measure the thickness of the retinal nerve fiber layer
Scanning laser polarimetry is the use of polarised light to measure the thickness of the retinal nerve fiber layer (RNFL) 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 (632.8nm) and argon (514nm).
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.
Concept of the instrument
The GDx nerve fiber analyzers measure the retinal nerve fiber layer (RNFL) thickness with a scanning laser polarimeter based on the birefringent properties of the RNFL. Measurement is obtained from a band 1.75 disc diameters concentric to the disc.
It projects a polarized beam of a light into the eye. As this light passes through the NFL tissue, it changes and slow. The detectors measure the change and convert it into thickness units that are graphically displayed. The GDx measure modulation around an ellipse just outside the optics disc and ratios of the thickest points either superiorly or inferiorly to the temporal or nasal regions.
The field of view is 15 degree and imaging should be performed through undilated pupil. The polarized laser scans the fundus and building a monochromatic image. The state of polarization of the light is change (retardation) as it passes through birefringent tissue (cornea and RNFL).
Corneal birefringent is eliminated (in part) by a proprietary ‘corneal compensator’. The amount of retardation of light reflected from the fundus is converted to RNFL thickness.
In Retinal scanning laser polarimetry (SLP), the cornea, lens, and retina are all treated as linear retarders (optical elements that introduce retardation to an illuminating beam).
A linear retarder has a slow axis and a fast axis, and the two axes are orthogonal to each other. Polarized light travels at higher speed when its electric field vector is aligned with the fast axis of a retarder.
In contrast, polarized light travels at lower speed when its electric field vector is aligned with the slow axis of a retarder.
Optical System
In the model, the measuring beam passed through three linear retarders: the corneal compensator (CC), the cornea (C), and a uniform radial retarder (R), that represented birefringent regions in the retina (e.g., peripapillary RNFL or macula). And polarization-preserving reflector (PPR).
Retarders
Firstly, the retardation (i.e., the change in polarization) is proportional to the RNFL thickness. In this instrument, there are four retarders in the measurement beam's path: 1. The first two linear retarders have equal retardance and form a VCC. 2. The third linear retarder is the combination of cornea and lens—the anterior segment 3. The fourth linear retarder, with radially distributed axes, is the retinal birefringent structure (RE; either peripapillary RNFL or the Henle fiber.
As polarized light passes through a form-birefringent medium, one of the two component waves traveling at 90 to each other becomes retarded relative to the other. The degree of the resulting phase shift is directly proportional to the number of microtubules the light passes through, which in turn, is directly proportional to RNFL thickness. The figure above illustrates this process.
The RNFL isn't the only form-birefringent structure in the eye. Anterior segment structures, such as the cornea, also phase-shift polarized light. So the latest instrument includes a compensating device or compensating corneal which is designed to remove the portion of the signal generated by the anterior segment.
This device consists of two optical retarders, which when rotated relative to each other, allow the operator to set the compensator to any value between 0nm and 120nm. Rotating the device to any axis can compensate for anterior segment birefringence in any orientation up to 120nm in magnitude.
The slow axis of R was oriented radially, and distance around R was measured from the horizontal nasal meridian by angle β. At each point, therefore, the fast axis of R was R = β + 90°. Radial variation in retardance was not analyzed. The measuring beam was reflected at a deeper layer and traveled back through the three retarders to the ellipsometer.
Reflection from the ocular fundus exhibits a high degree of polarization preservation, and the reflector in the model (polarization-preserving reflector [PPR]) was assumed to preserve completely the polarization state of the incident beam, except for a 180° phase change due to the reversal in direction. Each optical component in the model experienced a double pass of the measuring beam.
What is birefringent?
Birefringent is relared[check spelling] or characterized as a double refraction. In this picture we can see calcite crystal laid upon a paper with some letters showing the double refraction.
Clinical interpretation
Clinical Interpretation based on results from GDx Nerve Fiber Analyzer from Carl Zeiss Meditec.
Firstly, this instrument is used to measure thickness of nerve fiber layer in our retina. But, GDx give monochromatic image. Then this system will analyze and give colors for certain various thicknesses.
Presents RNFL thickness in colour with thick regions in red and yellow and thin regions in blue and green.
For healthy eye, the image will show yellow and red colour in superior and inferior at NFL regions. But, in glaucoma, the image is absence of red and yellow colours. Superiorly and inferiorly more uniform blue appearance. Picture indicates that the eye is at the advance stage of the disease.
Deviation map
GDx - Deviation map
TSNIT graph
The deviation map reveals the location and magnitude of RNFL thinning relative to a normal value. This normal value was generated as an average of people from various cultures. Defects are colour-coded based on probability of normality (e.g. yellow means that the probability is below 5% of that RNFL at that location is normal). A healthy eye has a clear deviation map.
A further representation is the TSNIT graph. TSNIT is stand for Temporal – Superior – Nasal – Inferior-Temporal. This graph displays the thickness values along the Calculation Circle from T to S, N and back to T. The area of normal values is shaded. Measurements for the left eye are labeled "OS", those for the right eye "OD". A defect is indicated if a measured value falls below the shaded area.
GDx Comparative database
A comprehensive database is essential for accurate glaucoma detection. In this instrument a database from 540 normal eyes is used. The subjects are multi-ethnic and 18–82 years old. The database also contains 262 glaucomatous eyes used by the NFI to discriminate between normal and glaucoma.
Related Research Articles
Polarization is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string (see image), for example, in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, and transverse sound waves in solids.
A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are the half-wave plate, which rotates the polarization direction of linearly polarized light, and the quarter-wave plate, which converts between different elliptical polarizations
Glaucoma is a group of eye diseases that lead to damage of the optic nerve, which transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of sight" because the loss of vision usually occurs slowly over a long period of time. A major risk factor for glaucoma is increased pressure within the eye, known as intraocular pressure (IOP). It is associated with old age, a family history of glaucoma, and certain medical conditions or medications. The word glaucoma comes from the Ancient Greek word γλαυκός, meaning 'gleaming, blue-green, gray'.
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Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.
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Ellipsometry is an optical technique for investigating the dielectric properties of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it to a model.
A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, known as polarized light. Polarizers are used in many optical techniques and instruments. Polarizers find applications in photography and LCD technology. In photography, a polarizing filter can be used to filter out reflections.
A Lyot filter, named for its inventor and French astronomer Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. Lyot filters are used in astronomy, particularly for solar astronomy, lasers, biomedical photonics and Raman chemical imaging.
The retinal nerve fiber layer (RNFL) or nerve fiber layer, stratum opticum, is part of the anatomy of the eye.
Ronald H. Silverman is an American ophthalmologist. He is currently Professor of Ophthalmic Science at Columbia University Medical Center. He is currently the director of the CUMC Basic Science Course in Ophthalmology, which takes place every January at the Harkness Eye Institute. He departed Weill Cornell Medical College in 2010, where he was Professor of Ophthalmology as well as a Dyson Scholar and the Research Director of the Bioacoustic Research Facility, Margaret M. Dyson Vision Research Institute at Weill Cornell.
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Retinal birefringence scanning (RBS) is a method for detecting the central fixation of the eye. The method can be used in pediatric ophthalmology for screening purposes. By simultaneously measuring the central fixation of both eyes, small- and large-angle strabismus can be detected. The method is not invasive and requires little cooperation by the patient, so it can be used for detecting strabismus in young children. The method provides a reliable detection of strabismus and has also been used for detecting certain kinds of amblyopia. RBS uses the human eye's birefringent properties to identify the position of the fovea and the direction of gaze, and thereby to measure any binocular misalignment.
Oculometer is a device that tracks eye movement. The oculometer computes eye movement by tracking corneal reflection relative to the center of the pupil. An oculometer, which can provide continuous measurements in real time, can be a research tool to understand gaze as well as cognitive function. Further, it can be applied for hands-free control. It has applications in flight training, cognitive assessment, disease diagnosis, and treatment. The oculometer relies on the principle that when a collimated light beam is incident on the eye, the direction in which the eye moves is proportional to the position of the reflection of that light beam from the cornea with respect to the center of the pupil. Eye movements can be accurately measured over a linear range of more than 20 with a resolution of 0.1.
The Heidelberg Retinal Tomography is a diagnostic procedure used in ophthalmology. The Heidelberg Retina Tomograph (HRT) is an ophthalmological confocal point scanning laser ophthalmoscope for examining the cornea and certain areas of the retina using different diagnostic modules. However, the most widely used area of application for HRT is the inspection of the optic nerve head (papilla) for early detection and follow-up of glaucoma. The procedure has established itself as an integral part of routine glaucoma diagnostics alongside the visual field examination (perimetry), the chamber angle examination (gonioscopy) and the measurement of intraocular pressure (tonometry). The HRT is the most widely used application of confocal scanning laser ophthalmoscopy.
Charles M. (2003). Ophthalmic Lasers. Philadelphia, Pennsylvania: Butterworth Heinemann
Josef Flammer, Melanie Eberle, Elisabeth Meier, Mona Pache: Glaukom. Verlag Hans Huber, ISBN3-456-83353-9.
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