Optimized and motion-compensated asymmetric spin echo acquisition methods for functional magnetic resonance imaging [electronic resource]
Keywords: Electrical Engineering.

Asymmetric spin echo (ASE) pulse sequences can be an excellent alternative in BOLD fMRI to combine the advantages of gradient echo (GRE) and spin echo (SE) pulse sequences which are challenged by either limited spatial specificity or functional sensitivity. However, ASE pulse sequences have been poorly understood and utilized in the context of an imaging system and they are highly susceptible to motion. ASE BOLD contrast is complexly related to intrinsic and extrinsic parameters. The theoretical derivation of the transverse magnetization and BOLD contrast and simulations with various intrinsic and extrinsic parameters, primarily determined by the blood vessel size and pulse sequence design, lead to an insightful understanding of the ASE BOLD contrast in promoting the advancement of ASE pulse sequences. ASE pulse sequences can be optimized by modulating TE and Echoshift that are highly dependent on T2- and T2'- weightings, in pursuit of acquiring high functional sensitivity, maintaining high spatial specificity, and recovering tSNR and functional activation in the regions of high susceptibility-induced field gradients. In-vivo experiments involving sensory and breath hold tasks demonstrate the effectiveness of the optimized ASE pulse sequences. For fMRI data analysis, spatial specificity is properly quantified by a functional resolution defined as a voxel width where an autocorrelation function of each slice reaches 70% of its maximum value or above. In our study, we find ASE pulse sequences provide higher spatial specificity compared to GRE pulse sequences, while SE pulse sequences compromise functional sensitivity for the sake of highest spatial specificity. Comparing the functional sensitivity among various imaging methods requires careful consideration while taking into account the intrinsic differences in functional resolutions. Thus, by equalizing functional resolutions a proper comparison of functional sensitivity among ASE, GRE, and SE methods can be made. Without equalizing functional resolutions, ASE pulse sequences are optimized with parameters TE = 65 ms and Echoshift = -40 ms providing enhanced functional sensitivity that behaves like GRE; yet with equalized functional resolutions, the optimal ASE approaches close to SE, preserving spatially localized components. Another challenge that ASE and SE pulse sequences encounter is in the significant reduction of SNR and tSNR affected by motion. In that regard, motion- compensated ASE pulse sequences can be flexibly designed by employing additional linear field gradients and compensating physical and physiological translational motion along the z axis that occurs between two RF pulses. Phantom experiments under motion- controlled environments and in-vivo experiments validate the improvement of SNR and tSNR resulting in enhanced functional activation in ASE BOLD fMRI compared to conventional ASE BOLD fMRI. In summary, the research presented in this thesis has contributed to improving ASE pulse sequences for high functional sensitivity and spatial specificity with motion- robustness in fMRI that can ultimately enhance brain functional mapping.