A scanning method and image reconstruction method for continuous tilt linear scanning variable pitch spiral CT

Abstract

A scanning method and image reconstruction method for a continuous tilted linear scan variable-pitch spiral CT, belonging to the field of ray tomography technology, includes the following steps: S1: the radiation source is translated along an inclined linear trajectory with an axial tilt angle γ(t), while the flat-panel detector remains stationary, completing a segment of the tilted linear scan in the tth scan cycle; S2: the radiation source and detector assembly are translated axially relative to the object being measured by an angle interval h(t), while the object is rotated by an angle interval Δθ; S3: the tilted linear scan in S1 is continued; this process is repeated until the variable-pitch spiral scan trajectory covers the object being measured. The reconstruction method includes: S1': obtaining a three-dimensional DBP image of the nth segment of the tilted linear scan using a differential backprojection operator; S2': performing a one-dimensional finite Hilbert inverse transform on the nth segment of the DBP image along the axial direction layer by layer; and S3': accumulating the reconstruction results of all segments of the tilted linear scan. The present invention can achieve variable-pitch scanning and imaging of long and transversely truncated and axially truncated objects, with high resolution and ease of implementation.

Description

Technical Field

[0001] The present invention belongs to the technical field of ray tomography, and in particular relates to a scanning method and an image reconstruction method of a continuous tilt linear scanning variable pitch spiral CT. Background Art

[0002] Computed tomography (CT) is a nondestructive testing technology that enables internal imaging of objects under test. It is widely used in numerous fields, including medicine, industrial testing, safety inspections, archaeology, agriculture, and geophysics. In practical CT testing, high-resolution imaging of large objects is often required. However, traditional CT testing requires that the object under test be strictly within the imaging field of view. If the object is too large, the geometric magnification ratio must be appropriately reduced, but this sacrifices imaging resolution. This is especially true when the object under test is difficult or impossible to separate into small samples, such as precious objects such as fossils, gemstones, and antiques. Achieving both a large imaging field of view and a high geometric magnification ratio (i.e., high resolution) has long been a key goal in the field of CT imaging.

[0003] Previous methods for expanding the field of view (FOV) of CT imaging primarily employed offset detector imaging geometry. However, this approach theoretically only allows for a maximum expansion of the field of view by a factor of two. Furthermore, as the FOV expands, the redundancy interval decreases, leading to deterioration in image quality. Therefore, in practice, the FOV cannot be expanded to its theoretical value.

[0004] Compared to traditional circular scanning modes, linear scanning is easier to control in terms of precision and engineering implementation. Patent Publication No. CN104809750A proposes a simple, low-cost, mobile / portable linear scanning CT system. This system utilizes a scanning method in which the X-ray source and detector move in parallel in different directions, showing potential in low-cost CT. To enable imaging of objects beyond the linear scanning CT field of view, Patent Publication No. CN106447740 proposes a method for reconstructing regions of interest (ROIs) in relatively parallel linear scanning CT. This method can partially image objects beyond the linear scanning CT field of view, but it cannot fully image the entire object. To increase both the geometric magnification ratio and the cross-sectional imaging field of view, Patent Publication No. CN111839568A proposes a novel large-field-of-view linear scanning CT system and image reconstruction method. This system achieves linear scanning by translating the X-ray source along a fixed linear trajectory using a fixed detector. Considering the limited length of a linear trajectory, which results in a limited reconstruction angle, a method is designed to rotate the workpiece by several angles to enable complete imaging of large transversely sized objects. However, since the system's multiple straight scanning trajectories are straight line segments within the same plane, it is unable to meet the data completeness condition in cone-beam CT imaging, achieve the expansion of the lateral field of view, and cannot achieve the imaging of axially long objects. In fact, if the system wants to achieve higher spatial resolution, that is, increase the geometric magnification ratio, the cone angle will become smaller. To avoid axial truncation artifacts, the usual measure is to cut off the data outside the cone beam artifact, which results in a narrowing of the axial imaging field of view. The above invention can also achieve scanning imaging of long objects by performing three-dimensional reconstruction and splicing block by block along the axial direction of the long object; however, this method not only has low scanning efficiency, but also has large registration errors. To achieve an expanded field of view for spiral cone-beam CT (SCT), that is, to resolve the issue of expanded field of view imaging with transverse truncation while also achieving axially truncated long object imaging, Zou Xiaobing proposed a transversely truncated, semi-covered spiral cone-beam CT method in his article "Research on Scanning Methods and Reconstruction Algorithms for Large-Field-of-View Spiral Cone-Beam Industrial CT." He also proposed rearranging the spiral cone-beam projection data into fan-beam projection data in a single layer, followed by reconstruction using a two-dimensional fan-beam BPF algorithm. He further proposed a dual-helical cone-beam CT scanning method, which ensures that the cross-section of the imaged area is completely covered by the beam at each projection angle. In fact, both the semi-covered spiral cone-beam CT imaging and dual-helical cone-beam CT scanning imaging studied by Zou Xiaobing are based on standard spiral trajectories. The essence of expanding the field of view is through detector bias. When the detector bias increases, the quality of the reconstructed image deteriorates.Patent publication number CN102004111B proposes a tilted multi-cone-beam linear trajectory CT imaging method, in which multiple cone beams are installed at different positions at an angle. This requires multiple sets of radiation sources and detectors. The object being inspected moves linearly through all the cone beams, and the detectors collect radiation passing through the object from different directions. This scanning method is costly and cannot obtain complete projection data for axially long objects. It is also unable to detect, accurately reconstruct, or expand the field of view of very long axial objects. Patent publication number CN111982939A proposes a movable multi-segment linear light source CT imaging system and method, which includes multiple movable scanning segments, each of which includes a linear distributed light source array and a linear detector array. The multiple scanning segments cooperate and are parallel to each other to achieve complete data acquisition and expanded field of view imaging. Summary of the Invention

[0005] To meet the demand for full-area high-resolution CT imaging of objects with large lateral dimensions and long axial lengths, the present invention aims to provide a variable-pitch spiral scanning CT system and image reconstruction method with continuous tilted linear scanning. First, a method for a novel non-standard spiral scanning trajectory is provided. Then, a mechanical device for implementing the scanning trajectory is provided. Finally, an image reconstruction method for the scanning trajectory is provided. This achieves the goals of both expanded lateral field of view imaging and high-resolution imaging of axially long objects, while simultaneously inheriting the advantages of previous linear scanning CT systems, such as simple structure, ease of implementation, and expanded imaging field of view.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A scanning method and an image reconstruction method for a continuous tilted linear scanning variable pitch spiral CT, characterized in that the scanning of the continuous tilted linear scanning spiral CT is a variable pitch scanning and a corresponding image reconstruction method for such scanning.

[0008] A scanning method for continuous tilt linear scanning variable pitch spiral CT, with the basic motion form of the mechanical motion system being:

[0009] S1: Set the number of spiral scans to N r The ray source moves along the inclined straight line trajectory with an axial inclination angle γ(t), and the flat panel detector does not move, completing the nth segment of the inclined straight line scan in the tth circle scan (where t is the spiral scan circle number, satisfying t=1,2,...,N r , n is the number of the inclined linear scanning segment, satisfying n=1,2,...,N r T, the relationship between the two is: t = ceil (n / T));

[0010] S2: The rotating stage controls the object to be measured to rotate at an angle interval Δθ and vertically rise or fall (in either direction) for a distance h(t);

[0011] S3: Continue the (n+1)th segment of the tilted linear scan in the tth circle scan of step S1 (where the t value is determined by the following formula: t=ceil(n / T)), and repeat this process until the continuous tilted linear scan covers the object to be measured, forming a variable pitch spiral scanning trajectory.

[0012] A scanning method for a continuous tilted linear scanning variable pitch spiral CT, wherein the tilted linear scanning trajectory of the ray source is expressed as:

[0013]

[0014] Among them, λ i is the coordinate of the ray source focus on the inclined straight line trajectory, that is, λ i ∈[-λ m ,λ m ], 1≤i≤N, N represents the number of sampling points in each segment of inclined linear scanning; θ n is the angle between the nth segment of the ray source's tilted linear translation trajectory and the positive direction of the x-axis of the fixed coordinate system, θ n =(n-1)·Δθ,n=1,2,...,N r T, where N r is the number of spiral scanning circles, T is the number of inclined straight line track segments required for scanning one circle, and Δθ is determined by the following relationship: Δθ=2arctan(u m / dod), and T can be determined by the following relationship: T = ceil(2π / Δθ), where ceil(·) means rounding up; is the rotation matrix of the nth segment of the ray source tilt linear translation trajectory; z(λ i ,n) is the coordinate of the ray source (1) on the z-axis of the fixed coordinate system, and its calculation formula is:

[0015]

[0016] Where H' is the z-direction height of the first (t-1) circle trajectory during the t-th spiral scanning process, satisfying h(t) is the height in z direction of the inclined straight line trajectory during the t-th spiral scanning process (excluding the redundant part where adjacent inclined straight line segments intersect), which satisfies h(t) = p(t) / T; p(t) is the variable pitch sequence, which is determined by the number of spiral scanning circles N. r Set different pitch values. 1≤t≤N r ;h p is the total height of the spiral scanning trajectory in the z direction,

[0017]

[0018] Where H(t) is the z-direction height of the inclined straight line trajectory during the t-th spiral scanning process (including the redundant part where adjacent inclined straight line segments intersect), satisfying Where L is the polygonal side length (intersection line) of the variable pitch spiral trajectory of the continuous tilt linear scan from a top-down perspective, and its calculation formula is: The calculation formula of the axial inclination angle γ(t) of a certain inclined straight line trajectory is:

[0019]

[0020] The radius R1' of the laterally expanded imaging field of view that can collect complete data in the present invention is:

[0021]

[0022] Here, min(·) represents a function for finding the minimum value.

[0023] An image reconstruction method for a continuously tilted linear scanning variable pitch spiral CT comprises the following steps:

[0024] S1': The three-dimensional DBP image of the nth segment of tilted linear scan is obtained by the differential back-projection (DBP) operator. The calculation formula is as follows:

[0025]

[0026] Where R and dod are the distances from the central axis of the rotating stage to the focus of the radiation source and the center of the flat-panel detector respectively; m is the half length of the tilted straight line scanning trajectory of the ray source; u is the row coordinate of the flat panel detector, u m is the half length of the flat panel detector in the row direction; v is the longitudinal coordinate of the flat panel detector; is the redundancy weighting function of the nth segment of tilted linear scan cone-beam projection data; is the cone beam projection data of the ray source at the position λ of the inclined straight line trajectory during the nth segment of the inclined straight line scan; z(λ) is the longitudinal coordinate of the ray source (1) on the flat panel detector during the nth segment of the inclined straight line scan, and the calculation formula is: z(λ)=λsin(γ(t))-H(t) / 2; the ray of the ray source passes through the point to be reconstructed The row coordinate of the flat panel detector is u * , and its calculation formula is:

[0027]

[0028] S2': Perform one-dimensional finite Hilbert inverse transform on the n-th segment DBP image layer by layer along the axial direction to obtain the three-dimensional reconstructed image of the n-th segment tilted linear scan

[0029]

[0030] Where y1 represents the one-dimensional Hilbert transform direction, y1∈]L y +ε y ,U y -ε y ], where [L y ,U y ] represents the finite interval of Hilbert transform, ε y is a small positive number; Indicates the reconstruction of the nth segment of the limited angle image The unknown constant to be calculated is obtained by finding the known Some position points, the mean value after finite Hilbert transformation is used as

[0031] S3': N is converted into r The reconstruction results of the T-segment tilted linear scan are accumulated to obtain a complete reconstructed image

[0032] The beneficial effects of the present invention are:

[0033] (1) Compared with relative linear scanning CT, planar multi-segment linear scanning CT, and standard spiral scanning CT, the scanning method of continuous tilted linear scanning variable pitch spiral CT provided by the present invention can not only expand the lateral imaging field of view to image objects with large cross-sections, but also can image long objects;

[0034] (2) Compared with standard spiral expanded field of view CT such as half-cover spiral scanning and double spiral scanning, the present invention has the advantages of simple scanning mode, easy high-precision control and implementation due to its partial characteristics of linear scanning. It can achieve adjustable expanded lateral field of view by adjusting the length of the tilted linear translation trajectory of the ray source; it can achieve adjustable axial imaging height by adjusting parameters such as the axial inclination angle of the tilted linear translation trajectory of the ray source, the length of the tilted linear translation trajectory of the ray source, and the number of scanning circles; it can conveniently control the number of sampling points of the tilted linear translation of the ray source beam to increase the number of projections, thereby improving imaging quality;

[0035] (3) The scanning method of the continuous tilted linear scanning variable pitch spiral CT provided by the present invention can meet the data completeness conditions for three-dimensional accurate CT image reconstruction and provide support for accurate reconstruction. Its image reconstruction method can solve the projection truncation problem of this scanning method and reconstruct a high-resolution three-dimensional image without truncation artifacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0037] Figure 1 The scanning method of the variable pitch spiral CT for continuous tilt linear scanning of the present invention is demonstrated;

[0038] Figure 2 A system composition diagram for implementing the method of the present invention is shown;

[0039] Figure 3 (a)-(c) respectively show schematic diagrams of a segment of tilted linear scanning under different viewing angles of the method of the present invention;

[0040] Figure 4 (a)-(c) respectively show the mechanical structure diagrams of different perspectives for implementing the method of the present invention;

[0041] Figure 5 (a)-(d) show schematic diagrams of the variable pitch spiral scanning trajectory of the present invention with three circles and six inclined straight lines in each circle under specific geometric parameters;

[0042] Figure 6 (a)-(d) show schematic diagrams of the variable pitch spiral scanning trajectory of the present invention with three circles and 10 inclined straight lines per circle under specific geometric parameters;

[0043] Figure 7 The image reconstruction results of the image reconstruction method of the present invention in numerical experiments are demonstrated. DETAILED DESCRIPTION

[0044] The embodiments of the present invention are described in detail below with reference to the accompanying drawings and examples.

[0045] A scanning method of continuous tilt linear scanning variable pitch spiral CT, such as Figure 1 The steps are shown as follows:

[0046] S1: Set the number of spiral scans to N r The ray source moves along the inclined straight line trajectory with an axial inclination angle γ(t), and the flat panel detector does not move, completing the nth segment of the inclined straight line scan in the tth circle scan (where t is the spiral scan circle number, satisfying t=1,2,...,N r , n is the number of the inclined linear scanning segment, satisfying n=1,2,...,N r T, the relationship between the two is: t = ceil (n / T));

[0047] S2: The rotating stage controls the object to be measured to rotate at an angle interval Δθ and vertically rise or fall (in either direction) for a distance h(t);

[0048] S3: Continue the (n+1)th segment of the tilted linear scan in the tth circle scan of step S1 (where the t value is determined by the following formula: t=ceil(n / T)), and repeat this process until the continuous tilted linear scan covers the object to be measured, forming a variable pitch spiral scanning trajectory.

[0049] The system components to implement this method are as follows Figure 2 As shown, it includes: a ray source, a flat-panel detector, a data acquisition system, a computing platform, a control system, and a mechanical motion system. Preferably, in the system implementing the method of the present invention, the control system 6 controls the tube voltage, tube current, and exposure time parameters of the ray source 1; controls the mechanical motion system 7 to achieve multi-axis precision motion; sends control instructions and data uplink and downlink to the data acquisition system 4, and the data acquisition system 4 performs high-speed acquisition, encoding, and transmission of the projection image of the flat-panel detector 3; the data acquisition system 4 transmits the acquired projection image in a specific format and stores it to the hard disk or memory of the computing platform 5, so that the image reconstruction method deployed in the computing platform 5 can be used to reconstruct the image.

[0050] Figure 3 (a)-(c) respectively show a schematic diagram of a tilted straight line scan under different viewing angles of the method of the present invention, and combined with Figure 1 The variable pitch spiral scanning trajectory in the figure, the inclined straight line scanning trajectory of the ray source 1 is expressed as:

[0051]

[0052] Where R and dod are the distances from the central axis of the rotating stage to the focus of the radiation source and the center of the flat-panel detector respectively; m is the half length of the tilted straight line scanning trajectory of the ray source 1; u is the row coordinate of the flat panel detector 3, u m is the half length of the flat panel detector in the row direction; v is the longitudinal coordinate of the flat panel detector 3; is the redundancy weighting function of the nth segment of tilted linear scan cone-beam projection data; is the cone beam projection data of the ray source at the position λ of the inclined straight line trajectory during the nth segment of the inclined straight line scan; z(λ) is the longitudinal coordinate of the ray source 1 on the flat panel detector 3 during the nth segment of the inclined straight line scan, and the calculation formula is: z(λ)=λsin(γ(t))-H(t) / 2; the ray of ray source 1 passes through the point to be reconstructed The row coordinate of the flat panel detector (3) is u * , and its calculation formula is:

[0053]

[0054] Where H' is the z-direction height of the first (t-1) circle trajectory during the t-th spiral scanning process, satisfying h(t) is the height in z direction of the inclined straight line trajectory during the t-th spiral scanning process (excluding the redundant part where adjacent inclined straight line segments intersect), which satisfies h(t) = p(t) / T; p(t) is the variable pitch sequence, which is determined by the number of spiral scanning circles N. r Set different pitch values. 1≤t≤N r ;h p is the total height of the spiral scanning trajectory in the z direction,

[0055]

[0056] Where H(t) is the z-direction height of the inclined straight line trajectory during the t-th spiral scanning process (including the redundant part where adjacent inclined straight line segments intersect), satisfying Where L is the polygonal side length (intersection line) of the variable pitch spiral trajectory of the continuous tilt linear scan from a top-down perspective, and its calculation formula is: The calculation formula of the axial inclination angle γ(t) of a certain inclined straight line trajectory is:

[0057]

[0058] The radius R1' of the laterally expanded imaging field of view that can collect complete data in the present invention is:

[0059]

[0060] Here, min(·) represents a function for finding the minimum value.

[0061] Figure 4 (a)-(c) respectively show the mechanical structure diagrams of the method of the present invention from different perspectives. The mechanical motion system 7 includes: a ray source linear motion module 7.1, a rotation and linear motion module 7.2 at the end face of the ray source column 8.1, a rotating table 7.4 and a linear motion module 7.3 at its bottom end, and a linear motion module 7.5 at the bottom end of the detector column 8.2. The mechanical motion system 7 is installed on the end face of the marble 8.3.

[0062] The radiation source linear motion module 7.1 is mounted on the rotation and linear motion module 7.2 on the end face of the radiation source column 8.1. The radiation source 1 is mounted on the slider of the linear motion module 7.1. The radiation source 1 can be precisely controlled by the radiation source linear motion module 7.1 to perform oblique translation motion a.1. At the same time, the radiation source 1 and the linear motion module 7.1 can be integrally controlled and precisely adjusted for elevation and axial inclination angle γ by the rotation and linear motion module 7.2.

[0063] In the spiral scanning process of the scanning method of the present invention, the rotation c.1 and lifting c.2 of the object 2 are completed by the rotating table 7.4, and the ray source 1 performs the oblique translation motion a.1 on the linear motion module 7.1. Figure 3 (a) Linear motion module 7.3 at the bottom of turntable 7.4 adjusts the turntable's horizontal position d.1 and advance / retract distance d.2. Detector column 8.2, fixed to the slider of linear motion module 7.5, precisely controls and adjusts the advance / retract distance e.1. Detector column 8.2 is equipped with linear motion module 7.6, which connects to the flat-panel detector to achieve its elevation / retraction f.1.

[0064] Furthermore, the ray source 1 is assembled on the linear motion module 7.1, and the linear motion module 7.1 is assembled on the rotation axis of the end face of the ray source column 8.1. The ray source 1 performs precise translational motion on the linear motion module 7.1 to realize translational scanning of the oblique trajectory. By adjusting the rotation angle of the linear motion module 7.1 on the ray source column 8.1, the axial inclination angle γ of the oblique trajectory can be adjusted. The axial inclination angle γ can be achieved by controlling the rotation and linear motion module 7.2.

[0065] An image reconstruction method for a continuously tilted linear scanning variable pitch spiral CT comprises the following steps:

[0066] S1': The three-dimensional DBP image of the nth segment of tilted linear scan is obtained by the differential back-projection (DBP) operator. The calculation formula is as follows:

[0067]

[0068] Where R and dod are the distances from the central axis of the rotating stage to the focus of the radiation source and the center of the flat-panel detector respectively; m is the half length of the inclined straight line scanning trajectory of the ray source (1); u is the row coordinate of the flat panel detector (3), u m is the half length of the flat panel detector in the row direction; v is the longitudinal coordinate of the flat panel detector (3); is the redundancy weighting function of the nth segment of tilted linear scan cone-beam projection data; is the cone beam projection data of the ray source at the position λ of the inclined straight line trajectory during the nth segment of the inclined straight line scanning; z(λ) is the longitudinal coordinate of the ray source (1) on the flat panel detector (3) during the nth segment of the inclined straight line scanning, and the calculation formula is: z(λ)=λsin(γ(t))-H(t) / 2; the ray of the ray source (1) passes through the point to be reconstructed The row coordinate of the flat panel detector (3) is u * , and its calculation formula is:

[0069]

[0070] S2': Perform one-dimensional finite Hilbert inverse transform on the n-th segment DBP image layer by layer along the axial direction to obtain the three-dimensional reconstructed image of the n-th segment tilted linear scan

[0071]

[0072] Where y1 represents the one-dimensional Hilbert transform direction, y1∈[L y +ε y ,U y -ε y ], where [L y ,U y ] represents the finite interval of Hilbert transform, ε y is a small positive number; Indicates the reconstruction of the nth segment of the limited angle image The unknown constant to be calculated is obtained by finding the known Some position points, the mean value after finite Hilbert transformation is used as

[0073] S3': N is converted into r The reconstruction results of the T-segment tilted linear scan are accumulated to obtain a complete reconstructed image

[0074] Furthermore, a variable pitch spiral scanning trajectory of continuous tilted linear scanning of the present invention is simulated. The simulation scanning parameters are shown in Table 1. The final obtained variable pitch spiral scanning trajectory of 3 circles and 6 segments of tilted linear scanning is shown in the following diagrams: Figure 5 As shown in (a)-(d).

[0075] Table 1. Simulation scan parameters

[0076]

[0077] Furthermore, some scanning parameters in Table 1 were modified as follows: the distance from the focus of the ray source to the central axis of the rotating stage R = 15 mm, the distance from the central axis of the rotating stage to the center of the flat panel detector dod = 190 mm, the half-length λ of the tilted linear translation trajectory of the ray source m =10mm, according to the above calculation formula, the rotation angle interval of the measured object Δθ = 36°, the number of inclined linear trajectory segments required for scanning one circle T = 10, the axial inclination angle sequence of the inclined linear trajectory γ(t) = [2.4706, 1.2359, 3.7031]°, the radius of the laterally expanded imaging field of view R1' = 4.2177mm, and the finally obtained variable pitch spiral scanning trajectory with 3 circles and 10 inclined linear scanning segments each, its various directional views are as follows Figure 6As shown in (a)-(d).

[0078] Furthermore, according to the simulation parameters shown in Table 1, the variable pitch helical scanning system of the present invention is used to perform numerical simulation scanning on the three-dimensional Shepp-Logan phantom to obtain cone beam projection data, and then the image reconstruction method of the present invention is used to perform image reconstruction.

[0079] Under the parameter conditions in Table 1, the object to be measured is a three-dimensional Shepp-Logan phantom with a cross-sectional size of 6.6162 mm and an axial height of 9 mm. Figure 7 The reconstruction results of the image reconstruction method of the variable-pitch spiral scanning CT system with continuous tilted linear scanning of the present invention are shown. It can be seen that the present invention is capable of scanning and imaging objects with a long axial length and a large cross-section.

[0080] The present invention is not limited to the above-mentioned specific embodiments, which are merely illustrative and not restrictive. A person skilled in the art, inspired by the present invention, may make many other forms without departing from the spirit of the present invention and the scope of protection of the claims, all of which fall within the protection of the present invention.

Claims

1. A scanning and image reconstruction method for continuous tilt linear scanning variable pitch spiral CT, characterized in that: The scanning method of the variable pitch spiral CT is specifically as follows: S1: Set the number of spiral scans to N r , the ray source (1) moves along the inclined straight line trajectory with an axial inclination angle γ(t), and the flat panel detector (3) remains stationary, completing the nth segment of the inclined straight line scan in the tth circle scan, where t is the spiral scan circle number, satisfying t=1,2,...,N r , n is the number of the inclined linear scanning segment, satisfying n=1,2,...,N r T, where T is the number of inclined straight line segments required to scan one circle; the relationship between t and n is: t = ceil (n / T); S2: The rotating stage controls the object to be measured (2) to rotate by an angle interval Δθ and vertically rise or fall by a certain distance; S3: Continue the n+1th segment of the tilted linear scan in the tth round of scanning in step S1, where the value of t is determined by the following formula: t=ceil(n / T), and repeat this process until the continuous tilted linear scan covers the object to be measured (2), forming a variable pitch spiral scanning trajectory; The image reconstruction method comprises the following steps: S1': The three-dimensional DBP image of the nth segment of tilted linear scanning is obtained by the differential back projection (DBP) operator The calculation formula is as follows: Where R and dod are the distances from the central axis of the rotating stage to the focus of the radiation source and the center of the flat-panel detector respectively; m is the half length of the inclined straight line scanning trajectory of the ray source (1); u is the row coordinate of the flat panel detector (3); v is the longitudinal coordinate of the flat panel detector (3); is the redundancy weighting function of the nth segment of tilted linear scan cone-beam projection data; is the cone beam projection data of the ray source at the position λ of the inclined straight line trajectory during the n-th inclined straight line scanning; z(λ) is the longitudinal coordinate of the ray source (1) on the flat panel detector (3) during the n-th inclined straight line scanning, and the calculation formula is: z(λ)=λsin(γ(t))-H(t) / 2, where H(t) is the z-direction height of the inclined straight line trajectory containing the redundant part of the intersection of adjacent inclined straight line segments during the t-th spiral scanning process, satisfying Ray source (1) The ray passes through the point to be reconstructed The row coordinate of the flat panel detector (3) is u * , and its calculation formula is: S2': Perform one-dimensional finite Hilbert inverse transform on the n-th segment DBP image layer by layer along the axial direction to obtain the three-dimensional reconstructed image of the n-th segment tilted linear scan : Where y1 represents the one-dimensional Hilbert transform direction, y1∈[L y +ε y ,U y -ε y ], where [L y ,U y ] represents the finite interval of Hilbert transform, ε y is a small positive number; Indicates the reconstruction of the nth segment of the limited angle image The unknown constant to be calculated is obtained by finding the known Some position points, the mean value after finite Hilbert transformation is used as S3': N is converted into r The reconstruction results of the T-segment tilted linear scan are accumulated to obtain a complete reconstructed image 2. The scanning and image reconstruction method of a continuous tilt linear scanning variable pitch spiral CT according to claim 1, characterized in that: The expression of the inclined straight line scanning trajectory of the ray source (1) is: Among them, λ i is the coordinate of the focus of the ray source (1) on the inclined straight line trajectory, that is, λ i ∈[-λ m ,λ m ], 1≤i≤N, N represents the number of sampling points in each segment of inclined linear scanning; θ n is the angle between the nth segment of the ray source's tilted linear translation trajectory and the positive direction of the x-axis of the fixed coordinate system, θ n =(n-1)·Δθ,n=1,2,...,N r T,,Δθ is determined by the following relationship:Δθ=2arctan(u m / dod), where u m is the half length of the flat panel detector in the row direction; T can be determined by the following relationship: T = ceil(2π / Δθ), where ceil(·) represents rounding up; is the rotation matrix of the nth segment of the ray source tilt linear translation trajectory; z(λ i ,n) is the coordinate of the ray source (1) on the z-axis of the fixed coordinate system, and its calculation formula is: Where H' is the z-direction height of the first t-1 turns of the spiral scanning process, satisfying is the z-direction height of the inclined straight line trajectory excluding the redundant part of the intersection of adjacent inclined straight line segments during the t-th spiral scanning process, which satisfies h(t)=p(t) / T; p(t) is the variable pitch sequence, which is determined by the number of spiral scanning circles N r Set different pitch values. h p is the total height of the spiral scanning trajectory in the z direction, Where L is the length of the polygonal side of the variable pitch spiral trajectory of the continuous tilt linear scan from a top view, and its calculation formula is: The calculation formula of the axial inclination angle γ(t) of a certain inclined straight line trajectory is: The radius of the laterally expanded imaging field of view R1' that can collect complete data is: Here, min(·) represents a function for finding the minimum value.

3. The scanning and image reconstruction method of a continuous tilt linear scanning variable pitch spiral CT according to claim 1, characterized in that: The oblique trajectory translation scanning of the ray source (1) is realized in the following manner: the ray source (1) is assembled on a linear motion module, the linear motion module is assembled on a rotation axis of the end face of a ray source column, the ray source (1) performs precise translation motion on the linear motion module to realize the oblique trajectory translation scanning, and adjusting the rotation angle of the linear motion module on the ray source column can adjust the axial inclination angle γ(t) of the oblique trajectory.

4. A device based on the scanning and image reconstruction method of continuous tilt linear scanning variable pitch spiral CT according to any one of claims 1 to 3, characterized in that: The invention comprises a ray source (1), a flat panel detector (3), a data acquisition system (4), a computing platform (5), a control system (6) and a mechanical motion system (7). The spiral scanning of the multi-segment oblique line combination is completed by the mechanical motion system (7).

Images