An improved method of densitometry of red‐free retinal nerve fibre layer photographs

Abstract

The red-free negatives of 53 right eyes (30 normal eyes and 23 glaucomatous eyes) and 51 left eyes (32 normal eyes and 19 glaucomatous eyes) were analysed using two different image densitometry techniques. The first technique measured the density from rectangular sample areas, while the second measured density from sector-shaped sample areas which more closely follow the course of the nerve fibres in the retina.Indices which measured the deviation of the data from a clinically determined normal 'gold standard'were calculated, and were used to determine the optimum sensitivity and specificity in separating normal from glaucomatous eyes. There is a significant difference between the data from the normal and glaucomatous groups of eyes, when measured from the sector sample areas. The relative efficacy of this technique is also shown by the improved values of sensitivity (from 42%-70% to 70%-91 Vo), although specificity remained fairly constant (from 66%-83% to 620/0-88%).Key words: Densitometry, glaucoma, image analysis, predictive value, red-free photography, retinal nerve fibre layer.Analysis of red-free photographs has become an established method of observing glaucomatous damage to the retinal nerve fibre layer (RNFL). A number of studies have used semi-subjective techniques to grade diffuse and wedge defects in thl RNFL.'.' These have shown an association be tween the degree of damage and the severity o glaucoma. However, a number of reports havc indicated a need for an objective method of gradine RNFL defects.*-" Densitometry has been used as a tool to measurc the RNFL1*-15 and sources of variance have beer identified.16 We have previously published reports on densitometry data from rectangular sample areas in the temporal retina,16 and have been able tc separate normal from abnormal eyes. This paper reports on changes to our data collection techniques that reflect better the course of the nerve fibres in the retina. Two data sampling and analysis techniques are presented, and their relative efficacy is determined by comparing the results against a clinical judgement of whether the subject is normal or has glaucomatous damage. Methods SubjectsNormal and abnormal (glaucomatous) subjects were selected from over 220 patients who have been studied for a period of five years. A subject with no visual field defects, a normal intraocular pressure ( < 22 mmHg), and a normal optic disc (C:D ratio less than 0.6) was classified as normal; a subject with visual field loss and abnormal optic disc cupping (C:D ratio > 0.6) was classified as glaucomatous (or abnormal). These abnormal subjects were chosen to cover a range of severity as would be seen in a glaucoma population. Full details of the classification From the Lions Eye Institute,

Remote sensing of blood oxygenation using red-eye pupil reflection

“…The study was however, restricted to subjects with very low pigmentation that provided sufficient visibility of the choroidal vessels within the retinal images and at the visible wavelengths suitable for vascular oximetry. Fundus reflection has also been used to quantify retinal pigments, including macular pigment (Elsner et al 1992) and melanin (Hunold andMalessa 1974, Delori andPflibsen 1989) as well as to study diseases and treatment efficacy; for example, in diabetes (Augsten et al 1998) and glaucoma (Cooper et al 1992, Eikelboom et al 1993. A review of fundus reflection characterisation and its application is presented in Berendschot et al (2003).…”