Method and device for determining the shape of the z-axis light intensity distribution of a ct tube focal spot

Abstract

The application discloses a CT tube focal point Z-direction light intensity distribution shape determination method and device, an axial scanning high-attenuation slotted flat plate model, and fixed angle moving bed scanning high-attenuation slotted flat plate model data. The axial scanning data is used to determine projection data, then, according to the fixed angle moving bed scanning data, the projection data is obtained by combining the axial scanning to construct a tube focal point Z-direction light intensity distribution model to determine simulated projection data. Based on the simulated projection data and actual scanning data, parameters of the constructed focal point Z-direction light intensity distribution model are solved. Finally, the focal point Z-direction light intensity distribution shape is calculated according to the model parameters. The CT tube focal point Z-direction light intensity distribution shape determination is realized.

Description

Technical Field

[0001] This invention relates to the field of CT system information processing technology, specifically to a method and apparatus for determining the Z-axis light intensity distribution shape of a CT tube focal spot. Background Technology

[0002] The intensity distribution of X-ray sources has a significant impact on image quality. Measuring the intensity distribution shape at the focal point of the X-ray tube is important for both evaluating tube performance and correcting for suboptimal intensity distribution shapes. While the defocusing effect in the Z-direction is often ignored, with the increase in CT scan rows and the advent of wide-body detectors, the effect of Z-direction defocusing has become more pronounced. Therefore, effectively measuring the intensity distribution shape in the Z-direction of the focal point is a problem that urgently needs to be addressed. Summary of the Invention

[0003] The main objective of this invention is to provide a method for determining the Z-axis light intensity distribution shape of a CT tube focal spot, in order to address the shortcomings of related technologies.

[0004] To achieve the above objectives, according to a first aspect of the present invention, a method for determining the Z-axis light intensity distribution shape of a CT tube focal spot is provided, comprising: placing a high-attenuation flat panel phantom with a slit in the CT scanning field of view; performing an axial scan and acquiring axial projection data; keeping the scanning table height constant; moving the flat panel phantom along the Z-axis to one side of the gantry; controlling the X-ray tube to rotate to a fixed angle perpendicular to the plane of the flat panel phantom; initiating a moving-table horizontal scan to move the slit of the flat panel phantom from one side of the gantry to the other side along the Z-axis; wherein, during the moving-table process, exposures are performed sequentially, each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit; acquiring a horizontal scan projection dataset corresponding to all exposure positions; the horizontal scan projection dataset containing measured projection data corresponding to multiple exposure positions; and based on the axial projection... The X-ray tube focuses on X-ray tubes. For each exposure position in the flat-scan projection dataset, projection simulation is performed to obtain simulated projection data corresponding to each exposure position. Using the simulated projection data of each exposure position as input and the measured projection data of the corresponding position as the target value, a one-to-one input-target data pair is constructed. An iterative optimization algorithm is used to solve for the globally optimal parameters of a preset focal point Z-axis light intensity distribution model. The obtained optimal parameters are then substituted into the focal point Z-axis light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal point in the Z-axis direction. The X-ray tube focal point Z-axis light intensity distribution model is constructed by superimposing a unit pulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit pulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

[0005] Optionally, based on the axial projection data, performing projection simulation for each exposure position in the planar projection dataset includes: determining the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit in the flat panel phantom, based on the axial projection data; and performing projection simulation for each exposure position in the planar projection dataset based on the known geometric parameters of the CT system, the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, to obtain the simulated projection data corresponding to each exposure position.

[0006] Optionally, based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel phantom, the vertical distance from the X-ray tube focal point to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position. This includes: discretizing the focal plane where the X-ray tube focal point is located along the Z-axis, with each discrete sampling point corresponding to a virtual focal point, and the light intensity of each virtual focal point being calculated by the focal point Z-axis light intensity distribution model; for each exposure position, determining the relative positional relationship between the flat panel phantom slit and each virtual focal point based on the coordinates of the slit plane of the flat panel phantom, the vertical distance, and the actual Z-axis width of the slit; and calculating the light intensity of the rays emitted by all virtual focal points after transmission through the slit to each channel of the detector based on the known geometric parameters of the CT system and the relative positional relationship, and summing the light intensity of the corresponding channels to obtain the simulated projection data for that exposure position.

[0007] Optionally, after acquiring the axial projection data, the method includes: performing correction on the axial projection data, and extracting the projection data of the middle row of the detector after correction; performing threshold segmentation on the middle row projection data, setting the pixel values ​​with intensity values ​​lower than a preset threshold to 0, to obtain preprocessed data, wherein the preset threshold is an intensity threshold for distinguishing the boundary between air and the flat panel phantom; performing a cleaning operation on the preprocessed data, and setting the pixel data corresponding to the area not completely covered by the flat panel phantom outside the slit area at each exposure angle to 0, to obtain cleaned projection data.

[0008] Optionally, the step of obtaining the planar position coordinates of the slit in the flat plate mold based on the axial projection data includes: selecting multiple exposure angles, calculating the angle between the focal point and the line connecting the two ends of the slit based on the rotation coordinates corresponding to the center of the X-ray tube at each exposure angle and the coverage width of the slit projection on the middle row channel; establishing a system of equations based on geometric relationships using the cosine theorem, and solving for the planar position coordinates of the left and right ends of the slit in the flat plate mold using the least squares method.

[0009] Optionally, determining the vertical distance from the X-ray tube focal point to the plane of the flat panel slit, and the actual Z-axis width of the flat panel slit includes: determining the distance from the fixed-angle light source to the plane of the flat panel slit based on the planar position coordinates of the flat panel slit and the position corresponding to the X-ray tube focal point at a fixed angle; extracting the single-angle projection frame corresponding to the X-ray tube rotating to the fixed angle from the axial projection data; extracting the projection data in the detector row direction, i.e., the Z-axis, from the single-angle projection frame, and measuring the projection coverage width of the flat panel slit in the Z-axis; and calculating the actual Z-axis width of the flat panel slit by combining the distance SDD from the X-ray tube focal point to the detector, the projection coverage width, and the vertical distance D in the CT system.

[0010] Optionally, the Z-axis light intensity distribution model at the X-ray tube focal point can be constructed by superimposing a unit impulse function with N Gaussian functions, including: Where A(z) is the intensity of the light at the focal point of the X-ray tube at position z; The intensity of the light source at the X-ray tube focal spot is represented by z; z is the offset position relative to the center of the X-ray tube focal spot. The position of the center of the spherical tank's focus is a known value; n is the Gaussian function index of the focus model; N is the total number of Gaussian functions. Let n be the magnitude of the Gaussian function. Let n be the standard deviation of the nth Gaussian function. It is the position of the center of symmetry of the nth Gaussian function.

[0011] According to a second aspect of the present invention, a system for determining the Z-axis light intensity distribution shape of a CT tube focal spot is provided, comprising: a projection data acquisition unit for placing a high-attenuation flat panel phantom with a slit in the CT scanning field of view, performing an axial scan and acquiring axial projection data; and a simulation projection data determination unit for maintaining a constant scanning bed height, moving the flat panel phantom along the Z-axis to one side of the gantry, controlling the X-ray tube to rotate to a fixed angle perpendicular to the plane of the flat panel phantom, and initiating a moving-bed planar scan to move the slit of the flat panel phantom from one side of the gantry to the other side along the Z-axis, wherein during the moving-bed process, exposures are performed sequentially, each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit, and a planar scan projection dataset corresponding to all exposure positions is acquired, the planar scan projection dataset containing measured projection data corresponding to multiple exposure positions; based on the axis The projection data is used to simulate projection at each exposure position in the flat-scan projection dataset to obtain simulated projection data corresponding to each exposure position. The light intensity distribution determination unit is used to construct a one-to-one input-target data pair by taking the simulated projection data of each exposure position as input and the measured projection data of the corresponding position as the target value. The global optimal parameters of the preset focal Z-direction light intensity distribution model are obtained by solving through an iterative optimization algorithm. The obtained optimal parameters are substituted into the focal Z-direction light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal Z direction. The X-ray tube focal Z-direction light intensity distribution model is constructed by superimposing a unit pulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit pulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

[0012] Optionally, based on the axial projection data, performing projection simulation for each exposure position in the planar projection dataset includes: determining the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit in the flat panel phantom, based on the axial projection data; and performing projection simulation for each exposure position in the planar projection dataset based on the known geometric parameters of the CT system, the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, to obtain the simulated projection data corresponding to each exposure position.

[0013] Optionally, based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel phantom, the vertical distance from the X-ray tube focal point to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position. This includes: discretizing the focal plane where the X-ray tube focal point is located along the Z-axis, with each discrete sampling point corresponding to a virtual focal point, and the light intensity of each virtual focal point being calculated by the focal point Z-axis light intensity distribution model; for each exposure position, determining the relative positional relationship between the flat panel phantom slit and each virtual focal point based on the coordinates of the slit plane of the flat panel phantom, the vertical distance, and the actual Z-axis width of the slit; and calculating the light intensity of the rays emitted by all virtual focal points after transmission through the slit to each channel of the detector based on the known geometric parameters of the CT system and the relative positional relationship, and summing the light intensity of the corresponding channels to obtain the simulated projection data for that exposure position.

[0014] This embodiment first scans an axially slit high-attenuation flat panel phantom. The X and Z widths of the slit and its distance from the light source are determined using projection data. Keeping the scanning bed height constant, the slit of the flat panel phantom is moved to one side of the gantry. The light source is adjusted to a direction perpendicular to the flat panel phantom for a fixed-angle planar scan. The Z-axis position of the slit is analyzed during each flat panel phantom exposure. Based on CT geometry, the dimensions of the flat panel phantom, its distance from the light source, and the constructed focal model, the ideal projection result of the flat panel phantom with the slit is calculated. The calculated ideal projection result and the measured result form a one-to-one input-target pair. This process continues until all data from the flat panel phantom slit scan from one side to the other in the Z direction are calculated, forming an ideal-target dataset pair. The parameters of the constructed defocus model with the Z-axis distribution are calculated using an iterative optimization method. Substituting these parameters into the defocus model yields the focal point's Z-axis light intensity distribution. This method effectively tests the shape of the focal point's Z-axis light intensity distribution, providing effective support for evaluating X-ray tube performance and correcting Z-axis defocus. Attached Figure Description

[0015] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1 This is a flowchart of the method for determining the Z-axis light intensity distribution shape of the CT tube focal spot according to an embodiment of the present invention; Figure 2 This is an application scenario diagram of the method for determining the Z-axis light intensity distribution shape of the CT tube focal spot according to an embodiment of the present invention. Detailed Implementation

[0017] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0018] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of the invention described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0020] According to embodiments of the present invention, a method for determining the Z-axis light intensity distribution shape at the focal point of a CT tube is provided, such as... Figure 1 As shown, steps 101 to 103 are included below: Step 101: Place the high-attenuation flat panel phantom with the slit in the CT scan field of view, perform an axial scan and acquire axial projection data; based on the axial projection data, determine the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit in the flat panel phantom.

[0021] In this step, the material for the flat phantom can be high-attenuation materials such as tungsten or molybdenum. (Reference) Figure 2 Axial scanning is the basic scanning mode in the field of CT, also known as tomographic scanning or sequential scanning. During the scanning process, the scanning bed remains stationary while the X-ray tube and detector rotate in a circle around the gantry and other centers. During the rotation, X-ray projections from different angles are continuously acquired at fixed angular intervals. After completing a single cycle of acquisition, the scanning bed moves a certain distance along the Z-axis (gantry axis) to acquire the next tomographic image.

[0022] Axial projection data refers to the raw X-ray intensity data collected by each channel unit of the detector at various exposure angles during axial scanning.

[0023] For example, a slit-covered flat phantom made of a high-attenuation material is scanned using preset conditions to obtain the intensity data of the scanned phantom. During scanning, the slit of the flat mold should be moved as close to the center in the Z direction as possible, and the scanning bed should be set to the lowest position.

[0024] By using multi-view projection data and trigonometric relationships, the spatial location, physical dimensions, and perpendicular distance to the focal point of the X-ray tube of the flat plate mold opening are determined, providing a precise geometric reference for subsequent fixed-angle scanning calculations.

[0025] Step 102: Keeping the scanning bed height constant, move the flat panel phantom along the Z-axis to one side of the frame. Control the X-ray tube to rotate to a fixed angle perpendicular to the plane of the flat panel phantom. Start the moving bed for horizontal scanning, causing the flat panel phantom slit to move from one side of the frame to the other along the Z-axis. During the moving bed process, exposures are performed sequentially, with each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit. Collect the horizontal scanning projection dataset corresponding to all exposure positions. The horizontal scanning projection dataset contains measured projection data corresponding to multiple exposure positions. Based on the axial projection data, perform projection simulation on each exposure position in the horizontal scanning projection dataset to obtain simulated projection data corresponding to each exposure position.

[0026] In this step, the exposure position refers to the physical spatial position of the slit in the flat panel phantom along the Z-axis during the fixed-angle moving bed scanning process. Each X-ray exposure corresponds to a physical position of the phantom opening in the Z-axis, i.e., the Z-axis position of the phantom corresponding to one exposure. Each exposure position corresponds to one frame of measured projection data.

[0027] With certain scanning conditions set and the scanning bed height kept constant, the slit portion of the flatbed mold is moved to one end of the scanning frame. Then, at a fixed angle where the light source is perpendicular to the flatbed mold, a low-speed horizontal scan is performed until the slit portion of the flatbed mold is re-scanned. The scanning frame is then moved from one end to the other to obtain the projected data. The projected data was subjected to bad channel interpolation and air correction, as well as correction for light intensity differences in each exposure. The corrected data was then cleaned to remove data that was not fully included in the scanning field of view due to the slit in the flat panel mold, resulting in the measured projected data. .

[0028] Furthermore, a model for the Z-axis light intensity distribution at the X-ray tube focal point is pre-constructed using the superposition of a pulse function and a Gaussian function as follows: A focal Z-axis intensity distribution model is constructed by using the unit impulse function to describe the main light source point light source and the sum of Gaussian functions with different parameters, where A(z) is the intensity of the focal light intensity at position z. The intensity of the point light source at the focal center is represented by z; z is the offset position relative to the focal center. The center of the focus is a known value; n is the Gaussian function index of the focus model; N is the total number of Gaussian functions. Let n be the magnitude of the Gaussian function. Let n be the standard deviation of the nth Gaussian function. It is the position of the center of symmetry of the nth Gaussian function.

[0029] By discretizing the constructed focal Z-axis intensity distribution model along the focal plane, the virtual focal intensity at each location can be obtained. The processed data can then be analyzed individually. For each exposure, the edge of the slit flat panel mold is extracted from the projection data to determine the positional relationship between the slits and the focal point. Then, the sum of the received focal light intensities in the channels covered by each row of slits is calculated. This yields the projection simulation result of the current slit flat panel mold position under the focal point intensity distribution model. This process is repeated for all actual measured exposures, and the simulated projection data is denoted as... .

[0030] Step 103: Using the simulated projection data of each exposure position as input and the measured projection data of the corresponding position as target value, construct a one-to-one input-target data pair. Solve the global optimal parameters of the preset focal Z-direction light intensity distribution model through an iterative optimization algorithm. Substitute the obtained optimal parameters into the focal Z-direction light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal Z direction. The X-ray tube focal Z-direction light intensity distribution model is constructed by superimposing a unit pulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit pulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

[0031] In this step, simulated projection data of the slotted flat mold is used. For input, the actual data collected by the slit mold. To achieve this, the mean parameters of the focal Z-direction light intensity distribution model are iteratively calculated using the simulated annealing optimization method. , , , The optimal value. The calculated parameters... , , , Substituting the value into the model formula in S10, the Z-direction light intensity distribution at the X-ray tube focal point can be obtained.

[0032] As an optional implementation of this embodiment, the projection simulation of each exposure position in the planar projection dataset based on the axial projection data includes: determining the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit in the flat panel phantom based on the axial projection data; and performing projection simulation on each exposure position in the planar projection dataset based on the known geometric parameters of the CT system, the planar position coordinates of the slit in the flat panel phantom, the vertical distance from the X-ray tube focus to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, to obtain the simulated projection data corresponding to each exposure position.

[0033] In this optional implementation, the axial projection data undergoes a first preprocessing step. This first preprocessing mainly involves interpolation and atmospheric correction of the bad channel data, as well as correction for exposure differences at each angle. The data from the middle row is then extracted from the preprocessed data. The middle row here refers to the position of the detector row corresponding to the optical plane formed by the center of the focal spot perpendicular to the Z direction, or the position of the row corresponding to the center of the focal spot.

[0034] Data of the middle row after preprocessing of the projection data of the flat mold with slots Further processing is performed. If the value is less than a certain threshold, it is set to 0. The data after this further preprocessing is called the second preprocessing, and the preprocessed data is denoted as: That is, the formula represents: Where: c is the pixel unit number of the middle row detector, v is the vth exposure angle, and T is the threshold, i.e. the threshold for distinguishing the boundary between air and the flat panel mode.

[0035] Furthermore, the obtained data is further cleaned. To reduce the impact of the slit frame on the projection, scan data obtained near the parallel angle between the light source and the flat panel are removed. Data from areas outside the slit region of the flat panel model that are not fully exposed at each exposure angle are set to 0 (the model is usually small and cannot cover the entire detection unit; setting the uncovered data to 0 is to accelerate calculation). The cleaned data is denoted as... .

[0036] Furthermore, when determining the planar position coordinates of the slit in the flat mold, the data is based on the preprocessed and cleaned projection data of the slit flat mold. Construct a model and calculate the location of the slit in the flat panel mold. First, assume that the coordinates of the left and right ends of the slit in the CT projection coordinates are respectively... and Then, the coordinates of the X-ray tube focal center at various exposure angles. The specific coordinate values ​​can be calculated based on the rotation angle; and at each exposure angle, the angle θ(v) between the two vectors formed by the focal position and the left and right ends of the slit in the flat mold can be calculated based on... The coverage area of ​​the slit projection of the flat mold on the middle row channel is calculated.

[0037] The equations for exposing two vectors and their included angle at various angles can be constructed using the law of cosines as follows: The coordinates of the flat panel module's position are constructed from data obtained from multiple selected exposure angles. and A series of equations were used to calculate the position coordinates of the left and right ends of the slit in the flat mold using the least squares method. and .

[0038] When determining the vertical distance from the X-ray tube focal point to the slit plane of the flat mold, the position of the light source during fixed-angle scanning is: Based on the position of the light source and the coordinates of the left and right sides of the slit in the flat plate and Calculate the distance from a fixed-angle light source to the plane of the slit in the flat plate. .

[0039] When determining the actual Z-direction width of the slit in the flat mold, the Z-direction width of the slit is calculated based on the distance from the light source to the slit plane. The axial scan projection data corresponding to the fixed-angle scan is found from the axial scan data. The Z-direction projection width of the slit in the flat mold is Wp, calculated from the direction of the projection data row. Where W is the Z-axis width of the slit. Let SDD be the Z-axis projection width of the detector at a fixed angle, and let SDD be the distance from the focal point to the detector.

[0040] Furthermore, when determining the simulated projection data, the position of the slit in the Z-axis of the flatbed phantom is analyzed during each exposure of the flatbed phantom. Based on the CT geometry, the size of the flatbed phantom, its position from the light source, and the constructed focal plane model, the projection result of the ideal flatbed phantom is calculated. That is, the constructed focal plane Z-axis intensity distribution model is discretized and sampled in the Z-axis of the focal plane to obtain the virtual focal light intensity at each position. The processed data is then analyzed one by one. For each exposure, the edge of the slit flat panel mold is extracted from the projection data to determine the positional relationship between the slits and the focal point. Then, the sum of the received focal light intensities in the channels covered by each row of slits is calculated. This yields the projection simulation result of the current slit flat panel mold position under the focal point intensity distribution model. This process is repeated for all actual measured exposures, resulting in the simulated projection data.

[0041] For example, the focal plane where the X-ray tube focal point is located is discretized along the Z-direction to obtain multiple discrete sampling points; the light intensity corresponding to each discrete sampling point is calculated by the focal point Z-direction light intensity distribution model and used as the virtual focal point light intensity; the Z-direction offset of each discrete sampling point relative to the X-ray tube focal point center is a known quantity; for each exposure position in the flat scan projection dataset, the Z-direction projection edge of the flat plate mold slit in the measured projection data corresponding to that exposure position is extracted; combined with the obtained actual Z-direction width of the flat plate mold slit and the vertical distance from the X-ray tube focal point to the flat plate mold slit plane, the Z-direction position of the flat plate mold slit relative to the X-ray tube focal point center at that exposure position is calculated; By combining the Z-axis offset of each virtual focus relative to the center of the X-ray tube focus, the Z-position of the slit in the flat panel phantom relative to the center of the X-ray tube focus, and the coordinates of the left and right planar positions of the slit in the flat panel phantom, the spatial relative geometric relationship between each virtual focus and the slit in the flat panel phantom is determined. Based on the known geometric parameters of the CT system, the light intensity of the X-rays emitted from all virtual focuses after being transmitted through the slit in the flat panel phantom is calculated and projected onto each channel of the detector. The light intensity received by all virtual focuses in the same channel is accumulated to obtain the simulated projection data corresponding to the exposure position.

[0042] For example, the virtual focus position is determined by the known position of the X-ray tube focus center and the Z-axis offset of discrete sampling; the slit space boundary is determined by the planar coordinates of the left and right ends of the slit, the actual Z-axis width of the slit, and the Z-axis position of the slit at the current exposure position; the detector pixel position is determined by the known geometric parameters of the CT system. For each detector channel, it is determined whether the X-rays emitted from each virtual focus can pass through the slit of the flat panel phantom to reach that channel; the intensity of all transmissible virtual focus light is accumulated to obtain the simulated light intensity value of that channel; after traversing all channels, the simulated projection data corresponding to the exposure position is obtained.

[0043] This embodiment first performs an axial scan of a high-attenuation flat panel phantom with a slit. The X and Z widths of the slit and its distance from the light source are determined using projection data. Keeping the scanning bed height constant, the slit of the flat panel phantom is moved to one side of the gantry. The light source is adjusted to a direction perpendicular to the flat panel phantom for a fixed-angle planar scan. The Z-axis position of the phantom slit is analyzed during each flat panel phantom exposure. Based on CT geometry, the dimensions of the flat panel phantom, its distance from the light source, and the constructed focal point model, the ideal projection result of the flat panel phantom is calculated. The calculated ideal projection result and the measured result form a one-to-one input-target data pair. The parameters of the constructed Z-axis defocus model are calculated using an iterative optimization method. Substituting these parameters into the defocus model yields the focal point Z-axis light intensity distribution. This method effectively tests the shape of the focal point Z-axis light intensity distribution, providing a highly effective basis for evaluating X-ray tube performance and correcting Z-axis defocus.

[0044] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0045] According to an embodiment of the present invention, a system for determining the Z-axis light intensity distribution shape of a CT tube focal spot is also provided, including a projection data acquisition unit for placing a high-attenuation flat panel phantom with a slit in the CT scanning field of view, performing an axial scan, and acquiring axial projection data; and a simulation projection data determination unit for maintaining a constant scanning bed height, moving the flat panel phantom along the Z-axis to one side of the gantry, controlling the X-ray tube to rotate to a fixed angle perpendicular to the plane of the flat panel phantom, and initiating a moving-bed planar scan to move the slit of the flat panel phantom from one side of the gantry to the other side along the Z-axis, wherein, during the moving-bed process, exposures are performed sequentially, each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit, and a planar scan projection dataset corresponding to all exposure positions is acquired, the planar scan projection dataset containing measured projection data corresponding to multiple exposure positions; based on the axis The projection data is used to simulate projection at each exposure position in the flat-scan projection dataset to obtain simulated projection data corresponding to each exposure position. The light intensity distribution determination unit is used to construct a one-to-one input-target data pair by taking the simulated projection data of each exposure position as input and the measured projection data of the corresponding position as the target value. The global optimal parameters of the preset focal Z-direction light intensity distribution model are obtained by solving through an iterative optimization algorithm. The obtained optimal parameters are substituted into the focal Z-direction light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal Z direction. The X-ray tube focal Z-direction light intensity distribution model is constructed by superimposing a unit pulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit pulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

[0046] As an optional implementation of this embodiment, based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel phantom, the vertical distance from the X-ray tube focal point to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, combined with a preset focal point Z-axis light intensity distribution model, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position. This includes: discretizing the focal plane where the X-ray tube focal point is located along the Z-axis, with each discrete sampling point corresponding to a virtual focal point, and the light intensity of each virtual focal point being calculated by the focal point Z-axis light intensity distribution model; for each exposure position, based on the coordinates of the slit plane of the flat panel phantom, the vertical distance, and the actual Z-axis width of the slit, determining the relative positional relationship between the slit of the flat panel phantom and each virtual focal point; based on the known geometric parameters of the CT system and the relative positional relationship, calculating the light intensity of the rays emitted from all virtual focal points after transmission through the slit to each channel of the detector, and summing the light intensity of the corresponding channels to obtain the simulated projection data for that exposure position.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for determining the shape of the Z-axis light intensity distribution at the focal point of a CT tube, characterized in that, include: A high-attenuation flat phantom with slits is placed in the CT scan field of view, and axial scanning is performed to acquire axial projection data. While maintaining a constant scanning bed height, the flat panel phantom is moved along the Z-axis to one side of the frame. The X-ray tube is rotated to a fixed angle perpendicular to the plane of the flat panel phantom. The moving bed is then activated to move the flat panel phantom with its slit along the Z-axis from one side of the frame to the other. During the moving bed process, exposures are performed sequentially, with each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit. A flat panel projection dataset corresponding to all exposure positions is collected. This dataset contains measured projection data corresponding to multiple exposure positions. Based on the axial projection data, projection simulations are performed on each exposure position in the flat panel projection dataset to obtain simulated projection data corresponding to each exposure position. Simulated projection data at each exposure position is used as input, and measured projection data at the corresponding position is used as target value to construct a one-to-one input-target data pair. The global optimal parameters of the preset focal Z-direction light intensity distribution model are obtained by solving through an iterative optimization algorithm. The obtained optimal parameters are substituted into the focal Z-direction light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal Z direction. The X-ray tube focal Z-direction light intensity distribution model is constructed by superimposing a unit pulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit pulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

2. The method for determining the Z-axis light intensity distribution shape of a CT tube focal spot according to claim 1, characterized in that, Based on the axial projection data, projection simulation is performed on each exposure position in the planar projection dataset, including: Based on the axial projection data, the planar position coordinates of the slit in the flat plate mold, the vertical distance from the focus of the X-ray tube to the slit plane of the flat plate mold, and the actual Z-direction width of the slit in the flat plate mold are determined. Based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel, the vertical distance from the X-ray tube focus to the slit plane of the flat panel, and the actual Z-axis width of the slit, combined with the preset Z-axis light intensity distribution model of the focus, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position.

3. The method for determining the Z-axis light intensity distribution shape of the CT tube focal spot as described in claim 2, characterized in that, Based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel phantom, the vertical distance from the X-ray tube focal spot to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, and combined with a preset focal spot Z-axis light intensity distribution model, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position, including: The focal plane where the X-ray tube focal point is located is discretized along the Z-direction. Each discrete sampling point corresponds to a virtual focal point, and the light intensity of each virtual focal point is calculated by the focal point Z-direction light intensity distribution model. For each exposure position, the relative positional relationship between the slit of the flat mold and each virtual focus is determined based on the plane position coordinates of the slit of the flat mold, the vertical distance, and the actual Z-axis width of the slit; Based on the known geometric parameters and relative positional relationships of the CT system, the light intensity of the rays emitted from all virtual focal points after transmission through the slits and projected onto each channel of the detector is calculated. The light intensity of the corresponding channels is then summed to obtain the simulated projection data of the exposure position.

4. The method for determining the Z-axis light intensity distribution shape of a CT tube focal spot according to claim 1, characterized in that, After acquiring the axial projection data, the methods include: The axial projection data is corrected, and the projection data of the middle row of the detector is extracted after the correction is completed; The projection data of the middle row is segmented by thresholding, and the pixel values ​​with intensity values ​​lower than a preset threshold are set to 0 to obtain preprocessed data. The preset threshold is the intensity threshold that distinguishes the boundary between air and the flat plate model. A cleaning operation is performed on the preprocessed data, and the pixel data corresponding to the area not fully covered by the flat plate mold outside the slit area under each exposure angle is set to 0 to obtain the cleaned projection data.

5. The method for determining the Z-axis light intensity distribution shape of a CT tube focal spot according to claim 4, characterized in that, The process of obtaining the planar position coordinates of the slit in the flat mold body based on the axial projection data includes: Multiple exposure angles are selected, and the angle between the focal point and the line connecting the two ends of the slit is calculated based on the rotation coordinates corresponding to the center of the X-ray tube at each exposure angle, combined with the coverage width of the slit projection of the flat mold on the middle row channel. Based on geometric relationships, a system of equations was established using the cosine theorem, and the coordinates of the planar positions of the left and right ends of the slit in the flat plate mold were obtained by using the least squares method.

6. The method for determining the Z-axis light intensity distribution shape of a CT tube focal spot according to claim 5, characterized in that, The vertical distance from the focal point of the X-ray tube to the slit plane of the flat mold, and the actual Z-axis width of the slit in the flat mold include: The distance from the fixed-angle light source to the plane of the flat plate slit is determined based on the planar position coordinates of the slit in the flat plate and the position of the X-ray tube focal point at a fixed angle. Extract the single-angle projection frame corresponding to when the X-ray tube rotates to the fixed angle from the axial projection data; In the single-angle projection frame, the projection data of the detector row direction, i.e., the Z-direction, is extracted, and the projection coverage width of the slit of the flat plate mold in the Z-direction is measured. By combining the distance SDD from the X-ray tube focal point to the detector, the projection coverage width, and the vertical distance D in the CT system, the actual Z-axis width of the slit in the flat panel phantom is calculated.

7. The method for determining the Z-axis light intensity distribution shape of a CT tube focal spot according to claim 1, characterized in that, The Z-axis light intensity distribution model at the X-ray tube focal point is constructed by superimposing a unit impulse function with N Gaussian functions, including: Where A(z) is the intensity of the light at the focal point of the X-ray tube at position z; The intensity of the light source at the X-ray tube focal spot is represented by z; z is the offset position relative to the center of the X-ray tube focal spot. The position of the center of the spherical tank's focus is a known value; n is the Gaussian function index of the focus model; N is the total number of Gaussian functions. Let n be the magnitude of the Gaussian function. Let n be the standard deviation of the nth Gaussian function. It is the position of the center of symmetry of the nth Gaussian function.

8. A system for determining the shape of Z-axis light intensity distribution at the focal point of a CT tube, characterized in that, include: The projection data acquisition unit is used to place a high-attenuation flat panel phantom with slits in the CT scanning field of view, perform axial scanning, and acquire axial projection data. The simulated projection data determination unit is used to maintain a constant scanning bed height, move the flat panel phantom along the Z-axis to one side of the frame, control the X-ray tube to rotate to a fixed angle perpendicular to the plane of the flat panel phantom, and initiate a moving bed horizontal scan to move the flat panel phantom slit along the Z-axis from one side of the frame to the other. During the moving bed process, exposures are performed sequentially, with each exposure corresponding to a Z-axis exposure position of the flat panel phantom slit. A horizontal scan projection dataset corresponding to all exposure positions is collected. The horizontal scan projection dataset contains measured projection data corresponding to multiple exposure positions. Based on the axial projection data, projection simulation is performed on each exposure position in the horizontal scan projection dataset to obtain simulated projection data corresponding to each exposure position. The light intensity distribution determination unit is used to construct a one-to-one input-target data pair by taking the simulated projection data of each exposure position as input and the measured projection data of the corresponding position as the target value. The global optimal parameters of the preset focal Z-direction light intensity distribution model are obtained by solving through an iterative optimization algorithm. The obtained optimal parameters are substituted into the focal Z-direction light intensity distribution model to calculate the light intensity distribution shape of the X-ray tube focal Z direction. The X-ray tube focal Z-direction light intensity distribution model is constructed by superimposing a unit impulse function and N Gaussian functions. The unsolved parameters of the model include the amplitude of the unit impulse function, the amplitude of each Gaussian function, the standard deviation, and the position of the center of symmetry.

9. The CT tube focal spot Z-axis light intensity distribution shape determination system according to claim 8, characterized in that, Based on the axial projection data, projection simulation is performed on each exposure position in the planar projection dataset, including: Based on the axial projection data, the planar position coordinates of the slit in the flat plate mold, the vertical distance from the focus of the X-ray tube to the slit plane of the flat plate mold, and the actual Z-direction width of the slit in the flat plate mold are determined. Based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel, the vertical distance from the X-ray tube focus to the slit plane of the flat panel, and the actual Z-axis width of the slit, combined with the preset Z-axis light intensity distribution model of the focus, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position.

10. The CT tube focal spot Z-axis light intensity distribution shape determination system according to claim 9, characterized in that, Based on the known geometric parameters of the CT system, the coordinates of the slit plane of the flat panel phantom, the vertical distance from the X-ray tube focal spot to the slit plane of the flat panel phantom, and the actual Z-axis width of the slit, and combined with a preset focal spot Z-axis light intensity distribution model, projection simulation is performed on each exposure position in the flat scan projection dataset to obtain the simulated projection data corresponding to each exposure position, including: The focal plane where the X-ray tube focal point is located is discretized along the Z-direction. Each discrete sampling point corresponds to a virtual focal point, and the light intensity of each virtual focal point is calculated by the focal point Z-direction light intensity distribution model. For each exposure position, the relative positional relationship between the slit of the flat mold and each virtual focus is determined based on the plane position coordinates of the slit of the flat mold, the vertical distance, and the actual Z-axis width of the slit; Based on the known geometric parameters and relative positional relationships of the CT system, the light intensity of the rays emitted from all virtual focal points after transmission through the slits and projected onto each channel of the detector is calculated. The light intensity of the corresponding channels is then summed to obtain the simulated projection data of the exposure position.

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