Susceptibility-Induced Increase of Apparent Diffusion Coefficient: BOLD Effect Behind Diffusion FMRI

Dmitry S. Novikov¹, Marco Reisert², Valerij G. Kiselev²

NMR diffusion measurements are confounded by heterogeneous microscopic magnetic susceptibility. It is known that apparent diffusion coefficient (ADC) is smaller in this case than the genuine diffusivity when diffusion is slow. We discover a more general effect: ADC over- or underestimates the genuine diffusivity when diffusion is fast depending on the pulse sequence. Our theory agrees with Monte Carlo simulations in computer-generated capillary network. The ~5% change in the simulated ADC quantitatively explains the observed DWI signal changes both in hypercapnia and under neuronal activation, suggesting that BOLD effect is sufficient for rationalizing the diffusion fMRI phenomenon.