EPI Phase Correction revisited - Beat Phenomena and Transient States

Motivation: For EPI, beat phenomena can occur and result in a slowly varying, erroneous phase component caused by mechanical resonances of the gradient system. Current phase correction approaches do not account for this error. Goal(s): Improving EPI correction by characterization of gradient-induced, time-varying errors of zeroth and first spatial order. Approach: EPI image projections of a spherical phantom were acquired by omitting the phase encoding blips. The resulting x-ky-data enable a characterization of zeroth- and first-order effects separately. Results: The mechanical properties of the gradient system result in a transient state during which an erroneous, temporally and spatially modulated magnetic field gradient occurs. Impact: EPI is a widely applied fast imaging technique but sensitive to gradient-induced magnetic field deviations. Therefore, phase correction is of utmost importance and may be improved by the proposed characterization of slowly varying, erroneous phases.