Biological Effects - RF/Microwave Interaction with Biological Tissues - page 131

Studies carried out by numerical simulation will be reviewed in Section
3.11.3. Another procedure consists of producing a phantom: a biological tissue
equivalent with the same permittivity and conductivity as brain and skull
tissue. Several papers in [116] describe the procedure and mention a number
of references. These phantoms make it possible to accomplish highly reliable
and precise estimation of the SAR in biological tissue. They are most fre-
quently human head models, because of both the main concern, about the
possible effects into the head, and the practical realizability of the model.
Phantom models of cube, sphere, and realistic human heads have been fabri-
cated. Measurements are then performed to estimate the SAR in the human
head models exposed to microwave sources by using a thermographic method.
More specifically, such phantoms offer the possibility of measuring the local
peak SAR. This is extremely interesting, in particular in the case of local exter-
nal microwave source, such as a cellular telephone.
A typical composition of a brain-equivalent phantom is given in Table 3.2.
It is in accordance with recommendations from the Action of the European
Cooperation in the field of Scientific and Technical Research (COST)244
[117]. The agar is used for maintaining the shape of the phantom by itself. The
relative permittivity is controlled with additional rate of polyethylene powder.
In order to mix water with the polyethylene, TX-151 is selected for stickiness.
The sodium azide is a preservative. In addition, the loss factor depends on the
concentration of sodium chloride. The relative permittivity and conductivity
can be controlled by the quantity of polyethylene powder and sodium
chloride.
The composition of a skull-equivalent solid phantom is shown in Table 3.3.
The glycerol is used as a solvent of the skull phantom. Skull tissue is a low-
loss medium; therefore, the solvent should be low loss and hydrophilic.
However, the relative permittivity would be too small with only glycerol, and
silicone emulsion is added. The constant can be controlled by the mixture rate
of the glycerol and silicone emulsion.
These phantoms do not need rigid shells because they contain and shape
the jelly material. Therefore it is easy to directly measure temperature rise on

TABLE 3.2 Composition of Brain-Equivalent Phantom

Material Amount (g)

Deionized water 3375.0

Agar 104.6

Sodium chloride 21.5

Sodium azide 2.0

TX-151 57.1

Polyethylene powder 548.1

Source: From [118].
Note: Batch is approximately 4000 cm 3 .