10 Principles Of Lesioning, Typical Lesion Sizes And Basic Procedures; Principles Of Lesioning - NeuroTherm NT1100 Operator's Manual

10 PRINCIPLES OF LESIONING, TYPICAL LESION SIZES AND BASIC PROCEDURES

Principles of Lesioning

10.1.1 The basic physical principles of radiofrequency ablation
Figure 10.1
conductive electrolytic media, and thus the patient's body becomes part of the RF circuit. This
current spreads out from the electrodes and flows through the electrolytic tissue medium of the
body. The active and dispersive electrodes have a similar physical role in delivering and
receiving the current, but functionally, because of their differing areas, they have very different
effects with regard to the RF heating process.
Figure 10.2
The active electrode with its smaller surface area has much
higher field densities in the tissue
field density causes significant heating near the active electrode
surface. The dispersive electrode has a much larger area, and,
as a consequence, the field density is much lower in the tissue
adjacent to it. This results in a lower radio frequency heating
effect, and thus if the dispersive electrode is large enough no appreciable heating will occur near it. In
fact, a large area surface plate to join to the skin with a conductive gel for good conductivity will not heat
appreciably even though this same radiofrequency current will cause intense heating near the much
smaller active electrode. It is recommended for most radio frequency procedures that the dispersive
electrode, therefore, should have an area of greater than 150 square centimeters to be safe from any
significant heat elevation when RF lesions delivering 50 watts or less are used.
The mechanism for radiofrequency heating is shown in figure 10.2. The electric field lines emanate from
the active electrode tip and are created by the voltage impressed upon it by the radiofrequency
generator. This electric field creates an electric force on the charged ions within the electrolytic medium
of the tissue. According to the physics laws this force produces a motion, and the motion is oscillatory at
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NEUROTHERM RADIO FREQUENCY LESION GENERATOR
MODEL NT1100
OPERATORS MANUAL
adjacent to it. This higher
®
Figure 10.1 shows the fundamental radiofrequency circuit.
The RF lesion generator or power source provides a
source of RF. It is connected by wires to 2 electrodes:
one inserted into the body, referred to as the active
electrode; another in contact with the surface of the body,
referred to as the dispersive electrode. This is the so-
called monopolar configuration. The RF voltage causes
current to flow through the wires, through the electrodes,
and to the patient's body. The patient's body is a
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