Akihiro Ishibazawa, MD, PhD
Research Fellow, Tufts New England Eye Center
Assistant Professor, Asahikawa Medical University
Nadia K. Waheed, MD
Associate Professor, Tufts New England Eye Center
Assistant Editor, RETINA
OCT angiography (OCTA) technology has developed rapidly with dramatic improvements in scanning speed, motion contrast imaging, and automatic layer segmentation to allow examination of the perfusion status of the retina and choroid noninvasively. OCTA is now utilized throughout the world, and we believe that it will become a part of routine clinical examination in the near future. Most OCT manufacturers have released their own OCTA systems (Figure 1) and are constantly continuing to improve them. In this article, we discuss the characteristics of each OCTA device and their clinical utility. We hope this article will help in the assessment of your OCTA needs.
Here are the units that we review in this piece:
- RTVue XR Avanti (Optovue)
- RS-3000 Advance (Nidek)
- Cirrus 5000 (Zeiss)
- PLEX Elite 9000 (Zeiss)
- Triton (Topcon)
- SPECTRALIS 2 (Heidelberg)
- OCT-HS100 (Canon)
The table below lists and compares the specs of each OCTA device. For images with a typical field of view (~6 mm) centered on the macula, all OCTA devices can clearly identify superficial and deep retinal vasculatures as well as the choriocapillaris. All devices nicely visualize choroidal neovascularization (CNV), one of the biggest benefits of OCTA (Figure 2), although manual adjustment of the segmentation is sometimes necessary. We can also analyze the status of chorioretinal vasculature quantitatively using unique software for each device, especially in the macula. Generally speaking, swept-source (SS)-OCTA systems have the advantage of wider field-of-view imaging. However, some limitations include their expense as well as some inherent production challenges of swept source lasers. The individual characteristics of each OCTA device are described and discussed below.
The RTVue XR Avanti (Optovue) was one of the first commercially available OCTA devices. It leads the pack in being the only device currently with vessel quantification software. Numerous scientific papers using this device have been published. The Avanti has an established track record of reliability for clinical and research use.
Prominent features of the Avanti include the built-in software “AngioAnalytics,” which is useful in analyzing vessel densities and the foveal avascular zone (FAZ). Vessel density can be automatically calculated within the sectors of the ETDRS grid and easily compared to the retinal thickness (Figure 3). The area and perimeter of the FAZ are also automatically measured. In addition, flow areas, such as CNV area, can be measured manually.
It is convenient for clinicians to have access to such analytic software on the device itself—the alternative is to rely on more complicated external software. Recent updates to the Avanti system include higher scanning density (400 x 400) on the 4.5 x 4.5-mm and 6 x 6-mm scans, and the addition of an automatic montage protocol (macula and disc area). This device also features a dual correction system that utilizes software-based as well as tracking-based image correction. The Avanti requires two scans (X-fast and Y-fast) to create the motion-corrected images. However, even with two scans, image acquisition is rapid.
The RS-3000 Advance (Nidek) is a spectral-domain (SD) OCT machine. The merit of this device is that the OCTA functionality can be additionally installed with a software update. However, the scan speed is slower than the other OCT devices (53,000 A-scan/sec) so it takes relatively longer to acquire the images. This system does incorporate a scanning laser ophthalmoscope (SLO) to display the fundus, which assists with accurate anatomic tracking. Moreover, the Nidek software allows overlay of microperimetry data (from the MP-3) onto the OCTA images and fundus images, and therefore allows correlation of functional testing and OCT/OCTA imaging (Figure 4).
Similar to the RS-3000 Advance, the Cirrus 5000 (spectral domain platform) and PLEX Elite 9000 (swept source platform) by Carl Zeiss Meditec also use a scanning laser ophthalmoscope (SLO) so that the fundus can be visualized scan specific areas. The image acquisition time on these two devices is quicker (~10 seconds, even for 12 x 12 images for the PLEX Elite), as is the processing time. The benefits of quick scanning include patient comfort, clinic efficiency, and easier rescanning if necessary. Additionally, the PLEX Elite and Cirrus are transverse-mounted devices as compared to the other devices, which are face-to-face. The transverse-mounted style can save space and enables the examiner to monitor the patient’s position easily.
The PLEX Elite has an inbuilt montaging program, which guides users to take images from the central, superotemporal, superonasal, inferonasal, and inferotemporal quadrants in sequence. The device then creates montage images that extend beyond the arcades. In retinal vascular diseases, such as diabetic retinopathy and branch retinal vein occlusion, this ability is helpful in detecting are of non-perfusion, intraretinal microvascular abnormalities, and neovascularization (Figure 5).
However, these wide-field OCTA images may not be optimal for use in segmental evaluation of the retinal deep plexuses because of their currently lower resolution and imprecise segmentation in the peripheral retina. Therefore, only full-thickness retina slabs should be used for wide-field montage evaluations. Such images could offer a clinical alternative to fluorescein angiography (FA).
Registered users can also access the “Advanced Retina Imaging Network” for further image analysis online. The PLEX Elite is FDA approved, but is available only for research at the time of writing. Moreover, some of the legacy Cirrus features, such as drusen volume and subretinal fluid segmentation, are not yet available on the PLEX Elite, although they are expected to be released soon with software updates.
The Cirrus 5000 has external software to analyze vascular length density, perfusion density, and FAZ metrics, but this software is not yet FDA approved. It retains many of the legacy Zeiss software such as the advanced RPE analysis software that allows volumetric evaluation of drusen, subretinal fluid and retinal pigment epithelial detachments.
The Triton (Topcon) is a SS-OCTA device. This device is FDA cleared in the U.S. for structural OCT imaging, but not for OCTA yet. It also has the ability to create wide-field OCTA images (Figure 6), at a resolution comparable to the PLEX Elite (scan density: 500 x 500 for PLEX Elite vs 512 x 512 for Triton for the 9 x 9-mm and 12 x 12-mm images). The Triton is the only SS-OCT added onto a multimodal platform, with the ability to take color fundus images, red-free, autofluorescence, and fluorescein angiography. It has the ability to register the OCTA images with color, FA and OCT images to allow true multimodal assessment of patients on one device. The device has a strong track record in the Asian and European markets, with the longest history of any swept source device. Similar to the other devices mentioned above, analytic tools are available only for research purposes at the current time.
The SPECTRALIS 2 (Heidelberg Engineering) is an SD-OCT platform with high image resolution (lateral: 5.7 μm, axial: 3.9 μm), and built-in software for projection artifact removal. These capabilities allow for high-quality imaging of all four vascular plexuses (i.e. the nerve fiber layer, superficial, intermediate and deep capillary plexuses). This is again on a robust multimodal platform, with the ability to take multicolor fundus images, autofluorescence, FA, and indocyanine green angiography, so the OCTA images can be overlaid on the other multimodal images. However, the scan time is still relatively long, as with the Triton.
Finally, the OCT-HS100 (Canon), which is not approved by the FDA yet, is a SD-OCT with the highest axial resolution (3 μm) among these devices. The most noteworthy characteristic of this device is OCTA “averaging.” Averaged OCTA images, which are automatically created from several consecutive OCTA images, can increase the signal / noise ratio, and provide higher contrast and more uniform images than from a single acquisition (Figure 7). Although multiple OCTA scans must be performed, this averaging technology can improve the imaging quality. Furthermore, Canon is now developing a new averaging technology using artificial intelligence, which may offer further improvements.
In summary, all manufactures are vying with each other to develop the best OCTA systems. Indeed, each OCTA device has pros and cons while the technology continues to evolve. We believe that OCTA will contribute to the routine noninvasive assessment of ocular perfusion, and provide better understanding of the pathogenesis of many ocular diseases. We hope that this guide was useful, and look forward to the continued advancements in OCTA imaging.
