Biomedical Engineering Reference
In-Depth Information
Fig. 7.1 Simulated variation
of e
0
and e
00
in the UWB
frequency range, IEEE
copyright [
39
]
permittivity variation of water depending on the incident frequency instead of the
constant value used in [
36
] for age related calculations. Figure
7.1
depicts an
example for the variation of e
0
and e
00
as a function of frequency for the brain tissue
material of a child of 7 years and a male of 55 years [
39
]. The basic set of tissue
parameters required for the calculation (e.g. s
1
s
4
, De
1
De
4
and a
1
a
4
) of the
4-Cole Cole approximation is taken from [
33
]. It should be observed that while e
0
depends on the variation of the tissue water content with age, e
00
is largely age
independent as the latter is determined by the conductivity (r). This can be seen in
Fig.
7.1
.
7.2.2 SAR Calculation Method
The Finite Integration Technique (FIT) is used as the volume discretization
approach for the described simulations. This technique is used to calculate the
absorption loss of the body tissues by discretising the Maxwell's curl equations in
a specified domain. The discretising volume element is chosen to be cubic, and
appropriate boundary conditions are applied in order to define the power absorbed
within that cube. Further information on the FIT model can be found in [
40
,
41
].
SAR is defined as the power absorbed by the mass contained within that dis-
cretised volume element as shown in (
7.8
)[
42
].
DW
qdV
d
SAR
¼
ð
7
:
8
Þ
dt
where DW is the power absorbed by the discretised volume element, q is the
density of the human tissue material, dt is the incremental time and dV is its
incremental volume. Present simulations use an IR-UWB signal pulse as the
excitation signal, and are conducted in order to calculate the 10 g averaged SAR so
as to compare it with the ICNIRP specifications for pulse transmission [
5
]. The
maximum
SAR
within
the
10 g
of
tissue
averaging
volume
is
taken
into
