Improving -inhomogeneity tolerance by resolving non-bijection in MP2RAGE R1 mapping: A 2D look-up table approach demonstrated at 3 T.

Abstract

Regarding in vivo, robust longitudinal relaxation rate (R1) mapping, the goal of the present paper is two-fold. First, to verify that non-bijective mapping in magnetization-prepared 2 rapid gradient echo (MP2RAGE) imaging can be resolved through a two-dimensional look-up table approach. Second, that the expanded parameter space from this can be used to improve -inhomogeneity tolerance without other prerequisites. By deriving a second contrast from the magnitude images of the MP2RAGE acquisition, ambiguities in the original MP2RAGE image resulting from non-bijective transfer curves can be resolved. Such ambiguities may occur when protocols are optimized, such as for higher -inhomogeneity tolerance. A 2D look-up table approach combines the available information to resolve these ambiguities during mapping. At 3 T, we acquired MP2RAGE images with standard acquisition parameters and (non-bijective) parameters optimized for -inhomogeneity tolerance. From 3 subjects across multiple sessions, we assessed the -inhomogeneity tolerance through excitation-pulse amplitude scalings. The R1 maps resulting from the -optimized protocols showed greatly reduced effects across images but without additional scanner time. Meanwhile, these maps could only successfully be derived by a 2D look-up table approach. We show that it is possible to optimize for -inhomogeneity tolerance in MP2RAGE through sequence-parameter settings, while still successfully estimating the R1 map with a two-dimensional look-up table approach. This without the need for an additional map. The increased parameter space enabled by the two-dimensional look-up table approach may further be used to adjust MP2RAGE acquisitions for improved scan times, signal-to-noise ratio, and/or contrast-to-noise ratio.