  Background: Magnetization transfer (MT) saturation reflects the additional
saturation of the MRI signal imposed by an MT pulse and is largely driven by
the saturation of the bound pool. This reduction of the bound polarization by
the MT pulse is less efficient than predicted by the differential B1-square law
of absorption. Thus, B1 inhomogeneities lead to a residual bias in the MT
saturation maps. We derive a heuristic correction to reduce this bias for a
widely used multi-parameter mapping protocol at 3T. Methods: The amplitude of
the 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 linear
correction term, which was determined separately for gray and white matter.
Results: The deviation of MT saturation from differential B1-square law is well
described by a linear decrease with the actual flip angle of the MT pulse. This
decrease showed no significant differences between gray and white matter. Thus,
the post hoc correction does not need to take different tissue types into
account. Bias-corrected MT saturation maps appeared more symmetric and
highlighted highly myelinated tracts. Discussion: Our correction involves a
calibration that is specific for the MT pulse. While it can also be used to
rescale nominal flip angles, different MT pulses and/or protocols will require
individual calibration. Conclusion: The suggested B1 correction of the MT maps
can be applied post hoc using an independently acquired flip angle map.
