Simultaneous mapping of T1and B1+ in human brain at 7T

PurposeTo map T1 and the local flip angle (B1+) in human brain using a single MP3RAGE sequence with 3 rapid acquisitions of gradient echoes (RAGEs).Theory and methodsA third RAGE with a relatively high flip angle was appended to an MP2RAGE sequence. Through curve fitting and a rational approximation for small flip angles and short TR, closed form solutions for T1 and B1+ were derived. The influence of different k-space encoding schemes on precision and whether edge enhancement artifacts could be reduced with a saturation pulse applied prior to the third RAGE were explored. Validation of T1 estimates was performed using single-slice inversion recovery (IR) and a subsequent region-of-interest–based comparison, whereas validation of B1+ was performed using a whole brain pixelwise comparison to a DREAM flip angle mapping protocol. Lastly, MP3RAGE was compared to T1-mapping by MP2RAGE with separate B1+ correction.ResultsWhole brain maps of T1 and B1+ at 1 mm isotropic resolution were obtained with MP3RAGE in 06:37 min. A linear–reverse centric–reverse centric phase-encoding order of the 3 RAGEs improved precision, and artifacts were successfully reduced with the saturation pulse. Estimations of T1 and B1+ deviated +2.5 ± 3.1% and −1.7 ± 8.6% from their respective references.ConclusionT1 and B1+ can be mapped simultaneously using MP3RAGE. The approach can be thought of as combining MP2RAGE with a dual flip angle T1-mapping protocol. Both maps can be solved for analytically and will be inherently co-registered at the high resolution associated with MPRAGE.