Decentration

Decentration artifact (Fig. 8‑1) is the most common type of artifact seen in peripapillary RNFL circular B-scans, occurring in 27.8% of Spectralis (Heidelberg Engineering, Heidelberg, Germany) B-scans, where a technician needs to manually center the RNFL circular scan over the optic nerve and when decentration is defined as imperfect alignment of the circular scan over the ONH by at least 10%. ² Decentration artifacts are less of an issue in machines where the software automatically determines the optic nerve location (e.g., Cirrus, Carl Zeiss Meditec, Dublin, CA). Since the RNFL normally thins farther away from the ONH, decentration may cause artifactually increased or decreased measurements of RNFL thickness for a given sector or clock hour, depending on whether the measured region is closer or farther away from the optic disc, respectively. For instance, if the scan circle is shifted superiorly, then the superior quadrant will appear artifactually thin (farther from the nerve head) and the inferior quadrant will appear artifactually thick (closer to the nerve head). It is evident that superior decentration may make it appear to the clinician as though the patient has glaucomatous thinning in the superior quadrant, when in fact there was no change at all.

Poor Signal

Poor signal (Fig. 8‑2 and Fig. 8‑6) is another common type of artifact seen in OCT scans, presenting in 5.1% of peripapillary RNFL circle scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² Poor signal often looks grainy or static-like, which makes it difficult to discern the layers of the retina. When the OCT attempts to segment these blurry or grainy images, it is difficult and sometimes impossible for the software to locate the boundaries of the retinal layers, thereby leading to false measurements.

Signal can be degraded by any of the ocular structures that the OCT light beam must pass in order to image the retina. This includes the cornea, anterior chamber, lens, and vitreous. The most common causes of poor signal include dry eyes ⁷ and cataract. ⁸ These conditions make it difficult for the OCT machine to take a clear picture of the back of the eye, just as it would be difficult for a camera to take a good photo of an object through frosted glass. Stein and associates sought to establish the effects of dry eye by taping normal subjects’ eyelids open to prevent blinking over the course of the testing period. They found significant reductions in both signal strength and RNFL thickness strictly by preventing blinking (Stratus OCT, version 3.0; Carl Zeiss Meditec, Inc., Dublin, CA). ⁷ Also using the Stratus OCT, Mwanza and associates found a 9.3 and 24.1% increase in peripapillary RNFL thickness and signal strength, respectively, in 45 glaucomatous and normal eyes after cataract removal. ⁸ These studies demonstrate that poor signal due to dry eyes and cataract may significantly impair accurate OCT measurement of RNFL thickness, and that strategies such as blinking and administration of artificial tears can improve signal quality if a scan needs to be retaken. Other less common causes of poor signal include anterior chamber cell or flare, posterior capsular opacification, and vitreous hemorrhage. In fact, it is even important to make sure that the OCT lens is cleaned, so as to avoid “smudge artifact” from dirty equipment degrading signal quality.

OCT machines automatically generate a quality score to reflect the signal strength of each scan. The range of scores differs among instruments, and each manufacturer defines a separate minimum acceptable value (Table 8‑1). For the Spectralis SD-OCT, quality scores range from 0 to 40, and a score less than 15 is considered poor. On the other hand, the Cirrus SD-OCT quality scores range from 0 to 10, and a score less than 6 is considered poor.

Motion Artifact

Motion artifact (Fig. 8‑3) presents in 0.2% of RNFL circular peripapillary B-scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² These artifacts result from subtle eye motions that occur during eye fixation, such as tremor, drifts, and microsaccades, or from larger movements, such as those due to head motion, heartbeat, respiration, or blinking. Eye-tracking technology can minimize errors from the former. Larger movements, however, can produce blurred, wavy images that move outside the boundary of the rectangular display box and preclude accurate segmentation.

Today, different OCT machines employ different methods of motion correction. Some machines have a second laser beam to detect eye movement. Other machines include the use of additional B-scans obtained during the examination as a framework for realigning individual B-scans. ⁹

So what can be done when there are wavy retinal layers or if the scan is cut off abnormally as a result of motion artifact? In the case of a tremor, an assistant can help to hold the patient’s head against the bar during image acquisition to minimize movement. Have the technician describe the importance of minimizing movement to the patient, so that he or she knows it will affect his or her diagnosis and management. Finally, it may be necessary to be patient and take multiple images, which can be difficult in a busy clinic. Having a well-trained technician can be one of the most important ways to minimize motion artifact.

Missing Data

In certain instances, the RNFL OCT fails to acquire data in an area of the scan (Fig. 8‑4). If this area of missing data is along the scan circle, the RNFL thickness measurements will be affected.

Cut Edge

Cut edge (Fig. 8‑5) artifacts occur in 0.2% of RNFL circular peripapillary B-scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² A cut edge artifact refers to the abrupt truncation of the lateral edge of the RNFL. This may take place at one or both ends of the OCT scan and most likely results from patient movement.

Anterior Misidentification

Anterior RNFL misidentification (Fig. 8‑6) presents in 3.2% of peripapillary RNFL scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² These occur when the segmentation algorithm fails to correctly identify the anterior border of the RNFL. On the printout, the top red line drifts anteriorly (upward) into the vitreous, resulting in falsely increased or decreased thickness measurements, respectively.

A common cause of this type of artifact is the presence of an epiretinal membrane (ERM) or posterior vitreous detachment (PVD). In one study, ERMs were found in 47.3% (26 out of 55) of peripapillary scans with artifacts and tended to increase measurements of RNFL thickness. ¹⁰ If there is suspicion for an ERM, a basic clinical examination with or without a macular OCT would confirm the presence of this pathology. Similarly, certain features of the vitreous can confuse anterior segmentation. Often in an area of vitreomacular traction (VMT), the hyaloid face appears as a hyperreflective line very near to the anterior boundary of the RNFL. In this case, the OCT may inaccurately segment the hyaloid face as the anterior RNFL. PVDs can induce other forms of artifact unrelated to segmentation errors, which is discussed further in the section Artifacts Due to Ocular Pathology Unrelated to Glaucoma.

Posterior Misidentification

Posterior RNFL misidentification (Fig. 8‑7) presents in 7.7% of peripapillary RNFL scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² These occur when the segmentation algorithm fails to correctly identify the posterior border of the RNFL, and this can lead to artifactually thick or thin RNFL measurements.

Glaucoma itself might contribute to this type of artifact, because glaucoma is associated with loss of RNFL reflectivity. Since OCT technology relies on differences in tissue reflectivity in order to construct the layers of the retina, loss of RNFL reflectivity means that the nerve fiber layer becomes less distinguishable from its surrounding layers. This change especially affects the interface between the RNFL and the less reflective deeper retinal layers. Theoretically, this would make glaucoma patients more prone to posterior misidentification artifact.

Incomplete Segmentation

Incomplete segmentation artifact (Fig. 8‑8) presents in 0.6% of peripapillary RNFL scans taken with the Spectralis SD-OCT machine (Heidelberg Engineering, Heidelberg, Germany). ² The anterior and posterior segmentation lines should be present from the left-most side of the image to the right-most side of the image. However, if the software cannot trace part of the scan, this is referred to as incomplete segmentation. This should be an easy artifact for the clinician to spot, as long as he or she looks at the scan.

Floor Effect

Physiologically, the RNFL thickness can never be 0 μm. In fact, even in the most advanced glaucoma patient, RNFL thickness values cannot be less than around 50 μm, because the RNFL is partly comprised of nonneuronal tissue such as blood vessels and glial tissue. Despite this, the OCT machine will often report thickness values as low as 0 μm, which is below the floor of around 50 μm (Fig. 8‑7 and Fig. 8‑10). This is an easy error to spot because the clinician can just look at the line graph and note that any points that drop to 0 μm are clearly artifacts. That being said, the RNFL may be very thin in that area, so physicians should refer back to the scan to understand why it was segmented incorrectly.

Vitreomacular Traction/Posterior Vitreous Detachment

With normal aging, the vitreous may detach from the ILM. Initially, there may be a stage of VMT or partial PVD, which can progress to a complete PVD. The precise effects of PVD on RNFL artifact in the literature are varied. However, the rates of PVD-associated artifact is estimated to be approximately 14%. As the posterior hyaloid pulls off the ILM, some patients show focal artifactual thickening or “tenting” of the RNFL in the region of the VMT due to the physical pull from the vitreous (Fig. 8‑9a, b). ¹⁰ Subsequently, as the traction is released and as the PVD progresses from a partial to complete PVD, there can be relative RNFL thinning as the anterior RNFL surface may drop back more posteriorly. This is an important confounding factor for clinicians to be aware of in order to avoid incorrectly interpreting the decrease in RNFL thickness following the progression from partial PVD to complete PVD as a sign of glaucomatous changes to the RNFL. ¹¹ It is important to know that the vitreous can create artifacts that can cause both falsely thickened and thinned RNFL. It is also important to know that this can change over time as the clinician follows a patient with sequential scans depending on the status of the vitreous traction. Another source of error during segmentation is if the detached or partially detached hyaloid face is misidentified as the anterior boundary of the RNFL (Fig. 8‑9b). ^{10 , 12} This is discussed further in the section Artifacts in Boundary Segmentation.

As an example, when monitoring a patient over time, if a focal decrease in RNFL thickness occurs at the same time as a newly diagnosed PVD, it would be prudent to carefully evaluate the scans and consider other diagnostic metrics prior to changing the patient’s therapy, as this could be due to artifact.