CN101126723B - Cone-beam CT beam hardening calibration method based on section profile reprojection - Google Patents
Cone-beam CT beam hardening calibration method based on section profile reprojection Download PDFInfo
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Abstract
The utility model discloses a CT beam-hardening correct method based on slice profile re-projection. The utility model adopts the technical proposal that: firstly, circular orbit cone beam CT is scanned on the parts to get a group of continuous multi-chromatic projective image data, and the image data is reconstructed to get a central slice image to get a plurality of close section profiles of the central slice parts; secondly, the section profiles of the central slice parts are re-projected according to the actual cone beam CT scanning imaging geometric relation to get the ray crossing length-multi-chromatic projection grey level corresponding data; thirdly, the corresponding data is merged with the central slice profile data; fourthly, the ray crossing length-multicolor projection gray level corresponding data is fitted to get a zero-crossing correction linear function; finally, the CT beam-hardening is corrected and calculated according to the exponential function and the correct linear function. The utility model has the advantages of having high flexibility in application, approximately correcting the multi-chromatic projection into monochromatic projection, and greatly reducing the beam-hardening artifacts.
Description
Technical field
The present invention relates to a kind of cone-beam CT beam hardening calibration method, belong to the CT technical field.
Background technology
(Cone-Beam Computed Tomography CBCT) utilizes cone beam of rays source and planar array detector to gather the data for projection of testee to Cone-Beam CT, is a kind of three dimensional CT technology that developed recently gets up.Compare with traditional two-dimensional ct, Cone-Beam CT has very high sweep velocity, the sectioning image that reconstructs has write down physical parameter such as the material, density of interior of articles each point and has distributed, have the section sequence continuously, in the section with section between characteristics such as identical, the precision height of spatial resolution, field such as look in reverse-engineering and industry has demonstrated wide application and development prospect.
In cone-beam CT system, the X ray that radiographic source sends has the energy distribution (this kind ray is called the polychrome ray) of certain limit, when ray and matter interaction, because the damping capacity of energy photons is greater than high-energy photons, cause the average energy of passing ray behind the certain thickness object to increase, the linear attenuation coefficient of ray average energy correspondence also no longer is constant but reduces gradually at this moment, causes projection value and crossing length also no longer linear.And the CT reconstruction algorithm to be based on X ray be monoenergetic spectrum hypothesis, directly replace monochromatic projection during reconstruction with the polychrome projection, cause on the sectioning image of rebuilding, presenting the pseudo-shadow (Cupping Artifacts) of cup-shaped, image can produce distortion when serious, make structure, size, density, one-tenth in this section grade physicochemical property interpretation and metering exactly, this phenomenon just is called beam hardening (Beam Hardening).
Beam hardening is a significant problem that must solve in the Cone-Beam CT practical application.At present, the beam hardening correction method of CT mainly is divided into monoenergetic method and dual intensity method two big classes.Because operational complicacy, the dual intensity method seldom is used in engineering practice.The monoenergetic method is easy to realize that practical application effect is also relatively good, therefore is widely studied.People such as Yang Min, Lu Hongnian, great distance are in " optical technology " (2003,29 (2): the method that proposes in article 177-182) " ray hardened correction is studied in the CT reconstruct " is exactly a kind of typical monoenergetic correction method, it proofreaies and correct thinking: utilize the wedge shape die body to obtain ray and run through relation curve between object length and the polychrome projection value, again this curve is carried out match, from true origin this curve is done tangent line then, set up the funtcional relationship of polychrome data and monochromatic data with this tangent line, thereby reach the purpose of hardening correcting.This method implements simply, but requires to have the die body with the identical material of tested test specimen, and this has just influenced the application flexibility of this method.
In addition, present beam hardening correction method majority is at two-dimensional ct, although the method that has can be generalized to Cone-Beam CT by transformation, the actual research of carrying out is still less.
Summary of the invention
In order to overcome the deficiency that prior art is used underaction, can not effectively be applicable to Cone-Beam CT, the invention provides a kind of cone-beam CT beam hardening and proofread and correct new method, effectively proofread and correct with the beam hardening problem that Cone-Beam CT is applied to occur in the actual Non-Destructive Testing based on section profile re-projection.
The cone-beam CT beam hardening that the present invention is directed to a kind of profile re-projection of cutting into slices of beam hardening problem proposition in the industrial Cone-Beam CT Non-Destructive Testing is proofreaied and correct new method, it is characterized in that comprising the following steps:
(1) detected part is justified the track cone-beam CT scan, obtain one group of continuous polychrome projecting image data from flat panel detector, these images process is necessary in acquisition process details in a play not acted out on stage, but told through dialogues correction, bad pixel correction and gain calibration, correction means can adopt the supporting program of flat panel detector manufacturer to carry out, and also can carry out according to known technology exploitation corresponding program voluntarily;
(2) cone-beam CT reconstruction is carried out in the polychrome projection and obtain a center sectioning image; If the center section does not comprise the maximum length that ray passes through object, promptly the maximum gradation value pixel of all projected images is not positioned at the center slice position, then near the maximum gradation value location of pixels of all projected images, rebuild the sectioning image of plurality of continuous, and utilize these sectioning images to generate the oblique section image of a maximum grayscale position of mistake according to the known method that dissects volume data;
(3) the anxious picture of centering adopts the related algorithm in the Digital Image Processing to carry out the profile extraction, obtains the center section part section profile of some sealings; If the oblique section image is arranged, then carry out same treatment, obtain the part oblique section profile of some sealings;
(4) anxious part section profile of centering carries out re-projection by actual cone beam CT scan imaging geometry, obtains data corresponding between ray crossing length one polychrome projection gray level; If part oblique section profile is arranged, then carry out same treatment, and will obtain data corresponding between ray crossing length one polychrome projection gray level and be merged into one group of data with the data that obtain by center section profile;
(5) adopt exponential function L=Aexp (BG
c)-A comes data corresponding between match ray crossing length one polychrome projection gray level, and wherein L is that ray passes through object length, and G is a polychrome projection gray level value, and A, B, C are fitting coefficients;
(6) data corresponding between ray crossing length one polychrome projection gray level are carried out linear fit near the part of initial point, obtain a correction linear function of crossing initial point;
(7) carry out the cone-beam CT beam hardening correction calculation according to exponential function and correction linear function: to each pixel of each width of cloth polychrome projected image, earlier its gray scale substitution exponential function is calculated corresponding length, then this length substitution is proofreaied and correct the approximate monochromatic projection gray level value after linear function obtains proofreading and correct.
The above-mentioned cone-beam CT beam hardening calibration method based on registration model emulation not only can be applied to Cone-Beam CT, can also be applied to fan beam CT.
In said method the 4th goes on foot, by the geometric relationship of cone-beam CT system imaging as can be known, ray source focus, center sectioning image and projection thereof are positioned on the same plane, and section profile re-projection computation purpose is exactly to pass through the length value of object for each the projection gray level value ray corresponding with it that obtains detector reception in this plane.Because oblique section profile re-projection is similar fully to center section profile re-projection, be that example describes in detail with the center section only below: center section profile re-projection is that ray and profile line segment are planar asked friendship in essence, then the length of passing through object by some intersection point calculation emerging rays of gained.At first choose a projection orientation, set up coordinate system as shown in Figure 3, wherein x
1O
1Y
1Become CT as coordinate system, X
2O
2Y
2Be the sectioning image coordinate system, factory is the translational movement between two coordinate systems, DSO be ray source focus to the rotation center distance, DOD is that rotation center arrives detector distance; Calculate ray then
With the intersection point of profile line segment, the gained intersection point by from the far and near ordering of ray source focus, just can be calculated the length that ray passes through object by simple odd even identification, wherein produce to cutting into slices on the projection row the in the detector center, z pixel, M is a detector row resolution.
The invention has the beneficial effects as follows: the cone-beam CT beam hardening calibration method that the present invention proposes based on section profile re-projection, compare with traditional fitting of a polynomial bearing calibration, this method does not need to make the wedge shape die body that is used to generate calibration model, the cad model and the relevant design information that also do not need part to be scanned, on using, have more dirigibility, and the polychrome projection approximation of Cone-Beam CT can be proofreaied and correct and be monochromatic projection, the beam hardening puppet shadow that carries out with the projection after proofreading and correct in the sectioning image of cone-beam CT reconstruction gained significantly reduces.
The present invention is further described below in conjunction with drawings and Examples.
Description of drawings
Fig. 1 is a beam hardening correction process flow diagram of the present invention:
Fig. 2 is the geometric relationship figure of cone-beam CT system imaging;
Fig. 3 is center section profile re-projection computational geometry synoptic diagram;
Fig. 4 is the linear gray scale comparison diagram of same position before and after the 50th layer of section of actual scanning cylindrical part proofreaied and correct.
Embodiment
To a material is the detected cylindrical part of iron, uses the inventive method and proofreaies and correct the pseudo-shadow of its cone-beam CT beam hardening, carries out following steps:
(1) detected part is justified the track cone-beam CT scan, part revolves three-sixth turn continuously, obtain one group of 360 continuous width of cloth polychrome projecting image data from the PaxScan2520 flat panel detector of Varian company, these images have adopted the supporting program of flat panel detector manufacturer to carry out necessary details in a play not acted out on stage, but told through dialogues correction, bad pixel correction and gain calibration in acquisition process;
(2) adopt the FDK algorithm that the partially sliced reconstruction of Cone-Beam CT is carried out in the polychrome projection, obtain a center sectioning image; Because center section does not comprise the day length that ray passes through object, promptly the maximum gradation value pixel of all projected images is not positioned at the center slice position, so near the maximum gradation value location of pixels of all projected images, rebuild 20 continuous sectioning images, and utilize these sectioning images to generate the oblique section image of a maximum grayscale position of mistake according to the known method that dissects volume data;
(3) anxious picture of centering and oblique section image adopt the OTSU algorithm in the Digital Image Processing to carry out the profile extraction respectively, obtain the center section part section profile and the part oblique section profile of some sealings;
(4) anxious part section profile of centering carries out re-projection by actual cone beam CT scan imaging geometry, obtains data corresponding between ray crossing length one polychrome projection gray level; Part oblique section profile is carried out same treatment, and will obtain between ray crossing length one polychrome projection gray level corresponding data and be merged into one group of data with the data that obtain by center section profile;
(5) adopt exponential function L=Aexp (BG
C)-A comes data corresponding between match ray crossing length one polychrome projection gray level, and the concrete grammar of match is a least square method, and wherein L is that ray passes through object length, and G is a polychrome projection gray level value, and A, B, C are fitting coefficients;
(6) adopt least square method that preceding 1/6 (near the part of initial point) of data corresponding between ray crossing length one polychrome projection gray level carried out linear fit, obtain a correction linear function of crossing initial point;
(7) carry out the cone-beam CT beam hardening correction calculation according to exponential function and correction linear function: to each pixel of each width of cloth polychrome projected image, earlier its gray scale substitution exponential function is calculated corresponding length, then this length substitution is proofreaied and correct the approximate monochromatic projection gray level value after linear function obtains proofreading and correct.
Fig. 4 is that the linear gray scale of same position compared before and after the 50th layer of section of cylindrical part proofreaied and correct, as seen the pseudo-shadow of the cup-shaped that is caused by beam hardening has been eliminated in bearing calibration provided by the invention substantially, sectioning image quality after the correction is obviously improved, the sharpness of image outline improves greatly, shows that the inventive method is practicable.
Claims (2)
1. based on the cone-beam CT beam hardening calibration method of section profile re-projection, it is characterized in that comprising the steps:
(a) detected part is justified the track cone-beam CT scan, obtain one group of continuous polychrome projecting image data, these images process is necessary in acquisition process details in a play not acted out on stage, but told through dialogues correction, bad pixel correction and gain calibration from flat panel detector;
(b) cone-beam CT reconstruction is carried out in the polychrome projection and obtain a center sectioning image; If the center section does not comprise the maximum length that ray passes through object, promptly the maximum gradation value pixel of all projected images is not positioned at the center slice position, then near the maximum gradation value location of pixels of all projected images, rebuild the sectioning image of plurality of continuous, and utilize these sectioning images according to dissecing the oblique section image that volume data generates a maximum grayscale position of mistake;
(c) the anxious picture of centering adopts the OTSU algorithm in the Digital Image Processing to carry out the profile extraction, obtains the center section part section profile of some sealings; If the oblique section image is arranged, then carry out same treatment, obtain the part oblique section profile of some sealings;
(d) anxious part section profile of centering carries out re-projection by actual cone beam CT scan imaging geometry, obtains one group of ray crossing length-polychrome projected pixel gray scale data one to one; If part oblique section profile is arranged, then carry out same treatment, obtain another group ray crossing length-polychrome projected pixel gray scale data one to one, and the end of adding these group data to first group of data is merged into one group of data;
(e) adopt exponential function L=Aexp (BG
C)-A comes data corresponding between match ray crossing length-polychrome projection gray level, and wherein L is that ray passes through object length, and G is a polychrome projection gray level value, and A, B, C are fitting coefficients;
(f) data corresponding between ray crossing length-polychrome projection gray level are carried out linear fit near the part of initial point, obtain a correction linear function of crossing initial point;
(g) carry out the cone-beam CT beam hardening correction calculation according to exponential function and correction linear function: to each pixel of each width of cloth polychrome projected image, earlier its gray scale substitution exponential function is calculated corresponding length, then this length substitution is proofreaied and correct the approximate monochromatic projection gray level value after linear function obtains proofreading and correct.
2. according to the cone-beam CT beam hardening calibration method based on section profile re-projection of claim 1, it is characterized in that described center section profile re-projection comprises the steps: at first to choose a projection orientation, sets up coordinate system, wherein X
1O
1Y
1Be CT imaging coordinate system, X
2O
2Y
2Be the sectioning image coordinate system, T is the translational movement between two coordinate systems, DSO be ray source focus to the rotation center distance, DOD is that rotation center arrives detector distance; Calculate ray then
With the intersection point of profile line segment, the gained intersection point by from the far and near ordering of ray source focus, just can be calculated the length that ray passes through object, wherein P by simple odd even identification
nBe n pixel on the detector center section projection row, M is a detector row resolution.
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| US8284892B2 (en) * | 2008-12-22 | 2012-10-09 | General Electric Company | System and method for image reconstruction |
| CN101533098B (en) * | 2009-04-08 | 2011-08-03 | 西北工业大学 | Method for reducing noise in hardening correction of CT beam |
| CN103134823B (en) * | 2013-03-21 | 2015-03-11 | 重庆大学 | Convolution-based X-ray CT system beam hardening correction method |
| CN107563972B (en) * | 2017-08-11 | 2020-06-09 | 重庆真测科技股份有限公司 | CT data correction method and step test piece |
| CN111539878B (en) * | 2020-03-25 | 2022-11-01 | 中国人民解放军战略支援部队信息工程大学 | Method and device for removing cone beam CT geometric artifacts based on mirror image projection similarity |
| CN111476734B (en) * | 2020-04-09 | 2023-10-03 | 航天特种材料及工艺技术研究所 | Method for reducing artifacts in CT reconstructed images |
| CN111487265B (en) * | 2020-05-25 | 2023-06-16 | 中国人民解放军战略支援部队信息工程大学 | Cone beam CT hardening artifact correction method combining projection consistency |
| CN116892893B (en) * | 2023-09-11 | 2023-12-12 | 上海福柯斯智能科技有限公司 | Industrial CT cone beam center projection point measuring method and storage medium |
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| US5023895A (en) * | 1989-03-02 | 1991-06-11 | Innovative Imaging Systems, Inc. | Three dimensional tomographic system |
| US5170439A (en) * | 1991-06-11 | 1992-12-08 | Picker International, Inc. | Cone beam reconstruction using combined circle and line orbits |
| EP1077430A2 (en) * | 1999-08-16 | 2001-02-21 | Analogic Corporation | Apparatus and method for reconstruction of volumetric images in a computed tomography system |
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