Improving B 1 + $$ {B}_1^{+} $$ -inhomogeneity tolerance by resolving non-bijection in MP2RAGE R1 mapping: A 2D look-up table approach demonstrated at 3 T.

Abstract

Purpose: 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 $B_1^{+}$ -inhomogeneity tolerance without other prerequisites.

Theory: 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 $B_1^{+}$ -inhomogeneity tolerance. A 2D look-up table approach combines the available information to resolve these ambiguities during mapping.

Methods: At 3 T, we acquired MP2RAGE images with standard acquisition parameters and (non-bijective) parameters optimized for $B_1^{+}$ -inhomogeneity tolerance. From 3 subjects across multiple sessions, we assessed the $B_1^{+}$ -inhomogeneity tolerance through excitation-pulse amplitude scalings.

Results: The R₁ maps resulting from the $B_1^{+}$ -optimized protocols showed greatly reduced $B_1^{+}$ effects across images but without additional scanner time. Meanwhile, these maps could only successfully be derived by a 2D look-up table approach.

Conclusion: We show that it is possible to optimize for $B_1^{+}$ -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 $B_1^{+}$ 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.