Comparison of acoustic transients with fringe washouts in OCT M-scans after RPE microsecond laser irradiation

The treatment of retinal pigment epithelium (RPE) related diseases, utilizing minimally invasive laser techniques like selective retina therapy (SRT) is highly demanded. However, due to the strong inter- and intraindividual variability of RPE absorption, as well as optical transmission, laser microsurgery requires reliable real-time feedback-controlled dosimetry (RFD) to prevent unwanted retinal overexposure. The formation of microbubbles around the strong absorbing melanosomes inside RPE cells, has been identified as the leading mechanism of RPE cell damage during low microsecond laser pulse exposure. Their formation and collapse cause measurable optoacoustic (OA) transients. In the presented experiment OA transients are compared to fringe-washouts in simultaneously recorded optical coherence tomography (OCT) M-scans directly following RPE laser irradiation. Ex-vivo porcine RPE-choroid-sclera explants were exposed to laser pulses of 8, 12, 16 and 20 μs duration and pulse energies ranging from 15 to 100 μJ (wavelength: 532 nm, exposure area: 120 × 120 μm2). Simultaneously, time-resolved OCT M-scans were recorded (central wavelength: 840 nm, scan rate: 77 kHz). Post irradiation, RPE cell damage was quantified using a calcein-AM viability assay and correlated with OA transients and fringe-washouts in OCT M-scans. The results show that the detection of fringe-washouts in OCT M-scans linearly scales with OA transients and correctly identifies the destruction of RPE cells. Furthermore, the findings indicate that the optical detection is more sensitive for SRT dosimetry than OA, because OCT reacts to fast dynamic changes of the scattering structure, which possibly are related to minute cell collapses yet indiscernible by OA transients.