Quantification of adult cerebral blood volume using the NIRS tissue oxygenation index.

Near-infrared spectroscopy (NIRS) is increasingly used as a non-invasive technique for monitoring cerebral oxygenation and haemodynamics . Simple continuous-wave (CW) NIRS systems utilising differential spectroscopy can measure quantitative changes in oxyand deoxyhaemoglobin (∆[O2Hb], ∆[HHb]) but only from an arbitrary baseline. Numerous studies of changes in cerebral oxygenation and haemodynamics in adults have been published but only few absolute quantitative measurements have been reported. Recent advances in the NIRS technology have enabled quantitative assessment of haemoglobin concentration in tissue using near-infrared (NIR) phase and time resolved systems; and absolute measurements of tissue saturation using phase, time or spatially resolved spectroscopy (SRS) systems 3, 4, 5, 6 . This paper suggests a way to use a commercially available spectrometer, which has both CW and SRS capabilities in order to measure absolute tissue haemoglobin (Hbtc) and hence cerebral blood volume (CBV). The methodology is based on that of Wyatt et al. 7 who developed a method for measuring absolute CBV, using NIRS measurements during controlled changes in inspired O2 fraction. By using NIRS measured tissue ∆[O2Hb] and comparing it to changes in arterial saturation (SaO2) measured with a pulse oximeter it is possible to calculate absolute Hbtc concentration. This is the so-called ‘desaturation method’ or ‘O2 method’ or ‘SaO2 method’ 8, 9, 10, 11 . The purpose of the present study was to compare measurements of CBV made using the conventional ‘SaO2 method’ with those using a new method employing the SRS derived absolute cerebral tissue oxygenation index (TOI), which will be called the ‘TOI method’.