Investigations on acoustic on-line monitoring of IR laser ablation of burned skin

Background and Objective: In burn surgery necrotic tissue has to be removed prior to grafting. Tangential excision causes high blood loss and destruction of viable tissue. Pulsed infrared laser ablation can overcome both problems because of its high precision and the superficial coagulation of the remaining tissue. We investigated the ablation noise to realize an acoustic feedback system for a selective removal of necrotic tissue. Materials and Methods: We studied free-running Er:YAG laser ablation of gelatin and burned skin. Schlieren laser flash photography was used to investigate the ablation dynamics generating the ablation noise. Acoustic signals were detected by a condenser microphone and a piezoelectric airborne transducer. Tissue discrimination was based on the evaluation of the normalized acoustic energy. Results: The ablation noise is mainly generated by shock wave emission and fast vaporization during the first part of the laser pulse. Frequency components of the ablation noise above 200 kHz are only detectable with the piezoelectric transducer. The normalized acoustic energy differs significantly between gelatin samples of different water content and between necrotic and vital tissue. Conclusions: Large bandwidth transducers are essential for an acoustic on-line monitoring of free-running Er:YAG laser ablation of burned skin. The normalized acoustic energy is a suitable parameter for the discrimination between necrotic and vital tissue.