Use of Near-Infrared Spectroscopy in the Neonatal Intensive Care Unit

2. Technique Of Near-Infrared Spectroscopy

The relative transparency of biological tissue to light in the near infrared part of the spectrum (700-1000 nm) enables photons to be detected following passage through tissue at distances of up to 8 cm. As the light passes through the head it is attenuated due to a combination of absorption and scattering. Brain contains three changing chromophores that are present in variable concentrations: oxyhaemoglobin, deoxyhaemoglobin and cytochrome oxidase. Attenuation of transmitted light in the brain due to other causes (muscle, bone, bilirubin, ...) will be assumed to be constant over the period of monitoring. Consequently any change in observed attenuation is due to a change in the concentration of these chromophores. Because of safe levels of light input, it is difficult to use source-detectors distances of more than 5 cm (Wolf, 2009) and because of scattering and acquiring enough depth of measurement the minimum interoptode distance will probably be around 1.5 to 2 cm (Faris, 1991). The depth measured with this method is around half of the source-detector distance, so between 1 and 2,5 cm (Choi, 2004). Different techniques are used to measure the cerebral oxygenation.