Numerical study of the influence of water evaporation on radiofrequency ablation

Abstract

Background: Radiofrequency ablation is a promising minimal invasive treatment for tumor. However, water loss due to evaporation has been a major issue blocking further RF energy transmission and correspondently eliminating the therapeutic outcome of the treatment.

Method: A 2D symmetric cylindrical mathematical model coupling the transport of the electrical current, heat, and the evaporation process in the tissue, has been developed to simulate the treatment process and investigate the influence of the excessive evaporation of the water on the treatment.

Results: Our results show that the largest specific absorption rate (QSAR) occurs at the edge of the circular surface of the electrode. When excessive evaporation takes place, the water dehydration rate in this region is the highest, and after a certain time, the dehydrated tissue blocks the electrical energy transmission in the radial direction. It is found that there is an interval as long as 65 s between the beginning of the evaporation and the increase of the tissue impedance. The model is further used to investigate whether purposely terminating the treatment for a while allowing diffusion of the liquid water into the evaporated region would help. Results show it has no obvious improvement enlarging the treatment volume. Treatment with the cooled-tip electrode is also studied. It is found that the cooling conditions of the inside agent greatly affect the water loss pattern. When the convection coefficient of the cooling agent increases, excessive evaporation will start from near the central axis of the tissue cylinder instead of the edge of the electrode, and the coagulation volume obviously enlarges before a sudden increase of the impedance. It is also found that a higher convection coefficient will extend the treatment time. Though the sudden increase of the tissue impedance could be delayed by a larger convection coefficient; the rate of the impedance increase is also more dramatic compared to the case with smaller convection coefficient.

Conclusion: The mathematical model simulates the water evaporation and diffusion during radiofrequency ablation and may be used for better clinical design of radiofrequency equipment and treatment protocol planning.

Figure 1

Schematic diagram of the eletrode and tissue. The gray cylinder represents the RF electrode, while the dark red cylinder represents the tissue.

Figure 2

Temperature distribution of two models at t = 600 s. (a) Temperature distribution without considering evaporation in the model at t = 600 s; (b) Temperature distribution with considering evaporation in the model at t = 600 s.

Figure 3

Transient temperatures at the monitoring points. P1, P2, P3 locate at the central axis of the tissue cylinder with 0 mm, 1 mm and 10 mm away from the contact surface of the electrode.

Figure 4

Q_SAR, electrical field and water content distributions of tissue. (a) The Q_SAR, electrical field and water content distribution at the beginning; (b) The pattern of Q_SAR, electrical field and water content distribution after heating for 600 s with considering evaporation.

Figure 5

The tissue impedance during treatment. X-axis stand for the time of RF treatment, Y-axis stands for the tissue impedance.

Figure 6

The tissue impedance via time, and temperature distribution at t = 656 s, 956 s and 1300 s. (a) Tissue impedance via time of both models. Point 1 is when the first heating stops; point 2 represents the start of the second treatment of RF; point 3 is when the second heating stops; (b) Temperature distribution of tissue at the end of first treatment, t = 656 s; (c) Temperature distribution of tissue at the beginning of second RF treatment, t = 956 s; (d) Temperature distribution at the end of second RF treatment, t = 1300 s.

Figure 7

Temperature and water content distribution at different cooling conditions. (a) Temperature and water content distributions under low convection coefficient (h = 25 W/m²K) at 150 s and 205 s. (b) Temperature and water content distributions under medium convection coefficient at 205 s and 476.5 s h = 100 W/m². (c) Temperature and water content distribution under constant temperature (T = 20°C) at 1400 s.

Figure 8

Transient temperature and impedance via time. (a) Transient temperature and (b) Transient impedance changes at the monitoring point under different treatment conditions.