Multiband accelerated 2D EPI for multi‐echo brain QSM at 3 T

Abstract Purpose Data for QSM are typically acquired using multi‐echo 3D gradient echo (GRE), but EPI can be used to accelerate QSM and provide shorter acquisition times. So far, EPI‐QSM has been limited to single‐echo acquisitions, which, for 3D GRE, are known to be less accurate than multi‐echo sequences. Therefore, we compared single‐echo and multi‐echo EPI‐QSM reconstructions across a range of parallel imaging and multiband acceleration factors. Methods Using 2D single‐shot EPI in the brain, we compared QSM from single‐echo and multi‐echo acquisitions across combined parallel‐imaging and multiband acceleration factors ranging from 2 to 16, with volume pulse TRs from 21.7 to 3.2 s, respectively. For single‐echo versus multi‐echo reconstructions, we investigated the effect of acceleration factors on regional susceptibility values, temporal noise, and image quality. We introduce a novel masking method based on thresholding the magnitude of the local field gradients to improve brain masking in challenging regions. Results At 1.6‐mm isotropic resolution, high‐quality QSM was achieved using multi‐echo 2D EPI with a combined acceleration factor of 16 and a TR of 3.2 s, which enables functional applications. With these high acceleration factors, single‐echo reconstructions are inaccurate and artefacted, rendering them unusable. Multi‐echo acquisitions greatly improve QSM quality, particularly at higher acceleration factors, provide more consistent regional susceptibility values across acceleration factors, and decrease temporal noise compared with single‐echo QSM reconstructions. Conclusion Multi‐echo acquisition is more robust for EPI‐QSM across parallel imaging and multiband acceleration factors than single‐echo acquisition. Multi‐echo EPI can be used for highly accelerated acquisition while preserving QSM accuracy and quality relative to gold‐standard 3D‐GRE QSM.