The near-infrared spectroscopy technique is being used for non-invasive in vivo measurements and quantifying of oxygenation of hemoglobin in skin blood microcirculations. The method utilizes a simple model for studying of skin oxygenation. The aim of this lecture is to show young researches and students some perspectives of near-infrared spectroscopy as a technique with great promise and a new medical tool for non-invasive diagnostics and monitoring of blood oxygenation in vivo.

For many years different methods have been used to measure and quantify the level of tissue oxygenation, oxyhemoglobin (HbO₂) saturation, blood volume changes and related topics^1-3. In recent years there has been a growing interest in the non-invasive study and quantitative in vivo measurements of blood volume and tissue oxygenation changes during exercise, post-exercise recovery and at rest by using the near-infrared spectroscopy (NIRS) technique³.

NIRS is a spectral analysis of transmitted of reflected light in the near-infrared spectral range (600-1000 nm) which is much less absorbed in biotissues than visible light (400-700 nm). The first publication on NIRS in biomedicine appeared in 1977⁴ and was orientated toward fundamental physiologycal studies with laboratory animals. This technique is based on different absorption of near-infrared light by the oxy- and deoxygenated forms of heme components (as hemoglobin (Hb), myoglobin (Mb), and cytochromes)⁵. The average optical power in NIRS clinical measurements is about 5-10 mW, i.e. the value of irradiation on the skin surface is less than 50 mW cm^-2 which is similar to the intensity of the sun on a sunny day⁶. That is the radiation which is used by NIRS prevents skin and other tissues from thermal damage. Besides, the results of NIRS measurements are in qualitatively good agreement with the physiological behavior of vascular system, which means that this method gives a possibility for accurate observation of mitochondrial metabolism changes and for some other physiological studies⁷. However, the best applications of these studies are clinical where they may use as a non-invasive tool for measurements of oxy- and deoxygemoglobin changes in capillary loops, large and small blood vessels of skin in vivo^8-10, as well as study of cerebral haemodynamics in newborn babies^11-13, children¹⁴, adults^15-17, and study of the oxygenation changes in muscles^{3, 17-20} and other biological tissues^21-24. The near-infrared light can penetrate even in muscles as deep as 10 cm and more²⁵, that also promotes a clinical application of this technique.

As an object of investigation by NIRS technique skin represents a complex heterogeneous medium consisting of different layers with different optical properties. In the figure 1 we represent the simplest model for description of human skin^26-27. The first layer - epidermis is about 80-100 mm thick without blood vessels. The next layer is the papillary dermis (thickness about 100 mm) which includes capillary loops generally oriented perpendicularly to the surface of the skin capillary loops²⁹. These superficial capillary loops with the inner diameter about 2-40 mm are perfused by slow-speed red blood cells that supply the tissue with oxygen and nutritive substances and remove waste metabolites^29-33. Deeper in upper blood plexus lie arterioles, venules and arteriovenous anastomosises with inner diameter of 10-40 mm³⁴ which take an active part in body temperature regulation³⁵. Therefore the non-invasive conditions for blood microcirculations measurements (i.e. measuring device should not be in physical contact with the tissue) should be satisfied, because any probe-tissue contacts may disturb the microcirculation flows conditions in the capillary network³⁶.

Fig.1. Skin tissue model. Optical properties of the different skin layers model as well as for other biotissues defined in Ref.26 (and references therein, see also ref.27-28).

Reticular dermis (about 1500 mm thick) also includs small arteries and veins (20-60 mm inner diameter) which are orientated almost perpendicularly to skin surface. These small vessels constitute routes for blood supply and drainage to veins and arteries (50-100 mm inner diameter) from deep blood plexus (220 mm thick) and subcutaneous fat (can be as much as 5 mm thick). Behavior of blood in skin capillaries of less than 100m m in diameter is non-Newtonian^{37, 38}, which present difficulties for a phantom modeling and computer simulation.

The application of Monte Carlo technique for simulation of light propagation started from determination of optical properties of photographic emulsions³⁹. However the first use of Monte Carlo model in the biomedical area was reported only in 1983⁴⁰. The method is based on the simulation of individual statistical pathways of the photons in tissue and requires information about anisotropy and absorption and scattering coefficients of tissue as input data. Nowadays Monte Carlo applications in biomedicine are innumerable^{27, 28, 41-50 (and other)}.

As mentioned above, the Monte Carlo method is approach for description of photon migration through highly scattering media. During their travel from laser source (area of light input) to detector area or output boundaries photons do not penetrate ballistically through turbid media including biotissues. They undergo approximately 100 more scattering events than absorption ones, or in other words, for many biotissues the light scattering coefficient m _s is much bigger than the absorption coefficient m _a.

It should be noted, that a Monte Carlo model of light propagation through high scattering turbid media is based on the follow assumptions:

1. photons are neutral ballistic particles and thus wave phenomena (coherence and interference) can be disregarded;
2. photons experience only elastic scattering events, i.e. the energy of photons doesn’t change during a scattering event (but in practice, in computer simulation packets of photons are being used, that is not similar to photons), inelastic scattering events, polarization and fluorescence effects are ignored;
3. the absorbed energy doesn’t change the optical properties of the tissue;
4. optical properties for each layer of tissue are described solely by:

1. n – refractive index;
2. the scattering coefficient m _s – which is probability of scatter per unit⁴²;
3. the absorption coefficient m _a – which is probability of photon absorption per unit⁴²;
4. factor of anisotropy g – which characterizes the average amount of scattering in a medium, specifies the shape of scattering function⁴³, and is the first moment of theprobability density function⁴², termed the "scattering phase function". g - equal to average cosine of scattering angle q - which is angle between and (0<q <p ): . Scattering of incident beam is symetrical relative to axis of incident beam (asymetrical scattering is ignored). In this case the phase function depends only on q ^{26, 43: . The scattering phase function gives the probability of photon scatter from initial propagation direction to a final direction . Often this phase function is taken to the Henyey-Greenstein scattering function51:} , which is originally used for Mie approximation of galactic light scattering upon particles with size comparable to wavelength of incident light.

The results of NIRS measurements including new microvasculature data of the skin are of interest to dermatologists, physiologists and some other physicians for many years, because these data provide a lot of useful information about physiological and pathological processes in tissues^52,53. Nowadays it is significant for monitoring problems and radiotherapy of deep-seated tumors, and monitoring of the cerebral oxygenation of newborn infants, that is not only academic interest.

IVM acknowledges financial supports under EPSRC grant GR/L89433, and U.C.CRDF grant RB1-230. The authors also would like to thank personally to Prof.Valery V. Tuchin for the possibility of present this lecture at the Internet session of the "Saratov Fall Meeting’98".

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