Introduction : Recently, radiofrequency ablation therapy (RFA) for liver tumors has been widely applied in clinical settings. In this study, the effect of radiofrequency ablation (RFA) on the intrahepatic vessels and bile ducts was examined experimentally and clinically based on the RFA principle of induction heating with an alternating current, as opposed to dielectric heating with electromagnetic waves and microwaves.
Methods : The RF-Generator (Model 500, RITA Medical Systems, Inc., Mountain View, CA) was used for RFA. For in vitro and in vivo experiments, Japanese white rabbits and an adult dog were used.
Results :
Exp. 1. A constant output of RFA to the target tissues elevated the temperatures to 100°C or higher in liver tissues, and ≤50°C in blood and bile. The impedances of the liver tissues, blood, and bile were 159±33.0 Ω, 136±20.7 Ω, and 73±17.6 Ω, respectively. The tissues with the higher impedance showed a higher temperature elevation.
Exp. 2. When RF output was applied to a series circuit formed by the liver tissues and blood, and to a series circuit formed by the liver tissues and bile, the voltage drop was highest in the liver tissues, followed by the blood and the bile.
Exp. 3. In vivo, after the RF output was applied to adult dog livers, the temperature of the liver parenchyma was elevated while that of the intrahepatic bile ducts remained nearly consistent at 40°C.
Exp. 4. RFA of the liver with hepatic blood flow occlusion by the Pringle maneuver resulted in a significantly higher temperature elevation compared with the liver without hepatic blood flow occlusion.
Exp. 5. A rabbit liver was examined histologically 10 days after RFA. Within the RFA thermocoagulation area, obstruction and rupture were found in the bile ducts (≤25 μm in diameter) and portal veins (≤50 μm in diameter). No abnormality was observed in larger bile ducts and vessels.
Clinical data of RFA. The effect of RFA on the intrahepatic portal veins, veins, and bile ducts was examined using dynamic computed tomography (CT) images. In the portal veins, narrowing was observed in only 2 of 32 veins. No narrowing was found in the hepatic veins. In the intrahepatic bile ducts, no biloma and biliary dilatation due to bile duct rupture distal to the RFA-ablated area were observed.
Discussion : According to Ohm's law, when current
I (ampere) flows between the puncture needle and counter electrode plates, and electric resistance is expressed as
R (Ω), the calorific values of the tissues can be expressed as
I2R (watts). The output needed for thermocoagulation of the liver tissues is often insufficient to warm the vessels and bile ducts despite RF induction heating because of the low impedance of the blood or bile. The voltage drop in the blood or bile is also lower than in the liver parenchyma tissues. The electric workload as a source of heat is greatest in the liver tissues, followed by the blood, and bile. These properties make the blood and bile harder to warm up than the liver parenchyma tissues. These were verified by in vivo experiment with a dog. Therefore, RFA is a safe and less-invasive therapy for treatment of the intrahepatic vessels and bile ducts.
View full abstract
