Comparison of fMRI with Accelerated Variable Density Spiral and EPI

Introduction In functional MRI, spiral trajectory has been proposed to reduce the signal dropout in the frontal orbital and lateral parietal region affected by the susceptibility-induced field gradient (SFG) [1-2]. With a spiral-in trajectory, the entire k-space except the origin is sampled before the prescribed echo time TE. This reduces T2* weighting and enhances the BOLD contrast in regions affected by the SFG. However, the spatial resolution that can be achieved with a spiral-in trajectory is limited by the need to sample the entire k-space before TE. In addition, the spatial resolution of the activation map in the SFG region may be compromised due to the spatial blurring introduced by the combination of a large B0 offset and a long readout window. Parallel imaging techniques can be used to undersample a spiral trajectory, therefore improving the spatial resolution of spiral fMRI. However, most existing spiral parallel imaging methods [3-5] require either a long computation time due to their iterative nature, or have difficulty working with variable density (VD) spiral datasets where spacing between adjacent spiral lines vary continuously across the k-space. In this work, a recently proposed rapid k-space-based parallel imaging method for VD spiral, Generalized GRAPPA for wider spiral bands (GROWL) is applied to fMRI for reduced off-resonance artifacts and potentially higher temporal/spatial resolution and larger volume coverage.