Imaging the RNFL

Question 1

What are 3 methods for imaging the retinal nerve fibre layer?

Answer 1

1. Scanning laser ophthalmoscopy (SLO)
2. Scanning laser tomography
3. Scanning laser polarimetry

Question 2

What are the indications for scanning laser ophthalmoscopy?

Answer 2

similar to colour fundus photography, but enhanced capabilities for evaluation of patients with medical retina pathology

Question 3

How do scanning laser ophthalmoscopes work?

Answer 3

• SLO devices employ a confocal (‘pinhole’) aperture, generating single point of laser light at a specific wavelength, scanned across the retina in a raster pattern (series of parallel horizontal lines)

Question 4

How can higher-contrast images be generated from scanning laser ophthalmoscopy + light scatter reduced?

Answer 4

only a small area of fundus illuminated at any time

Question 5

What are 2 common instrments used for scanning laser ophthalmoscopy?

Answer 5

1. Heidelberg Retina Angiograph-2
2. Nidek F-10

Question 6

What is a disadvantage of scanning laser ophthalmoscopy compared with fundus cameras?

Answer 6

fundus cameras typically have higher temporal and spatial resolution

Question 7

What are 2 indications for scanning laser tomography?

Answer 7

1. to distinguish normal optic disc anatomy from glaucomatous optic neuropathy
2. to monitor longitudinal or progressive change of glaucomatous optic neuropathy

Question 8

How does scanning laser tomography work?

Answer 8

confocal aperture in SLO devices allows acquisition of images at different focal planes within a tissue of interest (i.e. generation of a stack of en face images). 3D reconstruction then allows tomographic (corss sectional) visualisation of the fundus

Question 9

What is an example of an SLO device that allows for 3D reconstruction of the optic nerve head?

Answer 9

Heidelberg Retina Tomograph-3

Question 10

What are 3 cautions to bear in mind regarding scanning laser tomography?

Answer 10

1. lower resolution than OCT
2. media opacities and ability of pt to fiate can affect quality and variability
3. measurements influenced by acute changes in IOP and possibly cardiac cycle

Question 11

How is a false-colour topographic image created from scanning laser tomography?

Answer 11

the stack of confocal images is aligned and their reflectivities summed, generating the image

Question 12

How is optic nerve head morphology generated from scanning laser tomography (e.g. disc and cup area)?

Answer 12

software calculates detaield measurements from the stacked confocal images producing a topographic images
stereometric parameters compared with normative database + risk of glaucoma assessed using regression model

Question 13

What type of analysis is used for calculting the risk of glaucoma from the informaton on optic nerve morphology provided by scanning laser topography?

Answer 13

regression model - Moorfields Regression Analysis

Question 14

How many sections is the image obtained from scanning laser topography split into and how is each section scored?

Answer 14

6: green tick indicates within normal limits, yellow exclamation mark borderline, red cross is outside normal limits

Question 15

What are 2 indications for scanning laser polarimetry?

Answer 15

1. distinguish normal optic disc anatomy from glaucomatous optic neuropathy
2. monitor progression of glaucomatous optic neuropathy

Question 16

What is a key property of the RNFL due to the parallel arrangement of its axons?

Answer 16

birefringent: ray of light entering it is broken into two rays

Question 17

What are the basic principles of how scanning laser polarimetry works?

Answer 17

polarised light reflected from the RNFL undergoes a phase shift, depending on amount of birefringent material present. Scanning of polarised light across a region centred on the optic nerve head can be used to assess phase shifts and estimate RNFL thickness

Question 18

What is the only commercially available scanning laser polarimeter?

Answer 18

GDx

Question 19

How can newer models of scanning laser polarimeters improve the accuracy of results?

Answer 19

compensate for corneal birefringence

Question 20

How are the images from scanning laser polarimetry processed after acquisition? 3 forms of output

Answer 20

• nerve fibre thickness maps presented in colour-coded spectrum from blue to red
• deviation maps used to show the magnitude and location of RNFL defects using colour coded squares
• TSNIT graphs comparing RNFL thickness to a normative database

Question 21

What is the nerve fibre indicator (NFI) from scanning polarimetry?

Answer 21

global value based on the entire thickness map, used for discriminating normal vs glaucomatous eyes: normal 1-30, borderline 31-50, abnormal 51-100

Question 22

What is the benefit of advanced serial analysis with scanning laser polarimetry?

Answer 22

shows trend analysis over time from baseline in optic disc anatomy/optic neuropathy

Question 23

When may measurements from scanning laser polarimetry be erroneous?

Answer 23

in areas of peripallary atrophy (PPA) or chorioretinal scarring

Question 24

Do mild to moderate cataracts degrade the results from scanning laser polarimetry?

Answer 24

no