Functional contrast based on intermolecular double-quantum coherences: influence of the correlation distance.

A functional MRI (fMRI) study with visual stimulation of healthy subjects was performed at 3T exploiting intermolecular double-quantum coherences. The correlation distance, d(c), was varied between 60 and 300 microm for different evolution times, tau. Robust activation was obtained in all experiments with average signal changes (DeltaS(2)) = 8.4 +/- 0.7% and 9.4 +/- 0.8% for tau = 15 and 20 ms, respectively) exceeding those normally associated with conventional blood oxygen level-dependent (BOLD) fMRI. Relaxation-rate changes (DeltaR(2) = 0.33 +/- 0.36 s(-1) and DeltaR(2)* = 0.77 +/- 0.54 s(-1)) were similar to those commonly obtained for the extravascular BOLD effect. The number of activated voxels increased with increasing d(c) until a plateau was reached at approximately equal 120 microm. Similar trends were observed for the activation-induced percent signal change and for the maximal Z-scores. These effects were quantitatively explained by a reduced sensitivity at short d(c) due to increasing signal attenuation related to diffusion and an increasing amount of signal fluctuations in the fMRI time series due to imperfect suppression of unwanted coherence pathways. Consistent indications of a preferential selection of susceptibility changes in blood vessels of a particular size were not obtained.