The Future of Retinal Imaging

Daniel Q. Li; Netan Choudhry


Curr Opin Ophthalmol. 2020;31(3):199-206. 

In This Article

Adaptive Optics Scanning Laser Ophthalmoscopy Fluorescein Angiography

The human eye is an imperfect optic system, in which ocular aberrations, introduced mainly by the cornea and lens, limit the resolution of conventional retinal imaging techniques.[53] Adaptive optics is a technology that corrects aberrations in optical systems, and was initially introduced to improve the performance of astronomical telescopes by removing atmospheric distortions using a deformable mirror.[54] Adaptive optics technology was later introduced to retinal imaging in 1997, and in recent years has gained popularity in the ophthalmic research community because of its ability to drastically improve the quality of retinal images when coupled with conventional techniques, such as AO SLO.[55,56] The use of AO SLO with fluorescein angiography, the current clinical gold standard for assessing retinal and choroidal vascular diseases, was first reported in living human individuals by Pinhas et al.[57] in 2013. Compared with conventional fluorescein angiography, AO SLO-FA demonstrated appreciably greater resolution and contrast to visualize microscopic details of the retinal vasculature down to the level of quaternary branches and capillary networks. This translates to enhanced characterization of microvascular attributes, such as vessel diameter, branching pattern, tortuosity and capillary density, to an extent that had not been previously possible.

Clinical Applications

AO SLO-FA has been applied to study structural and functional changes in a variety of retinal vasculopathies, including diabetic retinopathy, retinal vein occlusion and sickle cell retinopathy. In diabetic retinopathy, AO SLO-FA can detect early, subclinical changes of prolonged hyperglycemia not visible with current generational imaging modalities.[58] Dubow et al.[59] expanded the anatomical classification of microaneurysms to include the saccular, fusiform and focal bulge designations proposed in previous studies, as well as three additional morphologies discernable with AO SLO-FA: saccular/fusiform, pedunculated and irregular microaneurysms. They also revealed intraluminal hypofluorescent regions that appeared in one-third of microaneurysms on AO SLO-FA, but not observable using conventional fluorescein angiography. These are hypothesized to be luminal clots derived from acute inflammatory cells subsequent to endothelial injury; however, further studies are needed to elucidate the relationship between these hypofluorescent regions and advanced disease. In retinal vein occlusions, AO SLO-FA revealed an array of microangiopathic features, such as nonperfused capillaries, leakage and microaneurysm not fully appreciated on conventional imaging.[60] Furthermore, AO SLO-FA revealed subclinical, but significantly reduced vessel densities in the asymptomatic fellow eyes of eyes affected with nonischemic central retinal vein occlusion, which may explain the increased lifetime risk of occlusion in fellow eyes of this condition. In sickle cell retinopathy, AO SLO-FA has demonstrated significantly higher capillary tortuosity compared to healthy controls.[61] Additional studies exploring the clinical application of capillary tortuosity as a metric for monitoring sickle cell retinopathy progression are currently underway.

Future Directions

The increased resolution of AO SLO-FA, compared to conventional fluorescein angiography, offers great opportunity for studying microvascular change on a histological scale, which may further our understanding of physiological and pathological vascular processes. As an emerging technology mostly confined to the research stage, further studies need to establish the accuracy and reproducibility of AO SLO-FA to establish validity and suitability for routine clinical implementation. It is also important to note that compared to conventional fluorescein angiography, AO SLO-FA has a smaller field of view and requires more time to acquire, making it most appropriate for focused questions and detailed analysis of capillary changes in a predefined region. Although image montage is an option to expand the field of view, it is labor intensive and time consuming. Further developments may focus on advancing the speed and field of view of AO SLO-FA imaging, whereas eye-tracking technology and software advances may make the results more readily available in a clinically relevant time course.