Biomedical Engineering Reference
In-Depth Information
Ta b l e 4 .
Doppler events percentage in each layer for the skin model
Skin layers
Wavelength Fibre distance
Papillary Superior Reticular
Inferior
Epidermis
Subcutis
(nm)
(mm)
dermis
blood net
dermis
blood net
0.00
0.00
41.07
51.19
7.24
0.50
0.00
0.14
0.00
35.10
53.06
10.31
1.37
0.17
635
0.25
0.00
31.98
55.23
11.10
1.49
0.21
1.20
0.00
19.43
56.21
19.51
4.08
0.77
0.00
0.00
41.73
48.84
8.93
0.51
0.00
0.14
0.00
30.85
54.86
12.97
1.33
0.00
785
0.25
0.00
27.69
56.42
14.38
1.51
0.00
1.20
0.00
18.81
55.44
22.24
3.51
0.00
0.00
0.00
38.76
49.36
10.35
1.54
0.00
0.14
0.00
26.66
55.91
15.09
2.33
0.00
830
0.25
0.00
23.09
57.05
17.27
2.59
0.00
1.20
0.00
16.57
53.06
25.15
5.218
0.00
Ta b l e 5 .
First order moment of Doppler power spectrum (M1) for skin model
M1 (Hz)
Fibre distance (mm)
Wavelength (nm)
0.00
0.14
0.25
1.20
3
.
57
×
10
18
2
.
51
×
10
19
3
.
71
×
10
19
1
.
63
×
10
20
635
3
.
45
×
10
18
2
.
13
×
10
19
3
.
23
×
10
19
1
.
05
×
10
20
785
5
.
30
×
10
18
2
.
66
×
10
19
3
.
93
×
10
19
1
.
06
×
10
20
830
Simulation results also demonstrate that M1 increases with the emitting-receiving
fibre distance (see table 5). Since M1 is proportional to the concentration of moving
RBCs times its average velocity, and both parameters increase with the fibre distance,
this was expected.
Regarding the wavelengths, M1 firstly decreases from 635 to 785 nm and then in-
creases for 830 nm. This can be explained if we look at the Doppler photons percentage
that exceeds the reticular dermis. This percentage is higher for 830 nm followed by
635 nm and lower for 785 nm, with the exception of the 0 mm fibre distance (table 4).
As the inferior blood net has the highest concentration of the high velocity component
of RBCs (30 mm/s), it results in higher Doppler shifts for 635 nm than for 785 nm
photons. Therefore M1 will be higher for 635 nm than for 785 nm. Table 6 shows the
results of the path tracking study for the skin model. It can be observed that the average
path number travelled by each photon, the mean path depth and the average path length
increase with emitting-receiving fibre distance. This occurs because increasing the fibre
distance a greater tissue volume is probed, and so, more scattering events occur. The
mean path number does not follow a general trend when increasing the wavelength of
the incident light.
The path depth increases with the laser light wavelength due to both skin absorp-
tion and scattering coefficients decrease with the wavelength, allowing the photons to
travel a longer path. This is in agreement with the mean depth of Doppler events results
showed in table 3. The average path length increases with the wavelength, excluding for
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