Removal of Biological Tissue by Conventional vs. Ultra Short Laser Pulses, as observed by Laser Ionization Mass Spectrometry (LIMS).

Research prject P 13756 Ultrashort Laser - Biological Tissue Interaction Wolfgang HUSINSKY 11.10.1999 The interaction of laser light with biological tissue is the basis for an immense and fast growing number of medical applications of lasers. Depending on the intensity of the laser radiation, it ranges from bio-stimulation (i.e. wound healing) to tissue removal by laser ablation or plasma induced processes. In particular, tissue removal by laser light has entered the modern operation room as a standard surgical technique in the last 20 years. The major underlying physical process used for tissue removal is generally called laser ablation (in some cases laser desorption). However, the nature, detailed mechanisms and physics of the laser ablation process vary according to tissue material and they also strongly depend on the kind of laser radiation used. The most important factors determining the actual physics of the ablation process are: a) the intensity b) the wavelength and c) the pulse duration of the radiation. Ibis ,physics" then is responsible for the suitability of particular laser system for a specific surgical application. Over the last decade the excimer laser, in particular the ArF excimer, has played a dominant role for non-thermal laser ablation in material processing and medicine, Correction of the cornea in order to compensate hyper- and myopia as well as astigmatism is only one of the well known applications of the technique, dermatology being another. Entirely new aspects and a new impetus for the use of lasers in medicine and, in particular, for surgical applications can be realized with ultra-short laser pulses, which are now available with pico- and femto-second laser sources. Ultra-short laser light offers many advantages, as for instance low thermal damage, the possibility of efficient interaction of light with long wavelengths and furthermore the potentiality of avoiding shock waves. In summary, the major advantages of this tissue ablation method are: 1) efficient ablation; 2) minimization of collateral damage; 3) ablation thresholds and -rates which are relatively insensitive to tissue type; 4) high control over ablation depth is achievable because only a small amount of tissue is ablated per pulse; 5) low operation noise level; and finally, 6) precise spatial control due to the multiphoton nature of the interaction. Detailed and extremely valuable information concerning the laser-tissue interaction process can be obtained by measuring the partial ablation rate (individually ablated masses) and the energy of the ablated particles, Only few investigations of this kind have been performed, so far. The following difficulties arise in connection with experiments of this kind: a) the measurements have to be performed in vacuum, b) most of the ablated particles are neutral and c) the complexity and high mass of the ablated molecules. Here again new ultra-short lasers offer immense new possibilities. One of the most sensitive methods nowadays available for detecting small particle concentrations - the laser post-ionization technique - in combination with time of flight spectroscopy can be used to measure the kind and energy of particles released during the laser-tissue interaction process. Ultra-short lasers for post-ionization extend the method for detection of complex molecules found for ablated biological materials. Over the last decade several models have been proposed for laser ablation, describing thermal ablation and non- thermal ablation (e.g. multiphoton excitation, photo-physical processes). In spite of these efforts, a more than crude understanding of the various processes leading to ablation is not available. The relevance of theses mechanisms for the particular cases and possible new mechanism will be determined by comparing the experimental data with the existing models as well as Molecular-Dynamics simulations.