Spectral reflectance imaging of the ocular fundus using a scanning laser ophthalmoscope
An investigation of the spectral reflectance of the fundus in normals and patients with common eye diseases was carried out using a multi wavelength scanning laser ophthalmoscope (SLO). With a knowledge of the spectral properties of the principal ocular pigments in the fundus, the appearance of the retina and choroid is predictable in normal subjects. The optic disc has a characteristic dark appearance when viewed with the SLO, and reasons for this may include a filtering effect of the confocal aperture to reduce the return of scattered light to the detector. In patients with macular exudates these features have a high reflectance in green-yellow light. An image processing method has been developed to quantify exudates and this was applied to SLO images and digitised colour slides of the same patients. Results show similar performance of processing in both image types with high accuracy (90% sensitivity for 95% specificity). A similar experiment was carried out on patients with macular drusen. These structures are of lower intensity than exudates and may have less well-defined borders. Again performance of the image processing methods showed broadly similar performance when comparing SLO images and digitised colour slides (60 to 68% sensitivity for 95% specificity). A study was carried out to assess the ability of the SLO to image the fundus in patients with cataract using a range of wavelengths. In conclusion the SLO offers the ability to image the fundus at selected wavelengths to enhance the desired features under investigation. As a fundus camera, used to generate digital images, it is unlikely to offer significant improvements over commercially available digital imaging CCD cameras. However, the SLO has properties which are likely to make it an ideal instrument for reflectometry, angiography and fundus topography. These applications are discussed in the final chapter.