B+1 -correction of magnetization transfer saturation maps optimized for 7T postmortem MRI of the brain

Abstract

Magnetization transfer saturation (MTsat) is a useful marker to probe tissue macromolecular content and myelination in the brain. The increasedB+1-inhomogeneity at ≥7 T and significantly larger saturation pulse flip angles which are often used for postmortem studies exceed the limits where previous 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 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.

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 MTsat maps.

Conclusion

The presented model and calibration approach effectively corrects for B+1 inhomogeneities in postmortem 7T data.