01 Nov 2023
Tohoku University employs oblique illumination to improve imaging of optic disk.
A normal fundus camera illuminates the inside of the eye by shining a ring-shape of light through the edges of a dilated pupil; and while the devices have become steadily more accessible, operating them still requires the expertise of an experienced technician.
Now a project at Japan's Tohoku University has developed a new fundus camera platform intended to be more straightforward to use, potentially allowing patients to take images of their own retina at home.
Published in Journal of Optical Microsystems, the findings may open a path to portable, inexpensive and wide-field fundus cameras that could revolutionize the screening of eye diseases, according to the project.
A key element in the new camera is the use of oblique illumination. Instead of illuminating the back of the eye homogeneously, asymmetric illumination is employed using light shone onto the retina at an angle. This approach means that shadows are cast by both the structures of the fundus and the interfaces of different types of tissues.
"This type of illumination provides a quasi-3D image and importantly improves the contrast of the image," noted the project in its published paper. "Phase gradient imaging of thick scattering samples is also possible. Imaging using oblique illumination is inexpensive and simple. Additionally, it is non-computational, so it allows for observing samples through a camera in real time."
Detection and diagnosis without an ophthalmologist
The trend towards giving patients more direct control over their own retinal imaging has been gathering pace for some time, as advances in optics and image analysis have made devices more compact and allowed valuable diagnostic information to be derived from images gathered outside clinical environments.
An early example was research at MIT Media Lab into how best to assist patients as they attempt to take a focused image of their own retina, using a pattern of dots visible to the patient to indicate when the camera is correctly aligned.
The new fundus camera delivers oblique illumination through the use of two infrared 850-nanometer LEDs, shining light through the white regions of the eye rather than directly through the pupil. Each source illuminates the retina at an angle of 45 degrees.
Illuminating the eye through the sclera avoids the need to dilate the pupil, which simplifies the diagnostic procedure. Using both LEDs at the same time provides homogeneous illumination, while employing only one creates oblique illumination.
The researchers designed the fundus camera using off-the-shelf optical components, simple 3D-printed pieces, and a commercial full HD webcam with minor modifications, according to the team. The handheld prototype device can stream images to a laptop so that the patient can position the camera over one of their eyes while looking at the laptop screen with the other, to ensure proper alignment.
In trials using the eyes of pigs, observation of the shadows cast by retina structures under oblique illumination was used to determine the nature of those structures, using algorithms analogous to those used in astronomy to analyze craters illuminated at an angle by the Sun. The team found that its measurements of the depth of the optical disk in pigs' eyes were in good agreement with those obtained using OCT.
"Our fabricated fundus camera could be an easy-to-handle and low-cost tool for remote detection and diagnosis of ocular diseases without the need of an ophthalmologist," said the project.