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Abstract
Rationale and Objectives
We determined whether heat distribution along a radiofrequency (RF) electrode would
be uniform when longer tip exposures are used and whether local temperature effects
would influence the shape of induced tissue coagulation.
Methods
Thermistors were embedded within 18-gauge RF electrodes at both ends and in the middle
of the exposed tip. The length of tip exposure varied from 1 to 7 cm. RF was applied
in vitro to pig liver for 6 min using a constant tip temperature, which was varied
in 10°C increments from 60°C to 110°C. Experiments were performed in triplicate. The
3- and 5-cm probes were used at a 90°C tip temperature to create lesions in live pig
liver and muscle using similar parameters. Temperature was measured throughout the
procedure. Observable coagulation necrosis was measured at the end of the treatment.
Regression analysis was used to evaluate the local temperature-lesion diameter relationship.
Results
Temperatures were not uniform along the tip exposure for any given trial. Temperature
variation increased with higher tip temperatures and longer tip exposures. The diameter
of local coagulation necrosis was a function of the local mean temperature. For in
vitro trials, no coagulation was seen when the local temperature was less than 50°C.
Temperatures above this threshold resulted in progressively greater lesion diameter,
with a minimum of 1 cm of necrosis occurring at 71°C. Additional increases in lesion
diameter (1.4–1.6 cm) were observed at approximately 90°C. Mathematical modeling demonstrated
a best-fit curve: lesion diameter (in cm) = [1.4 + 0.03 (tip exposure)] {1 - e[−0.067(local temp - 49.5°c]} r2 = .986, SD = 0.14 cm for each curve. In living tissue, less uniformity in the shape of coagulation
necrosis was seen around the electrodes. Local temperature-lesion diameter data fit
the same logarithmic relation, but the threshold for coagulation necrosis was 8.5°C
higher than for in vitro specimens.
Conclusion
Using a single-probe technique for RF-induced tissue necrosis, the diameter of tissue
coagulation may be predicted by the local temperature along the exposed electrode.
The uniformity of temperature decreases with increased tip exposures. This effect
may be partially corrected by creating lesions at higher tip temperatures, where necrosis
diameter is increased. Because effects are more pronounced in vivo, uniform volumes
of tissue necrosis are limited to tip exposures of 3 cm or less.
Key Words
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Article Info
Publication History
Accepted:
November 9,
1995
Received:
August 25,
1995
Identification
Copyright
© 1996 Association of University Radiologists. Published by Elsevier Inc.
