction area R1. While calculating 1 appropriate area | region ratio (eta) i1, 2nd appropriate area | region ratio (eta) i2 which is the ratio which 2nd appropriate area | region Ai2 occupies among 2nd exposure reduction area | region R2 is calculated.

  In step S10, for each scan condition candidate [Zs, pa, di, θ0ij] included in the second scan condition candidate C2, the scan condition candidate and the first and second calculated for the scan condition candidate are displayed. Are displayed in a list in association with the appropriate area ratios ηi1 and ηi2. At this time, the first and second appropriate area ratios ηi1 and ηi2 are displayed side by side in descending order of the total η. Then, the process proceeds to step S12.

  On the other hand, in step S11, each scan condition candidate [Zs, pa, di, θ0ij] included in the first scan condition candidate C1 is displayed as a list. Then, the process proceeds to step S12.

  FIG. 13 is an example of a list display of second scan condition candidates C2. In this example, the Z coordinate is taken on the horizontal axis, and the scan start position Zs, the scan end position Zf, the first and second exposure reduction regions R1, R2, and the first and second appropriate regions A1, A2 are graphed. it's shown. Also, a pie chart-like shape representing the X-ray tube angle θ at the scan start position Zs, and the positions Z1, Z2, Z1 ± ΔZ1, Z2 ± ΔZ2 at the center and both ends of the first and second exposure reduction regions R1, R2. A schematic diagram is displayed in association with each other. Such a display makes it easier to understand the operation status when a helical scan is performed according to each scan condition candidate, such as judgment materials when selecting scan conditions, whether or not it is necessary to change temporarily set parameters, etc. It becomes a judgment material.

  In step S12, the operator determines and inputs whether or not to change the temporarily set helical pitch pa. In the case of changing, the process returns to step S4 and the process is performed again. On the other hand, if not changed, the operator designates a desired scan condition candidate from the displayed list, and the designated scan condition candidate is selected and set as a scan condition used for the helical scan HS of the main scan. .

  From this point onward, as usual, the helical scan HS is performed according to the set scan conditions, the image is reconstructed based on the obtained projection data, and the tomographic image is displayed on the monitor 6.

  According to the first embodiment, since the X-ray CT apparatus 100 includes the scan condition candidate generation unit 67 as described above, it is included in each of the first and second exposure reduction regions R1 and R2. A scan condition that is considered to have an effect of reducing the exposure of the organ can be used as a candidate, and a plurality of highly radiation-sensitive organs are used by controlling the rotation angle of the X-ray tube 21 when performing a helical scan. It becomes possible to support the specification of the scanning conditions for efficiently reducing the exposure to the.

(Second embodiment)
FIG. 14 is a flowchart showing the flow of the scan condition setting process according to the second embodiment, and FIG. 15 is a flowchart showing the scan condition candidate generation process of step T5.

  In the second embodiment, when the X-ray start angle θ0ij is prepared and the scan condition candidate [Zs, pa, di, θ0ij] is generated for each condition of [Zs, pa, di], the angle ranges βi1, It is confirmed whether or not there is an angle range εi in which βi2, γi11, γi12, γi21, and γi22 overlap (step T508). If the angle range εi exists, a predetermined number of X-ray tube start angles θ0ij belonging to the angle range εi are prepared (step T508). If the angle range εi does not exist, the angle ranges βi1 and βi2 A predetermined number of X-ray tube start angles θ0ij belonging to the angle range βi3 where and are overlapped are prepared (step T510). That is, from the viewpoint of reducing exposure, the X-ray tube angles θ at all the z-direction positions of the first and second exposure reduction regions R1, R2 belong to the first and second specified angle ranges α1, α2, respectively. If the most ideal scan condition exists, such a scan condition is generated as a scan condition candidate. When such an ideal scan condition does not exist, the X-ray tube angle θ at least in the centers Z1 and Z2 of the first and second exposure reduction regions R1 and R2 is the first and second, respectively. Scan condition candidates belonging to the specified angle ranges α1 and α2 are generated.

  Then, the first and second appropriate area ratios ηi1 and ηi2 are calculated for all scan condition candidates (steps T6 and T7), and the sum or average of the first and second appropriate area ratios ηi1 and ηi2 is large. The scan condition candidates are arranged in order and displayed in a list (step T8).

  According to such a second embodiment, not only the most ideal scan condition candidates from the viewpoint of exposure reduction but also other scan condition candidates are displayed in a list, so that the scan conditions can be set only from the viewpoint of exposure reduction. In addition, it is possible to cope with a case where the user wants to select from the viewpoint of high-speed imaging depending on the helical pitch and the X-ray beam width.

(Third embodiment)
FIG. 16 is a flowchart showing the flow of the scan condition setting process according to the third embodiment, and FIG. 17 is a flowchart showing the scan condition candidate generation process of step U6.

  In the third embodiment, the temporary setting is performed not only for the helical pitch but also for the X-ray beam width (step U5). When generating the scan condition candidates, pa, pa ± Δp,... With a predetermined interval around the temporarily set helical pitch pa are prepared (step U601), and the temporarily set X Da, da ± Δd,... With a predetermined interval around the line beam width da are prepared (step U602). Then, for each condition of [Zs, pk, di], an X-ray start angle θ0ij is prepared and a scan condition candidate [Zs, pa, di, θ0ij] is generated.

  According to the third embodiment, it is possible to generate scan condition candidates that are close to the helical pitch or X-ray beam width desired by the operator.

  Although the embodiments of the invention have been described above, the embodiments of the invention are not limited to the above-described embodiments, and various additions and changes can be made without departing from the spirit of the invention.

  For example, when a helical scan is performed, the rotation angle of the X-ray tube at a predetermined position other than the center position in the first exposure reduction region or the other end positions belongs to the first specified angle range, In addition, only scan condition candidates are generated such that the rotation angle of the X-ray tube at a predetermined position other than the center position in the z direction or the other end positions in the second exposure reduction region falls within the second specified angle range. You may make it do. Alternatively, when the helical scan is performed, the rotation angle of the X-ray tube at both end positions in the z direction in the first exposure reduction region belongs to the first specified angle range, and in the second exposure reduction region. Only candidate scan conditions may be generated such that the rotation angle of the X-ray tube at both end positions in the z direction belongs to the second specified angle range.

  Further, for example, in the above embodiment, there are two exposure reduction regions, but of course, an embodiment in which there are three or more exposure reduction regions is also possible.

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 Unit 16 Supporting Unit 20 Scanning Gantry 21, 21A, 21B X-ray Tube 21f Focus 21fa First Focus 21fb First Focus 2 2 X-ray controller 23 Collimator 23a First collimator 23b Second collimator 24 X-ray detector 25 DAS
26 Rotating unit controller 29 Control controller 30 Slip ring 40 Subject 81 X-ray 100 X-ray CT apparatus

Claims (12)

When the X-ray beam width in the body axis direction of the subject is maximized, wide-coverage X-rays are applied so that X-rays are simultaneously irradiated to the highly sensitive organ and other parts of the subject. An X-ray CT apparatus equipped with an irradiable X-ray tube ,

Setting means for setting a scan range and first and second regions each containing different organs having high radiation sensitivity included in the scan range based on the position in the body axis direction;

When setting a scan condition for performing a helical scan at a constant X-ray tube rotational speed, a constant helical pitch, and a constant X-ray beam width in the body axis direction with respect to the scan range, A rotation angle of the X-ray tube at a predetermined position in the body axis direction in the first region belongs to the first angle range, and the X-ray at a predetermined position in the body axis direction in the second region. Generating means for determining an X-ray beam width smaller than the maximum X-ray beam width such that the rotation angle of the tube belongs to the second angle range;

An X-ray CT apparatus comprising: a scanning unit that performs a helical scan of the scan range in accordance with a scan condition including the predetermined helical pitch and the X-ray beam width .
The X-ray CT apparatus according to claim 1, wherein the X-ray tube has a first focal point and a second focal point at a position separated by a predetermined distance in the body axis direction .
The generation unit searches for a start angle of the X-ray tube when a helical scan is performed from a scan start position in the set scan range according to the helical pitch and the X-ray beam width in the body axis direction. , X-rays CT apparatus according to claim 1 or claim 2 for generating a candidate of the combination.
The predetermined position of the body axis direction in the body axis direction of the predetermined position and the second region in the first region, claim 1 is the center position of any one of claims 3 X-ray CT system.
The generating means includes a rotation angle of the X-ray tube at a predetermined position in the body axis direction in the first region at the predetermined helical pitch belonging to a first angle range, and at the second region. As a result of calculation of an X-ray beam width smaller than the maximum X-ray beam width such that the rotation angle of the X-ray tube at a predetermined position in the body axis direction belongs to the second angle range, no candidate is obtained. When the helical scan is performed with the predetermined helical pitch for which the X-ray beam width was not obtained , the rotation angle of the X-ray tube in the first region is within the first angle range. The ratio of the region including the position in the body axis direction to belong to, and the rotation angle of the X-ray tube in the second region in the body axis direction out of the second region. Calculate the percentage of the area containing the position X-ray CT apparatus according to claim 4 comprising calculating means for.
The calculation means includes a rotation angle of the X-ray tube at a predetermined position in the body axis direction in the first region at the predetermined helical pitch belonging to a first angle range, and at the second region. As a result of calculation of an X-ray beam width smaller than the maximum X-ray beam width such that the rotation angle of the X-ray tube at a predetermined position in the body axis direction belongs to the second angle range, no candidate is obtained. When the helical scan is performed at the predetermined helical pitch for which the X-ray beam width is not obtained , the rotation angle of the X-ray tube at both end positions of the first region is the first angular range. And a determination means for determining whether or not a combination candidate is included so that the rotation angle of the X-ray tube at both end positions of the second region belongs to the second angle range;

When the determination is negative, when a helical scan is performed according to the combination condition for the generated combination candidate, the rotation angle of the X-ray tube of the first region is the first combination. The ratio of the region including the position in the body axis direction that belongs to the angle range and the body in which the rotation angle of the X-ray tube belongs to the second angle range in the second region The X-ray CT apparatus according to claim 4 , further comprising calculation means for calculating a ratio of a region including the position in the axial direction.
The X-ray CT apparatus according to claim 6 , further comprising display means for displaying the generated combination candidate and the calculated ratio in association with the combination condition.
The X-ray CT apparatus according to claim 7 , wherein the display unit displays the generated combination candidates in order according to a predetermined rule based on the magnitude of the calculated ratio with respect to the combination condition. .
It said selection means, X-rays CT apparatus according to claim 7 or claim 8 selects a combination that has been designated by the operator among the displayed combination of candidates.
It said selecting means, based on the magnitude of the ratio of the calculated, X-rays CT apparatus according to any one of claims 8 claims 5 to select one of the candidates of the combinations.
The predetermined position of the body axis direction in the body axis direction of the predetermined position and the second region in the first region, claim 1 is a end positions according to any one of claims 3 X-ray CT system.
It said generating means, based on the specified helical pitch by the operator, the X-ray CT apparatus according to any one of claims 1 to 11 for obtaining the X-ray beam width of the body axis direction.

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