Artifacts and Anatomic Variations in Optical Coherence Tomography

Ahmet Akman

doi:10.4274/tjo.galenos.2019.78000

Artifacts and Anatomic Variations in Optical Coherence Tomography

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Review

Artifacts and Anatomic Variations in Optical Coherence Tomography

Turk J Ophthalmol 2020;50(2):99-106 DOI: 10.4274/tjo.galenos.2019.78000

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Abstract

In recent years, ophthalmologists widely depend on optical coherence tomography (OCT), which is an objective, reliable, and repeatable structural test for both early diagnosis of glaucoma and detecting progression of the disease. Using this technology, it is now possible to take measures of various anatomic structures and layers of the optic nerve head, peripapillary retinal nerve fiber layer, and macular area. Although OCT has these powerful capabilities in general, anatomical variations, artifacts related to the ocular pathologies, and issues with image acquisition can be present in up to one-third of scans. These anatomical variations and artifacts can be misleading to an interpreter and may lead to erroneous conclusions. This review focuses on the realization and prevention of most common anatomical variations and artifacts observed with OCT imaging. The concepts of floor effect and red and green diseases are also investigated.

Keywords: Optical coherence tomography, OCT artifacts, OCT anatomic variations, red disease, green disease

Introduction

Optical coherence tomography (OCT) has been widely used in recent years for both the diagnosis and follow-up of glaucoma, as well as in other areas of ophthalmology.^,^,^, When initiating treatment for patients with glaucoma, suspected glaucoma, or ocular hypertension, ophthalmologists generally base their decisions on OCT results. As with any newly introduced diagnostic method, it may take time to understand the limitations and sources of error of OCT. Evaluating results with knowledge of these limits and sources of errors will make OCT results more reliable in the diagnosis of new cases and analysis of progression.

For a sound evaluation of OCT data, physicians should not limit themselves to the colored images, tables, or maps that compare patient data with the normative database. The classifications in these images, tables, and maps are based on the manufacturer’s normative database and are open to various sources of error. For this reason, when evaluating OCT reports the physician should examine en-face images, temporal-superior-nasal-inferior-temporal (TSNIT) profiles, and the patient’s unprocessed scan results to ensure accurate data analysis. Knowing potential sources of error is critical at this stage to be able to make a correct decision.^,^,^, This is the only way we can distinguish anatomical variations and artifacts from true glaucomatous damage. Because retinal nerve fiber layer (RNFL) thickness can vary by race, this must also be considered starting from patient registration.

Overlooking OCT artifacts can cause patients who actually have glaucomatous damage to be evaluated as normal (i.e., false-negative diagnosis) or conversely, lead to false-positive diagnosis of individuals without glaucoma. False-positive diagnoses can lead to years of unnecessary treatment and follow-up. In addition to the adverse effects and expense associated with treatment, being diagnosed with a potentially blinding condition can cause patients serious psychological distress.

The aim of this review is to examine common OCT artifacts and anatomical variations that can lead to misdiagnosis and explain how we can correct these errors in cases where they may occur. The most widely used OCT images are those of the Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA) and Spectralis OCT (Heidelberg Engineering Inc., Heidelberg, Germany) instruments.

Figure 1. (Spectralis OCT) Example of a scan with low quality score for the left eye of a patient with cataract. Note that the quality coefficient (Q) is 26 on the right and 17 on the left. In the RNFL profile image of the left eye, RNFL thickness measurements are artificially high in the inferior and temporal regions due to incorrect detection of the RNFL border by the device algorithm
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 2. (Cirrus HD-OCT) Although signal strength is within normal range (≥ 6), there are significant motion artifacts in the deviation map of the right eye (note the breaks in the blood vessels). Similar motion artifacts are also present in the RNFL deviation map of the left eye. Average RNFL thickness is 85 μm in the right eye and 70 μm in the left eye. While the TSNIT profile and RNFL classification are within normal limits in the right eye, there are abnormalities in some sectors of the left eye
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 3. (Spectralis OCT) Segmentation artifact in the inferotemporal region in a myopic patient. Due to incorrect detection of the RNFL border by the device (in the upper right RNFL profile image), RNFL thickness measurement was artificially low in an area of approximately one sector, and peripapillary RNFL thickness was classified as abnormal in that sector. When the RNFL border is manually shifted to its normal position using the device settings, RNFL thickness returns to normal values and peripapillary RNFL thickness in the inferotemporal sector is also classified as normal
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 4. (Spectralis OCT) Segmentation error resulting from the passage of the scanning ring over an atrophic area in the superonasal region of a myopic patient with peripapillary atrophy. Values on the TSNIT profile are close to zero and peripapillary RNFL classification is borderline in that area
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 5. (Cirrus HD-OCT) The superior RNFL bundles of both eyes of this patient show split RNFL defect. On the TSNIT profile, the superior vertex in both eyes is split into two peaks separated by a valley. In addition, a displaced RNFL configuration which is more prominent in the left eye is observed in the inferior quadrants of both eyes. Average RNFL thickness is within normal limits on the right and borderline on the left
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 6. (Spectralis OCT) In the TSNIT profile of a myopic patient, RNFL thickness values in the nasal sectors appear low due to temporal displacement of the RNFL peaks. This occurs because the peaks do not align with expected positions in TSNIT graphs based on the normative database
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 7. (Spectralis OCT) A Weiss ring coinciding with the laser scanning region causes a segmentation artifact in the inferonasal sector of the right optic disc due to shadowing. On the TSNIT profile, it is seen that RNFL thickness has a value of 0 in this region (a). Upon movement of the eye, the Weiss ring also moved and the device’s algorithm performed segmentation correctly in the inferonasal region. The Weiss ring in the nasal region produces minimal shadowing that does not impair segmentation (b)
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 8. (Spectralis OCT) Areas of pronounced vitreoretinal traction are seen upon examination of the vitreoretinal interface in the RNFL profile of a patient being followed due to ocular hypertension. In the TSNIT profile, RNFL thickness is higher than expected normal values in certain areas in the superonasal region. Peripapillary RNFL thickness is classified as above normal in most sectors
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 9. (Spectralis OCT) Segmentation is completely disrupted in a patient with substantial ILM thickening. Peripapillary RNFL classification appears to be within normal limits in all sectors, and analysis of the sectors shows that average RNFL thickness values are much higher than expected average values. The same findings are seen in the TSNIT profile (a). Disc photography shows prominent glaucomatous pitting and peripapillary atrophy (b). There is significant glaucomatous visual field loss in the same eye (c). Note that OCT RNFL classification appears to be within normal limits in all sectors
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 10. (Cirrus HD-OCT) Quality score is within normal limits in a scan from a patient with optic nerve head drusen. However, note that although the diameter of the disc is normal, the cup volume value is 0. Despite significant RNFL thinning, neuroretinal rim thickness is higher than normal values in optic nerve head measurements
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 11. (Spectralis OCT) All sectors are green in the RNFL classification graph of a glaucoma patient with diabetic retinopathy and macular edema. In the TSNIT profile, the thickness curve is seen to be above normal limits in the inferotemporal, temporal, and superotemporal regions (a). Macular OCT analysis shows diabetic macular edema (b). Central 24-2 visual field testing demonstrates glaucomatous superior arcuate defect (c). Note that the sector classifications are completely within normal limits in the OCT RNFL examination of this patient
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

Figure 12. (Spectralis OCT) Peripapillary retinoschisis in the superior quadrant is apparent in a myopic patient (see RNFL profile image in the upper right). Note that the RNFL thickness curve is well above expected values in the location corresponding to the region of retinoschisis in the TSNIT profile
RNFL: Retinal nerve fiber layer, OCT: Optical coherence tomography

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