Segments of the Internal Carotid Artery

The internal carotid artery (ICA) can be divided into 7 segments:

• C1: Cervical segment - From the bifurcation to the carotid canal
  
  
• C2: Petrous segment - Enters the skull base at the carotid canal. Has vertical and horizontal subsegments, forming a genu.
  
  
• C3: Lacerum segment - Begins at the end of the petrous canal and ends at the petrolingual ligament. Travels above (not through) the foramen lacerum.
  
  
• C4: Cavernous segment - within the cavernous sinus. Has 3 subsegments: posterior vertical or ascending (longer), horizontal, and anterior vertical or ascending (shorter). The proximal dural ring forms the roof of the cavernous sinus and is continuous with the dura covering the inferolateral aspect of the anterior clinoid process.
  
  The meningohypophyseal trunk, also known as the posterior trunk, is usually seen on angiography arising posteriorly to supply the the pituitary gland (via the inferior hypophyseal artery) and the meninges overlying the tentorium (via the marginal tentorial artery, also known as the artery of Bernasconi-Cassinari) and the meninges overlying the clivus (via the clival branches).
  
  The persistent trigeminal artery (rare) typically arises from the distal posterior vertical segment.
  
  
• C5: Clinoid segment - The shortest segment. Bounded inferiorly by the proximal dural ring and distally by the distal dural ring, and located extradurally. Spans a wedge-shaped area along the superior aspect of the anterior genu.
  
  Internal carotid artery aneurysms arising from the clinoid segment have a lower risk of rupturing into the subarachnoid space than those arising from the supraclinoid segments.
  
  
• C6: Ophthalmic segment - Terminates just before the posterior communicating artery origin. It arches posteriorly and slightly superiorly and is located intradurally
  
  Internal carotid artery aneurysms arising from the supraclinoid segments have a higher risk of rupturing into the subarachnoid space than those arising from the clinoid segment.
  
  
• C7: Communicating (terminal) segment - Another small segment. Begins at the origin of the posterior communicating artery and ends at the bifurcation of the ICA into anterior and middle cerebral arteries.

Reference

• Alleyne CH, Coscarella E, Spetzler RF, Walker MT, Patel AC, Wallace RC. Microsurgical Anatomy of the Clinoidal Segment of the Internal Carotid Artery, Carotid Cave, and Paraclinoid Space. Barrow Quarterly - Volume 18, No. 1, 2002.
• Osborn AG, Chapters 3 and 4 in Diagnostic Cerebral Angiography (2nd ed). Lippincott, Williams, & Wilkins (1999).
• Thines L, Delmaire C, Le Gars D, Pruvo JP, Lejeune JP, Lehmann P, Francke JP. MRI location of the distal dural ring plane: anatomoradiological study and application to paraclinoid carotid artery aneurysms. Eur Radiol. 2006 Feb;16(2):479-88.