Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 168

1.0

41.5 ∞ C

10 - 1

42.0 ∞ C

10 - 2

43.0 ∞ C

42.5 ∞ C

C
44.0 ∞ C
44.5 ∞ C
45.5

43.5 ∞

10 - 3

∞ C

10 - 4

46.5 C

∞

10 - 5

0 100 200 400 500 600 700

300

Heating time (min)

FIGURE 4.27 Survival rate curve of biological cell.

It is clear that the temperature rises are strongly improved by introducing the
resonant circuit for implant coil design. In this experiment, a ferrite core appli-
cator is used to investigate the heating characteristic at 4 MHz. The output
power is normally 500 W. The heating tests have been conducted using a 20-
cm agar phantom cube that conforms to the guideline assigned by the QAC
in the Journal of the American Society of Hyperthermia Oncology . The implant
is put in the center of the phantom.

4.2.6 Detailed Theory of RF Dielectric Heating

In Section 4.2.2, an outline of the heating principle of RF capacitive coupling
applicator has been given. The human body, which consists of inhomogeneous
medium, was simplified by introducing a two-layer medium differing in dielec-
tric constant and conductivity. In addition, the permittivities of the two-layer
medium have been converted into the equivalent permittivity of a single-layer
medium. The electric power loss per unit volume, that is, the heating capacity,
is calculated using this equivalent permittivity. This approach is based on the
heterogeneous theory of Maxwell and Wagner (see Section 2.2).
In this section, the method for deriving expressions (4.4) and (4.5) is
described in detail. The beginner who reads this chapter for the first time may
skip this section. The derivation procedure consists of replacing each permit-
tivity of the two-layer medium by the permittivity of the equivalent mono-
layer medium. To do this, we derive the equivalent admittance Y between
parallel-plates and express the current I d flowing between the parallel-plate
electrodes considered as a static capacitance. This yields the equivalent
permittivity (4.4).