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Abstract
The technical feasibility
of commercially developing a safe and effective direct contact diathermy applicator
operating at the industrial, scientific, medical (ISM) frequency of 915 MHz
is demonstrated. The basic design consists of a circular waveguide which is
internally loaded with two orthogonal pairs of forward ridges to obtain
circular polarization and two rear ridges with a probe to excite the guide.
Two prototype designs are considered: the small applicator (15 cm diameter) has one
annular choke covered with a 2.5-cm thick microwave absorber, and the large
applicator (25 cm
diameter) has two additional concentric chokes to limit leakage radiation.
The performance of the applicators was evaluated in terms of the requirements
of a ORH microwave diathermy test protocol to
control stray radiation and deliver a thermally effective absorbed dose rate
to simulated muscle tissue of a phantom with a 1-cm or 2-cm fat layer. The
net power required to deliver a thermally effective 235-W/kg specific
absorption rate (SAR) to such a planar phantom was determined. For this net
power, leakage levels considerably less than 5 mW/cm² (at 5 cm from applicator-phantom
boundary) were obtained for the applicators in direct contact with the
phantom. If a small spacing (1
cm) between these applicators and planar phantoms is
introduced, the net power required to deliver an effective SAR to a phantom
and the associated leakage can become excessive. For the small applicator,
the required net power for inducing an SAR of 235 W/kg in muscle tissue of a
1-cm fat layer phantom is about 330 W and the leakage is about
120 mW/cm². For a 2-cm fat layer phantom, these values are somewhat
higher. For the large applicator, using a 1-cm fat layer phantom, the values
are about 200 W and about 17 mW/cm². Again, for a
2-cm fat layer phantom, these values are somewhat higher.
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