Kerala Journal of Ophthalmology

MAJOR REVIEW
Year
: 2016  |  Volume : 28  |  Issue : 3  |  Page : 158--163

Optical coherence tomography angiography


Remya Mareen Paulose1, Jay Chhablani2,  
1 Consultant, Vitreo Retina Services, Little Flower Hospital and Research Center, Angamaly, Kerala, India
2 Consultant, Vitreo-retina Services, L. V. Prasad Eye Institute, Hyderabad, Telangana, India

Correspondence Address:
Dr. Remya Mareen Paulose
Little Flower Hospital and Research Center, Angamaly, Kerala
India

Abstract

Optical coherence tomography angiography (OCTA) is a non-invasive angiography type without a contrast agent, which provides a detailed assessment of the retinal and choroidal vasculature and visualization of blood flow by detecting motions of erythrocytes, using serial optical coherence tomography B-scans. As compared to gold standard techniques such as fluorescein and indocyanin-green angiography, OCTA offers two major advantages: no dye is required and depth resolution is required is provided. As such OCTA has the potential to improve our abilities to diagnose and monitor ocular vascular diseases.



How to cite this article:
Paulose RM, Chhablani J. Optical coherence tomography angiography.Kerala J Ophthalmol 2016;28:158-163


How to cite this URL:
Paulose RM, Chhablani J. Optical coherence tomography angiography. Kerala J Ophthalmol [serial online] 2016 [cited 2019 Dec 13 ];28:158-163
Available from: http://www.kjophthal.com/text.asp?2016/28/3/158/205427


Full Text

 Introduction



Since the first demonstration in a biomedical application in 1991, optical coherence tomography (OCT)[1] has become an indispensable tool in routine ophthalmic practice for anatomical imaging and therapeutic monitoring. The latest spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT) machines use complex techniques to increase signal processing speed, resulting in faster scan acquisition and higher image resolution than is possible with older, time-domain OCT (TD-OCT) system. However, OCT is sensitive only to the backscattering light intensity and cannot distinguish vasculature from fibrous and other surrounding tissues. The search for functional extensions of OCT to provide contrast between small blood vessels and static tissue in addition to morphology led to the development of OCT angiography (OCTA).

OCTA exploits the specific image acquisition techniques of OCT scanners and by employing special image processing technology extracts details of retinal circulation. Thus, it helps the physicians to examine the retinal blood flow in a noninvasive way. The aim of this review is to provide some general information about OCTA including the algorithms used commercial devices with their specifications and the advantages and disadvantages of OCTA for retinal and choroidal vascular imaging.

 Technique of Optical Coherence Tomography Angiography



The various aspects of OCTA can be discussed under the following headings.

Optical coherence tomography systems

The OCTA is based on using moving blood cells within retinal vessels (motion contrast) to construct images of retinal microvasculature. In general, OCTA is based on the decorrelation of sequential B-scan signals from the same point of tissue scan to construct blood flow pattern creating the angiographic image. It therefore requires multiple OCT image acquisition at a very fast rate and additional motion correction to compensate for blur induced by saccadic eye movements. Conventional OCT device scanning speeds would result in too much trade-off between lower image quality and greatly increased scanning time. Due to the slow speed of TD systems and the challenge posed by eye motion, volumetric angiography was not feasible until the development of the two Fourier-domain OCT implementations:[2],[3],[4] SD-OCT [5],[6] and SS-OCT.[7],[8] The major features of these imaging systems are summarized in [Table 1]. With the continued improvement of OCT speeds due to hardware advances, methods for OCTA shifted from comparing adjacent A-scans to between sequential cross-sectional B-scans.{Table 1}

The SS-OCTA system has purported advantages of a better penetration of the deeper layers of the eye, a faster scanning speed (100 kHz A-scan rate), and wider field of view. It uses longer wavelength infrared light with less sensitivity roll-off with depth compared to conventional SD-OCT. Specific to the macula and disc scans, the SS-OCTA has a registration and tracking system allows for serial scans and monitoring. A team of international investigators determined that SS-OCTA was able to image significantly larger areas of choroidal neovascularization (CNV) more effectively than SD-OCTA.[9]

Technology overview

Image acquisition is based on the fact that structures in the eye are static, apart from blood flow. The platform for OCTA depends on high-definition OCT that provides rapid successive scanning of the retina at a given point of interest. By repeatedly capturing conventional OCT images, these changes in time allow the creation of an image contrast between perfused vessels and the surrounding tissue which does not have any intrinsic movement. Fourier transformation is applied to the raw data to obtain depth-resolved signals. After Fourier transformation, OCT signal contains both magnitude and phase information which have both been explored, either individually or combined, to develop angiographic methods for the purpose of contrasting the blood flow within living tissue.

The various angiographic approaches can be roughly categorized into three groups:

Angiography based on the phase of OCT signalAngiography based on the amplitude/intensity of OCT signalAngiography based on both the intensity and phase of OCT signal, i.e., complex signal.

Phase dependent (phase variance and Doppler variance)

The algorithms based on analyzing phase difference between OCT signals are phase variance [10] and Doppler variance [11] which was developed to detect small phase variations from microvascular flow. The major limitation was the artifacts introduced by phase noise in the OCT. Hence, these phase-based methods required very precise removal of background Doppler phase shifts which warranted the use of amplitude-based technology.

Intensity/amplitude dependent (speckle variance, split-spectrum amplitude-decorrelation angiography)

OCTA of retinal microvasculature in the human eye using methods based on amplitude or intensity was first demonstrated in 2012.[12] They used logarithmic intensity variance and differential logarithmic intensity variance to capture the microvascular network near the fovea. Speckle variance exploits the interference nature of OCT and denotes the variation of OCT reflectance amplitude over repeated B-scans at the same location. A key advantage of the speckle variance method is that it does not suffer from phase noise artifacts and does not require complex phase correction methods.[13],[14]

Another amplitude-based algorithm well suited to SS-OCT is correlation mapping where cross-correlation of a grid on adjacent B-scans was performed to identify vasculature (weak correlation) versus static tissue (strong correlation).[15] The decorrelation (1-correlation) depends on autocorrelation (comparison of a signal with itself) and cross-correlation (comparison at different time), the difference between these different signals as the variation with the time of the speckle pattern permit to detect the flow in the vessels. The split-spectrum amplitude-decorrelation angiography (SSADA) algorithm introduced by Jia et al., splits the OCT image into different spectral bands, thereby increasing the number of usable image frames and shortening the scan acquisition process. Compared with full-spectrum methods, SSADA improves the signal-to-noise ratio to provide clean and continuous imaging of the microvascular network with less noise inside the foveal avascular zone (FAZ).[16]

The OCT angiography ratio analysis is the latest addition based on SS-OCT and the intensity ratio calculation. Due to the use of SS-OCT, it is capable of delineating the choroid better.[17],[18]

Intensity and phase dependent/complex (optical microangiography)

Optical microangiography (OMAG) algorithm utilizes amplitude and phase OCT signal data to deliver the highest quality ultra-clear three-dimensional angiography images. OMAG has been shown to have ultra-high sensitivity to the microcirculation.[19],[20]

Zhang et al. compared the imaging performance of vascular networks using the various algorithms including OMAG, speckle variance, phase variance, SSADA, and correlation mapping. They concluded that OMAG yielded the best image contrast and vessel connectivity of the retinal vasculature.[21]

Commercial optical coherence tomography angiography machines [Table 2]{Table 2}

Currently, the different OCTA systems commercially available include:

ZEISS AngioPlex™Optovue AngioVue™ OCTA SystemTopcon Triton SS-OCT Angio™Heidelberg Spectralis ® OCTA.

 Optical Coherence Tomography Angiography Versus Conventional Angiography



Fluorescein angiography (FA) and indocyanine green angiography (ICGA) are the current “gold standards” for diagnosing vascular abnormality in clinical practice. Although the fluorescence of the injected dye improves the visualization of retinal capillaries, not all of the different layers of the retinal capillary network can be visualized using this bidimensional examination technique. Comparative findings suggest that the deeper capillary network in the retina is not visualized well by FA, possibly because of light scattering in the retina.[22] In addition, FA and ICGA do not offer precise view of neovascularization owing to the leakage of the dye along the walls of new blood vessels. The imaging of choriocapillaris and the neovascular vessels at avascular outer or deep retina mapping using OCTA is important, especially in cases of wet age-related macular degeneration (AMD) or polypoidal choroidal vasculopathy to detect abnormal core vessels.[23],[24],[25],[26],[27] The invasiveness of the dye injection in angiography combined with possible adverse reactions makes them unsuitable for widespread ophthalmic screening applications and frequent monitoring. Being a no injection, dye-free method for visualizing ocular vasculature OCTA is a promising tool for screening and long-term follow-up of retinal and choroidal vascular disorders [Figure 1]a,[Figure 1]b,[Figure 1]c.{Figure 1}

 Optical Coherence Tomography Angiography in Normal Eyes



Automatically obtained OCTA C-scan delineates the macular capillary bed clearly. This arcade can precisely delimitate the FAZ. The en face vascular image can include all the vessels seen throughout the retina or can be used to isolate the vessels in the inner retinal layers, the middle retina, and the outer retina. Using the depth-resolved data, retinal layer segmentation is performed to isolate specific retinal and choroidal layers. Thus, four en face zones are segmented, as shown in [Table 3].{Table 3}

 Artifacts in Optical Coherence Tomography Angiography



Although OCTA is a great imaging modality, interpretation of OCTA images should be done skeptically with regard to the possible artifacts. These artifacts can occur due to OCT image acquisition, intrinsic characteristics of the eye, eye motion, image processing, and display strategies. Flow projection artifacts are common, which arise from fluctuating shadows cast by flowing blood that result in variation of the OCT signal in deeper layers. This is particularly apparent in angiograms of the outer retina [Figure 2]. On the other hand, the lack of flow signal may be a result of shadowing and low OCT reflectance instead of true nonperfusion.{Figure 2}

 Clinical Aspects of Optical Coherence Tomography Angiography



OCTA provides depth-resolved information that can be used to isolate vascular structures in the different layers of the retina and to visualize them individually. This can be advantageous in a variety of retinal pathologies, such as exudative AMD, diabetic retinopathy (DR), and vascular occlusions as discussed below.

Optical coherence tomography angiography in wet age-related macular degeneration

One of the most useful clinical applications of OCTA may be the visualization and monitoring of CNV [Figure 3]. Palejwala et al.[28] reported the applicability of OCTA for early detection of CNV. In their series, they were able to detect early CNV (Type I), which was difficult to identify using conventional FA and SD-OCT. El Ameen et al.[29] characterized Type II CNV using OCTA. In their cohort of 14 patients, all demonstrated a hyperflow vascular lesion in the outer retina, with a glomerulus (4/14) or medusa shape (10/14), surrounded by a dark halo, proving that OCTA is highly sensitive in detecting CNV. There are reports [30],[31] where OCTA has been able to identify a distinct neovascular complex in retinal angiomatous proliferation (RAP) lesions. The neovascular complex of RAP appears as a small tuft of bright, high-flow tiny vessels with curvilinear morphology located in the outer retinal layers with a feeder vessel communicating with the inner retinal circulation.{Figure 3}

Another important clinical implication of OCTA is to observe the morphologic changes of CNV vessels over weeks after treatment with an intravitreal anti-vascular endothelial growth factor. Lumbroso et al. noted alternating regression and progression phases after intravitreal injection.[32] Twenty-four h after injection, there was a decrease in the dimensions of CNV with the loss of smaller vessels and narrowing of larger vessels. Between days 7 and 12, there was continued decrease in the size of CNV, whereas the central trunk remained unchanged. The maximum decrease in vessels was noted between days 13 and 18. Reproliferation was noted after day 28.

Optical coherence tomography angiography in diabetic retinopathy

In patients with DR, OCTA demonstrates retinal alterations including capillary dropout in the superficial and deep plexuses, FAZ enlargement, and microaneurysms [Figure 4]. The ability to separately examine the superficial and deep capillary plexuses with OCTA helps users to delineate retinal involvement in various diabetic lesions. Widening of the FAZ is best seen in the superficial plexus, whereas capillary dropout and microaneurysms are best appreciated in the deep plexus.[33] However, microaneurysms are visible on OCTA only in the presence of intravascular flow; therefore, those with slow flow or thrombosis will remain undetected. The detection of preretinal and prepapillary neovascularization is also facilitated with OCTA as these new vessels are not blurred by leakage in dye-based angiography.{Figure 4}

Optical coherence tomography angiography of vascular occlusion

OCTA demonstrates the features of both retinal artery and venous occlusion sufficiently to establish the diagnosis. In vascular occlusions, capillary telangiectasias, collateral vessels, microaneurysm, capillary nonperfusion, and the borders of ischemic retina are well delineated using OCTA.[34]

Optical coherence tomography angiography in macular telangiectasia

In macular telangiectasia (MacTel) 2, OCTA images are more revealing than FA as it clearly shows vascular rarefication or dilation, telangiectasia, neovascularization, and decreased capillary density more prominently in the deep retinal capillary plexus [Figure 5].[35]{Figure 5}

 Conclusion



OCTA is an important improvement over classic dye-based angiography as it enables better identification of nonperfused zones, capillary dropout areas, vascular dilatations, and neovascularization. Apart from the clinical imaging of chorioretinal pathology, it helps us to enhance our understanding of the pathogenesis and evolution of retinal disease. Being a noninvasive technology OCTA could be used for high-volume applications such as the routine screening of DR and regular follow-up of AMD. With the technological improvements that can be foreseen in the near future, we believe OCTA will become an important part of standard eye care.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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