Novel approach towards colour imaging using a scanning laser ophthalmoscope

Abstract

Aims-Conventional fundus imaging using a fundus camera produces colour fundus pictures. The scanning laser ophthalmoscope (SLO) has the advantages of lower levels of light exposure, improved contrast, and direct digital imaging but until now has produced monochromatic images as a laser of single wavelength is used. True representation of the fundus is possible by combining images taken using blue, green, and red lasers. Methods-A custom built SLO was used to capture blue, green, and red fundus images from suitable volunteers and patients with fundus disease. Images were corrected for eye movement and combined to form a colour image. Colour fundus photographs were taken using a fundus camera for comparison with the SLO image. Results-The background fundus and retinal vasculature had similar appearances with the two imaging modalities. Internal limiting membrane reflections were prominent with the SLO. Identification of new vessels in the diabetic fundus was easier with the SLO than the colour fundus photographs. Conclusion-A colour SLO oVers all the advantages of the present monochromatic imaging system with the added advantage of true colour representation of the fundus. (Br J Ophthalmol 1998;82:342-345) Confocal scanning laser technology has been in routine use for imaging of the fundus in a variety of diseases for a number of years.1-5 A prototype SLO was built in Aberdeen 6 and has been used to study device performance characteristics 7 and evaluate its potential in routine clinical diagnosis. In particular, recent work has studied the spectral reflectance of the fundus at wavelengths ranging from 488 nm to 830 nm. 8Colour fundus photography remains the gold standard for the documentation of fundus pathology. The SLO imaging system oVers some advantages over fundus photography such as improved image contrast by the selection of appropriate wavelength for the laser, 8 reduced power levels, and confocal imaging but the images are monochrome. In order to overcome this we have developed a colour SLO imaging system. Patients are imaged using three primary colour lasers and the resultant images are combined to form a true colour image. In this paper, we describe the imaging system and compare selected colour SLO images with colour fundus photographs. MethodsUsing the custom built Aberdeen SLO fundus images were captured at a resolution of 768 × 512 pixels using a frame grabber in an IBM compatible PC at blue (argon, 488 nm), green (coumarin dye, 547 nm), and red (diode, 670 nm) wavelengths. Image acquisition time was 40 ms per frame, but allowing for the change in wavelength the total imaging time was less than 1 minute. Raw images were registered to correct for eye movement using Visilog software (Noesis, France) running on a SUN Sparc station. The intensity of the registered images was then corrected to take into account the variation in the photodiode response at different wavelengths. These correction factors were computed from images of a uniformly reflective model eye with a fundus made of Spectral...

Improvements in Colour Fundus Imaging Using Scanning Laser Ophthalmoscopy

“…In the colour SLO images there is a bright central region surrounded by a darker ring corresponding to the neuroretinal rim. This feature is present at all three wavelengths and is consistent with other studies [10,12]. In photographic images the optic disc is brighter and the intensity distribution is more uniform.…”

Retinal Image Processing in Biometrics

“…Retinal imaging devices are primarily used in the earlier screening and diagnosis of ocular (retinal) diseases. The principal imaging technologies for the retina, are scanning laser ophthalmoscopy (Scanning laser ophthalmoscope -SLO) [4] and Optical Coherence Tomography (OCT) [5] and fundus camera imaging [6].…”

Laser imaging of the retina

“…True colour fundus imaging with a scanning laser ophthalmoscope is possible by sequentially imaging the fundus using blue, green, and red lasers 26. Colour SLO imaging has the obvious advantage of creating a colour view of the fundus with which clinicians are most familiar and for differentiating various pathological conditions such as cotton wool spots and drusen which have a similar colour.…”