Background: Magnetization transfer (MT) saturation reflects the additionalsaturation of the MRI signal imposed by an MT pulse and is largely driven bythe saturation of the bound pool. This reduction of the bound polarization bythe MT pulse is less efficient than predicted by the differential B1-square lawof absorption. Thus, B1 inhomogeneities lead to a residual bias in the MTsaturation maps. We derive a heuristic correction to reduce this bias for awidely used multi-parameter mapping protocol at 3T. Methods: The amplitude ofthe MT pulse was varied via the nominal flip angle to mimic variations in B1.The MT saturation's dependence on the actual flip angle features a linearcorrection term, which was determined separately for gray and white matter.Results: The deviation of MT saturation from differential B1-square law is welldescribed by a linear decrease with the actual flip angle of the MT pulse. Thisdecrease showed no significant differences between gray and white matter. Thus,the post hoc correction does not need to take different tissue types intoaccount. Bias-corrected MT saturation maps appeared more symmetric andhighlighted highly myelinated tracts. Discussion: Our correction involves acalibration that is specific for the MT pulse. While it can also be used torescale nominal flip angles, different MT pulses and/or protocols will requireindividual calibration. Conclusion: The suggested B1 correction of the MT mapscan be applied post hoc using an independently acquired flip angle map.<br
