Cortical depth-dependent gradient-echo and spin-echo BOLD fMRI at 9.4T.

To examine cortical depth-related spatial specificity and signal changes in gradient-echo (GE) and spin-echo (SE) blood oxygenation level-dependent (BOLD) fMRI signals, a well-established cat visual stimulation model was used at 9.4T. The GE BOLD signal percent change is the highest at the surface of the cortex containing pial vessels, and decreases as cortical depth increases. In contrast, the SE BOLD signal is more specific to parenchyma, showing the highest signal change in the middle cortical areas. The stimulation-induced DeltaR2* to DeltaR2 ratio is dependent on the vessel size, which is related to basal susceptibility effects. The averaged ratio of DeltaR2* to DeltaR2 in all active regions, including large vessels, is 3.3 +/- 0.5 (N = 6). The averaged ratio of DeltaR2* to DeltaR2 is 8.8 +/- 1.7 (N = 4) on the surface of the cortex with large pial draining vessels, and decreases to 1.9 +/- 0.1 on the middle cortical areas with parenchymal microvessels. DeltaR2*/DeltaR2 is closely related to basal susceptibility effects and can be used to differentiate tissue from vessel regions. Copyright 2004 Wiley-Liss, Inc.