The retinal blood vessels nourish the inner layers of the retina and they form trees
like networks (Forest) emerging at the optic disc and spreading over the retina
by branching and thinning. On the temporal retinal side, the networks ’hang around’
the macula by following double parabolic paths. Close to the optic disc rim, the
venous tree alternates with the arterial tree around the optic disc unless they
cross each other. The blood vessels vary in size from very large, down to capillaries
that are too fine to be captured by camera. The blood vessel networks are amendable
to quantitative and qualitative analysis by image analysis techniques.
The retinal vascular tree is known to be affected by ageing and diseases such as
hypertension and diabetes mellitus. Changes in vessel structure vary between arteries
and veins. Tortuosity has a very different course in arteries and veins while retinopathy
progresses, and some lesions are peculiar to only one type of vessels, e.g. focal
narrowing for arteries, beading for veins.