Biological Effects - RF/Microwave Interaction with Biological Tissues - page 91

1. Calorimetric methods particularly suited for in vitro measurements, in
which heating and cooling data can be analyzed to estimate the energy
absorbed by an exposed sample.
2. Thermometric methods , used to measure the temperature due to
microwaves with particular types of nonperturbing thermometers, with
only a few commercially available.
3. Thermographic techniques , used to measure temperature with particular
thermographic cameras.

This last technique is noninvasive and, as a consequence, especially attractive
for thermal measurements on human beings. As an example, human thermal
effects have been measured on faces exposed to the emission of a mobile tele-
phone handset using a thermographic camera to measure the thermal distri-
bution over the surface of the face and obtain quantitative thermometric data.
The camera was a forward-looking IR camera, measuring and imaging the
emitted IR radiation from the head, through a pattern of 10,000 thermocou-
ples located in the lens [7].
The rate of temperature change in the subcutaneous tissue in vitro exposed
to RF or microwave energy is related to the SAR as [1]

D
D

T
t

(

SAR +-- PPP

C

)

(3.4)

=

mc b

where D T is the temperature increase, D t the exposure duration, P m the meta-
bolic heating rate, P c the rate of heat loss per unit volume due to thermal con-
duction, P b the rate of heat loss per unit volume due to blood flow, and C the
specific heat. If before the exposure a steady-state condition exists such as

PPP
mc

=+ (3.5)

b

then during the initial period of exposure one has

D
D

T
t

SAR

= (3.6)

C

and the SAR can be determined from measurements of an increase in the
tissue temperature over a short period of time following the exposure. For
tissue phantoms and tissues in vitro, Eqn. (3.6) may be used as long as the
thermal conductivity can be neglected, that is, for short durations of exposure.
Several methods of SAR determination are based on thermal measurements
and utilization of this equation. The specific heat and density of various tissues
are summarized in Table 3.1.
More generally, from a macroscopic point of view, thermal effects resulting
from the absorption of EM waves inside biological tissues are described in
terms of the bioheat equation. Time-domain modeling should be necessary
when the exposure is pulsed microwave energy. Spatial approximations usually