Nerve Fibres: An Analysis Of SD-OCT (Fig2)

Recent advances in technology permit measurement of characteristics of the nerve fibres that make up the optic nerve. One of these is SD-OCT(Fig2), a non contact, non-invasive method that enables physicians to obtain in vivo, high resolution (upto 5 microns) cross-sectional images of the retina. It is based on principle of low-coherence interferometry(Fig 3). By imaging a 3D cube of data it allows a detailed evaluation of the peripapillary area.
Fig 2 : Cirrus HD OCT machine
In low-coherence interferometry, light is sent along two optical paths, one being the sample path (into the eye) and the other the reference path of the interferometer. The light source is an 840 nm super luminescent light emitting diode (SLD). Light returning from the sample and reference paths is combined at the detector, which is a spectrometer in SD-OCT. The spectrometer resolves the interference signals throughout the depth of each A-scan immediately by means of a Fourier transformation. This is possible because the spectrometer resolves the relative amplitudes and phases of the spectral components scattered back from all depths of each A-scan tissue sample, without varying the length of the reference path. Eliminating the necessity of moving a mechanical reference arm makes it possible to
RNFL thickness significantly decreased (p = 0.02) in the superior areas. The study detected that RNFL thickness was decreased in all quadrants in the NDR eyes as compared with normal eyes, however significant decreases were seen in the superior quadrant between the groups .In the other regions, RNFL thickness in NDR eyes showed a tendency towards thinning as compared with normal eyes, but there was no significant difference between the two