Carotid siphon

Atherosclerosis mainly affects large- and mediumsized arteries. Extracranial manifestations at the carotid bifurcation statistically dominate the intracranial arteries. Besides typical manifestations at the carotid siphon or the vertebrobasilar junction, atherosclerosis is occasionally also found in peripheral intracranial vessel segments. Typical sequelae of atherosclerosis are stenosing plaque formations, ulcerations, dilatations or the evolution of fusiform aneurysms, which can be accompanied by extensive formation of thrombus. 

The internal carotid artery starts as the carotid sinus at the bifurcation of the common carotid artery at the level of the thyroid cartilage. It runs up the neck, without any branches, to the base of the skull where it passes through the foramen lacerum to enter the carotid canal of the petrous bone. It then runs through the cavernous sinus in an S-shaped curve (the carotid siphon) pierces the dura and exits just medial to the anterior clinoid process. It then bifurcates into the anterior cerebral artery and the larger middle cerebral artery. 

Around the orbit. Branches of the external carotid artery can anastomose with branches of the ophthalmic artery if the internal carotid artery is severely stenosed or obstructed. Collateral flow from the external carotid artery into the orbit then passes retrogradely through the ophthalmic artery to fill the carotid siphon, middle cerebral artery and anterior cerebral artery. Sometimes flow may even reach the posterior cerebral artery and vertebrobasilar system. 

Fig. 4.4 Tandem lesion at the origin of the right internal carotid artery arrow) and at the carotid siphon arrow head)...

For evaluating carotid artery stenoses with MD-CTA, the scan range should include the aortic arch and the circle of Wilhs to ensure that stenoses at the origin of the CCA, at the carotid bulb and at the carotid siphon are reUably depicted (Fig. 10.4a,b). To avoid streak artefacts due to high contrast agent concentrations at the superior vena cava at the beginning of the injection, it is recommended to use a craniocaudal scan direction (de Monye et ah 2006). 

From the superior pharyngeal branch, the carotid branch arises to the carotid canal. This carotid branch ascends through the foramen lacerum and accompanies the internal carotid artery up to the cavernous sinus, where it anastomoses with the inferolateral trunk and with the recurrent artery of the foramen lacerum, arising from the C5 portion of the carotid siphon, which supplies the internal carotid artery wall and sympathetic nerve fibers. 

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