Spatially resolved extended phase graphs: modeling and design of multipulse sequences with parallel transmission.

A spatially resolved extended phase graph (SR-EPG) framework is proposed for prediction of echo amplitudes in the presence of spatially variable radio frequency (RF) fields. The method may be used to examine any regularly repeating pulse sequence and provides a design framework for parallel transmission (PTx) systems; in this work signal homogeneity in static pseudo-steady state (SPSS) turbo spin echo (TSE) imaging was investigated. Building on SR-EPG calculations with PTx, a dynamic RF-shimming approach is proposed in which, RF pulse amplitudes and phases are optimized on a per channel and per pulse basis to yield the desired signal response for all echoes. Results show significant improvements over "static" RF shimming (in which the relative amplitude/phase of the PTx channels are fixed for all pulses). SPSS-TSE imaging using dynamic RF shimming resulted in excellent image quality, both in phantoms and in vivo, and confirmed SR-EPG predictions.