Thermal Therapy - RF/Microwave Interaction with Biological Tissues - page 154

ature is maintained at 30-40°C for superficial tumors and at approximately
10°C for subsurface or deep tumors. The electrodes can be changed from 4 to
30 cm in diameter. Optimum electrodes are selected depending on the size
and location of the tumors. The present applicator system has a thermometry
system with four Teflon-coated probes of copper-constantan microthermo-
couples. The thermometry system with the microthermocouples is connected
to an automatic temperature-power feedback controller with an accuracy of
0.2°C. To protect the thermometry system from RF interference, a high RF
wave filter is inserted in the thermometry system and makes it possible to
measure temperature even during heating. The temperatures measured at four
points in the heated tissue are continuously displayed both graphically and
digitally on the computer screen. These data are also continuously recorded
on a floppy disc, and a hard copy can be obtained on the internal printer. The
power absorbed by the heated site is also continuously displayed graphically
and digitally and is recorded.
In the RF capacitive heating, the excessive heating of subcutaneous fat or
head skull should be noticed because an electric field becomes perpendicular
to these high-impedance layers of human body. It has been shown that a
patient with a subcutaneous fat layer of more than 1.5-2 cm thickness is diffi-
cult to be heated with this heating modality [5]. The advantage of this kind
of capacitive applicator system, however, is that it has wide applicability to
various anatomical sites, inducing relatively small systemic stress.
To more widely apply this applicator system to a portion of the thin sub-
cutaneous fat such as the neck, a localized heating technique is needed. Gen-
erally, in a dielectric applicator using a pair of circular conductive electrodes
in RF, a large-size electrode is needed to heat a deep-site tumor uniformly.
This means that, in the case of a capacitive applicator with a pair of circular
electrodes, a diameter a of applicator more than 1.5 times the space d between
both electrodes is needed to achieve uniform heating inside the human body.
If the height of the heating region is 15 cm, 22.5 cm is needed as the diameter
of the electrode to uniformly heat the human body in this area. If the diame-
ter is smaller while the volume remains constant, hot spots will arise under the
electrodes and heating will not be deep. To illustrate this, the electric field dis-
tributions using a cubic agar phantom (a medium considered equivalent to the
muscle) in the case of heating by the capacitive-coupling-type applicator are
shown in Figures 4.8 a , b [1]. It is found that the electric field concentrates in
the area under the pair of electrodes when the electrode diameter is small, as
in Figure 4.8 b . Actually, even in experiment, these parts are observed as hot
spots (local heat spots), as shown in Figure 4.9.
To solve this problem and achieve regional heating, a capacitive applicator
with a double electrode has been developed, as shown in Figure 4.10 [6]. By
introducing a subelectrode consisting of a ferrodielectric material under the
main electrode, the electric field can concentrate between a pair of subelec-
trodes of ferrodielectric materials. The size of the beam spot is proportional
to the diameter of the semicircular subelectrode. If the semicircular subelec-