The carotid siphon is an S-shaped part to the internal carotid artery ICA; it begins at the posterior bend of the cavernous ICA and ends at the ICA bifurcation.
Cavernous and supra clinoid portions of the ICA forms carotid siphon.
Cavernous portion contributes greater part of the carotid siphon, the cavernous portion consist of sub segments as (a) Posterior vertical, (b) Posterior bend, © Horizontal, (d) Anterior bend, and (e) Anterior vertical.
The siphon can have an open or a closed configuration.
A closed siphon anatomy can be attributed to exaggerated tortuosity of the ICA, can be seen in patients with advanced age or fibromuscular dysplasia. Clinical significance is during the endovascular navigation where it becomes difficult to negotiate catheter in such close configuration turn.
During DSA 'Haughton view' is used to open up the carotid siphon and to prevent overlapping of MCA branches within the Sylvian fissure. This view is also helpful for imaging of ICA and MCA aneurysms, PCom origin and anterior choroidal artery. The lateral arc is positioned as if the patient’s head is tilted away from the side of the injection and away from the xray tube. In simple words the X-ray tube should touch the shoulder on the side of interest. 1).
The carotid siphon morphology seems not to be related to PComA aneurysm formation or rupture 2).