Robust quantification of the longitudinal relaxation rate (R1)—a widely used proxy marker of myelin content—requires highly accurate and precise estimation of the RF transmit field (B1+). The Bloch-Siegert shift (BSS) is a B1+-mapping method that allows calibration data to be acquired with the same spoiled gradient-echo readout used for variable flip angle R1 mapping. Here we show that systematic differences in steady state phase, caused by the interleaved nature typically adopted, lead to bias or loss of precision, but that these effects can be corrected for using a multi-echo approach and GLM fitting to isolate the BSS phase.
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