Non-invasive optical monitoring of the newborn piglet brain using continuous-wave and frequency-domain spectroscopy

We have used continuous-wave (CW) and frequency-domain spectroscopy to inve stigate the optical properties of the newborn piglet brain in vivo and non- invasively. Three anaesthetized, intubated, ventilated and instrumented new born piglets were placed into a stereotaxic instrument for optimal experime ntal stability, reproducible probe-to-scalp optical contact and 3D adjustme nt of the optical probe. By measuring the absolute values of the brain abso rption and reduced scattering coefficients at two wavelengths (758 and 830 nm), frequency-domain spectroscopy provided absolute readings (in contrast to the relative readings of CW spectroscopy) of cerebral haemoglobin concen tration and saturation during experimentally induced perturbations in cereb ral haemodynamics and oxygenation. Such perturbations included a modulation of the inspired oxygen concentration, transient brain asphyxia, carotid ar tery occlusion and terminal brain asphyxia. The baseline cerebral haemoglob in saturation and concentration, measured with frequency-domain spectroscop y, were about 60% and 42 mu M respectively The cerebral saturation values r anged from a minimum of 17% (during transient brain asphyxia) to a maximum of 80% (during recovery from transient brain asphyxia). To analyse the CW o ptical data, we have (a) derived a mathematical relationship between the ce rebral optical properties and the differential pathlength factor and (b) in troduced a method based on the spatial dependence of the detected intensity (dc slope method). The analysis of the cerebral optical signals associated with the arterial pulse and with respiration demonstrates that motion arte facts can significantly affect the intensity recorded from a single optode pair. Motion artefacts can be strongly reduced by combining data from multi ple optodes to provide relative readings in the de slope method. We also re port significant biphasic changes (initial decrease and successive increase ) in the reduced scattering coefficient measured in the brain after the pig let had been sacrificed.