Importance of the new application and diagnostics

Chapter 1 OCT Angiography: Importance of the new application and diagnostics

Maria Cristina Savastano, MD1 - Marco Rispoli, MD1 Bruno Lumbroso, MD1

1Italian Macula Center, Rome

The analysis of the retinal choroidal vascular flow has always aroused interest in the ophthalmologic clinical world. In the last few years, the analysis of the retinal choroidal vasculature without the intravenous injection of a contrast medium has allowed for the study of many pathologies in various patients. OCT Angiography is a method based on high-resolution imaging techniques that are able to assess the retinal choroidal flow within vessels.1 Technically, OCT Angiography is based on an amplitude-decorrelation analysis. Introduced in 2005 by Barton et al.2, it can register decorrelation movements. To better understand this method, consider that this system can register the difference between static and moving signals. The only moving signal originates from blood tissue which is, therefore, registered as a decorrelation signal. Unlike fluorescein angiography, which is currently still considered the gold standard in retinal vascular imaging, OCT Angiography has reached an exponential application in clinical daily practice. This development is connected to the rapid execution of the diagnostic examination, without any risk for the patient, allowing for it to be easily performed even on children. Like any newly introduced method, OCT Angiography necessitates a new means for interpreting the images generated, with the aim of identifying the parameters required in reaching a correct diagnosis. An important concept is that OCT Angiography is three-dimensional and analyzes tissue, layer by layer, based on “en-face” or frontal images. This allows to obtain actual tomographies, which correspond to the retinalchoroidal vascular layer of interest. The first identified vascular planes were the superficial vascular plexus (SVP) and the deep vascular plexus (DVP).3 Figure 1 illustrates the comparison between vascular representation in fluorescein angiography (FAG) and OCT Angiography (OCT-A). What is most evident at first sight are the overlapping vascular planes, which can be observed with FAG. Instead, the superficial (SVP)

Figure 1. Comparison between vascular representation in fluorescein angiography (FAG) and OCT Angiography (OCT-A). FAG represents various overlapping vascular plexuses; OCT Angiography allows for scanning the various superficial (SVP) and deep (DVP) vascular planes separately.

Figure 2. Representation of several parallel scans that, distanced at 20 micron from the superficial capillary plexus (SCP) lead to the deep plexus (DCP). The blue circle in A highlights a detail of the retinal vessel. Following its development in B and C, note how this detail leads to the view of fan-shaped vascular fraying in D. This detail demonstrates how the plexuses are interconnected, as reported in the anatomical studies.

and deep (DVP) vascular planes can be distinguished through OCT Angiography. These are clearly represented separately and reveal different morphological features. The superficial vascular plexus, made up of large retinal vessels with a mean diameter of 120 μm, is situated in the group of ganglion cells and has a “spider web” structure. The deep vascular plexus, made up of vessels with a mean diameter of 60 μm, is situated in the internal core layer and external plexiform layer. The morphological aspect of this layer resembles small fans with many small horizontal and vertical interconnections. The superficial and deep vascular plexuses are interconnected between each other through a dense web of small vertical vessels that anastomose with each other (Figure 2). Anatomically, the retinal vascular layers are clearly described by Campbel et. al.

Anatomically, the retinal vascular layers are clearly described by Campbell et al. Their study illustrates the representation of the vascular retina in the radial peripapillary plexus (RPCP), as well as the superficial (SVP), intermediate (IVP), and deep vascular plexus (DVP).4 OCT Angiography confirmed the presence of these layers through in vivo analysis, although an analysis of the intermediate plexus finds limited application from a clinical viewpoint. Therefore, only the superficial and deep vascular plexuses are taken into consideration. The OCT Angiography study allows to not only identify the retinal vascular plexuses, but also the choriocapillaris and choroidal ones. Moreover, it allows to analyze the avascular area situated in correspondence of the outer core plexus. OCT Angiography is able to analyze the vascular flows within a precise range of 0.5 mm/sec and 2 mm/sec.1 This implies that the system is not able to identify flows that are too slow or too fast, therefore, they appear as areas without any flow. This happens for large choroid vessels that appear dark, since their flow is greater than the flow established (Figure 3). The importance of studying retinal vascular layers on different planes through OCT Angiography allows to identify anomalies in only several areas, which are often the starting point for an early diagnosis, as occurs with diabetic retinopathy. In fact, microcirculation in the early stages of diabetic retinopathy may primarily only affect the deep vascular plexus, leaving the superficial one intact. Moreover, identifying every plane, in turn, allows to precisely locate the position of the pathological process, as in the case of a neovascular membrane with flows that can be above or below the RPE. This precise distribution allows for a differential diagnosis: Type 1 neovascularization below the RPE, Type 2 neovascularization above the RPE. Another important aspect of the analysis performed through OCT Angiography is the morphology that can be identified without the indirect effect that derives from the use of a dye, as in FAG. A typical example could be the diffusion of the dye in the event of a defect in permeability. This effect – that, at a first

Figure 3. OCT Angiography allows to clearly identify the superficial vascular and the deep vascular plexuses. The avascular layer in a physiological condition does not have irregular flows. The choriocapillaris layer is clearly visible and presents a dense and regular capillary network without vascular flow in correspondence with the large choroidal vessels.

Figure 4. Clinical case of diffuse retinal epitheliopathy caused by chronic central serous retinopathy. In the B-scan (A), there is evidence of a pachychoroid with choroidal vessels that present an increase in diameter. An intraretinal edema affects both the inner and outer core layer, and the separation of the neuroepithelium is also visible in the foveal area. A thin flat separation of the RPE can be seen in the foveal area. Autofluorescence (B) shows signs of the involvement of the large retina with gravitational epitheliopathy phenomena. The Fluorescein angiography (C) shows large areas of hyperfluorescence where spread and impregnation phenomena are not well distinguished. The OCT Angiography analysis (D) shows one area with irregular flow (detail included in the red area), which represents a neovascular membrane.

Figure 5. Example of an artifact of segmentation and its correction. The OCT-A image on the left shows an irregular flow in the superior extra-foveal area (red arrow). On the bottom left, corresponding to the visible segmentation on the B-scan, an automatic segmentation defect (blue arrow) is visible. This segmentation artifact produces a flow deriving from the choriocapillaris layer and not from the avascular one. A manual correction (central image - blue arrow) produces a real image without any artifact (on the right - red arrow).

approach in an OCT Angiography assessment might even seem counterproductive – is interesting when determining the actual morphological aspect of CNV, as well as the peripapillary neovessels of a diabetic retinopathy or the presence of irregular flows in the acute or chronic stage of Central Serous Chorioretinopathy, and so on5 (Figure 4). Currently, many studies using OCT Angiography are being assessed in an attempt to identify special morphological markers designed to precociously diagnose or detect specific pathologies. Although OCT Angiography has many advantages, there are also some disadvantages to be considered. The images obtained must be interpreted after the automatic segmentation of the device is carefully assessed. The presence of any possible artifacts that can lead to an incorrect diagnosis should always be taken into consideration6 (Figure 5). Artifacts deriving from segmentation are not the only errors that may occur. To this regard, the fact that the learning curve is not always that short should be taken into consideration. Currently, scans of the posterior pole, and the area right beyond the arcades, can be performed, even if wide-angle assessment methods are being studied. In the event of inflammatory retinalchoroidal pathologies, a vascular analysis does not always allow for a correct interpretation of the clinical picture. As for structural OCTs, a good transparency of the diopter media and a minimal fixation ability by the patient are required. In conclusion, we can say that the introduction of OCT Angiography has, by now, become part of daily clinical practice. The study of different pathologies through OCT Angiography is unveiling new information that will probably generate new interpretations and diagnoses in the future. As for any new method implemented, the data collected must be studied and interpreted, in order to reach a correct diagnosis.