Scanning laser polarimetry

Scanning laser polarimetry is the use of polarised light to measure the thickness of the retinal nerve fiber layer as part of a glaucoma workup. The GDx-VCC is one example.

However a Dutch study found that while there is a correlation between standard automated perimetry and GDx VCC measurements in patients with glaucoma, suggesting that GDx VCC measurements relate well with functional loss in glaucoma, in healthy subjects, they found virtually no correlation between perimetry and GDx VCC measurements. This would cast doubt on its predictive value and suggests false positives. see : "The Relationship between Standard Automated Perimetry and GDx VCC Measurements", Nicolaas J. Reus and Hans G. Lemij.... From the Glaucoma Service, The Rotterdam Eye Hospital, Rotterdam, The Netherlands.

For overview, this first prototype of this instrument was developed about 10 years ago, and was first released commercially as the GDx Nerve fiber analyzer (Laser Diagnostic Technologies Inc). The second generation product is called the GDx Access. The field of view is 15 degree and imaging should be performed through an undilated pupil. The polarised laser scans the fundus, building a monochromatic image. The state of polarisation of the light is changed (retardation) as it passes through birefringent tissue (cornea and RNFL). Corneal birefringence is eliminated (in part) by a proprietary 'corneal compensator'. The amount of retardation of light reflected from the fundus is converted to RFNL thickness. Sub-optimal compensation of corneal birefringence is currently being addressed by the manufacturer with hardware and software modifications. The GDx scanning laser measures the thickness of the retinal nerve fiber layer, which is the very first part of your eye that is damaged by glaucoma.

Before we go any further, let us describe the basic GDx instrument. This instrument use a GaAIAs diode laser as a source of light. This diode will emit polarized light. The source is HeNe (632.8 nm) and argon (514 nm).

A polarization modulator in this instrument changes the polarization states of the laser output. The linearly polarized beam from the laser then passes through a rotating quarter-wave retarder.

A scanning unit in this instrument is used to move the beam horizontally and vertically on the retina. The focused beam is 35μm in diameter.

This instrument also has a polarization detector. It is used to detect polarized light that is reflected back from the cornea. It is also used to analyze the change in the polarization of the reflected radiation. This element consists of a second synchronously rotating quarter-wave retarder and a linear polarizer in front of the photo-detector. The output is then sampled, digitized, and stored by a computer.

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