Acoustic signal characteristics during IR laser ablation and their consequences for acoustic tissue discrimination

Abstract

IR laser ablation of skin is accompanied by acoustic signals the characteristics of which are closely linked to the ablation dynamics. A discrimination between different tissue layers, for example necrotic and vital tissue during laser burn debridement, is therefore possible by an analysis of the acoustic signal. We were able to discriminate tissue layers by evaluating the acoustic energy. To get a better understanding of the tissue specificity of the ablation noise, we investigated the correlation between sample water content, ablation dynamics, and characteristics of the acoustic signal. A free running Er:YAG laser with a maximum pulse energy of 2 J and a spot diameter of 5 mm was used to ablate gelatin samples with different water content. The ablation noise in air was detected using a piezoelectric transducer with a bandwidth of 1 MHz, and the acoustic signal generated inside the ablated sample was measured simultaneously ba a piezoelectric transducer in contact with the sample. Laser flash Schlieren photography was used to investigate the expansion velocity of the vapor plume and the velocity of the ejected material. We observed large differences between the ablation dynamics and material ejection velocity for gelatin samples with 70% and 90% water content. These differences cannot be explained by the small change of the gelatin absorption coefficient, but are largely related to differences of the mechanical properties of the sample. The different ablation dynamics are responsible for an increase of the acoustic energy by a factor of 10 for the sample with the higher water content.