Research On Rapid Image Reconstruction Of ART Algorithm Based On Joseph Projection Method

The digitalization of cultural relics is of great significance to their research and protection.However,conventional digital technology based on 3D laser scanning cannot provide objects' inner structure information.As a substitute,Computed Tomography,which could obtain geometric structure and material ingredient of objects by reconstructing gray images of the intersection slices in the non-destructive condition,is gradually applied into the field of cultural relic digitization.Reconstruction algorithm is the key to CT reconstruction process,including analytical method and iterative method.Analytical method can realize reconstructing relatively ideal image in a short time,but requires high completeness of the projection data and has weak noise immunity.Differently,iterative algorithm has strong noise immunity as well as able to obtain good reconstructed images under incomplete projection,which makes it gradually become research hotspot in CT algorithm.ART(Algebraic Reconstruction Techniques)is the typical algorithm in iterative algorithms.In ART algorithm,the calculation method of the projection coefficient has a great influence on reconstruction quality and speed.Conventional system matrix model confirms pixel's contribution to projection by calculating the intersection length of ray and pixel,in which Siddon algorithm is often used to calculate the projection coefficient,however,since it uses nearest neighbor interpolation,the reconstruction effect is not ideal.Joseph projection algorithm,which also belongs to line integral model,could bring better reconstruction quality than Siddon algorithm by using linear interpolation,but it bears shortcomings such as time-consuming computation and ill construction efficiency.On this research background,based on Joseph projection algorithm under ray-driven model,this dissertation will bring forward an efficient projection coefficient calculation method to improve ART.The primary research content as following:(1)A 2D rapid ART image reconstruction algorithm based on Joseph projection method is brought forward.In view of conventional Joseph algorithm's low efficiency in calculating the pixel index,this dissertation proposes a method to find the index based on distance increment.By calculating the increment in the other direction when the same distance is passed on X / Y in 2D coordinate system,and comparing it with pixel side length to estimate the condition of the ray passing through pixel,it could rapidly obtainpixel index so as to shorten time in calculating projection.The results of simulation experiment illustrate that this method doubles the time of reconstruction and speed up calculating projection coefficient with accelerating ration of 4.(2)A rapid Joseph projection algorithm under 3D cone beam is advanced.Based on the ray equation in 3D coordinate system,firstly,check rays' projection direction on XOY plane and the intersection of rays and the boundary of reconstruction area.Then calculate the distance increment on XOY plane when passing same distance on the projection direction,and figure out intersection of rays and voxel in the same layer by comparing this increment with voxel size to determine the intersection of the ray and the voxel in the same layer,and use bilinear interpolation to calculate the corresponding voxel weight factor.The same as above,we could obtain the increment on Z and compare it with the voxel size to confirm whether rays intersect the next voxel.It is approved that,compared with conventional ART algorithm of Joseph algorithm,this method could achieve overall reconstruction with the accelerating ration of 2.2 and speed up projection calculation with accelerating ration of 4.(3)The shortcoming of ART algorithm,much more calculation and reconstruction time,is especially evident when using in 3D case.Despite the algorithm is improved,the computation efficiency is still very low.Aiming at solve this problem,this dissertation uses Open MP technology to achieve multi-core parallel acceleration to 2D and 3D rapid ART algorithm.Dividing task according to the plane of the detector,and pre-calculating image /voxel which needs to be calculated for each ray at each angle,the problem that load between threads varies is resolved.In this six cores condition,experimental data shows that this method could obtain accelerating ration of up to 4.