T2 mapping at 0.55 T using Ultra-Fast Spin Echo MRI

Low-field T2 mapping MRI can democratize neuropediatric imaging by improving accessibility and providing quantitative biomarkers of brain development. \textbf{Purpose:} To evaluate the feasibility of high-resolution T2 mapping using a single-shot fast spin-echo (SS-FSE) sequence at 0.55~T in a healthy control cohort. \textbf{Study Type:} Prospective single-center study. \textbf{Population:} In vivo: ten healthy adults (18--43~years, 5 females/5 males). In vitro: NIST Phantom. \textbf{Field strength/sequence:} Multi-echo ultra-fast spin-echo at 0.55~T and 1.5~T. \textbf{Assessment:} Feasibility was first assessed in vitro using the NIST Phantom, comparing T2 relaxation times to spectrometer references at 0.55~T. Acquisition and T2-fitting parameters optimized in vitro were applied in vivo. Repeatability was evaluated by atlas-based analysis of white matter (WM) and cortical grey matter (GM) regions. Coefficients of variation (CoV) were computed across runs, sessions, and subjects. \textbf{Statistical Tests:} Wilcoxon signed-rank test with Bonferroni correction ($\alpha = 0.05/n_{ROI}$) assessed CoV differences. Pearson correlation coefficients quantified T2 associations. \textbf{Results:} In vitro, mono-exponential fitting under Gaussian--Rician noise yielded deviations $<12\%$ from reference values. In vivo, inter-subject CoV was 5.2\% (WM) and 17.7\% (GM), comparable to 1.5~T. Mean T2 times were 118~ms (WM) and 188~ms (GM) at 0.55~T, with a 16.5-minute acquisition. \textbf{Conclusion:} A rapid, robust high-resolution T2 mapping protocol at 0.55~T for HASTE MRI is presented, employing Gaussian noise-based fitting. We report the first normative T2 values for healthy adult brains at 0.55~T, demonstrating technical feasibility and reliability.