A method and system for focus tracking based on moire fringe phase detection

Abstract

This invention discloses a focus tracking method and system based on moiré fringe phase detection. The method includes: setting a focus tracking grating unit in a front collimator, the focus tracking grating unit being located at the Z-direction end of the main collimator unit and in a ray channel outside the main imaging channel and corresponding to the edge detector region; the focus tracking grating unit is used to generate moiré fringes using gratings; the generated moiré fringes are used to calculate the focus drift, thereby achieving focus tracking; wherein, the focus drift is obtained through one of the following two methods: method one, performing FFT on the current image and the reference image to extract the phase difference; method two, measuring the displacement of the moiré fringes spatially. In this invention, focus tracking is performed using moiré fringes, and the moiré fringe phase depends only on the position of the focus center. When the X-ray tube ages or the control system deviation causes the focus size to increase, the moiré phase remains unchanged, and recalibration is not required.

Description

Technical Field

[0001] This invention relates to the field of focus tracking processing technology, and in particular to a focus tracking method and system based on moiré fringe phase detection. Background Technology

[0002] In CT imaging, the stability of the X-ray tube focal spot directly affects image quality. During operation, due to factors such as anodic thermal expansion, tube shell mechanical deformation, and bearing wear, the focal spot will slowly drift in the X-axis (channel direction) and Z-axis (slice thickness direction), with a typical drift of 0.05~0.5mm. This focal spot drift can cause alignment drift between the collimator and the detector, producing artifacts in the image and thus affecting diagnosis.

[0003] The existing focus tracking technologies and their shortcomings are as follows: 1. Complementary Sensor Comparison Method (e.g., GE CN111435120B): This method involves placing adjacent sensor pairs on the detector with an attenuation layer in between. The focus drift is estimated by comparing the complementary responses of the signals from the two detectors during focus drift. This method relies on a specific detector design, and its sensitivity is limited by the detector pixel size and the geometry of the attenuation layer.

[0004] 2. Collimator protrusion / groove + edge detection method (e.g., US11419572B2): A protrusion or groove is added to the end of the collimator to narrow the end slot. An edge detector unit is used to detect changes in the radiation intensity distribution to determine focus drift. This method requires a custom-designed collimator structure, and the protrusion causes local radiation loss.

[0005] 3. Tracking hole + reference channel method (e.g., CN104783822): An additional tracking hole is created on the collimator, and the Z-axis adjustment of the collimator is driven by the dose ratio of the upper / lower reference channel and the reference channel of the detector. This method increases the manufacturing complexity of the collimator, and the tracking hole occupies additional space.

[0006] 4. Dynamic baffle scanning method (e.g., CN201710115526): During the scanning process, the collimator baffle is moved as a whole, and the focal position is determined by optimizing the edge layer photon response ratio and image quality. This method requires a mechanical scanning process and cannot be run in real time during normal scanning.

[0007] 5. Temperature-predicted drift method (e.g., GE US6185275): This method predicts the focal thermal drift position based on the X-ray tube temperature parameters and corrects for it during reconstruction. This method relies on the accuracy of the temperature-drift model and cannot capture drift caused by non-thermal factors.

[0008] The methods described above all pertain to signal ratio or edge intensity detection in the spatial domain, essentially utilizing the occlusion effect of the collimator / grating edges for position estimation. These methods face a common physical limitation: the X-ray tube focal spot has a finite size (typically 0.5–1.2 mm). When this finite-sized focal spot is projected onto the detector through any feature structure on the collimator (protrusions, grooves, tracking aperture edges, etc.), penumbra blur will occur. The formula for calculating the penumbra width W_pen is: W_pen = f×(SDD - d) / d Where f is the focal spot size, SDD is the distance from the focal spot to the detector, and d is the distance from the focal spot to the feature structure. Using typical parameters f=0.6mm, d=200mm, and SDD=1000mm, the penumbra width reaches 2.40mm. This means that the edges of any feature on the collimator are blurred on the detector into a gradual transition zone of approximately 2.40mm, far larger than a 1mm pixel size, leading to the following defects: 1. Limited edge position detection accuracy: The edge position is widened by the penumbra, resulting in extremely poor signal-to-noise ratio for subpixel localization; 2. Systematic errors caused by changes in focal size: When the focal size changes due to the aging of the X-ray tube, the change in penumbra width causes systematic drift in edge detection, making it impossible to distinguish between changes in focal position and changes in focal size; 3. Small features are completely obscured: The projection of millimeter- or even sub-millimeter-level features on the collimator is completely blurred by the penumbra and cannot be distinguished on the detector. Summary of the Invention

[0009] Technical Objective: To address the shortcomings of existing technologies, this invention discloses a focus tracking method and system based on moiré fringe phase detection. This invention utilizes moiré fringes for focus tracking; the moiré fringe phase depends only on the focal center position and is completely decoupled from the focal size. Even when the focal size increases due to X-ray tube aging or control system deviations, the moiré phase remains unchanged, eliminating the need for recalibration.

[0010] Technical solution: To achieve the above technical objectives, the present invention adopts the following technical solution.

[0011] A focus tracking method based on moiré fringe phase detection, characterized by the following steps: S1. A focus tracking grating unit is set in the front collimator, wherein the focus tracking grating unit is set at the Z-direction end of the main collimator unit and is located in the ray channel outside the main imaging channel and corresponding to the edge detector area; the focus tracking grating unit is used to generate moiré fringes using gratings; S2. Calculate the focus drift using the generated moiré fringes to achieve focus tracking; the focus drift is obtained through one of the following two methods: Method 1, perform FFT on the current image and the reference image to extract the phase difference; Method 2, measure the displacement of the moiré fringes in space.

[0012] Preferably, the focus tracking grating unit includes a first grating and a second grating; the first grating has a periodic slit structure with a first grating period p1 and is located at a distance d1 from the focus; the second grating has a periodic slit structure with a second grating period p2 and is placed at a distance Δd from the first grating along the ray direction and is located at a distance d1+Δd from the focus.

[0013] Preferably, the focus drift is calculated using method one, and the phase difference is extracted by performing FFT on the current image and the reference image respectively, including the following steps: Acquire a frame of moiré fringe image as a reference baseline image; Real-time detection of moiré stripe images, used as the current image; Perform FFT on the current image and the reference image respectively, extract the phase angle, and calculate the phase angle deviation Δφ_FFT between the current image and the reference image. Use the phase angle deviation Δφ_FFT to calculate the focus drift.

[0014] Preferably, the formula for calculating the focus drift is: , Where K_FFT is the frequency domain phase-shift conversion coefficient, P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2=d1+Δd is the distance between the second grating and the focal point.

[0015] Preferably, method two is used to calculate the focus drift, and the formula for calculating the focus drift is: , Where γ_eff is a coefficient that is only related to the spatial geometry of the system, P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2=d1+Δd is the distance between the second grating and the focal point.

[0016] Preferably, the formula for calculating the displacement Δxmoire of the moiré fringes on the detector is: , Where Δz1 is the translation of the first grating pattern projected onto the detector, Δz2 is the translation of the second grating pattern projected onto the detector; P1det is the projection period of the first grating onto the detector, and P2det is the projection period of the second grating onto the detector. The formula for calculating the phase change Δφ caused by moiré fringes is: , Wherein, Pmoire is the period of the moiré fringe.

[0017] The present invention also discloses a focus tracking system based on moiré fringe phase detection, used to implement the focus tracking method based on moiré fringe phase detection described above, comprising: an X-ray source, a front collimator, a detector, a signal processing unit, and a correction execution unit; The focal point of the X-ray source is located on the anode target surface, and is used to emit X-rays; The precollimator is positioned between the X-ray source and the subject. The precollimator includes a main collimating unit and a focus-tracking grating unit. The focus-tracking grating unit is located at the Z-direction end of the main collimating unit and in the X-ray channel outside the main imaging channel, corresponding to the edge detector region. The main collimating unit limits the X-ray irradiation range, and the focus-tracking grating unit generates moiré fringes using gratings. The focus-tracking grating unit includes a first grating and a second grating. The first grating has a periodic slit structure with a first grating period p1 and is located at a distance d1 from the focus. The second grating has a periodic slit structure with a second grating period p2 and is placed at a distance Δd from the first grating along the X-ray direction, located at a distance d1+Δd from the focus. The detector is positioned directly opposite the X-ray source to receive X-rays passing through the focus tracking grating unit and to detect moiré fringe signals. The signal processing unit is connected to the detector and is used to calculate the focus drift using the generated moiré fringes, thereby achieving focus tracking. The focus drift is obtained through one of the following two methods: Method 1, performing FFT on the current image and the reference image to extract the phase difference; Method 2, measuring the displacement of the moiré fringes in space.

[0018] The correction execution unit is connected to the signal processing unit and the front collimator, and is used to perform correction measures on the front collimator according to the focal drift amount.

[0019] Preferably, the detector includes a main imaging region and an edge detection region; the main imaging region is located within the coverage area of ​​the scintillator in the detector, the edge detection region is located at both ends of the detector in the Z direction, and the available width of the detector in the Z direction is greater than the projection length of the grid in the focus tracking grid unit onto the detector.

[0020] Preferably, the formula for calculating the second grating period p2 is: , Where P2det is the projection period of the second grating on the detector, and SDD is the distance from the focal point to the detector.

[0021] Beneficial effects: 1. This invention introduces a focus tracking grating unit, which uses gratings to generate moiré fringes and then calculates the focus drift to achieve focus tracking; the period of the moiré fringes is much larger than the penumbra width generated by the finite size of the focus, so the detection contrast is not affected by the blur of the focus size; 2. This invention utilizes moiré fringes for focus tracking. The phase of the moiré fringes depends only on the center position of the focus and is completely decoupled from the focus size. When the focus size increases due to tube aging or control system deviation, the moiré phase remains unchanged, eliminating the need for recalibration. Moiré fringe phase extraction is performed on a single-frame basis, requiring no additional scanning process, and the focus position can be tracked in real time during normal scanning. 3. This invention introduces a focus tracking grid unit, which includes a first grid and a second grid. The grid is only set at the end of the collimator and outside the main imaging channel. It does not block the X-rays in the imaging area, does not increase the patient's radiation dose, does not affect the main imaging quality, and only requires the addition of two micro-grids at the end of the front collimator. There is no need to modify the detector design or the X-ray tube design, so the modification cost is low. 4. This invention can be used in combination with existing mechanical and electromagnetic correction methods; it can still work normally in the fly-focus mode, and has good compatibility. Attached Figure Description

[0022] Figure 1 This is a schematic diagram illustrating the working principle of an embodiment of the present invention; Figure 2 A schematic diagram illustrating the principle of moiré fringe formation; Figure 3 This is a system structure diagram of an embodiment of the present invention; Among them, 1. X-ray source; 2. front collimator; 31. first grid bar; 32. second grid bar; 4. detector; 5. signal processing unit; 6. correction execution unit. Detailed Implementation

[0023] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0024] Example: As attached Figure 1As shown, a focus tracking method based on moiré fringe phase detection in this embodiment includes the following steps: S1. A focus tracking grating unit is set in the front collimator, wherein the focus tracking grating unit is set at the Z-direction end of the main collimator unit and is located in the ray channel outside the main imaging channel and corresponding to the edge detector area; the focus tracking grating unit is used to generate moiré fringes using gratings; The focus tracking grating unit includes a first grating 31 and a second grating 32. The first grating 31 has a periodic slit structure with a first grating period p1, is made of a high-absorption X-ray material, and is located at a distance d1 from the focus. The second grating 32 has a periodic slit structure with a second grating period p2, is placed at a distance Δd from the first grating along the ray direction, is located at a distance d1+Δd from the focus, and is made of a high-absorption X-ray material. The high-absorption X-ray material is tungsten or a tungsten alloy. The materials of the first grating 31 and the second grating 32 do not need to be the same; different materials or structural combinations are usually used according to functional requirements.

[0025] S2. Calculate the focus drift using the generated moiré fringes to achieve focus tracking; the focus drift is obtained through one of the following two methods: Method 1, perform FFT on the current image and the reference image to extract the phase difference; Method 2, measure the displacement of the moiré fringes in space.

[0026] The focus shift is calculated using Method 1, and the phase difference is extracted by performing FFT on the current image and the reference image respectively, including the following steps: Acquire a single frame of moiré fringe image as a reference image; there are no special requirements for the reference image, and any image under the specified exposure conditions can be used as a reference image.

[0027] Real-time detection of moiré stripe images, used as the current image; Perform FFT on both the current image and the reference image to extract the phase angle and calculate the phase angle deviation Δφ_FFT between the current image and the reference image. Under normal exposure conditions, the projected images of the gratings on the detector are in the spatial domain. FFT is used to convert the spatial domain to the frequency domain to obtain the phase angle deviation Δφ_FFT. The focus drift is then calculated using the phase angle deviation Δφ_FFT. The formula for calculating the focus drift is: , Where K_FFT is the frequency domain phase-shift conversion coefficient, with units of period / mm (or rad / mm normalized by 2π). P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2 = d1 + Δd is the distance between the second grating and the focal point.

[0028] The focus drift is calculated using Method 2. The formula for calculating the focus drift is as follows: , Where γ_eff is a coefficient that is only related to the spatial geometry of the system, P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2=d1+Δd is the distance between the second grating and the focal point.

[0029] After calculating the focal drift, the displacement of the moiré fringes on the detector and the phase change caused by the moiré fringes are then calculated. The process is as follows: When the focal point drifts by δf in the Z direction, the angle of incidence of the ray emitted from the focal point relative to the two grating strips changes. Because the two grating strips are at different distances from the focal point, the focal point drift causes different translations in the projected patterns of the two grating strips. The formula for calculating the translation Δz1 of the first grating projection pattern on the detector is as follows: , Where δf is the focal drift in the Z direction, SDD is the distance from the focal point to the detector, and d1 is the distance between the first grating and the focal point.

[0030] Similarly, the formula for calculating the translation Δz2 of the second grating projection pattern on the detector is: , Where d1+Δd is the distance between the second grating and the focal point.

[0031] The differential translation of the two projected patterns causes a phase change in the moiré fringes. The formula for calculating the displacement Δxmoire of the moiré fringes on the detector is: , Where P1det is the projection period of the first grating on the detector, and P2det is the projection period of the second grating on the detector.

[0032] The formula for calculating the phase change Δφ caused by moiré fringes is: , Wherein, Pmoire is the period of the moiré fringe.

[0033] In this embodiment, d1=200mm, SDD=1000mm, p1=0.5mm, p2=0.52mm, and Δd=35mm are defined. The sensitivity of this method, i.e., the displacement of the moiré fringes on the detector and the calculation process for the phase change caused by the moiré fringes, is as follows: When δf = 0.1 mm, the calculated results of the translation Δz1 of the first grating projection pattern on the detector and the translation Δz2 of the second grating projection pattern on the detector are as follows: , The calculated displacement Δxmoire of the moiré fringes on the detector is as follows: , The positive and negative signs here represent the direction of movement. Taking the absolute value, the fringe displacement Δxmoire ≈ 0.249 mm. because: , The corresponding envelope phase, that is, the calculation result of the phase change of the moiré fringes, is as follows: Δφ = 2π × 0.249 / Pmoire ≈ 0.812 rad or 4.65° In this invention, high-frequency grating information that the detector cannot distinguish is shifted to the low-frequency domain through the beat frequency effect, and the phase change caused by focus drift is extracted on the low-frequency moiré fringes.

[0034] After obtaining the focus drift amount, you only need to adjust the position of the collimator opening accordingly to compensate.

[0035] This invention introduces a focus-tracking grating unit, which uses gratings to generate moiré fringes and then calculates the focus drift to achieve focus tracking. The period of the moiré fringes is much larger than the penumbra width caused by the finite size of the focus, so the detection contrast is not affected by the blurring of the focus size. In contrast, the edges of traditional grooving / feature schemes are severely widened by the penumbra, and the detection accuracy is limited by the penumbra width. Furthermore, this invention uses moiré fringes for focus tracking, and the phase of the moiré fringes depends only on the center position of the focus, completely decoupled from the focus size. When the focus size increases due to tube aging or control system deviations, the moiré phase remains unchanged, eliminating the need for recalibration; while edge detection in grooving, tracking holes, and other schemes experiences systematic drift with changes in focus size.

[0036] Furthermore, the moiré fringe phase extraction is calculated in a single frame, requiring no additional scanning process, and can track the focus position in real time during normal scanning.

[0037] As attached Figure 3 As shown, this embodiment also discloses a focus tracking system based on moiré fringe phase detection, including: an X-ray source 1, a front collimator 2, a detector 4, a signal processing unit 5, and a correction execution unit 6; The focal point of the X-ray source is located on the anode target surface and is used to emit X-rays; the nominal size of the focal point of the X-ray source is 0.5mm~1.2mm.

[0038] The pre-collimator 2 is disposed between the X-ray source and the subject. The pre-collimator includes a main collimation unit and a focus tracking grid unit. The focus tracking grid unit is disposed at the Z-direction end of the main collimation unit and is located in the X-ray channel outside the main imaging channel and corresponding to the edge detector area. The main collimation unit is used to limit the irradiation range of X-rays, and the focus tracking grid unit is used to generate moiré fringes using the grid. The focus tracking grating unit includes a first grating 31 and a second grating 32; the first grating 31 and the second grating 32 are arranged in parallel; the first grating 31 has a periodic slit structure with a first grating period p1, is made of a high X-ray absorbing material, and is located at a distance d1 from the focus; the second grating 32 has a periodic slit structure with a second grating period p2, is placed at a distance Δd from the first grating along the ray direction, is located at a distance d1+Δd from the focus, and is made of a high X-ray absorbing material; the high X-ray absorbing material is tungsten or a tungsten alloy; the materials of the first grating 31 and the second grating 32 do not need to be the same, and different materials or structural combinations are usually used according to functional requirements.

[0039] In this design, both the first grating period p1 and the second grating period p2 are greater than twice the detector pixel size to ensure that moiré fringes can be reproduced. Furthermore, the penumbra width of the projections of the first grating 31 and the second grating 32 in the focus-tracking grating unit is less than the projection period of the gratings on the detector, preventing contrast collapse of the moiré fringes. Additionally, in the focus-tracking grating unit, the allowable ranges of d1 and Δd are determined based on the limited installation space of the actual CT system; simultaneously, for each given d1, different Δd will correspond to different detection sensitivities.

[0040] In this embodiment, a smaller grating period results in denser moiré fringes and increased detection sensitivity, but also reduces contrast and increases manufacturing difficulty. Therefore, the grating period needs to be limited according to the actual situation.

[0041] Because the moiré fringe period is much larger than the penumbra width, the blurring caused by the finite size of the focal spot has minimal impact on the moiré fringe contrast, thus achieving high-precision focal position tracking with penumbra immunity. Figure 2 Figure a shows the projection of the first absorption grating on the detector, Figure b shows the projection of the second absorption grating on the detector, Figure c shows the moiré fringes projected onto the detector by superimposing the projections, and Figure d shows the signal intensity curve of the moiré fringes.

[0042] This embodiment only uses a period of approximately 0.5 mm, which is easily achievable through conventional wire cutting processes, as an example. After determining the first grating period p1, the second grating period p2 is selected by matching the moiré fringe period Pmoire with the detector width constraint. The calculation formula is as follows: Where P2det is the projection period of the second grating strip on the detector, and SDD is the distance from the focal point to the detector. The process of determining the second grating period p2 includes: 1. Determine an ideal moiré fringe period, Pmoire, based on the effective width of the detector and the pixel size. The ideal moiré fringe period criterion is to ensure that the Fourier transform can stably extract the phase, requiring at least two complete periods (preferably three to five periods) within the detector width. 2. Calculate the projection period P1det of the first grating using the first grating period p1; 3. Using the formula for the period of moiré fringes: The projection period P2det of the second grating is calculated based on the obtained moiré fringe period Pmoire and the projection period P1det of the first grating. 4. Using the calculation formula: p2 is calculated based on the projection period P2det, d1, Δd and SDD of the second grating.

[0043] The detector 4 faces the X-ray source and is used to receive X-rays passing through the focus-tracking grating unit and detect moiré fringe signals. Detector 4 includes a main imaging region and an edge detection region; the main imaging region is located within the coverage area of ​​the scintillator in the detector, and the edge detection region is located at both ends of the detector in the Z direction, with the available width of the detector in the Z direction being greater than the projected length of the gratings in the focus-tracking grating unit onto the detector. The signal output by the edge detection region is a moiré fringe signal.

[0044] The signal processing unit 5 is connected to the detector 4 and is used to calculate the focus drift using the generated moiré fringes, thereby achieving focus tracking. The focus drift is obtained in one of the following two ways: Method 1, performing FFT on the current image and the reference image to extract the phase difference; Method 2, measuring the displacement of the moiré fringes in space.

[0045] The correction execution unit 6 is connected to the signal processing unit 5 and the front collimator 2, and is used to perform correction measures on the front collimator 2 according to the focal drift amount, including adjusting the Z-direction position of the main collimator unit and adjusting the electromagnetic deflection parameters of the X-ray tube.

[0046] This invention introduces a focus-tracking grating unit, which includes a first grating and a second grating. The grating is only located at the end of the collimator and outside the main imaging channel, so it does not block X-rays in the imaging area, does not increase the patient's radiation dose, and does not affect the quality of the main imaging. Moreover, it only requires the addition of two micro-gratings at the end of the front collimator, without modifying the detector design or the X-ray tube design, resulting in low modification costs. This invention can be used in combination with existing mechanical and electromagnetic correction methods. It can still work normally in fly-focus mode, showing good compatibility.

[0047] In the embodiments of this application, the terms "first" and "second" (if they exist) are used only as name identifiers and do not represent the order of first and second.

[0048] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A focus tracking method based on moiré fringe phase detection, characterized in that, Includes the following steps: S1. A focus tracking grating unit is set in the front collimator, wherein the focus tracking grating unit is set at the Z-direction end of the main collimator unit and is located in the ray channel outside the main imaging channel and corresponding to the edge detector area; the focus tracking grating unit is used to generate moiré fringes using gratings; S2. Calculate the focus drift using the generated moiré fringes to achieve focus tracking; the focus drift is obtained through one of the following two methods: Method 1, perform FFT on the current image and the reference image to extract the phase difference; Method 2, measure the displacement of the moiré fringes in space.

2. The focus tracking method based on moiré fringe phase detection according to claim 1, characterized in that: The focus tracking grating unit includes a first grating and a second grating; the first grating has a periodic slit structure with a first grating period p1 and is located at a distance d1 from the focus; the second grating has a periodic slit structure with a second grating period p2 and is placed at a distance Δd from the first grating along the ray direction and is located at a distance d1+Δd from the focus.

3. The focus tracking method based on moiré fringe phase detection according to claim 1, characterized in that: The focus shift is calculated using Method 1, and the phase difference is extracted by performing FFT on the current image and the reference image respectively, including the following steps: Acquire a frame of moiré fringe image as a reference baseline image; Real-time detection of moiré stripe images, used as the current image; Perform FFT on the current image and the reference image respectively, extract the phase angle, and calculate the phase angle deviation Δφ_FFT between the current image and the reference image. Use the phase angle deviation Δφ_FFT to calculate the focus drift.

4. The focus tracking method based on moiré fringe phase detection according to claim 3, characterized in that: The formula for calculating focus drift is: ， Where K_FFT is the frequency domain phase-shift conversion coefficient, P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2=d1+Δd is the distance between the second grating and the focal point.

5. The focus tracking method based on moiré fringe phase detection according to claim 1, characterized in that: The focus drift is calculated using Method 2. The formula for calculating the focus drift is as follows: ， Where γ_eff is a coefficient that is only related to the spatial geometry of the system, P1det is the projection period of the first grating on the detector, P2det is the projection period of the second grating on the detector, SDD is the distance from the focal point to the detector, d1 is the distance between the first grating and the focal point, and d2=d1+Δd is the distance between the second grating and the focal point.

6. The focus tracking method based on moiré fringe phase detection according to claim 1, characterized in that: The formula for calculating the displacement Δxmoire of the moiré fringes on the detector is: ， Where Δz1 is the translation of the first grating pattern projected onto the detector, Δz2 is the translation of the second grating pattern projected onto the detector; P1det is the projection period of the first grating onto the detector, and P2det is the projection period of the second grating onto the detector. The formula for calculating the phase change Δφ caused by moiré fringes is: ， Wherein, Pmoire is the period of the moiré fringe.

7. A focus tracking system based on moiré fringe phase detection, used to implement the focus tracking method based on moiré fringe phase detection as described in any one of claims 1-6, characterized in that: include: X-ray source, collimator, detector, signal processing unit, and correction execution unit; The focal point of the X-ray source is located on the anode target surface, and is used to emit X-rays; The precollimator is positioned between the X-ray source and the subject. The precollimator includes a main collimating unit and a focus-tracking grating unit. The focus-tracking grating unit is located at the Z-direction end of the main collimating unit and in the X-ray channel outside the main imaging channel, corresponding to the edge detector region. The main collimating unit limits the X-ray irradiation range, and the focus-tracking grating unit generates moiré fringes using gratings. The focus-tracking grating unit includes a first grating and a second grating. The first grating has a periodic slit structure with a first grating period p1 and is located at a distance d1 from the focus. The second grating has a periodic slit structure with a second grating period p2 and is placed at a distance Δd from the first grating along the X-ray direction, located at a distance d1+Δd from the focus. The detector is positioned directly opposite the X-ray source to receive X-rays passing through the focus tracking grating unit and to detect moiré fringe signals. The signal processing unit is connected to the detector and is used to calculate the focus drift using the generated moiré fringes, thereby achieving focus tracking. The focus drift is obtained through one of the following two methods: Method 1, performing FFT on the current image and the reference image to extract the phase difference; Method 2, measuring the displacement of the moiré fringes in space. The correction execution unit is connected to the signal processing unit and the front collimator, and is used to perform correction measures on the front collimator according to the focal drift amount.

8. A focus tracking system based on moiré fringe phase detection according to claim 7, characterized in that: The detector includes a main imaging region and an edge detection region; the main imaging region is located within the coverage area of ​​the scintillator in the detector, and the edge detection region is located at both ends of the detector in the Z direction, and the available width of the detector in the Z direction is greater than the projection length of the grating in the focus tracking grating unit onto the detector.

9. A focus tracking system based on moiré fringe phase detection according to claim 7, characterized in that: The formula for calculating the second grating period p2 is: ， Where P2det is the projection period of the second grating on the detector, and SDD is the distance from the focal point to the detector.

Images