Understanding variations between devices can matter during ophthalmic evaluations.
This article was reviewed by Joel S. Schuman, MD
It goes without saying that the features that are paramount for any optical coherence tomography (OCT) instruments are image quality and the abilities to identify artifacts and interpret scans regardless of the desired structures to be viewed or the disease process involved.
The various commercially available devices work in different ways and understanding those differences may matter during evaluations.
Truth in imaging
In order to ensure image quality in spectral-domain OCT, the signal quality must be highest in the top third of the scanning window and the tissue must be as flat as possible.
The condition of the ocular media, including a dry cornea, refractive errors, cataracts, and vitreous opacities, adversely affect the image quality, according to Joel S. Schuman, MD,
Acceptable signal strengths and quality scores vary among four of the most widely used OCT instruments, e.g., for the Cirrus (Carl Zeiss Meditec) and Avanti (Optovue), signal strengths of 6 or greater and 40 or greater, respectively, are acceptable and for Spectralis (Heidelberg Engineering) and Triton (Topcon), quality scores of 20 or greater and 30 or greater are acceptable, according to Dr. Schuman.
In instruments in which acceptable images are not obtained, the results are segmentation errors, i.e., incorrect information about tissue characteristics.
Ferreting out the faults
A number of these faults can play tricks on the eyes.
Blood vessel shadows are the products of every vessel containing blood. Mirror images or inverted images can result from the manner in which the image was acquired.
Blinking artifacts, which appear as black bands across an image, indicate that the signal was interrupted during a blink or a segmentation error occurred, and eye movement artifacts can distort the shapes of the vessels in an image, said Dr. Schuman, professor of neuroscience and physiology, Neural Science and Electrical and Computer Engineering, and chairman of ophthalmology, NYU Langone Medical Center, NYU School of Medicine, New York.
This health of this structure is assessed in a circular area around the optic nerve head. This area of measurement can be used on any of the previously mentioned instruments to obtain a robust assessment of the RNFL.