At the completion of this CPD activity, optometrists will have developed their knowledge of optical coherence tomography angiography (OCT-A). Including:
- Recognise the limitations of OCT-A, including common artefacts, compared to fluorescein angiography
- Appreciate the core technical principles of OCT-A
- Understand the evolving clinical applications of OCT-A
NOTE: Optomery Australia members can enter their details at the bottom of this article to have it automatically added to their Learning Plan.
It has been almost 10 years since OCT-A was introduced commercially. For optometrists considering a ‘new’ technology to sink their teeth into, DR ANGELICA LY outlines the clinical utility of OCT-A and highlights its emerging role in the modern optometric practice.
Dr Angelica Ly
BOptom (Hons) GradCertOcTher PhD
Lead Clinician (Macula), Centre for Eye Health
Lecturer, UNSW School of Optometry and Vision Science
Eye disease impacting on retinal vasculature is increasing due to the aging population and the rise of systemic diseases such as diabetes. Consequently, the ability to non- invasively evaluate retinal vasculature helps primary eyecare providers to make informed decisions regarding the management of their patients.
Optical coherence tomography angiography (OCT-A) represents the latest innovation capable of providing remarkable, high-resolution, in vivo images of the ocular microvasculature without the need for intravenous dye.
In essence, the technology works via repeat scanning of the same transverse location. Subsequent scans are analysed for differences in signal amplitude, intensity, or phase variance – known as ‘decorrelation’ – which are subsequently presented as blood flow.
Applied hundreds of times in adjacent locations, an en face view can be generated and adjusted to project the decorrelation signals confined to specific layers (Figure 1).1-4
The aim of this article is to introduce readers to the usefulness of OCT-A via a specifically curated case series. It is not intended to ‘teach’ readers how to interpret OCT-A, but rather, to acquaint them with the possible applications, which may be helpful to those considering investing in the technology.
Prospective buyers’ guide to the technical principles of OCT-A
As a non-invasive imaging modality, the key advantage of OCT-A is that images can be acquired multiple times in single or different visits with minimal fuss. The device is often well-tolerated by patients, based on the same core technology as structural OCT, and provides direct visualisation of vessel morphology. On the other hand, and in contrast to fluorescein or indocyanine green angiography, OCT-A does not provide any information on vessel leakage or filling speed.
For practitioners thinking of purchasing a device, it’s important to point out that different manufacturers typically use different OCT-A proprietary algorithms. AngioVue of the RTVue XR Avanti by Optovue uses split-spectrum amplitude decorrelation angiography. AngioPlex OCT-A associated with the Cirrus HD-OCT 6000 by Zeiss uses optical microangiography. These varying algorithms can cause subtle variations in the output images, so it may be worth comparing or ‘trying before buying.’
Other technical points of difference include scan speed, scan area and resolution. These hardware parameters significantly affect acquisition time and so, they may limit the day-to-day usefulness of the purchased device, especially in uncooperative patients, older patients with media opacities or individuals with poor fixation/nystagmus.
Software differences, including the pre-set slab views, output quantitative metrics (‘analytics’), and the user interface, are also important considerations. At a minimum, the operator should be able to review the cross-sectional, structural B-scans in tandem with en face images.
Common sources of misinterpretation or unusable oct-a data include:
- Motion artefacts
- Projection artefacts
- Segmentation errors
Motion artefacts – Most OCT-A devices will have some form of in-built eye tracking or motion correction, but sensitivity varies, and, at times, these corrective features can themselves create new artefacts or contribute significantly to the scan acquisition time.
Projection artefacts – The impression of superficial vessels on to deeper layers, also represents a major limitation of OCT-A technology so some form of a ‘projection artefact removal’ feature is often included. It can be helpful to be able to toggle on or off this feature as required.
Segmentation errors – As described above, the pre-set slab views or automated segmentation boundaries are one consideration. However, the operator should also be able to manually adjust these boundaries by location, thickness, and shape e.g.: to align with the curvature of the RPE or to lay flat horizontally.
As with standard structural OCT technology, OCT-A segmentation errors are often unavoidable especially in cases of retinal pathology.
Manual segmentation correction options and the functionality of these tools are becoming increasingly sophisticated. Some devices provide the option of ‘auto-propagating’ changes made on one B-scan to adjacent slices.
Finally, other features including change analysis (and instrument differences in their ability to align and register follow-up scans against baseline, the presentation of these findings and associated analyses/reporting functions), an anterior segment or glaucoma module may also interest certain practitioners.
In short, OCT-A is a complex, rapidly evolving technology. In addition to reviewing the technical specifications of a brochure, prospective users are encouraged to ask device representatives to demonstrate the usefulness and ease of use of the associated OCT-A software before buying.
Case 1: Polypoidal choroidal vasculopathy
A 70-year-old Asian male with a known history of central serous chorioretinopathy in his left eye returned for a scheduled follow-up assessment. He reported a clear view on the Amsler grid and denied any new visual or ocular symptoms.
Entering unaided visual acuities were significantly reduced, especially in the left eye, at 6/9.5 right eye and 6/30 (improved to 6/15 with pinhole) left eye. The right retina appeared healthy. The left eye showed hypopigmentary abnormalities at the superior macula with associated subfoveal subretinal fluid. OCT-A showed choroidal neovascularisation (including a polypoidal lesion coincident with a notched pigment epithelial detachment and connecting branching vascular network) just above and temporal to the left macula (Figure 2).
Outcome
The patient was referred and seen promptly by a retinal specialist, who performed both fluorescein and indocyanine green angiography to confirm the presence of occult choroidal neovascularisation and associated polypoidal lesions. He was diagnosed with polypoidal choroidal vasculopathy and treated immediately via intravitreal therapy using Eylea and will be followed closely.
Key lesson
OCT-A in conjunction with structural OCT imaging is a useful screening tool for distinguishing proliferative from non-proliferative disease, including polypoidal features
Case 2: Abnormal retinal neovascularisation
A 48-year-old female was referred for further investigation of a lesion in her right eye, first noted and managed some 20 years ago by an ophthalmologist overseas. She reported systemic hypertension and was otherwise in good general health. She denied any new changes in her vision.
Unaided visual acuities were 6/9.5-2 (improved to 6/9.5 with pinhole) right eye and 6/7.5+2 left eye. Examination showed a right visually significant epiretinal membrane, likely longstanding in nature and secondary to former chorioretinitis or uveitis. There was associated fibrosis, foveoschisis and disruption of the subfoveal ellipsoid zone, explaining the reduced visual acuity. Abnormal retinal neovascularisation was also observed among the superotemporal midperiphery with no obvious signs of retinal vascular occlusion or vasculitis (Figure 3).
The left posterior eye was unremarkable.
Outcome
The patient was referred non-urgently to an ophthalmologist. The ophthalmologist reported that the cause of the abnormal retinal neovascularisation was deemed unclear; however, wide field Optos retinal angiography was performed to assess the full extent of retinal ischemia and treatment options including sectoral panretinal laser treatment with or without intraocular anti-VEGF injections were presented.
The option of epiretinal membrane peel surgery was also discussed with the patient on the presumption that surgery would be to protect her existing level of vision and prevent future, sight threatening vitreous haemorrhage, tractional retinal detachment or further worsening of the ERM.
Key lesson
OCT-A forms a useful screening tool for abnormal retinal vasculature but is currently limited by field of view.
Case 3: Sub-clinical disease
A 55-year-old asymptomatic female in good health, taking prescription medication for rosacea only, was referred for a macular assessment.
Entering unaided visual acuities were 6/7.5 right eye and 6/6 left eye. Amsler grid testing was unremarkable in both eyes and Mars contrast sensitivity was similarly within normal limits at 1.76 log units in each eye. Fundus examination showed focal RPE pigmentary abnormalities at the left inferotemporal macula with no associated drusen, and a corresponding flat irregular pigment epithelium detachment using structural OCT and underlying thick choroid. OCT-A at baseline did not show any convincing evidence of neovascularisation.
Outcome
The patient was followed regularly for the next two years. Follow up assessments showed the development and eventual resolution of associated subretinal fluid and possible macular neovascularisation using OCT-A. The patient will continue ongoing monitoring and have her images regularly reviewed virtually by an ophthalmologist.
Key lesson
OCT-A can be a helpful monitoring tool especially of ‘sub-clinical’ disease, such as non-exudative macular neovascularisation.
In this fast-paced technological world, eyecare professionals face ever-increasing pressure to apply the newest, ‘latest and greatest’ gadget to patient care. Clinical testing, including ocular imaging and OCT-A, can be used for disease diagnosis, screening, and monitoring.5 This brief case series highlights the value and emerging role of OCT-A in optometric practice.
Disclosures: The author has no conflicts to declare.
References
1. Tan ACS, Tan GS, Denniston AK et al. An overview of the clinical applications of optical coherence tomography angiography. Eye (Lond) 2018; 32: 262-286.
2. Spaide RF, Fujimoto JG, Waheed NK et al. Optical coherence tomography angiography. Prog Retin Eye Res 2018; 64: 1-55.
3. Spaide RF, Fujimoto JG, Waheed NK. Image Artifacts in Optical Coherence Tomography Angiography. Retina 2015; 35: 2163-2180.
4. 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; 7: 42201.
5. Power M, Fell G, Wright M. Principles for high-quality, high-value testing. Evid Based Med 2013; 18: 5-10.
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