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

Purpose
To 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 methods
A 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.

Results
Whole 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.

Conclusion
T₁ 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.