Most commercially available OCT and OCTA devices can provide widefield OCTA through a combination of faster speeds, tracking, and the ability to montage images. This may revolutionize clinicians’ ability to assess and quantify retinal vascular disease.
By Lynda Charters
Most of the available optical coherence tomography (OCT) and OCT angiography (OCTA) devices provide clinicians with a 30-degree x 30-degree field of view. This is adequate when performing OCT and visualizing the macula. However, when OCT is performed to evaluate the changes in retinal vascular diseases, this size field of view is inadequate, according to Nadia Khalida Waheed, MD, MPH.
Increasing the field of view to evaluate those retinal vascular diseases is the next logical step. However, a number of problems become evident, Dr. Waheed pointed out. She is associate professor of ophthalmology, New England Eye Center, Tufts University School of Medicine, Boston.
For example, Dr. Waheen demonstrated that when the scanning area is increased from 3 mm x 3 mm to 6 x 6 mm and then to 8 mm x 8 mm, the loss of the fine vascular resolution is marked with each step.
There are a couple of ways to surmount this problem. One approach, she noted, is to begin with a machine that has a higher speed as in spectral-domain OCT (SD-OCT) or swept-source OCT (SS-OCT) technology. Images that are 15 mm x 9 mm and 12 mm x 12 mm obtained from SS-OCT provide good resolution of the small blood vessels, Dr. Waheed pointed out.
Creating a montage of images is another approach to solving the problem of decreased resolution. “A montage of images has much better fine vascular resolution than fluorescein angiography (FA) images or large single-acquisition images,” Dr. Waheed said.
She and her colleagues have combined both methods for addressing losses in resolution.
“We used a higher speed SS-OCT machine and obtained five 12-mm x 12-mm OCTA scans and montaged them together into a single larger scan,” Dr. Waheed said. “This enabled us to see vascular dropout or early neovascularization much faster than they could be seen in a standard 3-mm x 3-mm or 6-mm x 6-mm scan,” Dr. Waheed explained.
This high level of resolution allows quantification of areas of non-perfusion or ischemia, she added. The areas of resolution then can be seen to match up well with the level of retinopathy.
Another available technology that fills the need for more extensive views in the eye is ultrawide FA. A comparison of SS-OCT images that were montaged into a larger image and images obtained using ultrawide FA showed that the former had better resolution than the latter. However, the SS-OCT images do not capture as extensive a view as the ultrawide FA when the images are montaged, Dr. Waheen reported.
“Imaging could theoretically go much further out to the equator or beyond,” Dr. Waheed explained. “However, as the field becomes increasingly wider, the problem that arises is that a spherical surface is attempting to be represented as a flat surface. We know from cartography that this leads to a great deal of distortion, especially in the peripheral areas where structures seem to occupy a larger area than they truly occupy.”
Dr. Waheed and colleagues, in collaboration with Ruikan (Ricky) Wang, PhD, director, Biophotonics and Imaging Laboratory, Department of Bioengineering, University of Washington, Seattle, have attempted to correct for this peripheral distortion that appears when montaging images to obtain a wider field of view.
“The distortion can be corrected by displaying the image as a virtual three-dimensional projection or as a flat corrected image,” Dr. Waheed said, noting that after the correction software is applied, the areas of non-perfusion in the periphery appear much smaller than in the uncorrected image.
The question then arises about how much correction actually occurs?
In the macula, there is not much difference, but going past the macular arcades, there is “quite a significant difference,” Dr. Waheed emphasized. She and her colleagues found differences ranging from 12% to 20% in the areas outside the macula in the montaged images.
After the distortion is corrected in the ultrawide FA images, various factors, such as vessel density and vessel complexity, can be quantified. “This is increasingly important in patients with retinal vascular diseases, and when conducting clinical trials or providing clinical care for these patients with retinal vascular diseases,” Dr. Waheed added.
“Most commercially available OCT and OCTA devices can provide widefield OCTA through a combination of faster speeds, tracking, and the ability to montage images,” Dr. Waheed concluded. “In the future, this may revolutionize our ability to assess and quantify retinal vascular disease either in the clinical setting or in a clinical trial to evaluate the efficacy of certain drugs.”
Nadia Khalida Waheed, MD, MPH
This article was adapted from a presentation that Dr. Waheed delivered at the Retina Subspecialty Day held prior to the 2017 American Academy of Ophthalmology meeting. Dr. Waheed is a consultant for Optovue, Karl Zeiss, and Carl Zeiss Meditec and has done contracted research for Topcon, Nidek, and Heidelberg Engineering.