Abstract
Tissue longitudinal relaxation characterized by recovery time T(1) or rate R(1) 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(1) and myelination, and its dependence on B(0) field strength, is still not well known. It has been theorized that in much of brain tissue, T(1) 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(1)-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(1) were obtained in several brain structures at 3 and 7 T, which allowed distinguishing between contributions from macromolecular content and iron to tissue T(1). Based on this, the T(1) of MP in white matter, indirectly determined by assuming a field independent T(1) of free water, was shown to increase approximately linearly with B(0). This study advances our understanding of the T(1) 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(1) mapping methods for improved reproducibility in the human brain.