This document describes the anatomy of the major arteries of the anterior and posterior cerebral circulations. It discusses the course and branches of the internal carotid artery as it passes through the petrous, cavernous, and clinoid segments. It also describes the anterior, middle, and anterior cerebral arteries, their branches, and vascular territories. Finally, it summarizes the vertebral and posterior cerebral arteries that make up the posterior circulation.

1. Intracranial Arterial Anatomy

5. Anterior Circulation
• The internal carotid arteries
• The anterior cerebral arteries
• The anterior communicating artery
• The middle cerebral arteries

6. Internal Carotid Artery

9. The Petrous ICA Segment (C2)
• contained within the carotid canal of the temporal bone and is L-shaped
• as it enters the skull at the exocranial opening of the carotid canal, the ICA lies just
in front of the internal jugular vein
• has a short vertical segment, then a genu or "knee“ where it turns anteromedially
in front of the cochlea, and a longer horizontal segment
• exits the carotid canal at the petrous apex
Has two small but important branches
• Vidian artery, also known as the artery of the pterygoid canal: anastomoses with
branches of the external carotid artery
• Caroticotympanic artery is a small ICA branch that supplies the middle ear

11. The Lacerum ICA Segment (C3)
• short segment that lies just above the foramen lacerum and extends from the
petrous apex to the cavernous sinus (CS)
• covered by the trigeminal ganglion of CN V and has no branches

12. The Cavernous ICA Segment (C4)
• has three subsegments connected by two genua
(1) a short posterior ascending (vertical) segment
(2) the posterior genu
(3) a longer horizontal segment
(4) an anterior genu
(5) an anterior vertical ascending (subclinoid) segment
As the cavernous ICA courses anteriorly, it also courses medially
Therefore, on anteroposterior or coronal views, the posterior genu is lateral to the
anterior genu

14. The Cavernous ICA Segment (C4)
• The abducens nerve (CN VI) is inferolateral to the ICA and is the only cranial nerve that lies
inside the CS itself
Has two important branches
• meningohypophyseal trunk arises from the posterior genu, supplying the pituitary gland,
tentorium, and clival dura
• inferolateral trunk (ILT) arises from the lateral aspect of the intracavernous ICA and
supplies cranial nerves and CS dura.
Via branches that pass through the adjacent basilar foramina, the ILT anastomoses freely
with branches from the ECA that arise in the pterygopalatine fossa.
• This important connection between the external and internal carotid circulations may
provide a source of collateral blood flow in the case of ICA occlusion

15. The Clinoid ICA Segment (C5)
• short interdural segment that lies between the proximal and distal dural rings of the
CS
• terminates as the ICA exits the CS and enters the cranial cavity adjacent to the
anterior clinoid process
• has no important branches unless the ophthalmic artery originates within the CS
and not in the proximal intracranial (C6) segment.

16. The Ophthalmic ICA Segment (C6)
• first ICA segment that lies wholly within the subarachnoid space
• extends from the distal dural ring to just below the posterior communicating artery
(PCoA) origin
Has two important branches
• The ophthalmic artery (OA) arises from the anterosuperior aspect of the ICA, then
passes anteriorly through the optic canal together with CN II
- has extensive anastomoses with ECA branches in and around the orbit and
lacrimal gland
• The superior hypophyseal artery arises from the posterior aspect of the C6 ICA
segment and supplies the anterior pituitary lobe (adenohypophysis) and
infundibular stalk as well as the optic chiasm

19. The Communicating ICA Segment (C7)
• last ICA segment and extends from just below the PCoA origin to the terminal ICA
bifurcation into the ACA and MCA
• as it courses posterosuperiorly, the ICA passes between the optic and oculomotor nerves
The most distal ICA segment has two important branches
• PCoA joins the anterior to the posterior circulation
o A number of perforating arteries arise from the PCoA to supply the basal brain structures including the
hypothalamus.
• The anterior choroidal artery (AChA) arises 1 or 2 mm above the PCoA and initially courses
posteromedially, then turns laterally in the suprasellar cistern to enter the choroidal fissure
of the temporal horn
o The AChA territory is reciprocal with that of the posterolateral and posteromedial choroidal arteries (both
are branches of the PCA) but usually includes the medial temporal lobe, basal ganglia, and intralenticular
limb of the internal capsule.

20. Anterior Cerebral Artery
• begins at the terminal portion of the internal carotid artery (after the
ophthalmic branch is given off) on the medial part of the Sylvian
fissure
• travels in an anteromedial course, superior to the optic nerve (CN II)
towards the longitudinal cerebral fissure (anastomoses with the
contralateral counterpart via the short anterior communicating artery
(AComm))
• the paired arteries then travel through the longitudinal cerebral
fissure along the genu of the corpus callosum.

22. Anterior Cerebral Artery
• smaller, more medial terminal branch of the supraclinoid ICA
• runs mostly in the interhemispheric fissure and has three defined
segments

23. Anterior Cerebral Artery
A1 (Horizontal) ACA Segment
• extends medially over the optic chiasm and nerves to the midline,
where it is joined to the contralateral ACA by the ACoA
• two important groups of branches arise from the A1 segment
- the medial lenticulostriate arteries pass superiorly through the
anterior perforated substance to supply the medial basal ganglia.
- the recurrent artery of Heubner arises from the distal A1 or
proximal A2 ACA segment and curves backward above the horizontal
ACA, then joins the medial lenticulostriate arteries to supply the
inferomedial basal ganglia and anterior limb of the internal capsule.

24. Anterior Cerebral Artery
A2 (Vertical) Segment
• courses superiorly in the interhemispheric fissure, extending from the A1-
ACoA junction to the corpus callosum rostrum
• has two cortical branches, the orbitofrontal and frontopolar arteries, that
supply the undersurface and inferomedial aspect of the frontal lobe
A3 (Callosal) Segment
• curves anteriorly around the corpus callosum genu, then divides into the
two terminal ACA branches, the pericallosal and callosomarginal arteries
-pericallosal artery is the larger of the two terminal branches, running
posteriorly between the dorsal surface of the corpus callosum and
cingulate gyrus. Thecallosomarginal artery courses over the cingulate
gyrus within the cingulate sulcus

27. Anterior Cerebral Artery
• The cortical ACA branches supply the anterior two-thirds of the
medial hemispheres and corpus callosum, the inferomedial surface of
the frontal lobe, and the anterior two-thirds of the cerebral convexity
adjacent to the interhemispheric fissure
• The penetrating ACA branches (mainly the medial lenticulostriate arteries)
supply the medial basal ganglia, corpus callosum genu, and anterior limb of
the internal capsule.

29. Anterior Communicating Artery
• short, slender vessel that runs horizontally between the anterior
cerebral arteries
• crosses the ventral aspect of the median longitudinal fissure and is
located anterior to the optic chiasm and posteromedial to the
olfactory tracts
• forms the anterior bridge between the left and right halves of the
anterior circuit
• completes the anterior part of the anastomotic ring known as the
circle of Willis

30. Middle Cerebral Artery
• largest terminal branch of the internal carotid artery
• travels through the Sylvian (lateral) fissure before coursing in a
posterosuperior direction on the island of Reil (insula)
• subsequently divides to supply the lateral cortical surfaces along with
the insula

31. Middle Cerebral Artery
• Central branches
o relatively small and include the lenticulostriate arteries that pass through the
anterior perforated substance to supply the lentiform nucleus and the
posterior limb of the internal capsule
• Cortical branches
o frontal arteries perfuse the inferior frontal, middle, and precentral gyri
o orbital branches supply the lateral orbital parts of the frontal lobe, as well as
the frontal gyrus
o parietal branch supply the inferior parietal lobe, the inferior part of the
superior parietal lobe, and the postcentral gyrus receive blood from the
o several temporal arteries then go on to perfuse the lateral aspect of the
temporal lobe

32. Middle Cerebral Artery
M1 (Horizontal) Segment
• extends laterally from the ICA bifurcation toward the sylvian (lateral cerebral) fissure
• typically bi- or trifurcates just before it enters the sylvian fissure
The most important branches
• lateral lenticulostriate arteries which supply the lateral putamen, caudate nucleus, and
external capsule
• anterior temporal artery supplies the tip of the temporal lobe
M2 (Insular) Segments
• the postbifurcation MCA trunks turn posterosuperiorly in the sylvian fissure, following a
gentle curve (the genu or "knee" of the MCA)
• several branches—the M2 or insular MCA segments—arise from the postbifurcation
trunks and sweep upward over the surface of the insula

33. Middle Cerebral Artery
M3 (Opercular) Segments
• The MCA branches loop at or near the top of the sylvian fissure, then
course laterally under the parts ("opercula") of the frontal, parietal,
and temporal lobes that hang over and enclose the sylvian fissure
M4 (Cortical) Segments
• The MCA branches become the M4 segments when they exit the
sylvian fissure and ramify over the lateral surface of the cerebral
hemisphere
• There is considerable variation in the cortical MCA branching patterns

35. Middle Cerebral Artery
Vascular Territory
• largest vascular territory of any of the major cerebral arteries
• supplies most of the lateral surface of the cerebral hemisphere with
the exception of a thin strip at the vertex (supplied by the ACA) and
the occipital and posteroinferior parietal lobes (supplied by the PCA)
• Its penetrating branches supply most of the lateral basal brain
structures

38. Posterior Circulation
• The vertebral arteries
• The basilar artery and its branches
• The posterior cerebral arteries
• And the posterior communicating arteries

39. Vertebral Artery
• gain access to the cranial vault via the foramen magnum anterolateral to
the brainstem
• gives off a posterior inferior cerebellar artery
• contributes to the formation of the anterior spinal artery via tributaries
that converge in the midline anterior to the medulla oblongata
• contributes meningeal branches near the foramen magnum that supplies
the falx cerebelli and the surrounding bone
• may give off the posterior spinal artery; although this vessel usually arises
from the posterior inferior cerebellar artery
• gives off medullary arteries that perfuse the medulla oblongata
• the vertebral arteries unite in the midline at the pontomedullary junction
to form the basilar artery

40. Vertebral Artery
V1 (Extraosseous) Segment
• arises from the ipsilateral subclavian artery and courses posterosuperiorly to
enter the C6 transverse forame
• unnamed segmental branches arise from V1 to supply the cervical musculature
and lower cervical spinal cord
V2 (Foraminal) Segment
• courses superiorly through the C6-C3 transverse foramina until it reaches C2,
where it first turns superolaterally through the "inverted L" of the transverse
foramen and then turns upward to pass through the C1 transverse foramen (8-24)
• an anterior meningeal artery and additional unnamed segmental branches arise
from V2

41. Vertebral Artery
V3 (Extraspinal) Segment
• begins after the VA exits the C1 transverse foramen
• lies on top of the C1 ring, curving posteromedially around the atlantooccipital joint
before making a sharp anterosuperior turn to pierce the dura at the foramen magnum
• the only major V3 branch is the posterior meningeal artery
V4 (Intradural) Segment
• courses superomedially behind the clivus and in front of the medulla
• gives off small anterior and posterior spinal arteries and medullary perforating branches
• the posterior inferior cerebellar artery (PICA) arises from the distal VA, curves
around/over the tonsil, and gives off the perforating medullary, choroid, tonsillar, and
inferior cerebellar branches

45. Basilar Artery
• important vessel found in the pontine cistern
• posterior to the clivus and anterior to the pons, as it ascends in the
basilar groove
• its branches are responsible for supplying the pons, cerebellum,
internal ear, and other nearby structures

46. Basilar Artery
Three major branches
• Anterior inferior cerebellar
• Superior cerebellar
• Internal auditory (Labyrinthine)
There are also smaller pontine and posteromedial (paramedian)
arteries that arise from the lateral surface and distal bifurcation of the
artery, respectively.
The basilar artery ends by dividing into two posterior cerebral arteries

47. Basilar Artery
• courses superiorly in the prepontine cistern, lying between the clivus in front and the pons
behind. It terminates in the interpeduncular fossa by dividing into the two posterior cerebral
arteries.
• numerous small but critical basilar perforating arteries arise from the entire dorsal surface of the
BA to supply the pons and midbrain.
The first major named BA branch is the anterior inferior cerebellar artery (AICA)
• arises from the proximal BA and courses ventromedially to CNs VII and VIII, frequently looping
into the internal auditory meatus. It
• supplies both nerves as well as a relatively thin strip of the cerebellar hemisphere that lies
directly behind the petrous temporal bone.
One or more (usually two to four) superior cerebellar arteries (SCAs) originate from each side of the
distal BA, course laterally below CN III, then curve posterolaterally around the midbrain just below
the tentorium
• ramify over the surface of the superior cerebellum and upper vermis, curving into the great
horizontal fissure.

49. Basilar Artery
Vascular Territory
• The vertebrobasilar system normally supplies all of the posterior fossa
structures as well as the midbrain, posterior thalami, occipital lobes,
most of the inferior and posterolateral surfaces of the temporal lobe,
and upper cervical spinal cord

51. Posterior Cerebral Artery
• terminal branches arising from the bifurcation of the basilar artery
• division takes place behind the dorsum sellae
• separated from the superior cerebellar artery by the oculomotor nerve (CN
III)
• continues in a course lateral to the midbrain (adjacent to the trochlear
nerve, CN IV)
• gives off the posterior communicating artery, which completes the circle of
Willis
• vessel then continues to course around the cerebral peduncles toward the
tentorial aspect of the cerebrum, here it supplies the occipital and
temporal lobes

52. Posterior Cerebral Artery
P1 (Precommunicating) Segment
• extends laterally from the basilar artery (BA) bifurcation to the
junction with the posterior communicating artery (PCoA)
• lies above the oculomotor nerve (CN III) and has perforating branches
(the posterior thalamoperforating arteries) that course
posterosuperiorly in the interpeduncular fossa to enter the
undersurface of the midbrain

53. Posterior Cerebral Artery
P2 (Ambient) Segment
• extends from the P1-PCoA junction, running in the ambient (perimesencephalic) cistern as it sweeps
posterolaterally around the midbrain
• lies above the tentorium and the cisternal segment of the trochlear nerve (CN IV)
• two major cortical branches—the anterior and posterior temporal arteries—arise from the P2 PCA segment
and pass laterally toward the inferior surface of the temporal lobe
Several smaller but important branches also arise from the P2 PCA segment
• Thalamogeniculate arteries and peduncular perforating arteries arise from the proximal P2 and pass directly
superiorly into the midbrain
• The medial posterior choroidal artery (PChA) and the lateral PChA also arise from the P2 segment.
• The medial PChA curves around the brainstem and courses superomedially to enter the tela choroidea and
roof of the third ventricle.
• The lateral PChA enters the lateral ventricle and travels with the choroid plexus, curving around the pulvinar
of the thalamus.
• The lateral PChA shares a reciprocal relationship with the AChA, a branch from the ICA

54. Posterior Cerebral Artery
P3 (Quadrigeminal) Segment
• short segment that lies entirely within the quadrigeminal cistern
• begins behind the midbrain and ends where the PCA enters the
calcarine fissure of the occipital lobe
P4 (Calcarine) Segment
• terminates within the calcarine fissure, where it divides into two
terminal PCA trunks
• the medial trunk gives off the medial occipital artery, parietooccipital
artery, calcarine artery, and posterior splenial arteries, whereas the
lateral trunk gives rise to the lateral occipital artery.

56. Posterior Cerebral Artery
The branches of the posterior cerebral artery bring oxygenated blood
to the following areas:
• Anterior thalamus and subthalamus
• Lateral wall of the third ventricle and inferior horn of the lateral
ventricle
• Choroid plexus of third and lateral ventricles
• Globus pallidus
• Lateral and medial geniculate bodies

57. Posterior Cerebral Artery
• supplies most of the inferior surface of the cerebral hemisphere, with
the exception of the temporal tip and frontal lobe
• also supplies the occipital lobe, posterior one-third of the medial
hemisphere and corpus callosum, and most of the choroid plexus
• penetrating PCA branches are the major vascular supply to the
midbrain and posterior thalami

61. Posterior Communicating Artery
• long, slender vessel originating from the posterior cerebral artery
• much longer than its anterior counterpart - the anterior
communicating artery
• medial to the uncus of the temporal lobe and lateral to the
mammillary bodies of the hypothalamus
• distal part of the vessel may overlap the proximal part of the optic
tract
• completes the circle of Willis posteriorly
• additionally, it gives tributaries to the optic tract, cerebral peduncles,
internal capsule, and thalamus