Scanning laser polarimetry
The scanning laser polarimetry is a method to determine with the aid of polarized light, the layer thickness of the optically transparent material. The GDx scanning laser polarimetry of the nerve fiber layer of the retina detects the three-dimensional profile of the optic nerve head and the thickness of the nerve fiber layer of the retina in its environment.
Problem
When glaucoma ( glaucoma) go through the high pressure in the eye continuously insidious optic nerve fibers based, which are not modeled. Man is, however, equipped with an excess of these fibers so that first damage in the form of visual field defects only become noticeable when more than half of the fibers has perished. To observe this loss is very difficult because the retina is transparent. Only in the red-free light at the slit lamp, the nerve fiber layer is slightly visible to the eye doctor, but this requires very large experience. Polarimetry is an objective method to assess this decline.
Measurement methods
The measurement is performed with a laser scanner. A simple laser scanner works as follows: First, a single point on the retina is illuminated with a laser beam. Then measuring how much light is reflected from that point. This process is repeated sequentially for a large number of points ( up to 100,000 ). The measurement takes about 2 seconds per eye. The result can be directly visualized in the retinal image. The reflectivity is color coded: yellow means that much has been reflected, dark brown little. The reds are the corresponding intermediates. Information about the actual Papillenfarbe, like a color photo Reflektivitätsdiagramm not provide, because the beam is monochromatic ( one color) is. The fundus image is generally used for the evaluation of image quality.
Polarimetry in the polarization property of light is used. The measuring beam passes through the retina fiber layer ( RNFL ) and is reflected from the fundus. The light beam is split into two polarization states. Both polarization states run at different speeds through this nerve fiber layer. The resulting delay of polarization to another is dependent on the layer thickness, so that compute these and can be represented in false colors. Red and yellow indicate thick nerve fibers, blue and green thin regions.
Evaluations
At the initial assessment you like to compare the fiber layer thickness with a normal value, which was formed from measurements of different people of different cultures in the deviation map. A color display features and regions of the probability of the extent to which a measured value is still normal ( eg yellow: <5% probability).
A further graph showing the fiber layer thickness in TSNIT diagram. Here is the layer thickness of a circular path starting in the T sector (Temporal ), via S (superior, above), N ( nasal, nose), I ( Inferior, below) and shown again T. A shaded background is the range of standard thicknesses. The corresponding standard values are therefore in the shaded area.
Numerically, the values can be displayed from this circle in a table. OD is the right one for the left eye OS. Here, the average values for the total circuit ( TSNIT average) and for the upper ( superiorly average) and lower ( Inferior Average) sector are given. From the statistical values of a nerve fiber index ( NFI) can be determined. Values up to 30 are considered normal, values from 51 as abnormal.
More interesting is beyond the comparison of measurements at longer intervals of 1 year or more. This can be assessed objectively whether the glaucoma causes a damage to the nerve fibers in the course of time.
The measurement of the nerve fiber layer of the retina is not supported currently (December 2010) in Germany by health insurance.