Ophthalmologists and optometrists considering investing in OCT need to understand the strengths and weaknesses of the technology, and how it complements fundus imaging and other diagnostic tests that contribute to a complete eye exam.
Optical coherence tomography (OCT) technology has, for many years, been regarded as the future for eye care professionals who wish to differentiate themselves through clinical excellence.
But like colour fundus photography before it, OCT isn’t everything. Ophthalmologists and optometrists considering an OCT investment need to understand both the strengths and weaknesses of the technology and how it complements fundus imaging and other diagnostic tests that contribute to a complete eye examination.
One logical option is a modular system, which allows the addition of components as the practice grows. Spectralis SD-OCT is one such system, with options for Blue Peak Autofluorescence and Multi Colour Imaging to be added subsequent to the initial purchase.
Clinically, glaucomatous damage is typically diagnosed via a combination of structural and functional measurements…
Below the Surface
The strength of SD-OCT lies below the surface. This three-dimensional imaging technology provides supplementary information to ophthalmoscopy or fundus imaging. It certainly doesn’t replace it. That is why it is so important to be able to capture a diagnostic-quality fundus image and SD-OCT scan simultaneously. You will want to place your OCT cross-section directly through the pathology that you identify on the fundus image, in order to look for additional information below the surface which will either confirm or refute your diagnosis. Point-to-point correlation between fundus and OCT scans, during the initial examination, as well as follow-up scans, will give you more confidence to track small changes over time.
Active Eye Tracking
An instrument may seem to be easy to use during a demonstration when imaging a ‘young, well trained patient’, but the real world is full of difficult patients with small pupils, problems with fixation or who just simply can’t sit still. The Spectralis uses active eye tracking to make imaging the most difficult patients easier. The system uses a dual beam, known as TruTrack Technology: one beam tracks the eye’s movements and guides the other beam which captures the OCT image (Figure 1).
By using this active eye tracking, the Spectralis has been shown to measure changes as small as one micron. It has the ability to set the original scan as a baseline, then align follow up scans to it in real time as they are acquired, rather than after they are acquired. This becomes vital in monitoring progression in diseases like glaucoma, where small changes may occur. Clinicians can have greater confidence that they are measuring true change rather than change due to inter test variability.
Multi-colour Fundus Imaging
Multi-colour Scanning Laser Imaging brings a new dimension to non-mydriatic fundus imaging. Simultaneous imaging
with multiple laser colours selectively captures and portraits diagnostic information originating from different retinal structures within a single examination. Multi-colour imaging delivers high-contrast, detailed images even in difficult patients including those with cataracts or nystagmus, flash free. Spectralis combines MultiColor and SD-OCT in a single device creating a new diagnostic platform for simultaneous colour fundus and cross-sectional OCT imaging (Figure 2).
Blue Laser Autofluorescence Imaging
Without the need for dye, non-invasive blue laser autofluorescence imaging in combination with SD-OCT provides metabolic as well as structural information about the retina. Taking advantage of the natural fluorescent properties of lipofuscin, a blue laser captures fundus autofluorescence (FAF) images. Accumulation of lipofuscin is associated with a number of retinal diseases, including both wet and dry forms of AMD. Characteristic autofluorescence patterns seen in blue laser images can clearly show the extent of geographic atrophy (GA), helping you to refer patients for appropriate treatment and manage patient expectations (Figure 3).
Infrared Fundus Imaging
Since infrared light is invisible, patients are spared the jolting flash that accompanies traditional fundus photography. This is particularly valuable when imaging children and light sensitive patients. Infrared reflectance imaging may offer better visualisation of epiretinal membranes and cystoid macular oedema compared to fundus photography and red-free imaging. Spectralis fundus imaging uses confocal scanning laser technology to minimise scattered light and sharpen focus, especially in patients with media opacities such as cataract (Figure 4).
Enhanced 3D detail
Three-dimensional OCT images may be enhanced with the application of a red/green tool and 3D glasses. In the OCT Plus model stereo pair images can be acquired at two angles at the touch of a button, offering three dimensional retinal images when viewed with red green glasses or a prismatic viewer.
Full Depth OCT imaging
Full Depth Imaging (FDI)-OCT and Enhanced Depth Imaging (EDI)-OCT are both additional, innovative diagnostic imaging modalities that come as standard in every Spectralis OCT model. EDI-OCT enables high resolution OCT imaging of external retinal layers, the choroid and lamina cribrosa. You can view structural changes ‘beyond the RPE’ in a reproducible fashion, which is more difficult to capture with standard OCT (Figure 5).
Anterior Segment Imaging
An optional Anterior Segment Module allows you to capture high-resolution images of the cornea, sclera and anterior chamber angles. The Anterior Segment Module offers a 16mm SD-OCT Scan for dual-angle imaging and interactive measurement tools (Figures 6a, b and c).
Glaucoma patients
Glaucomatous damage is often marked by retinal thinning in the zone surrounding the fovea and extending toward the optic nerve head. Clinically, glaucomatous damage is typically diagnosed via a combination of structural and functional measurements. Spectralis Posterior Pole Asymmetry Analysis maps retinal thickness and ganglion cell layer thickness across the posterior pole and graphs asymmetry both between hemispheres and between eyes. RNFL thickness can be compared to age matched normals. (Figure 7).
(Click here to view Figure 7.)
Combined with the Heidelberg Edge Perimeter’s flicker-defined-Form or Standard Perimetry, you can combine visual fields with imaging data about the ONH, retinal ganglion cell and retinal nerve fibre layer thickness.
Conclusion
OCT isn’t everything. While SD-OCT provides enhanced detail below the surface, simultaneous confocal scanning laser ophthalmoscopy increases your diagnostic capability with point-to-point correlation of detail on the surface, which is possible with dual-beam active eye tracking built into the Spectralis OCT. With full depth OCT imaging
and posterior pole analysis, as well as infrared and stereo fundus imaging functionality, your practice will be able to offer a comprehensive eye health
check that truly differentiates you from your competition. The upgradeable design of the Spectralis platform also allows you to add MultiColour and Blue Laser Auto fluorescence fundus imaging modalities, anterior segment imaging and HEP combined structure function reports seamlessly as your practice grows.
Carey Hazelbank has over 25 years industry experience in ophthalmic instrumentation. He is General Manager at Heidelberg Engineering in Australia and has represented Heidelberg engineering in Australia since 2002 as a director of Optech Diagnostic and Surgical.
References
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Menke et al. Am J Ophthalmol 2009; 147: 467-472.
Hassenstein and Meyer. Clinical and Experimental Ophthalmology 2009; 37: 130-143.
Samjay Asrani MD, Whitepaper: “Novel Strategies in Glaucoma Diagnosis and Management.”
Priel. Journal of Ophthalmic Photography 2007, Volume 29 : 62-71
