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

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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 T2 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 T2 typically used for optimal BOLD contrast, these gradients had much smaller effects at both fields, consistent with the decreasing blood T2 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 T2, significantly reduces nonspecific mapping signals from large vessels and significantly accentuates microvasculature contributions. Magn Reson Med 49:1019–1027, 2003.