Undersampled Radial STEAM MRI: Methodological Developments and Applications
by Andreas Merrem
Date of Examination:2018-03-05
Date of issue:2018-03-09
Advisor:Prof. Dr. Jens Frahm
Referee:Prof. Dr. Jens Frahm
Referee:Prof. Dr. Marina Bennati
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Abstract
English
The diffusion-weighted (DW) single-shot stimulated echo acquisition mode (STEAM) sequence offers DW MRI without susceptibility artifacts which severely hamper diagnostic applications when using conventional echo-planar imaging techniques. This thesis describes the development and in vivo application of novel DW STEAM MRI techniques which overcome previous deficiencies and, in particular, improve the signal-to-noise ratio. This was achieved using undersampled radial k-space trajectories and iterative image reconstruction methods. As a basic technique for data acquisition, a radial single-shot STEAM sequence with an improved suppression of image artifacts was developed and applied to cardiac black-blood MRI. For DW imaging of the brain, this sequence was extended by a DW spin echo and combined with a nonlinear inverse image reconstruction algorithm including a new regularization method for the coil sensitivities. Studies of healthy volunteers and patients yielded good diagnostic image quality and spatial resolution within clinically feasible imaging times. The first application of a DW single-shot STEAM sequence to prostate MRI was achieved using multi-shot radial data acquisition and iterative image reconstruction with a new regularization of motion-associated phase maps. The measurement of the apparent diffusion coefficient was validated using numerical simulations and in vivo experiments which showed an improved accuracy compared to previous reconstruction methods. MRI studies of healthy volunteers indicated a strong potential for diagnostic DW imaging even near the boundary of the prostate where echo-planar imaging suffers from severe image distortions. Additionally, a new method for DW STEAM MRI with inner-volume excitation of multiple sections was developed and applied to the prostate. Thereby, artifacts originating from abdominal tissues were successfully removed.
Keywords: magnetic resonance imaging; iterative image reconstruction; diffusion-weighted MRI; brain; prostate; stimulated echo; STEAM MRI; parallel imaging; multi-shot acquisition