Biophysics of the photoablation process

Besides the coagulation, where the body digests the necrotic tissue and direct evaporation of tissue, the photoablation effect turns out to be very important in tissue removal.In the case of high tissue absorption the process channels in photoablation can either be photochemical (bond breaking) or fast thermal. In the case of transparent media, a plasma formation due to high irradiances and an optical breakdown is necessary for ablation or photodisruption.All the process channels lead to a fast microscale explosion and to Shockwaves. For soft tissue the main process channel is the fast thermal explosion.Assuming that tissue will be disintegrated, if the energy deposited within a single laser pulse is larger than a material specific threshold, the thresholds for the radiant exposure and ablation rates respectively can be calculated.There is a large difference, whether the laser radiation is applied to the tissue surface in noncontact or through a fibre in contact. In contact the ‘fast thermal explosion’ happens in a closed chamber and hence the photohydraulic effect will support the photoablation.The thermally damaged zone in the surrounding tissue depends on the optical penetration depth mainly in cases that the pulse duration is shorter than a critical time given by the heat conductivity. Pulsed lasers can be used ‘non-thermally’ only if the average power is less than a tenth of a watt. With a higher amount of average power a pulsed laser will act comparable to a cw laser.