In Vivo and In Vitro Imaging of Pigmented Structures in the Rat Eye with Polarization Sensitive Optical Coherence Tomography

Abstract

 

Purpose

 

To demonstrate polarization sensitive (PS) optical coherence tomography (OCT) for investigating pigmentation in the eyes of different rat strains in vivo. To correlate the findings in PS-OCT images to histological analysis of the same tissues.

 

Methods

 

A prototype high-speed PS-OCT system was modified for imaging the rat eye. Healthy male rats of both pigmented (Brown Norway, Long Evans) and albino (Sprague Dawley) strains were anaesthetized and densely sampled PS-OCT raster scans covering an area of ~1.5 mm x 1.5 mm centered at the papilla were acquired. In addition to conventional OCT images based on reflectivity, cross-sectional PS-OCT images displaying the birefringent and depolarizing tissue properties were computed from the 3D data of the ocular structures. The eyes were enucleated post mortem for histological analysis. After fixation, cryosectioning and staining, confocal microscopy imaging of ocular structures in the posterior globe was performed. In vitro PS-OCT imaging was performed in unstained histological sections in order to compare tissue appearance to histology.

 

Results

 

In reflectivity images, the retinal layers and the choroid can be observed. In the albino rat eye, light penetration through the non-pigmented choroid is increased and the backscatter signal from the sclera is stronger. Depolarization was investigated in degree of polarization uniformity (DOPU) images. High DOPU values indicating uniform polarization characteristics were observed in most tissues. Depolarization, i.e. low DOPU values, was observed in structures containing melanin granules, namely the RPE/choroid complex in Brown Norway and Long Evans rats. In addition, the remnant of the hyaloid artery exhibited depolarization in the Brown Norway rat retina. Overlap of depolarization and melanin pigmentation in the RPE/choroid complex and the hyaloid artery remnant was confirmed using histological analysis.

 

Conclusions

 

PS-OCT enables quantitative imaging of polarizing tissue properties in addition to conventional OCT imaging based on reflectivity. Ocular structures containing melanin granules appeared depolarizing in PS-OCT images. Since pigmented structures such as the RPE and the choroid play key roles in age-related macular degeneration (AMD), PS-OCT may provide a promising technology for in vivo imaging of (micro)structural changes in disease models of AMD.

 

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 596 microscopy: confocal/tunneling