Monochromatic-beam-based Dynamic X-ray Microtomography

Dynamic X-ray microtomography has played an important role in the in-suit and nondestructive study in the fields of material science and biomedical science.The usual illumination mode is white beam or undulator beam in the world,but these illumination modes will result in loss of quantitative information or strong radiation dose.In order to acquire quantitative information,a new imaging technique named as X-ray phase contrast computed tomography(XPCT)is developed in the monochromatic illumination mode,which is particularly suitable for low-Z elements.But the flux density of monochromatic illumination is relatively low,then it usually takes about ten minutes to acquire one data set,which limits its application in the investigation of the dynamic process of samples.In order to improve the temporal resolution of CT and maintain the signal-to-noise ratio of the acquired data,the method of reducing projections of one CT data set is applied in this thesis.Meanwhile,iterative reconstruction algorithms based on the compressed sensing theory are used to keep the reconstructed image quality.Combining the algebraic reconstruction method and total variation(ART-TV)of the desired image,the reconstructed image quality maintains high with a undersampled data set.According to high collection efficiency of dynamic CT,we proposed an efficient ordered subset expectation maximization–TV(OSEM-TV)reconstruction algorithm.The computing efficiency of OSEM-TV is an order of magnitude higher than that of ART-TV under limited projections as a whole.Finally,a dynamic CT experimental platform with a 2 Hz temporal resolution is established and the characteristics of respiration of insects are investigated.This thesis is dedicated to carry out the study of the dynamic CT and its applications at the X-ray Imaging and Biomedical Application beamline of Shanghai Synchrotron Radiation Facility.The main contributions are summarized as follows:1 The reconstruction algorithm of undersampled data is established and developed.Using the ART-TV method,the reconstructed image quality remains high.The algorithm has been studied through experimental research of actual samples.Insitu and in-vivo observation of live ants reduced the time of CT data acquisition to 17 seconds.The analysis results show that the quantitative phase information of the ant is acquired using the phase retrieval algorithm and ART-TV algorithm.Compared to the filtered back projection(FBP)algorithm,the ART-TV could effectively suppress streak artifacts caused by under sampling and acquire the 3D structures of ants.Research shows that ART-TV combined with XPCT can reconstruct the quantitative information of samples with high image quality from undersampled data.Meanwhile,it can effectively improve the efficiency of data collection,reduce the radiation dose.It is also beneficial to the application of CT in the study of the dynamic characteristics of samples.2 The effective reconstruction algorithm of undersampled data is established and developed,which has solved the key technique of reconstructing the mass data acquired from the dynamic X-ray microtomography quickly.The ART-TV algorithm is faced with the problem of huge time consumption in image reconstruction,which is contradictory to the characteristic of high speed data acquisition of dynamic CT.In order to improve the reconstruction speed,the statistical iterative algorithm OSEM is used to replace ART as the data constraint.The simulated results show that the OSEM-TV algorithm can obtain the similar reconstruction result with that of ART-TV under different noise levels,but the reconstruction speed is increased by an order of magnitude.At the same time,the experimental study was carried out on the standard samples and in vivo samples.The experimental results show that the time consumption of reconstructing one image using OSEM-TV is several minutes at the optimal projection number and exposure time.Compared with the ART-TV,the speed-up ratio reaches about 30.Further through CUDA(Compute Unified Device Architecture)parallel computing technology,the reconstruction time is less than two minutes,then we achieve the efficient reconstruction of dynamic microtomography.3 Applying monochromatic-beam-based dynamic X-ray microtomography in the study on respiration of insects.We revealed the anisotropic shrinkage of the air sac in suit first.A dynamic CT experimental platform with a 2 Hz temporal resolution is established and the characteristics of respiration of insects are investigated.We acquired and analyzed the 3D dynamic evolution of the air sac during the respiration of insects.This anisotropic shrinkage is beneficial to the air flowing in trachea and increase of the efficiency of gas exchange.The study has deepened the understanding of the respiratory characteristics of insects and provided new knowledge of insects.