Short gradient coil design and peripheral nerve stimulation properties.

摘要

The technological and physiological limits of magnetic resonance imaging (MRI) gradient coils are addressed in this thesis. The overall goal is to find relationships between coil technical performances, physical dimensions, and peripheral nerve stimulation (PNS) properties. This goal is achieved in three steps. The first step explores the design guideline for minimum-length optimal-performance coils given any specific imaging application. The second step examines PNS properties of gradient coils of various sizes using experimental and theoretical approaches. The last step is the optimization of gradient waveforms for various coils subject to their PNS constraints.;A scaling law was discovered that the shortest length of a maximum-performance gradient coil is a simple linear function of coil diameter and diameter of homogeneous gradient spherical volume (DSV). Given any imaging application, this simple guideline incorporated with the shortest length formula for minimum-cost homogeneous magnets enables easy estimation and adjustment of optimized and matched MRI systems.;A human volunteer study on three different body-size coils and one head-size coil was performed. It was found that the average measured stimulation threshold increases while DSV decreases: PNS parameters are linear functions of DSV. Moreover, chronaxie, a parameter commonly thought of as a single-value, nerve-specific constant, was discovered to vary inversely linearly with DSV. A modified linear model was developed accordingly, which may permit the prediction of PNS properties for gradient coils of arbitrary DSV.;Different analytical approaches were used to calculate the induced electric field pattern of each coil used for the experimental study. The results were correlated with the experimental observations in order to develop linear models which offered possible physical interpretation of the experimental data. More sophisticated electric field calculations and experiments would be necessary for a more fundamental understanding of the experimental outcomes.;A new method was developed for optimal readout gradient design of two commonly used fast imaging sequences: Echo-Planar-Imaging and Steady-State Free-Precession sequences, for various sampling methods. For various coils, minimum echo spacing values subject to PNS constraints and the corresponding gradient sequence parameters were compared against the values achievable subject only to hardware limitations over the typical clinical spatial resolution range.