Theoretical aspects of motion sensitivity and compensation in echo-planar imaging.

Magnetic resonance (MR) imaging can be performed on or below the time scale of most anatomic motion via echo-planar imaging (EPI) techniques and their derivatives. The goal is to image rapidly and reduce artifacts that typically result from view-to-view changes in the spatial distribution of spins due to motion. However, the required time-dependent magnetic field gradient waveforms remain sensitive to the dephasing effects of motion. Sources of motion artifact are simulated for spins moving along the imaging axes and are shown to be an important source of reduced image quality in EPI. A novel method of EPI is proposed that (a) refocuses single or multiple derivatives of motion at all echoes and (b) prevents accumulation of velocity (or higher derivative)--induced dephasing along the phase-encoding axis by moment nulling all phase-encoding-step waveforms about a single instant of time. Theoretical EPI sequences with considerable reductions in ghosts, blurring, and signal loss due to motion sensitivity are produced and compared with other EPI methods. Their time efficiency is presented as a function of available (relative) gradient strength for a variety of sequence waveforms.