  Purpose: Echo-planar imaging (EPI) with blip-up/down acquisition (BUDA) can
provide high-quality images with minimal distortions by using two readout
trains with opposing phase-encoding gradients. Because of the need for two
separate acquisitions, BUDA doubles the scan time and degrades the temporal
resolution when compared to single-shot EPI, presenting a major challenge for
many applications, particularly functional MRI (fMRI). This study aims at
overcoming this challenge by developing an echo-shifted EPI BUDA (esEPI-BUDA)
technique to acquire both blip-up and blip-down datasets in a single shot.
Methods: A three-dimensional (3D) esEPI-BUDA pulse sequence was designed by
using an echo-shifting strategy to produce two EPI readout trains. These
readout trains produced a pair of k-space datasets whose k-space trajectories
were interleaved with opposite phase-encoding gradient directions. The two
k-space datasets were separately reconstructed using a 3D SENSE algorithm, from
which time-resolved B0-field maps were derived using TOPUP in FSL and then
input into a forward model of joint parallel imaging reconstruction to correct
for geometric distortion. In addition, Hankel structured low-rank constraint
was incorporated into the reconstruction framework to improve image quality by
mitigating the phase errors between the two interleaved k-space datasets.
Results: The 3D esEPI-BUDA technique was demonstrated in a phantom and an fMRI
study on healthy human subjects. Geometric distortions were effectively
corrected in both phantom and human brain images. In the fMRI study, the visual
activation volumes and their BOLD responses were comparable to those from
conventional 3D echo-planar images. Conclusion: The improved imaging efficiency
and dynamic distortion correction capability afforded by 3D esEPI-BUDA are
expected to benefit many EPI applications.
