ADD-ONS / ANATOMY
Anatomy of cerebral arteries
Created 10/11/2022, last revision 12/01/2023
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 their origin but follow symmetrical courses
- 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)
- internal carotid artery (ICA)
- occasionally, the bifurcation can be found higher (C2-3 level) or lower (C6-7 level), and the bifurcation level may be asymmetric
- 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)
- ventral
- 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):
- 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
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)
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
- 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
- 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)
- shorter, thinner, and fewer in number than the lateral lenticulostriate arteries, which arise from the M1 segment of MCA
- the recurrent artery of Heubner (RAH; AKA the medial striate artery) is the largest perforating branch routinely seen on angiography
- 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
- medial lenticulostriate arteries – arise from the A1 segment and supply the globus pallidus and medial portion of the putamen
- 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; it may look like an ACA “quadrifurcation”
- 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
- 4/5 segment (pericallosal artery) – continues above the body of the corpus callosum
- A1 segment (horizontal/pre-communicating) originates from the internal carotid artery and extends to the AComA
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)
→ clinical presentation of various brainstem syndromes see here
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)
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)
- 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)
- 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
- 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
- thalamo-subthalamic arteries (paramedian branches)
- main cortical branches
- temporal branches
- anterior inferior temporal artery (AITA)
- middle inferior temporal artery (MITA)
- posterior inferior temporal artery (PITA)
- anterior inferior temporal artery (AITA)
- occipital branches
- calcarine artery (CA)
- parieto-occipital artery (PoA)
- splenial artery
- calcarine artery (CA)
- temporal branches
Vertebral artery (VA)
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
- branches for the tonsil, and the lower vermis (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 (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)
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
Circle of Willis
- intracranially, the basal arteries are interconnected to form 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 originates at the anterior perforating 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 (segment A1)
- perforating branches (from the AComA)
- thalamoperforating and thalamogeniculate arteries (from the basilar tip, proximal PCA, and PComA)
- the circle of Willis allows redirection of the blood flow between both sides of the brain and between the vertebrobasilar and the internal carotid artery systems
- 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
- 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