A 3T investigation of B1+-inhomogeneity tolerance in MP2RAGE-based R1-mapping by calculating second contrast that permits previously problematic sequence parameters

Abstract

PurposeRegarding in vivo, robust R1-mapping, the goal of the present paper is twofold. First, to verify that non-bijective mapping in MP2RAGE imaging can be resolved through a 2D look-up-table approach. Second, that the expanded parameter space from this can be used to improve B1+-inhomogeneity tolerance without other prerequisites. TheoryBy 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 optimised, e.g., for higher B1+-inhomogeneity tolerance. A 2D look-up-table approach combines the available information to resolve these ambiguities during mapping. MethodsAt 3T, we acquired MP2RAGE images with standard acquisition parameters and (non-bijective) parameters optimised for B1+-inhomogeneity tolerance. From three subjects across multiple sessions, we assessed the B1+-inhomogeneity tolerance through excitation pulse amplitude scalings. ResultsThe R1-maps resulting from the B1+-optimised protocols showed greatly reduced B1+-effects across images, but without additional scanner time. Meanwhile these maps could only successfully be derived by a 2D look-up-table approach. ConclusionWe show that it is possible to optimise for B1+-inhomogeneity tolerance in MP2RAGE through sequence parameter settings, while still successfully estimating the R1-map with a 2D look-up-table approach. This without the need for an additional B1+-map. The increased parameter space enabled by the 2D look-up-table approach may further be used to adjust MP2RAGE acquisitions for improved scan times, SNR and/or CNR.