An overview of the clinical applications of optical coherence tomography angiography

Abstract

Optical coherence tomography angiography (OCTA) has emerged as a novel, non-invasive imaging modality that allows the detailed study of flow within the vascular structures of the eye. Compared to conventional dye angiography, OCTA can produce more detailed, higher resolution images of the vasculature without the added risk of dye injection. In our review, we discuss the advantages and disadvantages of this new technology in comparison to conventional dye angiography. We provide an overview of the current OCTA technology available, compare the various commercial OCTA machines technical specifications and discuss some future software improvements. An approach to the interpretation of OCTA images by correlating images to other multimodal imaging with attention to identifying potential artefacts will be outlined and may be useful to ophthalmologists, particularly those who are currently still unfamiliar with this new technology. This review is based on a search of peer-reviewed published papers relevant to OCTA according to our current knowledge, up to January 2017, available on the PubMed database. Currently, many of the published studies have focused on OCTA imaging of the retina, in particular, the use of OCTA in the diagnosis and management of common retinal diseases such as age-related macular degeneration and retinal vascular diseases. In addition, we describe clinical applications for OCTA imaging in inflammatory diseases, optic nerve diseases and anterior segment diseases. This review is based on both the current literature and the clinical experience of our individual authors, with an emphasis on the clinical applications of this imaging technology.

Figure 1

OCTA of a single normal eye showing variations in the scan area and algorithms. Cross-sectional OCTA images of the superficial vascular plexus segmentation (first row) and deep vascular plexus (third row). En face OCTA images of the superficial vascular plexus segmentation (second row) and deep vascular plexus (bottom row). (a) An 8 × 8 mm scan taken with the AngioVue (Fremont, CA, USA) RTVue XR Avanti processed with the SADA algorithm; (b) A 6 × 6 mm scan taken with the Angioplex (Dublin, CA, USA) CIRRUS HD-OCT Model 5000 processed with the optical microangiography algorithm; (c) A 3 × 3 mm scan taken with DRI-OCT Triton (Tokyo, Japan) swept source OCT processed with the OCTA-RA algorithm. On the automated segmentation of the deep vascular plexus of both the Angiovue and Angioplex, some projection artefact from the superficial layer is observed. A full color version of this figure is available at the Eye journal online.

Figure 2

A summary of an approach to OCTA interpretation.

Figure 3

Examples of common artefacts seen on OCTA. (a) Motion artefact seen by black vertical lines caused by blinking (yellow arrowhead) and eye movements (green arrowhead). (b) An example of a projection artefact (yellow boxes) of the superficial vessels seen in the deep vascular plexus segmentation. Comparing the deep vascular plexus segmentation, all the projection artefact seen can be accounted for by the more superficial vessels (green boxes). (c) Unmasking artefact seen as an area of high flow (middle of crosshairs) on en face OCTA (left), cross-sectional OCTA (middle) showed a focal area of atrophy with underlying hyper-transmission of the signal (yellow arrow). Area of high flow on en face OCTA can be accounted for by an area of atrophy causing the underlying choroidal vessels to be seen as an area of unmasking artefact. This is confirmed by the corresponding area of hypo-autofluorescence seen on fundus autofluorescence (right). (d) En face OCTA (left) corresponding to cross-sectional OCTA (middle top) showing a straight segmentation line that does not capture polyps (blue arrow) seen at the peak of the pigment epithelial detachment. Alternatively, when using the RPE fit segmentation the area of polyps (orange arrow) are then seen on en face OCTA

Figure 4

Multimodal images including OCTA images of the 3 subtypes of neovascular age-related macular degeneration. (CFP, colour fundus photo; FA, fundus fluorescein angiography; ICGA, indocyanine green angiography; OCTA, topical coherence tomography angiography; OCT, optical coherence tomography). (a) Type 1 neovascularisation (NV) (yellow arrows) with a vascularised pigment epithelial detachment seen on CFP, stippled hyperfluorescence and late leakage on FA, a plaque on ICGA and a vascular network seen on en face OCTA with a corresponding area of abnormal flow seen under the RPE on cross-sectional OCTA. (b) Type 2 NV (green arrows) with a greyish membrane seen on CFP, early lacy hyperfluorescence with late leakage seen on FA and a vascular network seen on en face OCTA with abnormal flow seen above the RPE on cross-sectional OCTA. (c) Type 3 NV (blue arrows) seen with associated atrophy on CFP, pinpoint leakage on FA with an area of abnormal flow on en face OCTA corresponding to a linear area of abnormal flow in the deep retina seen on cross-sectional OCTA seen below a large patch of geographic atrophy.

Figure 5

OCTA images showing different neovascularisation (NV) responses to treatment with intravitreal anti- vascular endothelial growth factor therapy (IVT). (a) En face OCTA (top row) shows after one IVT, there is reduction in the overall size of the type 2 NV with the regression of the smaller peripheral anastomosis leaving the larger calibre vessel trunks. After three IVTs the lesion size remains stable with the persistence of the larger calibre vessel trunks with a reduction in the dark halo surrounding the vascular lesion. Corresponding cross-sectional OCTA (second row) that show the reduction in the area of abnormal flow (red overlay) during the course of treatment. (b) En face OCTA with colour-coded density mapping showing the reduction in size of the type 1 NV (red) from baseline and after six IVTs with corresponding cross-sectional OCTAs showing a reduction in abnormal flow (red overlay) from baseline.

Figure 6

Multimodal images including OCTA images of mixed subtypes of neovascular age-related macular degeneration. (CFP, colour fundus photo; FAF, fundus autofluorescence; FA, fundus fluorescein angiography, ICGA=indocyanine green angiography; OCTA, topical coherence tomography angiography, OCT=optical coherence tomography). (a) Type 1 neovascularisation (NV) with polypoidal choroidal vasculopathy. Polyps (blue arrows) seen as orange nodules on CFP, focal leakage on FA, clusters of hypercyanescence on ICGA and a focal area of increased flow surrounded by a halo of decreased flow signal on en face OCTA with a corresponding area of abnormal flow directly under the RPE seen on cross-sectional OCTA. An associated branching vascular network or type 1 NV (orange arrows) seen as stippled hyperfluorescence on FA, a plaque on ICGA and a vascular network on OCTA corresponding to shallow, irregular pigment epithelial detachment containing abnormal flow seen on cross-sectional OCTA. (b) A mixed type 2 and type 1 NV with the subretinal type 2 component (yellow circles and arrow) and the subretinal pigment epithelial type 1 component (green circles and arrows).

Figure 7

OCTA features in diabetic retinopathy. (a) An eye with severe non-proliferative diabetic retinopathy with microaneurysms surrounding the fovea as seen on fluorescein angiography (left) and the corresponding 6x6 (middle) and 3 × 3 (right) en face OCTA of the superficial segmentation. An enlarged FAZ is also noted (yellow arrows). (b) An eye with diabetic macula oedema and an enlarged FAZ (green arrow) with disruption of the normal vasculature inferiorly as seen on en face OCTA with superficial segmentation (left) and corresponding cross-sectional OCTA middle (top) and similarly with deep segmentation (right and middle bottom). Both the cystic spaces from diabetic macula oedema and areas of non-perfusion are seen as dark areas on the deep segmentation en face OCTA. A full color version of this figure is available at the Eye journal online.

Figure 8

OCTA features of branch retinal vein occlusion. (a) Fundus fluorescein angiography (FA) showing an ischaemic branch retinal vein occlusion with neovascularisation and areas of capillary non-perfusion. (b) The areas of non-perfusion corresponding to the FA (yellow box) are seen clearly on en face OCTA. (c) An area of neovascularisation leaking on FA (green box) is seen on en face OCTA as a small vascular tuft of high flow growing into the posterior hyaloid as seen on cross-sectional OCTA. A full color version of this figure is available at the Eye journal online.

Figure 9

OCTA identifies neovascular membrane secondary to puntate inner chorioretinopathy (PIC). (a) En face OCTA shows an area of absent flow (yellow circle) on the choriocapillary segmentation seen to correspond with a hyper-reflective inflammatory lesion (yellow arrow) on cross-sectional OCTA with absent flow. (b) Another PIC lesion seen on colour fundus photo (top left), the corresponding en face OCTA shows a secondary choroidal neovascularisation (CNV) (blue circle), with the corresponding cross-sectional OCTA showing an area of abnormal flow (blue arrow) on the hyper-reflective inflammatory lesion. After 1 intravitreal anti- vascular endothelial growth factor therapy (IVT) (bottom row), there is regression of the CNV seen on both en face and cross-sectional OCTA (blue circle and arrow).

Figure 10

OCTA findings in a patient with non-arteritic ischaemic optic neuropathy and ipsilateral visual loss. The sectorial optic disc swelling (a), is associated on OCTA (AngioVue, Optovue) with tortuous radial peri-papillary capillaries and vascular dropout in the optic nerve head (b). Optic nerve head swelling in a patient with idiopathic intracranial hypertension and preserved visual function (c). Despite the severe optic disc swelling and peri-papillary haemorrhages (c), OCTA evaluation (AngioVue, Optovue), discloses only limited vascular dropout in the optic nerve head region (d and e). A full color version of this figure is available at the Eye journal online.

Figure 11

Optical coherence tomography angiography of the cornea. (a) Fungal keratitis with chronic inflammation and corneal vascularisation. (b) Optical coherence tomography angiography imaging may be useful to guide fine-needle diathermy and anti-VEGF therapy to reduce corneal vascularisation before corneal transplantation, and risk of corneal graft rejection. (c) Interstitial keratitis with deep corneal vascularisation. (d) Optical coherence tomography angiography reveals deeper vessels not obvious on slit-lamp microscopy. A full color version of this figure is available at the Eye journal online.