Biological Effects - RF/Microwave Interaction with Biological Tissues - page 132

TABLE 3.3 Composition of Skull-Equivalent Phantom

Material Amount (g)

Silicone emulsion 250.0

Glycerol 200.0

TX-151 12.0

Agar 20.0

Polyethylene powder 130.0

Source: From [118].
Note: Batch is approximately 500 cm 3 .

TABLE 3.4 Average Parameter Values of Phantom

Parameter Brain Skull

Relative permittivity 43.1 17.3

Conductivity (S m -1 ) 0.83 0.25

Density (kg m -3 )

880 910

Source: From [118].
Note: Conductivity and relative permittivity are values meas-
ured at 900 MHz.

a section or surface of the phantom. The SAR is measured either by using an
E -field probe or by thermographic methods. Equation (3.6) gives the relation
between the SAR and temperature.
The relative permittivity and conductivity of these phantoms have been
measured from 200 to 3000 MHz [118]. The measured values are in very good
agreement with the COST244 brain and skull tissue. The average values of the
parameters are given in Table 3.4.
The thermal characteristic of the phantom is very important and the pre-
cautions to be taken for thermographic analysis of phantoms are the same as
for actual human heads. Several methods are based on thermal measurements:
(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 irradiated sample; (2) thermometric methods , used to measure
the temperature due to microwaves with particular types of nonperturbing
thermometer, but only a few are commercially available; and (3) thermo-
graphic techniques , used to measure the temperature with particular thermo-
graphic cameras. In thermography, when using a thermographic camera,
emissivity of the object is an important parameter. It is defined as the ratio of
the radiation flux per unit area of the emitter to that of a blackbody radiator
at the same temperature and under the same conditions. In short, it measures
how much radiation is emitted by the object compared to that emitted by a
perfect blackbody. Normally, emissivities range from approximately 0.1 to 0.95.