Sponsored by NIDEKThe Role of OCT Angiography in Assessing
Retinal and Choroidal Vasculature
OCT-based angiography offers several unique and novel tools for evaluating
retinal diseases that may impact how we make decisions in the clinic.
BY DANIELA BACHERINI, MD, PHD
OCT has become an indispensable tool for objectively assessing retinal structure. Increasingly, it isbeing recognized that the microvasculature ofthe retina and choroid plays an important role inmany ocular pathologies, implying a need to supplement OCT data with angiography. For the past few decades, clinicians have relied on a combination of angiography under fluorescein (FA) or indocyanine green (ICG) contrast dye to detect diseasefeatures. Yet, FA and ICG are associated with several well-known
limitations, most prominently the requirement for intravenous contrast dye that may cause a variety of complications, including nausea, vomiting, pruritis, urticaria, and, in rare instances, anaphylaxis.1
The emergence of OCT angiography (OCT-A) thus offers
an intriguing potential to expand the role of evaluating themicrovasculature in entities such as age-related maculardegeneration, diabetic retinopathy, retinal vein occlusion, andothers.2 Fundamentally, OCT-A differs from contrast-based
angiography in how the images are captured. Whereas FA or
ICG angiography tracks movement of intravenously injected
dye through various filters, thereby acting as a surrogate
marker of blood flow, OCT-A images are 3D reconstructions ofnumerous A-scans over the same tissue area. Laser light reflects
off the surface of moving red blood cells, and analysis by theaccompanying software depicts movement of the red blood cells
as a function of their differing positions between serial scans.
The above is a vast oversimplification of the technical nuancesof OCT-A, which have now been described in several excellent
publications.3,4 With OCT-A, the need for intravenous dye iseliminated; it has an improved safety profile, and the scan canbe performed faster and easier. Some OCT machines, such as
the NIDEK RS-3000 Advance 2 are now equipped with onboard
OCT-A capabilities, making them even more convenient for busy
clinicians and their patients and further expanding their utility.
These kinds of multifunctional platforms also offer some uniquefeatures that portend to impact clinical decision-making andenhance research capabilities.

SEGMENTED ANALYSIS OF RETINAL VASCULATURE
One interesting feature of the AngioScan software (NIDEK) on
the RS-3000 Advance 2 is that it can provide segmented mapsof the chorioretinal vasculature. By comparison, FA images are
2D in nature, and so, these layers are depicted together. WithOCT-A, the resulting image can easily be segmented to evaluate
the retinal flow layer by layer;5,6 specifically the latest softwareon the RS-3000 Advance 2 allows for segmentation of the superficial capillary plexus (SCP), deep capillary plexus (DCP), OuterRetina (OR), OR + Choriocapillaris (ORCC), Choriocapillaris,and Choroid Layers, which cannot be separately visualized byother techniques.
This segmentation may be useful in classifying choroidal
neovascularization (CNV) as type 1 (occult, below the retinal
pigment epithelium), type 2 (classic, in the OR), type 3 (retinal
angiomatous proliferation lesions), or type 4 (mixed type 1 andtype 2). In this regard, we have found that the ORCC layer isparticularly useful, as this is where the majority of CNV lesions
are found (Figure 1).
More recently, evidence has emerged that alterations in the DCP
correlate with visual acuity outcomes in many retinal diseases. We
recently demonstrated a positive correlation between the restoration of the DCP and improved vision after surgery to repair macular hole (Paper submitted). At a minimum, this recent researchsuggests that it may be useful to evaluate the DCP before and aftersurgery as an additional metric for assessing the outcome.7

CONNECTING STRUCTURE AND FUNCTION
Because the OCT-A function is integrated into the existing platform of the RS-3000 Advance 2, it is possible to perform angiography and traditional OCT imaging at the same time. Furthermore,
the RS-3000 Advance 2 is equipped with a scanning laser ophthalmoscopy eye tracer, thereby allowing compensation for eye movements resulting in a more accurate scan and higher image quality.
This functionality also increases the reproducibility of the scanninglocation during follow-up.
There is an additional possibility to connect the RS-3000
Advance 2 with other diagnostic modalities, such as the MP-3
Microperimeter (NIDEK). The multimodal integrated evaluation
permits assessment of both structure and retinal function during the same examination (Figure 2). In our clinic, we have foundthat, in some surgical cases, despite anatomic success in repairing
retinal tissue, visual acuity gains may not be apparent. In thesecases, multimodal evaluation, including retinal sensitivity assessment, helps us to understand exactly what is going on from afunctional perspective and to connect that with vascular andstructural details.
MARCH 2019 | INSERT TO RETINA TODAY 1

FAZ ASSESSMENT
The foveal avascular zone (FAZ), as the name would imply, isthe area in the central part of the fovea where vessels are absent,
and it is surrounded by the perifoveal capillary plexus. The
AngioScan software on the RS-3000 Advance 2 automaticallydetects the FAZ, providing quantitative metrics, including

its area, perimeter, and circularity. There is already literature
supporting the notion that FAZ size may be a biomarker forvisual changes in diabetic retinopathy, branch retinal vein
occlusion, and other retinal diseases.

VESSEL DENSITY AND PERFUSION DENSITY MAPS
With the AngioScan software, the angiography image can beconverted into two color-coded types of vascular density maps,
which differ in how they are calculated. The vessel density map
provides a display of total linear millimeter of vessels per square
millimeter regardless of the vessels’ original thickness. The perfusion density map takes into account the original thickness ofthe vessel when calculating density to display the percentage oftissue that is perfused by blood flow (Figure 3).
Each of these may be important, as one or the other mightshow alterations depending on the particular pathology and/or
the evolutionary stage of the disease.

CONCLUSIONS
Figure 1. OCT-A in a case of neovascular age-related macular degeneration. A
large CNV is evident in the ORCC layer.

Figure 2. Overlay of the microperimetry onto the choroidal slab on OCT-A inStargardt disease.

Figure 3. Vessel density map (left) and perfusion density map (right).
2 INSERT TO RETINA TODAY | MARCH 2019

Based on what we currently know about OCT-A, this new toolfor assessing the retinal and choroidal vasculature does not yet
supplant the role of FA or ICG angiography. Indeed, it is likelythat there will be a role for each of the three, at least for theforeseeable future. For instance, dye leakage on FA is a diagnostic
sign in retinal vasculitis, and this is not visible on OCT-A due tothe working principle of the imaging. Furthermore, the role ofOCT-A for peripheral retina is as of yet undefined, and this, too,is better visualized with contrast-based angiography.
At the same time, OCT-A offers novel mechanisms forassessing chorioretinal vasculature in a noninvasive way that is
not possible with other modalities. OCT-A also has intriguing
potential for adding to clinical decision-making. I have foundthe device to be easy to use and understand; the interface is very
user-friendly, thus making it suitable for a wide range of clinicalapplications. In addition, the connectivity and multimodal
capabilities of this platform technology make it an exciting
addition to the research setting. n1. Yannuzzi LA, Rohrer, MA, Tindel LJ, et al. Fluorescein angiography complication survey. Ophthalmology. 1986;93:611-617.
2. Mastropasqua R, Di Antonio L, Di Staso S, et al. Optical coherence tomography angiography in retinal vascular diseases andchoroidal neovascularization. J Ophthalmology. 2015;2015:343515.
3. Koustenis A, Harris A, Gross J, et al. Optical coherence tomography angiography: an overview of the technology and an assessment of applications for clinical research. Br J Ophthalmol. 2017;101(1):16-20.
4. Spaide RF, Klancnik JM, Cooney MJ. Retinal vascular layers imaged by fluorescein angiography and optical coherence tomography angiography. JAMA Ophthalmol. 2015;133(1):45.
5. Savastano MC, Lumbroso B, Rispoli M. In vivo characterization of retinal vascularization morphology using optical coherence
tomography angiography. Retina. 2015;35:2196-2203.
6. Campbell JP, Zhang M, Hwang TS, et al. Detailed vascular anatomy of the human retina by projection-resolved optical coherence tomography angiography. Sci Rep. 2017.10;7:42201.
7. Kim YJ, Jo J, Lee JY, et al. Macular capillary plexuses after macular hole surgery: an optical coherence tomography angiography
study. Br J Ophthalmol. 2018;102(7):966-970.

Daniela Bacherini, MD, PhD ABD
n F aculty, Department of Ophthalmology, University of Florence, Azienda
Ospedaliero Universitaria Careggi, Florence, Italyn L ivorno Hospital, Eye Clinic, Livorno, Italyn daniela.bacherini@gmail.com
n F inancial disclosures: None acknowledged