B1+ -correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

Abstract

Purpose:

Magnetization transfer saturation ( ) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased -inhomogeneity at T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous

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 dependence of 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 is proposed to correct bias in

by mapping it to the reference value of the saturation pulse flip angle. Methods:

was estimated voxel-wise in five postmortem chimpanzee brains. “Individual-based global parameters” were obtained by calculating the mean

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 was not entirely independent of the underlying tissue and . Individual-based correction parameters and a group-based global correction parameter () led to visible, quantifiable reductions of -biases in high-resolution

maps. Conclusion:

The presented model and calibration approach effectively corrects for inhomogeneities in postmortem 7T data.