Biological Effects - RF/Microwave Interaction with Biological Tissues - page 93

study to take into account all the components of power. As an example, an in
vivo experiment was conducted on rabbits to measure evoked potentials in
the presence and absence of microwave exposure of the spinal cord, respec-
tively. An electrical stimulus was applied to the peripheral nervous system of
a rabbit while an electrode in the cortex measured the impulse response—the
evoked potential. The spinal cord was exposed at 4.2 GHz by an implanted
microantenna. The purpose of the experiment was to distinguish between
thermal and possible nonthermal effects. A statistical treatment of the
recorded data showed that there was a microwave effect after a long period
of exposure and that the induced variations were reversible. Calculating the
power deposition, the bioheat equation in the time domain indicated that
microwave exposure resulted in a temperature increase within the spinal
cord. The conclusion was that no nonthermal effect was observed in the
experiment [8].
A number of specific thermal effects have been observed [12-14]. They
depend, of course, on the spatial distribution of the SAR [15, 16]. It is esti-
mated that a SAR of about 1 W kg -1 produces an increase of 1°C in the human
body temperature, taking thermoregulation into account [9]. Behavior is the
most sensitive measure of effects [17]. In some experiments, it has been
observed that pulsed fields produce a detectable effect at power levels smaller
than continuous waves [8, 17].
On the other hand, several modeling methods and tools are now available.
Finite-difference time-domain (FDTD) techniques offer broadband analysis
and computational efficiency. Recent anatomical models of the head yield a
better accuracy. Such models have greatly improved data visualization [18].
The level of SAR influences the thermal effects [19-23]:

• One watt per kilogram yields an increase of 1°C in the human body, taking
thermal regulation into account.
• Ocular damage (cataracts) has been observed at 100 mW cm -2

and more

above 1 GHz.
• Corneal damage has been observed in monkeys for a SAR of 2.6 W kg -1
at 2.45 GHz; it is interesting to observe that, with drug pretreatment such
as phenylthiazines and miotics [8], damage has been observed for a ten
times lower value of SAR, in the range 0.26-0.5 W kg -1 .
• Retinal damage has been observed on monkeys with a SAR of 4 W kg -1
in the range 1.25-2.45 GHz with pulsed fields.
• The SAR above 15 W kg -1

produces malformations, with more than 5°C

temperature increase.

For mobile communication fields, the interaction between a handset antenna
and the human body is of special interest [18, 24-26].
Hyperthermia is the name given to one of the medical treatments for cancer
to heat tumors up to therapeutic temperatures (43-45°C) without overheat-