Functional MRI (fMRI) is an indirect measure of blood flow and neuronal activity based on changes in the local magnetic field (T2*). when neurons are active there is an increase on blood flow to the region, which increases the amount of oxygenated hemoglobin in the capillary beds. The amount of oxygen delivered by the hemodynamic response to neuronal activity exceeds the amount required by the tissue, thus increasing the ratio of oxygenated to deoxygenated hemoglobin in the venous beds compared to the resting state. At rest, deoxygenated hemoglobin causes a slight disturbance in the local magnetic field, which is attenuated by increased presence of diamagnetic oxygenated hemoglobin during neuronal activity, thereby causing longer T2* and increased signal intensity. The signal change is very small, but is reliably measured by subtracting images collected at rest from images collected during activity.