Optoacoustic on-line dosimetry during selective RPE treatment

The selective RPE treatment (SRT) is a new method, which targets retinal diseases associated with disorders in the retinal pigment epithelium (RPE). By applying a train of μs laser pulses, it is possible to selectively damage RPE cells while sparing the adjacent photoreceptors and the neural retina. Due to the ophthalmoscopic invisibility of the RPE effects we investigated an optoacoustic (OA) on-line dosimetry system to monitor RPE damage non-invasively. Material and Methods: For in vitro experiments porcine RPE was irradiated with a Nd:YLF laser pulse train (527nm, 1.7μs, 5-40μJ, 30 pulses, 100 Hz). Pressure waves (optoacoustic transients) generated at the RPE were measured with a piezoelectric transducer. The RPE cell damage was visualised by fluorescence microscopy by means of the vitality stain CalceinAM. During 27 patient treatments (527nm, 1.7μs, 50-150μJ, 30 pulses, 100 Hz) the optoacoustic signals were measured with an ultrasonic transducer embedded in the contact lens. The RPE leakage was visualized by fluorescein and ICG angiography. Results: In vitro: Below the RPE cell damage threshold, the optoacoustic transients from each single pulse are almost similar. With RPE damage, fluctuations of the individual transients are observed during the pulse train. These fluctuations can be explained by statistical irregular microbubble formation around the strong light absorbing melanosomes inside the RPE cells, which occur after the temperature exceeds the vaporization threshold. The transient microbubbles probably lead to RPE cell disruption. An optoacoustic value (OA-value) calculated from the fluctuations was defined in order to assess RPE damage. If optoacoustic pulse-to-pulse fluctuations were measured, RPE leakage was observed in fluorescein and ICG angiography. In 96% of the irradiated areas, RPE-leakage in fluorescein angiography and OA-value correlated. The stronger the optoacoustic pulse-to-pulse fluctuations, thus the higher the OA-value, the more intense angiographic leakage was observed in ICG-angiography. Conclusion: A non-invasive optoacoustic on-line dosimetry control to monitor RPE damage during SRT was developed. In order to avoid invasive angiography, it is currently evaluated in a multicenter clinical SRT study.