3-D optical coherence tomography gaining ground, but 2-D optical coherence tomography may be mainstay

August 15, 2008

Three-dimensional optical coherence tomography (OCT) has improved on the speed with which scans are obtained and has added more detailed information about the retina in various retinal diseases. Two-dimensional OCT, however, should remain a staple in retinal practice for some time to come.

Key Points

Paris-Time-domain two-dimensional (2-D) optical coherence tomography (OCT) (Stratus OCT, Carl Zeiss Meditec) appears to be a staple in retinal practice, at least for the time being, according to Alain Gaudric, MD, head of the Department of Ophthalmology, Lariboisière Hospital, Paris.

With the advent of OCT outside of research facilities, the popularity of the technology has risen dramatically since the first OCT machine was used in 1996. The 2-D OCT was introduced in 2003, and, according to Dr. Gaudric, "considerably improved the accuracy of the diagnosis of vitreomacular diseases and allowed the description of new pathologic conditions." As a result, 2-D OCT has become the standard examination for all macular diseases.

With the introduction of the faster three-dimensional (3-D) OCT, the future of 2-D OCT can be debated.

In patients with vitreomacular traction syndrome (VMTS), 2-D OCT rapidly proved very effective in showing abnormal vitreomacular adherence and traction that characterize VMTS, he said. With 2-D OCT, details of this condition are refined because the technology shows the intraretinal cystoid spaces more clearly. The next-generation 3-D OCT can display 3-D reconstruction images of vitreomacular attachment. In this case, Dr. Gaudric said that 2-D OCT should continue to be useful for diagnosing and monitoring VMTS.

In cases of epiretinal membranes, 2-D OCT is advantageous in that the instrument can detect and quantify the amount of retinal thickening induced by epiretinal membrane contraction. That information is valuable for determining a correlation between the thickness of the macula and visual acuity (VA), he said. Although 2-D OCT has been valuable in the detection of small epiretinal membranes on a thickened retina, the technology has not provided information that would change surgical decisions. The details and boundaries of the membranes are demonstrated more clearly with 3-D OCT.

"[The technology associated with] 2-D OCT will still be sufficient to appreciate whether epiretinal membrane constriction has a measurable effect on retinal thickening," Dr. Gaudric said. "On the other hand, by showing photoreceptor structure more clearly, 3-D OCT will be able to explain why in some cases there is little or no improvement in visual acuity, despite the successful removal of the epiretinal membrane."

In the assessment of macular edema (ME), 3-D OCT may prove to be more advantageous by overcoming some limitations inherent in 2-D OCT, he said. The precise registration of 3-D OCT scans will correlate the hard exudates or capillary nonperfusion with the VA and also will allow the evolution of the macular thickness to be monitored following lengthy treatment.

Improved resolution

The 3-D OCT also provides better resolution of the outer retinal layer that might identify defects in the photoreceptors that may be responsible for persistently decreased vision, despite the fact that the macular thickness returned to normal.

Further, the neuronal volume can be assessed by developing software that allows the subtraction of empty spaces from the increased macular volume.

Dr. Gaudric said that these advances in 3-D OCT technology will foster research, but 2-D OCT can efficiently monitor effectiveness of medical therapy, laser, and injection of intravitreal drugs to treat ME.

In patients with age-related macular degeneration (AMD), OCT surpassed fluorescein angiography for managing choroidal neovascularization because the technology can show the dramatic effect of antiangiogenic drugs.

"Spectral-domain OCT will provide segmentation of the different retinal layers and make it possible to differentiate between the retinal pigment epithelium and macular surface mapping and also to calculate macular retinal volume and the volume of subretinal fluid," he said.

The caveat is that these capabilities, though attractive, probably will not influence how retina specialists determine when to re-treat patients with AMD. Because of the unknowns in using the relatively new anti-vascular endothelial growth factor drugs, spectral-domain OCT should add to the body of details provided by the latest generation technology.

For all the reasons cited, Dr. Gaudric said that 2-D OCT will be used in clinical practice for the foreseeable future. He said he sees 3-D OCT as "an immediate improvement between 2-D OCT and the future ultra-high-resolution OCT. This prospect is extremely exciting for clinical research. For current practice, however, 2-D OCT will certainly remain in use for several years."

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