**4. NIRS Instrumentation**

114 Infrared Spectroscopy – Life and Biomedical Sciences

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-

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

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

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.

The amount of oxygen in the arterial blood depends upon the inspired oxygen and the pulmonary gas exchange. It depends on the arterial blood gas partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2). The units of partial pressures may cause some confusion. Three types of units are in use. One unit is kiloPascal (kPa) equivalent to 7.5006 mmHg (or Torr). It can be measured also in %, when 100% corresponds to atmospheric pressure of 760 mmHg (Torr), i.e 1% corresponds to 7.6 Torr (mmHg). The arterial hemoglobin saturation (SaO2 ) is measured in %. The normal value is about 95%. A typical oxygen carrying capacity of the blood is 19.4 ml of O2 per dl of blood with 19.1 ml O2/dl carried by hemoglobin and only 0.3 ml O2/dl dissolved in plasma (Cope, 1991). It should be noted that the oxygen delivery to the tissues is by diffusion and the hemoglobin acts as a

A typical averaged value for adult cerebral blood flow (CBF) is 47.7 ml/100 ml/min (Frackowiak et al., 1980) corresponding to total oxygen delivery 9.25 ml O2/100 ml/min. (Cope, 1991). Typical oxygen consumption of the adult brain is 4.2 ml O2/100 ml/min (Frackowiak et al., 1980). CBF, cerebral blood volume (CBV) and cerebral oxygen extraction (COE) are significantly greater in grey matter compared to white matter in normal human adults (Lammertsma et al., 1983; cope, 1991). The CBF and the cerebral blood volume (CBV) of grey matter in normal human adults is approximately 2.5 times that of white matter, while the cerebral oxygen extractions (COE) are 0.37 and 0.41 for grey and white matter respectively (Lammertsma et al., 1983; cope, 1991). Only part of the arterial oxygen which

1000 nm, i.e. the light that penetrates superficial layers of the human body.

(Sato et al., 1976); therefore, its contribution is often neglected.

buffer to maintain plasma's oxygen which is extracted by the tissue.

detected across a thick layer of tissue.

Toomim.

**3. Oxygen utilization** 

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

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

A further distinction among the instruments can be made. The simplest are the photometers, which use single-distance and CW, light, usually with one sensor (channel).

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

In the following we will compare two instruments: the INVOS Cerebral Oximeter of Somanetics (www.somanetics.com; Thavasothy et al., 2002), and the hemoencelograph (HEG) (Toomim and Marsh, 1999; Toomim et al., 2004).

#### **4.1 The INVOS oximeter**

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

Probing Brain Oxygenation Waveforms with Near Infrared Spectroscopy (NIRS) 117

What is the HEG measuring? According to the producers: "The HEG ratio is the basis of blood flow training. A normalized basis for HEG was established using measurements at Fp1 of 154 adult attendees at professional society meetings. A normalized reference value of 100 (SD=20) was thus established and served to calibrate all further spectrophotometers". The calibration was achieved by multiplying the intensities ratio by 200. We have shown (Gersten et. al. 2006) that one can relate the readings of the HEG to rSO2 and even calibrate it

We started this research in Israel in the Pulmonary Unit of the Soroka University Hospital in Beer-Sheva. The study protocol was approved by the Helsinki (Ethics) Committee of the Soroka University Hospital. The investigation conforms with the principles outlined in the Declaration of Helsinki. The nature of the study was explained, and all subjects gave written

At the beginning our research was based on capnometer measurements of end tidal CO2 (EtCO2). The capnometer measures the CO2 concentration of the expired air. During the inspiration or breath holding the capnometer indications are zero. The capnometer enable us

We continued the research in the USA using the INVOS Cerebral Oximeter model 5100B of Somanetics Corporation and a capnometer of Better Physiology, Ltd. Both devices are noninvasive. The INVOS Cerebral Oximeter is based on most recent technological developments of near infrared spectroscopy (NIRS). With this device data are collected of regional oxygen saturation (rSO2) near the forehead with two optical sensors (for more details see www.somanetics.com). In addition to rSO2 one can determine the Blood Volume Index (BVI), which is an indicator of blood changes in the brain. This is a relative quantity,

The fastest recording rate of the 5100B oximeter is every 12 seconds from both left and right sensors. This time is much longer then the average period of about 4 seconds of normal respiration. In order to detect oxygenation periodicity we had to study respiration periods of about 36 seconds or larger (i.e. 3 data points or more for each breathing period). This is still a rather small amount of data points per respiration period. We have compensated for this small number by using a cubic spline interpolation of the data points, adding new interpolation points through this method. The cubic spline interpolation is a very effective

We found six people well acquainted with yoga pranayama, who could easily perform breathing exercises with periods around 36 seconds. All of them performed the following

They were asked to breathe in the following way: to inhale for 4 units of time (UOT), to hold the breath for 16 UOT and to exhale for 8 UOT, this we denote as the 4:16:8 (pranayama)

The unit of time (UOT) is about 1 second. The yoga practitioners develop an internal feeling of UOT which they employ in their practices. They learn to feel their pulse or they learn to count in a constant pace. Often they practice with eyes closed. In order not to distract or

separately for each individual.

consent to participate.

to follow the breathing periodicity.

method of smooth interpolation.

routine which lasted for 15 minutes.

routine.

which could not be normalized with our oximeter.

**5. Experimental detection of oxygen waveforms** 

wavelengths, 730 and 810 nm. The sensor, ( "SomaSensor"), is applied to the forehead with an integrated medical-grade adhesive. Two sensors can be placed in the forehead near Fp1 and Fp2. The spatially resolved spectroscopy (SRS) method is applied by using in the sensor two source-detector distances: a "near" (shallow), 3 cm from the source and a "far" (deep), 4 cm from the source. Both sample almost equally the shallow layers in the tissue volumes directly under the light sources and detectors in the sensor, but the distant "far" penetrates deeper into the brain. Using the SRS method, subtraction of the near signal from the far should leave a signal originating predominantly in the brain cortex. The measurement takes place in real time, providing an immediate indication of a change in the critical balance of oxygen delivery and oxygen consumption.

According to the producers: "Using the model at a 4 cm source-detector spacing and no signal subtraction, the overlying tissue and skull contribute, on average, about 45 percent of the signal while 55% is cerebral in origin. Subtracting the data from the 3 cm spacing (as the Oximeter does) reduces this extracerebral contribution to less than 15 percent. While the potential exists to develop an instrument that will reduce the extracerebral contribution to zero, subject-dependent variations in anatomy and physiology will likely cause variations of ±10%. While the extracerebral contribution is not zero, the noninvasive Somanetics INVOS Cerebral Oximeter provides a "predominately cerebral" measurement where over 85 percent of the signal, on average, is exclusively from the brain" (www.somanetics.com).

The INVOS Cerebral Oximeter is an important tool in surgery rooms, saving lives and expenses. The producers explain: " Declining cerebral oximeter values occur frequently in cardiac surgery and reflect the changing haemodynamic profile of the balance between brain oxygen delivery and consumption. Since low rSO2 values correlate with adverse neurological and other outcomes, continuous assessment is a valuable patient management tool. Declining or low cerebral oximeter values are corrected with simple interventions".

#### **4.2 The HEG**

The hemoencephalograph (HEG) is a single-distance CW spectrophotometer, which uses NIR light with two wavelengths, 660 and 850 nm. The light source consist of closely spaced emitting diodes (LED optodes). The source and an optode light receiver are mounted on a headband. The distance between the source and receiver is 3 cm.

The HEG measures the ratio of the intensity of the 660 nm light to the intensity of the 850 nm light.

The HEG is not intended to measure rSO2. Nevertheless it is an important tool in the biofeedback research.

Hershel Toomim, the inventor of HEG has noticed that he can influence the outcome by looking at the results. Since then many people were able to increase the HEG readings via such a biofeedback.

The HEG became an important tool for training local brain oxygenation. The HEG is a very sensitive device. The distance between the source and receiver is the same as the distance of the shallow detector of the Somanetics INVOS Cerebral Oximeter. Therefore the INVOS Oximeter covers larger brain tissue and is more stable and less influenced by biofeedback.

What is the HEG measuring? According to the producers: "The HEG ratio is the basis of blood flow training. A normalized basis for HEG was established using measurements at Fp1 of 154 adult attendees at professional society meetings. A normalized reference value of 100 (SD=20) was thus established and served to calibrate all further spectrophotometers". The calibration was achieved by multiplying the intensities ratio by 200. We have shown (Gersten et. al. 2006) that one can relate the readings of the HEG to rSO2 and even calibrate it separately for each individual.
