A cone beam CT image reconstruction method

Abstract

The invention discloses a cone beam CT image reconstruction method, comprising the following steps: S10, measuring the projection images of water of different thicknesses under a preset radiation dose, and obtaining a comparison table of the relationship between pixel values ​​and water thickness values ​​according to the projection images; S20, rotating and collecting the projection images of the object to be measured at different angles under the same radiation dose as that in step S10; S30, according to the pixel values ​​of the projection image obtained in step S20, finding the corresponding water thickness value in the comparison table obtained in step S10, and forming an equivalent projection water thickness image; S40, reconstructing the equivalent projection water thickness image formed in step S30, and obtaining a three-dimensional matrix of the equivalent water thickness of each voxel of the object to be measured; S50, according to the numerical relationship between the material coefficient and the water thickness value, obtaining a three-dimensional matrix of the material coefficient; S60, according to the three-dimensional matrix of the material coefficient obtained in step S50, calculating the CT value of each voxel of the object to be measured, and obtaining the final cone beam CT image.

Description

Technical Field

[0001] The present invention relates to the field of medical imaging technology, and in particular to a cone beam CT image reconstruction method. Background Art

[0002] Cone beam CT (CBCT), which emerged in modern times, has been widely used in interventional treatment, oral digital imaging and image-guided radiotherapy due to its compact structure and three-dimensional rapid imaging.

[0003] like Figure 1 As shown, a cone beam CT device is usually composed of a rotating gantry, an X-ray source, and a detection plate, wherein the X-ray source and the detection plate are relatively installed on the rotating gantry and rotate with the rotating gantry.

[0004] Cone beam CT uses multiple X-ray pulse exposures on a rotating trajectory to reconstruct a three-dimensional image using projection data at multiple angles. Common reconstruction methods include:

[0005] a. Filtered back projection method: Filtered back projection method is a spatial domain processing technology based on Fourier transform theory. Its characteristic is to filter the projection at each acquisition projection angle (frequency domain filtering or time domain convolution), and then perform back projection reconstruction based on the geometric relationship of the projection.

[0006] b. Iterative reconstruction method: First, make an original estimate of the X-ray photon distribution. On this basis, estimate the possible counts obtained by the detector in each projection direction, that is, the orthographic projection. Then compare the orthographic projection data with the projection data actually collected by the detector to update the original estimated data. Repeat this process until the next generation of iteration results are infinitely close.

[0007] Using a filtered back-projection algorithm or an iterative reconstruction algorithm, an accurate tomographic image can be obtained, which can reflect the distribution of X-ray attenuation values ​​of tissues at different positions in the human body, that is, CBCT values. Ideally, the CBCT values ​​are accurate, but in actual imaging, they are often affected by scattered rays and ray hardening. If no compensation is made, the reconstructed image will produce serious artifacts and seriously affect the image quality.

[0008] X-ray scattering occurs when X-rays interact with matter. In addition to being absorbed by the matter, they may also be scattered by the matter. A large amount of scattered signals reaches the flat-panel detector, causing a large amount of scattering noise in the CBCT projection image and severe cupping artifacts in the reconstructed image.

[0009] The beam hardening effect means that the X-rays produced by the CT tube have a certain spectrum width. After the continuous energy spectrum X-rays pass through the human body, low-energy rays are easily absorbed, while high-energy rays are easier to pass through. During the propagation of the rays, the average energy will become higher and the rays will gradually become harder. This is called the beam hardening effect, which can produce strip-shaped, radial or cup-shaped artifacts.

[0010] Therefore, there is an urgent need to provide a cone beam CT image reconstruction method to reduce the influence of scattered rays and ray hardening on the reconstruction results. Summary of the invention

[0011] The object of the present invention is to provide a cone beam CT image reconstruction method to solve the above problems.

[0012] The present invention solves the technical problem by adopting the following technical solution:

[0013] A cone beam CT image reconstruction method comprises the following steps:

[0014] S10, measuring the projection images of water of different thicknesses under a preset radiation dose, and obtaining a comparison table of the relationship between pixel values ​​and water thickness values ​​according to the projection images;

[0015] S20, under the same radiation dose as step S10, rotating and collecting projection images of the object under test at different angles;

[0016] S30, according to the pixel value of the projection image obtained in step S20, find the corresponding water thickness value in the comparison table obtained in step S10 to form an equivalent projection water thickness image;

[0017] S40, reconstructing the equivalent projection water thickness image formed in step S30 to obtain a three-dimensional matrix of the equivalent water thickness of each voxel of the measured object;

[0018] S50, obtaining a three-dimensional matrix of material coefficients according to the numerical relationship between the material coefficients and the water thickness value;

[0019] S60 , calculating the CT value of each voxel of the object to be measured according to the three-dimensional matrix of material coefficients obtained in step S50 , and obtaining a final cone beam CT image.

[0020] Furthermore, methods for reconstructing the equivalent projection water thickness image include but are not limited to a matrix method, a back projection method, a filtered projection method, and an iterative reconstruction method.

[0021] The present invention has the following beneficial effects: the cone beam CT image reconstruction method of the present invention converts the process of solving the attenuation coefficient of a certain volume element in the human body into the process of solving the thickness value of water that produces the same attenuation effect, thereby eliminating the projection brightness deviation caused by scattered rays and ray hardening effects, and reducing the impact on the image quality of cone beam CT reconstruction. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural diagram of a cone beam CT device in the prior art;

[0023] Figure 2 is a flow chart of the cone beam CT image reconstruction method of the present invention;

[0024] Figure 3 Schematic diagram of constructing the equivalent projected water thickness image in the present invention.

[0025] The markings in the figure are as follows: 1-rotating frame; 2-rotating axis; 3-detection plate; 4-X-ray source. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below in conjunction with the embodiments and drawings.

[0027] Example 1

[0028] This embodiment provides a cone-beam CT image reconstruction method for reducing the influence of scattered rays and ray hardening on the reconstruction result of cone-beam CT images.

[0029] In the field of medical imaging technology, the CT value (in Hounsfield Unit, HU) of a certain spatial pixel (x, y, z) in a cone beam CT image can be calculated by the following formula:

[0030]

[0031] In the formula, μ Water is the linear attenuation coefficient of X-rays of water at 73keV, in cm 2 / g;μ x (x, y, z) is the X-ray linear attenuation coefficient at a certain spatial pixel (x, y, z).

[0032] Therefore, by obtaining μ x (x, y, z) and μ Water The relationship between μ and y can be used to calculate the CT value of a certain spatial pixel (x, y, z), and cone beam CT image reconstruction can be regarded as solving μ x (x, y, z) process.

[0033] When performing cone beam CT image acquisition, the projection image can be regarded as the intensity distribution of X-rays after they pass through the object being measured. Specifically, the relationship between the intensity of X-rays with a preset dose before and after they pass through a material A with a thickness of l can be expressed by the X-ray intensity attenuation formula:

[0034]

[0035] Where I1 is the intensity of the outgoing X-ray, I0 is the intensity of the incident X-ray, μ x is the linear attenuation coefficient of the material passed through, and l is the thickness of the material passed through by the ray.

[0036] Under the same radiation dose, the linear attenuation coefficient μ of material A is x Linear attenuation coefficient with water μ water The relationship can be expressed as:

[0037] μ x = b·μ water

[0038] Wherein, b is the material coefficient, and the value of b can be measured according to the test.

[0039] Then the CT value of substance A can be expressed as:

[0040]

[0041] According to the X-ray intensity attenuation formula, the attenuation of X-rays after passing through a unit thickness of material A can be expressed as:

[0042]

[0043] The attenuation of X-rays after passing through h unit thickness of water can be expressed as:

[0044]

[0045] Comparing the above two equations, we can find that when the incident light intensity I0 and the outgoing light intensity I1 are the same in both equations, then the attenuation effect of 1 unit of material A and h units of water on X-rays is the same, that is, the material coefficient b and the water thickness h are numerically equivalent. Therefore, the attenuation coefficient μ of a certain voxel in the human body can be solved. x The process can be converted into solving the thickness h of water that produces the same attenuation effect.

[0046] Based on the above theory, this embodiment proposes a cone beam CT image reconstruction method, including the following steps:

[0047] S10, measuring the projection images of water of different thicknesses under a preset radiation dose, and obtaining a relation comparison table between pixel values ​​(I) and water thickness values ​​(h) according to the projection images;

[0048] In this embodiment, a preset dose of X-rays is used to irradiate water of different thicknesses to obtain projection images of water of different thicknesses, and a comparison table of the relationship between pixel values ​​and water thickness values ​​is obtained based on the pixel values ​​of the projection images of water of different thicknesses.

[0049] S20, under the same radiation dose as step S10, rotating and collecting projection images of the object under test at different angles;

[0050] S30, according to the pixel value of the projection image obtained in step S20, find the corresponding water thickness value in the comparison table obtained in step S10 to form an equivalent projection water thickness image;

[0051] In this embodiment, if Figure 2 As shown, each pixel in the projection image corresponds to a pixel value. According to the pixel value in the comparison table obtained in step S10, the corresponding water thickness value is found, and the water thickness value is used to replace the pixel value of the pixel in the projection image to obtain an equivalent projection water thickness image.

[0052] S40, reconstructing the equivalent projection water thickness image formed in step S30 to obtain a three-dimensional matrix of equivalent water thickness of each voxel;

[0053] In this embodiment, methods for reconstructing the equivalent projection water thickness image include but are not limited to a matrix method, a back-projection method, a filtered projection method, and an iterative reconstruction method.

[0054] S50, obtaining a three-dimensional matrix of material coefficients according to the numerical relationship between the material coefficients and the water thickness value;

[0055] In this embodiment, it can be seen from the above that the material coefficient b and the water thickness value h are numerically equivalent, so the three-dimensional matrix of the equivalent water thickness of each voxel obtained in step S40 is the three-dimensional matrix of the material coefficient.

[0056] S60 , calculating the CT value of each voxel of the object to be measured according to the three-dimensional matrix of material coefficients obtained in step S50 , and obtaining a final cone beam CT image.

[0057] In this embodiment, the CT value of each voxel is calculated according to the formula CT=(b-1)×1000, and finally a cone beam CT image is obtained, and the reconstruction is completed.

[0058] The cone beam CT image reconstruction method of the present invention converts the process of solving the attenuation coefficient of a certain volume element in the human body into the process of solving the thickness value of water that produces the same attenuation effect, thereby eliminating the projection brightness deviation caused by scattered rays and ray hardening effects, and reducing the impact on the image quality of cone beam CT reconstruction.

[0059] The order of the above embodiments is only for the convenience of description and does not represent the advantages or disadvantages of the embodiments.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cone beam CT image reconstruction method, characterized in that: The following steps are involved: S10, measuring the projection images of water of different thicknesses under a preset radiation dose, and obtaining a comparison table of the relationship between pixel values ​​and water thickness values ​​according to the projection images; S20, under the same radiation dose as step S10, rotating and collecting projection images of the object under test at different angles; S30, according to the pixel value of the projection image obtained in step S20, find the corresponding water thickness value in the comparison table obtained in step S10 to form an equivalent projection water thickness image; S40, reconstructing the equivalent projection water thickness image formed in step S30 to obtain a three-dimensional matrix of the equivalent water thickness of each voxel of the measured object; S50, obtaining a three-dimensional matrix of material coefficients according to the numerical relationship between the material coefficients and the water thickness value; S60, calculating the CT value of each voxel of the object to be measured according to the three-dimensional matrix of material coefficients obtained in step S50, and obtaining a final cone beam CT image; In step S60, the CT value of each voxel is calculated by the following formula: in, is the linear attenuation coefficient of X-rays of water at 73keV, in units of ; b is the material coefficient.

2. The cone beam CT image reconstruction method according to claim 1, characterized in that: Methods for reconstructing equivalent projection water thickness images include matrix method, back-projection method, filter projection method and iterative reconstruction method.

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