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
CT angiography
Anatomy of cerebral arteries

Anterior circulation

  • common carotid arteries (CCA) differ in their 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 of both CCAs is found     Common origin of the left common carotid artery and the brachiocephalic trunk
  • CCA splits commonly at the C4 level (C3-C5 range, upper border of the thyroid cartilage) to:
    • internal carotid artery (ICA)
      • usually lies dorsolateral to the ECA, and its origin is enlarged (carotid bulb or sinus with the carotid body)
      • has no branches on the neck
      • enters the skull via the carotid canal, which is situated anteromedially to the jugular foramen
    • external carotid artery (ECA)
  • occasionally, the bifurcation can be found higher (C2-3 level) or lower (C6-7 level), and the bifurcation level may be asymmetric
Carotid bifurcation
  • ECA usually has 8 branches:
    • ventral
      • superior thyroid artery (a.thyreoidea superior) – arising from its anterior aspect, the first branch seen 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 branch
      • 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 anastomoses with the inferior thyroid artery (arising from the thyrocervical trunk of the subclavian artery)
    • the terminal branch of the facial artery anastomoses with the ophthalmic artery (from the ICA)
    • posterior auricular artery anastomoses with the occipital artery
    • one of the branches of the superficial temporal artery anastomoses with lacrimal and palpebral branches of the ophthalmic artery

Posterior circulation

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

Intracranial cerebral arteries

The circle of Willis

Anterior circulation

Internal carotid artery (ICA)
  • ICA usually courses posterior, lateral, or posterolateral to the ECA
  • the ICA enters the skull via the carotid canal (C1, extracranial segment)
  • it turns anteromedially as the C2 (petrous) segment  to run through the petrous temporal bone
    • in 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 with the posterior tympanic branch of the stylomastoid artery
  • ICA exits the carotid canal superiorly (C3, lacerum segment)
  • later it turns anteriorly and enters the medial part of the cavernous sinus (C4, cavernous segment)
  • then ICA turns superiorly (C5, clinoid segment)
  • finally, it turns posteriorly along the anterior clinoid process and continues as a C6 segment (ophthalmic)
    • 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 divides into two arteries – anterior cerebral anterior (ACA) and middle cerebral artery (MCA)
  • there are several classifications of ICA segments (see tab) – preferably use descriptive terms instead of simple labels (C1-7)
Bouthillier classification
  • C1: cervical segment
  • C2: petrous (horizontal) segment
    • caroticotympanic artery
  • C3: lacerum segment
  • C4: cavernous segment
    • meningohypophyseal trunk
    • inferolateral trunk
  • C5: clinoid segment
  • C6: ophthalmic (supraclinoid) segment
    • ophthalmic artery
    • superior hypophyseal artery
  • C7: communicating (terminal) segment
    • posterior communicating artery
    • anterior choroidal artery
    • anterior cerebral artery
    • middle cerebral artery
Bouthillier’s classification
Gibo 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
Gibo classification
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”)
NYU Langone Medical Center classification of ICA segments
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 that corresponds to the clinoidal portion of the artery
  • C4 – the cavernous portion of the artery that ends at the posterior genu of the ICA
  • C5 – the part of the ICA lateroinferior to the cavernous sinus in the lacerum foramen; it does not include the petrous segment
Fisher classification of ICA segments
Middle cerebral artery (MCA)
  • middle cerebral artery (MCA) is (together with ACA) a terminal branch of the internal carotid artery
  • the MCA supplies many deep brain structures, the majority 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 middle cerebral artery into four segments (M1-M4)
    • M1  (sphenoidal/horizontal) segment – originates at the terminal bifurcation, travels along the posterior margin of the lesser wing of sphenoid bone (ala minor ossis sphenoidal), and usually 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 determined as the end of the M1 segment (bifurcation usually starts here but can be placed more distally) = it is more practical to use bifurcation as a distal border of the M1 segment
    • M2 (insular) segment begins at the point of MCA bifurcation (or genu) and comprises a superior and inferior trunk. It 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 travels over the surface of the cerebral hemisphere
  • 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
  • ACA supplies the medial aspect of the cerebral hemispheres back 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
        • shorter, thinner, and fewer in number than the lateral lenticulostriate arteries, which arise from the M1 segment of 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) is the largest perforating branch routinely seen on angiography Lenticulostriate arteries
        • it 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) segmentextends 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   Callosomarginal artery originating from proximal A3 segment
      • sometimes A2 segment is very short and  the pericallosal and callosomarginal trunks arise just distal to the AComA;  it may look like an ACA “quadrifurcation”
    • 4/5 segment (pericallosal artery) – continues above the body of the corpus callosum
Anterior cerebral artery segments
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 and anterior (ICA) circulation is connected via the posterior communicating artery (PComA)
  • the arteries supplying the brainstem can be divided 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)

→ a detailed description of the vascular supply of the brainstem see here

→ 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 medullar branches (from distal VA and anterior spinal artery)
    • anteromedial  – corticospinal tract, part of medial  lemniscus, reticular formation (RF), medial longitudinal fasciculus (MLF), hypoglossus nucleus
    • anterolateral – pyramidal tract
  • lateral medullar 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 nucleus, etc.
Pons
  • perforators arising from BA, AICA, SCA, and pontine branches
    • with regard to the anatomy of the pons, there is only a limited number of posterior pontine perforators (from the SCA); pons is mainly supplied by the anterior pontine arteries and the lateral (long) pontine arteries
  • anterior pontin­e arteries are divided to:
    • 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 from anteromedial perforators
  • lateral pontine perforators – lateral parts 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
  • perforators arising from PCA, SCA, posterior choroidal artery
  • anterior mesencephalic arteries
    • anteromedial – decussation of superior peduncles, red nucleus (ncl. ruber), substantia grisea centralis, 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 colliculus, 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
  • 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    Paramedian thalamic infarction
      • B – Percheron artery – unilateral, dominant, single artery arising from P1 segment, supplying both medial thalami  Bithalamic infarction caused by the Percheron artery occlusion
      • C – perforators arising from the bridging artery, which connects the P1 segments bilaterally
  • thalamogeniculate artery (P2 segment)
  • posterolateral and posteromedial choroidal artery  (P2 segment)
Thalamic arterial supply
  • isolated thalamic infarcts (lacunar strokes) are usually caused by arteriolopathy or ostial atherosclerosis of perforating branches  Thalamic lacunar strokes
  • in the case of thromboembolic PCA occlusion, other areas besides the thalamus, are most commonly affected (occipital, temporal lobe)  Infarction caused by P1 segment occlusion (both thalamus and occipital lobe are affected) Infarction caused by P2 segment occlusion (lateral thalamus and occipital lobe are affected)
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 (only in 15%) – accompanies n. 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)
Vertebrobasilar arteries
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 goes above the tentorium to supply the inferomedial temporal lobe and the occipital lobe (including the visual cortex)
    • in the initial segments, it gives off branches to the middle part of the mesencephalon, a substantial part of the thalamus, and the lateral geniculate nucleus   Infarction caused by the proximal P2 segment occlusion (lateral thalamus and occipital lobe are affected)
    • the macular part of the visual cortex is usually supplied from both PCA and 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 the 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   Infarction caused by the P2-segment occlusion
  • P3 segment (quadrigeminal)​ and P4 segment (cortical)
Posterior cerebral artery segments

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

Posterior cerebral artery segments
  • 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 variant of the norm
  • significant branches:
    • PICA (can originate from BA as well)
    • anterior and posterior spinal arteries
  • atherosclerosis typically involves the V0 and V4 segments Stenosis of V4 segment on TCCD and CTA
  • dissection commonly affects the V3 segment
Posterior inferior cerebellar artery (PICA)
  • usually arises from VA; anatomic variants are common
    • unilateral or bilateral aplasia
    • hypoplastic VA may end as PICA without communication with contralateral VA
    • may originate from the BA (10%)
  • segments:
    • anterior, lateral, and posterior medullary
    • supratonsillar
  • branches
    • perforators in the anterior and lateral segments
    • there are branches for the tonsil, and the lower vermis arises from the supratonsillar segment
Infarction in PICA territory
  • etiopathogenesis of stroke in PICA territory
    • thromboembolism from the proximal VA stenosis (usually V0 segment)
    • thromboembolism from the distal VA stenosis (V4 segment)
    • thromboembolism or hypoperfusion caused by VA dissection (V3 segment)
    • embolization from the heart and aortic arch
  • clinical presentation
    • cerebellar infarction with ataxia and vertigo (usually from concurrent brainstem lesion)
    • lateral oblongata syndrome (Wallenberg syndrome)
      • the most common posterior circulation ischemic stroke syndrome
      • symptoms occur as a result of 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%)
    • from the vertebrobasilar junction or middle segment of the BA
  • tissue supplied by the AICA is variable; 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)
Infarct in AICA territory
 Superior cerebellar artery (SCA)
  • the superior cerebellar artery (SCA) arises 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 (perforating branches)
    • middle cerebellar peduncle
    • dentate nucleus
  • variants:
    • SCA, unlike AICA or PICA, is only rarely absent
    • duplication (uni/bilateral) is quite often
Infarct in SCA territory
Circle of Willis
  • intracranially, the basal arteries are interconnected, forming the arterial circle of Willis
  • it is a heptagon consisting of the following:
    • 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 arises at the anterior perforated substance and runs back 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 (A1 segment)
    • perforating branches (from the AComA)
    • thalamoperforating and thalamogeniculate arteries (from the basilar tip, proximal PCA, and PComA)
  • the circle of Willis allows redirecting the blood flow between both sides of the brain and between the vertebrobasilar and the internal carotid artery systems
The circle of Willis
  • a complete circle is present in < 30% of patients; individual anatomical variants are common
    • different diameter of the vessels on the right and left side
    • the predominance of carotid or vertebral blood flow
    • significant asymmetry of the whole circuit
    • absence/hypoplasia of one of the arteries
      • AComA is absent in 1% of cases
      • the proximal segment of the ACA is absent in approximately 10%
      • PComA is absent or hypoplastic in 30% of cases
The circle of Willis variants

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