  We introduce wave encoded acquisition and reconstruction techniques for
highly accelerated echo planar imaging (EPI) with reduced g-factor penalty and
image artifacts. Wave-EPI involves playing sinusoidal gradients during the EPI
readout while employing interslice shifts as in blipped-CAIPI acquisitions.
This spreads the aliasing in all spatial directions, thereby taking better
advantage of 3D coil sensitivity profiles. The amount of voxel spreading that
can be achieved by the wave gradients during the short EPI readout period is
constrained by the slew rate of the gradient coils and peripheral nerve
stimulation (PNS) monitor. We propose to use a half-cycle sinusoidal gradient
to increase the amount of voxel spreading that can be achieved while respecting
the slew and stimulation constraints. Extending wave-EPI to multi-shot
acquisition minimizes geometric distortion and voxel blurring at high in-plane
resolution, while structured low-rank regularization mitigates shot-to-shot
phase variations without additional navigators. We propose to use different
point spread functions (PSFs) for the k-space lines with positive and negative
polarities, which are calibrated with a FLEET-based reference scan and allow
for addressing gradient imperfections. Wave-EPI provided whole-brain
single-shot gradient echo (GE) and multi-shot spin echo (SE) EPI acquisitions
at high acceleration factors and was combined with g-Slider slab encoding to
boost the SNR level in 1mm isotropic diffusion imaging. Relative to
blipped-CAIPI, wave-EPI reduced average and maximum g-factors by up to 1.21-
and 1.37-fold, respectively. In conclusion, wave-EPI allows highly accelerated
single- and multi-shot EPI with reduced g-factor and artifacts and may
facilitate clinical and neuroscientific applications of EPI by improving the
spatial and temporal resolution in functional and diffusion imaging.
