Page 63 - Computational Retinal Image Analysis

5 Conclusions 53

state defined by the magnitude of the birefringence, the relative orientation of light
direction and the optic axis and the thickness of the material traversed by the light.
Non-isotropic crystals, such as quartz and calcite, have very well defined birefrin-
gence that is commonly exploited in optical instruments to control polarization. But
form birefringence exists in biological materials where anisotropy is due to aligned
cells; such as are found in the cornea, the lens and the retinal nerve fiber layer [63]. In
consequence, linearly polarized light incident upon the cornea will be converted into
elliptically polarized light transmitted through the lens and cornea. Furthermore this
change in polarization state varies across the aperture of the pupil with the variation
in the orientation of the cells and the thickness of the layers of cells that compose
the cornea and lens. In consequence polarization modification by the lens and cornea
varies both across the pupil of the eye and across the retina and also between eyes. In
consequence polarization cannot be reliably used to remove specular reflections, but
with care it can be used to provide some suppression of reflections and to extract ad-
ditional information the retina [64]: for example polarimetric imaging of the retinal
nerve-fiber layer exploits the form birefringence arising from the co-alignment of
nerve fibers to provide an estimate of the thickness of the RNFL [65] although this
requires a careful calibration of the effects of birefringence of the cornea and lens and
OCT is now the most commonly used technique for this measurement.
We have so far described the importance of polarization on specular reflections,
but most of the light used to form an image of the retina is unpolarized, even when
the illumination is polarized. This is because light incident upon the complex struc-
ture of the retina tends to undergo many random reflections (scattering events) from
an extended volume of the many biological components of the retina, such as the
blood cells, nerve fibers, connective tissue etc. and these many reflections tend to
randomize the light-ray polarization orientation such that for many light rays the
resultant effect is to produce depolarization of light. In consequence, images of the
retina are characterized typically by unpolarized light due to multiple scattering com-
bined with strong polarization effects due to the cornea, lens, retinal nerve fiber layer
and specular reflections from blood vessels, the optic nerve head and nerves. Also
particularly notable is the central reflex within images of blood vessels which varies
between images. The origin of this reflex may be associated with specular reflections
from the vessels, overlying tissue of from the alignment of blood cells that is associ-
ated with laminar flow of blood [66]. The reflex can be partially attenuated using
cross polarized light—although with erratic efficiency.

5 Conclusions
In this chapter we have reviewed the main retinal imaging instruments and the how
their imaging modalities interact with the physics of light in the eye. The optics and
geometry impose limits on the transverse and axial achievable resolution and intro-
duce difficulties in achieving reflex-free illumination and imaging of the retina. We dis-
cussed how they produce the specific characteristics of recorded images in particular