DIS - Ocular Imaging II - Week 4

Question 1

What is an en face image?

Answer 1

A surface scan

Question 2

Briefly describe how OCT works.

Answer 2

Uses ultrasound, and is based on the concept of different tissue having different refractive indices

Question 3

List two historical eye scans.

Answer 3

Polarimetry

Scanning laser ophthalmoscopy

Question 4

List the two forms of OCT and their resolution and depth. Name which is superior.

Answer 4

Time domain - resolution 8um, depth 15um
Spectral domain - resolution 2um, depth 8um
Spectral domain is superior

Question 5

List the two types of light sources that can be used for OCT. Describe them in terms of their wavelengths.

Answer 5

Standard laser - fixed wavelength OCT

Swept-source OCT - variable wavelength

Question 6

What does OCT give a reflection of and what is this dependent on?

Answer 6

A reflection or scatter profile that depends on the refractive index of tissue, and correlates well with anatomical features

Question 7

What kind of depth can a single laser OCT resolve to and can it resolve the choroid?

Answer 7

Fixed depth - usually cannot see the choroid

Question 8

Describe three advantages of multiple wavelengths during a single sweep in OCT.

Answer 8

Better resolution
Deeper penetration
Faster image acquisition

Question 9

When using multiple wavelengths in OCT, is there more or less movement artefact and noise?

Answer 9

Less

Question 10

What are black lines in an OCT scan? Give three examples.

Answer 10

An artefact

• shadows from blood vessels
• shadows from tissue
• echo

Question 11

Is a whole eye OCT possible?

Answer 11

Yesd

Question 12

Define ASOCT.

Answer 12

Anterior segment OCT

Question 13

List four things an ASOCT scan can assess.

Answer 13

Angle and structures
Corneal thickness
Corneal oedema (bullae)
Choroidal swelling

Question 14

What wavelengths are needed to visualise deeper structures of the eye?

Answer 14

Longer

Question 15

List the three clinically important structures seen on a retinal OCT scan.

Answer 15

RNFL/GCC
ISe (PR)
RPE/BM

Question 16

How is photoreceptor integrity judged?

Answer 16

By the presence of ISe

Question 17

How does the ISe appear on an OCT?

Answer 17

A thin white line

Question 18

How does the outer segment (OS) appear on an OCT scan?

Answer 18

A thin black line under the ISe and above the RPE

Question 19

How does the RPE appear on an OCT acan?

Answer 19

A thin white linebelow the black outer segment line

Question 20

What is the ISe exactly? What elsa is it known as?

Answer 20

The interface between the outer segment and inner segment

Ellipsoid zone

Question 21

List three appearances of the ISe on an OCT scan indicating a retinal disorder. What does this mean for the photoreceptors?

Answer 21

Thickening
Gaps
Disruptions to the ISe
May indicate local dropout of photoreceptors

Question 22

What is missing ISe an early sign of?

Answer 22

Photoreceptor (OR) disease

Question 23

On an OCT scan, what do dark, optically homogenous regions in the retina indicate?

Answer 23

Fluid

Question 24

How does a choroidal neovascular membrane appear on an OCT scan? Describe the ISe as well (2).

Answer 24

It creates fluid with a thickening and breakup of the overlying ISe

Question 25

Where does fluid accumulate in cystoid macula oedema (what layer)? What disease is it common in?

Answer 25

Fluid in Henle’s layer

Common in diabetes

Question 26

What are drusen, where do they form, what appearance does the interior have on an OCT and how do they affect the ISe?

Answer 26

Lumps that form sub-RPE
Interior has a milky appearance
ISe is intact

Question 27

What is soft drusen?

Answer 27

Choroidal neovascular membrane (CNVM)

Question 28

Can an OCT detect a choroidal neovascular membrane not visible in a retinal photo?

Answer 28

Yesd

Question 29

Can an OCT differentiate cotton wool spots, drusen, and hard exudates or are they too similar distinguish?

Answer 29

It can differentiate them by their layer and appearance

Question 30

List 6 limitations for OCT scans.

Answer 30

• 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

Question 31

What is common in people with large discs getting OCT scans?

Answer 31

Red disease - SLO

Question 32

What can hide early, thin RNFL defects?

Answer 32

Averaging over quadrants/clockfaces

Question 33

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?

Answer 33

En-face age-norm deviations

Noisy - gives a lot of red disease

Question 34

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

Answer 34

1 - 26%
2 - 5%
3 - 1%

Question 35

What is red disease?

Answer 35

A false positive

Question 36

In what percentage of high and low myopes is red disease present? Explain why.

Answer 36

High myope - 20%
Low myope - 10%
This is because myopes have thinned retinal layers due to larger globe

Question 37

Explain the reliability of a TSNIT plot for myopes. Give 2 alternatives.

Answer 37

Its unreliable

GCC/NRR scans are better

Question 38

What type of OCT is best for detecting early glaucoma?

Answer 38

tdOCT

Question 39

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.

Answer 39

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

Question 40

Aside from being unable to detect certain kinds of losses early, give another reason why GCC scans fail to detect RNFL loss.

Answer 40

About 50% of all RGCs reside in the macula

But the periphery can be undersampled

Question 41

What do studies suggest is better for early glaucoma, TSNIT or GCC/NRR? Does this finding hold for everyone? Explain.

Answer 41

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