Data-Driven Retrospective Correction of B1 Field Inhomogeneity in Fast Macromolecular Proton Fraction and R1 Mapping

Correction of
${B}_{1}$
field non-uniformity is critical for many quantitative MRI methods including variable flip angle (VFA)
${T}_{1}$
mapping and single-point macromolecular proton fraction (MPF) mapping. The latter method showed promising results as a fast and robust quantitative myelin imaging approach and involves VFA-based
${R}_{1}=1/{T}_{1}$
map reconstruction as an intermediate processing step. The need for
${B}_{1}$
correction restricts applications of the above methods, since
${B}_{1}$
mapping sequences increase the examination time and are not commonly available in clinics. A new algorithm was developed to enable retrospective data-driven simultaneous
${B}_{1}$
correction in VFA
${R}_{1}$
and single-point MPF mapping. The principle of the algorithm is based on different mathematical dependences of
${B}_{1}$
-related errors in
${R}_{1}$
and MPF allowing extraction of a surrogate
${B}_{1}$
field map from uncorrected
${R}_{1}$
and MPF maps. To validate the method, whole-brain
${R}_{1}$
and MPF maps with isotropic 1.25 mm
³
resolution were obtained on a 3 T MRI scanner from 11 volunteers. Mean parameter values in segmented brain tissues were compared between three reconstruction options including the absence of correction, actual
${B}_{1}$
correction, and surrogate
${B}_{1}$
correction. Surrogate
${B}_{1}$
maps closely reproduced actual patterns of
${B}_{1}$
inhomogeneity. Without correction,
${B}_{1}$
non-uniformity caused highly significant biases in
${R}_{1}$
and MPF (
${P} < 0.001$
). Surrogate
${B}_{1}$
field correction reduced the biases in both
${R}_{1}$
and MPF to a non-significant level (
$0.1\le {P}\le 0.8$
). The described algorithm obviates the use of dedicated
${B}_{1}$
mapping sequences in fast single-point MPF mapping and provides an alternative solution for correction of
${B}_{1}$
non-uniformities in VFA
${R}_{1}$
mapping.