The arterial system is responsible for the transport of oxygenated blood to organs and tissues.
Some pathological conditions can interrupt or reduce the blood supply,
for breach of continuity of this system.
We can enumerate the trauma,
the arteritis and atherosclerosis.
Atherosclerosis,
in particular,
can lead to progressive reduction of the volume of blood to an organ,
and is associated with widely known and proven risk factors,
such as hypertension,
obesity,
diabetes,
hypercholesterolemia,
sedentary lifestyle,
chronic kidney disease,
smoking,
among others.
The body has defense mechanisms to maintain the blood supply.
We can enumerate the resistance fall of peripheral bed (by vasodilation),
compensatory increase in blood pressure,
increased heart rate,
compensatory arterial enlargement and formation of collateral circulation.
For collateral circulation mean a set of arteries that provide an alternative flow circuit,
in order to perfuse ischemic or hypoxic bed due to stenosis or occlusion of a core network circulation.
This mechanism occurs throughout the human body,
as the need for each organ and tissue.
Color Doppler is an excellent tool for screening of stenoses or occlusions in the territory of the carotid and vertebral arteries.
On pathologic examination,
several ways of collateral blood circulation can be established in order to maintain normal cerebral perfusion.
Some of collateral circulation pathways can be observed by color Doppler sonography and should be recognized by the examiner.
In obstructive disease of the internal carotid artery (ICA),
the collateral circulation is important to maintain adequate cerebral perfusion.
The primary collateral via the circle of Willis is,
with the possibility to redistribute the flow from the ICA through the contralateral anterior communicating artery or arteries from vertebrobasilar via the posterior communicating artery.
Secondary collaterals routes include the external carotid artery by anastomosis of eleptomeníngeas artery (not assigned to ultrasound) and ophthalmic artery on the surface of the brain.