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what are the 3 types of modern imaging techniques out there

- Scanning Laser Ophthalmoscope (SLO)
- Scanning Laser Polarimetry (SLP)
- Optical Coherence Tomography (OCT)


when was the Scanning Laser Ophthalmoscope (SLO) developed
how does it work

First developed in early 1990s. Now a very established technology and is the maturest of all the imaging techniques

Employs monochromatic laser light (single frequency):
One ‘spot’ illuminated at any one time, which scans across the region of interest (raster scanning). Reflected light is collected by a light detector.


which type of setup of most Scanning Laser Ophthalmoscope (SLO) use and how does this work

Confocal SLO

They use a pinhole set up in the optics. e.g. A pinhole in front of the detector (conjugate to laser plane of focus), light shines in very small areas of the retina, improving contrast.


what is the main commercial SLO instrument called
how many optoms use this in secondary/tertiary care in glaucoma

Heidelberg Retina Tomograph (HRT)

About 2-3%


how does the Heidelberg Retina Tomograph (HRT) work and what is it able to provide from this

Uses SLO technology to obtain multiple confocal ‘slices’

to provide pseudo ‘quantifiable’ 3-D topographies of ONH and retina


how does the Heidelberg Retina Tomograph (HRT) produce it's image acquisition

An image in one focal plane is obtained (x-y). The instrument then changes the plane of focus to achieve a series of x-y images at increasing depth (z)

Scan is centred on the optic disc and a variable number of slices are obtained at a rate of 16 per mm of scan depth

Depth of scanning varies from 0.5mm to 4mm depending on the eye

Three consecutive scans are obtained within ≈ 2sec (up to 192 slices) to improve reproducibility

so this allows to produce a stereoscopic photograph
by using the confocal set up in different planes i.e. images at different depths and so you end up with a pancake affect
it uses 32 structures to reconstruct the eye in a 3D way


what does the Heidelberg Retina Tomograph (HRT) do to improve reproducibility of its images

Three consecutive scans are obtained within ≈ 2sec (up to 192 slices) to improve reproducibility.


what is the disadvantage of the Heidelberg Retina Tomograph (HRT)

the imaging device is not as quick to picking up the slices of scans, it is difficult because the eye is constantly moving (micro saccades)
so these imaging techniques are just not fast enough in it's image acquisition


what is the real strength of the SLO and how does it do this

By quantifying what is happening at and outside the ONH

It does this by quantifying the neuroretinal rim area (RA):
Corrects for Disc (ONH) Size by using the Moorfields Regression Analysis: MRA software


what does the Scanning Laser Polarimetry (SLP) attempt to do

it attempts to measure the peripapillary RNFL thickness in the area around the optic nerve

as it is assumed that the thickness is a type of surrogate of the optic nerve damage leading to the back to the brain

it tries to see what the experts see in the red free photographs


what does the Scanning Laser Polarimetry (SLP) use to obtain its image of the RNFL
what technique does this method use and how it is done

Employs polarised diode laser light

Retardation of Birefringent Light

Birefringence: Property of a medium of having two different refractive indices depending on the angle of incidence of light.
Retardation: Difference in speed between two rays of light affected by different refractive indices.

Microtubules supporting nerve fibres are tubular and have a parallel arrangement; these produce linear birefringence of the incoming polarised light.


what is the difference between what a SLO uses and what a SLP uses to obtain its image

SLO uses monochromatic laser

SLP uses technology called birefringence


explain how Scanning Laser Polarimetry (SLP) produces its image acquisition

The speed that light travels through ‘tube’-like birefringent media (RNFL) depends on the polarization of the light.
There are 2 types of polarised light:
Light polarized parallel to the tube axis travels at a different speed than light perpendicular to that axis.
The difference in travel time (for the two differently polarized light beams) is called the retardation

so light slows down as you change the direction of the light going into the tissue


how will the Scanning Laser Polarimetry (SLP) image acquisition be different if the RNFL was thin and if it was thick

the 2 types of polarised lights which are perpendicular to each other will run equivalent to each other
it doesnt matter with orientation you enter the lights, they both arrive at the source as the same time
= no retardation

Thick (i.e. healthy):
we would assume the 2 types of polarised lights to have a bigger difference
= a large retardation


what is a commercial instrument of the Scanning Laser Polarimetry (SLP) called

GDx Variable Corneal Compensator (VCC) by zeiss


what problem did the Scanning Laser Polarimetry (SLP) have and how did the GDx try to resolve this problem

the type of light source is also influenced by the cornea (which was even more influenced by monochromatic light of the SLO)

so the GDx contains a variable Corneal Compensator (VCC)
which has a a calibration algorithm for correcting for what happens at the cornea (very birefringent)


how is the results/image produced by the Scanning Laser Polarimetry (SLP) GDx Variable Corneal Compensator (VCC) interpreted

- Blue: low retardation areas - ‘thinner’ RNFL
- Yellow to red : greater retardation areas (healthier RNFL area)

The RNFL is thicker at the superior and inferior poles of the ONH.


what issue did the GDx Scanning Laser Polarimetry (SLP) have
what was released to resolve this and what was the outcome

Frequently ‘weird’ images – atypical scans = was difficult to get good images

Appears to occur more in eyes with lightly pigmented fundi, myopia, and in eyes of elderly subjects

New (enhanced!) corneal compensator (GDx ECC)
Seems to reduce atypical scans. But is it just a fix for ‘flawed’ technology?


what was Scanning Laser Polarimetry (SLP) replaced by and why

It has good studies to support it

research found that a px that got a damaged ONH or RNFL with the SLP was not found to be glaucomatous


what is Optical Coherence Tomography (OCT)

High-resolution cross-sectional optical imaging of ocular structures


what technology does the OCT employ
and how does it use this

Employs low-coherence interferometry to achieve a high resolution

Uses a ultrasound technique but is looking at correlation between light waves

Coherence describes the correlation between light waves
When interfering, two waves can add together to create a larger wave (constructive interference) or subtract from each other to create a smaller wave (destructive interference)
Interferometry studies the pattern of interference of waves.
Based on a classic Michelson interferometer
The light beam scans transversely across the eye, similar to ultrasound, and produces a cross-sectional image of the tissue of interest


how does the OCT produce its image acquisition

A Scans
Axial (longitudinal) measurement that gives information about depth

B Scans
Series of A scans taken at different transverse points and finally reconstructed into a 2-D image of tissue
i.e. a slice taken into the retina


what was the commercial name of the first type of OCT instrument and which type of technique did this use

used time domain
this used a more mechanical process

it did this by:
taking 3 circular scans centred on the ONH, which are averaged to produce a cylinder of information, which is ‘straightened’ and viewed in cross-section


what is a better, new gold standard of OCT commercial name
explain how it's technology is better than the Stratus OCT machine

Spectral Domain (SD) OCT

Newer OCT devices use spectral-domain (SD) technology
this uses a more computerised trick, which is quicker than the stratus at image acquisition

No scanning mirror is used: instead collects all reflections at once and uses spectral Fourier analysis to process information and eliminate motion artefacts


what type of images did the gold standard Spectral Domain (SD) OCT produce

- high-resolution scans of the macula

- cross-sectional images along orthogonal planes of the ONH generated from 3-D OCT data

- volume rendering from the 3-D OCT data


name 3 manufacturers of the Spectral Domain (SD) OCT
and what ocular structure is this type of OCT good for

Cirrus (by zeiss)

Good for retina and glaucoma


what are the 4 advantages of modern imaging techniques

‘Sharper’ and more detailed images
e.g. SLO: Unlike conventional indirect/fundus imaging, only a small region of the fundus is illuminated at any one time, which allows sharper images to be captured. OCT: Incredible resolution

‘Better’ and ‘easier’ image acquisition
e.g. SLO and OCT: Reduces ‘light scatter’ through cloudy media. Can capture image through small pupils’ SLO: Can capture images through retinal hyperpigmentation, blood, heavy exudation or sub-retinal fluid

‘Deeper’ structures in the retina can be evaluated
‘New’ SD-OCT: Getting closer and closer to lamina cribrosa – Is this the important structure for glaucoma?

‘Automated’ processes and image properties can be better ‘quantified’