BRAIN OR VEIN-OXYGENATION OR FLOW - ON SIGNAL PHYSIOLOGY IN FUNCTIONAL MRI OF HUMAN BRAIN ACTIVATION

Stimulus-related signal changes in functional MRI of human brain activ ation not only reflect associated adjustments of cerebral blood Bow an d oxygen consumption, but strongly depend on the MRI technique chosen and the actual experimental setting. A list of relevant parameters inc ludes static field homogeneity of the magnet, MR pulse sequence and si gnal type, TE, TR, flip angle, gradient strengths, gradient waveforms, receiver bandwidth and voxel size. In principle, a local signal incre ase during functional activation may reflect a regional change in cere bral blood flow or deoxyhemoglobin concentration or both. This ambigui ty was demonstrated using long TE FLASH MRI at high spatial resolution . Subsequently, experimental strategies were evaluated that either dis criminate MRI effects in large vessels from those in the cortical micr ovasculature or separate changes in blood flow velocity from those in blood oxygenation. Examples comprise studies of the human visual and m otor cortex.