Modelling transverse relaxometry using myelin-induced off-resonance fields and magnetisation transfer

We present a new Monte Carlo MR simulator, which in addition to simulating the random diffusion of water restricted by any obstacles also simulates (1) the off-resonance field due to myelin and (2) signal loss due to magnetisation transfer. We use this simulator to study the interaction of diffusion, relaxation, and magnetisation transfer. We simulate gradient and spin echo measurements in white matter. The myelin off-resonance field is found to greatly reduce T2* due to dephasing of the signal between compartments, but barely affects T2. Magnetisation transfer at tissue boundaries explains T2 differences between intra-, extra-axonal, and myelin water.