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 Table of Contents  
REVIEW ARTICLE
Year : 2017  |  Volume : 55  |  Issue : 2  |  Page : 107-112

Optical coherence tomography angiography: A general view


Uma Eye Clinic, Chennai, Tamil Nadu, India

Date of Web Publication26-Dec-2017

Correspondence Address:
Dr. Arulmozhi Varman
Uma Eye Clinic, No 182, O Block, 2nd Avenue, Anna Nagar, Chennai - 600 040, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/tjosr.tjosr_21_17

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  Abstract 


Optical coherence tomography angiography (OCTA) is a new, non-invasive imaging technique that generates angiography images in a matter of seconds. Since it detects vascular compromise even before clinical picture sets in , it is a very useful tool in the diagnosis of retinal vascular pathology.In this review we introduce the technology and compare it with the current angiographic gold standards, fluorescein angiography (FA) and indocyanine green angiography (ICGA). Finally we summarize its potential application to retinal vascular diseases..Its current limitations include a relatively small field of view, inability to show leakage & motion artefacts.Published studies hint at OCTA's potential efficacy in the evaluation of common ophthalmologic diseases such as age related macular degeneration (AMD), diabetic retinopathy, artery and vein occlusions, and glaucoma.

Keywords: Age-related macular degeneration, diabetic retinopathy, fundus fluorescein angiography, optical coherence tomography angiography, retinal vascular disorders


How to cite this article:
Varman A, Muralidharan R, Balakumar D. Optical coherence tomography angiography: A general view. TNOA J Ophthalmic Sci Res 2017;55:107-12

How to cite this URL:
Varman A, Muralidharan R, Balakumar D. Optical coherence tomography angiography: A general view. TNOA J Ophthalmic Sci Res [serial online] 2017 [cited 2018 May 22];55:107-12. Available from: http://www.tnoajosr.com/text.asp?2017/55/2/107/221449




  Introduction Top


Optical coherence tomography angiography (OCTA) is an imaging modality for the retinal vasculature based on noninvasive angiography without a contrast agent. It provides a detailed assessment of the retinal and choroidal vasculature. It is based on the principle as follows:

  1. Mobility of erythrocytes within retinal vasculature and
  2. Serial B-scans which visualize blood flow within a vascular column.[1],[2],[3]


OCTA is a useful analytical modality for imaging the microvasculature in central macular diseases including diabetic maculopathy, idiopathic juxtafoveal telangiectasias, retinal vascular occlusive disorders, macular edema, and age-related macular degeneration (ARMD). Apart from this, OCTA has also emerged as a successful modality in the diagnosis of optic nerve head disorders.[4],[5],[6],[7],[8],[9],[10],[11]

In addition, OCTA has been of beneficial value in disorders such as polypoidal choroidal vasculopathy, sickle-cell retinopathy, and central serous retinopathy.[12],[13],[14],[15]

Here, in this review article, we are highlighting the general information about OCT angiographic techniques, protocols followed for screening, the benefits and pitfalls of retinal vasculature, and choroidal vasculature imaging [Table 1]. The latest technological advances provide a noninvasive 3D mapping of the circulatory vessels at the deeper retinal levels and choroidal level.[16]
Table 1: Comparison between Fundus fluorescein angiography and OCT angiography

Click here to view



  Principles of Optical Coherence Tomography Angiography Top


The prototype modality of imaging is the RTvue XR Avanti by Optovue. The image acquisition ranges to 70,000 A-scans per se cond with a tissue resolution of 5 mm axially and a beam width of 15 mm.

  1. The algorithm followed for the detection of blood flow within the vascular columns is the comparison between serial consecutive B-scans using the property of contrast of motion. This is the split-spectrum amplitude-decorrelation angiography (SSADA).[16],[17],[18],[19]


  2. The mobility of erythrocytes and moving blood within the vascular columns result in a variation of reflectivity over successive scans resulting in a high decorrelation between the various time frames and images obtained. The elimination of the sequential decorrelated frames of images and immobile outliers from the frame provides better resolution and image clarity, as the tissue motion artifacts are eliminated.

    In addition to this, the spectrum of the source of illumination is divided into 4 components, in an attempt to eliminate the noise present in the image, and the decorrelation step is assessed individually by each of the components, respectively.

    This SSADA algorithm results in lesser noise and high transverse resolution.

  3. The highly organized retina is a structure that has got segmental vascular supply and retinal segmentation for various layers [Figure 1]. The complex data analysis performed on the images acquired processes the vessel density and index of flow separately for 4 layers of the retinal choroidal complex. These 4 areas of in-depth scanning include [Figure 2]:
  4. Figure 1: The location of different en face zones in relation to histology of the human retina. The four en face zones include (i) the superficial plexus, the capillary network in ganglion cell layer and nerve fiber layer,(ii) the deep plexus, a network of capillaries in the inner plexiform layer with offshoot of 55 μ, (iii) the outer retina (photoreceptors), and (iv) the choriocapillaris (choroid) with offshoot of 30 μ

    Click here to view
    Figure 2: Optical coherence tomography angiogram fields of view and segmentation layers on AngioVue

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    1. The vasculature in the ganglion cell layer which comprises the superficial plexus
    2. The network of blood vessels in between the outer boundary of the inner plexiform layer and the central zone of the outer plexiform layer
    3. The layer of photoreceptors which is relatively avascular
    4. A 30 micron offshoot for the vasculature of the choroid.


Along with this, the installed software processing provides detailed information regarding the indices of perfusion in the following areas [Figure 2]:

  1. The central 3 mm of the parafoveal zone
  2. The surrounding 3–6 mm of the perifoveal zone
  3. The foveal avascular zone is automatically calculated by an exclusion analysis from the measurements of perfusion index.


Although the OCTA is a reliable tool for both imaging and diagnosis of various retinal pathologies, only little is known about the normative database for different age groups.

However, Matsunaga et al. demonstrated that the OCTA provides detailed high-resolution images obtained from healthy individuals comparable to the routine fundus fluorescein angiography (FFA) techniques.[20]


  Optical Coherence Tomography Angiography in Diabetics Top


OCTA has been identified as a valuable tool in the diagnosis of early and advanced changes in diabetic retinopathy (DR), one of the most common conditions known to cause retinal vasculopathy and visual morbidity.[21]

Ishibazawa et al. have demonstrated the value of OCTA in the management of DR by clearly demarcating the areas of capillary nonperfusion and microaneurysms. The OCTA also provides detailed information regarding the layers of the retinal capillary plexus [Figure 3] and [Figure 4]. This is important in the micro-evaluation of the status of the retinal vasculature and also for prognostication.[6]
Figure 3: Image of a patient with mild nonproliferative diabetic retinopathy and diabetic macular edema. Image on the left showing the en face image of the superficial plexus with distortion of foveal avascular zone. Image on the right showing the en face image of the deep plexus with distortion of foveal avascular zone, more profound than in superficial plexus. This happens due do accumulation of fluid in retinal layers

Click here to view
Figure 4: Optical coherence tomography angiography of microaneurysms in nonproliferative diabetic retinopathy. A patient with nonproliferative diabetic retinopathy imaged using the optical coherence tomography angiography. Aneurysms are circled in yellow. Foveal avascular zone appears enlarged

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There are variations in the region of the foveal avascular zone (FAZ) as studied by Takase et al. where they concluded that diabetic eyes demonstrated a larger region of FAZ as compared to eyes of normal healthy individuals irrespective of the presence of DR.[22]

A study performed by de Carlo et al. concluded that foveal microvascular changes which were not identified clinically were better delineated in the imaging modality pertaining to OCTA. They concluded that the rate of FAZ remodeling was as high as 86% as compared to the normal eye study (11%) and also the areas of capillary nonperfusion were demonstrated better with OCTA in diabetic eyes.[23]


  Optical Coherence Tomography Angiography in Age-Related Macular Degeneration Top


ARMD is the most common cause of defective vision in individuals in the sixth and seventh decades of life in the western countries.[24] Depending on the morphology and clinical characteristics, ARMD is broadly classified into dry ARMD and wet ARMD.

Wet ARMD is further classified into:

  1. Type 1 is a choroidal neovascular membrane seen in between the RPE and the Bruch's membrane
  2. Type 2 is a choroidal neovascular membrane extending in between the RPE and the neurosensory retina above
  3. Type 3 is a rare entity called retinal angiomatous proliferans (RAP).[25]


A study conducted by Palejwala et al.[26] demonstrated the early detection of choroidal neovascular membrane (CNVM) with the help of OCT angiographic techniques. They concluded that OCTA is a better modality of investigation as compared to spectral domain-OCT or FFA for the detection of type 1 CNVM [Figure 5].
Figure 5: Optical coherence tomography angiography of a patient with choroidal neovascular. The optical coherence tomography angiography image of superficial plexus, with associated vessel changes (left). “Medusa head” appearance of neovascular membrane in the photoreceptor zone (white arrows) (middle). Neovascular membrane extends in the deeper choroid (red arrows) with extensive arborization (right)

Click here to view


Another study conducted by El Ameen et al.[27] helped in the classification of type 2 CNVM in a sample of 14 patients. All the individuals showed a vascular lesion with an increased flow in the outer retinal layers, where 4 out of the 14 patients showed a glomerulus configuration and the rest showed a medusa-shaped lesion which in turn was surrounded by a dark halo, thereby demonstrating that OCTA is a better modality of investigation for the detection of CNVM.

Other studies conducted proved that OCTA is the modality of choice for the identification of complex vascular anastomotic vessels within RAP lesions. This complex appears as a group of tiny vessels with increased blood flow located in the outer retinal layers with a communicating vessel with the inner retinal complex.[28],[29]

A study conducted by Jia et al. concluded that the OCTA is a valuable tool for providing detailed information regarding the rate of blood flow and the extent of CNVM.[2]

Moult et al. studied the variations of choriocapillary networks around a CNVM.[30]

de Carlo et al. concluded that the specificity of the detection of a CNVM with the help of OCTA modality of imaging was as high as 91% as compared to the conventional FFA, whereas the sensitivity was only about 50%.

Currently, intravitreal injection of anti-vascular endothelial growth factor (VEGF) is the mainstay of therapy for the management of CNVM in wet ARMD.[31] A study conducted by Lumbroso et al. demonstrated various phases of the CNVM during the period of treatment with intravitreal anti-VEGF therapy. A considerable decrease in dimension of the CNVM with disappearance of small caliber vessels and attenuation of large caliber vessels was noted as early as 24 h following intravitreal anti-VEGF. The maximum amount of regression was observed in between 13 and 18 days and reproliferation was noted at the end of 4 weeks.[32]


  Optical Coherence Tomography Angiography in Retinal Vessel Occlusion Top


OCTA has also emerged as a useful tool in the imaging and prognostication of visual impairment that has occurred as a result of retinal vascular occlusion [Figure 6]. Poor vascular perfusion and ischemic changes in the retina occur as a result of thrombosis of the retinal veins and it is well known that this is the most common cause of central/branch retinal venous occlusion.[33]
Figure 6: A case of an inferior hemi central vein occlusion of the left eye demonstrating areas of capillary nonperfusion at superficial retinal layer, deeper retinal and choroidal layers. Neovascular membrane extends in the deeper choroid (red arrows) with extensive arborization (right)

Click here to view


In a study conducted by Kashani et al., 26 eyes with RVO were studied and it was concluded that OCTA demonstrated equal efficacy clinically and anatomically in the demonstration of edema, intraretinal leakage, areas of poor perfusion, dilatation and tortuosity of vessels as well as retinal atrophy, as compared to the conventional FFA techniques.[34]

A study conducted by Bonini Filho et al.[35] concluded that microvessels of the retinal vasculature can be studied in detail in individuals with nonarteritic retinal artery occlusive disorders and OCTA is beneficial in clearly delineating the retinal vascular plexuses at various levels.


  Optical Coherence Tomography Angiography in Central Serous Chorioretinopathy Top


Central serous retinopathy (CSR) is an idiopathic well-demarcated area of serous retinal detachment, most often restricted to the central macular region due to increased permeability of serous fluid through the retinal pigment epithelium (RPE) [Figure 7].[36] CSR primarily occurs in healthy individuals between 25 and 55 years.[37]
Figure 7: Optical coherence tomography angiography image of a case of central serous retinopathy demonstrating an area of apparent reduction in the blood flow at the level of choriocapillaris

Click here to view


The areas of leakage, along with the pooling of subretinal fluid, RPE defects, and the leakage of dye through the choroid have been most widely demonstrated by conventional FFA.

A study conducted by Bonino et al. concluded that OCTA has better sensitivity and specificity in the diagnosis of an underlying CNV in individuals with chronic or recurrent CSR.[14]


  Optical Coherence Tomography Angiography in Idiopathic Juxtafoveolar Retinal Telangiectasis Top


Neurodegenerative changes occurring in the macular area result in a central disorder called macular telangiectasia type 2 or MacTel2, along with which there is a significant loss of Muller cells.[38],[39],[40] The neuroretinal changes that occur in the region affected include inner retinal cyst formation and whitening, crystalline deposition in the retinal nerve fiber layer, outer retinal cavitation, destruction of the external limiting membrane, retinal pigment epithelial proliferation, and occasionally a subretinal neovascularization.[41]

A study conducted by Zeimer et al.[42] noted that the pathology affecting the retinal vasculature was located in the deeper capillary networks and these included widening of intervascular spaces and dilatation of vasculature, reduction in the capillary vasculature density, and an establishment of anastomotic vessels toward superficial vascular plexus. Contraction of the surrounding retinal vasculature was invariably associated with RPE proliferation.

OCTA findings and FFA findings were comparable in demonstrating that a deeper capillary plexus of vessels were abnormal in the initial stages of the disease and with time, changes appeared in the superficial plexus of vessels, finally resulting in abnormal vasculature in the outer retinal layers and a pattern which was irregular in the underlying deeper choriocapillaris [Figure 8].
Figure 8: Optical coherence tomography angiography of a case of idiopathic juxtafoveal telangiectasia demonstrating enlargement of vessels and larger intervascular spaces, dilated, dendritic appearance of vessels and the presence of anastomoses toward the superficial capillary network

Click here to view


Limitations of optical coherence tomography angiography

Although OCTA has been a breakthrough in complex vascular imaging, it has the following limitations.

  1. The screening of the area of interest is limited to a maximum of 8 mm × 8 mm, thereby limiting the field of view
  2. Since OCTA is based on the principle of relative motility within the vascular columns, there remains a high possibility of motion artifacts, which appears as white lines when the fixation of the patient is lost and as black lines when the patient blinks where no movement is detected.



  Summary Top


OCTA has the following advantages as compared to conventional imaging techniques for retinal vascular disorders:

  1. Detects vascular compromise even before clinical picture sets in
  2. Decreases disease morbidity by early identification and management
  3. Noninvasive technique
  4. Rapid and better delineation of structures.


What's next?

Along with the usage of this complex technique for detailed imaging of retinal and choroidal vasculature; it is also beneficial in detection of decrease in optic nerve head perfusion even before the clinical disease sets in. Apart from this, a provision to track the relative eye movements that can correct the subtle eye movements provides more reproducible means of determining retinal perfusion by correcting the motion artifacts. Sooner, the increase in relative velocity of scanning would provide images with better resolution and large field of view which is currently restricted to a screening area of 8 mm. Usage of monochromatic source of light of higher wavelength allows better penetration and provides detailed information of deeper structures.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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