Page 49 - Computational Retinal Image Analysis

3 Ophthalmic instruments 39

3.6 Handheld retinal cameras
In the recent years there has been an increased interest in low-cost, handheld retinal
cameras. Some of these devices result from an emphasis on portability while provid-
ing useful imaging quality, while some are focused on very low cost, and some are
even based on attaching an adaptor to smartphones. In general, the compromises
involved in the optical design of such devices mean it is difficult to yield consistent
and good image quality, especially for moderate to high fields of view. Common
problems are low contrast and strong reflection artifacts. Although there are no rigor-
ous comparisons on performance, some of these devices are not capable of producing
images with sufficient quality to enable their use in clinical settings.

3.7 Ultrawide field imaging

Imaging with an ultrawide field of view is important for a complete exploration of
the retina, in particular for the detection of diseases that occur in the retinal periph-
ery. Achieving ultrawide field of view however compromises the widely adopted
Gullstrand principle, and normally it is not possible to acquire reflex-free images
with a fundus camera. The Retcam achieves widefield, artifact free imaging by use
of optical contact between the optical head and the cornea to prevent corneal reflec-
tions, but is targeted at imaging of the retinas of neonates, for which the eye contact
can be acceptable. For non-contact eye imaging, the freedom provided by the design
of the Laser Scanning Ophthalmoscope, such as the Optos SLO (which produced
the Optomap image in Fig. 12b) can provide high-quality ultrawide field imaging. A
common approach to extend the field of view is by acquiring a navigated sequence
of narrow-field images, which are stitched with post-detection image processing to
yield a single wide-field image. This requires however some expertise in operation
of the camera and increases significantly the time required to obtain an appropriate
set of images.

3.8 Optical coherence tomography
Optical coherence tomography (OCT) is a non-invasive high-resolution optical imaging
technology based on constructive interference between light back-reflected by the sam-
ple under investigation and some “reference” light. OCT has the potential to generate,
in real-time, cross-section images of the sample, i.e., two-dimensional images in the XZ
or YZ space (X,Y: transversal (lateral) coordinate, Z: longitudinal (axial) coordinate).
Confocal microscopy, as described in Section 3.5 can also be used to image the retina of
the human eye, however due to the limited numerical aperture of the eye and its optical
aberrations, the axial resolution of a confocal microscope is inferior to that provided by
OCT. In confocal microscopy, both lateral and axial resolutions are determined by the
numerical aperture of the eye lens, whereas in OCT, the axial resolution is decoupled
from the optical characteristics of the eye being exclusively determined by the spectral
range of the optical source employed. Consequently, the retina of the human eye can
be imaged with an axial resolution of at least 100 times better than that provided by a