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

IntroductionQuantitative 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 (TRMP2RAGE) than those typically used previously. MethodsT1 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 TRMP2RAGE 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 TRMP2RAGE (4000ms) were compared across white matter (WM), cortical grey matter, and deep grey matter regions. ResultsThe T1 values were insensitive to the choice of different TRMP2RAGE. 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. ConclusionWe 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 TRMP2RAGE 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.