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

Abstract

PurposeMagnetization transfer saturation (MTsat) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increased [Formula] -inhomogeneity at [≥] 7T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous MTsat [Formula] 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. TheoryThe [Formula] dependence of 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 MTsat by mapping it to the reference value of the saturation pulse flip angle. MethodsC 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. ResultsThe linear calibration model described the data well, though C was not entirely independent of the underlying tissue and [Formula]. Individual-based and group-based global correction parameters (C = 1.2) led to visible, quantifiable reductions of [Formula]-biases in high resolution MTsat maps. ConclusionThe presented model and calibration approach effectively corrects for [Formula] in-homogeneities in postmortem 7T data.