Real-time Correction of Respiration-Induced B0 Fluctuations in Oscillating Steady State fMRI using FID Navigators (FIDNavs)

Motivation: Signal fluctuations due to respiration negatively impact functional contrast for OSSI steady-state fMRI. Goal(s): The objective of our study is to develop a method for compensating for these changes prospectively. Approach: We modified a 2D OSSI sequence for real-time processing, employing FIDNavs to monitor respiration-induced B0 fluctuations. These B0 fluctuations are estimated on a per-shot basis accounting for shot-dependent phase variations introduced by eddy currents. The estimated B0 change is then used to adjust the periodic phase cycling (excitation and receive) to remove signal fluctuations. Results: Our approach resulted in an increased number of active voxels, increased tSNR, and enhanced functional contrast. Impact: This study proposes a robust zeroth-order (bulk) prospective correction method for OSSI, a phase-cycled steady-state fMRI sequence. Our approach is effective in reducing B0-induced signal changes, allowing realization of theoretical SNR gains in complex steady-state MRI sequences.