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. $\mathbf{\text{Purpose:}}$ 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. $\mathbf{\text{Study}}\mathbf{\text{Type:}}$ Prospective single-center study. $\mathbf{\text{Population:}}$ In vivo: ten healthy adults (18–43~years, 5 females/5 males). In vitro: NIST Phantom. $\mathbf{\text{Field}}\mathbf{\text{strength/sequence:}}$ Multi-echo ultra-fast spin-echo at 0.55~T and 1.5~T. $\mathbf{\text{Assessment:}}$ 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. $\mathbf{\text{Statistical}}\mathbf{\text{Tests:}}$ Wilcoxon signed-rank test with Bonferroni correction ($\alpha =0.05\mathrm{/}{n}_{ROI}$) assessed CoV differences. Pearson correlation coefficients quantified T2 associations. $\mathbf{\text{Results:}}$ In vitro, mono-exponential fitting under Gaussian–Rician noise yielded deviations $<12\mathrm{\%}$ 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. $\mathbf{\text{Conclusion:}}$ 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.