Toward faster and quieter MRI

This thesis represents a study of partially parallel imaging and active passive shielding with the goal of making MRI faster and quieter. We present the design and optimization of rectangular surface rf coils used for partially parallel imaging. The effect of passive copper shielding applied to the outside of an actively shielded gradient is studied with the aim of substantially reducing induced eddy currents in the cryostat inner bore. A ladder rf coil design with all-neighbors coupling included is considered. First, it is shown that, due to far-neighbor couplings, exact decoupling and degeneracy of normal modes are impossible. A technique for minimizing the frequency spread by appropriate choice of capacitors is investigated. A special surface coil design with an additional "parasitic" loop is considered. This coil is shown to have four exact degenerate normal modes. The theoretical predictions are compared with experimental results. A g-factor calculation of two and four rectangular surface coils is considered. It is shown that g-factors can be minimized by changing the relative orientation of the coils. A new regularization technique for SENSE image reconstruction is considered. This technique requires no prior knowledge of the likelihood of the image. Our numerical simulations applied to simulated data acquired with a four-element phased array coils and reduction factor (R = 4) show that there is a reasonable trade-off between signal and aliasing for a given weighting factor. Two configurations of passive shielding are analyzed. The first uses a cylindrical copper cover applied to the outside gradient assembly circumference, and the second extends the copper over the ends of the gradient assembly. For a z-gradient, a 2mm thick copper layer on the gradient circumference reduces the power deposited in the cryostat inner bore by 26.7dB for 1kHz harmonic gradient excitation. Extending the passive shield to cover the ends of the gradient reduces cryostat inner bore power deposition by more than 22dB for the same frequency. For a transverse gradient a finite element calculation shows that a 2mm copper shield without end caps reduces the eddy currents on the cryostat inner bore by a factor of 30 at the same frequency.