CT system z-axis position adjustment tool and adjustment method thereof

Abstract

The present invention belongs to the field of X-ray CT systems, and in particular relates to a z-position adjustment tool for a CT system and an adjustment method thereof, comprising an X-ray shielding plate, an X-ray transmission window for transmitting X-rays, a shielding plate frame for supporting the X-ray shielding plate, and a horizontal position fine-adjustment mechanism for adjusting the horizontal position of the X-ray shielding plate. One side of the shielding plate frame is provided with a support frame for fixing to a patient bed frame, and the support frame and the shielding plate frame are connected by a pitch angle fine-adjustment mechanism for adjusting the pitch angle of the X-ray shielding plate. The present invention adjusts the detector and the X-ray generator to the same vertical plane through an adjustment tool. The present invention has low cost, simpler operation, more accurate results, reusability, simpler and more reliable installation and fixation of the detector, a high degree of automation, and better image quality, especially for multi-segment heart scans and three-dimensional reconstruction images.

Description

Technical Field

[0001] The present invention belongs to the field of X-ray CT systems, and in particular relates to a CT system z-axis position adjustment tool and an adjustment method thereof. Background Art

[0002] Currently, X-ray CT systems have been widely used in medical diagnosis, industrial flaw detection and other fields.

[0003] Aligning the components of the CT system's imaging chain to the same vertical plane (generally referred to as the scan plane) is a necessary adjustment step before using the CT system. The goal is to ensure that the scan plane is in the vertical direction and that the X-ray tube focus is aligned with the center of the detector. This ensures that, without tilting the gantry, the images within a single scan and from multiple consecutive scans are all located in the vertical plane and aligned with each other, preventing misalignment in the tilted direction. This ensures that the detector's geometric efficiency is maximized and the detected signal achieves the highest signal-to-noise ratio.

[0004] Figure 1 and Figure 2 is a schematic diagram of the CT system. Figure 1 As shown, the imaging chain of the CT system includes an X-ray tube 10, a collimator 11, and a detector 12. The rotation plane refers to the plane where the focal center of the X-ray tube 10 and the z-axis of the detector 12 are located, and the z-axis refers to the direction perpendicular to the rotation plane.

[0005] The traditional method for z-axis adjustment and alignment of the imaging chain uses film and an appropriate collimator opening. The film is placed on the dust seal at the bottom of the gantry aperture (scanning window). The film is then exposed twice, at the 0-degree position and the 180-degree position. The exposed area on the 180-degree exposure film is narrower and darker, while the exposed area on the 0-degree exposure film is wider and lighter. The distribution of the 0-degree exposure band on either side of the 180-degree exposure band is measured to calculate the offset of the X-ray tube relative to the beam limiter. The detector offset relative to the X-ray tube and beam limiter is then adjusted. This step can be performed using film or, for multi-row detector systems, directly using data collected by the detectors to determine whether the projection of the X-ray beam through the collimator 11 on the detector is centered.

[0006] The above method has the advantages of simple technology and intuitive and easy-to-understand operation, but its disadvantages are:

[0007] The above method requires the use of disposable film, which is not environmentally friendly, and the operation process is purely manual measurement and calculation. The boundary between the two exposures on the film is not clear and sharp, and the result is greatly affected by the operator's subjective influence.

[0008] Furthermore, the above method uses a collimator as a reference for z-axis adjustment, which requires that the z-axis mounting positions of the X-ray tube and detector are adjustable, increasing the difficulty of mechanical design and reducing the rigidity of the detector installation. This will cause the detector to deform more during high-speed rotation, ultimately affecting image quality.

[0009] There is also a method in which the X-ray tube and the detector centerline are not adjusted to be in the same vertical plane, but only the focus position is adjusted so that the focus and the detector centerline are approximately in the same plane.

[0010] While this method is simple and fast to adjust, the plane in which the X-ray tube focal point and the detector centerline lie is not vertical, nor perpendicular to the system's rotation axis or the direction of motion of the patient bed during scanning. This ultimately leads to reduced image quality, particularly for cardiac scans and 3D reconstructions. Summary of the Invention

[0011] In order to address the defects and deficiencies in the above-mentioned prior art, the present invention provides a low-cost, reusable, automated CT system z-position adjustment tool and adjustment method for position adjustment of imaging chain components of an X-ray CT system and calibration of the CT system.

[0012] The technical solution of the present invention is: a z-axis position adjustment tool for a CT system, comprising an X-ray shielding sheet, an X-ray transmission window for transmitting X-rays, a shielding sheet frame for supporting the X-ray shielding sheet, and a horizontal position fine-adjustment mechanism for adjusting the horizontal position of the X-ray shielding sheet. One side of the shielding sheet frame is provided with a support frame for fixing to a patient bed frame, and the support frame and the shielding sheet frame are connected by a pitch angle fine-adjustment mechanism for adjusting the pitch angle of the X-ray shielding sheet.

[0013] Preferably, the number of the X-ray transmission windows is two, and the two X-ray transmission windows are symmetrically distributed about a Z-axis passing through the geometric center of the X-ray shielding sheet.

[0014] The centers of the two X-ray transmission windows are located at the center position of the X-ray shielding sheet in the z direction, and are equidistant from the center point of the X-ray shielding sheet in the x direction, and the two X-ray transmission windows have the same size.

[0015] Preferably, the horizontal position fine-adjustment mechanism is two horizontal pressing plates installed on the shielding plate frame, which are used to adjust the horizontal position of the X-ray shielding plate so that it is always in a horizontal direction.

[0016] Preferably, the horizontal position fine-tuning mechanism is two fine-tuning rulers installed on the shielding plate frame, which are used to adjust the horizontal position of the X-ray shielding plate so that the center line of the two X-ray transmission windows on the X-ray shielding plate and the center line of the detector are located on parallel vertical planes.

[0017] Preferably, the shielding sheet frame is provided with support bars that match the X-ray shielding sheet, and both ends of the X-ray shielding sheet are supported on the support bars and pressed by a horizontal position fine-tuning mechanism.

[0018] Preferably, the pitch angle fine-tuning mechanism includes a support vertical plate fixed on one side of the shielding film frame, an adjusting bolt installed on the upper side of the support vertical plate and a plurality of limit bolts. The support frame is connected to the support vertical plate through the limit bolts, and the angle of the support frame is adjusted by adjusting the bolts, thereby realizing the pitch angle adjustment of the X-ray shielding film.

[0019] The present invention can adjust the pitch, lateral, longitudinal and torsion of the X-ray shield to ensure that the X-ray shield is in a horizontal direction, and the connecting line of the centers of the X-ray transmission windows of the X-ray shield is parallel to the plane where the center line of the detector is located, and the distances from the centers of the two X-ray transmission windows to the straight line described below are equal, which is the line connecting the center channel and the focus of the X-ray tube when the X-ray tube is at the 0 degree position.

[0020] A method for adjusting the position of an X-ray tube and a detector using a z-position adjustment tool of a CT system includes the following steps:

[0021] 1) Ensure that there are no obstructions in the CT system scanning field and perform a single exposure of air data;

[0022] 2) Place the adjustment tool in a position within the scanning field of view so that the projection of the X-ray through the window is within the detector receiving range;

[0023] 3) Two exposures are performed when the X-ray tube of the CT system is at the 0-degree position and the 180-degree position respectively;

[0024] 4) Gain correction is performed on the 0-degree and 180-degree data using the air data. Then, the offset of the X-ray tube in the z direction is obtained based on the X-ray distribution projected on the detector by the X-rays on the X-ray shield through the window and the positional relationship between the X-ray tube and the detector.

[0025] 5) adjusting the focus of the X-ray tube according to the offset of the X-ray tube so that the distance between the focus and the center line of the detector in a direction perpendicular to the vertical plane where the center line of the detector is located meets the preset indicators of the system.

[0026] Preferably, the positional relationship between the X-ray tube and the detector in step 4) refers to the positional relationship between the X-ray tube focus, the X-ray shield, the X-ray transmission window, and the detector, and the offset of the X-ray tube in the z direction refers to the offset of the X-ray tube focus relative to the detector centerline in the z direction;

[0027] (1) Determine whether the system adjustment tool has been properly placed based on the distance between the projection center of gravity of the two X-ray transmission windows and the central channel. If the projection center of gravity of the X-ray transmission window on the detector is significantly different from the distance to the central channel, fine-tune the position of the shielding sheet so that the distance from the center of the two X-ray transmission windows to the central channel beam is equal;

[0028] (2) Based on the geometric position of the projection center of the two X-ray transmission windows on the detector and the known distance between the centers of the two X-ray transmission windows, calculate the length a of the projection of the line connecting the center of the X-ray transmission window and the focus of the X-ray tube on the vertical plane (i.e., the XY plane);

[0029]

[0030] in:

[0031] a represents the length of the projection of the line from the focus of the X-ray tube to the center of the X-ray transmission window on the vertical plane;

[0032] d represents the distance between the centers of the two X-ray transmission windows on the adjustment tool; e represents the distance between the center of gravity of the projection of the X-ray transmission window on the adjustment tool and the central channel. When the adjustment tool is centered, the distance between the centers of the left and right X-ray transmission windows and the central channel is e1 = e2 = e, and the unit is the number of detector channels.

[0033] θ represents the fan angle of a single channel of the detector relative to the focus of the X-ray tube in the XY plane (that is, the above two are projected into the XY plane).

[0034] Preferably, the step of obtaining the offset of the X-ray tube focus based on the intensity distribution of the projection of the X-ray transmission window on the X-ray shielding sheet on the detector, the X-ray tube focus, the positional relationship between the vertical plane where the center line of the detector is located, and the distance between the adjustment tool and the X-ray tube focus also includes:

[0035] Calculate the z-direction offset g0 and g2 of the center of gravity of the projection of the X-ray through the window on the detector when the X-ray tube focus is at 0 degrees and 180 degrees. 180 ;

[0036] When the X-ray tube is at 0 degrees and 180 degrees, determine the lengths a0 and a1 of the projection of the line from the center of the X-ray transmission window to the focus of the X-ray tube on the vertical plane.180 ;

[0037] The z-direction offset s of the X-ray tube focus is obtained according to the following formula:

[0038]

[0039] in,

[0040] a0 represents the length of the projection on the vertical plane of the line from the focus of the X-ray tube to the center of the X-ray transmission window when the X-ray tube is at 0 degrees exposure;

[0041] a 180 It indicates the length of the projection of the line from the focus of the X-ray tube to the center of the X-ray transmission window on the vertical plane when the X-ray tube is at 180 degrees exposure;

[0042] g0 represents the z-direction offset of the center of gravity of the projection of the X-ray transmission window on the adjustment tool on the detector when the X-ray tube is at 0 degrees exposure, that is, the distance from the center line of the detector;

[0043] g 180 It indicates the z-direction offset of the center of gravity of the projection of the X-ray transmission window on the adjustment tool on the detector when the X-ray tube is at 180 degrees exposure, that is, the distance from the center line of the detector.

[0044] Preferably, after determining the z-direction offset of the X-ray tube focus and making corresponding adjustments, the X-ray shield of the collimator is moved in small steps from the extreme position on one side to the center of the X-ray beam, and the detector response data is collected at the same time, and the outputs of the corresponding detector data channel and beam position tracking channel are recorded to determine the collimator shield position corresponding to each opening and the target value of the beam tracking.

[0045] The technical solution of the present invention for adjusting the position of the imaging chain components of the CT system using the CT system z-direction adjustment tool and method further includes the following steps:

[0046] Ensure that there are no obstructions within the rack aperture and perform an air scan as a baseline;

[0047] Place the adjustment tool at a certain height away from the rotation center, and adjust the tool as much as possible to make the X-ray transmission window parallel to the plane where the z-axis of the detector is located;

[0048] The CT system is set so that the X-ray tube of the CT system is at 0 degrees, and the tool of the present invention is exposed. The data including the adjustment tool can be normalized using the data of the air scan, and the center of gravity of the projection area of the X-ray through the window on the detector can be calculated by the intensity of the transmission projection.

[0049] According to the center of gravity of the projection of the X-ray transmission window on the detector, it is determined whether the line connecting the centers of the X-ray transmission window is parallel to the plane where the z-axis center line of the detector is located, and whether the distances to the line connecting the center of gravity channel and the focus of the detector are equal. Preferably, appropriate adjustments can be made using a fine-tuning device based on the deviation so that the X-ray transmission window is close to the expected position;

[0050] The CT system is set so that the X-ray tube of the CT system is at 180 degrees, and the tool of the present invention is exposed. Preferably, the data including the adjustment tool can be normalized using the data of the air scan, and the center of gravity of the projection area of the X-ray window on the detector can be calculated by the intensity of the transmission projection;

[0051] Compared with the prior art, the present invention has the following beneficial effects and advantages:

[0052] 1. The system adjustment tool of the present invention is reusable and easy to carry, thereby saving costs.

[0053] 2. The present invention can use common metal materials, such as steel, and is therefore more environmentally friendly.

[0054] 3. The solution of the present invention does not require manual measurement. The offset of the X-ray tube can be calculated through the system's built-in software, and the operator is prompted to adjust the direction and distance. Therefore, the operation is simpler and the results are more accurate.

[0055] 4. The adjustment tool and method of the present invention use the z-axis centerline of the detector as a reference and do not require the detector to be adjustable in the z-axis. Therefore, the installation and fixation of the detector are simpler and more reliable.

[0056] 5. The adjustment tool and method of the present invention can ensure that the X-ray tube focus and the detector centerline are in the same vertical plane, and perpendicular to the device rotation axis and the direction of patient bed entry, thereby improving image quality, especially for multi-segment cardiac scans and three-dimensional reconstructed images. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] To help understand the content of this disclosure, the following description of the patent drawings is provided:

[0058] Figure 1 and Figure 2 This is a simple schematic diagram of the imaging chain device layout of a CT system;

[0059] Among them, 10 is an X-ray tube, 11 is a collimator, and 12 is a detector;

[0060] Figure 3 is a schematic structural diagram of a z-position adjustment tool for a CT system according to the present invention;

[0061] 1 is the shielding plate frame, 2 is the X-ray shielding plate, 3 is the horizontal position fine-tuning mechanism, 4 is the pitch angle fine-tuning mechanism, 5 is the support frame; 6 is the X-ray transmission window;

[0062] Figure 4 This is a schematic diagram of the X-ray distribution on the detector when the X-ray tube is offset;

[0063] Figure 5 It is the triangular relationship between the focus, the center of the X-ray transmission window and the center of gravity of the detector projection projected into the xy plane;

[0064] Figure 6 is the geometric relationship between the focus, the center of the X-ray transmission window, the center of gravity of the detector projection, and the center line of the detector projected into the y2-z plane; the straight line y1 is the straight line connecting the focus, the center of the X-ray transmission window, and the center of gravity of the detector projection, and the straight line y2 is the projection of the straight line y1 into the XY plane;

[0065] Figure 7 This is a schematic diagram of the X-ray projection distribution of the shielding sheet and the X-ray transmission window on the detector;

[0066] Figure 8 It is a schematic diagram of the z-direction adjustment method of the CT system of the present invention. DETAILED DESCRIPTION

[0067] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments. However, the present invention is not limited to the following specific embodiments.

[0068] like Figure 1 and 2 As shown, the imaging chain of the X-ray CT system mainly includes three components: an X-ray tube 10, a collimator 11 and a detector 12, which rotate around a unified rotation center during scanning.

[0069] like Figure 3 As shown, the CT system z-axis adjustment tool includes a shield frame 1, an X-ray shield 2, a horizontal position fine-adjustment mechanism 3, a pitch angle fine-adjustment mechanism 4, and a support frame 5. The X-ray shield 2 is provided with an X-ray transmission window 6, and the support frame 5 is mounted on the mold frame of the patient bed.

[0070] There are two X-ray transmission windows 6 , which should be evenly distributed on the X-ray shielding sheet 2 and be symmetrical with respect to the center of the X-ray shielding sheet 2 (in the x direction).

[0071] The adjustment method using the above-mentioned CT system z-position adjustment tool includes the following steps: (1) placing the X-ray shield and the bracket on the patient support or auxiliary positioning device so that the X-ray shield and the window deviate from the ISO center by an appropriate distance, such as 130 mm, in the height direction; (2) controlling the temperature of the X-ray tube at a low level, placing the X-ray tube at the 0-degree position for a low-dose static exposure, and collecting data; (3) controlling the temperature of the X-ray tube at a low level, placing the X-ray tube at the 180-degree position for a low-dose static exposure, and collecting data; (4) preprocessing and analyzing the data, calculating the offset value of the X-ray tube focal spot, and providing adjustment suggestions; (5) mechanically adjusting the X-ray tube position according to the adjustment suggestions; repeating steps (2) to (5) until the X-ray tube focal spot position meets the requirements.

[0072] like Figure 4 As shown in the figure, when the z-axis adjustment tool of the CT system is placed in the field of view of the detector and at a certain height from the rotation center, the X-rays passing through the window will form a projection on the detector. The placement of the adjustment tool does not require the z-axis to be aligned with the center of the detector, but requires that there should be no twisting or pitching as much as possible, and the x-axis should be as central as possible.

[0073] like Figure 5 As shown in the figure, when the z-axis adjustment tool of the CT system is within the field of view, the angle between the projection of the ray passing through the center of the X-ray transmission window and the projection of the ray passing through the central channel on the XY plane can be calculated based on the geometric position of the projection center of gravity. The left and right sides can be calculated separately.

[0074] β1=e1·θ

[0075] β2=e2·θ

[0076] Where e1 is the number of offset detector channels, and θ is the fan angle corresponding to each channel in the XY plane (ie, the ray beam is projected onto the XY plane).

[0077] The lengths a1 and a2 of the projections of the lines connecting the focal point and the centers of the two X-ray transmission windows on the XY plane can be obtained by solving the following equations:

[0078]

[0079] in:

[0080] d is the distance between the centers of the two X-ray transmission windows of the z-axis adjustment tool of the CT system;

[0081] d1 and d2 are the lengths of the projections on the XY plane of the vertical line segments from the centers of the two X-ray transmission windows to the ray beam passing through the central channel;

[0082] a1, a2 are the lengths of the projections of the line segments from the focus to the centers of the two X-ray transmission windows on the XY plane;

[0083] In particular, when the tools are placed symmetrically on the left and right, a can be calculated using the following formula:

[0084]

[0085] in:

[0086] a is the length of the projection of the line from the focus of the X-ray tube to the center of the X-ray transmission window on the vertical plane (i.e., the XY plane) (the same for the left and right X-ray transmission windows);

[0087] e is the number of detector channels (equal on the left and right) by which the projected X-ray beam at the center of the window is offset from the projected X-ray beam at the center of the channel after the two described below (i.e., the X-ray beam at the center of the window and the X-ray beam at the center channel) are projected into the XY plane.

[0088] Figure 6 It expresses the geometric relationship between the focus, the center of the X-ray transmission window, the center of the detector projection, and the center line of the detector projected into the y2-z plane; the straight line y1 is the straight line connecting the focus, the center of the X-ray transmission window, and the center of the detector projection, and the straight line y2 is defined as the projection of the straight line y1 into the XY plane. Figure 6 As shown, for each X-ray transmission window, the ratio of the distance from the center of the X-ray transmission window to the focus in the z-direction to the distance from the center of gravity of the projection of the X-ray transmission window on the detector to the focus in the z-direction is equal to the ratio described below, which is the ratio of the distance from the center of the X-ray transmission window to the focus in the y2-direction to the distance from the center of gravity of the projection of the X-ray transmission window on the detector to the focus in the y2-direction.

[0089] For the exposure of the X-ray tube at 0 degrees and 180 degrees:

[0090]

[0091] in:

[0092] f is the distance in the z-direction between the center of the X-ray transmission window and the focal point;

[0093] s is the distance in the z-direction from the focus to the center line of the detector (i.e., the origin of the z-axis);

[0094] b is the length of the projection of the line from the focus to the detector's Z-axis origin on the vertical plane (the Z-axis is consistent with the direction of movement of the scanning bed and is perpendicular to the vertical plane);

[0095] a0, a180 It is the length of the projection of the line from the focus of the X-ray tube to the center of the X-ray transmission window on the vertical plane when the X-ray tube is at 0 degrees and 180 degrees;

[0096] g0, g 180 It is the absolute value of the z-coordinate of the center of gravity of the projection of the X-ray through the window on the detector when the X-ray tube is at 0 degrees and 180 degrees.

[0097] Therefore, the distance s between the focus and the vertical plane where the detector centerline is located can be obtained by the following formula:

[0098]

[0099] The result is the z-adjustment distance of the X-ray tube.

[0100] Figure 7 The figure shows a schematic diagram of the X-ray projection distribution of the shielding sheet and the X-ray transmission window on the detector.

[0101] like Figure 8 As shown, in the overall process of the CT system z-position adjustment method, the order of 0-degree exposure and 180-degree exposure can be reversed without affecting the result.

Claims

1. A z-position adjustment tool for a CT system, characterized by: The device comprises an X-ray shielding sheet, two X-ray transmission windows for transmitting X-rays, a shielding sheet frame for supporting the X-ray shielding sheet, and a horizontal position fine-adjustment mechanism for adjusting the horizontal position of the X-ray shielding sheet. A support frame for fixing to a patient bed frame is provided on one side of the shielding sheet frame, and the support frame and the shielding sheet frame are connected by a pitch angle fine-adjustment mechanism for adjusting the pitch angle of the X-ray shielding sheet. There are two X-ray transmission windows, and the two X-ray transmission windows are symmetrically distributed about the Z-axis passing through the geometric center of the X-ray shielding sheet; The adjustment method of the z-position adjustment tool of the CT system is used to calibrate the position of the tube and the detector, and includes the following steps: 1) Ensure that there are no obstructions in the CT system scanning field and perform a single exposure of air data; 2) Place the adjustment tool in a position within the scanning field of view so that the projection of the X-ray through the window is within the detector receiving range; 3) Determine whether the system adjustment tool has been properly placed based on the distance between the projected center of gravity of the two X-ray transmission windows and the central channel. If the projected center of gravity of the X-ray transmission window on the detector is significantly different from the distance to the central channel, fine-tune the position of the shielding sheet to make the distance between the centers of the two X-ray transmission windows and the central channel equal; 4) Based on the geometric position of the projection center of the two X-ray transmission windows on the detector and the known distance between the centers of the two X-ray transmission windows, calculate the length a of the projection of the line connecting the center of the X-ray transmission window and the focus of the tube on the vertical plane (i.e., the XY plane); in: a represents the length of the projection of the line from the focus of the tube to the center of the X-ray transmission window on the vertical plane; d represents the distance between the centers of the two X-ray transparent windows on the adjustment tool; e represents the distance between the center of gravity of the X-ray projection through the window and the central channel in the projection of the adjustment tool on the detector, and the unit is the number of detector channels; θ represents the fan angle of a single channel of the detector relative to the focus of the tube in the XY plane (that is, the two aforementioned channels are projected into the XY plane); 5) Two exposures are performed when the CT system tube is at the 0-degree position and the 180-degree position respectively; 6) Gain correction is performed on the 0-degree and 180-degree data using the air data. The offset of the tube in the z direction is then obtained based on the X-ray distribution projected onto the detector by the X-rays on the X-ray shield through the window and the positional relationship between the tube and the detector. 7) adjusting the focus of the tube according to the offset of the tube so that the distance between the focus and the center line of the detector in a direction perpendicular to the vertical plane where the center line of the detector is located meets the preset indicators of the system; In step 6), the positional relationship between the tube and the detector refers to the positional relationship between the tube focus, the X-ray shield, the X-ray transmission window, and the detector. The offset of the tube in the z direction refers to the offset of the tube focus relative to the detector's z-axis centerline. In step 6), the offset of the tube focus is obtained based on the intensity distribution of the projection of the X-ray transmission window on the X-ray shield on the detector, the positional relationship between the tube focus and the vertical plane where the center line of the detector is located, and the distance between the adjustment tool and the tube focus, which also includes: Calculate the z-direction offset g0 and g2 of the center of gravity of the projection of the X-ray through the window on the detector when the focus of the tube is at 0 degrees and 180 degrees. 180 ; When the tube is at 0 degrees and 180 degrees, determine the lengths a0 and a1 of the projection of the line from the center of the X-ray window to the focus of the tube on the vertical plane. 180 ; The z-direction offset s of the tube focus is obtained according to the following formula: in, a0 represents the length of the projection on the vertical plane of the line from the focus of the tube to the center of the X-ray transmission window when the tube is at 0 degrees exposure; a 180 It indicates the length of the projection of the line from the focus of the tube to the center of the X-ray transmission window on the vertical plane when the tube is at 180 degrees exposure; g0 represents the z-direction offset of the center of gravity of the projection of the tube from the transmission window on the adjustment tool onto the detector when the tube is at 0 degrees exposure, that is, the distance from the center line of the detector; g 180 It indicates the z-direction offset of the center of gravity of the projection from the transmission window on the adjustment tool onto the detector when the tube is at 180 degrees exposure, that is, the distance from the center line of the detector.

2. The CT system z-position adjustment tool according to claim 1, wherein: The horizontal position fine-adjustment mechanism is composed of two horizontal pressing plates installed on the shielding plate frame, which are used to adjust the horizontal position of the X-ray shielding plate so that it is always in a horizontal direction.

3. The CT system z-position adjustment tool according to claim 1, wherein: The horizontal position fine-tuning mechanism is two fine-tuning rulers installed on the shielding plate frame, which are used to adjust the horizontal position of the X-ray shielding plate so that the center line of the two X-ray transmission windows on the X-ray shielding plate and the center line of the detector are located on parallel vertical planes.

4. The CT system z-position adjustment tool according to claim 3, wherein: The shielding sheet frame is provided with a support bar that matches the X-ray shielding sheet, and both ends of the X-ray shielding sheet are supported on the support bar and pressed by a horizontal position fine-tuning mechanism.

5. The CT system z-position adjustment tool according to claim 1, wherein: The pitch angle fine-tuning mechanism includes a support vertical plate fixed on one side of the shielding film frame, an adjusting bolt installed on the upper side of the support vertical plate and a plurality of limit bolts. The support frame is connected to the support vertical plate through the limit bolts, and the angle of the support frame is adjusted by adjusting the bolts, thereby realizing the pitch angle adjustment of the X-ray shielding film.

6. The method for adjusting the z-position adjustment tool of a CT system according to claim 1, wherein: After determining the z-direction offset of the tube focus and making corresponding adjustments, the collimator's X-ray shield is moved in small steps from the extreme position on one side to the center of the X-ray beam, and the detector response data is collected at the same time. The outputs of the corresponding detector data channel and beam position tracking channel are recorded to determine the collimator shield position corresponding to each opening and the target value of beam tracking.

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