Intracranial Arterial System Circle of Willis


The internal carotid artery (ICA) can be conceptually divided into three main segments: cervical, petrosal, and intracranial. The cervical portion extends from the carotid bifurcation to the base of the skull. This segment has no branches. The petrosal segment begins after the artery enters the base of the skull and travels through the carotid canal in the petrous portion of the temporal bone. The artery then crosses the foramen lacerum and enters the cavernous sinus. The intracranial portion begins distal to the petrosal segment and proximal to the anterior clinoid process. At this point, the presellar and juxtasellar portions of the ICA are distinguished. The juxtasellar portions lies within the cavernous sinus and is closely associated with cranial nerves III, IV, VI, and the ophthalmic and maxillary divisions of cranial nerve V. Meningohypophyseal branches arise from the presellar and juxtasellar portions to supply the meninges and the posterior lobe of the hypophysis. The ICA pierces the dura mater medial to the anterior clinoid process to become supraclinoid.

The first major branch of the ICA, the ophthalmic artery, arises at the level of the anterior clinoid process ( ..Fig,,22:3 ). After initially running intracranially, this artery traverses the optic canal and then goes to the orbit. The most important of the ocular branches is the central retinal artery. Numerous anastomotic channels between the internal and external carotid arteries ultimately involve the ophthalmic artery.

After giving rise to the ophthalmic artery, the ICA gives rise to the posterior communicating artery (PCoA) and then the anterior choroidal artery (AChA). The AChA has a proximal or cisternal segment that courses posteromedially below the optic tract and medial to the uncus of the temporal lobe. The artery then turns laterally and passes through the crural cistern and around the cerebral peduncle to enter the choroidal fissure of the temporal lobe. y The distal or plexal segment of the AChA starts at the choroidal fissure and goes posteriorly in a cleft of the temporal horn. The AChA may terminate near the lateral

Figure 22-3 Lateral internal carotid artery angiogram, arterial phase. 1, Internal carotid artery. 2, Ophthalmic artery. 3, Anterior cerebral artery. 4, Orbitofrontal branch. 5, Frontopolar branch. 6, Callosomarginal branch. 7, Pericallosal branch. 8, Middle cerebral artery branches/Courtesy of Karen S. Caldemeyer, M.D.)

geniculate body or extend around the pulvinar of the thalamus. y There is a rich anastomotic relationship between the AChA and the lateral posterior choroidal, posterior communicating, and posterior cerebral arteries. Therefore, the regions supplied by the AChA and its branches can be extensive, but also variable. Possible regions supplied by this artery include the choroid plexus of the temporal horn, the amygdaloid nucleus, the piriform cortex and uncus of the temporal lobe, the hippocampus and dentate gyri, the lateral geniculate body, the optic tract and the origin of the optic radiations, the genu and inferior and medial parts of the posterior limb of the internal capsule, the medial globus pallidus, tail of the caudate nucleus, and the upper brain stem.

After giving off the AChA, the ICA bifurcates to form the anterior cerebral and middle cerebral arteries (...Figs 2.2.i3 and 22-4. ).


The circle of Willis is an arterial anastomosis at the base of the brain that connects the internal carotid and vertebrobasilar arterial systems. The circle surrounds the ventral surface of the diencephalon adjacent to the optic nerves and tracts. The anterior portion of the circle of Willis consists of the two internal carotid arteries, the anterior cerebral arteries, and the anterior communicating artery. The posterior portion of the circle consists of the proximal portions of the posterior cerebral arteries and the two posterior communicating arteries.


The anterior cerebral artery (ACA) arises below the anterior perforated substance and runs anteromedially to

Figure 22-4 AP internal carotid artery arteriogram, arterial phase. 1, Internal carotid artery. 2, Anterior cerebral artery. 3, Middle cerebral artery. 4, Medial lenticulostriate a(t:ourtesy of Karen S. Caldemeyer, M.D.)

the interhemispheric fissure, where it joins the opposite ACA by way of the anterior communicating artery (ACoA). The ACA supplies the medial surface of the cerebrum and the upper border of the frontal and parietal lobes. y

The ACA is often divided into A1 and A2 segments. The A1 segment is the horizontal, proximal portion that extends from the origin of the ACA to its union with the contralateral ACA by way of the ACoA. The A2 segment is the portion of the ACA that is distal to the ACoA. It runs superiorly into the interhemispheric fissure, coursing around the genu of the corpus callosum. It has an important relationship anatomically to two other midline structures, lying above the optic chiasm and the pituitary gland.

The ACA gives rise to medial lenticulostriate, pericallosal, and hemispheric branches (see Fig. 22-4 ). The medial lenticulostriate branches include basal branches, which supply the dorsal aspect of the optic chiasm and hypothalamus, and the medial striate artery (recurrent artery of Heubner), which supplies the anteroinferior limb of the internal capsule and the anterior aspects of the putamen and caudate nuclei. The callosal branches arise from the pericallosal artery, which is that portion of the ACA distal to the ACoA. Some reserve the term pericallosal artery for the segment beyond the origin of the callosomarginal artery. The ACA and the pericallosal arteries also supply the septum pellucidum and the fornix. The hemispheric branches supply the medial surface of the hemisphere and include the orbitofrontal, frontopolar, internal frontal (anterior, middle, and posterior), paracentral, and internal parietal (superior and inferior) branches.


The middle cerebral artery (MCA), the largest branch of the ICA, arises below the medial part of the anterior perforated substance. It supplies most of the lateral surface of the cerebral hemisphere including the lateral frontal, parietal and temporal lobes, insula, claustrum, and extreme capsule. y

The MCA is divided into proximal, sylvian, and distal segments. The posterior-superior aspect of the proximal segment gives rise to the lenticulostriate arteries that supply the corona radiata, external capsule, claustrum, putamen, part of the globus pallidus, body of the caudate nucleus, and superior portion of the anterior and posterior limbs of the internal capsule. Other branches that may arise from the horizontal segment are the orbitofrontal and the anterior temporal arteries, but there are many variations. The sylvian segment includes all of the branches on the insula of Reil and in the sylvian fissure. After the takeoff of the anterior temporal artery, the main trunk of the MCA usually bifurcates into two branches: a superior and inferior division. Less common patterns include a trifurcation into superior, middle, and inferior divisions and a ramification pattern into four or more trunks. One branch gives rise to the anterior or proximal group of arteries, and the other branch gives rise to the posterior or distal group. The anterior group includes the orbitofrontal, precentral, central, and anterior parietal arteries. The posterior group includes the posterior parietal, posterior temporal, and the angular or terminal arteries.


The posterior communicating artery (PCoA) arises from the posteromedial aspect of the intracranial internal carotid artery and runs caudally and medially, above the oculomotor nerve to join the posterior cerebral artery. The two PCoAs from each side provide a connection between the carotid and vertebrobasilar systems. The posterior communicating arteries supply the anterior and medial portions of the thalamus, and the walls of the third ventricle.


The posterior cerebral arteries (PCAs) usually arise as the terminal bifurcation of the basilar artery ventral to the midbrain ( ..Fig..22-5 ). Approximately 15 to 22 percent of people have a fetal (embryonic) origin of the PCA from the ICA. y , y The PCA supplies the occipital lobes, inferomedial portions of the temporal lobes, midbrain, thalamus, and deep structures including the choroid plexus and ependyma of the third and lateral ventricles. The first branches are the small thalamoperforating arteries that penetrate and supply the midbrain, thalamus, and lateral geniculate body. The next branches are the medial and lateral posterior choroidal arteries that supply the posterior portion of the thalamus and the choroid plexus. An anterior division of the PCA gives rise to the inferior temporal arteries (hippocampal, anterior, middle, posterior, and common temporal arteries) that supply the inferior portions of the temporal lobes. A posterior division includes the two terminal branches of the PCA; the parieto-occipital artery, which supplies part of the cuneus and precuneus, the superior occipital gyrus, and occasionally, the precentral and superior

Figure 22-5 AP vertebral angiogram. 1, Vertebral artery. 2, Posterior inferior cerebellar artery. 3, Anterior inferior cerebellar artery. 4, Basilar artery. 5, Superior cerebellar artery. 6, Posterior cdrebr^(Coudesy of Karen S. Caldemeyer, M.D.)

parietal lobule; and the calcarine artery, which supplies the visual cortex, inferior cuneus, and part of the lingual gyrus. The splenial arteries may arise directly from the PCA or from the parieto-occipital artery.

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