Classification of Rabbit Photocoagulation Lesions in Optical Coherence Tomography and Class-related Coagulation Temperatures

Abstract

 

Purpose

 

The rabbit is the most common animal model for photocoagulation, particularly in histological studies. Rabbit optical coherence tomographic (OCT) findings differ from those in humans. This study presents six OCT-morphological endpoints of retinal photocoagulation lesions in rabbits and matched temperatures.

 

Methods

 

We applied 1022 photocoagulation lesions of 133 µm diameter to six eyes of three rabbits and varied laser powers and exposure times. At the times of 1 hour, 1, 4 and 12 weeks after treatment, we examined fundus color images, Spectralis® OCT and infrared and autofluorescence images. We grouped the lesions according to OCT morphological criteria, measured funduscopic lesion diameters after one hour and greatest linear diameters (GLD) with OCT at all times. Our modified 532 nm photocoagulator facilitated temperature measurements based on optoacoustics. Peak temperatures at the end of irradiation in the lesion center at the retinal pigment epithelium level were evaluated for every lesion.

 

Results

 

We detected six OCT-morphological lesion classes that ranged from minimal reflectivity increases at the outer nuclear layer level (class 1) to full thickness signal increases with neurosensory detachment and an optically empty space in the center (class 6). Ophthalmoscopical visibility was 17% (class 1) to > 95% (class 4-6). Median diameters ranged from 0 (invisible, classes 1 and 2) to 200 µm (class 6). GLD's were 150 (class 1) µm to 400 µm (class 6) after 1 hour, which shrunk to 100 - 280 µm after 12 weeks, respectively. All 3 parameters increased significantly with the OCT class. For lesions with 200 ms exposure time, the peak temperatures were 60 to 80° C for class 2 to 6 lesions.

 

Conclusions

 

The validity of the OCT classes is supported by increasing ophthalmoscopical visibility, diameters and GLD's. These OCT endpoints allow standardized evaluation of photocoagulation lesion in rabbits and are applicable in repeated examinations, in contrast to histology. Moreover, it allows to estimate lesion end peak temperatures according to the presented values. Classes 1 and 2 allow recognition and differentiation of "sub-threshold" lesions, which are of particular interest. Complete retinal restoration was not observed in our study.

 

Keywords: 578 laser • 688 retina • 551 imaging/image analysis: non-clinical