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Chen-Hsin Sun, Tin Tun, John Mark de Leon, Michael Girard, Tin Aung, Nicholas Strouthidis; Measurement of minimum rim width (MRW) using Cirrus Optic Nerve Head Volumes in a Chinese Population. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2269.
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© ARVO (1962-2015); The Authors (2016-present)
The minimum rim width (MRW), the shortest distance between Bruch’s membrane opening (BMO) and internal limiting membrane (ILM), has recently been proposed as the most accurate OCT measurement of neuroretinal rim. We describe a method of calculating MRW from Cirrus volumes; we measure MRW in a cohort of normal Singapore Chinese eyes to characterize its distribution.
99 right eyes were selected from the Singapore Chinese Eye Study database. Median age was 53 years (range 44 - 76) and 52 subjects were male. Each eye was imaged using the Cirrus OCT system using a standard 4x4x4mm cube acquisition protocol. Within each volume, the ILM and RPE/BM were automatically segmented using the native Cirrus algorithm. 36 interpolated radial B-scans were extracted from each volume. The innermost termination of the RPE/BM signal was assumed to be the BMO, thereby yielding 72 data points per 5 degree increment. We developed an algorithm, coded using Matlab, which measured the shortest distance between the coordinates of each BMO point and the ILM. The extracted MRW measurements were plotted across each eye quadrant to characterize the sectoral distribution of MRW. Mean MRW of each eye was calculated and associations with age were evaluated using linear regression. Furthermore sectoral distribution and mean MRW derived from the 4 'cardinal' B-scans versus 36 B-scans were compared.
The distribution of MRW measurements was consistent with the expected distribution with the rim thickest inferiorly, followed by superior, nasal and temporal sectors. Average of Mean MRW across this cohort was 294um +/- 62um. Linear regression identified that mean MRW decreases with increasing age (p<0.05). Paired T-test of mean MRW computed from 36 B-scans and 4 B-scans showed no statistically significant difference (p=0.69). The plots of sectoral distribution of MRW computed using 36 B-scans demonstrated a similar distribution as MRW computed from 4 B-scans.
It is possible to extract MRW measurements using Cirrus volumes containing automatic segmentations. We demonstrate that the sectoral MRW distribution follows the expected pattern in these eyes and that MRW appears to decrease with age. Our results suggest that MRW derived from 4 cardinal B-scans may be sufficient for assessment for global and sectoral analyses. The utility of this parameter in glaucoma case finding will need to be assessed.
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