NeuroTherm NT1100 Operator's Manual page 82

NEUROTHERM RADIO FREQUENCY LESION GENERATOR 10-2
®
MODEL NT1100
OPERATORS MANUAL
the frequency of the RF current. It is this radio frequency motion which causes the ions to rub against
the surrounding fluid medium, causing friction which results in the tissue heating. The temperature at
any point is controlled by the frictionally induced power dissipation at that point, mediated by thermal
diffusion and thermal convection. The thermal diffusion and convection are typically caused by blood
circulation.
The temperature distribution around the electrode tip can be calculated by making certain simplifying
assumptions. One of the assumptions is that the medium is homogeneous and that the factors of
thermal diffusion and circulation are also uniform. Under these circumstances one can calculate the
isotherms (surfaces of constant temperature) surrounding the electrode tip for a given impressed
radiofrequency voltage. These isothermal surfaces are critical to determining the lesion size. As it is
known that living tissue will be permanently destroyed for sustained temperatures of approximately 45 C,
the 45 degree isotherm can be considered to be the outer border of the lesion since tissue within this
volume will be thermally destroyed and tissue outside of this volume will experience lower temperatures
than are necessary for cellular destruction. Isothermal surfaces are indicated by the -- lines in figure
10.2.
It is important to understand that the radiofrequency field, and thus power dissipation in the tissue
actually heats the tissue as opposed to the electrode itself. The heated tissue in turn raises the
temperature of the electrode tip and thus heats the tip. Therefore, it is not to the electrode tip which
heats the tissue, but rather the tissue which heats the electrode tip. If the electrode is properly designed
so as not to sink away too much of the thermal energy, the electrode will give an accurate representation
of the tissue temperature at it's surface. It is for this reason that thermal monitoring of the radiofrequency
tip is a good indication of the hottest portion of the lesion volumes, namely the isotherm that lies closest
to the surface of the electrode. With the simplifying assumptions of a homogeneous medium, the lesion
size represented by the 45 degree isotherm increases with increasing tip temperature and also increases
with increasing tip dimension particularly the radius of the electrode tip.
Document 109.00 10September2012 Issue 06