The advance towards in vivo histology benefits greatly from the quantification of specific physical parameters1. The longitudinal relaxation rate (R1) has proven a reliable surrogate for myelination, facilitating investigation of the relationship between brain microstructure and function in vivo2–5. R1 maps can be estimated by combining spoiled gradient-echo volumes, acquired with variable flip angles, and calibration data correcting for flip angle inhomogeneities. The Bloch-Siegert shift (BSS) approach6 is a relatively time-efficient method that allows the calibration data to be acquired with an identical gradient-echo readout thereby matching distortions across all data needed to map R1. However, it requires data to be acquired at two off-resonance frequencies to remove B0 dependence up to second order and suffers from high specific-absorption-rate (SAR). Here, we investigate a modified BSS-based B1+ mapping approach that aimed to overcome these shortcomings by using a single offset frequency and a multi-echo readout.
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