Page 87 - Computational Retinal Image Analysis

CHAPTER

Automatic landmark detection

in fundus photography 5

a
c
b
Jeffrey Wigdahl , Pedro Guimarães , Alfredo Ruggeri
a VisonQuest Biomedical LLC, Albuquerque, NM, United States,
b Chair for Clinical Bioinformatics, Saarland University, Saarbrücken, Germany,
c Department of Information Engineering, University of Padua, Padua, Italy

1 Background
This section provides background anatomical information for the retinal land-
marks of interest, the optic disc and the macula lutea (including the fovea centralis).
Researchers have been using this information to help them extract the pertinent fea-
tures that become the basis for their automatic detection algorithms. It is recognized
that the main vessel arcades also serve as retinal landmarks, but the amount of infor-
mation and developed techniques for vessel segmentation require a separate chapter
to properly cover.

1.1 Optic disc
The optic nerve head (optic disc (OD)) is the most distinguishing feature of the ret-
ina. It does not have the same layers as the rest of the retina, containing only the
nerve fiber layer and the internal limiting membrane. This accounts for the pale,
yellowish appearance (Fig. 1), as there is no retinal pigment epithelial layer with
the dark melanin to absorb light. There are no photoreceptors in this region which
makes it our natural blind spot. This is also the region where all of the ganglion cell
axons converge and exit the eye through the optic nerve to the brain. The OD also
contains the optic cup, which presents in fundus photography as a bright central
region, and whose inner and outer layers become the retinal neural and pigmented
layers respectively.
The average optic disc is an oval with a major axis of approximately 1.8 mm
and minor axis of 1.5 mm [1]. It shares a geometric relation with the fovea, gener-
ally being located 3–4 mm to its nasal side and 3° above. The optic disc is also the
entry point for the main retinal vessels, the central retinal artery and vein. The vessel
arcades leave vertically from the optic disc before curving into a parabolic shape
around the macula.

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