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Jonathan P. Greenberg, Theodore Smith, Tomas R. Burke, Tobias Duncker, Janet R. Sparrow, Francois C. Delori; Quantitative Autofluorescence Measurements with the Scanning Laser Ophthalmoscope (SLO). Invest. Ophthalmol. Vis. Sci. 2011;52(14):4032.
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© ARVO (1962-2015); The Authors (2016-present)
To evaluate the feasibility and reproducibility of quantitative measurements obtained from fundus autofluorescence (AF) images.
AF images were acquired with SLO’s (HRA2 and Spectralis, Heidelberg Eng.) modified by insertion of an internal AF reference to account for variable laser power and detector sensitivity. All subjects (10-60 y/o) had normal retinal status and clear media. A 20 s bleaching period (488 nm) preceded each test. Each 30° image was a mean of 9 images without histogram stretching. Image analysis yielded mean gray levels (GL) in fixed retinal areas. Quantified autofluorescence (qAF) was derived accounting for the calibrated reference, the zero GL, and the magnification (refractive error). Images from subjects and from fixed patterns were used to test linearity of the system, field uniformity, effect of refractive error, and reproducibility.
The linear detection range was found to generally extend up to 175 GL. Field uniformity in 5 subjects was better than 5% in a central 20°-diameter circle but dropped to 15±4% at the edge of the field. Using model eyes to simulate refractive errors, an inverse relationship was found between the recorded AF and the square of the image magnification. Different day reproducibily was evaluated (1-2 images/session, different sensitivities) in 17 eyes (13 subjects): the median absolute difference in qAF expressed in % of their mean was 4.1 and 5.1 % for the fovea and a temporal site, respectively (IRQ: 6.9 and 7.4). Measurements with the Spectralis and the HRA2 gave similar qAF (p>0.2). Median inter-instrument reproducibily in 9 eyes (6 subjects) was 7.4 and 8.0 %, respectively (IRQ: 6.8 and 6.8).
qAF measurements can be achieved with reasonable reproducibility. Variability appears to have an ocular rather than an instrumental origin. Based on our better understanding of the variability, we are developing optimal protocols for these measurements. For retinal degenerations, this method may enhance our understanding of disease processes, and may serve as a diagnostic aid, as a more sensitive marker of disease progression and as a tool to monitor the effects of pharmaceutical interventions.
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