Interleaved snapshot echo planar imaging of mouse brain at 7.0 T

Single-shot echo planar imaging (EPI) of a mouse brain at high field is very challenging. Large susceptibility-induced gradients affect much of the brain volume, causing severe image deformations and signal loss. Segmented EPI and other conventional multi-shot approaches alleviate these problems but suffer from lower temporal resolution and motion artifacts. We demonstrate that interleaved snapshot EPI represents a simple and robust alternative approach and one that is particularly suitable for high-field T2*-weighted functional imaging of a mouse brain. Similarly to segmented multi-shot techniques, it significantly reduces the susceptibility-related artifacts. At the same time, it preserves the high temporal resolution and the snapshot capability of a conventional EPI by acquiring entire image within a single TR period. We discuss implementation details of the interleaved snapshot EPI sequence and the trade-offs involved between the imaging efficiency, the number of interleaved excitation-acquisition blocks and the artifact reduction. To document the sequence utility, murine brain in vivo imaging with the interleaved snapshot EPI method was compared with a conventional EPI. We found that at least five interleaved blocks were necessary to restore the signal in most cortical areas. We also show that a standard global shimming procedure provides sufficient homogeneity for multi-slice interleaved snapshot EPI acquisition. In contrast, the conventional EPI of comparable image quality would be limited to a single slice with highly optimized local shim. Finally, an in vitro comparison with turbo FLASH acquisition shows the interleaved snapshot EPI to have superior time resolution and signal-to-noise ratio.