Relationship between roll-off occurrence and spatial distribution of dehydrated tissue during RF ablation with cooled electrodes

Abstract

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.

Review of the mathematical functions used to model the temperature dependence of electrical and thermal conductivities of biological tissue in radiofrequency ablation

“…Finally, when the tissue around the electrode middle zone rises to a temperature over 100ºC, i.e. when the electrode is completely surrounded by dehydrated tissue, impedance increases abruptly and roll-off occurs (zone 3) [30].…”

“…The impedance evolution up to first roll-off was used to compare the different approaches, since impedance increases abruptly and the subsequent roll-off occurs when the electrode is completely surrounded by dehydrated tissue (100ºC) [30]. The evolution of the short diameter of the thermal lesion for up to 6 minutes was also used as a comparison.…”

“…The notation h(99) and h(100) refers to enthalpy at 99 and 100ºC, respectively. The characteristics of materials used in the model are shown in Table 2 [12,19,29,30].…”

“…This allows initial impedance (≈100  to match the experimental mean value obtained from [30]. (2) Assessed at 37ºC [12].…”

Comparison of Enthalpy Method and Water Fraction Method to Mathematically Model Water Vaporization During RF Ablation

“…Rolloff time, i.e. when impedance suddenly rises, is a crucial phenomenon which denotes when the electrode is entirely surrounded by dehydrated tissue [16].…”