Sensitivity of near infrared spectroscopy to cerebral and extra-cerebral oxygenation changes is determined by emitter-detector separation

Objective. To examine the effect of two emitter-detector separations (2.7 a nd 5.5 cm) on the detection oi changes in cerebral and extra-cerebral tissu e oxygenation using near infrared spectroscopy (NIRS). Methods, Two NIR det ectors were placed on the scalp 2.7 and 5.5 cm from a single NIR emitter. C hanges in deoxyhaemoglobin (HHb), oxyhaemoglobin (O(2)Hb),oxidised cytochro me C oxidase (CYt) and total haemoglobin (tHb) were recorded from each dete ctor during the induction of cerebral oligaemia (transition from hypercapni a to hypocapnia) and scalp hyperaemia (following release of a scalp tourniq uet). Results. Cerebral oligaemia (mean decrease in middle cerebral artery blood flow velocity of 44%) induced by a mean reduction in end tidal CO2 of 18 mmHg was accompanied by a significant increase in the spectroscopic sig nal for HHb and a decrease in the O(2)Hb signal. The signal change per unit photon path length detected at 5.5 cm was significantly greater for HHb (p = 0.007) than that detected at 2.7 cm. In contrast, the increase in all ch romophores detected at 5.5 cm during scalp hyperaemia was significantly les s than that detected at 2.7 cm (p < 0.001). Conclusions. The differing sens itivity of the proximal and distal channels to changes in cerebral and extr acerebral oxygenation is compatible with theoretical models of NIR light tr ansmission in the adult head and may provide a basis for spatially resolvin g these changes. The optimal emitter-detector separation for adult NIRS req uires further investigation and may differ between individuals.