Optical Coherence Tomography
Optical coherence tomography is a non-contact imaging method that produces cross-sectional images of the retina, optic nerve head and anterior segment, with a resolution of 3 to 7 micrometres. It works like an ultrasound scan, but it uses near infrared light, and measures the time delay and strength of light reflected from each layer of tissue. It is the most widely used imaging test in eye clinics.
Current instruments use spectral domain or swept source technology, and capture 20,000 to 400,000 depth scans per second, so that a scan of the macula takes 1 to 5 seconds. Each scan builds a map of retinal thickness and shows each retinal layer, the vitreoretinal interface, and the choroid (with enhanced depth or swept source methods).
The optic nerve scan measures the thickness of the retinal nerve fibre layer, the neuroretinal rim, and the cup, and the macular scan measures the ganglion cell layer. Optical coherence tomography angiography uses repeated scans to show blood flow, and gives images of the retinal and choroidal vessels without dye. Anterior segment instruments image the cornea, anterior chamber angle and lens.
The test takes 5 to 10 minutes. It does not need dilation in most patients, although dilation improves image quality when the pupil is small. It has changed the diagnosis and management of macular disease, and it detects macular oedema, fluid, and traction that are invisible on examination. For glaucoma, the nerve fibre layer measurement detects loss of tissue years before visual field loss is found, with a sensitivity and specificity of about 80 to 90%.
Indications.
- Macular diseases: macular oedema (diabetic, vein occlusion, post-surgical), age related macular degeneration, macular hole, epiretinal membrane, vitreomacular traction, central serous chorioretinopathy, and macular telangiectasia.
- Diagnosis and monitoring of neovascular macular degeneration and diabetic macular oedema during injection treatment.
- Glaucoma: diagnosis, and progression of thinning of the nerve fibre layer, rim and ganglion cell layer.
- Optic nerve disease: papilloedema (to separate from pseudopapilloedema), optic neuritis, ischaemic neuropathy, and compressive lesions.
- Inherited retinal disease and drug toxicity (for example, hydroxychloroquine).
- Corneal and anterior segment disease: corneal thickness and layers, keratoconus, angle closure, and after surgery.
- Retinal detachment, vitreous disease, and retinal tumours.
- Neurological disease: multiple sclerosis and neurodegenerative disease (research and selected clinical use).
Not indicated in place of an eye examination, and not useful when the media are too opaque to take a scan.
Contraindications and precautions.
- No contraindications. The test uses low power infrared light and is safe in pregnancy and in children.
- The patient needs to hold the head on the rest and fixate on a target for a few seconds, so very poor fixation, tremor, severe photophobia, nystagmus, or learning difficulty make scans difficult.
- Dense cataract, corneal opacity, vitreous haemorrhage, and small pupils reduce signal strength.
- Dilation is used when needed, with its usual precautions (narrow angles).
- Patients with a very high refractive error or severe scoliosis may need adjustment of the instrument.
- Bedside scanning is possible with hand held devices for infants and bedbound patients.
Preparation and technique.
The patient is seated with the chin and forehead against the rest. The operator selects the scan type (macular volume, optic disc, retinal nerve fibre layer, or anterior segment), and the patient looks at a fixation target while the instrument aligns with the pupil. A real time image is checked, and the scan is repeated if the signal strength is low or the patient blinks. Both eyes are scanned.
The instrument compares the thickness measures with an age matched normal database and flags values that are outside the normal range. The scans are reviewed by the clinician, with the raw images checked for errors in segmentation (layer detection) before measurements are used.
