Heat sink effect on tumor ablation characteristics as observed in monopolar radiofrequency, bipolar radiofrequency, and microwave, using ex vivo calf liver model

Krishna Pillai, Javid Akhter, Terence C Chua, Mena Shehata, Nayef Alzahrani, Issan Al-Alem, David L Morris, Krishna Pillai, Javid Akhter, Terence C Chua, Mena Shehata, Nayef Alzahrani, Issan Al-Alem, David L Morris

Abstract

Thermal ablation of liver tumors near large blood vessels is affected by the cooling effect of blood flow, leading to incomplete ablation. Hence, we conducted a comparative investigation of heat sink effect in monopolar (MP) and bipolar (BP) radiofrequency ablation (RFA), and microwave (MW) ablation devices.With a perfused calf liver, the ablative performances (volume, mass, density, dimensions), with and without heat sink, were measured. Heat sink was present when the ablative tip of the probes were 8.0 mm close to a major hepatic vein and absent when >30 mm away. Temperatures (T1 and T2) on either side of the hepatic vein near the tip of the probes, heating probe temperature (T3), outlet perfusate temperature (T4), and ablation time were monitored.With or without heat sink, BP radiofrequency ablated a larger volume and mass, compared with MP RFA or MW ablation, with latter device producing the highest density of tissue ablated. MW ablation produced an ellipsoidal shape while radiofrequency devices produced spheres.Percentage heat sink effect in Bipolar radiofrequency : Mono-polar radiofrequency : Microwave was (Volume) 33:41:22; (mass) 23:56:34; (density) 9.0:26:18; and (relative elipscity) 5.8:12.9:1.3, indicating that BP and MW devices were less affected.Percentage heat sink effect on time (minutes) to reach maximum temperature (W) = 13.28:9.2:29.8; time at maximum temperature (X) is 87:66:16.66; temperature difference (Y) between the thermal probes (T3) and the temperature (T1 + T2)/2 on either side of the hepatic vessel was 100:87:20; and temperature difference between the (T1 + T2)/2 and temperature of outlet circulating solution (T4), Z was 20.33:30.23:37.5.MW and BP radiofrequencies were less affected by heat sink while MP RFA was the most affected. With a single ablation, BP radiofrequency ablated a larger volume and mass regardless of heat sink.

Conflict of interest statement

The authors have no funding and conflicts of interest to disclose.

Figures

FIGURE 1
FIGURE 1
General set-up for performing the experiment using monopolar (MP) radiofrequency ablation device. The set-up was similar for microwave (MW) ablation and for bipolar (BP) radiofrequencyablation devices, 2 antennas were used that pierced the liver parallel to each other. In the case of BP and MW, there was no dispersive electrode.
FIGURE 2
FIGURE 2
(A) Strategic placement of temperature sensors T1 and T2 for monopolar (MP) radiofrequency ablation alongside the hepatic portal vein. (B) Similar arrangement carried out with bipolar (BP) radiofrequency ablation. For microwave, a similar arrangement as for MP was carried out except temperature sensor T3 that was not required because it was already monitored by the generator device with continuous recording. The placement of temperature sensors T3 for both the MP and the BP are also shown Figure 2A and B.
FIGURE 3
FIGURE 3
(A) Three types of ablation devices, monopolar and bipolar radiofrequency (2 identical probes but only 1 is displayed) and microwave, that have been used in this experiment. (B)–(D) Ablated tissues (longitudinal section) using monopolar radiofrequency, bipolar radiofrequency, and microwave devices, respectively. Both monopolar and bipolar radiofrequency ablation produce a more spherical shape compared with microwave. The red arrows denote the position of the antennas in the tissue.
FIGURE 4
FIGURE 4
(A)–(C) Comparison of liver tumor ablative parameters that were measured in the absence and presence of heat sink. (D) Graphical representation of the percentage heat sink effect on measured ablative parameters. BP = bipolar, D = density, ELI = elipscity, -HS = heat sink absent, +HS = heat sink present, LAD = lateral dimension, LOD = longitudinal dimension, M = mass, MP = monopolar, MW = microwave, V = volume
FIGURE 5
FIGURE 5
Temperature profile with time, (A) and (B) for BP in perfused liver and (C) and (D) for MP RFA. BP = bipolar, MP = monopolar, RFA = radiofrequency ablation, W = time in minutes to reach maximum temperature, X = time at which maximum temperature was maintained, Y = temperature difference between (T1 + T2)/2, and T3, T1, and T2 are temperatures on either side of the perfused hepatic portal vein, respectively, and T3 is the temperature of the ablative thermal probe, Z = temperature difference between (T1 + T2)/2 and outlet circulating fluid temperature T4. Tmax = maximum temperature reached during ablation.
FIGURE 6
FIGURE 6
Temperature profile with time, (A) and (B) for MW ablation of perfused liver while (C) shows a comparative heat sink effect on various time and temperature profile in BP RFA, MP RFA, and MW devices. BP = bipolar, MP = monopolar, MW = microwave, RFA = radiofrequency ablation, W = time in minutes to reach maximum temperature, X = time at which maximum temperature was maintained, Y = temperature difference between (T1 + T2)/2 and T3, T1, and T2 are temperatures on either side of the perfused hepatic portal vein, and T3 is the temperature of the ablative thermal probe, Z = temperature difference between (T1 + T2)/2 and outlet circulating fluid temperature T4, Tmax = maximum temperature reached during ablation, Xf = time at which maximum temperature fluctuates.

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