Recognizing Common OCT Artifacts for Successful Clinical Management of Glaucoma

By Osamah Saeedi, MD, MS

Serial high-resolution optical coherence tomography (OCT) scans of the retinal nerve fiber layer (RNFL) and the macula sectors are essential to detect glaucomatous progression. This advanced tool can alter our clinical decision-making for glaucoma suspects and those with existing disease. It is imperative that we understand the limits and sources of errors within a technology and how to use a platform’s tools to increase the accuracy and reliability of these measures.¹

For a complete evaluation of OCT data, physicians should not rely solely on the printout and the colored images, tables, or maps that compare patient data with the reference database. Instead, I highly recommend studying the information on screen, evaluating enface images, temporal-superior-nasal-inferior-temporal (TSNIT) profiles, and the unprocessed scan results to ensure accurate data analysis. To make the right decisions for patient care, clinicians need to be aware of and well-versed in potential sources of error. Common OCT artifacts and anatomical variations such as poor scan quality, errors of segmentation, and errors related to posterior vitreous detachment (PVD), for example, may lead to misdiagnosis. These artifacts must be recognized and considered.^2-5

The Future is Now
Gone are the days of looking at the RNFL and deciding “yes or no” in diagnosing glaucoma. Older OCT software gave simple circle scans that determined what areas were thin compared with a reference database. Today, we can perform macular analysis, determine asymmetry, and segment the macular layers to identify the ganglion cell layer. In addition, we are on the cusp of using OCT Angiography not just for glaucoma diagnosis, but also to help us in disease management.

The first thing I do when viewing OCT is assess the scans for artifacts. Assessing only the red and green on the pie chart does not provide complete information. Paying attention to the raw data is necessary to see PVD artifact or segmentation error. I assess RNFL and minimum rim width (MRW) at baseline, as well as the baseline macular analyses, identify asymmetry, and then progression. OCT is a more sensitive tool for determining progression as compared to functional testing, such as visual fields, thereby potentially enabling earlier diagnosis and improved outcomes.

Common Errors
In the older single circle scans, decentration of the optic nerve head is a common error, so we must ensure the optic nerve head is in the center of the ring. Notably, this error has been nearly eliminated with the glaucoma module premium edition (GMPE). Posterior Vitreous Detachments (PVD) can cause both errors of segmentation but also can result in thickened or edematous RNFL, particularly in diabetics. These problems can easily be seen by looking at the raw data. Errors from poor delineation of the RNFL can occur, as well as those stemming from an overall poor-quality scan.

Scan Quality
Scan quality is an internal metric calculated within the system; for example, with the SPECTRALIS OCT (Heidelberg Engineering) good quality is indicated numerically as 20 to 25 and excellent is >25. This platform incorporates simultaneous scanning laser ophthalmoscope imaging and active eye tracking that uses a second laser beam to track the eye during scanning to avoid motion artifact. This feature allows averaging of b-scans after internal compensation for eye movement artifacts, fixation errors, and head tilt. The number of averaged b-scans after active eye tracking compensation is referred to as ART; the higher this number, the better. (Figure 1)

PVD Effect on RNFL
Additionally, SPECTRALIS allows us to look at the vitreous’ effect on the RNFL. Patients with diabetes often have RNFL edema, with the vitreous pulling up and causing traction at the time of PVD. As it releases, the RNFL thins. When this occurs, that particular quadrant of the RNFL is less reliable for determining progression. To confirm progression, I look at the raw data of the PPole scan thickness as determined by the GMPE and compare hemifields for asymmetry. (Case 1, Figure 2)

Segmenting Artifact
In order for the GMPE software to provide a horizontal representation of the retinal curvature, the algorithm must flatten the retinal pigment epithelium. This may be imperfect, particularly around areas where there is a vessel and a shadowing effect. This is why high-quality scans are so important. When looking at progression, the plot will identify the thickness of a RNFL (black line) as compared to age-matched controls at individual locations and the baseline (gray). As I examine these lines, that curvature flattening artifact may show a thicker nerve fiber layer. The platform allows a hands-on experience with the data to improve the ability to assess progression. By simply clicking a button, I can access the RNFL screen and use a circle tool to fix the area to see if there is an area of thinning. (Case 2, Figures 3-6)

Foveal Positioning Error
OCT platforms with original, standard software could sometimes have a foveal positioning error. GMPE allows for identifying the fovea pre-scan, therefore eliminating this issue. We do have patients, however, with scans that were acquired using non-GMPE software. In comparing their older and new scans, we see a difference in the foveal position versus the optic nerve. There also may be a difference in RNFL thinning between scans taken with the two software versions. One reason for this is a change in the reference database. This can potentially affect our diagnosis; therefore, we can simply correct for the degree offset by using a tool to manually reposition the scan and fix the discrepancy. (Case 3, Figure 7)

Tips for Highly Myopic Eyes
As we know from numerous epidemiological studies,⁶ patients with high myopia, an epidemic condition, are at a higher risk for glaucoma. Because the RNFL may be thinner to begin with in patients with longer axial lengths, it presents a challenge in identifying early glaucoma compared with glaucoma suspects.

A recent study helps put this in perspective, giving us some things to keep in mind⁷:

• temporal RNFL thickness increases with increasing axial length
• supra-nasal BMO-MRW sector decreases with increasing axial length
• myopic eyes often have temporally located peak RNFL thickness resulting in a higher rate of false positives based on comparisons to reference databases
• use all three circles to avoid areas of peripapillary atrophy

(Case 4, Figure 8)

OCT Angiography in Managing Glaucoma
As a novel noninvasive imaging technology, OCT Angiography can provide additional information about the vasculature in the retina and optic nerve head, identifying areas of microvascular dropout, complementing visual fields and OCT.⁸ It is safe and can be performed at the same time as OCT, may have the potential to monitor progression in eyes with advanced glaucomatous damage, and can help offer insight into glaucoma patients at risk of faster progression, thereby informing our management decisions.

Conclusion
Ophthalmic imaging is crucial in our clinical diagnosis of glaucoma, but the limitations of the technology regarding common artifacts must be understood. Hence physicians should assess scans for artifacts and not solely rely on printouts. Correcting segmentation errors will allow for more accurate RNFL or macular thickness measurements and ultimately better clinical care.

Osamah Saeedi, MD, MS, is a Professor in the Department of Ophthalmology and Visual Sciences, Director of Clinical Research; Director of the Glaucoma Division; and Vice Chair for Academic Affairs, University of Maryland School of Medicine.

Reference

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