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      Effect of ocular magnification on macular measurements made using spectral domain optical coherence tomography

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          Abstract

          Aim:

          The aim of the present study was to study the effect of ocular magnification on macular measurements made using spectral domain optical coherence tomography (OCT).

          Materials and Methods:

          One hundred and fifty-one subjects were included from the normative study of foveal morphology carried out at our hospital. Subjects underwent comprehensive eye examination and macular scanning using Cirrus high-definition OCT and axial length (AXL) measurement. Macular cube 512 × 128 scan protocol was used for scanning the macula. Automated measurements of the fovea namely foveal diameter, foveal slope (lateral measurements) and foveal depth (axial measurement) were taken. A correction factor for ocular magnification was done using the formula t = p × q × s, where “ t” is the corrected measurement, “ p” is the magnification of OCT, “ q” is the ocular magnification, and “ s” is the measurement on OCT without correction. The difference between corrected and uncorrected measurements was evaluated for statistical significance.

          Results:

          Mean AXL was 22.95 ± 0.78 mm. Refractive error ranged from −3D to +4D. Mean difference between measured and corrected foveal diameter, slope and depth was 166.05 ± 95.37 µm ( P < 0.001), 0.81° ± 0.53° ( P < 0.001) and 0.05 ± 0.49 µm ( P = 0.178) respectively. AXL lesser than the OCT calibrated value of 24.46 mm showed an increased foveal diameter ( r = 0.961, P < 0.001) and a reduced foveal slope ( r = −0.863, P < 0.001) than the corrected value.

          Conclusion:

          Lateral measurements made on OCT varied with AXL s other than the OCT calibrated value of 24.46 mm. Therefore, to estimate the actual dimensions of a retinal lesion using OCT, especially lateral dimensions, we recommend correction for the ocular magnification factor.

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          Most cited references24

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          Optical coherence tomography.

          A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.
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            Improvements on Littmann's method of determining the size of retinal features by fundus photography.

            Littmann's formula relating the size of a retinal feature to its measured image size on a telecentric fundus camera film is widely used. It requires only the corneal radius, ametropia, and Littmann's factor q obtained from nomograms or tables. These procedures are here computerized for practitioners' convenience. Basic optical principles are discussed, showing q to be a constant fraction of the theoretical ocular dimension k', the distance from the eye's second principal point to the retina. If the eye's axial length is known, three new methods of determining q become available: (a) simply reducing the axial length by a constant 1.82 mm; (b) constructing a personalized schematic eye, given additional data; (c) ray tracing through this eye to extend calculations to peripheral retinal areas. Results of all these evaluations for 12 subjects of known ocular dimensions are presented for comparison. Method (a), the simplest, is arguably the most reliable. It shows good agreement with Littmann's supplementary procedure when the eye's axial length is known.
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              Choroidal thickness in normal eyes measured using Cirrus HD optical coherence tomography.

              To examine choroidal thickness and area in healthy eyes using spectral-domain optical coherence tomography (SD-OCT). Retrospective, observational case series. Thirty-four eyes (34 subjects), with no retinal or choroidal disease, underwent high-definition raster scanning using SD-OCT with frame enhancement software. Choroidal thickness was measured from the posterior edge of the retinal pigment epithelium to the choroid/sclera junction at 500-microm intervals up to 2500 microm temporal and nasal to the fovea. The central 1-mm area of the choroid was also measured, along with foveal thickness of the retina. All measurements were performed by 2 independent observers. Statistical analysis was used to correlate inter-observer findings, choroidal thickness and area measurements with age, and choroidal thickness with retinal foveal thickness. The 34 subjects had a mean age of 51.1 years. Reliable measurements of choroidal thickness were obtainable in 74% of eyes examined. Choroidal thickness and area measurements had strong inter-observer correlation (r = 0.92, P < .0001 and r = 0.93, P < .0001 respectively). Area had a moderate negative correlation with age (r = -0.62, P < .0001) that was comparable to the correlation between mean subfoveal choroidal thickness and age (r = -0.61, P < .0001). Retinal and choroidal thickness were found to be poorly correlated (r = -0.23, P = .18). Mean choroidal thickness showed a pattern of thinnest choroid nasally, thickening in the subfoveal region, and then thinning again temporally. Mean subfoveal choroidal thickness was found to be 272 microm (SD, +/- 81 microm). Choroidal thickness can be measured using SD-OCT high-definition raster scans in the majority of eyes. Choroidal thickness across the macula demonstrates a thin choroid nasally, thickest subfoveally, and again thinner temporally, and a trend toward decreasing choroidal thickness with age. Copyright (c) 2010 Elsevier Inc. All rights reserved.
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                Author and article information

                Journal
                Indian J Ophthalmol
                Indian J Ophthalmol
                IJO
                Indian Journal of Ophthalmology
                Medknow Publications & Media Pvt Ltd (India )
                0301-4738
                1998-3689
                May 2015
                : 63
                : 5
                : 427-431
                Affiliations
                [1 ]Department of Vitreoretinal Services, Medical Research Foundation, Chennai, Tamil Nadu, India
                [2 ]Department of Vitreoretinal Services, Elite School of Optometry, Chennai, Tamil Nadu, India
                [3 ]Department of Vitreoretinal Services, Birla Institute of Technology and Science, Pilani, Rajasthan, India
                Author notes
                Correspondence to: Dr. Muna Bhende, No. 18, College Road, Chennai - 600 006, Tamil Nadu, India. E-mail: drmuna@ 123456snmail.org
                Article
                IJO-63-427
                10.4103/0301-4738.159877
                4501140
                26139805
                b4c4fa03-3c04-4e97-bbab-d7aeab8f80de
                Copyright: © Indian Journal of Ophthalmology

                This is an open-access article distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

                History
                : 18 October 2014
                : 22 March 2015
                Categories
                Original Article

                Ophthalmology & Optometry
                artifact,foveal diameter,foveal slope,ocular magnification,optical coherence tomography

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