Biological Effects - RF/Microwave Interaction with Biological Tissues - page 89

osition per unit mass. This is why the specific absorption rate (SAR) is used as
the dosimetric measure at those frequencies. It is expressed in watts per kilo-
gram or derived units (e.g., milliwatts per kilogram). In some respect, the gen-
eralized use of the SAR at microwaves is unfortunate because the SAR is
based on absorption only, which raises questions about using this parameter
for evaluating effects that may be of another nature than absorption. The pos-
sibility of nonthermal effects is a controversial issue and having only the SAR
for evaluating all the biological effects does not help in reducing the contro-
versy. In fact, the SAR may be a valid quantitative measure of interaction
mechanisms other than absorptive ones when the mechanism is dependent on
the intensity of the E field, except, however, when the direction of the E field
is of importance with respect to the biological structure. Similarly, the SAR
concept may not be sufficient for the interactions directly through the H field.
With these limitations in mind, the SAR concept has proven to be a simple
and useful tool in quantifying the interactions of RF/microwave radiation with
living systems, enabling comparison of experimentally observed biological
effects for various species under various exposure conditions, providing the
only means, however imperfect, of extrapolating the animal data into poten-
tial hazards to humans exposed to RF radiation, and facilitating, planning, and
effectively executing the therapeutic hyperthermic treatment [1].
The SAR is defined as the time derivative of the incremental energy dW
absorbed by or dissipated in an incremental mass dm contained in a volume
element dV of a given density r [2, 3]:

d
dt

dW
dm

d
dt

dW
dV

SAR = Ê
Ë

ˆ
¯

Ê
Ë

ˆ
¯

= Ê
Ë

ˆ
¯

È
Í

˘
˙

(3.1)

( ) r

or, using the Poynting vector theorem for sinusoidal varying electromagnetic
fields:

s
r

we e
r

≤

SAR = Ê
Ë

ˆ
¯

Ê
Ë

ˆ
¯

2 2

0

E = E (3.2)

i i

2 2

where is the peak value of the internal electric field (in volts per meter).
The average SAR is defined as the ratio of the total power absorbed in the
exposed body to the mass in which it is absorbed, which is not necessarily that
of the total body: The SAR is the ratio of absorbed power by absorbing mass .
The local SAR refers to the value within a defined unit volume or unit mass,
which can be arbitrarily small.
When extrapolating the results of, for instance, animal experiments to
human exposure, the conditions of EM similitude can be applied [4, 5]. These
conditions are often used in a reduced form termed frequency scaling . They
enable us to use results obtained with a given object and adapt them to predict
the results to be obtained with another object, similar in form to the first and
differing only by a scale factor. Using reduced scale models is of common use
in a variety of fields, such as hydraulic and mechanical engineering. It is of
common practice also in electrical engineering, for instance when construct-

E i