Biomedical Engineering Reference
In-Depth Information
consideration in order to demonstrate the worst-case scenario. The Specific
Absorption (SA) per pulse, which is being used to introduce additional limitations
for pulsed transmissions in the ICNIRP limitations, is computed using:
SA
¼
SAR
T
p
ð
7
:
9
Þ
where T
p
is the pulse duration.
It should be noted that heat sources such as the electronic components used in
the implanted circuitry also affect the SAR variation in the body tissues. The
influence of these heat sources are not considered in the performed simulations,
since the main purpose of this chapter is to determine the effect of the IR-UWB
electromagnetic field on the SAR variations.
7.2.3 Temperature Variation Based on Bio Heat Model
When exposed to an electromagnetic field, the absorbed power by the body tissues
causes a temperature increase. A temperature increase exceeding 1-2 C in the
human body tissue can cause adverse health effects, such as a heat stroke [
43
]. In
addition to the study of SAR variations, this chapter also analyses the temperature
variation in the human head when it is exposed to IR-UWB transmission from an
implanted transmitter. The temperature of the body tissues is modelled using the
bio heat equation in (
7.10
)[
44
]:
Cp
dT
dt
¼r
k
r
T
ð
Þþ
q
SAR
þ
A
BT
T
b
ð
Þ
ð
7
:
10
Þ
where K is the thermal conductivity, Cp denotes the specific heat, A is the basal
metabolic rate, B is the term associated with blood perfusion, q is the tissue density
in
kg
m
3
and
rð
k
r
T
Þ
represents the thermal spatial diffusion term for heat transfer
through conduction at temperature T in degrees Celsius. The bio heat equation in
(
7.10
) is solved with the boundary conditions given in (
7.11
), namely:
K
dT
dn
¼
h
ð
T
T
a
Þ
ð
7
:
11
Þ
where T
a
is the temperature of the surrounding environment, n is the unit vector
normal to the surface of interest and h is the convection coefficient for heat
exchange with the external environment.
The human body tries to regulate its core temperature by various mechanisms in
order to keep it at approximately 37 C. The effect of thermo regulatory mecha-
nisms causes the tissue specific basal metabolic rate (A) and blood perfusion
