Microvascular BOLD contribution at 4 and 7 T in the human brain: gradient-echo and spin-echo fMRI with suppression of blood effects. Magnetic resonance

The BOLD signal consists of an intravascular (IV) and an extravas-cular (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 in-vestigated 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, consis-tent 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 sur-rounding both large and small vessels dominated. These results suggest that spin-echo BOLD fMRI at 4 and 7 T, with TE approx-imating tissue T2, significantly reduces nonspecific mapping sig-nals from large vessels and significantly accentuates microvas-culature contributions. Magn Reson Med 49:1019–1027, 2003