Microvascular BOLD contribution at 4 and 7 T in the human brain: Gradient‐echo and spin‐echo fMRI with suppression of blood effects

Abstract

The BOLD signal consists of an intravascular (IV) and an extravascular (EV) component from both small and large vessels. Their relative contributions are dependent on field strength, imaging technique, and echo time. The IV and EV contributions were investigated in the human visual cortex at 4 and 7 T using spin-echo and gradient-echo BOLD fMRI with and without suppression of blood effects. Spin-echo acquisition suppresses EV BOLD from large veins and reflects predominantly blood T 2 changes and EV BOLD signal from small blood vessels. At a short echo time (32 ms), diffusion gradient-based suppression of blood signals resulted in a 75% and 20% decrease in spin-echo BOLD changes at 4 T and 7 T, respectively. However, at echo times (55-65 ms) approximating tissue T 2 typically used for optimal BOLD contrast, these gradients had much smaller effects at both fields, consistent with the decreasing blood T 2 with increasing field strength. Gradient-echo BOLD percent changes, with relatively long echo times at both fields, were virtually unaffected by gradients that attenuated the blood contribution because the EV BOLD surrounding both large and small vessels dominated. These results suggest that spin-echo BOLD fMRI at 4 and 7 T, with TE approximating tissue T 2 , significantly reduces nonspecific mapping signals from large vessels and significantly accentuates microvasculature contributions.

Reproducibility of functional MRI localization within the human somatosensory cortex

“…Recent work by Zhang et al (18) in monkeys at 9.4 Tesla (T) using an in-plane resolution of 0.625 mm found a mean inter-session difference in digit location of 0.55 mm. This improvement in comparison to our results is likely to be due to the smaller voxel size (of interest, the mean difference is similar to the voxel size, as we found), and higher SNR and higher spatial specificity of the BOLD response at higher fields (19).…”

High spatial correspondence at a columnar level between activation and resting state fMRI signals and local field potentials

“…At high fields, the relative contributions of both intravascular signals and the effects of larger vessels on extravascular dephasing to the overall fMRI signal changes are substantially diminished (19,20). The increase in the intrinsic width of the PSF of BOLD signals at 1.5 T and 3 T may be attributed to the presence of greater large vessel contributions to signal dephasing of the extravascular water at those lower fields (21,22). Moreover, it is likely that measurements acquired at lower field and resolution also reflected the intrinsic resolution limitations of the image acquisitions.…”

Ultrahigh field magnetic resonance imaging and spectroscopy

“…The long tagging times, on the other hand, may lead to the tagged spins appearing in the venous side, thus leading to false activation in the veins. At high magnetic fields, the spin-spin relaxation rate (T 2 ) of venous blood is very short [30,55] so that this effect can be selectively eliminated by a brief delay after excitations of spins but before image acquisition. Accordingly, perfusionbased fMRI maps have been shown to co-localize with Mn ϩ2 uptake [56], a marker of calcium dependent synaptic activity [57].…”