3D-OCT Improves Image Capture, Viewing, and Analysis
Fourier-domain system virtually eliminates eye movement artifact.
ROCHELLE NATALONI, CONTRIBUTING WRITER
The 3D OCT-1000 (Topcon Inc., Paramus, NJ) is up to 100 times faster and 25% more accurate than conventional optical coherence tomography (OCT) systems. Unlike conventional–time-domain OCT systems, the 3D OCT-1000 is a spectral or Fourier-domain system. This means it records the interferometric information with an internal spectrometer instead of moving mirrors.
This distinction, says Yale L. Fisher, MD, of Vitreous Retina Macula Consultants of New York, is the basis for its dramatic increase in scanning speed and imaging sensitivity. Dr. Fisher, who was integral in the development of the Topcon system, is director of the surgical retinal service at Manhattan Eye, Ear & Throat Hospital and clinical professor of ophthalmology at the New York Hospital - Cornell Medical Center.
Richard F. Spaide, MD, in practice with Dr. Fisher, says the spectral scanner is "the heart" of the 3D system. "Acquiring a 3D image takes less than 2 seconds more than a typical B-scan done with a time-domain instrument, but for the patient, the time in the chair seems less with the 3D OCT," he says.
The problem with time-domain systems, according to Dr. Fisher is that — just as their name implies — they take too much time. "There's a small moving mirror in the known-distance arm that has to move back and forth and looks at each A-scan as it goes across. During the time that the mirror is moving, it slows down the process of information gathering so there is time for the patient to move his or her eyes, which of course results in artifacts," he explains. The speed with which the 3D OCT-1000 captures data results in reduced patient eye movement, increased comfort, and the virtual elimination of artifacts resulting from eye movement, he says.
|Drs. Walsh and Sadda share royalties from software licensed to Topcon by the Doheny Eye Institute. Drs. Fisher and Spaide have no financial interest in the information contained in this article.|
FEATURES AND BENEFITS
Topcon has combined spectral-domain OCT technology with a nonmydriatic retinal camera for an extremely versatile retinal evaluation tool. The system provides infrared alignment viewing with split mechanism, internal matrix LCD for stable fixation, and adjustments for integral signal optimization. Retinal specialists say these features make the system user-friendly. There are also improvements in capturing, viewing, and analyzing images with the 3D OCT-1000.
Figure 1. Vascularized pigment epithelial detachment before (top) and after (bottom) intravitreal ranibizumab injection. The 3D image shows marked resolution of retinal elevation.
"You can collect a vast quantity of information, and once you have all of this information, instead of viewing it as A-scans, you can compress them and basically create a 3D projection image that looks like a relief map," says Dr. Fisher.
The system's software enables the user to click on any location on the map to examine an area of interest (Figure 1). The system even makes it possible to remove specific layers of the retina for a better view. "For instance, if you want to look at the [retinal pigment epithelium] RPE layer but not the neurosensory layer, you can literally strip off from the 3D image all of the neurosensory layers and view only the strong reflections from the RPE," says Dr. Fisher. This, he adds, offers the opportunity to look at drusen as a field of boulders and count them. "Users can simply strip off a layer and then count the elevations. Unlike with a photograph, this system allows you to see the drusen in cross-section. You might even be able to get a larger field of view in a 3D image looking at all of the elevations of the RPE with the drusen underneath it."
Alex Walsh, MD, assistant professor of ophthalmology, Doheny Eye Institute, Keck School of Medicine, at the University of Southern California, Los Angeles, explains that the 3D OCT system has higher axial resolution, which enables it to see smaller structures and provides more complete coverage of the macula than conventional OCT, along the lines of about 100% vs 5%. "We did a study that demonstrates 15% better sensitivity for 3D OCT over conventional time-domain OCT for clinically relevant findings such as epiretinal membranes and drusen," says Dr. Walsh (Figure 2). "It is also beneficial to be able to compare results from 1 visit to the next, being as the 3D-OCT can align data better than time-domain OCT."
In the Doheny study, 50 eyes of 28 consecutive patients were imaged with a time-domain OCT device using a high-resolution radial line protocol (6 × 512 A-scans) and the Topcon Fourier-domain OCT instrument using both a 3D-OCT protocol (128 × 512 A-scans) and a high-resolution B-scan protocol (1 × 1024 A-scans). The study concluded that subjective and objective measures of image quality suggest that the image quality of Fourier-domain OCT is superior to time-domain OCT.
FIGURES 1 AND 2 PROVIDED BY VITREOUS-RETINA-MACULA CONSULTANTS OF NEW YORK.
REGISTRATION CAPABILITY AND IMAGE PROCESSING
Data collection is an important component of the 3D OCT, but perhaps even more so is the system's registration capability. "It's nice to have an image, but it's even more valuable to know exactly where the picture came from to be able to refer back to it and track changes," says Dr. Fisher. "Registration is markedly improved with this system."
There are clear advantages to image processing with the 3D OCT-1000 as well, says Dr. Fisher, "For instance, if the macula was imaged, but you notice something questionable outside the central fovea during the exam, once the material has been collected and image processed, all you have to do is point the cursor at the questionable area and you will immediately get a B-scan of that area. So extrafoveal lesions, which were perhaps not the center of focus when the exam started, can be looked at."
"This system has made a huge difference in my practice," says Dr. Fisher. "Once you have great registration and know exactly where something is coming from, you can go back to that same area and take another picture of it and know that if you align the blood vessel or the projected image with the previous exam, you're going to be in exactly the same place or as close as possible as you were the last time."
The system scans a large area with densely sampled data to ensure that the area of concern is captured. The software allows a direct comparison between the raster scans on the retinal image to the corresponding OCT tomographic layers. The accurate retinal registration permits subsequent scans of the same area to be repeated for better patient management.
Dr. Spaide also notes the system's resolution and registration benefits. "The physician can get high-quality, high-resolution scans that can be assembled into a 3D image. This gives the physician the ability to evaluate each B-scan or to look at the greater whole by looking at the 3D image. Since the entire macular region is scanned and the data is saved as one block, the physician can go back and look at any region within a 6 × 6 mm block. This makes it easier to evaluate areas of the retina in retrospect," he says.
Dr. Walsh says the system has what is needed to monitor age-related macular degeneration (AMD) and vitreomacular traction (VMT). "3D-OCT covers more of the macula, so we can be sure in AMD that we're not missing something that might require treatment. We have seen several patients where traditional OCT did not detect subretinal fluid, while 3D-OCT detected fluid that was subsequently treated with an injection," he explains.
In VMT, Dr. Walsh says, the way in which the vitreous attaches to the underlying retina can assist in preoperative planning. "This can be very helpful in patients with diabetes or even routine posterior vitreous detachments where monitoring of VMT may assist in determining the best therapeutic."
"The Topcon 3D OCT is excellent when it comes to imaging vitreoretinal problems," says Dr. Walsh. "The machine can image vitreous really well — in addition it makes a stack of 128 B-scan images. These images can be assembled into a 3D image or can be individually evaluated. It is almost impossible to miss pathology because of the density of sampled points. Evaluation of macular diseases in general is made much easier because of the same strategy. It is very difficult for a technician to miss a macular problem when doing a scan with this system."
Srinivas R. Sadda, MD, associate professor of ophthalmology, Doheny Eye Institute, describes 2 situations in which the 3D OCT-1000 is useful for finding pathology missed by other diagnostic equipment. "The Topcon system is excellent for identification of subretinal fluid and retinal cysts in patients who are receiving anti-vascular endothelial growth factor therapy.
Figure 2. Epiretinal membrane. The 3D image provides unprecedented visualization of the membrane.
I have several patients where this pathology was missed by time-domain stratus OCT but was evident on 3D-OCT," he says. "The system has also been helpful in identifying subtle abnormalities in patients with unexplained vision loss," Dr. Sadda adds.
Physicians, technicians, and patients share equally in the 3D system's advantages, says Dr. Sadda. "For the physician, there is a more comprehensive view of the macula, and the color photo allows for a better correlation between fundus findings and OCT features. Technicians benefit from the speed with which the scans can be obtained, and patients benefit from a better understanding of their disease, which is made possible with the 3D image of the ocular anatomy."
PRACTICE AND RESOURCE MANAGEMENT
Dr. Sadda says that it takes less time to obtain a scan with the 3D OCT 1000 than a high-resolution radial-line image, and the entire macula is assessed, so there is less chance of having to rescan the patient because the lesion of interest was missed.
Dr. Sadda suspects that as the system's software matures, more reliable change analysis will be possible.
"I think with the next generation of machines it will be even easier and quicker to tell if treatment is working. Doing an accurate change analysis requires that the software that comes with these machines accurately identifies the features that the physician wants quantified," he says.
Dr. Walsh says that although the 3D OCT scans take a few more minutes to accomplish than traditional time-domain OCT, the fact that they offer so much more information makes them more efficient. He also points out the usefulness of being able to use the 3D color images for patient education.
"3D reconstructions help patients to better understand their disease. This is advantageous to the practitioner who wants to maximize patient satisfaction, and in practices where color photos are used to document disease stages, 3D OCT's color fundus imaging is also good," he adds.
Retinal specialists interviewed for this article agree that when integrated with the Topcon nonmydriatic retinal camera, the 3D OCT-1000 is an excellent resource for detecting and monitoring retinal disease.
"Eventually," says Dr. Fisher, "we will all switch to Fourier domain because it offers better resolution, improved registration, and better image quality, and there's not as much interpolation of the image quality." RP