Optical Coherence Tomography Angiography (OCTA) is a new and emerging technology that is exciting Ophthalmic Clinicians and imaging Scientists across the World. OCTA generates 3 dimensional, depth resolved images of blood flow in the retina by using motion contrast and without the need for invasive intravenous dye injection (fluorescein angiography).
Conventional Optical Coherence Tomography (OCT), developed in the early 1990’s, has revolutionised ophthalmic imaging, in much the same way as fluorescein angiography did when it was first used in 1959 to assess retinal blood circulation. OCT provides cross-sectional and 3D imaging of the retina and optic nerve head with micrometer-scale depth resolution1. Structural OCT allows clinicians to monitor subtle changes in the layers of the retina. It does not, however, show blood flow, leaking blood vessels or occluded blood vessels. Fluorescein angiography is required to delineate changes in retinal blood flow and aide diagnosis of many retinal conditions.
OCTA relies on rapid OCT scanning of the retina and compares repeated scans acquired at the same position, assessing change. Image acquisition is based on the fact that structures in the eye are static, apart from blood flow. And so it is the change in blood flow that is assessed with repeated OCT scans. Mapping these areas of blood flow by point to point comparison of two or more OCT volumetric cubes provides detailed images of the vasculature of the retina without injection of dye2. OCTA is an acquisition of retinal function AND structure in tandem. 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. An amplitude decorrelation (variation) algorithm is applied to the software that gives us the vascular image (figure 1a). Actual visualisation is very similar to visualising a fluorescein angiogram image (figure 1b).
OCTA provides depth resolved images that can be used to pinpoint very accurately pathology within the retinal layers. The retina is a complex structure. Microscopically, there are 10 layers in the retina. With the advent of high resolution OCT imaging, new layers have recently been discovered3. The main layers are pigment epithelium and light-sensitive cells (photoreceptor layer). These are followed by external limiting membrane, outer nuclear layer, outer plexiform (synaptic) layer, inner nuclear layer, inner plexiform layer, ganglion cell layer, nerve fibre layer and inner limiting membrane. OCTA allows you to literally scroll down through the retinal layers. This has never been possible in the past with conventional, in vivo, imaging. In addition, the OCTA image can be cross-registered with conventional structural OCT images to provide a direct comparison with structural and vascular images (figure 2).
As with all new (and still emerging technology) there are advantages, disadvantages and limitations. Advantages; It’s non-invasive and therefore a safer diagnostic procedure. Fluorescein angiography requires the intravenous injection of sodium fluorescein, a procedure not done without risk (anaphylaxis). OCTA is quick to perform (compared to a full ocular angiography study). It is highly repeatable with accurate results. Identification of pathology (and classification of disease) is very accurate. Correct diagnosis is essential when planning treatment. OCTA provides high resolution depth resolved imaging. It is an acquisition of structure and function (blood flow) imaged in tandem and co-registration of conventional OCT images with OCTA images give precise location of pathology.
Limitations; OCTA imaging is only possible with a small field of view (typically 15 degrees). Wide field imaging with fluorescein angiography is possible (up to 102 degrees). Peripheral imaging is therefore very difficult using OCTA. Peripheral imaging is essential in assessing diabetic retinopathy and other retinal vascular diseases. Haemorrhages are a common feature of many retinal diseases. If blood overlies that part of the retina that is being imaged with OCTA – the signal from the blood vessels and surrounding tissue is blocked. Eye movement during acquisition can degrade the image. It is essential to minimize any unwanted eye movement during image acquisition. Unwanted eye movement can be as a result of a head tremor or poor fixation of the eye being imaged (ie the patient is unable to fixate onto a target). Large blood vessels can cause ‘ghosting’ or shadow artefact, more so in the deeper retinal layers. Because multiple scans are required, there is an increase in time to perform the scan (40 – 60 secs compared with typically 20 seconds). This requires the patient to fixate for longer – remember, these patients already have diminished visual acuity. OCTA imaging is based on blood flow and this is detected above a minimum threshold. A fibrotic lesion or scar on the retina may exhibit slow blood flow and there would be no decorrelation signal and therefore no worthwhile image will be generated. OCTA imaging can not differentiate between arterial or venous circulation. Conventional fluorescein angiography can. This is important when assessing vascular changes. OCTA imaging is also unable to exhibit leak of dye.
Spaide et al4 compared OCTA with fluorescein angiography in 12 normal eyes. OCTA provided improved visualisation of all the vascular layers including the deep capillary network that were not well visualised on conventional angiography. In other published papers, OCTA imaging was comparable with FFA.
There is debate amongst clinicians as to whether OCTA will actually replace the gold standard procedure of fluorescein angiography to assess retinal vasculature. Intravenous Angiography is still relevant with a strong evidence base and can help guide OCTA position, correlating leak with structure. For now, OCTA is still an emerging technology but should be used as part of a multi-modal approach to ophthalmic imaging in assessing retinal structure and function.
Figure 1a. Visulisation of OCT Angiography in a patient with a choroidal neovascular membrane – typically seen in wet form age related macular degeneration. Field of view 15 degrees.
Figure 1b. Visulisation of fluorescein angiography in the same patient. Field of view 35 degrees.
Figure 2. Structural OCTA image of same patient. Note the neovascular membrane (arrow) deep within the retinal layers. This corresponds very accurately with image seen in figure 1a.
All images courtesy of Christopher Mody, Heidelberg Engineering.
References:
- Huang et al. Clinical OCT Angiography Atlas. Jaypee Brothers Press
- Waheed et al. www.retinal physician.com
- www.heidelbergengineering.com/international/2016/04/06/know-your-retinal-layers.
- JAMA Ophthalmol.2015 Jan;133(1):45-50