B-field dependence of MRI T relaxation in human brain.

Tissue longitudinal relaxation characterized by recovery time T or rate R is a fundamental MRI contrast mechanism that is increasingly being used to study the brain's myelination patterns in both health and disease. Nevertheless, the quantitative relationship between T and myelination, and its dependence on B field strength, is still not well known. It has been theorized that in much of brain tissue, T field-dependence is driven by that of macromolecular protons (MP) through a mechanism called magnetization transfer (MT). Despite the explanatory power of this theory and substantial support from in-vitro experiments at low fields (<3 ​T), in-vivo evidence across clinically relevant field strengths is lacking. In this study, T-weighted MRI was acquired in a group of eight healthy volunteers at four clinically relevant field strengths (0.55, 1.5, 3 and 7 ​T) using the same pulse sequence at a single site, and jointly analyzed based on the two-pool model of MT. MP fraction and free-water pool T were obtained in several brain structures at 3 and 7 ​T, which allowed distinguishing between contributions from macromolecular content and iron to tissue T. Based on this, the T of MP in white matter, indirectly determined by assuming a field independent T of free water, was shown to increase approximately linearly with B. This study advances our understanding of the T contrast mechanism and its relation to brain myelin content across the wide range of currently available MRI strengths, and it has the potential to inform design of T mapping methods for improved reproducibility in the human brain.