Berjano, E. (2012). Relationship between roll-off occurrence and spatial distribution of dehydrated tissue during RF ablation with cooled electrodes. International Journal of Hyperthermia. 28 (1) AbstractPurpose: To study the relationship between roll-off (sudden increase in impedance) and spatial distribution of dehydrated tissue during RF ablation using a cooled electrode (temperatures around 100ºC). Methods:We used a double approach: 1) theoretical modeling based on the Finite Element Method; and 2) 20 ablations using an experimental study on ex-vivo excised bovine liver in which we measured impedance progress and temperature at three points close to the electrode surface: 0.5 (T1), 1.5 (T2) and 2.5 (T3) mm from the tip. T2 was located exactly at the center of the 30 mm long electrode.Results: Temperatures at T1 and T3 quickly rose to 100ºC (at ≈ 20 and 40 s, respectively), while at the rise at T2 was somewhat slower, stabilized around 50 s and reached a maximum value of 99ºC at about 60 s. Impedance reached a minimum of 65  (plateau), began increasing at 50 s and continued rising throughout the procedure, reaching a value equal to the initial value at 70 s. Likewise, computed impedance dropped to ≈73  (plateau), began increasing at 50 s and reached an impedance value equal to the initial value at ≈78 s, which approximately coincided with the time when the entire zone surrounding the electrode was within the 100ºC isotherm. Conclusion:There is a close relationship between the moment at which roll-off occurs and the time when the entire electrode is completely encircled by the dehydrated tissue. The midelectrode zone is the last in which tissue dessication occurs.

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The acute tissue damage produced by the CO2 laser was similar to that induced by microelectrodes in cutting mode. The tissue damage resulting from the use of the microelectrode in coagulation mode was comparatively greater.

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Control of thyroid function should be standardised for several years after treatment, particularly in patients with risk factors, such as those treated with telecobalt therapy, those with post-operative complications and for whom the thyroid parenchyma is included in the irradiated area (laryngeal or pharyngeal location and bilateral cervical radiation).

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The Cool-tip is one of the most widely employed electrodes in radiofrequency (RF) ablation (RFA) of hepatic tumors. This electrode creates reliable geometry and coagulation zones. Despite the advantages of this electrode, during the ablation is produced a phenomenon called roll-off in which impedance increases, energy deposition completely stops and the lesion size cannot be increased. Consequently, the thermal lesion size is smaller and the tumors which can be ablated are smaller too. In this research we studied theoretical and experimentally the electrical-thermal performance of the Cool-tip electrode during RFA of hepatic tissue. Mainly, we were interested in the occurrence of the roll-off and its relationship with the tissue temperatures around the electrode. The theoretical model included the vaporization of the tissue and the variation of the thermal and electrical conductivities with temperature. The model was solved numerically using COMSOL Multiphysics software. For the experimental part we conducted a study in ex vivo liver tissue. The experimental and theoretical results showed that the roll-off is totally related when temperatures around 100 °C surrounds the tissue close to the center of the Cool-tip. The knowledge of this fact brings a powerful tool to analyze alternative methods or techniques to avoid the roll-off.