Diffusion functional MRI (dfMRI) is a promising technique to map functional
activations by acquiring diffusion-weighed spin-echo images. In previous
studies, dfMRI showed higher spatial accuracy at activation mapping compared to
classic functional MRI approaches. However, it remains unclear whether dfMRI
measures result from changes in the intra-/extracellular environment, perfusion
and/or T2 values. We designed an acquisition/quantification scheme to
disentangle such effects in the motor cortex during a finger tapping paradigm.
dfMRI was acquired at specific diffusion weightings to selectively suppress
perfusion and free-water diffusion, then times series of the apparent diffusion
coefficient (ADC-fMRI) and of the perfusion signal fraction (IVIM-fMRI) were
derived. ADC-fMRI provided ADC estimates sensitive to changes in perfusion and
free-water volume, but not to T2/T2* values. With IVIM-fMRI we isolated the
perfusion contribution to ADC, while suppressing T2 effects. Compared to
conventional gradient-echo BOLD fMRI, activation maps obtained with dfMRI and
ADC-fMRI had smaller clusters, and the spatial overlap between the three
techniques was below 50%. Increases of perfusion fractions were observed during
task in both dfMRI and ADC-fMRI activations. Perfusion effects were more
prominent with ADC-fMRI than with dfMRI but were significant in less than 25%
of activation ROIs. Taken together, our results suggest that the sensitivity to
task of dfMRI derives from a decrease of hindered diffusion and an increase of
the pseudo-diffusion signal fraction, leading to different, more confined
spatial activation patterns compared to classic functional MRI.Comment: Submitted to peer-reviewed journa