Unit 3 - Retinal imaging Flashcards
What is an OCT?
- Non-contact
- Topographical
- Biomicoscopic device producing High res, cross-sectional digital images in vivo
What are top 3 most reflective layers on OCT?
- RNFL
- Ellipsoid zone (junction between inner segment and outer segments)
- RPE/Bruchs
What spacial resolution does OCT give?
as good as 2um
Current instruments use which wavelenght of what type of light?
IR light of about 840nm
What absorbs light between 200-600nm?
haemoglobin
What absorbs light above 1000nm?
Water in tissue
What does dopper OCT measure?
Velocity of blood flow in eye
What spacing are the ETDRS grids?
1,3,6mm
What is the average retinal thickness?
250um
Which people tend to have thinner retinas?
- Myopes
- Older patients
- Women
- African patients
What percentage of OCT scans have artefacts?
90%
What percentage of OCT scans have clinically significant artefacts?
5-8%
How many A-scans per second are taken in time domain, spectral and swept source OCTs?
400
15000- 40k
100k
What is the axial resolution in time domain, spectral and swept source OCTs?
10mics
6-7mics
3mics
What drug may cause IRF?
Latanoprost
What do vitelliform lesions look like on OCT?
Yellow, round elevated lesions below neurosensory retina
What do pseudcysts look like on OCT and what causes them?
Round or oval cystic spaces
Represent a loss of muller cells and may be a precursor to a macula hole
What do tubulations look like on OCT?
round/oval lesions surrounded by hyperflectivity caused by a rearrangement of the photoreceptor layer in response to injury.
What causes ring flare on fundus photography?
Being too close
What causes a blue haze on fundus photography?
Being too far away
How do you deal with IOLs on fundus photography?
Set exposure lower
How can you take A/C photos?
Pull back and focus on pupil margin.
What colour light do you use to take RNFL photos?
Short wavelength i.e. blue
What colour light do you use to take choroid photos?
Long wavelength i.e. red
What blocks green light?
Pigment in RPE, useful for viewing nerve fibre layer, diabetic retinopathy, ERMs and PED.
What do types of digital sensor are there?
Charged coupled devices
Complimentary Metal Oxide Semiconductors (CMOS)
What are the advantages of CMOS?
Easier to produce
Use less power
Chips of choice
What are the advantages of CCDs?
Produce very high quality images but very expensive to produce.
What types of picture file formats are there?
- JPEG (Joint photographic experts group)
- TIFF (Tagged image file format)
Which are bigger JPEG or TIFF?
TIFF
What images are captured in diabetic screening?
2 x 45 degrees once centred on macula one on disc
What size are microanuerysms?
30um
What resolution must be set for diabetic screening?
30um
What screen should images be viewed on?
1200 x 1080, laptops are considered unsuitable.
