OCT & OCT ANGIO

What are OCT & OCT Angiography?

Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that produces an in vivo cross-sectional view of the retina. OCT uses a principle known as interferometry to create a cross-sectional map of the retina accurate to within 10–15 microns. First introduced in 1991, OCT has found many uses beyond ophthalmology, where it is used to image certain non-transparent tissues. Because the eye is transparent — the retina can be viewed directly through the pupil — OCT has become a hugely popular ophthalmic diagnostic tool.

Retina

OCT is valuable in diagnosing many retinal conditions, particularly when the ocular media are clear. It can be especially helpful in diagnosing the following pathologies:

• Macular hole

• Macular pucker/epiretinal membrane

• Vitreomacular traction

• Macular oedema and exudates

• Detachments of the neurosensory retina

• Detachments of the retinal pigment epithelium (e.g., central serous chorioretinopathy or age-related macular degeneration)

• Retinoschisis

• Pachychoroid

• Choroidal tumours

In some cases, OCT alone can provide the diagnosis (as with a macular hole). In other disorders, particularly retinal vascular disease, additional tests may be helpful (such as Fluorescein Angiography or Indocyanine Green Angiography).

Optic nerve

OCT is increasingly popular in the assessment of optic nerve disorders, as it measures the thickness of the retinal nerve fibre layer and ganglion cell layer accurately and reproducibly, aiding the evaluation of:

• Glaucoma

• Optic neuritis

• Non-glaucomatous optic neuropathies

• Alzheimer’s disease

The anterior segment

Anterior segment OCT uses light of a longer wavelength than traditional posterior segment OCT. This longer wavelength is absorbed more and penetrates less, allowing the structures of the anterior segment (the cornea, anterior chamber, iris and angle) to be visualised.

Limitations

Because OCT uses light waves (unlike ultrasound, which uses sound waves), opacities in the ocular media can interfere with optimal imaging. OCT is therefore of limited value where there is vitreous haemorrhage, dense cataract or corneal opacity.

As with most diagnostic tests, patient cooperation is essential, since movement can degrade the quality of the image. Newer machines have shorter acquisition times, which helps reduce motion-related artefacts.

Optical Coherence Tomography Angiography (OCTA)

Optical coherence tomography angiography (OCTA) is a functional extension of OCT that provides information about the retinal and choroidal circulations without the need for dye injections. With the recent development of high-speed OCT systems and efficient algorithms, OCTA has become clinically practical.

The key difference between the two is that OCT images anatomical structure, whereas OCTA images vascular structure.

Motion detection

OCTA identifies blood vessels by detecting the changes in the OCT reflectance signal caused by blood flow. Flowing red blood cells produce more variation in the OCT signal between repeated scans than static tissue does. Several approaches have been described for quantifying this change, based on the intensity, the phase, or both the intensity and phase of the OCT signal. Studies have also shown that the resulting OCTA signal relates to the speed of the flowing red blood cells: faster flow produces a stronger flow signal, up to a saturation limit.

Athens EyeCare Clinic offers the most modern OCT and OCT Angio equipment available: the Solix OCT from Optovue, the long-standing leader in OCT innovation and the company that invented OCT Angiography.