Biomedical Engineering Reference
In-Depth Information
Thermal rise observed in experiments vs. simulation
100
90
80
Experimental : Heater
Simulation : Heater
Experimental : Mid-vitreous
Simulation : Mid-vitreous
70
60
50
40
30
20
10
0
0
10
20
30
40
Time, minutes
50
60
70
80
Figure 17.5 Comparison between observed experimental results and computationally derived results for an
experiment designed to validate the computational models. (From Gosalia K, Weiland J, Humayun M, and Lazzi G.
IEEE Transactions on Biomedical Engineering, 51(8): 1469-1477, 2004. With permission.)
where
n
is perpendicular to the skin surface and the right hand expression models the heat losses
from the surface of the skin due to convection and radiation, which is proportional to the difference
between skin temperature (
T
(
x, y, z
)
) and external environmental temperature (
T
a
).
For all the computations performed in the example above, the temperature of blood was
assumed to be constant at 37
8
C, while
H
a
is the heat convection coefficient and is assumed to be
10.5 W/(m
2
.
8
C). The thermal parameters for all the tissues in the head model have been directly
obtained from previous studies (DeMarco et al., 2003; Bernardi et al., 2003).
In order to validate the thermal method and model used,
in vivo
experiments conducted with dogs
were simulated, and experimental and computational results were compared. The experiment com-
prised of mechanically holding a heater probe (1.4
1.4
1.0 mm in size) dissipating 500 mW in the
vitreous cavity of the eye of the dog for 2 h (Gosalia et al., 2004; Piyathaisere et al., 2003). The
experimental set up included thermocouples to measure the temperature rise at different locations in
the vitreous cavity and the retina during this period. Figure 17.5 shows the comparison between the
experimentally observed and the simulated results for temperature rise at the retina and the vitreous
cavity. The uncertainty in the exact locations of the thermocouples during the actual experiment is the
likely cause of the small difference between simulated and experimental results.
17.5.1
Heat and the Telemetry System
As mentioned in the preceding paragraphs, the wireless telemetry system can be a source of thermal
rise since it causes deposition of electromagnetic (EM) power in the head and eye tissues. Using the
FDTD technique, the deposited EM power can be quantified in terms of the specific absorption rate
(SAR) and several studies have quantified the thermal effects in the human head and eye tissues
based on the evaluated SAR using the bio-heat equation (DeMarco et al., 2003; Bernardi et al.,
1998, 2000; Hirata et al., 2000). SAR is expressed as
s
*
2
=
2
ðÞ
for conductivity
s
, electric field
*
,
Search WWH ::
Custom Search