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Noninvasive monitoring of excimer laser ablation by time-resolved reflectometry.

BACKGROUND
Current excimer laser photorefractive procedures use empiric etch rates to determine specific changes in corneal shape. A real-time analytic method for monitoring the tissue ablation process may be useful in tailoring energy delivery to a specific patient and in detecting detrimental phenomena such as corneal desiccation.


METHODS
We monitored 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 synthetic polymer that undergoes an incubation phase where no ablation occurs for an initial finite number of laser pulses), and bovine cornea. Observed reflectivity changes during irradiation of PMMA were compared to profilometric ablation depth measurements. Corneal ablation was performed 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 pulses. 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 reflectometry provided real-time information about target ablation. This technique may have diagnostic utility during laser corneal surgical procedures.