**2. Fundamentals of NIRS**

The study of the human brain made a big step forward with the introduction of noninvasive techniques, among them the near-infrared spectroscopy (NIRS). This technique allows to measure the oxygenation of the brain tissue (Alfano et al., 1997, 1998; Chance 1998, 1998a; Delpy and Cope, 1997; Hoshi, 2003; Obrig, 2003; Rolfe, 2000; Strangman et al., 2002).

The light, with wavelengths 650-1000 nm, penetrates superficial layers of the human body, among them the skin, the scull and the brain. It is either scattered within the tissue or absorbed by absorbers present in the tissue (chromophores).

The visible light has wavelengths 400-700 nm, some individuals can see up to 760 nm. Formally, the red light extends within the wavelengths of 630-760 nm, and the near infrared

Probing Brain Oxygenation Waveforms with Near Infrared Spectroscopy (NIRS) 115

arrives in the brain is absorbed and utilized. The fraction which is utilized, known as the

OEF=( SaO2 – SvO2)/SaO2 ,

According to Derdeyn et al (Derdeyn et al, 2002) the EOF, measured in their normal control subjects, was 0.41±0.03. Assuming SaO2 equal to 0.95 the SvO2 will be, using Eq. (6), equal to 0.56±0.03. In the brain tissue the absorption of the oxygenated and deoxygenated hemoglobin is mostly venous. Assuming a 75% venous contribution, the brain tissue regional oxygen saturation (rSO2) in the frontal region will be about 66±3%, a mean value which is observed in experiments. With a decrease of CBF there is a bigger demand for oxygen and EOF will increase (Kissack et al, 2005). Accordingly, with an increase of CBF the

Several types of NIRS equipment, based on different methods, are commercially available. They measure the concentrations of Hb, HbO2 and the total hemoglobin tHb. If the redox state of CytOx is also taken into account, then measurements with 3 wavelengths has to be

Three types of instruments are in use according to the used method : continuous intensity, time resolved and intensity modulated. For details, see (Delpy and Cope, 1997). Most of the commercial instruments utilize continuous wave (CW) light. In combination with the modified Lambert-Beer law it allows to measure changes in Hb and HbO2. In a biological tissue, quantification of the NIRS signal is difficult. Different methods have been proposed to improve the resolution. One of them is the spatially resolved spectroscopy (SRS), which uses CW light and a multi-distance approach. With this method the rSO2 (the absolute ratio

A further distinction among the instruments can be made. The simplest are the photometers,

The oximeters are more sophisticated. They use multi-distance (SRS) techniques with CW and usually two sensors (channels). For details, see (Ferrari et al., 2004; Delpy and Cope, 1997; Rolfe, 2000). Recently several groups have begun to use multi-channel CW imaging systems generating images of a larger area of the subject's head with high temporal resolution up to 10 Hz (Ferrari et al., 2004; Miura et al., 2001; Obrig and Villringer, 2003;

In the following we will compare two instruments: the INVOS Cerebral Oximeter of Somanetics (www.somanetics.com; Thavasothy et al., 2002), and the hemoencelograph

The Somanetics INVOS Cerebral Oximeter (ICO) system measures regional hemoglobin oxygen saturation (rSO2) of the brain in the area underlying the sensor and uses two

done. Instruments with 2 wavelengths do not evaluate the CytOx contribution.

of HbO2 to the total Hb content-tHb), can be evaluated (Suzuki et al., 1999).

which use single-distance and CW, light, usually with one sensor (channel).

where SaO 2 and SvO2 are the arterial and venous oxygen saturations respectively.

oxygen extraction fraction (OEF), is defined as

OEF will decrease.

**4. NIRS Instrumentation** 

Quaresima el al., 2001a. 2002a).

**4.1 The INVOS oximeter** 

(HEG) (Toomim and Marsh, 1999; Toomim et al., 2004).

light within 760-1400 nm. However, these terms are not precise, and are used differently in various studies. Here we will refer to the near infrared light as having the spectrum of 650- 1000 nm, i.e. the light that penetrates superficial layers of the human body.

The dominant chromophore in tissue is water. It absorbs strongly below 300 nm and above 1000 nm. The visible part of the light spectrum, between 400 and 650 nm, is almost nontransparent due to strong absorption of hemoglobin and melanin. Only in the NIR light region of 650-1000 nm, the overall absorption is sufficiently low, and the NIR light can be detected across a thick layer of tissue.

In the following pages, we will be concerned with utilizing the NIR light to determine the oxygenation of the brain tissue. In the rather transparent NIR region, there are many absorbing light chromophores, but only three are important as far as the oxygenation is concerned. They are the hemoglobin (HbO2), the deoxyhemoglobin (Hb) and cytochrome oxidase (CtOx), Hb and HbO2 (which carries the oxygen) are found inside the red blood cells. CtOx is the enzyme which ends the cellular respiratory chain, and is located in the mitochondrial membrane. Quantitatively important is the difference between the absorption spectra of the oxidised and reduced forms of CtOx. The concentration of cytochrome oxidase in living tissue is usually at least an order of magnitude below that of hemoglobin (Sato et al., 1976); therefore, its contribution is often neglected.

In the last 20 years there was an enormous growth in the instrumentation and applications of NIRS. A separate section will be devoted to the instrumentation. We will compare two NIRS devices: the INVOS Brain Oximeter (IBO) of Somanetics and the HEG device of H. Toomim.
