NIRS in the temporal region--strong influence of external carotid artery.

NIRS in transmission mode has been shown to provide useful information on intra cerebral oxygenation in neonates1,2, and is beginning to be applied to adults using reflectance oximetry3,4. The relative contribution of intra-and extra-cranial blood pools to the MRS signal is not known, but initial results during selective carotid injection suggested a greater extra-cranial component than theoretical data had suggested. This study investigates the response of NIRS using temporal and frontal optodes to a change in external carotid flow (teeth clenching) and in internal carotid flow (rise in CO2) using Doppler measurement of blood velocity.