DIS - Ocular Imaging II - Week 4 Flashcards
What is an en face image?
A surface scan
Briefly describe how OCT works.
Uses ultrasound, and is based on the concept of different tissue having different refractive indices
List two historical eye scans.
Polarimetry
Scanning laser ophthalmoscopy
List the two forms of OCT and their resolution and depth. Name which is superior.
Time domain - resolution 8um, depth 15um
Spectral domain - resolution 2um, depth 8um
Spectral domain is superior
List the two types of light sources that can be used for OCT. Describe them in terms of their wavelengths.
Standard laser - fixed wavelength OCT
Swept-source OCT - variable wavelength
What does OCT give a reflection of and what is this dependent on?
A reflection or scatter profile that depends on the refractive index of tissue, and correlates well with anatomical features
What kind of depth can a single laser OCT resolve to and can it resolve the choroid?
Fixed depth - usually cannot see the choroid
Describe three advantages of multiple wavelengths during a single sweep in OCT.
Better resolution
Deeper penetration
Faster image acquisition
When using multiple wavelengths in OCT, is there more or less movement artefact and noise?
Less
What are black lines in an OCT scan? Give three examples.
An artefact
- shadows from blood vessels
- shadows from tissue
- echo
Is a whole eye OCT possible?
Yesd
Define ASOCT.
Anterior segment OCT
List four things an ASOCT scan can assess.
Angle and structures
Corneal thickness
Corneal oedema (bullae)
Choroidal swelling
What wavelengths are needed to visualise deeper structures of the eye?
Longer
List the three clinically important structures seen on a retinal OCT scan.
RNFL/GCC
ISe (PR)
RPE/BM
How is photoreceptor integrity judged?
By the presence of ISe
How does the ISe appear on an OCT?
A thin white line
How does the outer segment (OS) appear on an OCT scan?
A thin black line under the ISe and above the RPE
How does the RPE appear on an OCT acan?
A thin white linebelow the black outer segment line
What is the ISe exactly? What elsa is it known as?
The interface between the outer segment and inner segment
Ellipsoid zone
List three appearances of the ISe on an OCT scan indicating a retinal disorder. What does this mean for the photoreceptors?
Thickening
Gaps
Disruptions to the ISe
May indicate local dropout of photoreceptors
What is missing ISe an early sign of?
Photoreceptor (OR) disease
On an OCT scan, what do dark, optically homogenous regions in the retina indicate?
Fluid
How does a choroidal neovascular membrane appear on an OCT scan? Describe the ISe as well (2).
It creates fluid with a thickening and breakup of the overlying ISe
Where does fluid accumulate in cystoid macula oedema (what layer)? What disease is it common in?
Fluid in Henle’s layer
Common in diabetes
What are drusen, where do they form, what appearance does the interior have on an OCT and how do they affect the ISe?
Lumps that form sub-RPE
Interior has a milky appearance
ISe is intact
What is soft drusen?
Choroidal neovascular membrane (CNVM)
Can an OCT detect a choroidal neovascular membrane not visible in a retinal photo?
Yesd
Can an OCT differentiate cotton wool spots, drusen, and hard exudates or are they too similar distinguish?
It can differentiate them by their layer and appearance
List 6 limitations for OCT scans.
- Eye movement instability
- Inadequate racial database
- Inadequate database for high refractive error (> ±4.00D)
- Age limitations (<18, >70)
- Presence of other disease
- Large discs, especially for SLO
What is common in people with large discs getting OCT scans?
Red disease - SLO
What can hide early, thin RNFL defects?
Averaging over quadrants/clockfaces
What is the best means of finding early localised loss using OCT scans? Is this noisy or clear? Does it give a lot of red or green disease?
En-face age-norm deviations
Noisy - gives a lot of red disease
What is the frequency of red disease for the following: 1 clock hour red 2 clock hour red 3 clock hour red ah ah ah
1 - 26%
2 - 5%
3 - 1%
What is red disease?
A false positive
In what percentage of high and low myopes is red disease present? Explain why.
High myope - 20%
Low myope - 10%
This is because myopes have thinned retinal layers due to larger globe
Explain the reliability of a TSNIT plot for myopes. Give 2 alternatives.
Its unreliable
GCC/NRR scans are better
What type of OCT is best for detecting early glaucoma?
tdOCT
Why do TSNIT/GCC scans fail to detect all RNFL loss? Explain in terms of the types of losses (2) and what they do refractive indices. Explain which of the two types the OCT has limited capacity to find early.
Diffuse loss gives a swiss cheese/honeycomb type loss
This causes a minor disruption to refractive index as its spread out
The OCT has limited capacity to find diffuse loss early
This is opposed to local loss, which causes more disruption to refractive index, and is easier to pick up
Aside from being unable to detect certain kinds of losses early, give another reason why GCC scans fail to detect RNFL loss.
About 50% of all RGCs reside in the macula
But the periphery can be undersampled
What do studies suggest is better for early glaucoma, TSNIT or GCC/NRR? Does this finding hold for everyone? Explain.
TSNIT is better for early glaucoma
This finding doesnt hold for myopes - GCC/NRR better
Ideally, you’d do all the scans anyways
VF needs to be done as well
