In vivo investigation of near infrared retinal lesions utilizing two adaptive optics enhanced imaging modalities

Abstract

ABSTRACT Near threshold retinal lesions were created in the eyes of non-human primate (NHP) subjects in the near infrared (NIR) wavelength range of 1100 to 1319 nm, with 80 to 100 ms laser exposures. Two new in vivo imagining techniques, Adaptive Optic enhanced-Spectral Domain Optical Coherence Tomography (AO-SDOCT) and Adaptive Optic enhanced confocal Scanning Laser Ophthalmoscope imagery (AOcSLO) we re utilized to pinpoint areas of chronic damage within the retinal layers resulting from laser e xposure. Advantages and limitations of each technology with regard to the study of laser retinal tissue interaction are highlighted. Keywords: Adaptive Optics, MVL, NIR, laser threshold, in vivo imaging, confocal, scanning laser ophthalmoscope, SLO, Spectral Domain, OCT 1. INTRODUCTION High-fidelity imaging of lesions in the NHP retina is vital for determining damage thresholds for laser exposure. Retinal damage thresholds are traditionally determined from direct funduscopic observation with an ophthalmoscopic camera. The ability to discern a change from healthy to denatured retinal tissue in vivo is limited primarily by the optical transfer function (OTF) of the eye, with observable lesions of 25 um in extent (or larger) typically reported as being a minimally visible lesion (MVL).[1] However, damage widths which correspond to the extent of a single cell at the RPE and photoreceptor layers in the retina for ultrashort laser exposures have been reported with histological confirmation by Toth.[2] Recent observations of changes in autofluorescence in the retina also suggest that unresolved damage as a result of subthreshold exposure levels can exist without detection via traditional imaging methods in threshold studies. [3-5] These observations and the lack of data reported for similarly sized lesions resulting from photothermal exposures accentuate the possibility that damage which is not directly observable through standard funduscopic observation is present at exposure levels which are “subthreshold.” Given the 25-um size purported to be the smallest repeatedly observable lesion in thermal studies, the observation of small lesions in ultrashort laser studies and the subthreshold detection of changes in autofluorescence of RPE cells it is apparent that a method of observation which is both more sensitive (improved spatial resolution) and specific (able to isolate damaged tissue within the layers of the retina) is needed. Therefore, imaging techniques which overcome the li mitations of the OTF of the eye and which enable imaging at depth in turbid media are highly desired for use in retinal laser threshold studies. Two adaptive-optic-enabled retinal imaging systems were delivered to Brooks City-Base as the product of Air Force Research Laboratory sponsored Small Business Initiative for Re search (SBIR) technology de velopment contracts. The first system developed by PSI (Andover, MA) is a spectral domain optical coherence tomography (AO-SDOCT) system. Details of this system are published elsewhere.[6]. The second system, developed by IRIS Electro-Optics (Berkeley, CA), is a confocal scanning ophthalmoscope (AOcSLO). [7] A schematic for each system is shown in Figure 1. This paper details the initial use of these imaging systems to observe laser damage in the NHP retina which were produced by exposure to NIR laser wavelengths.