Scanning laser polarimetry

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The scanning laser polarimetry is a procedure with the aid of polarized light to determine the layer thickness of optically transparent materials. The GDx scanning laser polarimetry of the nerve fiber layer of the retina records the three-dimensional profile of the optic nerve head and the thickness of the nerve fiber layer of the retina in its vicinity.

Problem

In glaucoma, optic nerve fibers that are not reproduced gradually perish due to the high eye pressure. However, humans are endowed with an excess of these fibers, so that the first damage in the form of visual field defects only becomes noticeable when more than half of the fibers have perished. Observing this loss is very difficult because the retina is transparent. The nerve fiber layer is only slightly visible to the ophthalmologist in the red-free light on the slit lamp, which, however, requires a great deal of experience. Polarimetry is an objective way of assessing this decay.

Measurement method

Fundus image with TSNI sectors

The measurement is carried out with a laser scanner. A simple laser scanner works as follows: First, a single point on the retina is illuminated with a laser beam. It then measures how much light is reflected from that point. This process is repeated successively for a large number of points (up to 100,000). The measurement takes about 2 seconds per eye. The result can be made directly visible in the fundus image. The reflectivity is color-coded: yellow means that much has been reflected, dark brown little. The red tones form the corresponding intermediate levels. The reflectivity diagram does not provide information about the actual papilla color, as is the case with a color photo, since the laser beam is monochromatic (single color). The fundus image is generally used to assess image quality.

In polarimetry, the polarization property of light is used. The measuring beam runs through the retinal fiber layer (RNFL) and is reflected from the fundus. The light beam is split into two polarization states. Both polarization states run through this nerve fiber layer at different speeds. The resulting delay from one polarization to the other depends on the layer thickness, so that it can be calculated and displayed in false colors. Red and yellow indicate thick nerve fibers, blue and green thin regions.

Evaluations

GDx - deviation representation

For an initial assessment, one likes to compare the fiber layer thickness with a normal value, which was formed from measurements by various people from different cultures, in the deviation display. A colored representation indicates regions and the size of the probability to what extent a measured value is still normal (e.g. yellow: <5% probability).

TSNIT diagram

Another graphic representation is the fiber layer thickness in the TSNIT diagram. This layer thickness of a circular path starting in sector T (temporal), over S (superior, above), N (nasal, nose), I (inferior, below) and again T is shown. The range of standard layer thicknesses is shaded. Values ​​corresponding to the standard are therefore in the shaded area.

The values ​​from this circle can be shown numerically in a table. OD stands for the right eye, OS for the left eye. The average values ​​for the entire district (TSNIT average) and for the upper (superior average) and lower (inferior average) sectors are given here. A nerve fiber index (NFI) can be determined from the statistical values. Values ​​up to 30 are considered normal, values ​​from 51 are considered abnormal.

RNFL summary OD value OS value
TSNIT average 47.1 45.5
Superior average 56.2 54.0
Inferior average 59.6 56.4
TSNIT standard deviation 22.2 20.4
Symmetry comparison 0.92
NFI 21st 27

It is also more interesting to compare measurements at longer intervals of 1 year and more. This makes it possible to objectively assess whether the glaucoma causes damage to the nerve fibers over time.

The measurement of the nerve fiber layer of the retina is currently (December 2010) not covered by health insurance in Germany.

literature

  • Josef Flammer, Melanie Eberle, Elisabeth Meier, Mona Pache: Glaucoma. A manual for those affected, an introduction for those interested, a reference work for those in a hurry. Hans Huber, Bern et al. 2000, ISBN 3-456-83353-9 .

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