SD OCT Reveals Details of Posterior Segment Structures

Spectral-domain optical coherence tomography (SD OCT) is a recently introduced modality that has drawn much interest from research and clinical ophthalmologists. Many anterior segment surgeons, however, do not yet know what this new technology offers or how it differs from time-domain OCT (TD OCT), which has grown familiar over the decade since its introduction.

What can we see with SD OCT (also known as Fourier-domain OCT) that we cannot see with TD OCT? With SD OCT, we can study not only anatomy but also virtual histology. In the normal retina with SD OCT, we see new layers and new features whose significance is still being discussed. We do not yet know how much these new features reflect the actual pathologic anatomy and how much are only artifacts caused by optical or electronic effects.

The images provided by the new SD OCT devices (Table 1) are not only much sharper than TD OCT images—they are different. The huge quantity of new data can be perplexing to the clinical eye and mind. An ophthalmologist who is already using TD OCT will not immediately be able to understand all the information in SD OCT scans.

In pathologic retinas, these elements can offer new difficulties in interpretation. The interpretation of SD OCT images is not as easy and intuitive as with TD OCT.

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On the other hand, with SD OCT there is a reduction of movement artifacts compared with TD OCT, there is more detail at the vitreoretinal interface, and there is better delineation of all the retinal layers.

It is notable that OCT experts prefer to use the grayscale instead of color images (Figure 1).

In the outer retina, SD OCT allows us to see the external limiting membrane and the junctions between the internal and outer segment of the photoreceptors. It reveals early damage to the photoreceptors and allows better assessment of outer nuclear layer thickness and of the length of the outer segment of photoreceptors, especially in the foveal area.

SD OCT also allows better visualization of the three-layer structure of the pigment epithelium, Bruch's membrane, the choroid, and the choriocapillaris (Figure 2).

In patients with age-related macular degeneration, SD OCT can show the thinning and disruption of the outer retinal layer in geographic atrophy and the new vessels penetrating deeper layers of the retina in the neovascular form of the disease (Figure 3).

Another feature of SD OCT is the ability to perform retinal segmentation, creating a virtual 3-D in vivo dissection of the retina. It can isolate interfaces or layers of the retina and subtract retinal or vitreal layers from an image (Figure 4). This is a new feature that was not possible with TD OCT.

It will take time for ophthalmologists to learn to use the advanced imaging capabilities with SD OCT. It is certain, however, that this new modality will add to our understanding of the retina and provide diagnostic information that was never before available to clinicians and researchers.

Images in this article were created using the Spectral OCT/SLO (Opko/OTI, Miami, Florida).

Bruno Lumbroso, MD, a past president of the Italian Society of Retina, practices in Rome. Professor Lumbroso states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +39 0637518008; E-mail: bruno.lumbroso@libero.it.