CAPÍTULO IV: ANÁLISIS DE RESULTADOS: PRESENTACIÓN Y DISCUSIÓN
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4.1.3 Optical properties of various tissue types
The main constituents of biological tissue which contribute towards absorption in the near infrared are water, fat and haemoglobin. While the former two remain fairly constant over
short time-scales, the concentrations of oxygenated and deoxygenated haemoglobin change according to the function and metabolism of the tissue. Thus the corresponding changes in absorption can provide clinically useful physiological information. This section discusses the absorption properties of these tissue constituents as well as measurements of the absorption and scattering coefficients of a variety of specific tissues.
Water
The absorption spectrum of water [Cooper 1996] in the range 600-1050 nm is shown in Figure 4-1. Significant transmission through tissue is only possible from the UV (about 200 nm, not shown in spectrum) to the near infrared (about 935 nm). The absorption drops again beyond 1000 nm, but there are currently no efficient detectors available at such long wavelengths. 0.05 - V 0.04 - 0 0.03 - C 1 0.02 - 0.00 600 700 800 900 1000 W a ve le n g th [nm ]
Figure 4-1 Absorption spectrum of water.
Note that although the absorption coefficient of water is rather low in this ‘water transmis sion window’, it is still a significant contributor to the overall attenuation as its concentration is very high in biological tissue. The average content in the neonatal brain is 90% [Fillerup 1967], and 80% in the adult brain [Woodard 1986].
Lipids
Lipids, or fat, make up about 5% of the neonatal brain’s weight. Figure 4 -2 represents the absorption spectrum of pork fat [Conway 1984], which is thought to be largely identical to that of human lipids. The absorption coefficient, which is of the same order of magnitude as for water, is low at shorter wavelengths (down to about 600 nm, not shown in spectrum), with a strong peak at about 930 nm. However, because of the low content of lipids in the brain, the effect on the overall absorption is rather small.
0.05 - F 0.04 - Ü Q 0 0.03 - c Ü CO 0.02 - 3 1 < 0.01 - 0.00 - 800 850 900 950 W a v e le n g th [nm ]
Figure 4-2 Absorption spectrum of fat.
1000 1050
Haemoglobin
Haemoglobin molecules within the red blood cells (Erythrocytes) carry 97% of the oxygen in the blood, while the remaining 3% is dissolved in the plasma.
100 - Og un lo ade d to tissue s 75 - 50 - CM 25 - 0 - 25 50 75 0 100 PO 2 [m m H g]
Figure 4-3 Oxygen-haemoglobin dissociation curve. Note that the oxygen satura tion of blood leaving the lungs (the arterial saturation) is about 97%, while that of venous blood is still about 67%. Hence approximately 30% of the haemoglobin- bound oxygen is unloaded in the tissues during one cycle through the body.
Each haemoglobin molecule consists of four iron-containing haeme groups as well as the protein globin. It is the iron to which the oxygen atoms easily bind, causing the haemoglo bin molecule to assume a new three-dimensional shape. In the oxygenated state haemoglobin is referred to as oxyhaemoglobin (Hb0 2), and in the reduced state it is called
[Severinghaus 1979], which relates the percentage saturation of haemoglobin (SO2) to the
partial pressure of oxygen dissolved in blood (PO2).
0.10 I « 0.08 0 1 i 0.06 0 1 O 0 04 HbO.
i
0.02 I w 0.00 500 600 700 800 900 1000 W a v e le n g th [nm ] - 0.003 - H 0.002 - R 0.001 - 0.000 700 800 900 W a v e le n g th [nm ] 1000Figure 4-4 Absorption spectra of oxy- and deoxyhaemoglobin in the ranges 450- 1000 nm (top), and 650-1050 nm (bottom) [Cope 1991].
The spectra of oxy- and deoxyhaemoglobin, expressed in terms of the specific extinction coefficient, can he seen in Figure 4-4. While both absorb strongly in the blue and green regions of the visible spectrum, the absorption of deoxyhaemoglobin is slightly stronger beyond about 690 nm. Hence venous blood appears in a darker red than the arterial blood. Note the isobestic point at about 800 nm, where the two curves intersect. If one is able to accurately determine the tissue absorption coefficients at, say, 20 nm, to either side of this
wavelength it is then possible to determine both blood voliune and oxygenation [Cope 1988]. The strong increase in absorption below 600 nm sets a lower limit for spectroscopic or imaging measurements. Optical properties of blood in the wavelength range 400-2500 nm are discussed in great detail by [Roggan 1999].
There is a number of other tissue chromophores, such as melanin, cytochrome c oxidase, myoglobin, etc. These can be largely ignored in the near infrared (but not in the visible!) regime, as they contribute little to the overall attenuation. The combined ‘absorp tion window’ lies in the range 600-900 nm. Taking into account the increase in scattering at lower wavelengths and a rapid drop in quantum efficiency of practical detectors at longer wavelengths, the ‘useful’ range is approximately 650-900 nm. Table 4-1 below summarises the absorption and scattering coefficients of various tissue types relevant to this project, together with the corresponding reference source for the data.
Tissue type Sample X[nm] Pa [mm'^l \Ls' [mm^] Reference
neonatal grey matter in vitro 650-900 0.04-0.08 0.4-0.9 [van der Zee 1993] neonatal white matter in vitro 650-900 0.04-0.07 0.5-1.2 [van der Zee 1993]
adult brain in vitro 700-900 0.1-0.2 2-5 [Sterenborg 1989]
adult grey matter in vivo 811 0.018-0.019 0.48-0.74 [Bevilacqua 1999]
adult grey matter in vivo 849 0.018-0.019 0.45-0.74 [Bevilacqua 1999]
adult white matter in vivo 849 0.013 0.98 [Bevilacqua 1999]
adult skull in vivo 849 0.022 0.91 [Bevilacqua 1999]
adult grey matter in vitro 650-900 0.04-0.06 1.9-2.2 [van der Zee 1993]
adult white matter in vitro 650-900 0.02-0.03 8-10 [van der Zee 1993]
pig brain in vitro 630 0.026 5.7 [Patterson 1987]
pig skull in vitro 650-950 0.04-0.05 2.63-1.32 [Firbank 1992]
healthy breast tissue in vitro 700-900 0.022-0.075 0.53-1.42 [Peters 1990]
breast carcinoma in vitro 700-900 0.045-0.050 0.89-1.18 [Peters 1990]
healthy breast tissue in vivo 800 0.002-0.003 0.72-1.22 [Mitic 1994]
Note the low scattering of the neonatal as compared to the adult brain, as well the smaller differences between grey and white matter. These facts combined with the small neonatal head size (approximately 6-12 cm across), and the thinner clear CSF layer (see section 2.1) suggest that light is much more likely to penetrate deep into the neonatal, as compared to adult, white matter - thus supporting the feasibility of a neonatal brain imaging modality. The thickness of the CSF layer is relevant because it exhibits low absorption and almost no scattering, therefore acting as a light guide which can channel photons around the head without much penetration into the grey and white matter [Firbank 1995a, Okada 1995].
Finally, it has to be noted that most measurements presented in the table were made using in vitro tissue samples, which can yield very different results as compared with in vivo results. In addition, the measurement techniques currently available still contain many sources of error, and most published data have been obtained with only a small number of samples. The data must therefore only be taken as being approximate. Consequently it is important for the field that investigators continue to explore the optical properties of the human neonatal head.