## 4 Discussion
Regarding the IMF, local RPE cell destruction (e.g., lesion 3 (262 mJ/cm\({}^{2}\)\(\cong\) 29 \(\upmu\)J) in Figure 8b) may be attributed to local peaks in the laser beam profile, in addition to local pigment differences. For laser treatments that are intended to selectively damage the RPE, the influence of the top-hat beam quality is of fundamental importance. Whereas local intensity peaks can lead to local over-treatment and mechanical rupture, local dips might not lead to any damage at all. In terms of clinical application, the influence of the IMF in this respect is interesting to discuss, because in our experiments the laser was applied to the uncovered RPE situated within an artificial eye with an optically \"perfect\" cornea and lens. When applied to patients, however, ocular aberrations, cloudiness of the ocular media, and other effects need to be considered. The beam profile of the SRT laser is thus (additionally) reshaped on its way to the RPE, which can strengthen or weaken intensity peaks. In addition, the laser can undergo scattering caused by edema, which may smooth out local intensity peaks. It is therefore difficult to precisely define what IMF value is reasonable, although homogenous exposure (IMF = 1) that excludes ocular influences is desirable. The correlation between the exposure threshold values for the OCT signal loss in M-scans and RPE cell damage supports the ability of the Spectralis Centaurus system for predictive microsecond laser dosimetry via OCT fringe washout analysis. A similar average threshold for fringe washouts in M-scans would also allow the evaluation of sequential events (e.g., three consecutive fringe washouts) as proposed by [PERSON] et al. (SRT in ramp mode combined with a fast algorithm-based SD-OCT treatment stop). This would make the SRT dosing more robust, without risking over-treatment despite the tissue's variable exposure thresholds [28]. This methodology will be investigated in further experiments with the Spectralis Centaurus system. Figure 8: (**a**) Calcein-AM image of porcine RPE-choroid-scera explant No. 7. Focus is on the treatment lesions in lines three and eight which were applied with laser pulses of 10 \(\upmu\)s duration (white rectangles with lesions 2 to 6 (white-dashed)). When comparing sections (**b**) and (**c**), variations for the threshold of RPE cell damage can be observed. Whereas for treatment line three, a distinctive lesion can be observed at a pulse energy of 51 \(\upmu\)J 460 mJ/cm\({}^{2}\), a similar distinctive lesion for treatment line eight is already present at a pulse energy of 44 \(\upmu\)J 397 mJ/cm\({}^{2}\). Furthermore, for each lesion in sections (**b**,**c**), the corresponding SD-OCT M-scan, as well as an explant classification according to the confusion matrix, is presented. Local RPE cell destructions that may be related to intraindividual melanin concentrations or local intensity peaks in the beam profile are marked with white triangles (sample classification with a \(\kappa\)-value of 13). The algorithm is partially able to detect MBF through a few damaged RPE cells (e.g., lesion four (**c**)). A remaining open question is whether the OCT signal loss is caused solely by MBF or whether pure thermal effects, like fast tissue expansion or coagulation, are also detected. Recently, [PERSON] et al. concluded that thermal cell damage is likely to be the primary cellular damage mechanism. According to this work, MBF sets in at slightly higher radiant exposure levels and leads to superimposed mechanical damage [40]. Based on ex-vivo experiments, [PERSON] et al. found approximately equal thresholds for MBF and RPE cell damage at 10 \(\upmu\)s, whereas at 20 \(\upmu\)s the MBF threshold was ~10% higher than for cell damage [18]. Analogous results were recently found by [PERSON] et al.