$B_1^{+}$ -correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

Purpose: Magnetization transfer saturation ( $\text{MTsat}$ ) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased $B_1^{+}$ -inhomogeneity at $\ge 7$ T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous $\text{MTsat}$ $B_1^{+}$ 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.

Theory: The $B_1^{+}$ dependence of $\text{MTsat}$ 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 $C$ is proposed to correct bias in $\text{MTsat}$ by mapping it to the reference value of the saturation pulse flip angle.

Methods: $C$ was estimated voxel-wise in five postmortem chimpanzee brains. "Individual-based global parameters" were obtained by calculating the mean $C$ within individual specimen brains and "group-based global parameters" by calculating the means of the individual-based global parameters across the five brains.

Results: The linear calibration model described the data well, though $C$ was not entirely independent of the underlying tissue and $B_1^{+}$ . Individual-based correction parameters and a group-based global correction parameter ( $C=1.2$ ) led to visible, quantifiable reductions of $B_1^{+}$ -biases in high-resolution $\text{MTsat}$ maps.

Conclusion: The presented model and calibration approach effectively corrects for $B_1^{+}$ inhomogeneities in postmortem 7T data.