Integrated OCT and SLO for Small Animal Retinal Imaging

Scanning Laser Ophthalmoscopy (SLO) and Coherence Tomography (OCT) are complimentary retinal imaging modalities. Integration of SLO and OCT allows for both fluorescent detection and depth- resolved structural imaging of the retinal cell layers to be performed in-vivo. System customization is required to image rodents used in medical research by vision scientists. We are investigating multimodal SLO/OCT imaging of a rodent model of Stargardt's Macular Dystrophy which is characterized by retinal degeneration and accumulation of toxic autofluorescent lipofuscin deposits. Our new findings demonstrate the ability to track fundus autofluorescence and retinal degeneration concurrently.

The integration of FDOCT with cSLO is facilitated by the similarities and differences of the two sub-systems's details. Since both FDOCT and cSLO are raster scanning modalities, a common set of galvanometer mounted mirrors can be used to scan the beam on the sample. Aligning the beams to be co-linear ensures that the same region on the sample is imaged. The FDOCT sub-system operates in the infrared to optimize tissue penetration. By selecting the cSLO to operate in the visible wavelength, simple heat control optics can be used to combine the two systems [1-5].The integrated FDOCT-cSLO system is schematically presented in Figure 1.

Figure 1: System setup

Representative cSLO image acquired from an albino mouse isshown in Figure 2.

The quality and brightness of the cSLO image of an Albino ABCA4 mouse (Figure 3b) is similar to that of the control mouse (Figure 3a). However, in the fundus autofluorescence (FAF) image, we can clearly see the auto fluorescence on the albino ABCA4 which is due to much higher amount of lipofuscin accumulation compared to the control mouse's FAF image.

Figure 3: cSLO of mouse retina (albino)

Figure 4: Timeline of Fluorescence measurements from abca4 -/- mouse.

Alignment of the field of view with the OCT system is time consuming because of the slow volume acquisition rate (several seconds), and was greatly facilitated by the use of the cSLO sub-system (~2 frames per second). The OCT, once aligned to the right location on the retina based on the landmarks of the blood vessels, can be used to acquire 3D volumetric images, as it is shown in Figure 5. In this example, we used a custom system designed at SFU with an 86nm bandwidth. The importance of the FDOCT is that it can be used for quantitative imaging of retinal degeneration, and thinning of layers.

Figure 5: Comparison of Fundus imaging via cSLO and FDOCT

In Figure 6, averaged FDOCT B-scan of the retinal cell layers is shown. We are examining the retinal diseases (such as glaucoma), which affects the retinal nerve fiber layer, we would make measurements on the top portion (top arrows). In another case study, we examine diseases, which causes the loss of photo receptors (such as stardardt's macular dystrophy), and we would measure the changes on the thickness of the outer nuclear layer (ONL).

Figure 6: (a) SLO image with the optic nerve head aligned at the center; (b) OCT summed voxel projection fundus image; (c) a 3D volumetric data set at the same location as shown by the OCT summed voxel projection fundus image; (d) 20 frames averaged on an ABCA4 mouse retina and the ONL thickness at approximately 430um away from the ONH is measured (~44 um +- 1um).


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