Simulation of the temperature distribution of kidney stones induced by thulium fiber laser and Ho: YAG laser lithotripsy.

Abstract

Simulation studies on temperature distribution in laser ablation help predict ablation rates, laser settings, and thermal damage. Despite the limited number of reported numerical studies on the temperature distribution of kidney fluid, there is no simulation study for kidney stone temperature distribution. We employ a numerical approach to study the kidney stone temperature distribution and predict ablation rates, which is an important parameter for clinical lithotripsy. The study looked at how the thulium fiber laser and the Ho:YAG laser differ in terms of temperature profile and ablation depth of kidney stones like calcium oxide monohydrate. The ablation depth increased from 152.7 µm to 489.7 µm when the TFL laser (operated at 10 Hz repetition rate and 1 ms pulse width) fluence increased from 764 J/cm² to 1146 J/cm². Correspondingly, the depth increased from 21 µm to 68 µm for the Ho: YAG laser operated at 3 Hz and 0.22 ms pulse width. We attribute this to an increase in temperature with laser energy. We further investigated the effect of pulse width on ablation depth by considering three different TFL pulse widths: 0.5 ms, 0.75 ms, and 1 ms. There was a decrease in ablation depths from 402.5 µm to 242.6 µm when the pulse width increased from 0.5 ms to 1 ms. Because of lower water absorption coefficients, the Ho:YAG laser (70 mJ/10 Hz) produced a smaller ablation depth and temperature profile than the thulium fiber laser (70 mJ/10 Hz). Experimental results from the literature validated the simulation. We found that the Ho:YAG laser worked better for ablation when it was set to 0.2 J/100 Hz for the Ho:YAG laser and 0.4 J/50 Hz for the TFL laser, which were clinical laser settings that we found in the literature. This indicates that, in addition to laser absorption by water, the laser parameters also significantly influence temperature distribution and ablation.