Instrumento 003 Evaluación de productos académicos:

The absorption coefficient (μa) describes the effectiveness of light absorption by a

chromophore. When light radiation is incident on matter composed of discrete electrical charges, the charges are forced to oscillate at the frequency of the incident electric field. When infrared radiation is incident on a system of matter, resonance will occur, whereby energy is transferred from the incident field to the system and its amplitude of vibration is greatly increased. The excited state of the atoms or molecules usually lose their energy by colliding with one another within 10-12seconds, thereby raising the kinetic energy of the other particles involved in the collisions. Hence, the energy associated with the incident field is most often dissipated as heat within the medium. This process is known as absorption.

The overall effect of absorption is a reduction in the intensity of the light beam travelling the medium. A relationship between the absorption of light in a purely absorbing medium and the thickness of the medium was first determined as the Beer-Lambert law

. . . 0 0 aL C L I I e I e in base of e or . . 010 C L I I  in base on 10 (1-4)

where I0 is the incident intensity, I is the transmitted intensity, (the log ratio of which is

the attenuation of light), C is the chromophore concentration, L is the path length of light through the medium, and α and ε is known as the specific absorption coefficient for base of e and 10, respectively. The ability to perform spectroscopy in-vivo relies on the fact that there are few absorbers of NIR light in biological tissue allowing adequate light penetration of the tissue. The chief endogenous absorbers are water, oxygenated hemoglobin, deoxygenated hemoglobin and lipids.

a)

Water

The average water content of neonatal brain is estimated to be 85 % of the mass of the brain and 80 % of the adult brain.85 Because of its high concentration in most biological tissues, water is considered to be one of the most important chromophores in tissue spectroscopy measurements. The extinction coefficient for water shows a general trend of increase with increasing wavelength. Between 600 and 900 nm there exists a region of relatively low absorption. Within this NIR range (600-900 nm), water has two distinct features in its absorption spectrum: one at 740 nm and another at 840 nm. Above 900 nm the absorption coefficient increases fairly rapidly to a peak at about 970 nm. In fact, for wavelengths of light greater than 900 nm, absorption by water is strong enough to limit the propagation of light to less than 1 centimeter, effectively forming the upper- limit on what is termed the "optical window" of NIR. Within tissue, water binds to a variety of molecules such as other water molecules, proteins (collagen, myosin, actin), mineral deposits in bone and phospholipids bilayers. The hydrogen bonding with these molecules changes the NIR spectral features derived from O-H vibrations.

b)

Hemoglobin

Hemoglobin is the strongest endogenous absorber of NIR light. Hemoglobin is a large iron-containing protein found only in the blood, and constitutes approximately 40- 45% of the whole blood. It is responsible for delivering oxygen from the lungs to body

tissues and returning waste gases, such as carbon dioxide, to the lungs to be exhaled. Carbon dioxide is transported in the blood from the tissue to the lungs by one of three methods86: (i) About 5-7% is dissolved directly into the blood (ii) 10% binding to proteins, particularly haemoglobin; and (iii) 85% carried as a bicarbonate ion. Both Hb and HbO2 exhibit strong absorptions at wavelengths shorter than 600 nm: effectively

placing a lower-limit constraint on the range of wavelengths suitable for tissue interrogation. Another feature to note is the isosbestic point where the specific extinction coefficients of these two forms of hemoglobin are the same. Within the NIR range, the specific absorption spectra of Hb and HbO2 cross at 800 nm but the marked differences

between them at other NIR wavelengths have allowed the determination of tissue blood oxygenation with NIRS. The temperature-dependent changes of the hemoglobin NIR absorption spectra have been investigated87, 88. It has been shown that by increasing the temperature in the range of 20-40°C, the amplitude of Hb and HbO2 decreased by 0.15-

0.18% °C-1 and less than 0.05% °C-1 and shifted by 0.08 nm °C-1 (red-shifted for 760nm) and 0.15 nm °C-1, respectively.87 The amplitude changes in Hb are likely to have more significant effect on the total absorption in the range of interest than those for HbO2.

c)

Lipids and Other Absorbing Compounds

Lipids constitute about 5% of the total wet weight of a newborn infant’s brain. This percentage increases to 8% of the grey matter and 17% of the white matter in adulthood. The specific absorption of NIR light by lipids is similar in magnitude to that of water. Lipid will not significantly add to the overall extinction coefficient of brain tissue as it is only present at approximately one tenth the proportion compared to water. Other negligible absorbers include some other enzymes involved in cellular respiration and some proteins found in blood plasma, none of which are abundant enough to contribute significantly to in-vivo spectroscopy.