Optimal flip angles for in vivo liver 3D T 1 mapping and B 1+ mapping at 3T
Gabriela Belsley, Damian J. Tyler, Matthew D. Robson, Elizabeth M. Tunnicliffe
Abstract
Purpose
The spoiled gradient recalled echo (SPGR) sequence with variable flip angles (FAs) enables whole liver T1 mapping at high spatial resolutions but is strongly affected by B1+ inhomogeneities. The aim of this work was to study how the precision of acquired T1 maps is affected by the T1 and B1+ ranges observed in the liver at 3T, as well as how noise propagates from the acquired signals into the resulting T1 map.
Theory
The T1 variance was estimated through the Fisher information matrix with a total noise variance including, for the first time, the B1+ map noise as well as contributions from the SPGR noise.
Methods
Simulations were used to find the optimal FAs for both the B1+ mapping and T1 mapping. The simulations results were validated in 10 volunteers.
Results
Four optimized SPGR FAs of 2°, 2°, 15°, and 15° (TR = 4.1 ms) and B1+ map FAs of 65° and 130° achieved a T1 coefficient of variation of 6.2 ± 1.7% across 10 volunteers and validated our theoretical model. Four optimal FAs outperformed five uniformly spaced FAs, saving the patient one breath-hold. For the liver B1+ and T1 parameter space at 3T, a higher return in T1 precision was obtained by investing FAs in the SPGR acquisition rather than in the B1+ map.
Conclusion
A novel framework was developed and validated to calculate the SPGR T1 variance. This framework efficiently identifies optimal FA values and determines the total number of SPGR and B1+ measurements needed to achieve a desired T1 precision.