NONINVASIVE MONITORING OF EXCIMER-LASER ABLATION BY TIME-RESOLVED REFLECTOMETRY

BACKGROUND: Current excimer laser photorefractive procedures use empir ic etch rates to determine specific changes in corneal shape. A real-t ime analytic method for monitoring the tissue ablation process may be useful in tailoring energy delivery to a specific patient and in detec ting detrimental phenomena such as corneal desiccation. METHODS: We mo nitored excimer laser ablation by studying the amplitude and temporal characteristics of ArF laser pulses reflected from the ablation site. Two target materials were used: polymethylmethacrylate (PMMA, a synthe tic polymer that undergoes an incubation phase where no ablation occur s for an initial finite number of laser pulses), and bovine cornea. Ob served reflectivity changes during irradiation of PMMA were compared t o profilometric ablation depth measurements. Corneal ablation Was perf ormed both with and without nitrogen gas flow at the ablation site to study the effect of tissue desiccation. RESULTS: For ablation of PMMA at 160 mJ/cm2, the incubation phase included the initial eight laser p ulses. For corneal tissue ablation at a fluence of 125 mJ/cm2, flowing nitrogen gas caused significant shortening and amplitude reduction in the reflected laser signals. CONCLUSIONS: Noninvasive time-resolved r eflectometry provided real-time information about target ablation. Thi s technique may have diagnostic utility during laser corneal surgical procedures.