Dual-Echo bSSFP for Rapid T2-Sensitive Quantitative Contrast Imaging at Ultra-Low Field.

OBJECTIVE: Rapid characterization of transverse-relaxation-sensitive MRI contrast is important for evaluating tissue-dependent signal behavior, but remains challenging in ultra-low-field (ULF) MRI because of limited signal-to-noise ratio (SNR) and acquisition-efficiency constraints. This study aims to develop a rapid, high-SNR, sequence-specific T2-sensitive quantitative contrast imaging method for ULF MRI. METHODS: A balanced dual-echo steady-state (bDESS) sequence was developed to acquire two echoes at predefined echo times within each repetition of a balanced steady-state free precession acquisition. A logarithmic-ratio operator, based on a mono-exponential attenuation approximation, was used to derive a sequence-specific T2-sensitive contrast index, termed T2SDI, from the two echo magnitudes. The proposed method was implemented on a custom-built 6.5 mT MRI system and evaluated using numerical simulations, CuSO₄ phantom experiments, and in vivo brain imaging. RESULTS: Numerical simulations showed that T2SDI exhibited a monotonic and approximately linear dependence on T2 under controlled field-inhomogeneity conditions. Phantom experiments confirmed that bDESS-derived T2SDI increased with CPMG-measured reference T2 and showed higher SNR than dual-echo SPGR. The method was further demonstrated in vivo by generating T2SDI maps of the human brain. CONCLUSION: This study presents a sequence-specific, index-based method for rapid T2-sensitive quantitative contrast imaging in ULF MRI. SIGNIFICANCE: The proposed dual-echo bSSFP/bDESS method provides a high-SNR and time-efficient strategy for sequence-specific T2-sensitive contrast characterization at ultra-low field.