Optical coherence tomography has given ophthalmologists a window into the eye that they never had before, and the features of the various OCT devices on the market continue to multiply. With OCTs’ new modules and optional features, physicians can use the devices to evaluate the effects of surgery, diagnose conditions and follow the progression of disease and therapies. Here’s a look at the latest features available on OCT units that can help you better manage patients.
Bioptogen
The Spectralis OCT is a spectral-domain (also called Fourier-domain) optical coherence tomography system that allows high-speed, high-resolution cross-sectional imaging of the retina, offering the following imaging modes: - Simultaneous OCT and infrared reflectance imaging. The only ultra-widefield retinal imaging device with integrated, UWF-guided swept source OCT, Silverstone produces a 200° single-capture optomap image in less than ½ second and enables guided OCT scanning across the retina and into the far periphery. Imaging modalities and image viewing options are detailed below. Purpose:: Optical coherence tomography (OCT) is a noninvasive imaging technique clinically used for visualization of in vivo structures such as the human retina. Using statistical analyses, this study compares the retinal thickness (RT) measurements between two different spectral-domain optical coherence tomography (SD-OCT) devices (Cirrus HD-OCT, and the OPKO Spectral OCT/SLO) in both.
Bioptigen’s Envisu C-Class spectral domain OCT systems are mobile devices with handheld imaging heads that capture images at 32,000 lines per second down to an axial resolution that the company says depends on the unit purchased. The resolution is 5 to 6 µm (high-res light source unit) or 3 µm (very high-res light source unit, but with slightly less depth of field).
Heidelberg says the Spectralis’ new multicolor imaging function can allow clinicians to see certain disease signs better than if they were viewed in simply black and white.
“The system provides flexibility, in terms of software function, a higher resolution available through the VHR light source and a very good signal-to-noise ratio so the clinician can get images in real time scanned over a high-density volume,” says Eric Buckland, PhD, chief executive officer of Bioptigen. “Also, you can take the imaging to the patient, rather than having to take the patient to the imaging—that’s a key functionality difference with the system.”
Dr. Buckland notes that, because of its mobility and handheld imaging head, the Envisu C-Class is the only OCT cleared by the Food and Drug Administration for the imaging of children down to the prematurity age group and the only one approved for use on anesthetized patients. “It’s cleared for use under anesthesia because it can take images from whatever angle is necessary,” he explains. “It has sufficient ergonomics to allow that sort of imaging without any change to the image quality.”
The Envisu, the standard package of which consists of a lens for retinal imaging and one for imaging the anterior segment, provides images only—no normative data or measurements of normative data. “Our system is really a visualization tool,” says Dr. Buckland. “If one wants to image pathology, understand epithelial and stromal thickness, or determine if a candidate is a good candidate for DSAEK or DALK, our system can show them the detail they need.” For information, visit bioptigen.com.
Carl Zeiss Meditec
Carl Zeiss Meditec says the new FastTrac retina tracking system makes it easier to follow pathology from prior exams (top) to current ones (bottom). |
Carl Zeiss Meditec recently introduced four new OCT models, and the company says each offers particular features that may appeal to different practices.
• Cirrus HD-OCT 5000 and 500. The 5000 model has a feature called FastTrac, which is retina tracking technology that Carl Zeiss Meditec’s U.S. President and CEO Ralf Kuschnereit, PhD, says serves a twofold purpose: “While the OCT scan is being done, the independent system is tracking the retina,” he says. “And if the patient moves or blinks, this is taken care of, corrected or re-scanned so that there is no motion artifact in the final image. This is especially important for subspecialists who may have patients who find it hard to fixate. The other advantage of the tracking is if you have subsequent scans of different visits, you can position the HD high-resolution raster scan in exactly the same position so that you can better track the progression of the disease.” The tracking can also be turned off for speedier imaging in patients who can sit still, since using the tracking can make an exam longer than not using it. The model 500 doesn’t have FastTrac, but has a much faster OCT scanner than previous models, which the company says makes it easier to align a patient to the system for accurate imaging.
• Cirrus photo 800 and 600. These are both combinations of a fundus camera and Cirrus OCT in one unit. The 800 increases the feature count by also offering fluorescein and indocyanine green angiography capability. “If you want to provide advanced care and need versatility but maybe have a space constraint, you might not want to have a separate fundus camera and OCT,” says Dr. Kuschnereit. “By combining Cirrus OCT images with fundus, angiography or autofluorescence images all on one screen, it’s a great way to review the disease state of the patient.”
The new Cirrus models also offer software to analyze such features as the retinal nerve fiber layer, ganglion cell layer and optic nerve head. Available anterior segment imaging allows analysis of the angle and the cornea. For information on CZM’s new OCTs, visit www.meditec.zeiss.com/Cirrus.
Heidelberg Technology
The newest addition to the Heidelberg Spectralis OCT is multicolor scanning laser imaging. Multicolor imaging uses several laser wavelengths (blue, red and green) at the same time to allow the physician to capture and display diagnostic information from different parts of the retina during one OCT exam. “It’s very sharply detailed because the Spectralis is confocal, and blocks light from outside of the focal plane, which allows it to get sharp images,” explains Adam Doherty, Spectralis product manager. “Each laser color goes to a certain depth, giving information from different layers of the retina, with blue being more anterior and red more posterior. The clinician can look at the multicolor image or at each color image separately. The multicolor image is helpful because when you take an image of the fundus and you see blood, cotton wool spots and a vitreous hemorrhage in black and white, sometimes those three things can look very similar. But in multicolor you can see the blood is red and isn’t a cotton wool spot, and the vitreous will be darker and not resemble blood.”
The other feature that’s being manufactured but isn’t released yet is the ultra widefield lens. “The non-contact lens allows the OCT to go out over 100 degrees of the retina,” explains Mr. Doherty. “This makes it easier to get images but also makes getting the images easier on the patient. Normally, a retinal photographer would have to do seven field sweeps, especially for diabetic patients, taking images around the posterior pole with a 30-degree lens, then send the series for studies to get a picture of the whole posterior pole. This lens allows the doctor to perform those sweeps, but now out to the far periphery. This may allow physicians to catch disease states in the periphery, such as choroidal ruptures and retinal tears, earlier. Also, if a physician is going to perform a laser treatment in the periphery, he can use one or two of these images to see where he’s going to do the treatment.” Visit heidelbergengineering.com or call 1 (800) 931-2230.
Optopol/Canon
The Optos OCT device combines the technolgies of optical coherence tomography and scanning laser ophthalmoscopy in one unit, allowing the separate images from both devices to be registered to each other pixel-by-pixel.
The Optopol Copernicus HR Spectral OCT scans at 52,000 A-scans per second with an axial resolution of 3 µm. The latest feature available to the clinician is the ability to select “choroid” or “vitreous” mode for a particular scan. “For an exam, the clinician can focus the sensitivity of the spectrometer on one end of the spectrum or the other,” explains Arkaydiusz Chalecki, the Optopol product manager for OCT. “By focusing, you can get increased sensitivity in the upper part, the vitreous, or the lower part, the choroid. So, for example, in the vitreous mode it can capture floaters, the vitreous or a detachment of the ILM. Though being able to focus on the choroid or vitreous doesn’t matter for most patients, if someone has a specific disease of deformation of the retinal structure, one of these modes can enable us to get better details. If a patient has photoreceptor or RPE problems, the choroid mode gives better details in that area.”
“Another area where the device’s focusing ability is useful is in the setting of media opacity,” Mr. Chalecki continues. “There are a number of patients with cataract or other media opacities that can block the light from an OCT, making for a diminished signal. This system helps get as much data as possible for the doctor.”
The Optopol OCT allows clinicians to switch between a choroid-focused mode and a vitreous-focused one depending on the patient’s particular pathology.
The Copernicus also has a progression module to help the clinician follow a disease. “Since the device always does a very dense 3D raster scan, it is able to recognize blood vessels from one exam to the next,” says Mr. Chalecki. “The system uses the vessels to overlay the exams above each other, so even if a patient doesn’t fixate as well one time or the scan area is shifted from the last session, using the vessels as landmarks we can capture all the scans within the same volume. This allows us to precisely measure the thickness of structures over time and compare the same cross-section slices from different exams.” In the United States, Canon distributes Optopol’s OCT, and can be reached at 1 (800) 970-7227. For information, visit
optopol.com.
Optos
Optos says its OCT SLO helps clinicians make better diagnoses by marrying a confocal scanning laser ophthalmoscope with an OCT. “This generates a fundus image as well as an OCT image,” explains Glenn Erickson, director of North American product specialists at Optos. “What makes the OCT SLO different is that it uses a single light source for the beam that’s sent into the eye, rather than two different ones. This gives us pixel-to-pixel registration on the fly between the SLO and the OCT. So, for the user, the two images, the fundus and the OCT, are registered to each other. Anything the user does to one image, he sees on the other. This is in the hardware, and they don’t need to be married up afterwards in the software. This means you always know exactly where you are in the image. For example, with a macular hole, while scanning through it, because you have the SLO in front of you, you can move your scan line into the hole and you see the OCT on the other side of the screen. It allows you to know whether it’s open or closed, because you’re seeing it on the screen. If you didn’t have this, you wouldn’t know if it were open or closed, because you wouldn’t be sure where you were scanning on the retinal surface during an exam.”
The OCT SLO also has functions for tracking glaucoma progression, though Mr. Erickson says the device does the retinal nerve fiber layer test a little differently. “We use vessel registration and tracking,” he says. “We track back to the same place around the nerve head where the measurement circle was the first time, based on where it was intersecting the blood vessels. This is important because if you’re looking for change you want to go the exact place on the retina—in this case the nerve fiber layer measurement—and look for change over time.” For information, visit optos.com or call 1 (800) 854-3039.
Opko Spectral Oct Slo User Manual Online
Optovue
The latest additions to the Optovue line of devices are the Total Corneal Power module and the RTVue VTRAC Premier real-time, active-tracking OCT.
“Total Corneal Power comes in when the surgeon or staff is doing preop IOL calculations in post-refractive surgery patients,” says Mike Scott, senior product marketing manager for Optovue. “Standard keratometry measures the anterior curvature of the cornea and then makes broad assumptions to extrapolate the cornea power to be used in IOL selection. However, in a post-LASIK patient, for example, the anterior curvature has changed, so if you try to use the same assumptions, the surgery may result in an unexpected postoperative surprise. TCP takes a direct measurement of the front surface and a direct measurement of the back surface of the cornea to calculate the anterior, posterior and net cornea powers. Mr. Scott says the surgeon can then take those corneal powers and enter them into a special OCT-based IOL power calculator for eyes with previous refractive surgery that is available at coollab.net/index.php?id=852.
Though the RTVue system already has vessel registration to allow change and trend analysis, Mr. Scott says that the VTRAC Premier system allows surgeons to get images with even more detail. “It has a scan depth of approximately 3 mm, which gives a larger window into the retina,” he explains. “Combined with its Noise Reduction Technology, the system allows for deep choroidal imaging and measurement, while also providing rich detail in the vitreous. Also, in patients with fixation drift, the real-time, active eye tracking helps capture their images.” The RTVue also has ganglion cell complex analysis, which the company says has been used in more than six years of published studies and clinical use. The GCC and thickness measurements can be compared to a large normative database, as well.
In addition to the RTVue VTRAC Premier, Optovue also offers the iVue, a more compact OCT device. The iVue allows imaging of the anterior segment, posterior pole and optic disc; as well as retinal thickness measurements and optic disk and peripapillary retinal nerve fiber layer assessment. The device’s anterior segment module is offered as standard and provides pachymetric measurements of a 6-mm diameter area of the central cornea, as well as visualization and measurement of the angle. For information visit optovue.com or call 1 (866) 344-8948.
Topcon
Opko Spectral Oct Slo User Manual Instructions
The Topcon 3D OCT combines a high-resolution digital fundus camera with the OCT to give the clinician different views of the retina.
The company says the OCT portion of the exam uses its proprietary FastMap software system to allow dynamic viewing of 2D, 3D and fundus imaging. Topcon says FastMap can help when faced with complex pathologies such as vitreous traction, macular edema and retinoschisis. The software also allows the physician or technician to export its images and 3D movies to other devices for presentation purposes. The OCT system can capture images of the fovea and optic nerve in a single scan, and has a choroid reference mode for providing high-resolution views of the choroid. For capturing exam images of high myopes and hyperopes, the system can work with a diopter compensation lens and can provide an extended scanning depth of 2.3 mm.
By employing a non-mydriatic color fundus camera in the OCT system, clinicians may be able to visualize conditions that might not be picked up on an OCT, such as disc hemorrhages, the company says. The camera has a 45-degree field of view and can also provide stereo photos for analysis of optic disk changes. To help orient themselves to areas of pathology that are displayed in the system, clinicians can use the 3D OCT’s PinPoint Registration to link the location of the OCT data points to specific points in the fundus image.
For following patients with glaucoma or retinal pathology, the FastMap software also uses layer detection algorithms to measure such features as total retinal thickness and retinal nerve fiber layer thickness. The physician or technician can manually adjust the imaging grids that are measured in these exams, and can also manually register serial exams or allow the computer to register them automatically for comparison purposes. For information, visit topconmedical.com or call 1 (800) 223-1130.
For many ophthalmologists, OCT technology has become an integral part of their day-to-day practice. The new modules and add-ons available on the various OCT devices should help doctors diagnose and manage disease more quickly, and with even more accuracy. REVIEW
IRENE A. BARBAZETTO, MD · SANDRINE A. ZWEIFEL, MD · MICHAEL ENGELBERT, MD, PhD · K. BAILEY FREUND, MD · JASON S. SLAKTER, MD
Over the last decade, optical coherence tomography has become an important imaging technology used in diagnosing and following macular pathologies. It has complemented, and in some cases replaced, fluorescein angiography in many instances, especially in the diagnosis and management of various retinal disorders, including macular edema and AMD.
ENTER FOURIER-DOMAIN OCT
The introduction of spectral-domain or Fourier-domain OCT (vs time-domain OCT, which characterized all previously commercially available machines) has taken image resolution, acquisition time, registration, and display options to new levels. As clinicians have a variety of new devices to choose from, it is worthwhile to take a closer look at the different features, including imaging capabilities, before making the decision of which system to purchase.
All devices currently available offer a high theoretical axial image resolution between 3 to 7 μm, with the 3D SD-OCT (Bioptigen) and Copernicus (Optopol Technology SA) marking the higher end of the spectrum and Spectralis (Heidelberg Engineering) representing the lower end, with 7-μm 'optical' resolution (but 3.5-μm 'digital' resolution). The transverse image resolution ranges between 10 and 25 μm. Greater variability can be found looking at the scanning speed with the Copernicus HR (Optopol Technology SA) acquiring 55 000 A-scans per second, the Spectralis acquiring 40 000, the Cirrus (Carl Zeiss Meditec, Inc.) acquiring 27 000 A-scans per second, and the 3D OCT-1000 (Topcon Medical Systems, Inc.) acquiring 18 000 A-scans per second.
Despite these improved acquisition times, motion error and poor image contrast remain problems. Thus, theoretical axial and transverse resolution specified by the manufacturer do not necessarily translate into crisper, more detailed images. Most devices improve contrast and reduce motion errors through frame averaging. Also the Spectralis allows for real-time eye tracking to eliminate motion artifacts.
Another variable is the devices' capability of transverse image registration (see, for example, the Topcon 3D OCT-1000, and the Heidelberg Spectralis), which allows for a direct, side-by-side comparison of different visits and easy monitoring of localized anatomic changes.
| Irene A. Barbazetto, MD, is a retina fellow at the Harkness Eye Institute of Columbia University. Sandrine A. Zweifel, MD, is a medical retina fellow at the Vitreous-Retina-Macula Associates of New York (VRMNY). Michael Engelbert, MD, PhD, is a clinical fellow at VRMNY. K. Bailey Freund, MD, and Jason S. Slakter, MD, are partners in Vitreous-Retina-Macula Consultants of New York. The authors report no financial interest in any products mentioned here. Dr. Slakter can be reached via e-mail at jslakter@aol.com. |
HEAD-TO-HEAD COMPARISON
When comparing actual images taken with different devices, it is not surprising that the resolution and the contrast seem to be superior for machines with axial image resolution of 3 to 4 μm, higher scanning speed, and real-time eye tracking. However, these differences may not play as important a role for standard applications, since many of the images appear rather comparable when it comes to standard diagnostic applications (Figures 2, 3, and 4). Sayanagi et al. compared Stratus OCT images of AMD patients to studies obtained with spectral-domain devices for the detection of typical neovascular patterns and concluded that all 4 spectral-domain devices were superior in delineating sub-RPE fluid.1 However, they did find differences when comparing detection of subretinal fluid and intraretinal changes, including cysts, with the Spectralis and Cirrus performing best in their study.
Opko Spectral Oct Slo User Manual 2017
Figure 1. Comparison of OCT images of a patients with neovascular age-related macular degeneration obtained with the RTVue OCT (OptoVue) and the Cirrus OCT (Zeiss Meditec, Inc.).
Figure 2. Comparison of images obtained with 3 different spectral-domain OCT devices (Topcon 3D OCT-1000, Zeiss Cirrus, Heidelberg Spectralis) of both eyes of the same patient with early AMD changes taken just minutes apart.
Figure 3. Comparison of images obtained with 3 different spectral-domain OCTs (Heidelberg Spectralis, Optovue RTVue, Topcon 3D OCT-1000) and with 1 time-domain OCT (Zeiss Stratus) of both eyes of the same patient with a history of central serous chorioretinopathy in both eyes.
Figure 4. The same set of images as shown in Figure 3 in pseudo color.
In general, software has become an equally, if not more important part of the package. These applications include, among others, retinal and macular thickness maps and 3D reconstruction of the vitreoretinal interface, a feature that can be helpful in surgical planning (Figure 5). Also, C-scan or 'en face' representation of the retinal layers and choroid adds a new dimension to visualization. Curved C-scans may be set to conform to the contours of the internal limiting membrane, the retinal pigment epithelium, or a spherical approximation to the RPE surface called the RPE fit, depending on whether the structures of interest are located in the inner or outer retina (Figure 6). En face OCT images will likely be helpful in assessing progression of drusen or geographic atrophy in AMD or photoreceptor loss in retinal dystrophies or AZOOR.
Figure 5. Comparison of horizontal B-scan images and 3D images of a patient with neovascular age-related macular degeneration obtained with Heidelberg Spectralis, Zeiss Cirrus, Topcon 3D OCT-1000.
Figure 6. En face OCT (C-scan) through the choroid (top left) and corresponding B-scan (top right) of the same patient as shown in Figure 5. The double blue lines represent the curved optical slab, which is then overlaid onto the scanning laser ophthalmoscopy image. En face OCT (C-scan) through the outer nuclear layer of a patient with long-standing neovascular age-related macular degeneration (bottom left) with an interconnected network of tubules (outer retinal tubulation). The corresponding B-scan (bottom right) shows the location of these tubular structures within the outer retina.
One has to keep in mind that the measurements with different machines are not always interchangeable from device to device and that previous time-domain devices use different reference points for analysis with respect to the retina or pigment epithelium. A recent study by Wolf-Schnurrbusch et al. showed that, of six tested spectral-domain and time-domain OCT devices, the Cirrus and Spectralis showed similar values for serial measurements of central retinal thickness in 'normal' patients, but those values were higher compared to other devices, which did not include the RPE layer into the segmentation algorithm when calculating retinal thickness.2 It may be of interest that, of all the devices tested in this study, the Spectralis had the lowest coefficients of variation for repeated measurements and allowed for the most accurate repeat measurements, possibly due to the combination of image registration and eye tracking.
The fact that time- and spectral-domain machines will differ both qualitatively and quantitatively has been corroborated by several studies.3-5 In 'real life,' this will mainly affect users who are in the process of upgrading their current equipment, as not only retinal thickness but also nerve-fiber layer analyses will no longer be comparable.
OTHER APPLICATIONS
These issues are also relevant in clinical trials with OCT measurement endpoints and for practices using different imaging devices in different offices. However, if thickness determination is based on identical anatomical boundaries, thickness measurements may be comparable.6 This is of import for retrospective studies that follow patients over a long enough time span that parallels the evolution of OCT.7
Advanced 3D viewing for surgical planning and patient education may be as important for some users, particularly when this feature can be performed on a networked computer in an examination or consultation room. Most devices provide desktop review software to allow for varying degrees of 3D viewing, visit-to-visit comparisons, and advanced image analysis on networked computers within the office. If, instead, the practice chooses to upload representative B-scans, thickness maps, and change analyses to a pre-existing image database, they will lose many of these advanced features.
With regard to patient tolerance and office workflow, ease of use and acquisition times may vary substantially from device to device. Certain systems require considerably more training than others in order for the user to obtain high-quality images. Also, while useful for obtaining higher quality images, real-time eye tracking lengthens the acquisition time and is very difficult to perform in patients with poor fixation. The size or 'footprint' of the systems vary considerably, making some units, most notably the Cirrus, more suited for small examination rooms.
Several companies offer combined application devices, most notably Heidelberg Engineering, which offers the Spectralis as part of their HRA package. It includes fluorescein and indocyanine green angiography, as well as fundus autofluorescence imaging — a space-saving approach, but of concern in case of any need for repair, as in all multifunction machines. The Topcon 3D OCT-1000 takes a nonmydriatic color fundus image of the posterior pole at the end of the scan; however, the limited resolution may not necessarily replace the need for a standard digital fundus camera in many instances. The Spectral OCT SLO (OPKO/OTI) offers additional microperimetry — a useful, but also time-consuming tool in assessing retinal function.
Other additional features make some of the devices more suitable for research, like the 3D SD-OCT (Bioptigen, Inc.), which has specific applications for scanning small animal eyes, anterior-segment examinations, and tissue cultures. The machine has two engines, one at 840 nm for imaging the retina in small animals (rodents, rabbits), larger animals (such as dogs and pigs), and humans, and one at 1310 nm, which is suited for tissue imaging, anterior segment imaging, small animal, external, and ex vivo imaging.
Several devices allow for retinal nerve fiber layer analysis and can be used to monitor glaucoma patients and their progression. Not all devices, however, offer a normative database. Others combine not only retina and glaucoma applications but also allow for anterior-segment visualization and corneal topography like the RTVue OCT (Optovue) and the 3D SD-OCT. Some of the most recent devices broadened the applications even further and measure blood flow using a Doppler OCT.
Most recently approved among OCT devices is the OPKO/OTI Spectral OCT SLO Combination Imaging System, which received FDA 510(k) clearance in January 2009 and is manufactured in Canada. Similar to the Spectralis, this machine is a combination of OCT and scanning laser ophthalmoscope (SLO). Its features include real-time 3D imaging and an optional anterior-segment OCT module. In August 2008, Optopol Technology received market approval in Japan for its SPOCT Copernicus HR spectral computed tomography system, which includes retinal- and RNFL-thickness maps, ocular motion tracking, and optional Doppler imaging.
CONCLUSION
In summary, spectral-domain OCTs allow not only for greater image resolution at much higher speeds than the previous generation time-domain devices. They also have the potential to improve patient care and surgical planning, as well as to broaden the understanding of pathologies and their mechanisms. Although differences in image quality can be found between devices of different manufactures, it is safe to say that all images obtained with SD-OCT devices exceed the previous standard by far. Therefore, one should carefully review the individual technical specifications and analyze how the devices will be most efficiently used in the individual office or research lab with its specific needs before making the investment, which might be quite substantial in some cases. Software updates, technical support, image storage, and networking capabilities, as well as ease of use and available ancillary imaging capacities, may prove to be as or even more important than small differences in picture quality. RP
REFERENCES
- Sayanagi K, Sharma S, Yamamoto T, Kaiser PK. Comparison of spectral-domain versus time-domain optical coherence tomography in management of age-related macular degeneration with ranibizumab. Ophthalmology. 2009;116:947-955.
- Wolf-Schnurrbusch UE, Ceklic L, Brinkmann CK, et al. Macular thickness measurements in healthy eyes using six different optical coherence tomography instruments. Invest Ophthalmol Vis Sci. 2009 Feb 21. [Epub ahead of print]
- Leung CK, Cheung CY, Weinreb RN, Lee G, Lin D, Pang CP, Lam DS. Comparison of macular thickness measurements between time domain and spectral domain optical coherence tomography. Invest Ophthalmol Vis Sci. 2008;49:4893-4897.
- Forooghian F, Cukras C, Meyerle CB, Chew EY, Wong WT. Evaluation of time domain and spectral domain optical coherence tomography in the measurement of diabetic macular edema. Invest Ophthalmol Vis Sci. 2008;49:4290-4296.
- Forte R, Cennamo GL, Finelli ML, de Crecchio G. Comparison of time domain Stratus OCT and spectral domain SLO/OCT for assessment of macular thickness and volume. Eye. 2008 Dec 12. [Epub ahead of print]
- Engelbert M, Zweifel S, Imamura Y, Fisher YL. Spectral and time domain OCT measure identical retinal thickness if identical boundaries are selected for analysis. Eye. 2009 Apr 17. [Epub ahead of print]
- Engelbert M, Zweifel S, Freund KB. 'Treat and extend' dosing of intravitreal anti-vascular anti-endothelial growth factor agents for retinal angiomatous proliferation (RAP/type 3 neovascularization. Retina, accepted for publication.