Optimal flip angles for in vivo liver 3DT1 mapping and B1+ mapping at 3T

Abstract

PurposeThe spoiled gradient recalled echo (SPGR) sequence with variable flip angles (FAs) enables whole liver mapping at high spatial resolutions but is strongly affected by inhomogeneities. The aim of this work was to study how the precision of acquired maps is affected by the and ranges observed in the liver at 3T, as well as how noise propagates from the acquired signals into the resulting map.TheoryThe variance was estimated through the Fisher information matrix with a total noise variance including, for the first time, the map noise as well as contributions from the SPGR noise.MethodsSimulations were used to find the optimal FAs for both the mapping and mapping. The simulations results were validated in 10 volunteers.ResultsFour optimized SPGR FAs of 2°, 2°, 15°, and 15° (TR = 4.1 ms) and map FAs of 65° and 130° achieved a 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 and parameter space at 3T, a higher return in precision was obtained by investing FAs in the SPGR acquisition rather than in the map.ConclusionA novel framework was developed and validated to calculate the SPGR variance. This framework efficiently identifies optimal FA values and determines the total number of SPGR and measurements needed to achieve a desired precision.