Feasibility of tagged MRI at 0.55 T

To determine the feasibility of tagged MRI at 0.55 T and tag persistence for cardiac and speech production applications. This is particularly challenging due to the short muscle T₁ at 0.55 T (701 ms) and requires advanced techniques for maximizing initial tag contrast and signal-to-noise ratio (SNR)-efficient imaging.

Tagged MRI was performed using phase-sensitive tagging (REALTAG) two-dimensional grid-based preparation and spiral balanced steady-state free precession (bSSFP) imaging. Experiments were performed in healthy adult volunteers using a 0.55 T MRI system with high-performance gradients on two functional anatomies. Cardiac scans had 7.5-mm tag spacing, 1.3-mm² in-plane resolution, and were acquired during about 18s breath-holds. Speech scans had 10-mm tag spacing, 2-mm² spatial resolution, and were acquired in real time. Resulting tagged images were compared with conventional gradient-recalled echo (GRE). Tag persistence and tag contrast-to-noise ratio were measured and compared.

REALTAG preparation and phase-sensitive reconstruction resulted in taglines that recover quickly when compared to 1.5 T, for both bSSFP and GRE imaging approaches. The bSSFP approach, at optimal flip angles, provided a 1.35-fold improved SNR and 1.6-fold improved tag persistence. For a tag contrast-to-noise ratio threshold of 6, the tag persistence was about 506 ms for GRE and about 851 ms for bSSFP.

Tagged MRI is feasible at 0.55 T and can be used for functional analysis of muscle motion such as cardiac and speech applications, despite the short T₁. Spiral bSSFP along with REALTAG preparation pulses provide adequate SNR and contrast to visualize cardiac systolic mechanics and to visualize tongue deformation during short speech utterances.