Design And Implementation Of The Local Linear Embedding Algorithm For Cone-beam CT Geometry Calibration

X-CT is an abbreviation of X-ray Computed Tomography(CT)technique,which can be used to visualize the internal structure information of objects.Therefore,X-CT has many advantages such as lossless,non-contact and high resolution.However,if there is an error in the geometrical structure of X-CT system or device,the geometric artifact will be generated in CT reconstruction image.The existence of geometrical artifact will reduce the imaging quality of CT reconstruction image.Therefore,how to calibrate the geometrical artifact in CT reconstruction image has great significance to improve the imaging quality of X-CT system.In recent years,Cone-beam Computed Tomography(CBCT)has become a hotspot in the field of X-CT with its rapid speed and isotropy.This paper mainly focuses on the geometric calibration of CBCT system.The geometric artifact of CT system or device is mainly caused by the seven geometrical errors in CBCT system,include the error of X-ray source to rotation center distance、the horizontal offset error of rotation center、the error of rotation center to detector distance、the horizontal and vertical errors of detector、rotation angle around the normal vector of detector、rotation angle around the center row of detector、rotation angle around the center column of detector.Each geometric error will have different effect on the projection of CBCT system.Therefore,in order to calibrate geometric parameters of CBCT system and reduce the geometric errors,thereby removing the geometric artifact in CT reconstruction image,this paper adopt the geometric calibration method based on the local linear embedding(LLE)algorithm,which developed in recent years,to calibrate the geometric parameters of CBCT system.The advantage of the geometric calibration method based on the local linear embedding algorithm compared with other geometric calibration methods is that this method is less dependent on initial value and can be calibrated for any kind of the geometric errors.Firstly,the geometric calibration method based on the local linear embedding algorithm is studied on the basis of the basic principle of X-CT in this paper.This method mainly used the local linear embedding algorithm to calculate the geometric parameters through the nearest neighbor points and then the geometric parameters of the CT system are obtained after several iterations.Then,this paper designed a CBCT geometric correction method based on local linear embedding algorithm by analyzing the influence of geometrical errors on the projection of CBCT system.Based on this,this paper developed the software platform for geometric calibration of CBCT system.This software adopt the modular design concept,and was divided into five modules to develop.The five modules include data processing module、projection matrix calculation module、image reconstruction module、scanning geometric calibration module and software interface module.In the end,though analyzing the reason of the time-consuming problem of CBCT system,this paper perfected the design of geometric calibration software and improved the calibration efficiency of CBCT system geometric calibration software by using parallel processing technology based on OpenMP.