Signal strength important factor when analyzing OCT scans

Take-home message: Optical coherence tomography scans can be useful to follow glaucoma, but clinicians should be aware of scan quality.

Reviewed by Donald L. Budenz, MD, MPH

Chapel Hill, NC-Clinicians can use optical coherence tomography (OCT) scans along with functional testing and clinical judgement to follow patients with glaucoma and those suspected of having glaucoma, said Donald L. Budenz, MD, MPH.

A good starting point with OCT scans is to consider scan quality, said Dr. Budenz, Kittner Family Distinguished Professor and chairman, University of North Carolina School of Medicine, Chapel Hill, NC.

The characteristics of good scan quality are signal strength, evidence of segmentation failure, and blocked signal. Blocked signal can be caused by media opacities, pupil edge, drying of the cornea, a smudge on the lens, posterior vitreous detachment, and blinking.

Of the scan-quality related features, “signal strength is the easiest to assess,” Dr. Budenz said.

Focusing on three commonly used OCT platforms in the United States, a signal strength equal to or above 6 is desirable for the Cirrus HD-OCT (Carl Zeiss Meditec)-however, an 8 or above is even better, he said.

For the Spectralis OCT (Heidelberg Engineering), the quality score should be greater than or equal to 20.

For the Optovue, the signal strength index should be greater than or equal to 30 (Table 1).

Another pearl that Dr. Budenz shared is that if the signal strength is lower, you can get artefactual thinning. To make his point, Dr. Budenz described a patient with mild corneal haze and steroid-induced elevated IOP after corneal transplant that he was asked to evaluate. The OCT showed retinal nerve fiber layer (RNFL) thickness that was artefactually thin due to the opacified cornea.

As a result of the corneal haze, the OCT image only had a signal strength of 3.

Blocked signal

Blocked signal

Clinicians should also stay aware of the possibility of a blocked signal, which can come from a variety of sources, Dr. Budenz said.

“You might catch the edge of the pupil if you’re doing OCTs undilated, or even a smudged lens can cause blocked signals,” he said.

In patients with moderate to high myopia, optic disc parameters show fewer false positives than RNFL thickness, ganglion cell layer measurements, or total retinal thickness measurements.

For this reason, he said, the optic disc parameters may be better to use when analyzing OCTs in these patients.

Donald L. Budenz, MD, MPH

P: 984/974-2020

This article was adapted from Dr. Budenz’s presentation during Glaucoma Subspecialty Day at the 2015 meeting of the American Academy of Ophthalmology. Dr. Budenz is a consultant with Alcon Laboratories and Ivantis.