NeuroTherm NT1100 Operator's Manual page 83

Figure 10.3
Figure 10.3 shows a schematic diagram of the temperature of the tissue as a function of distance
from the electrode tip. It should be noted that the temperature is not isotropic for a non-spherical
electrode, and thus these curves may differ at different orientations on a non-spherical electrode
tip. The temperature at the surface of the electrode, such as T2, measures very nearly the
hottest tissue nearby, and the tissue at greater distances falls off until it asymptotically
approaches body temperature at large distances from the electrode tip. By raising the
radiofrequency voltage, one will increase the temperature to T1 near the electrode tip, and thus
the distances to the 45 C isotherm will increase accordingly. With knowledge of these
characteristics and the temperature vs.distance curve, one can therefore judged the size of the
lesion volumes by choosing the appropriate tip temperature for a given tip geometry. This is the
reason temperature measurement has been historically essential to produce consistent and
quantified lesion volumes. (See section 10.2 for typical Lesion sizes).
Measurement of the tip temperature has another very important benefit. By avoiding tip
temperatures near 100 C (the temperature at which water boils), one avoids the undesired effects
of charring, sticking or the formation of a haemorrhage or explosive gas which may be also be
accompanied by sparking. In the early days of neurosurgical lesions, before reliable tip
temperature measurement was possible, neurosurgeons would establish the end point of their
lesion making by listening for the "popping" lesion. The popping was caused by the tip
temperature exceeding 100 C and the subsequent gas formation at the tip to the electrode. This
obviously was not a controlled lesion technique and led to unpredictable and dangerous
destructive conditions.
In pain management there are now well-established prescriptions for appropriate electrode size
and tip temperature to achieve desired lesion volumes. It has been historically clear that
prescriptions which involve power and current did not have lasting value, but rather prescriptions
that involve temperature, electrode size and accounting for the heat washout caused by blood
flow. The importance of temperature control was not always recognized. For instance, in the
early days of percutaneous cervical cordotomy, elaborate prescriptions of current, power and
time for making RF lesions were established. The subsequent clinical results were not consistent
in the early days of cordotomies, and it was only when temperature was measured at the tip of
the cordotomy electrode that consistency and reproducibility was finally achieved.
surface can then be referred to as the equilibrium.
Lesion Size. Leaving the radiofrequency power turned on indefinitely beyond 60 seconds will not
increase the equilibrium lesion size. In the past, so-called time-dependent lesions were made in
which a certain power was held by the radiofrequency generator for 10-20 seconds. This too led
Document 109.00
NEUROTHERM RADIO FREQUENCY LESION GENERATOR
OPERATORS MANUAL
Figure 10.4
®
MODEL NT1100
Another important aspect of controlled radiofrequency
lesion making is illustrated in Figure 10.4. This shows
experimental data of the increase in lesion size for a
fixed electrode geometry and a fixed tip temperature.
The lesion size in this situation is defined as the width
of the prolate ellipsoidal width of the prolate ellipsoidal
lesion volume. The graph clearly shows that for
constant tip temperature the lesion size grows and
asymptotically reaches a maximum value in a time
between 30 and 60 seconds. The 45C isothermal
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