Systematic analysis of relaxometric constant in brain using temperature-dependent relaxometry and susceptibility: Toward 7T chi-separation

Motivation: Important physical parameter, relaxometric constant$$$\;{D_r}$$$, linking magnetic susceptibility to induced transverse relaxation acceleration (i.e.,$$$\;{R2'}$$$) has not yet been fully understood in brain. Goal(s): To investigate underlying mechanisms affecting relaxometric constant in brain using temperature-dependent relaxometry and susceptibility and explore a better field-strength correction for ultra-high-field MRI. Approach: 3T and 7T R2*/R2'/quantitative-susceptibility maps were acquired from a post-mortem brain at different temperatures and analyzed based on the physical model. Results: In human brain, effects of temperature-dependent water susceptibility, water diffusion, and field strength on Dr were observed, and a field-strength correction coefficient was calculated, generating consistent chi-separation maps at both 3T and 7T. Impact: A better understanding of relaxometric constant in brain can provide better insight on effects of susceptibility sources (e.g., iron and myelin), on MR relaxometry, improving quantification accuracy of those biological substances using MRI.