Cerebral near infrared spectroscopy: emitter-detector separation must be increased

We have compared the effect of increasing optode separation (range 0.7- 5.5cm) on the sensitivity of near infrared spectroscopy (NIRS) to discretereductions in scalp and cerebral oxygenation in 10 healthy men (mean age32, range 26-39 yr) using multichannel NIRS. During cerebral oligaemia (amean reduction in middle cerebral artery flow velocity of 47%) induced by amean reduction in end-tidal PCO2 of 2.4 kPa, the decrease in oxyhaemoglobindetected by NIRS became significantly greater with increasing optodeseparation (P < 0.0001). In response to scalp hyperaemia induced byinflation and release of a pneumatic scalp tourniquet, increases inoxyhaemoglobin became significantly smaller with increasing optodeseparation (P < 0.0002). These results are consistent with theoreticalmodels of the behaviour of NIR light in the adult head and support theconcept of using multi-detector NIRS to separate intra- and extracranialNIR signal changes. However, the emitter-detector separation used bycurrently available cerebral oximeters is not large enough to provideoptimal spatial resolution.