Page 88 - Computational Retinal Image Analysis

80 CHAPTER 5 Automatic landmark detection in fundus photography

FIG. 1
Typical retinal image taken using a Canon CR2. Overlays highlight the optic disc, optic cup,
macula and fovea regions.

1.2 Macula lutea
The macula or macula lutea is located near the center of the retina of the human eye.
It appears as a darkened oval region with a yellow tint. The tonality of the macula
derives from lutein and zeaxanthin, two xanthophyll pigments present throughout the
entire retina but with higher concentration in this region. These pigments are gradu-
ally accumulated from the diet, and are nearly non-existent at birth. The macula’s
color acts as a filter, reducing UV radiation damage and chromatic aberration.
The macula is approximately 5.5 mm in diameter and is subdivided into peri-
fovea, parafovea, fovea, and foveola [2]. These areas are not easily differentiated.
The parafovea and perifovea are the inner and outer surrounding peripheric areas to
the fovea, respectively. The fovea is the shallow pit depression in the center of the
macula about 1.5 mm in diameter. Within it, retinal layers are displaced, which leaves
only photoreceptors at its center, the foveola. These center regions also lack blood
vessels. With no other cells and no blood vessels, light passes unobstructed to the
photoreceptors. Within the fovea the highest concentration of cones can be found,
and at its center no rods exist. Thus, the fovea and foveola play a major part in vision
enabling color and detail discrimination.
Curiously, clinical and anatomical definitions of macula and fovea differ consid-
erably. Macula is typically a clinical term that describes the darker coloration seen in
fundus photography at the anatomical fovea. The clinical fovea is then the center of
this area, and what anatomically is described as the foveola.

2 Fovea and disc detection/segmentation—Utility
The following section lists the different use cases for detecting the OD as part of a
pipeline of image understanding and/or disease detection.