Quantitative T1 and Effective Proton Density (PD*) mapping in children and adults at 7T from an MP2RAGE sequence optimised for uniform T1-weighted (UNI) and FLuid And White matter Suppression (FLAWS) contrasts

Abstract

Introduction Quantitative MRI is important for non-invasive tissue characterisation. In previous work we developed a clinically feasible multi-contrast protocol for T₁-weighted imaging based on the MP2RAGE sequence that was optimised for both children and adults. It was demonstrated that a range of Fluid And White Matter Suppression (FLAWS) related contrasts could be produced while maintaining T₁-weighted uniform image (UNI) quality, a challenge at higher field strengths. Here we introduce an approach to use these images to calculate effective proton density (PD^*) and quantitative T₁ relaxation maps especially for shorter repetition times (TR_MP2RAGE) than those typically used previously. Methods T₁ and PD^* were estimated from the analytical equations of the MP2RAGE signal derived for partial Fourier acquisitions. The sensitivity of the fitting results was evaluated with respect to the TR_MP2RAGE and B₁⁺ effects on both excitation flip angles and inversion efficiency and compared to vendor T₁ maps which do not use B₁⁺ information. Data acquired for a range of individuals (aged 10-54 years) at the shortest TR_MP2RAGE (4000ms) were compared across white matter (WM), cortical grey matter, and deep grey matter regions. Results The T₁ values were insensitive to the choice of different TR_MP2RAGE. The results were similar to the vendor T₁ maps if the B₁⁺ effects on the excitation flip angle and inversion efficiency were not included in the fits. T₁ values varied over development into adulthood, especially for the deep grey matter regions whereas only a very small difference was observed for WM T₁. Effective PD maps were produced which did not show a significant difference between children and adults for the age range included. Conclusion We produced PD^* maps and improved the accuracy of T₁ maps from an MP2RAGE protocol that is optimised for UNI and FLAWS-related contrasts in a single scan at 7T by incorporating the excitation flip angle and inversion efficiency related effects of B₁⁺ in the fitting. This multi-parametric protocol made it possible to acquire high resolution images (0.65mm iso) in children and adults within a clinically feasible duration (7:18 min:s). The combination of analytical equations utilizing B₁⁺ maps led to T₁ fits that were consistent at different TR_MP2RAGE values. Average WM T₁ values of adults and children were very similar (1092ms vs 1117ms) while expected reductions in T₁ with age were found for GM especially for deep GM.