Regularized Estimation of Main and RF Field Inhomogeneity and Longitudinal Relaxation Rate in Magnetic Resonance Imaging.

Field Mapping;Magnetic Resonance Imaging;Penalized-likelihood Estimation;B1 Mapping;Electrical Engineering;Engineering;Electrical Engineering: Systems

Funai, Amanda K.Scott, Clayton D. ;

University of Michigan

关键词: Field Mapping;    Magnetic Resonance Imaging;    Penalized-likelihood Estimation;    B1 Mapping;    Electrical Engineering;    Engineering;    Electrical Engineering: Systems;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/86473/afunai_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In designing pulses and algorithms for magnetic resonance imaging, several simplifications to the Bloch equation are used. However, as magnetic resonance (MR) imaging requires higher temporal resolution and faster pulses are used, simplifications such as uniform main field (B0) strength and uniform radio-frequency (RF) transmit coil field (B1) strength no longer apply. Ignoring these non-uniformities can cause significant distortions. Accurate maps of the main and RF transmit coil field inhomogeneity are required for accurate pulse design and imaging.Standard estimation methods yield noisy maps, particularly in image regions having low spin density, and ignore other important factors, such as slice selection effects in B1 mapping and T2 effects in B0 mapping. This thesis uses more accurate signal models for the MR scans to derive iterative regularized estimators that show improvements over the conventional unregularized methods through Cramer-Rao Bound analysis, simulations, and real MR data.In fast MR imaging with long readout times, field inhomogeneity causes image distortion and blurring. This thesis first describes regularized methods for estimation of the off-resonance frequency at each voxel from two or more MR scans having different echo times, using algorithms that decrease monotonically a regularized least-squares cost function.A second challenge is that RF transmit coils produce non-uniform field strengths, so an excitation pulse will produce tip angles that vary substantially over the field of view. Thisthesis secondly describes a regularized method for B1 map estimation for each coil and for two or more tip angles. Using these scans and known slice profile, the iterative algorithm estimates both the magnitude and phase of each coil’s B1 map.To circumvent the challenge in conventional B1 mapping sequences of an long repetition time, this thesis thirdly describes a regularized method for joint B1 and T1 map estimation using a regularized method based on a penalized-likelihood cost function using the steady-state incoherent (SSI) imaging sequence with several scans with varying tip angles or repetition times.