O49 Autodyne and acoustic monitoring of the process of evaporating the multilayer biomaterials by CO 2 laser emission

Abstract

Introduction: Laser ablation of biotissues is based on local heating results in phase transitions, intensive evaporation, substance mass emission in the form of drop-vapor mixture and tissue particles and laser-induced acoustic emission. These conditions create prerequisites for using of light scattering and optoacoustics techniques for monitoring of laser tissue ablation. Materials and Methods: Surgical CO2 laser was used to vaporize biomodels on the basis of gelatin with the different liquidwater content (60%-90%); biotissues of pig in vitro and of rat in vivo. Diagnostics of evaporating processes of the samples was accomplished by an autodyne method and by an optoacoustical method. The procedure of signal processing and extracting the informational component included fast Fourier transformation of the recorded signal and extraction the acoustic spectrum and the dependence of autodyne signal on the Doppler shift. As the quantitative characteristic of the obtained spectrum we used the spectral power of the signal and weighted mean frequency. According to the behavior of these values was made the conclusion about the approach of the moment the laser emission transmits through the boundary of the samples. Results: The autodyne signals, obtained during the cutting of biotissues by laser emission with the power density in the range from 1 to 10 kW/cm 2 were investigated. Tests on animals showed that it is possible to define the moment of the intersection the different biotissues by the laser beam. The analysis of the spectra of acoustical signals during the contact (piezoelectric ceramic) and remote (highly sensitive microphone) sounding of the laser evaporation zone of gelatin based model media and of biological tissues was made. It showed that during the laser cutting of multilayer gelatinous models a variation in the water content of layers leads to an abrupt change of the spectral power of the acoustical signal. Conclusion: It was established that interaction of CO2 laser radiation with the different biotissues possessed nonstationary nature and was manifested in a change in both the temporary form of autodyne and acoustic signals and their spectral characteristics. The developed algorithms for extracting information component of autodyne and acoustical signals allow to determine the moment of laser beam passage through the interface of biological tissues layers. The experiments have shown that the autodyne and acoustics methods can be used simultaneously to organizing the feedback in CO2 laser surgical systems. This work is performed with the support of Grant Russian Basic Research Fund No. 10 07 00605.