The intracranial internal carotid artery (ICA) has petrous, presellar, intracavernous, and supraclinoid segments. In the illustrations and legend keys we further subdivide those segments into the vertical and horizontal petrous, presellar (Fischer C5), horizontal intracavernous (Fischer C4), anterior genu (Fischer C3), as well as the proximal and distal supraclinoid segments. This is the nomenclature we find most helpful in discussing the anatomy and the relevant pathology.

After entering the skull base the internal carotid artery travels within the vertical and horizontal segments of the petrous carotid canal before emerging near the petrous apex. It courses over the foramen lacerum in the distal horizontal segment and then assumes a vertical orientation to become the presellar or Fischer C5 segment. Near the posterior aspect of the sella turcica the ICA changes from a vertical to an anteriorly projecting horizontal path to become the Fischer C4 or horizontal intracavernous segment. The size and extent of the cavernous sinus is variable and therefore the level where the ICA pierces the dura is also variable. For the purposes of this atlas, we will consider that the ICA pierces the dura of the cavernous sinus near the Fischer C5-C4 junction.

The intracavernous ICA gives rise to several small branches. The meningohypophyseal artery (MHA) trunk most often arises from the posterior wall of the ICA at the Fischer C5-C4 junction. This normally small vessel can often be seen on high-resolution two-dimensional digital subtraction angiography (2D DSA).