This patient underwent an fMRI to localize language preoperatively. A motor paradigm was also performed (shown here)
BOLD (Blood Oxygenation Level Dependent) Imaging is a functional MRI (fMRI) technique to delineate regional activity. It uses heavily T2* weighted sequences to detect the different concentration of oxygenated versus deoxygenated haemoglobin. It is possible to detect these differences because deoxygenated hemoglobin is paramagnetic whereas oxygenated hemoglobin is not, and therefore the former will cause signal drop out (just as all other paramagnetic compounds cause signal drop out on T2* sequences… think of haemosiderin).
It has been shown that neuronal activity is closely coupled with cerebral blood flow (CBF), and that when CBF is increased there is a disproportionate increase in the amount of oxygenated haemoglobin. The T2* signal returned from a portion of tissue will therefore correlate with CBF. (see graph here)
Patients are asked to perform specific tasks while being imaged, such as motor activity (see image) or verbal tasks. The relevant parts of the brain will therefore demonstrate increased signal.
For more on the limitations of the technique please visit Radiopaedia.org here.Credit: Dr Frank Gaillard