Imaging of the retinal nerve fibre layer for glaucoma

Abstract

Background: Glaucoma is a group of diseases characterised by retinal ganglion cell dysfunction and death. Detection of glaucoma and its progression are based on identification of abnormalities or changes in the optic nerve head (ONH) or the retinal nerve fibre layer (RNFL), either functional or structural. This review will focus on the identification of structural abnormalities in the RNFL associated with glaucoma.

Discussion: A variety of new techniques have been created and developed to move beyond photography, which generally requires subjective interpretation, to quantitative retinal imaging to measure RNFL loss. Scanning laser polarimetry uses polarised light to measure the RNFL birefringence to estimate tissue thickness. Optical coherence tomography (OCT) uses low-coherence light to create high-resolution tomographic images of the retina from backscattered light in order to measure the tissue thickness of the retinal layers and intraretinal structures. Segmentation algorithms are used to measure the thickness of the retinal nerve fibre layer directly from the OCT images. In addition to these clinically available technologies, new techniques are in the research stages. Polarisation-sensitive OCT has been developed that combines the strengths of scanning laser polarimetry with those of OCT. Ultra-fast techniques for OCT have been created for research devices. The continued utilisation of imaging devices into the clinic is refining glaucoma assessment. In the past 20 years glaucoma has gone from a disease diagnosed and followed using highly subjective techniques to one measured quantitatively and increasingly objectively.

Figure 1

Scanning laser polarimetry (SLP) birefringence maps. The map on the left represents a typical healthy patient’s SLP scan (GDx, Carl Zeiss Meditec, Dublin, California), showing bright thick superior and inferior nerve bundles. The centre map represents a patient with significant nerve fibre loss due to glaucoma. The map on the right displays an abnormal birefringence pattern due to error caused by ocular opacity resulting in poor image quality. The retinal nerve fibre layer thickness colour scale is included. I, inferior; N, nasal; S, superior; T, temporal.

Figure 2

Optical coherence tomography retinal nerve fibre layer (RNFL) dataset visualisation. The upper left is an en-face image of a spectral domain optical coherence tomography (SD-OCT) 3D data cube, created by averaging the reflectance along each A-scan. The lower left is the RNFL thickness map around the optic nerve (the blue spot in the centre) along with the colour scale. The two B-scans on the right are circular scans generated in two different ways. The time domain optical coherence tomography (TD-OCT) is gathered solely as a single circular scan shown. The SD-OCT image is sampled in postprocessing from the 3D data set along the red circle shown on the en-face image. Note the improvement in image clarity with SD-OCT’s increased resolution. The location along the circle is labelled below the B-scans. I, inferior; N, nasal; S, superior; T, temporal.