Parallel imaging with asymmetric acceleration to reduce Gibbs artifacts and to increase signal-to-noise ratio of the...

Parallel imaging with accelerated acquisition was noted to pronounce Gibbs artifacts which appear as ripples propagated in the phase-encoding (PE) direction near the susceptibility-affected region in echo-planar imaging (EPI). Using the extended EPI sequence, which collected extended readouts outside the regular data sampling time, the pronounced Gibbs artifact was analyzed and found to be caused by an increased echo shift in the pre-echo time (T(E)) of accelerated parallel imaging. This was also confirmed by theoretical derivation of the echo shift caused by the inplane susceptibility gradient in the PE direction (ISG(PE)). A new EPI sequence was developed to reduce the Gibbs artifact and to restore the signal level toward that of nonaccelerated parallel imaging by asymmetrically accelerating only the post-T(E) sampling time and by using the extended EPI in the pre-T(E). The nonaccelerated portion in the pre-T(E) used the delay for the optimum blood oxygen level dependent (BOLD) sensitivity at 3 T, maintaining the same slice coverage as the symmetrical acceleration in both pre-T(E) and post-T(E). The increased data sampling points resulted in an increase of the signal-to-noise ratio (SNR). The restored signal and enhanced SNR of the proposed method were confirmed to deliver a better BOLD functional MRI (fMRI) result in the breath holding experiment.