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Imaging Technologies for the Diagnosis of Glaucoma
Figure 1: Print-out of a Spectral-domain Optical Coherence Tomograph
light scattering in the eye leading to a poor signal-to-noise ratio and MRA improves the diagnostic accuracy of the HRT by taking
atypical retardation patterns (ARPs).
9–11
The reason for ARPs is into consideration that neuroretinal rim area is affected by disc size
unknown, but as ARPs occur more in glaucomatous eyes than in and age.
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normal eyes it has been hypothesised that it results from low signal-
to-noise ratio when decreased reflectivity is present.
9
An analysis of The GPS is an automated approach to the optic disc classifying
the contribution of backscattered light from various depths to the procedure that eliminates operator-dependent factors, which are a
total retardation map using spectral-domain OCT found that atypical source of variability. It is based on five glaucoma-specific
retardation patterns in SLT are associated with deep penetration of parameters of the 3D shape of the optic disc and peri-papillary
the probing light beam into the strongly birefringent sclera.
12
To RNFL and provides disease probability values.
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improve the accuracy, first the already-mentioned VCC,
13
then the
software-based ECC
14
were introduced. It appears that GDx ECC The glaucomatous change can be assessed by topographic change
performs better than VCC in the detection of glaucoma in early analysis (TCA). TCA provides localised, objective and quantitative
stages of disease.
11,15
information about changes in the volume of the neuroretinal tissue.
TCA describes significant repeatable changes in the neuroretinal rim
Confocal Scanning Laser Ophthalmoscopy volume by comparing the variability within a baseline examination
Confocal scanning laser ophthalmoscopy (CSLO) is an imaging tool with that between baseline examination and follow-up examination.
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that is designed to create a quantitative, 3D topographic picture of
the ONH and the posterior segment surface. The currently used Optical Coherence Tomography
instrument, the HRT (Heidelberg Engineering GmBH, Dossenheim, OCT uses a scanning interferometer and a coherent infrared light
Germany), uses a 670nm diode laser beam to scan the surface of the (of 820–870nm) to obtain cross-sectional retinal images based on
posterior segment. Based on the measurements of the ONH, the the reflectivity of the different retinal layers down to the retinal
instrument generates a number of stereometric parameters, such as pigment epithelium.
20
OCT is a non-invasive, cross-sectional
rim volume, rim area, cup shape or cup-to-disc ratio, that allow imaging technique that allows in vivo measurements of tissue
evaluation of the ONH for glaucomatous damage. The relevant ONH thickness. The most commonly used time-domain OCT device
parameters are automatically generated by the instrument’s (Stratus TD-OCT model 3000, Carl Zeiss Meditec, Inc. Dublin) has an
software after identification of the optic disc border by the operator. axial resolution of approximately 10 microns and a transversal
It allows optic disc assessment to detect structural glaucomatous resolution of approximately 20 microns.
changes up to eight years earlier than visual field examination.
16
With the OCT the topography of the ONH can be assessed; however,
The latest version of the CLSO, the HRT III, provides a large ethnic- the most important aspect of the OCT is the quantification of the
selectable normative database and includes data analysis tools RNFL thickness, which is measured using peri-papillary scanning
such as Moorfields regression analysis (MRA) and the Glaucoma around the optic disc. Because the RNFL is one of the layers with the
Probability Score (GPS). Other technical improvements in the greatest reflectance and because of its anterior location, it can be
scaling and alignment of images have been made. automatically segmented and measured by computer algorithms.
MRA is a linear regression that takes into account the relationship There are major recent developments of OCT, such as spectral-
between optic disc size and rim area or cup-to-disc ratio. The domain optical coherence tomography (SD-OCT), which permits
EUROPEAN OPHTHALMIC REVIEW 17
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