Investigation of wavelength-dependent tissue ablation: visible (λ=532 nm) vs IR (λ=2.01 μm)

Abstract

Laser prostatectomy with various lasers has been shown to be effective in the treatment of benign prostate hyperplasia. However, the impact of laser parameters on tissue ablation is still in question. The aim of this study is to experimentally characterize laser-tissue interactions in terms of wavelength by comparing visible (λ= 532 nm) and infrared (λ= 2.01 μm) spectra. Porcine kidney tissue was used as it has thermal properties and glandular structure similar to human prostatic tissue. Q-switched 532 nm (GreenLight TM HPS) and continuous-wave (CW) 2.01 μm (custom-made Tm:YAG) lasers were employed to remove soft tissue under various settings (power, working distance, and treatment speed). For both laser systems, ablation rate increased with power and was maximized at 4 mm/s. The 532 nm laser generated approximately 30% (p<0.005) higher ablation efficiency than the IR laser. A comparable ablation depth was found between the two wavelengths, but the 532nm laser generated relatively wider (up to 30%; p<0.005) craters. Owing to constant heating due the CW mode, the IR laser induced 20% thicker coagulation depth than the 532 nm (0.94 vs. 0.8 mm at 100 W; p<0.005). Histology also confirmed coagulation depth in response to each wavelength. Due to light absorption in aqueous environment, the IR laser exhibited a dramatic decrease in power transmission and ablation volume with increasing working distance whereas the 532 nm laser maintained relatively constant features. In conclusion, the characteristics of tissue ablation were contingent upon the applied wavelengths due to optical properties and laser parameters.