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

• Published in: IEEE transactions on medical imaging Vol. 40; no. 12; pp. 3473 - 3484
• Main Author: Yarnykh, Vasily L.
• Format: Journal Article
• Language: English
• Published: IEEE 01.12.2021
• Subjects: <italic xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">B ₁ field; <italic xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">T ₁ relaxation; Indexes; macromolecular proton fraction (MPF); Magnetic resonance imaging; Magnetization; magnetization transfer; Mathematical model; myelin; Nonhomogeneous media; Protons; quantitative MRI; Radio frequency
• ISSN: 0278-0062; 1558-254X; 1558-254X
• DOI: 10.1109/TMI.2021.3088258

Correction of <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> field non-uniformity is critical for many quantitative MRI methods including variable flip angle (VFA) <inline-formula> <tex-math notation="LaTeX">{T}_{1} </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">{R}_{1}=1/{T}_{1} </tex-math></inline-formula> map reconstruction as an intermediate processing step. The need for <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> correction restricts applications of the above methods, since <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> mapping sequences increase the examination time and are not commonly available in clinics. A new algorithm was developed to enable retrospective data-driven simultaneous <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> correction in VFA <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> and single-point MPF mapping. The principle of the algorithm is based on different mathematical dependences of <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula>-related errors in <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> and MPF allowing extraction of a surrogate <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> field map from uncorrected <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> and MPF maps. To validate the method, whole-brain <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> correction, and surrogate <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> correction. Surrogate <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> maps closely reproduced actual patterns of <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> inhomogeneity. Without correction, <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> non-uniformity caused highly significant biases in <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> and MPF (<inline-formula> <tex-math notation="LaTeX">{P} < 0.001 </tex-math></inline-formula>). Surrogate <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> field correction reduced the biases in both <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> and MPF to a non-significant level (<inline-formula> <tex-math notation="LaTeX">0.1\le {P}\le 0.8 </tex-math></inline-formula>). The described algorithm obviates the use of dedicated <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> mapping sequences in fast single-point MPF mapping and provides an alternative solution for correction of <inline-formula> <tex-math notation="LaTeX">{B}_{1} </tex-math></inline-formula> non-uniformities in VFA <inline-formula> <tex-math notation="LaTeX">{R}_{1} </tex-math></inline-formula> mapping.