Advanced techniques for four-dimensional motion-resolved abdominal diffusion-weighted imaging

Abstract

4D computerized tomography (4D-CT), a technique for visualizing abdominal motion, is used for dose planning in abdominal radiotherapy. Abdominal four-dimensional magnetic resonance imaging (4D-MRI) is expected as an alternative technique to 4D-CT due to its non-invasive property and high contrast in soft tissue. Among the various 4D-MRI techniques, 4D diffusion-weighted imaging (4D-DWI) attracts lots of attention since it can offer more functional information and better delineate both the tumor and organs at risk. To date, only one study explored and verified the feasibility of 4D-DWI with single-shot echo-planar imaging (ss-EPI), termed 4D-DW-ss-EPI. However, the reconstructed images suffer from geometric distortions due to strong susceptibility effects and blurring artefacts due to T2* decay and large amplitude intervals for each bin. This thesis aims to develop advanced 4D-DWI techniques for solving the drawbacks of 4D-DW-ss-EPI step by step using ss-EPI data acquired from PROPELLER-EPI blades. Three novel 4D-DWI techniques were proposed and referred to as 4D-DW-PROPELLER-EPI, 4D-DW-SCOPER-EPI, and 4D-DW-MB-SCOPER-EPI, respectively. The first technique,4D-DW-PROPELLER-EPI, unaliases each blade and then uses conventional PROPELLER-EPI reconstruction to achieve distortion-free images. The second technique, 4D-DW-SCOPER-EPI, applies an alternative reconstruction method, called self-calibrated and collaborative PROPELLER-EPI reconstruction (SCOPER), to improve the acquisition efficiency of the first technique, thereby offering a way of reducing acquisition time with a limited SNR tolerance. It combines blades into a large single matrix, sets up a signal equation, and solves the image by an iterative method. The last technique, 4D-DW-MB-SCOPER-EPI, employs the multi-band (MB) technique to further reduce the acquisition time without the undersampling-related ratio of signal-to-noise (SNR) penalty. Simulation and in vivo experiments were both performed to test the proposed 4D-DWI techniques except 4D-DW-MB-SCOPER-EPI, which is only tested by in vivo experiments. Computer-aid evaluations and virtual assessments were performed to evaluate the results. The results show that 4D-DW-PROPELLER-EPI can solve the problem of geometric distortions, but the reconstructed images still suffer from blurring artefacts due to slight residual off-resonance artefacts. Moreover, 18-min data acquisition is required for the coverage of 160 mm in the SI direction, which is hard for patients in poor conditions and children. 4D-DW-SCOPER-EPI can improve acquisition efficiency and the acquisition time can be further reduced by 22% compared with 4D-DW-PROPELLER-EPI. 4D-DW-MB-SCOPER-EPI can further reduce the acquisition time to 50% compared with 4D-DW-SCOPER-EPI, whereas the SNR loss caused by slice separation needs to be improved further. For the three proposed techniques, the shorter the acquisition time is, the more complex the post-processing is. In conclusion, the thesis proposed the three in-depth 4D-DWI techniques, which may help clinical abdominal radiotherapy.