B1+$$ {B}_1^{+} $$‐correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

Magnetization transfer saturation ( MTsat$$ \mathrm{MTsat} $$ ) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased B1+$$ {B}_1^{+} $$ -inhomogeneity at ≥7$$ \ge 7 $$ T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous MTsat$$ \mathrm{MTsat} $$B1+$$ {B}_1^{+} $$ correction methods are applicable. Here, we develop a calibration-based correction model and procedure, and validate and evaluate it in postmortem 7T data of whole chimpanzee brains.The B1+$$ {B}_1^{+} $$ dependence of MTsat$$ \mathrm{MTsat} $$ was investigated by varying the off-resonance saturation pulse flip angle. For the range of saturation pulse flip angles applied in typical experiments on postmortem tissue, the dependence was close to linear. A linear model with a single calibration constant C$$ C $$ is proposed to correct bias in MTsat$$ \mathrm{MTsat} $$ by mapping it to the reference value of the saturation pulse flip angle.C$$ C $$ was estimated voxel-wise in five postmortem chimpanzee brains. "Individual-based global parameters" were obtained by calculating the mean C$$ C $$ within individual specimen brains and "group-based global parameters" by calculating the means of the individual-based global parameters across the five brains.The linear calibration model described the data well, though C$$ C $$ was not entirely independent of the underlying tissue and B1+$$ {B}_1^{+} $$ . Individual-based correction parameters and a group-based global correction parameter ( C=1.2$$ C=1.2 $$ ) led to visible, quantifiable reductions of B1+$$ {B}_1^{+} $$ -biases in high-resolution MTsat$$ \mathrm{MTsat} $$ maps.The presented model and calibration approach effectively corrects for B1+$$ {B}_1^{+} $$ inhomogeneities in postmortem 7T data.