Optoacoustic detection of selective RPE cell damage during μs-laser irradiation

Objective: The selective damage of the retinal pigment epithelium (RPE) with repetitive μs laser pulses is a new technique for the treatment of several retinal diseases. RPE can selectively be damaged by simultaneously sparing off the adjacent photoreceptor tissue. Objective of this study is to investigate whether optoacoustic (OA) transients occurring during irradiation might be used to control the invisible treatment effect. Setup: A train of frequency doubled Nd:YLF laser pulses (527 nm, 1.7 μs pulse length, 500 Hz rep. rate) were applied via a laser slit lamp on porcine RPE samples. The acoustic transients were recorded with a broadband transducer. Results: At low radiant exposures (< 100 mJ/cm²) we found a bipolar pressure transient due to thermoelastic expansion of the RPE. The pressure waves from the individual pulses of one pulse train show nearly identical transients. The transients differ slightly from different sites on the sample. At higher radiant exposures (> 150 mJ/cm²), the OA transients differ from pulse to pulse within a pulse train, which can be attributed to microbubble formation around the strong absorbing melanosomes inside the RPE cells. FFT spectra of the OA transients show slight differences in the frequency spectrum with the different radiant exposures.