Field dependence of T2 relaxation due to diffusion through an inhomogeneous medium.

Motivation: Brain iron in the form of ferritin is an important contributor to R2 relaxation. We aim to understand why this contribution was shown to be linear with field strength in our previous study (ISMRM2023:4342), contrary to the expected quadratic dependence. Goal(s): To explain R2 (=1/T2) field dependence and the role of iron distribution in R2 relaxation. Approach: Simulate effects on R2 from diffusion through field inhomogeneities from tissue iron. Results: Simulations explain both the magnitude and field dependence of diffusion mediated R2 relaxation due to iron clustered at the spatial scale of cells. With increasing field, the dependence transitions from B02 to B00.66. Impact: This study helps to explain a previously observed linear dependence of R2 relaxation on the field strength (ISMRM2023:4342) by simulating the effects of diffusion through the iron-induced field inhomogeneities.