The ICA usually originates from the CCA at C3–4 level. This segment has two distinct parts:

There are no named branches that arise from the cervical segment. There can be considerable variation in the level of the carotid bifurcation. A medial origin of the ICA is common and may require an oblique projection to view the bifurcation clearly. Tortuosity and looping of the ICA are also visualized frequently and may present technical challenges to endovascular procedures.

Anomalies of the cervical ICA are uncommon with a congenital absence of the ICA being an extremely rare example. An absent or hypoplastic bony carotid canal on CT scan can present a clue to distinguish a developmentally hypoplastic ICA rather than an occlusion of a normally developed vessel [9]. Two important anastomotic channels that can originate from the cervical ICA are the persistent hypoglossal artery, which may connect it to the basilar artery, and the proatlantal intersegmental artery, which could anastomose with the vertebral artery.

The petrous ICA has a vertical subsegment, a genu, and a horizontal subsegment. It may give off two small branches. The first of these is the Vidian artery (artery of the pterygoid canal) that more often arises from the ECA. As described in the previous section, it is an important anastomotic connection between the ICA and the IMAX, a branch of the ECA. The second is the caroticotympanic artery that supplies the middle ear cavity and may anastomose with the inferior tympanic artery, a branch of the APA.

Anomalies of the petrous ICA are rare but of clinical importance. An aberrant ICA may rarely be seen and can present as a retrotympanic pulsatile mass or can be found incidentally where significant bleeding from ear surgery may be encountered (see Chap. 16). The artery may also be mistaken for a middle ear tumor. A persistent otic artery is another anastomotic connection between the ICA and the basilar artery, which is extremely rare [10].

This segment begins at the end of the petrous carotid canal, courses above the foramen lacerum, and ends at the petrolingual ligament. Usually, it has no branches, although rarely the Vidian artery may originate from the C2–C3 segment junction.

The cavernous ICA begins at the petrolingual ligament and ends as it leaves the cavernous sinus through the dural ring. It is the most medial structure in the cavernous sinus, and the CN III, IV, and V1 and V2 run lateral to it, whereas CN VI runs inferolaterally. It is visualized well on a lateral view where it can be seen to have a posterior genu, a horizontal segment, and an anterior genu. This segment gives off three branches. The posterior trunk or the meningohypophyseal trunk (MHT) arises from the posterior genu and is often seen on angiography. Its branches include the inferior hypophyseal artery which supplies the pituitary gland, the marginal tentorial artery (MTA) which supplies the tentorium, and the clival branches to the clivus. The MTA, also known as the artery of Bernasconi–Cassinari, is sometimes involved in the pathologic supply of arteriovenous malformations or tumors, typically associated with tentorial meningiomas [11]. The lateral trunk or the inferolateral trunk (ILT) supplies the CN III, IV, and VI and dura of the cavernous sinus. It anastomoses with branches of the internal maxillary artery and the middle meningeal artery. In addition to these the cavernous ICA also gives off small capsular branches, which supply the pituitary gland. Anatomical variants of the cavernous ICA include increased tortuosity in this segment and paramedian ICAs. The latter, also known as “kissing” ICAs, occurs when the two ICAs course medially through the sella turcica. An important anomalous connection is a persistent trigeminal artery, which is the most common carotid–basilar anastomosis [12]. It arises from the posterior genu and is associated with increased prevalence of other vascular anomalies and aneurysms. The posterior genu is also often the site of direct carotid–cavernous fistulae formation [13].

This is the shortest ICA segment and lies between the proximal and distal dural ring, making it an interdural structure. It has no named branches but may give off small capsular arteries. Rarely, the ophthalmic artery may arise from this segment.

The first intradural segment of the ICA, the ophthalmic segment is surrounded by CSF. The first major intracranial ICA branch, the ophthalmic artery arises from this segment. It runs anteriorly through the optic canal and gives off three types of branches. The ocular branches provide arterial supply to the choroid and retina and include the central retinal artery and the ciliary arteries. Orbital branches of the ophthalmic artery supply the extraocular muscles and the orbit periosteum [14]. An important orbital branch is the lacrimal artery which may anastomose with the middle meningeal artery through the recurrent meningeal artery. An important but rare anatomic variant is a middle meningeal artery which may arise from the ophthalmic artery [15]. Extraorbital branches of the ophthalmic are also important sites of anastomoses with the ECA circulation through ethmoidal and facial arteries. The other important ICA branches from the C6 segments are the superior hypophyseal arteries which may be one or more in number. The C5 and 6 segments together make up the carotid siphon and are best visualized on lateral views on angiography [16].

The C7 segment is the terminal ICA segment and begins proximal to the origin of the posterior communicating artery (PCom) and ends at the ICA bifurcation into the anterior and middle cerebral arteries. The first major branch from this segment is the PCom that arises from the posterior aspect of the ICA, runs above CN III, and gives off several small perforating branches. An important anatomical variant of the PCom is when it may supply the entire PCA territory in the absence of a P1 segment, also called a “fetal PCA” (Fig. 2.6). Another frequently encountered variant is a dilatation at the origin of the PCom called an infundibulum. It is important to distinguish these structures from true aneurysms. The second major branch of this segment is the anterior choroidal artery (AChA), which arises from the posteromedial aspect of the C7 segment. It has two segments, the cisternal and intraventricular. It has a variable but important territory of vascular supply including the optic tract, the posterior limb of the internal capsule, cerebral peduncle, medial temporal lobe, and choroid plexus. Rarely, a hypoplastic or hyperplastic AChA may be present. Another rare anomaly is when an AChA may arise proximal to the PCom.