Retinal Imaging for the Comprehensive Ophthalmologist

Experts discuss the applications of FA and OCT.

BY RACHEL M. RENSHAW, EXECUTIVE EDITOR

With the recent approvals of next-generation spectral-domain optical coherence tomography (OCT) systems, ophthalmologists may be asking whether OCT technology is replacing fluorescein angiography (FA) as the new gold standard for retinal imaging. However, while there are several companies that have spectral-domain technology, not all of the systems have been made available and many comprehensive ophthalmologists have yet to acquire a time-domain OCT system. This article will discuss the position of FA and OCT technology, as well as future considerations for the comprehensive ophthalmologist.

The Standard of Care

According to the specialists interviewed for this article, FA remains the standard of care for most ophthalmology practices.

"Fluorescein angiography is what physicians have looked at for many years — they are trained to identify pathologies through this technology," says Dirk-Uwe G. Bartsch, Ph.D., director of the Retinal Image Laboratories at the Jacobs Retina Center and associate adjunct professor at the Shiley Eye Center of the University of California San Diego. "It is a dynamic imaging system and can be used to detect numerous ocular pathologies."

Howard F. Fine, M.D., who practices ophthalmology with Vitreous-Retina-Macula Consultants of New York and the LuEsther T. Mertz Retina Research Center of the Manhattan Eye, Ear and Throat Hospital, agrees. "FA is the gold standard for the initial diagnosis of several common retinal conditions such as exudative age-related macular degeneration (AMD) and proliferative diabetic retinopathy. Additionally, FA provides information regarding circulation, which OCT does not."

Having OCT to complement the findings of FA has revolutionized retinal care, but the standard time-domain OCT technology falls short in some areas.

"The downside of OCT in the current configuration (time-domain) is that you still don't know exactly where [on the retina] the scan was taken," Dr. Bartsch says. "There are also a lot of artifacts in standard OCT images. There have been many studies showing that that artifacts result from the software aligning the images automatically, which takes away the user's ability to identify pathology."

Dr. Bartsch says that while OCT provides structural information, OCT alone does not provide dynamic information. "If you see fluid underneath the retina with OCT, you don't know if it's being actively perfused or if it's fluid that has been there for some time. With FA, the clinician can differentiate this," he says. "If you want to look at perfusion and blood flow, you need FA."

OCT vs. FA

The question of whether FA is sufficient for a comprehensive ophthalmologist is complicated, according to Dr. Fine. "It really depends on many local factors, primarily dealing with how accessible a retinal specialist is to the comprehensive practice," he says. "In a way, it might even be easier for most comprehensive ophthalmologists to have/read/interpret OCT rather than FA. But this is very practice dependent."

Dr. Bartsch proposes that it may make more sense for general ophthalmologists to have FA if they can only have one or the other, because a variety of diseases can be imaged with the technology. "General ophthalmologists also may not be comfortable performing intravitreal injections, so they are probably sending their patients with AMD to a retinal specialist," he says.

The advantages to FA include the dynamic nature of the photography, which, as noted earlier by Dr. Bartsch, is important when identifying active perfusion. The distinct disadvantages include the need for needles and the potential for fluorescein allergies. FA cannot differentiate between staining (such as in a patient with AMD who has scarring) and active leakage.

OCT has the advantage of being a non-invasive technique, quick and reproducible and easy to interpret, says Dr. Fine. But each has its limitations and the ideal situation is to use the technologies together.

When the generalist is deciding whether he needs an OCT in the office, Dr. Fine advises that the following be considered:

► How accessible is a retinal specialist? (Keep in mind the urgency of many conditions detected by OCT, e.g., exudative AMD, cystoid macular edema from retinovascular disease, Irvine Gass syndrome, macular holes, macular pucker.)

► Who will perform the OCT studies (e.g., technician, nurse)?

► Will the results of the OCT change your management? (Will these patients be referred to a retinal specialist regardless?)

► How much will the OCT be used? Will it pay for itself?

Spectral-domain OCT

The new spectral-domain, or Fourier-domain, OCT systems are based on a platform that is designed to capture images much more quickly. All of the systems either available or in development have B-scan imaging, produce 3-D images and offer some other form on non-OCT imaging (see Table for comparisons).

Dr. Bartsch says that one of the most important factors to consider when purchasing this type of equipment is image quality. "A simultaneous, high-speed image capture feature of spectral-domain OCTs can be an advantage and a disadvantage," he says. "If the signal processing is lacking, the images can be more grainy than with time-domain systems."

The 3-D maps generated by spectral-domain OCT provide a view of the retinal layers, as well as point-to-point correlation between the OCT and non-OCT images, says Dr. Fine. These features assist the retinal specialist in treatment and management decisions as they provide additional information not previously available in OCT and FA alone. If time-domain OCTs are used along with FA, the information is more difficult to use effectively, because the images are never able to be taken simultaneously, as with some spectral-domain systems.

Can all of this additional information be practically applied?

"For every new generation of ophthalmic instrument, there's a vast amount of information that comes to the observer and, in the beginning, we don't quite know what to do with it," Dr. Bartsch says. "Based on my experience, I would say that it may take about 1 or 2 years for a system to be developed that can condense the information [received with spectral-domain technology] into something useful for the clinician. The first OCTs came out years ago and you had the single scans. Pretty pictures, but what were we to do with them? OCT really becomes useful when you have a normative database to help understand what's going on."

Considering New Technology

Whether considering purchasing your first OCT machine, upgrading to spectral-domain or continuing to rely on the gold-standard FA, Dr. Bartsch provides practical advice. "A general ophthalmologist has maybe 1 to 2 minutes of time to look at the images on an FA, OCT or spectral-domain system because patient flow is much heavier than for a retinal specialist," he says. "So if it takes you longer than that, ultimately this information is not helping you and you may as well not do it. But if you can get the information in 1 to 2 minutes, then you have something useful." OM

Dirk-Uve Bartsch, Ph.D., has received support software and discounted hardware from Carl Zeiss Meditec, OTI and Heidelberg Engineering. Howard F. Fine, M.D., has no financial interest in the information contained in this article.