Dynamic OCT Signal Loss for Determining RPE Radiant Exposure Damage Thresholds in Microsecond Laser Microsurgery

Christian Burri,Dirk Theisen-Kunde,Alessa Hutfilz,Ralf Brinkmann,Andreas Ebneter,Simon Salzmann,Lorenz Grimm,Boris Považay,Patrik Arnold,Christoph Meier

doi:10.3390/app11125535

Abstract

Optical microsurgery of the retinal pigment epithelium (RPE) requires reliable real-time dosimetry to prevent unwanted overexposure of the neuroretina. The system used in this experiment implements optical coherence tomography (OCT) to detect the intentional elimination of RPE cells. We evaluated the performance of OCT dosimetry in terms of its ability to detect RPE cell damage caused by microsecond laser pulses of varying duration. Therefore, ex-vivo porcine RPE choroid sclera explants were embedded in an artificial eye and exposed to single laser pulses of 2–20 µs duration (wavelength: 532 nm, exposure area: 120 × 120 µm2, intensity modulation factor: 1.3). Simultaneously, time-resolved OCT M-scans were recorded (central wavelength: 870 nm, scan rate: 33 kHz). Post-irradiation, RPE cell damage was quantified using a calcein-AM viability assay and compared with an OCT-dosimetry algorithm. The results of our experiments show that the OCT-based analysis successfully predicts RPE cell damage. At its optimal operating point, the algorithm achieved a sensitivity of 89% and specificity of 94% for pulses of 6 µs duration and demonstrated the ability to precisely control radiant exposure of a wide range of pulse durations towards selective real-time laser microsurgery.

Highlights

In the early 1950s, light coagulation was introduced as an outstanding optical therapeutic tool for remote tissue manipulation to prevent retinal detachment after retinal break formation [1]
We evaluated the performance of optical coherence tomography (OCT) dosimetry in terms of its ability to detect retinal pigment epithelium (RPE) cell damage caused by microsecond laser pulses of varying duration
Our results for porcine RPE damage exposure thresholds were in good agreement with data of 1 and 3 μs laser pulses reported by Brinkmann et al and 5 and 50 μs laser pulses found by Schuele et al Recently published mean RPE damage exposure thresholds on lightly pigmented porcine eyes reported by Seifert et al for laser pulses of 2 to 50 μs duration are consistent with our findings for 2 μs pulses

Summary

Introduction

In the early 1950s, light coagulation was introduced as an outstanding optical therapeutic tool for remote tissue manipulation to prevent retinal detachment after retinal break formation [1]. The delicate multi-layer retinal structure, consisting of the retinal pigment epithelium (RPE), Bruch’s membrane, choriocapillaris and the otherwise healthy overlying neuroretina, including the sensitive photoreceptor cells, are collaterally damaged by laser pulses in the millisecond range, due to the relatively slow heating process and the relatively fast heat dissipation into the directly unheated neighboring tissue This indirect heating resulting from excessive power in turn leads to scotoma, reduced night vision and disruption of the retinal anatomy through scarring [5,6]. It is not possible to use LPC near the fovea because of the high risk of permanent loss of central vision [7,8] These disadvantages have led to the development of new laser treatment modalities, which aim to produce specific therapeutic results without damaging photoreceptors and the neural retina by selectively acting on target structures. The rapid mechanical expansion and collapse of these microbubbles causes RPE cell-wall disruption, followed by immediate or delayed cell death [15]

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Conclusion