Assessment and correction of B1-induced errors in magnetization transfer ratio measurements

The magnetization transfer ratio (MTR) is strongly related to the field strength (B₁) of the saturation pulse. B₁ variations therefore can result in significant MTR variations and can affect histogram analysis, particularly if data from a large volume of interest are included. A multicenter study was performed to determine the typical range of B₁ errors and the corresponding MTR variations in brain tissue of healthy volunteers. Seven subjects were included at each center resulting in a total cohort of 28 subjects. Additionally, numerical simulations were done to study this relationship more generally for pulsed saturation. It could be demonstrated, both theoretically and empirically, that for typical B₁ errors there is a linear relationship between B₁ error and the corresponding MTR change. In addition, for proton density-weighted sequences, this relationship seems to be largely independent of the underlying relaxation properties. Mean B₁ errors in the entire brain were typically in the range between -3% and -7%. Due to different coil characteristics, significant MTR differences between different scanners and sites were observed. Using a simple correction scheme that is based on a linear regression analysis between MTR and B₁ data it was possible to reduce the intersubject variation by ∼50%. Furthermore, interscanner variation could be reduced such that no significant differences between scanners could be detected. The correction scheme may be useful when investigating MTR as an outcome measure in single or multicenter studies.