Macular edema may affect signal strength, reliability of OCT imaging

Issue: December 10, 2010

Optical coherence tomography may be used to identify and follow macular edema, but the pathology itself may affect the quality of the image.

According to a study by Hossein Ashraf, MD, and colleagues at Shiraz University of Medical Sciences, Iran, as macular edema increased, as measured by change in central point thickness, signal strength decreased.

Macular edema in 57 patients was followed with serial OCT imaging. In the study, Stratus OCT (Carl Zeiss Meditec) was used to obtain all images; the device automatically grades signal strength on a 1 to 10 scale.

In a separate portion of the study, the investigators sought to determine whether macular edema has a similar effect on measurements of retinal nerve fiber layer thickness.

“In patients with [macular edema],” the study authors wrote, “[signal strength] obtained during macular OCT examination was significantly lower than that obtained during retinal nerve fiber layer OCT examination.”

Effect of macular edema

According to Dr. Ashraf and colleagues, OCT signal strength is dependent on absorption and scatter to determine an image in target tissue, and macular edema affects absorption and scattering, which changes the optical characteristics of the back of the eye.

Additionally, signal strength is also affected by what the authors dubbed the “intraretinal fraction of the optical pathway,” or the degree of signal blockage represented by retinal tissue.

In the context of a macular scan using OCT in a normal, healthy eye, light enters from the anterior and is reflected back primarily by the retinal pigment epithelium. Because a healthy retina is transparent and lacks pigment other than hemoglobin and luteal pigment, it acts as only part of the optical pathway, the intraretinal fraction, of OCT light sources entering the eye and reflecting back.

However, the study authors noted, signal strength is affected by interaction with retinal tissue in both healthy and pathologic states. This interaction directly affects signal strength, as well as alters the absorption and scatter properties of the retinal tissue, which further affects signal strength.

As a result, edematous tissue decreases the extent of light source penetration, as well as alters the pathway of any light that enters the eye or seeks to return to the OCT device. Deviations in the optical characteristics of retinal tissue may affect OCT image contrast and, therefore, signal strength.

“In our routine practice, sometimes we see images which seem clear and give at least useful structural information but with a low signal strength, and we don’t know to what extent these images are reliable with regard to quantitative information,” Dr. Ashraf said.

Clinical relevance

While most experts agree that OCT cannot reliably be used as a sole determinant of clinical decision making, the results of the current study should reinforce that point, Dr. Ashraf said. However, what may warrant additional consideration is just how reliable images taken from an eye with significant edema really are.

“A corrected signal strength based on degree of macular edema may compensate for this error. However, different patterns of edema may affect signal strength differently because they can have different optical characteristics,” Dr. Ashraf said.

Decreased signal strength may become important, for instance, in attempting to determine retinal nerve fiber layer thickness or other thickness measurements. – by Bryan Bechtel

Reference:

Hosseini H, Razeghinejad MR, et al. Effect of macular edema on optical coherence tomography signal strength. Retina. 2010;30(7):1084-1089.

Hossein Ashraf, MD, can be reached at Poostchi Ophthalmology Research Center, Zand Avenue, Poostch Street, Shiraz, Iran; e-mail: hosseinashraf@yahoo.com.