Biomedical Engineering Reference
In-Depth Information
3.2
Skin
Skin Simulations.
The mean depth of the Doppler events per photon, the percentage of
Doppler shifted photons detected and the mean of Doppler scattering events per photon
obtained in Monte Carlo simulations, for the skin model, are presented in table 3.
Ta b l e 3 .
The mean depth of the Doppler events for each photon, the percentage of Doppler events
detected and the mean of Doppler scattering events per photon for the skin model
Mean Doppler Detected Doppler Mean Doppler
Fibre distance
depth (mm)
(%)
scattering
mm
Wavelength (nm)
635 785 830
635
785
830
635 785 830
0.00
0.24 0.25 0.27 1.94
1.97
3.54 1.19 1.16 1.15
0.14
0.27 0.29 0.31 9.62 10.71 14.54 1.23 1.22 1.23
0.25
0.28 0.30 0.33 15.45 16.29 20.39 1.26 1.23 1.25
1.20
0.37 0.38 0.41 47.70 41.89 43.45 1.49 1.41 1.46
The mean depth for the Doppler events, the percentage of Doppler shifted photons
detected and the mean Doppler scattering per photon increase with fibre distance, as for
the phantom model, because a larger emitting-receiving separation allows sampling a
larger volume.
Furthermore, the mean measurement depth also increases with the wavelength. This
is due to both skin absorption and scattering coefficients decrease with the wavelength,
allowing the photons to travel a longer path. Similar results were obtained by Fredriks-
son
et al.
[10], with a measurement depth slightly smaller, but with the same order of
magnitude. It can be noticed that the mean depth of the Doppler shifted photons never
reaches the reticular dermis or the layers below this one, since reticular dermis lies at a
depth of 1.175 mm and the mean depth predicted for the Doppler events is always lower
than this value. In addition, detected photons reached the superior blood net dermis only
when detected with the 1.2 mm fibre distance.
The percentage of Doppler shifted detected photons also increases with the wave-
length excluding for the 1.2 mm emitting-receiving fibre distance. This may be related
to the distribution of the Doppler events percentage in each layer (cf. table 4). It can be
seen that the reticular dermis is the 2nd layer with the most detected Doppler photons
for 1.2 mm fibre distance (for 785 and 830 nm laser light), whereas for the other fibre
distances the 2nd layer with the most detected Doppler photons is the papillary dermis.
This proves that the photons detected at 1.2 mm from the emitting fibre cross a higher
volume of blood. Besides, the Doppler events percentage in the inferior blood net (for
1.2 mm fibre distance) decreases going from 635 to 785 nm laser light and increases
going from 785 to 830 nm laser light. This layer has the second higher blood concen-
tration when compared with the other layers. The higher volume of blood crossed and
the increasing of the blood absorption coefficient with the wavelength may cause this
non-linearity.
In opposition, the mean Doppler scattering event does not follow a general trend
when increasing the wavelength of the incident light but is smaller than 1.5, which
means that there are few photons that suffer multiple Doppler shifts.
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