Variable flip angle T1 mapping for quantitative liver imaging at 0.55 T.

PURPOSE: Investigating the feasibility of variable flip angle (VFA) T1 mapping for quantitative liver imaging at 0.55 T and evaluating the impact of RF spoiling correction and deep learning-based (DL) reconstruction on accuracy. METHODS: A VFA protocol was developed and combined with DL reconstruction to improve quantitative robustness at 0.55 T. Phantom experiments validated T1 estimates against multi-TI inversion-recovery spin-echo reference measurements. Bloch simulations characterized RF spoiling-induced bias and derived 0.55 T-specific calibration coefficients. Twelve healthy subjects were scanned to assess field-strength-dependent effects of B1+ inhomogeneity. RESULTS: In vivo, DL reconstruction reduced noise-related variability of voxel-wise hepatic T1 estimates compared to conventional reconstructions in the same subject. At 0.55 T, B1+ maps showed the highest transmit homogeneity, with liver flip angle variation of 98.2 ± 2.9%, compared with 94.6 ± 7.4% at 1.5 T and 100.7 ± 9.8% at 3 T. Accordingly, B1+ correction had minimal impact on hepatic T1 distributions at 0.55 T, whereas more pronounced effects were observed at higher field strengths. Phantom experiments demonstrated improved agreement of VFA-derived T1 values with the IR-SE reference after 0.55 T-specific RF spoiling correction, supporting the accuracy of the corrected VFA approach. Bland-Altman analysis confirmed that RF spoiling correction had a greater impact on phantom T1 accuracy than B1+ correction. CONCLUSION: At 0.55 T, RF spoiling and reconstruction strategies are the main determinants of quantitative accuracy, while transmit field inhomogeneity plays a minor role. Combined RF spoiling correction and DL reconstruction enables robust VFA T1 mapping without B1+ correction. Reduced low-field T1 values improve VFA model conditioning and reduce sensitivity to flip angle deviations, supporting VFA-based T1 mapping for low-field MRI.