Study Of Imaging Methods For Stationary/Semi-stationary CT

Computed tomography is one of the most commonly used imaging modality for diagnosis.Thanks to the development of small-sized X-ray tube during the past few years,the feasibility of stationary CT is increased in terms of technical aspects and cost.Stationary CT is composed of multiple X-ray sources which are distributed at different positions to obtain projection data from different angles without the rotation of gantry.Stationary CT possesses several potential benefits.Firstly,it avoids the image degradation caused by mechanical instability and enlarged X-ray focal spots along the motion direction.Secondly,the temporal resolution can be improved because the projection data from different angles can be obtained in short time without rotation of the gantry.Thirdly,the sources can be individually controlled,which allows to design various acquisition sequential for further improve the image quality.This research focuses on the imaging methods of stationary CT,including reconstruction algorithm,dynamic imaging and scatter correction.The main contributions are introduced as follows.(1)A Katsevich-type reconstruction algorithm was proposed for the helical stationary CT.The proposed algorithm is advantageous in terms of reconstruction accuracy and efficiency because data rebinning for ring detector is not required.To solve the data truncation problem in stationary CT,a moving scan method was proposed.It is a method that moves the table or gantry during scanning,thus reshaping the effective source trajectory so that a sufficient dataset can be obtained.(2)Existing reconstruction algorithms for time-resolved CT are mainly accomplished by shorten the time interval of scanning,while shorter time interval sometimes means less data for reconstruction and reduces image quality.We have proposed a temporal-weighting iterative reconstruction(TWIR)algorithm to solve this problem.The algorithm doesn't truncate the projection data,but assigns different weight to each projection data according to the reliability of projection data in temporal dimension.TWIR algorithm enables more data for reconstruction.Results suggest that this algorithm improves the image quality as well as temporal resolution in time-resolved imaging.For the stationary CT,a interlaced acquisition sequence was proposed.As the sources in stationary CT can be individually controlled,the acquisition sequence can be interlaced instead of sequential to make the projections data less redundant in short temporal window.Thus the image quality and temporal resolution can be improved.Results show that this interlaced acquisition sequential improves the image quality for each time frame.(3)Scatter reduces the image quality in computed tomography(CT).Anti-scatter-grid cannot be used in stationary CT.In this study,an optimization-based scatter estimation algorithm(OSE)is proposed to estimate and correct scatter.In the concept of primary modulation,the primary is modulated,but the scatter remains smooth by inserting a modulator between the X-ray source and the object.In the proposed algorithm,an objective function is designed for separating the scatter and primary.Prior knowledge is incorporated in the optimization-based framework to improve the accuracy of the estimation:(1)the primary is always positive;(2)the primary is locally smooth and the scatter is smooth;(3)location of penumbra can be determined;and(4)scatter-contaminated data provide knowledge about which part is smooth.Result shows that the proposed OSE algorithm improves the robustness and accuracy in scatter estimation and correction.This method is promising for scatter correction of various kinds of X-ray imaging modalities,such as X-ray radiography,cone beam CT,and the fourth-generation CT.