It is often overlooked that iron concentrations, as determined, e.g., via Quantitative Susceptibility Mapping (QSM), reflect the mass of iron per unit volume. Consequently, structural atrophy alone (i.e. decreased volume) increases the tissue iron concentration if the total mass of iron remains constant.
In this work, we present a technique to assess the mass of regional tissue iron in milligrams (mg). We retrospectively applied the technique to data from a recently published 2-year longitudinal study, in which we had investigated iron concentration changes in Multiple Sclerosis (MS).