Digital fundus imaging with a scanning laser ophthalmoscope
A prototype scanning laser ophthalmoscope was constructed based on the original principles of Webb et al (1987 and 1990). The basic design was, however, altered in a number of ways to give the flexibility needed for what was primarily a research instrument with a requirement to use lasers of various wavelengths and to carry out confocal imaging with a variety of slice thicknesses. Some of the performance characteristics of the prototype SLO, such as modulation transfer function, axial resolution and uniformity of illumination were measured and are presented. The SLO can operate in three different modes; direct, confocal and indirect. The direct mode imaging has a large depth of focus and collects light from all layers of the fundus. High contrast imaging is possible due to scattered light being significantly reduced as only a single spot is illuminated at a time. Confocal imaging further improves the contrast by collecting light only from a small slice of the fundus. The capacity to carry out confocal imaging is a significant advantage over fundus cameras. By moving the confocal aperture, light from different layers could be collected enabling the visualisation of various lesions. The indirect mode collects only the scattered light, avoiding the directly reflected light. The clinical value of the equipment was evaluated with a small number of patients with various pathological conditions from the Eye Out-patient Clinic of the Aberdeen Royal Infirmary. The images from the SLO were compared with fundus photographs. The confocal capability, together with the penetrating capacity of near infrared illumination, was utilised to demonstrate the ability to visualise deep lying structures such as choroidal vessels and intra- retinal abnormalities. Near infrared illumination was found useful for imaging through media opacities, such as cataracts. The indirect mode was found useful for visualising structures such as drusen and epiretinal membrane. It is concluded that near infrared monochromatic imaging with the SLO offers a 'new' picture of the fundus. The confocal capability provides the ability to produce 3-D images of the fundus. The near infrared reflective characteristics of many normal and pathological tissues appear to offer new clinical insights into fundal pathology. However, more work is needed to fully appreciate the clinical potential of this new imaging modality.