Intro to OCT Flashcards
what does OCT stand for
optical coherence tomography
what is OCT
a non-invasive imaging technique that allows very detailed information to be acquired about living structures within the eye
what are the advantages of OCT
non-contact, fast, reliable, usually does not require mydriasis, using non-ionising radiation, aids recognition and monitoring of very subtle changes to eye structures.
what is the difference between OCT and ultrasound
OCT uses reflective sound rather than reflective light
what is an A-scan
produces a 1D image of the eye , used to measure axial length
what is a B-scan
constructed from multiple A-scans taken across various planes
how does OCT work
uses low coherence infrared light. light split into two beams, one beam goes to a reference mirror, the other to the tissue being imaged, back-scattered light from both surfaces combine, interference occurs and an image generated.
what are the two types of OCT scanners
time domain OCT and spectral (Fourier) domain OCT
what is time domain OCT
reference mirror is moved to detect reflection from various layers one depth at a time to produce axial A-scan.
what are the disadvantages
Inefficient as depends on how fast the mirror can move
what is spectral (Fourier) domain
the reference mirror is kept stationary and reflections from all layers/depths are detected simultaneously as spectrum to produce an A-scan
what are the advantages of Spectral (fourier) domain
faster and provides higher resolution with fewer motion artefacts
what structure allows all layers of the eye to be identified
spectrometer
how can you create a 3D image
combining multiple B-scans
what is the resolution that OCT works at
3-10um
what is the resolution of MRI
1um
What is the resolution of CT
500um
what is the resolution of ultrasound
150um
what orientation does an OCT scan usually show the vitreous
superiorly
how is the foveal identified
by the depression and reduced retinal thickness
what does the seudo colour light show
the backscattered light
what happens in OCT
changes in optical density are identified
what do bright colours or light grey correspond to
features with highly reflectivity
what are examples of highly reflective structures
retinal nerve fibre layer, plexiform layers, retinal pigment epithelium and choroid.
adjacent structures with large differences in RI too.
what do dark colours and blue represent
little or no reflectivity
what is the photoreceptor integrity line
boundary of the inner segment and outer segment
what are the reasons for poor quality OCT
blinking, poor fixation or nystagmus, media opacities and dry eye
what colour does the vitreous usually appear
black
what could you find on the vitreous
floaters or clouding
what does PVD stand for
posterior vitreous detachment
what is asteroid hylosis
accumulation of calcium or lipids
what are we looking for when scanning the retina
areas of hyper or hypo-reflectivity , intra or sub-retina fluid, specific layers that may be involved, the thickness and distribution of individual layers
true or false, retinal blood vessels block the reflections from the RPE and choriocapillaris
true
