WO2021031968A1 - 一种ct***扫描动态调节方法 - Google Patents

一种ct***扫描动态调节方法 Download PDF

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WO2021031968A1
WO2021031968A1 PCT/CN2020/108809 CN2020108809W WO2021031968A1 WO 2021031968 A1 WO2021031968 A1 WO 2021031968A1 CN 2020108809 W CN2020108809 W CN 2020108809W WO 2021031968 A1 WO2021031968 A1 WO 2021031968A1
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cone beam
axis
opening
scanned
area
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曾凯
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南京安科医疗科技有限公司
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
    • A61B6/03—Computed tomography [CT]
    • A61B6/032—Transmission computed tomography [CT]
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/06—Diaphragms
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
    • A61B6/40—Arrangements for generating radiation specially adapted for radiation diagnosis
    • A61B6/4064—Arrangements for generating radiation specially adapted for radiation diagnosis specially adapted for producing a particular type of beam
    • A61B6/4085—Cone-beams

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  • the invention relates to a scanning dynamic adjustment method of a CT system, which belongs to the technical field of medical imaging.
  • the current mainstream CT system uses a multi-row detector, which usually uses a cone X-ray beam to scan the human body. Because the spiral scanning can get better image quality, the scanning speed is fast, and the patient keeps steady and continuous movement on the bed, therefore, the spiral scanning has a wide range of applications in clinical use.
  • the vast majority of CT examinations use spiral scanning. But this scanning method is not perfect, and there are still areas that need improvement.
  • the system uses an alignment blade to control the size of the beam. The movement of the two blades is usually symmetrical with respect to the center of the opening, so the coverage of the ray beam is usually symmetrical with respect to the central plane.
  • the US patent proposes a mechanism that can switch the ray beam.
  • the boundary of the ray beam is to block the rays by rotating two independent elliptical cylinders to achieve the purpose of cone-ray beam adjustment.
  • the adjustment here is fixed.
  • the range is not adaptively adjusted according to the shape of the object.
  • the rotation of the elliptical cylinder makes the edge of the ray beam block or open, and the two sides cooperate at the same time to control the shape of the cone beam.
  • the conventional system does not have the ability to dynamically adjust the cone beam, which makes the invalid scanning area of multi-slice CT larger and increases the radiation dose to the patient.
  • the method mentioned in the aforementioned US patent requires an additional movement mechanism. This requires not only more space but also more complex control.
  • the method based on the elliptic cylinder is limited to the size of the elliptical cylinder, and the control of the cone beam is limited, especially for CT systems with large cone angles of 64 rows and above.
  • the technical problem to be solved by the present invention is to provide a method for dynamically adjusting the scanning of a CT system, which can effectively control the shape of the ray beam, so as to minimize the invalid scanning area of the spiral CT scan and reduce the overall scanning dose.
  • a CT system scan dynamic adjustment method a CT system scan dynamic adjustment method, is characterized in that the method adjusts two relatively independent opening baffle A and opening baffle B in the CT system to adjust the cone of X-ray source
  • the bundle adjustment is specifically divided into the following three stages:
  • the opening shutter A When the scan is not started, the opening shutter A is in a fully closed state, that is, the opening shutter A completely blocks the cone beam emitted by the X-ray source, and the opening shutter B is in a fully opened state; when the cone beam emitted by the X-ray source is detected When the beam touches the area to be scanned, scanning starts, and the opening stopper A is driven to move to the left.
  • the distances between the opening stopper A and the opening stopper B and the central axis of the cone beam satisfy the formula:
  • dx A and dx B represent the opening flaps A and The distance between the aperture plate B and the central axis of the cone beam, sid represents the distance between the X-ray source and the central axis of the area to be scanned, R represents the radius of the area to be scanned, and z represents the central axis of the cone beam and The distance between the start or end position of the area to be scanned, s2b represents the distance between the X-ray source and the aperture cover A or the aperture cover B, and x_max represents the cone beam when the aperture cover A and the aperture cover B are fully opened X-axis positive coordinate value;
  • the coverage of the cone beam on the X axis is [g(dx A ), g(dx B )], g(dx A ), g(dx B ) represent the left and right boundaries of the cone beam, respectively
  • the coordinate value on the X axis, when g(dx A ) -g(dx B ), that is, when the opening shutter A is fully opened, the scanning enters the intermediate stage;
  • Both the opening baffle A and the opening baffle B are in a fully opened state, that is, the cone beam emitted by the X-ray source is not blocked;
  • the scanning enters the end stage.
  • the opening stopper B is driven to move to the right, and the opening stopper A is fully opened.
  • the opening stopper A and the opening stopper B are respectively The distance from the central axis of the cone beam satisfies the formula:
  • the cross section of the area to be scanned at each time is regarded as an ellipse, and the long and short axes of the ellipse at each time are the respective time when the patient is supine.
  • the expression of R is:
  • R( ⁇ ) represents the distance between the center of the ellipse and the point of intersection at the beginning or end.
  • the point of intersection is the point of intersection between the line of the tube and the center of the ellipse and the boundary of the ellipse.
  • a and b represent the length of the ellipse at each moment.
  • Half axis and short half axis, ⁇ is expressed as the angle between the line connecting the tube and the center of the ellipse and the y axis of the coordinate system.
  • the coordinate values g(dx A ) and g(dx B ) of the left and right edges of the cone beam on the X axis at the beginning of the scan are as follows:
  • sid represents the distance between the X-ray source and the central axis of the area to be scanned
  • R represents the radius of the area to be scanned
  • z represents the distance between the central axis of the cone beam and the start or end position of the area to be scanned
  • x_max represents The coordinate value of the cone beam in the positive direction of the X-axis when the opening baffle A and the opening baffle B are fully opened.
  • the coordinate values g(dx A ) and g(dx B ) of the left and right edges of the cone beam on the X axis at the intermediate stage of the scan are as follows:
  • x_max represents the positive coordinate value of the cone beam on the X axis when the aperture flap A and the aperture flap B are fully opened
  • -x_max means the cone beam is negative on the X axis when the aperture flap A and the aperture flap B are fully opened.
  • the coordinate value of the direction is the positive coordinate value of the cone beam on the X axis when the aperture flap A and the aperture flap B are fully opened.
  • the coordinate values g(dx A ) and g(dx B ) of the left and right edges of the cone beam on the X axis are as follows:
  • sid represents the distance between the X-ray source and the central axis of the area to be scanned
  • R represents the radius of the area to be scanned
  • z represents the distance between the central axis of the cone beam and the start or end position of the area to be scanned
  • x_max represents The coordinate value of the cone beam in the positive direction of the X-axis when the opening flap A and the opening flap B are fully opened.
  • the present invention adopts the above technical solutions and has the following technical effects:
  • the present invention realizes the change of the shape of the cone beam through the left and right movement of the opening baffle piece group, so as to reach the area near the start and end positions of the cone beam, and minimize the irradiation of the invalid area.
  • the present invention proposes a method that can adjust the shape of the cone beam of the X-ray system. By adjusting the shape of the cone beam, the radiation dose to the ineffective scanning area can be reduced, and the image quality of the effective area can be guaranteed.
  • the present invention uses independently moving blades to realize the control of the ray beam, without adding additional mechanisms, and the independently moving blades have lower cost and easier control.
  • Figure 1 shows the scanning area and effective scanning range in traditional cone beam spiral scanning.
  • Figure 2 is the whole process of scanning, the shape diagram of the cone beam modulation at the beginning and end positions.
  • Figure 3 shows the coverage of the cone ray beam at each stage of the scan; among them, (a), (b), (c) are the middle, start, and end stages of the scan, respectively.
  • Fig. 4 is the modulation relationship between the movement of the aperture flap and the range of the cone beam.
  • Fig. 5 is a schematic diagram of the beginning of scanning.
  • Figure 6 is a schematic diagram of the scanning end stage.
  • FIG. 7 is a schematic diagram of the cross section of the area to be scanned as an ellipse at the beginning or end of scanning.
  • This patent proposes a cone beam adjustment method, which can effectively control the shape/range of the cone beam.
  • the specific realization can be achieved by adjusting the shape of the cone beam through an independently moving collimator blade in the X-ray optical path.
  • Figure 2 shows the shape diagram of the cone beam modulation at the beginning and end of the scanning process. When the left blade is opened and the right is closed, the cone beam on the right is blocked and the ray beam is deflected to the left. On the contrary, the cone beam on the left is attenuated.
  • the opening blade of the collimator in the CT system is driven by a motor to switch the ray opening in different scanning modes.
  • the two blades usually move in symmetrical coordination.
  • the original stepping motor can also be used to drive the two open blades to move independently, so as to achieve the purpose of modulating the cone beam.
  • the present invention achieves the goal of adjusting the cone beam by changing the control mode of the open blade.
  • the specific coordinate system definition is shown in FIG. 4. When the filter is shifted in the lateral direction, the positions of the left and right borders of the cone beam are changed.
  • the specific correspondence is the g function.
  • the patient's area to be scanned is regarded as a cylindrical shape, and the point where the central axis of the cone beam intersects with the central axis of the area to be scanned is defined as the origin of the coordinate system.
  • the direction in which the cone beam moves to the area to be scanned is the positive X axis, cone
  • the coverage of the beam on the X axis is [g(dx A ), g(dx B )], g(dx A ), g(dx B ) represent the coordinates of the left and right edges of the cone beam on the X axis, respectively value.
  • the modulation of the cone beam in the scanning process can be calculated according to the relationship of FIG. 5 and FIG. 6.
  • the distance between the center axis of the cone beam and the scan start position is z.
  • the left and right boundaries of the cone beam satisfy the following formula, and the distances dx A and dx B of the opening blade relative to the central axis of the cone beam also satisfy the following relationship.
  • the distance between the center axis of the cone beam and the end of the scan is z.
  • the left and right boundaries of the cone beam satisfy the following formula, and the distances dx A and dx B of the opening blade relative to the central axis of the cone beam also satisfy the following relationship.
  • the overall scanning dose is reduced.
  • the section of the patient's area to be scanned at each time at the beginning and end stage is regarded as an ellipse
  • R at the beginning or end stage is expressed as the distance between the center of the ellipse section and the point of intersection, and the point of intersection is the tube and The intersection of the center line of the ellipse section and the boundary of the ellipse is shown in Figure 7.
  • the long and short axes of the ellipse section are the horizontal and vertical widths when the patient is supine.
  • ⁇ is the angle between the line connecting the tube and the center of the ellipse section and the coordinate axis y axis.

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Abstract

一种CT***扫描动态调节方法，通过调节CT***中两个相对独立的开口挡片A和开口挡片B，对X射线源发出的锥形束进行调节，具体分为三个阶段：开始阶段：当检测到X射线源发出的锥形束接触到待扫描区域时，扫描开始，驱动开口挡片A打开，锥形束在X轴上的覆盖范围为[g（dx A）,g（dx B）]，当g（dx A）＝-g（dx B）时，进入中间阶段；中间阶段：开口挡片A和开口挡片B均处于完全打开状态；结束阶段：当-g（dx A）>g（dx B）时，进入结束阶段，驱动开口挡片B开始关闭，开口挡片A处于完全打开状态。根据患者待扫描区域的体型，对锥形束的形状/范围进行有效的控制，使得在临床螺旋CT扫描中，能够尽可能的减少无效扫描区域，减少整体的扫描剂量，并保证有效区域的图像质量。

Description

一种CT***扫描动态调节方法 技术领域
本发明涉及一种CT***扫描动态调节方法，属于医学成像技术领域。
背景技术
目前主流的CT***采用是多排探测器，这个通常使用的是锥形X射线束对人体进行扫描。由于螺旋扫描能够得到更好的图像质量，扫描速度快，病人在床上保持平稳和连续的运动，因此，螺旋扫描在临床使用中有着广泛的应用。绝大多数CT检查都是采用的螺旋扫描方式。但是这个扫描方式并不完美，还有需要改进的方面。比如在现有的大多数***中，***是通过一对准直刀片来对射线束的大小来进行控制的。这两片刀片的运动通常是相对于开口中心对称移动，因此射线束的覆盖范围通常是相对于中心平面对称的。但是由于锥形束的特点，为了完全覆盖扫描区域，必须要多扫描一段范围，如图1所示。普通CT***的螺旋扫描会因为无效扫描区域的影像，给病人带来额外的辐射剂量。这个问题在16排一下的CT***中并不明显，因为锥形束的锥角很小，但是对于现在越来越多排数的多排CT***，这个问题变得尤为突出，比如64排及以上***，如果不采用任何措施，无效扫描区域会变得很大。
为了降低无效扫描区域，理想情况下的调节应该是可以开关或者调制锥形束的形状，避开不需要扫描的区域。这就需要把射线束调节成非对称的形状，通常需要额外的机构来对射线束进行控制。美国专利(US7113570)提出了一种能够开关射线束的机构，射线束的边界是通过旋转两个独立的椭圆柱来对射线进行遮挡，以达到锥形射线束调节的目的，这里调节是固定的范围，并没有根据物体的形状做自适应的调节。椭圆柱的转动使得射线束的边缘挡住或者打开，两边同时配合就能起到控制锥形束形状的作用。
常规的***并不具备对锥形束动态调节的能力，使得多排CT的无效扫描区 域变大，增加对病人的辐射剂量。上述美国专利中所提到的方法，需要增加额外的运动机构。这个不仅需要更大的空间也需要更复杂的控制。而且基于椭圆柱的方法，限于椭圆柱的尺寸，对锥形束的控制有限，尤其是对于64排及以上的大锥角的CT***。
发明内容
本发明所要解决的技术问题是：提供一种CT***扫描动态调节方法，能够有效控制射线束形状，使得螺旋CT扫描的无效扫描区域降到最低，减少整个扫描剂量。
本发明为解决上述技术问题采用以下技术方案：
一种CT***扫描动态调节方法，CT***扫描动态调节方法，其特征在于，该方法通过调节CT***中两个相对独立的开口挡片A和开口挡片B，对X射线源发出的锥形束进行调节，具体分为以下三个阶段：
扫描开始阶段：
扫描未开始时，开口挡片A处于完全关闭状态，即开口挡片A将X射线源发出的锥形束完全挡住，开口挡片B处于完全打开状态；当检测到X射线源发出的锥形束接触到待扫描区域时，扫描开始，驱动开口挡片A往左移动，开口挡片A、开口挡片B分别与锥形束的中轴线之间的距离满足公式：
Figure PCTCN2020108809-appb-000001
Figure PCTCN2020108809-appb-000002
定义锥形束的中轴线与待扫描区域的中轴线相交的点为坐标系原点，锥形束向待扫描区域移动的方向为X轴正向，dx A、dx B分别表示开口挡片A、开口挡片B与锥形束的中轴线之间的距离，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离，s2b表示X射线源与开口挡片A或开口挡片B之间的距离，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值；
扫描开始阶段，锥形束在X轴上的覆盖范围为[g(dx A),g(dx B)]，g(dx A)、g(dx B)分别表示锥形束左边界、右边界在X轴上的坐标值，当g(dx A)＝-g(dx B)时，即开口挡片A完全打开时，扫描进入中间阶段；
扫描中间阶段：
开口挡片A和开口挡片B均处于完全打开状态，即X射线源发出的锥形束未被遮挡；
扫描结束阶段：
当-g(dx A)>g(dx B)时，扫描进入结束阶段，此时驱动开口挡片B往右移动，开口挡片A处于完全打开状态，开口挡片A、开口挡片B分别与锥形束的中轴线之间的距离满足公式：
Figure PCTCN2020108809-appb-000003
Figure PCTCN2020108809-appb-000004
作为本发明的一种优选方案，所述扫描开始或者结束阶段，将待扫描区域在每个时刻的截面视为椭圆形，每个时刻椭圆形的长、短轴分别为患者仰卧时每个时刻对应的水平、垂直方向的宽度，则R的表达式为：
R(θ)＝(1-w(θ))a+w(θ)b
Figure PCTCN2020108809-appb-000005
其中，R(θ)表示在开始或者结束阶段椭圆形中心与交点之间的距离，交点为球管与椭圆形中心连线和椭圆边界的交点，a、b分别表示每个时刻椭圆形的长半轴、短半轴，θ表示为球管与椭圆截面中心连线与坐标系y轴之间的夹角。
作为本发明的一种优选方案，所述扫描开始阶段，锥形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
Figure PCTCN2020108809-appb-000006
g(dx B)＝x_max
其中，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离， x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值。
作为本发明的一种优选方案，所述扫描中间阶段，锥形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
g(dx A)＝-x_max
g(dx B)＝x_max
其中，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值，-x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴负向的坐标值。
作为本发明的一种优选方案，所述扫描结束阶段，锥形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
g(dx A)＝-x_max
Figure PCTCN2020108809-appb-000007
其中，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值。
本发明采用以上技术方案与现有技术相比，具有以下技术效果：
1、本发明通过开口挡片片组的左右移动，实现对锥形束形状的改变，来达到锥形束在开始和结束位置附近的区域，尽量减少对无效区域的照射。
2、本发明提出了一种能够对X射线***锥形束形状进行调节的方法，通过对锥形束形状的调节可以减少对无效扫描区域的辐射剂量，并保证有效区域的图像质量。
3、本发明利用独立运动的刀片实现射线束的控制，不需要增加额外的机构，独立运动的刀片成本更低，更容易控制。
附图说明
图1是传统锥形束螺旋扫描中的扫描区域和有效扫描范围。
图2是扫描的整个过程，开始和结束位置锥形束调制的形状图。
图3是扫描各阶段锥形射线束的覆盖范围；其中，(a)、(b)、(c)分别是扫描中间、开始、结束阶段。
图4是开口挡片移动和锥形束范围的调制关系。
图5是扫描开始阶段的示意图。
图6是扫描结束阶段的示意图。
图7是扫描开始或结束阶段待扫描区域截面视为椭圆形示意图。
具体实施方式
下面详细描述本发明的实施方式，所述实施方式的示例在附图中示出。下面通过参考附图描述的实施方式是示例性的，仅用于解释本发明，而不能解释为对本发明的限制。
本专利提出了一种锥形束调节方法，能够对锥形束的形状/范围进行有效的控制。使得在常用的临床螺旋CT扫描中，能够尽可能的减少无效扫描区域，减少整体的扫描剂量，并保证有效区域的图像质量。具体的实现可以通过在X射线的光路中通过独立运动的准直器刀片对锥形束的形状进行调节。如图2所示，为扫描的整个过程，开始和结束位置锥形束调制的形状图。左刀片打开，右边关上，右边的锥形束就被挡住，射线束往左偏，反之，左边的锥形束就被衰减。扫描开始的时候，准直器左边刀片在右侧，右边刀片打开，开口向右偏移，这样锥形束的右边保持正常输出，左侧基本都被衰减，随着球管的平移，准直器的开口左边也逐渐打开，锥形束全部打开。随着球管移动到扫描区域的末端，准直器开口右边刀片向左逐渐关闭，使得射线束的右侧基本关闭。上述的过程就可以有效的控制无效扫描区域的辐射剂量。图3的(a)、(b)、(c)分别是扫描中间、开始和结束阶段，锥形射线束的覆盖范围。
通常CT***中的准直器的开口刀片是由电机驱动的，用以切换不同扫描模式下的射线开口。两个刀片通常是对称的协调运动。这里同样可以利用原有的步进电机来驱动这两个开口刀片独立运动，达到调制锥形束的目的。本发明通过改变开口刀片的控制方式实现调节锥形束的目标，具体的坐标系定义如图4所示。当滤波器在横向有偏移的时候，锥形束的左边界和右边界的位置被改变。具体的对应关系是g函数。
将患者待扫描区域视为圆柱形，定义锥形束的中轴线与待扫描区域的中轴线相交的点为坐标系原点，锥形束向待扫描区域移动的方向为X轴正向，锥形束在X轴上的覆盖范围为[g(dx A),g(dx B)]，g(dx A)、g(dx B)分别表示锥形束左边界、右边界在X轴上的坐标值。
在扫描的过程中的锥形束的调制可以按照图5和图6的关系来计算。在扫描开始阶段，假设锥形束的中轴线距离扫描开始位置的距离是z。锥形束的左边界和右边界满足以下公式，开口刀片相对于锥形束的中轴线距离dx A和dx B也是满足以下关系。
Figure PCTCN2020108809-appb-000008
g(dx A)＝x
g(dx B)＝x_max
则
Figure PCTCN2020108809-appb-000009
Figure PCTCN2020108809-appb-000010
在扫描结束阶段，假设锥形束的中轴线距离扫描结束位置的距离是z。锥形束的左边界和右边界满足以下公式，开口刀片相对于锥形束的中轴线距离dx A和dx B也是满足以下关系。
g(dx A)＝-x_max
g(dx B)＝x
则
Figure PCTCN2020108809-appb-000011
Figure PCTCN2020108809-appb-000012
为了进一步减少开始阶段以及结束阶段的无效扫描区域，减少整体的扫描剂量。在开始以及结束阶段，将患者待扫描区域在开始以及结束阶段每个时刻的截面视为椭圆形，则R在开始或者结束阶段表示为椭圆截面中心与交点之间的距离，交点为球管与椭圆截面中心连线和椭圆边界的交点，如图7所示。
椭圆截面的长、短轴分别为患者仰卧时水平、垂直方向的宽度。
R(θ)＝(1-w(θ))a+w(θ)b
Figure PCTCN2020108809-appb-000013
其中，θ表示为球管与椭圆截面中心连线与坐标轴y轴之间的夹角。
以上实施例仅为说明本发明的技术思想，不能以此限定本发明的保护范围，凡是按照本发明提出的技术思想，在技术方案基础上所做的任何改动，均落入本发明保护范围之内。

Claims (5)

  1. 一种CT***扫描动态调节方法，其特征在于，该方法通过调节CT***中两个相对独立的开口挡片A和开口挡片B，对X射线源发出的锥形束进行调节，具体分为以下三个阶段：
    扫描开始阶段：
    扫描未开始时，开口挡片A处于完全关闭状态，即开口挡片A将X射线源发出的锥形束完全挡住，开口挡片B处于完全打开状态；当检测到X射线源发出的锥形束接触到待扫描区域时，扫描开始，驱动开口挡片A往左移动，开口挡片A、开口挡片B分别与锥形束的中轴线之间的距离满足公式：
    Figure PCTCN2020108809-appb-100001
    Figure PCTCN2020108809-appb-100002
    定义锥形束的中轴线与待扫描区域的中轴线相交的点为坐标系原点，锥形束向待扫描区域移动的方向为X轴正向，dx A、dx B分别表示开口挡片A、开口挡片B与锥形束的中轴线之间的距离，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离，s2b表示X射线源与开口挡片A或开口挡片B之间的距离，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值；
    扫描开始阶段，锥形束在X轴上的覆盖范围为[g(dx A),g(dx B)]，g(dx A)、g(dx B)分别表示锥形束左边界、右边界在X轴上的坐标值，当g(dx A)＝-g(dx B)时，即开口挡片A完全打开时，扫描进入中间阶段；
    扫描中间阶段：
    开口挡片A和开口挡片B均处于完全打开状态，即X射线源发出的锥形束未被遮挡；
    扫描结束阶段：
    当-g(dx A)>g(dx B)时，扫描进入结束阶段，此时驱动开口挡片B往右移动，开口挡片A处于完全打开状态，开口挡片A、开口挡片B分别与锥形束的中轴线之间的距离满足公式：
    Figure PCTCN2020108809-appb-100003
    Figure PCTCN2020108809-appb-100004
  2. 根据权利要求1所述CT***扫描动态调节方法，其特征在于，所述扫描开始或者结束阶段，将待扫描区域在每个时刻的截面视为椭圆形，每个时刻椭圆形的长、短轴分别为患者仰卧时每个时刻对应的水平、垂直方向的宽度，则R的表达式为：
    R(θ)＝(1-w(θ))a+w(θ)b
    Figure PCTCN2020108809-appb-100005
    其中，R(θ)表示在开始或者结束阶段椭圆形中心与交点之间的距离，交点为球管与椭圆形中心连线和椭圆边界的交点，a、b分别表示每个时刻椭圆形的长半轴、短半轴，θ表示为球管与椭圆截面中心连线与坐标系y轴之间的夹角。
  3. 根据权利要求1所述CT***扫描动态调节方法，其特征在于，所述扫描开始阶段，锥 形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
    Figure PCTCN2020108809-appb-100006
    g(dx B)＝x_max
    其中，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值。
  4. 根据权利要求1所述CT***扫描动态调节方法，其特征在于，所述扫描中间阶段，锥形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
    g(dx A)＝-x_max
    g(dx B)＝x_max
    其中，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值，-x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴负向的坐标值。
  5. 根据权利要求1所述CT***扫描动态调节方法，其特征在于，所述扫描结束阶段，锥形束左边界、右边界在X轴上的坐标值g(dx A)、g(dx B)，公式为：
    g(dx A)＝-x_max
    Figure PCTCN2020108809-appb-100007
    其中，sid表示X射线源与待扫描区域的中轴线之间的距离，R表示待扫描区域的半径，z表示锥形束的中轴线与待扫描区域开始或结束位置之间的距离，x_max表示开口挡片A和开口挡片B完全打开时锥形束在X轴正向的坐标值。
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