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BOLD contrast; Correlation distance; Distant dipolar field; Intermolecular double-quantum coherence (iDQC)
Abstract:
A functional MRI (fMRI) study with visual stimulation of healthy subjects was performed at 3T exploiting intermolecular double-quantum coherences. The correlation distance, dc, was varied between 60 and 300 μm for different evolution times, τ. Robust activation was obtained in all experiments with average signal changes (ΔS2) = 8.4 ± 0.7% and 9.4 ± 0.8% for τ = 15 and 20 ms, respectively) exceeding those normally associated with conventional blood oxygen level-dependent (BOLD) fMRI. Relaxation-rate changes (ΔR2 = 0.33 ± 0.36 s -1 and ΔR2* = 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 ≈120 μm. 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 dc 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. © 2007 Wiley-Liss, Inc.
