Our study confirms that reduction of speckle noise in optical coherence tomography by alignment and averaging of multiple OCT scans from identical retinal locations results in improved imaging of the retina. The averaged images correlate well with the known pathology of clinical cases, and yield new information of the outer layers of the retina most notably with the OCT3. With this technique, three hyperreflective and two hyporeflective layers are discernible in the outermost part of the scan, and the middle hyperreflective layer can be attributed to the level of outer photoreceptor pigment epithelium interdigitising cells in agreement with histological studies.8,9
In the case of single scan OCT2, this layering is not detectable and the whole band has simply been allocated to the retinal pigment epithelium/choriocapillaris.
With the new averaged images, we were also able to demonstrate subtle, previously undocumented alterations of intraretinal architecture after ischaemic injury, leaving no opthalmoscopically visible changes of the neurosensory retina, even with OCT2 images.
The correlation between histology and OCT is not straightforward because the two techniques rely on different principles to produce contrast. The histological preparation makes use of artificial stains to achieve contrast or freeze fracturing of tissue to expose its natural surfaces of mechanical cleavage, whereas OCT displays contrast in reflectivity and attenuation as observed from the centre of the pupil. Thus, only variations in tissue refractive index at surfaces that are close to orthogonal to the direction of view will be detected. These are found at the vitreous retina interface and in the plexiform layers of the retina, but variations in refractive index could also be the origin of the hyperreflective signals from the inner/outer photoreceptor junctions and the zone of interdigitising outer segments/pigment epithelium. The orientation and hence the reflectivity of some retinal structures varies, however, with eccentricity from the fovea, Furthermore, the reflectivity varies in relation to disease. Such effects have scarcely been researched.
The functional subdivision of the retina is closely related to its histological layering and hence the correlation between histology and OCT is of fundamental interest.10,11
Comparison with histology indicates that the fibre layer of Henle contributes to a wide hyporeflective OCT layer which also includes the outer nuclear layer and some of the photoreceptor layer, the only interruption of which is seen as a resolution limited thin band of moderate reflectivity near the transition between the outer and inner segments, presumably the outer limiting membrane.1,2
The hyporeflectivity of the photoreceptors might be attributed to their elongated morphology where light is internally reflected, in analogy with optical fibres.1,2
The photoreceptors extend through layers of markedly varying reflectivity as their outer section forms part of several layers of the RPE associated complex. When the neurosensory retina is split from the retinal pigment epithelium, microscopic examination demonstrates fragments of pigment found among the photoreceptors.9
This is compatible with our observations in central serous chorioretinopathy (figs 3 and 4) which indicate that the level of splitting is at the tips of the photoreceptors and causes the villous processes of the RPE to be torn from the RPE cell bodies. It thus appears that the interdigitising layers of photoreceptor outer segments and RPE cells produce a distinct OCT reflection line. Corresponding findings have previously been made of an artificial serous detachment in a porcine eye using experimental ultra high resolution OCT technique.3
Additionally, patients with stage 3–4 macular holes demonstrate that only the outermost of the three high intensity layers of the RPE photoreceptor complex seen in OCT is attributable to the RPE as it presents after pathological denudation (not shown).
Impaired venous outflow results in increased transmural pressure and a reduction in blood flow (case 4). The resulting thickening of the retina and the ischaemia, as indicated by the presence of angiographic hypoperfusion and cotton wool spots, is largely reversible, as only a shallow relative scotoma was detectable after resolution. Averaged OCT images demonstrated clear evidence of selective structural damage to the inner retina, the normal layered pattern of which was absent despite the retina being of normal thickness. By comparison, the more severe ischaemia of cilioretinal artery pseudo-occlusion was associated with a better preservation of retinal layering but a significant reduction of retinal thickness (case 5) and pronounced irreversible functional deficit of the involved area.
In conclusion, OCT averaging enables recovery of detailed structural information about the retina. The non-invasive in vivo OCT image is not related to histological preparations in a straightforward manner, and the interpretation of OCT requires consideration of the specific interaction between light and tissue in the retina. Selected cases of highly localised pathological change show an excellent correlation with the known sites of injury. The method demonstrated in the present study allows upgrading of widely available clinical instrumentation.