EVALUATION OF LTK LESIONS BY OPTICAL LOW- COHERENCE TOMOGRAPHY (OCT) AND POLARIZATION MICROSCOPY AFTER SIRIUS-RED STAINING

Background: Information on the extent and degree of the thermal effect produced is of great importance for control of the laser dosage in la ser thermokeratoplasty (LTK) and for postoperative follow-up. We inves tigated on acute LTK effects which information images obtained by opti cal low coherence tomography (OCT) offer compared to those obtained by polarization microscopy. Methods: Porcine eyes were irradiated throug h a 400 mu m quartz fiber using light from a laser diode emitting up t o 300 mW at a wavelength of 1.86 mu m. Thermal lesions of varying stre ngth were scanned using an experimental OCT device with about 25 mu m lateral and 20 mu m axial resolution. Histologic evaluation of the sca nned areas was done by polarization microscopy after Sirius-Red staini ng, and similar lesions were also analyzed by TEM. Results: Both metho ds differentiated three damage zones: a transition zone, a zone of mod erate coagulation, and a central zone of strong coagulation. In the tr ansition zone,increased birefringence was seen in polarization microsc opy, which correlated with increased light scattering seen in the DCT images,ln the moderately coagulated zone, a decrease in birefringence was associated with an even stronger increase of the OCT signal, In th e central zone,a loss of the fibrillar tissue structure was observed, which led to a complete loss of birefringence and a strong reduction o f the OCT signal. Conclusions: Although OCT does not provide the detai led information on thermal changes of tissue seen by the histologic me thod, it offers information on the extent and degree of tissue changes without preparation artifacts and provides a non-invasive method of i mmediate and follow-up control of LTK lesions, A quantitative analysis of changes in corneal thickness and curvature is much simpler than by a slit lamp. Time-resolved measurements of corneal light scattering m ay be used for on-line control of the laser-light dosage during LTK.