ADD-ONS / ANATOMY

Anatomy of cerebral arteries

Created 10/11/2022, last revision 20/09/2023

This chapter summarizes the basics of vascular anatomy necessary for the clinical work of strokologists and neurosonologists. Anatomical variants of extra- and intracranial arteries are discussed here. For detailed anatomy, see, e.g., KENHUB

Extracranial cerebral arteries

The extracranial cerebral arteries include:

brachiocephalic trunk (BCT)
proximal sections of the subclavian arteries (SA) up to the origin of the vertebral artery
common carotid arteries (CCA)
internal carotid arteries (ICA)
external carotid arteries (ECA)
vertebral arteries (VA) – from the origin to the atlas (including the atlas loop)

common carotid artery	6-7 mm
internal carotid artery	4-5.2 mm
vertebral artery	3-4 mm
ophthalmic artery	1 mm

Anterior circulation

common carotid arteries (CCA) differ in origin but follow symmetrical courses
- the right CCA originates from the brachiocephalic trunk (BCT) at the level of the sternoclavicular joint
- the left ACC originates directly from the aortic arch
- sometimes a common outflow for both CCAs is found
the CCA stypically bifurcates at the C4 level (within C3-C5 range, at the upper border of the thyroid cartilage) into:
- internal carotid artery (ICA)
  - usually situated dorsolateral to the ECA, its origin is enlarged (forming the carotid bulb or sinus with the carotid body)
  - has no branches on the neck
  - enters the skull via the carotid canal, located anteromedially to the jugular foramen
- external carotid artery (ECA)
the bifurcation level can occasionally be found higher (C2-3 level) or lower (C6-7 level), and may be asymmetric

Branches of external carotid artery

ECA typically has 8 branches:
- ventral
  - superior thyroid artery (a.thyreoidea superior) – arises from its anterior aspect; the first branch visible on ultrasound
  - lingual artery (a.lingualis)
  - facial artery (a.facialis)
- dorsal
  - occipital artery (a. occipitalis) – common feeding artery in dural AV fistula (DAVF)
  - posterior auricular artery (a.auricularis posterior)
- medial
  - ascending pharyngeal artery (a.pharyngea ascendens)
- ECA terminates as two branches
  - maxillary artery (a.maxillaris)
  - superficial temporal artery (STA) (a.temp. superficialis)
ACE branches help to differentiate ICA from ECA on the ultrasound
anastomoses:
- the superior thyroid artery → inferior thyroid artery (originating from the thyrocervical trunk of the subclavian artery)
- the terminal branch of the facial artery → ophthalmic artery (from the ICA)
- posterior auricular artery → occipital artery
- branches of the superficial temporal artery → lacrimal and palpebral branches of the ophthalmic artery

Posterior circulation

the subclavian artery (SA) originates from the brachiocephalic trunk on the right side and directly from the aorta on the left side
- dopplerimaging typically shows a triphasic, resistant flow
vertebral artery (VA) usually originates from the SA
- in approx. 5% of cases, the left VA may originate directly from the aortic arch
dominant left VA with right-sided hypoplasia is frequently observed
extracranial segments (V0-3):
- V0 – origin
- V1 (preforaminal segment) – segment prior to entering the transverse foramen (most commonly at the C6 level)
- V2 – intraforaminal segment
- V3 – atlas loop (C2-dura)

Intracranial cerebral arteries

Territories 1

Territories 2

Territories 3

Territories 4

Territories 5

Territories 6

Territories 7

Territories 8

Territories 9

Territories 10

Territories 11

Anterior circulation

Internal carotid artery (ICA)

ihe internal carotid artery (ICA) usually courses posterior, lateral, or posterolateral to the ECA
the ICA enters the skull via the carotid canal (C1, extracranial segment)
it then turns anteromedially as the C2 (petrous) segment and traverses the petrous temporal bone
- within the petrous part of the temporal bone, the artery runs medially
- it gives off small branches to the middle ear cavity (caroticotympanic arteries)
- caroticotympanic arteries anastomose with the anterior tympanic branch of the internal maxillary and the posterior tympanic branch of the stylomastoid artery
ICA exits the carotid canal superiorly (C3, lacerum segment)
subsequently, it turns anteriorly and enters the medial part of the cavernous sinus (C4, cavernous segment)
the artery then turns superiorly (C5, clinoid segment)
finally, it turns posteriorly along the anterior clinoid process, continuing as the C6 (ophthalmic) segment
- the ophthalmic artery arises in this segment and enters the orbit together with the optic nerve via the optic canal
the terminal ICA (C7 segment) begins proximal to the origin of the PComA
- distal to the ophthalmic artery, the posterior communicating artery (PComA) and anterior choroidal artery arise (AChA may originate from MCA)
- the terminal segment bifurcates into the anterior cerebral anterior (ACA) and the middle cerebral artery (MCA)
there are several classifications of ICA segments (see tab); it is preferable to use descriptive terms rather than simple labels (C1-7)

Bouthillier classification

C1: cervical segment (starts at carotid bufurcation)
C2: petrous (horizontal) segment (starts at carotid canal)
- caroticotympanic artery
C3: lacerum segment (starts at posterior edge of foramen lacerum)
C4: cavernous segment (starts at the superior margin of petrolingual ligament)
- meningohypophyseal trunk
- inferolateral trunk
C5: clinoid segment (starts at proximal dural ring / anterior clinoid process)
C6: ophthalmic (supraclinoid) segment (starts at distal dural ring / cavernous sinus roof)
- ophthalmic artery
- superior hypophyseal artery
C7: communicating (terminal) segment ((starts proximal to the origin of the PComA)
- posterior communicating artery (PComA)
- anterior choroidal artery
- anterior cerebral artery
- middle cerebral artery

Gibo classification

The GIBO (Glasscock-Ince-Brackmann-Osborn) classification

C1: extracranial segment
C2: petrous segment
C3: cavernous segment (terminates at the distal dural ring)
C4: supraclinoid segment (entire intradural portion, with 3 subsegments)
- ophthalmic
- communicating
- choroidal

NYU Langone Medical Center classification

C1 – cervical segment
C2 – petrous segment
C3 – cavernous segment
C4 – paraophthalmic segment
C5 – posterior communicating segment
C6 – choroidal segment
C7 – terminus segment (“carotid terminus”)

Fisher classification

C1 – the portion of the ICA from its termination to the origin of the PComA
C2 – the part of the ICA between the origin of the PComA and the origin of the ophthalmic artery (OA)
C3 – the anterior genu of the ICA, corresponding to the clinoid portion of the artery
C4 – the cavernous portion of the artery that terminates at the posterior genu of the ICA
C5 – the part of the ICA lateroinferior to the cavernous sinus in the foramen lacerum; it does not include the petrous segment

Middle cerebral artery (MCA)

→ see complete anatomy at KENHUB

middle cerebral artery (MCA) is (together with ACA) a terminal branch of the internal carotid artery
the MCA supplies numerous deep brain structures, a significant portion of the lateral surface of the cerebral hemispheres, and the temporal pole of the brain
MCA travels from the base of the brain through the lateral sulcus (of Sylvius) and terminates on the lateral surface of the brain; the surgical classification divides the MCA into four segments (M1-M4)
- M1 (sphenoidal/horizontal/Sylvian) segment – originates at the terminal bifurcation of the ICA, travels along the posterior margin of the lesser wing of sphenoid bone, and typically has 2 terminal branches
  - MCA gives off small arteries penetrating the anterior perforated substance (substantia perforata anterior) – central branches/lateral lenticulostriate arteries, which supply the anterior part of the thalamus, internal capsule, and the basal ganglia
  - MCA communicates with the posterior cerebral artery (PCA) via the posterior communicating artery (PComA)
  - originally, the MCA genu was considered the end of the M1 segment (bifurcation usually starts here but can be placed more distally) = it is more practical to use the bifurcation as a distal border of the M1 segment
- M2 (insular) segment begins at the point of MCA bifurcation (or genu) and comprises superior and inferior trunks. It gives off insular and temporal branches and terminates at the level of the circular sulcus of the insula
- M3 (opercular) segment – ascends toward the surface of the brain. It courses over the inner surfaces of the parietal and temporal portions of the insular operculum and reaches the surface of the Sylvian fissure
- M4 (terminal, cortical) segment – starts at the external surface of the Sylvian fissure and extends over the convexity of the hemisphere
  - precentral, central, postcentral, and other cortical branches

MCA branches

Structures supplied by MCA

anterior commissure
internal capsule
caudate nucleus, putamen, globus pallidus
temporal pole, insula, lateral aspect of the orbital surface of the frontal lobe, opercular surfaces of frontal, parietal, and temporal lobes, inferior and middle frontal gyri, precentral and postcentral gyri, superior and inferior parietal lobules, superior, lateral surface of the temporal lobe, superior part of the lateral surface of the occipital lobe

Anterior cerebral artery (ACA)

the anterior cerebral artery (ACA) is a terminal branch of the ICA
the anterior cerebral artery (ACA) supplies the medial aspect of the cerebral hemispheres up to the parietal lobe
the left and right anterior cerebral arteries are connected by the anterior communicating artery (AComA), forming the anterior portion of the circle of Willis
ACA is divided into 5 segments:
- A1 segment (horizontal/pre-communicating) originates from the internal carotid artery and extends to the AComA
  - medial lenticulostriate arteries – arise from the A1 segment and supply the globus pallidus and medial portion of the putamen
    - medial LSAa are shorter, thinner, and fewer in number than the lateral lenticulostriate arteries, which arise from the M1 segment of the MCA
    - should not be confused with the perforating branches from the A1/A2 intersection (including the recurrent artery of Heubner)
  - the recurrent artery of Heubner (RAH; AKA the medial striate artery) – the largest perforating branch routinely seen on angiography
    - may arise distal to the AComA, at the level of the AComA, or less frequently proximal to the AComA
    - 0.8 mm in diameter
    - it supplies the head of the caudate nucleus, the medial portion of globus pallidus, the anterior crus of the internal capsule, the anterior hypothalamus, the basal nucleus of Meynert
    - its occlusion can occur during clipping of the AComA aneurysm
    - variants: absent or duplicated/triplicated/quadruplicated
- A2 (infracallosal) segment – extends from the AComA along the rostrum of the corpus callosum to the genu
  - orbitofrontal artery arises near AComA
  - frontopolar artery
- A3 (precallosal) segment – extends around the genu of the corpus callosum and bifurcates into callosomarginal and pericallosal arteries (continuation of ACA)
  - callosomarginal artery can arise anywhere along the A1-A4 segments; it further branches into the medial frontal arteries (anterior, intermediate, posterior), and the paracentral artery
  - sometimes A2 segment is very short and the pericallosal and callosomarginal trunks arise just distal to the AComA; this may look like an ACA “quadrifurcation”
- 4/5 segment (pericallosal artery) – continues above the body of the corpus callosum

Callosomarginal and pericallosal arteries

Ophthalmic artery (OA)

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Posterior circulation

the posterior (vertebrobasilar) circulation supplies the medulla oblongata, pons, mesencephalon, cerebellum, and partially the thalamus
- posterior (VA-BA) and anterior (ICA) circulation is connected via the posterior communicating artery (PComA)
the arteries supplying the brainstem can be categorized into anterior (paramedian), lateral (short circumflex), and posterior (long circumflex) perforating branches
these branches arise from the vertebral arteries, basilar artery (BA), posterior inferior cerebellar artery (PICA), anterior inferior cerebellar artery (AICA), superior cerebellar artery (SCA), and the P1 and P2 segments of the posterior cerebral artery (PCA)

→ clinical presentation of various brainstem syndromes see here

Medulla oblongata

oblongata is supplied by 3 groups of perforating arteries arising from the anterior and posterior spinal arteries, VA and PICA
anterior medullary branches (from distal VA and anterior spinal artery)
- anteromedial – corticospinal tract, part of medial lemniscus, reticular formation (RF), medial longitudinal fasciculus (MLF), hypoglossal nucleus
- anterolateral – pyramidal tract
lateral medullary branches (from VA and PICA, more cranially from BA and AICA
- caudal, medial, rostral – spinothalamic tract, spinocerebellar tract, vestibular et cochlear nuclei, spinal trigeminal nucleus
posterior medullar branches (from the posterior spinal artery and PICA)
- caudal and rostral – gracile and cuneate nuclei, etc.

Pons

perforators arising from BA, AICA, SCA, and pontine branches
- with regard to the anatomy of the pons, there are only a limited number of posterior pontine perforators (from the SCA); the pons is mainly supplied by the anterior pontine arteries and the lateral (long) pontine arteries
anterior pontine arteries are divided into:
- anteromedial – the abducens nucleus, medial longitudinal fasciculus (MLF), raphe nuclei, paramedial part of the reticular formation, the medial aspect of pyramidal, corticobulbar and corticopontine pathways, medial parts of pontine nuclei and sometimes part of the medial lemniscus
- anterolateral – lateral and ventral parts of the pyramidal tract and lateral aspects of tracts and nuclei, whose medial parts are supplied by anteromedial perforators
lateral pontine perforators – lateral portions of the abducens and vestibulocochlear nuclei, superior olivary complex, spinothalamic tract, lateral and medial lemniscus, spinal and principal nuclei of the trigeminal nerve, superior vestibular nucleus, lateral pyramidal fibers, pontine nuclei

Mesencephalon

midbrain is supplied by perforators arising from the PCA, SCA, and posterior choroidal artery
anterior mesencephalic arteries
- anteromedial – decussation of superior peduncles, red nucleus (ncl. ruber), central substantia grisea, medial longitudinal fasciculus (MLF), the nuclei of the oculomotor and trochlear nerves, dorsal raphe nuclei
- anterolateral (peduncular) – corticopontine, corticospinal, and corticobulbar tracts, most of the substantia nigra
lateral mesencephalic arteries – supply the spinothalamic tract and the lateral lemniscus
posterior mesencephalic arteries (superior et inferior colliculus)

Thalamus

arterial supply of the thalamus is usually provided by multiple, small, perforating vessels arising from the posterior communicant (PComA) and from the P1 and P2 segments of the posterior cerebral arteries (PCAs)

polar (thalamotuberal) artery supplies the anterior portion of the thalamus (from PComA)
paramedian branches (from P1 segment) – 3 subtypes were described:
- - A – multiple small perforating arteries arising from bilateral P1 segments
  - B – Percheron artery – unilateral, dominant, single artery arising from the P1 segment, supplying both medial thalami
  - C – perforators arising from the bridging artery, which connects the P1 segments bilaterally
thalamogeniculate artery (from P2 segment)
posterolateral and posteromedial choroidal artery (from P2 segment)

isolated thalamic infarcts (lacunar strokes) are usually caused by arteriolopathy or ostial atherosclerosis of perforating branches
in the case of thromboembolic PCA occlusion, areas other than the thalamus, such as the occipital and temporal lobes, are most commonly affected

arterie - kmen 1-2

vertebral artery

anterior spinal artery (ASA)

posterior spinal artery

arterie - kmen 2-2

posterior spinal artery

anterior spinal artery (ASA)

vertebral artery

arterie - kmen 3-2

anterior spinal artery (ASA)

vertebral artery

vertebral artery
(V4 segment)

posterior inferior cerebellar artery (PICA)

arterie - kmen 4-2

vertebral artery + caudal BA perforators

posterior inferior cerebellar artery (PICA)

arterie - kmen 5-2

long circumferential branches from BA + AICA

short circumferential branches from BA

paramedian perforators from BA

arterie - kmen 6-2

short circumferential branches from BA

long circumferential branches from BA + SCA

paramedian perforators from BA

basilar artery

arterie - kmen 7-2

PCA

basilar artery

branches from P1 segment + SCA

short nad long branches from P1 + posteromedial choroid artery

anteromedial branches from BA and P1 segment

arterie - kmen 8-2

thalamogeniculate artery

short nad long branches from P1 + posteromedial choroid artery

branches from P1 segment + SCA

anteromedial branches from BA and P1 segment

Basilar artery

unpaired artery, formed by the confluence of the vertebral arteries (V4 segments)
localized in the pontine basilar sulcus
branches:
- anterior inferior cerebellar artery (AICA)
- labyrinthine artery (originating from the BA in approx. 15% of cases) – accompanies CN VIII through the internal acoustic meatus to the inner ear; most commonly (in 85% of cases), it originates from AICA
- superior cerebellar artery (SCA) – arises from the rostral portion of the basilar artery, just below the PCA
- perforating arteries supplying the brainstem
BA terminates by branching into posterior cerebral arteries (PCAs)

Posterior cerebral artery (ACP)

posterior cerebral artery (PCA) is a terminal branch of the basilar artery
PCA crosses the oculomotor nerve, curves around the cerebral peduncle, and ascends above the tentorium to supply the inferomedial temporal lobe and the occipital lobe (including the visual cortex)
- in the initial segments, the PCA gives off branches to the middle part of the mesencephalon, a substantial portion of the thalamus, and the lateral geniculate nucleus
- the macular part of the visual cortex is usually supplied from both the PCA and the MCA
P1 segment (pre-communicating)
- originates at the termination of the basilar artery and ends at the origin of the PComA
- occlusion of the P1 segment (proximal to PComA) leads to infarction of the cerebral peduncle, thalamus, medial temporal, and occipital lobes
P2 segment (post-communicating)
- terminates as it enters the quadrigeminal cistern
- peduncles and thalamus are spared in occlusions localized in the distal P2 segment
P3 segment (quadrigeminal) and P4 segment (cortical)

1 – thalamo-subthalamic arteries (paramedian branches), 2 – posteromedial choroidal arteries, 3 – thalamogeniculate artery, 4 – posterolateral choroidal arteries, 5 – polar (thalamotuberal) artery, 6 – temporal branches, 7 – occipitotemporal artery , 8 – parieto-occipital artery and calcarine artery

PCA branches

main subcortical branches (P1/P2 segment)
- thalamo-subthalamic arteries (paramedian branches)
  - these branches may arise from only one PCA supplying both thalami (bilateral infarction occurs when such PCA is occluded)
- posteromedial choroidal arteries
- thalamogeniculate artery
- posterolateral choroidal arteries

main cortical branches
- temporal branches
  - anterior inferior temporal artery (AITA)
  - middle inferior temporal artery (MITA)
  - posterior inferior temporal artery (PITA)
- occipital branches
  - calcarine artery (CA)
  - parieto-occipital artery (PoA)
  - splenial artery

Vertebral artery (VA)

hypoplasia (usually on the right side) is a common anatomical variant
significant branches:
- posterior inferior cerebellar artery (PICA) (may also originate from the BA)
- anterior and posterior spinal arteries → Blood Supply of the Spinal Cord
atherosclerosis typically involves the V0 and V4 segments
dissection predominantly affects the V3 segment

Posterior inferior cerebellar artery (PICA)

usually arises from the VA; anatomic variants are common
- unilateral or bilateral aplasia
- a hypoplastic VA may terminate as the PICA without communication with the contralateral VA
- may also originate from the BA (10% of cases)
segments:
- anterior, lateral, and posterior medullary
- supratonsillar
branches
- perforators in the anterior and lateral segments
- branches supplying the tonsil and the lower vermis (originating from the supratonsillar segment)

etiopathogenesis of stroke in PICA territory
- thromboembolism from the proximal VA stenosis (usually V0 segment)
- thromboembolism from the distal VA stenosis (typically V4 segment)
- thromboembolism or hypoperfusion caused by the VA dissection (V3 segment)
- embolization from the heart or aortic arch
clinical presentation
- cerebellar infarction with ataxia and vertigo (usually resulting from a concurrent brainstem lesion)
- lateral oblongata syndrome (Wallenberg syndrome)
  - the most common posterior circulation ischemic stroke syndrome
  - symptoms occur due to damage to the lateral segment of the medulla,posterior to the inferior olivary nucleus

A.cerebelli inferior anterior (AICA)

the anterior inferior cerebellar artery (AICA) contributes to the blood supply to the cerebellum
origin:
- most commonly from the proximal segment (lower third) of the basilar artery (75%)
- alternatively from the vertebrobasilar junction or middle segment of the BA
area supplied by the AICA is variable and it usually includes:
- middle cerebellar peduncle
- pons
- flocculus
- the anteroinferior surface of the cerebellum
- cranial nerves VII and VIII (via internal auditory branch = labyrinthine artery)

Labyrinthine artery

Superior cerebellar artery (SCA)

the superior cerebellar artery (SCA) originates from the rostral portion of the basilar artery, just below the PCA
it runs at the upper edge of the pons, past the crura cerebri, towards the upper surface of the cerebellum
it supplies:
- the superior cerebellar peduncle and superior vermis
- the superior surface of the cerebellar hemisphere (to the great horizontal fissure)
- dorsolateral part of the midbrain and pons (via perforating branches)
- middle cerebellar peduncle
- dentate nucleus
variants:
- SCA, unlike AICA or PICA, is only rarely absent
- duplication (uni/bilateral) is quite often

Circle of Willis

intracranially, the basal arteries are interconnected to form the arterial circle of Willis
it is a heptagon consisting of the following components:
- left and right ICA
- left and right A1 segments, connected by an unpaired AComA
- left and right P1 segments
- left and right PComA (connecting ICA and P1 segment on each side)
  - PCOM originates at the anterior perforating substance and runs posteriorly through the interpeduncular cistern
- basilar artery tip
branches of the circle of Willis also supply the optic chiasm and tracts, infundibulum, hypothalamus, and other structures at the base of the brain
- medial lenticulostriate arteries (from A1 segment)
- perforating branches (from the AComA)
- thalamoperforating and thalamogeniculate arteries (from the basilar tip, proximal PCA, and PComA)
the circle of Willis allows for the redirection of blood flow between both sides of the brain and between the vertebrobasilar and carotid circulations)

a complete circle is present in < 30% of patients; individual anatomical variants are common
- different diameter of the vessels on the right and left sides
- predominance of carotid or vertebral blood flow
- significant asymmetry of the entire circuit
- absence/hypoplasia of one of the arteries
  - AComA is absent (1%)
  - the proximal segment of the ACA is absent (10%)
  - PComA is absent or hypoplastic (30%)