Page 34 - Computational Retinal Image Analysis

24 CHAPTER 3 The physics, instruments and modalities of retinal imaging

spatial frequencies such that the contrast of smaller features in the image is reduced
in relation to their spatial frequency content and above the optical cutoff frequency,
ν = Dn , where D is pupil diameter, the contrast is reduced to zero. This is in addition
c
λ f
to contrast reduction by scattering in the ocular media [15], which in many cases
dominates, particularly in the eyes of older subjects, and is a major issue in image
analysis based on measurement of image contrast.
A practical limit on spatial resolution is associated with simple blurring due to
incorrect focusing of the ophthalmoscope including correction for refractive error of
the eye—an error greater than about 0.5 Diopters can produce a significant reduction
in contrast of the smaller features. Furthermore, as for any optical instrument the eye
has a small depth of field over which sharp focus is achieved and this decreases with
increasing pupil diameter according to the laws of diffraction and the aberrations of
the eye [16]. Typically the depth of field is about 50 μm; that is, less than the thick-
ness of the retina, but the lack of sharp features in the retina means that variations
in focus within retinal images is not generally obvious, except around the optic disc
(particularly when it is distorted by high intra-ocular pressure) or as a result of se-
vere distortions of diseased retinas. Recording multiple images with a small range
of focuses can be useful so that post hoc assessment can select the best-focused im-
age. This is particularly important where accurate quantification of contrast of, for
example, blood vessels, is important.

2.3 Glare, contrast and image quality

The reflectivity of the eye fundus is rather low and typically below 1% in the blue to
about 10% at red and infrared wavelengths [17] (see Section 2.5). This means that
reflections from the surfaces of the cornea and eye lens may be significantly stron-
ger than the light received from the retina, and may appear as localized reflections
as well as glare. Thus, mitigation of reflections is a major issue for retinal imaging.
Four so-called Purkinje reflections, depicted in Fig. 3, originate at the outer and
inner surfaces of the cornea and the anterior and posterior surfaces of the eye lens.
The refractive index change is the greatest for the first Purkinje reflection from front
surface of the cornea and so, according to the Fresnel relations, the intensity of this
reflection is much greater than the other three Purkinje reflections.
Cross-polarized illumination and imaging provides a simple mechanism to at-
tenuate these reflections. Laser light is normally polarized, but light from LEDs,
flash lamps and incandescent filaments, such as are used in fundus cameras and
indirect ophthalmoscopes is naturally unpolarized and can be simply polarized us-
ing polarizers. Transmission of light through biological media and scattering tend to
depolarize light however and so this rejection of reflected or scattered light through
cross-polarized imaging is imperfect. See Section 4 for more details.
When polarized light is reflected at a surface or undergoes a single or a small
number of scattering events (from blood cells for example) it tends to retain its po-
larization. Conversely, light that is scattered multiple times is effectively depolar-
ized. This is the case for light diffused and backscattered within the retinal volume,