Plume Emission, Shock Wave and Surface Wave Formation During Excimer Laser Ablation of the Cornea

Abstract

Excimer lasers are now used for corneal surgery; however, the physical processes occurring during photoablation of the cornea are incompletely understood. High speed laser-based photographic arrangement was constructed. The temporal resolution was better than 1 ns. The setup could work as a Schlieren arrangement, which is sensitive to the refractive index change caused by the shock wave propagating in the air above the eye. With minor changes the setup was converted into a shadowgraph, which could detect the ablation plume and the waves propagating on the surface of the eye. Due to the impact of the excimer laser pulse onto the surface of the cornea, a shock wave was generated in the air. The shadowgraph clearly showed the ejection of the ablated cornea. The ejection velocity of the plume was found to be over 600 m/s. It was shown for the first time that the recoil forces of the plume are generating a wave on the surface of the eye. The laser-based high speed photographic arrangement is a powerful arrangement in the study of physical effects occurring during photoablation of the cornea. (Refract Corneal Surg (suppl) 1993;9:S111-S115)

Mechanisms of Pulsed Laser Ablation of Biological Tissues

“…The delay between laser irradiation and the onset of particle ejection was measured between 90 and 280 ns for Q-switched Er:YAG laser (λ = 2.94 μm) ablation of skin, liver, and agar and decreased with increasing radiant exposure. , A delay of 70 ns was observed for ArF excimer laser (λ = 193 nm) ablation of corneal tissue . Processes contributing to these delays include the spinodal decomposition underlying the phase explosion, the straining of the tissue matrix required to achieve tissue fracture, and the acceleration of the plume material.…”

“…To date, most investigations of the plume dynamics and acoustic phenomena associated with pulsed laser ablation of biological tissues have been performed experimentally. Particulate matter in the ablation plume has been visualized using bright-field photography − or photographic recording of scattered light. , Schlieren photography has enabled the visualization of vapor and gaseous ablation products in addition to the ejected particles. ,, Resonance absorption methods , have been used to detect specific ablation products and chemical reactions in the plume . Other techniques employed to analyze the composition of the ablation products include mass spectrometry and gas chromatography .…”

“…Other techniques employed to analyze the composition of the ablation products include mass spectrometry and gas chromatography . Specific data on the ablation dynamics, including the time delay between laser irradiation and the onset of tissue removal, the velocities of acoustic transients, vapor plume, and particulate matter, and the duration of postpulse material ejection, have been obtained by photographic techniques − ,, or by use of a probe laser beam directed parallel to the target surface. ,,,− Probe beam techniques have also provided quantitative time-resolved information regarding plume transmission 240 and the shape and amplitude of acoustic trasients. ,− …”

“…The ablation process continues with the ejection of condensed material mixed with vapor. This applies both to tissues, in which case the ejected material mostly consists of particulate fragments, − , and to liquids, in which case small droplets are ejected. , The sequence of events in the early phase of Q-switched Er:YAG laser (λ = 2.94 μm) ablation of water is shown in Figure . The vapor plume and the external shock wave driven by the vapor expansion are both visible on the Schlieren photographs in the upper row of the figure.…”

“…In both Q-switched and free-running laser ablation of soft tissues, material ejection continues for a considerable time after the end of the laser irradiation. The time period during which particles were detected close to the target surface after application of nanosecond pulses ranges from a few microseconds to several milliseconds for both UV and IR wavelengths. ,,,,,,, Thus, postpulse ablation can last 3−5 orders of magnitude longer than the laser pulse duration. Similar durations for postpulse ablation were reported for microsecond laser exposures. ,, Postpulse ablation generally lasts longer for mechanically weaker tissues, larger radiant exposures, and larger laser beam diameters.…”