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J. M. Beach, G. H. Halldorsson, R. A. Karlsson, S. H. Hardarson, T. Eysteinsson, J. A. Benediktsson, E. Stefansson; Wide Field High Resolution Retinal Imaging Oximeter. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3839.
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We have previously reported non-invasive retinal oximeter [Hardarson et al. IOVS 2006]. Here we show an improved design for dual-wavelength retinal oximetry that substantially increases both the area and resolving power of an imaging oximeter.
The system consists of a fundus camera, an image splitting component and a digital camera. The recorded area of the retina is set by a 5 x 10 mm rectangular window. The image is split into separate optical paths by a dichroic mirror mounted within an interchangable filter cube. Narrow band (5 nm) images at 586nm and 605nm are obtained with interferences filters. Duplicate images at both wavelength are formed side-by-side on the image sensor. Retinal images were obtained using the minimum flash setting with the pupil dilated. Recorded fields of view and the number of pixels across the diameter of first order vessels (n = 5) were evaluated using the new system (fundus camera set to 35o field of view (FOV),) and using the previous system [Hardarson et al. IOVS 2006]. Average pixel gray level values from the same area of the retina were determined for each system to compare dynamic ranges of images.
Previous system the recorded area was 13.8 (mm2) and is 31.2 (mm2) with the new system. The recorded area may be increased to 50.9 (mm2) using the fundus camera in 50° FOV. The average vessel diameter was 4.8 pixels with the previous system and increases to 13.2 pixels with the new system using the fundus camera in 35° FOV. At a 20° FOV the venule diameter is 20 pixels. Average pixel gray level values with was 970 and increases to 5480. Results of automated analysis of vessel saturation are seen in the attached figure.
The new format significantly increases the recorded field of view of the retinal surface, enabling more of the primary and branched vessels to be examined in single images, and also enables more accurate measurement of the vessel diameter. Higher quantum efficiency of the image sensor is a primary factor contributing to our higher dynamic range, which corresponds to a significant increase in measurement sensitivity.
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