Biomedical Engineering Reference
In-Depth Information
Ta b l e 1 .
Optical properties for the six skin layers and oxygenated blood (hematocrit=42%) used
in skin simulations for 635, 785 and 830 nm
μ
a
mm
−
1
μ
s
mm
−
1
g
Wavelength (nm)
635 785
830
635 785 830
635
785
830
Epidermis
0.15 0.1
0.0122
4.8
3.5 1.81 0.85
0.85
0.9
Papillary dermis
0.15 0.1
0.0122
3
2
1.81 0.85
0.85
0.9
Superior blood net 0.15 0.1
0.0122
3
2
1.81 0.85
0.85
0.9
Reticular dermis
0.15 0.1
0.0122
3
2
1.81 0.85
0.85
0.9
Inferior blood net 0.15 0.1
0.0122
3
2
1.81 0.85
0.85
0.9
Subcutis
0.15 0.1 0.00856 2.4
2
1.12 0.85
0.85
0.9
Blood
0.34 0.5
0.52
2.13
2
2
0.991 0.991 0.991
The optical properties of deoxygenated and oxygenated blood are equal except
for the absorption coefficient (see table 5 from [10]). However, this value is rather
independent of the level of oxygenation of the blood, once the chosen wavelengths
are close to the 800 nm isobestic point of oxygenated and deoxygenated haemoglobin.
Rat Brain Model.
The rat hippocampus consists of several substances such as grey
matter, white matter and blood vessels, among others. The blood percentage is nearly
4.5% and the white matter is up to 4% of the blood volume. As the percentage of white
matter is very low we considered that the hippocampus has 95.5% of grey matter and
4.5% of blood (3.6% of oxygenated blood and 0.9% of deoxygenated blood) [10] [13].
The optical properties chosen in the simulations for the 785 nm wavelength were
based on Fredriksson
et al.
[10]. The absorption coefficients used were 0.2, 0.5 and 0.64
mm
−
1
for grey matter, oxygenated and deoxygenated blood, respectively. The scatter-
ing coefficients were 0.78
mm
−
1
for the grey matter and 2
mm
−
1
for oxygenated
and deoxygenated blood and the anisotropy factor was 0.900 for grey matter and 0.991
for oxygenated and deoxygenated blood [10]. Concerning the scattering functions, the
blood (oxygenated and deoxygenated) was modeled with the Gegenbauer kernel scat-
tering phase function, with
α
=1
and
g
=0
.
948
mm
−
1
[10]. For grey matter the
Henyey-Greenstein phase function was used with
g
=0
.
85
mm
−
1
[10]. For blood
a hematocrit equal to 42% was considered. The refractive index was set to 1.4 to all
components and the laser light was simulated as a pencil beam with a perpendicular
entrance in the tissue. The path tracking was recorded with
1
/μ
s
resolution, where
μ
s
is the reduced scattering coefficient. The numerical aperture (NA) of the optical fibres
is 0.11.
The simulations were performed only for the 785 nm laser light wavelength due to
the absence of information concerning optical properties of grey matter, oxygenated
and deoxygenated blood for the 1300 nm laser light beam.
Monte Carlo Simulations.
For the simulations, Monte Carlo software MONTCARL
from Frits de Mul was used [14] [15] (see also www.demul.net/frits).
2.3
Measurements
Phantom Measurements.
Measurements were carried out with the non invasive pro-
totype, with the 635 nm laser light, in the built phantom. The milk and two different
Search WWH ::
Custom Search