Integrating a negative relaxometric constant in the χ-separation model improves the correlation between χ negative and myelin metrics

Motivation: χ-separation methods rely on assuming a single relaxometric constant (Dr) for both positive and negative susceptibility contributions and its value impacts on the resulting maps. Goal(s): This study proposes to introduce a physiological value for the negative relaxometric constant (Dr,neg) and tests the correlation between the new negative susceptibility map and myelin metrics (bound pool fraction (BPF), T2b). Approach: We calculated Dr,neg in the body of corpus callosum and tested the correlation between negative susceptibility and BPF,T2b in white matter for both new and standard methods. Results: Only the new method captured a positive correlation with BPF and a negative correlation with T2b. Impact: The integration of a physiologically plausible negative relaxometric constant in the χ-separation model will impact on how we assess the presence of positive and negative magnetic susceptibility sources in the brain, i.e. iron and myelin.