Optical Coherence Tomography (OCT)
Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that renders an in vivo cross sectional view of the retina. OCT utilizes a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. OCT was first introduced in 19911 and has found many uses outside of ophthalmology, where it has been used to image certain non-transparent tissues. Due to the transparency of the eye (i.e. the retina can be viewed through the pupil), OCT has gained wide popularity as an ophthalmic diagnostic tool.
Time Domain vs. Spectral Domain2:
From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices.
Spectral domain technology, on the other hand, scans approximately 20,000-40,000 scans per second. This increased scan rate and number diminishes the likelihood of motion artefact, enhances the resolution and decreases the chance of missing lesions. Whereas most time domain OCTs are accurate to 10-15 microns, newer spectral domain machines may approach 3 micron resolution. Whereas most time domain OCTs image 6 radial slices, spectral domain systems continuously image a 6mm area. This diminishes the chance of inadvertently missing pathology.
Uses:
OCT showing both macular oedema and sub retinal fluid in a diabetic patient
OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in diagnosing:
-
macular hole
-
macular pucker
-
vitreomacular traction
-
macular oedema
-
Detachments of the neurosensory retina and retinal pigment epithelium (e.g. central serous retinopathy or age-related macular degeneration)
In some cases, OCT alone may yield the diagnosis (e.g. macular hole). Yet, in other disorders, especially retinal vascular disorders, it may be helpful to order additional tests (e.g. fluorescein angiogram).
Optic neuropathies:
OCT is gaining increasing popularity when evaluating optic nerve disorders such as glaucoma. OCT can accurately and reproducibly evaluate the nerve fibre layer thickness.
Anterior segment:
Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris and angle) can be visualized.
Limitations:
Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT will be limited the setting of vitreous haemorrhage, dense cataract or corneal opacities.
As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. With newer machines (i.e. spectral domain), acquisition time is shorter which may result in fewer motion related artefacts.
The quality of the image is also dependant on the operator of the machine. Early models of OCT relied on the operator to accurately place the image over the desired pathology. When serial images were acquired over time (e.g. during treatment for AMD with anti-VEGF therapy), later images could be taken that were off axis compared to earlier images. Newer technologies, such as spectral domain acquisition or eye tracking equipment, limit the likelihood of acquisition error.