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Sarah MacIver, Jerome Sherman, Natalie Hutchings; Clinical Utility of Thresholding Segmentation in Ultra-Widefield (Optos®) Fundus Autofluorescence Images. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5513.
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Fundus Autofluorescence (AF) is an imaging modality that indicates retinal pigment epithelium (RPE) integrity with hypo-AF signifying RPE loss /atrophy and hyper-AF signifying RPE metabolic stress. This study investigates the clinical utility of thresholding segmentation for identifying areas of hyper/hypoAF in panoramic AF (PAF) images
A retrospective review was carried out for 20 PAF images of normal (n=2) and diseased (n=18) retinas, including maculopathies, posterior uveitis, and other peripheral retinal disorders. All PAF images were captured with the Optos 200Tx (200° ultra-widefield). Image analysis was carried out using Image J (NIH Image). A circular region of interest (ROI) centered around the macula and free from imaging artifacts was selected, eliminating the far periphery The maximum entropy threshold (MET) and the mean and SD of the grey level in ROI was determined. Four levels of AF were identified: High AF: pixels above MET +1SD, High-Mod AF: pixels above MET, Mod-Low AF: pixels below MET-1SD, and Low AF: pixels below mean gray-1SD. The percentage area of the ROI containing each AF classification of pixels was determined
In normal images the MET and mean threshold were at a similar grey level increment. In diseased eyes, the average grey level for the MET was significantly higher than the average mean threshold (MET=118.8; Mean T= 73.3; paired t-test p<0.0001). The percentage area of the whole image showing mod-high and high AF was smaller for those cases with pathology (Mean [Range]: High AF 5.53% [0.43-14.11] Mod-High AF 15.49% [2.24-36.42]) than in those without pathology (Mean: High AF 20.28% Mod-High AF 49.53%), Cases with pathology had large areas with mod-low AF (Mean 65.10%) than those without pathology (Mean 33.80%). However, the percentage area of each AF classification was dependent on the condition. In diseased eyes, smaller percentage areas of the ROI exhibited moderate AF than in the normal eyes.
Segmentation using the MET and mean grey level offers an objective assessment of fundus AF images. MET and mean threshold were similar in normal eyes and showed large areas of moderate AF. Diseased eyes showed larger areas of High- or Low-AF (condition dependent) than moderate AF. These techniques offer potential for quantitative, longitudinal assessment of AF and, hence, RPE integrity
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