OCT Flashcards
List the retinal layers from innermost to outermost
Inner Limiting Membrane
Nerve Fibre Layer
Ganglion Cell Layer
Inner Plexiform Layer
Inner Nuclear Layer
Outer Plexiform Layer
Outer Nuclear Layer
Outer Limiting Membrane
Inner Photoreceptor Segments
Outer Photoreceptor Segments
RPE
Choroid
How does OCT work?
Interferometry:
IR light is sent towards a beamsplitter which sends some light to a mirror (at a known distance) and some towards the retina.
The delay in light returning to the detector from the retina allows an image to be build up via interference patterns when compared with the light returning from the mirror.
A longer delay indicates further depth, and longer wavelengths can be used to penetrate deeper.
What do bright bands in an OCT image represent?
Layers with high reflectivity (RPE, NFL, plexiform layers)
What do dark bands in an OCT image represent?
Layers with low reflectivity (cell bodies, blood vessels)
What is an A-scan?
Vertical, 1D scan
What is a B-scan?
Multiple A-scans of varying depths combined, allowing for a 3D image to be created
How many A-scans are in a B-scan?
512-1024
What was the first type of OCT? What was its light source peak and how many A-scans per sec could it do?
Time-domain (TD)
Light source peak: 830nm
A-scans per sec: 400
What was the second type of OCT? What was its light source peak and how many A-scans per sec could it do?
Spectral-domain (SD)
Light source peak: 830nm
A-scans per sec: 20,000-40,000
What additional features does an SD-OCT have over a TD OCT?
Takes fundus photo
Takes multiple B scans in one area and averages them giving better resolution and reduced noise, motion artefact and chance of missing lesions
What is the most modern type of OCT? What is its light source peak and how many A-scans per sec could it do?
Swept-source (SS)
Light source peak: 1050-1060nm
A-scans per sec: 100,000-400,000
What advantages does an SS-OCT have over the older versions?
Increased light source peak allows for deeper tissue penetration
What are the disadvantages of TD OCT?
Time-consuming
Only produced cross-sectional images
What are the limitations of OCT?
Requires patient cooperation: no blinks or movement
Opacities can interfere with images
Operator error can result in unfocused scans
What are the advantages of OCT?
Non-contact
Relatively easy for patient
Allows easy and instant imaging of cornea, retina, choroid, optic nerve head and anterior chamber angle
Can monitor over time
High repeatability