Lynette Masters Neuroradiologist

“BOLD” IMAGING• increased neural activity stimulates a local increase inCBF and metabolic rate.• however, the rise in CBF is much higher than the rise inCMRO² (metabolic rate)• so, the oxygen extraction fraction falls• and there is a higher concentration of oxygenated blood(and concomitant decrease in deoxyHb) in the capillariesand veins• this leads to a slight increase in local MR signal• 70% of the blood lies within the capillary network andvenules. Therefore the signal changes detected in fMRI arethought to predominantly reflect regional deoxygenationstate of the venous system