VASCULAR ANATOMY OF BRAIN

G PARTHASARATHY

• Anterior circulation - Internal carotid arteries• Posterior circulation - Vertebral and Basilar arteries• Meninges - External carotid arteries contribute

Internal carotid artery

• Common carotid artery bifurcates at level of upper border of thyroid cartilage (C3 – C4 disc level).

• ICA lies posterior and lateral to ECA.• Divided into cervical, petrous, cavernous,

supraclinoid segments.

Cervical segment: no named branches arise.Petrous segment: Intraosseous Begins at carotid canal, traverses foramen

lacerum. BranchesVidian artery (anastomoses with ECA)Caroticotympanic artery (supplies middle ear)

Cavernous segment: Branches Posterior trunk (meningohypophyseal artery) - supplies pituitary, tentorium, clival dura. Inferolateral trunk - supplies 3rd, 4th, 5th cranial nerves.

- anastomoses with ECA branches through foramen rotundum, ovale, spinosum.

Cavernous and supraclinoid segments – “Carotid siphon”.

Supraclinoid segment: ICA pierces dura, enters sub-arachnoid space near

anterior clinoid process. Branches Ophthalmic artery – passes through optic canal into orbit. Posterior communicating artery – links ICA with Posterior

cerebral artery (PCA can arise directly from ICA- “fetal arrangement”).

Anterior choroidal artery – supplies posterior internal capsule, basal ganglia, choroid plexus, medial temporal lobe.

ICA segments in lateral angiogram

Terminal branches Anterior cerebral artery: Horizontal/Precommunicating A1 segment Vertical/Postcommunicating A2 segment Distal A3 segment- gives off cortical branches.• Perforating arteries arise from A1- medial lenticulostriate

arteries.• Recurrent artery of Heubner arises from distal A1 or proximal A2.• Anatomical variations Hypoplasia/aplasia of A1 segment Fusion of A2 segments in midline- “azygos” anterior cerebral

artery.

Middle cerebral artery: Runs laterally in stem of lateral sulcus. Horizontal segment M1, Insular segment M2, Opercular

segment M3, Cortical branches M4. Branches Perforating branches (M1 segment)- medial and lateral

lenticulostriate arteries supply basal ganglia and capsular region. Temporal, frontal, angular, parietal branches (M4 segment)-

supply most of the lateral surface except a narrow superomedial strip.

MRA showing MCA segments

CTA

• Sylvian triangle – formed by the branches of MCA within sylvian fissure on outer surface of insula (form a loop) upon reaching the upper margin of insula.

• Serves as landmark for localising supratentorial masses.

• Sylvian point – most medial point where the last cortical MCA branch (usually angular artery) turns inferiorly to exit sylvian fissure.

• This point approximates the apex of insula, posterior limit of lateral sulcus.

Vertebro – Basilar system VA arises from first part of Subclavian artery. Segments and branches: Extaosseous V1 (origin → C6): segmental cervical muscular, spinal

branches. Foraminal V2 (C6 → C1): Anterior meningeal artery Extraspinal V3 (C1 → foramen magnum): Posterior meningeal

artery Intradural V4 (intracranial): - anterior and posterior spinal arteries - perforating branches to medulla.

- Posterior inferior cerebellar artery- gives off perforating medullary, choroid, tonsillar, cerebellar branches.

Vertebral arteries fuse behind the clivus and in front of lower pons to form Basilar artery.

Branches: Anterior inferior cerebellar arteries- loops in the

CP angle and gives off internal auditory arteries. Superior cerebellar arteries- pass around

brainstem to fan out over superior surface of cerebellar hemispheres.

Posterior cerebral arteries – terminal BA branches.

Posterior Cerebral Artery Bifurcation of BA- either V-shaped (caudal fusion) or T-shaped (cranial

fusion). Basilar tip aneurysms more common with caudal fusion. Segments: P1- precommunicating P2- ambient P3- quadrigeminal P4- calcarine Branches: Perforating branches- posterior thalamoperforating areteries (P1),

thalamogeniculate arteries (P2). Ventricular/ choroidal branches (P2). Cortical branches- anterior and posterior temporal arteries (P2).

Distal PCA divides into 2 terminal trunkso Medial branches- medial occipital artery, calcarine artery.o Lateral branches- lateral occipital artery, temporal arteries. Fetal origin of PCA:• Large Posterior communicating artery gives direct origin to

PCA.• P1 segment hypoplastic or absent.• Absent PCA on vertebral angiogram usually due to “fetal”

origin, not occlusion.• Injection of ipsilateral carotid artery confirms presence of fetal

PCA.

External Carotid artery

• Ascending pharyngeal artery- gives branches to dura mater of posterior cranial fossa.

• Occipital artery supplies dura through a petromastoid branch.

• Maxillary artery gives off middle meningeal accessory meningeal, deep temporal arteries.

Anastomotic pathways

• The Circle of Willis• Extracranial – Intracranial anastomoses• Leptomeningeal collaterals

Circle of Willis Central arterial anastomotic ring of brain- lies above sella in

suprasellar cistern. Anteriorly Internal carotid arteries- distal part Anterior cerebral arteries- precommunicating A1 segment Anterior communicating artery Posteriorly Basilar artery- distal part Posterior cerebral arteries- precommunicating P1 segment Posterior communicating arteries A1 segments course above optic nerves, PcoAs course below optic

tracts.

• CTA/ MRA best for imaging entire COW.• DSA requires multiple views, cross-compression of contralateral

carotid artery to visualize AcoA.• Vascular territory- entire central base of brain

(hypothalamus,internal capsule,optic tracts,thalamus,midbrain).• Complete COW found only in 40% of people.• Variations: Absent or hypoplastic components (A1,PcoA). Fetal origin of PCA. Persistent caroticovertebral anastomoses (trigeminal,

hypoglossal).

Extracranial- Intracranial anastomoses: Between ECA and ICA:o Facial arteryo Middle meningeal arteryo Ophthalmic arteryo Superficial temporal artery Between ECA and Posterior circulation:o Occipital arteryo Ascending pharyngeal arteryo Vertebral arteryLeptomeningeal (pial) collaterals:• End-to-end anastomoses between distal branches of

intracerebral arteries.• Provide collateral flow across watershed zones.

Dural venous sinuses

• Endothelial lined trabeculated venous channels encased within dural reflections.

• Receive blood from superficial and deep veins. Communicate with extracranial veins directly.

• Contain arachnoid granulations, villi – return CSF to venous circulation.

• Obtain source images for MR venogram perpendicular to main axis of dural sinus (eg.coronal for superior sagittal sinus).

• MRV, CTV excellent for general overview but DSA best for detailed dilineation.

Superior sagittal sinus: Occupies upper convex attached margin of falx cerebri. Originates from ascending frontal veins anteriorly- collects

superficial cortical veins- terminates at venous sinus confluence.

Important hemispheric tributary- Vein of Trolard. Inferior sagittal sinus: Smaller channel in inferior free margin of falx. Receives tributaries from corpus callosum. Joins with Vein of Galen to form straight sinus. Straight sinus: Runs from falcotentorial apex posteroinferiorly to sinus

confluence. Receives tributaries from falx, tentorium, and cerebral

hemispheres.

Venous sinus confluence (Torcular Herophili): Formed from union of superior sagittal, straight, and transverse

sinuses. Often asymmetric, interconnections between TS highly variable. Transverse (lateral) sinuses: Situated in the posterior part of attached margins of tentorium

cerebelli. Right TS usually continuation of superior sagittal sinus, left TS a

continuation of straight sinus. Extends from internal occipital protubertance to posteroinferior

angle of parietal bone. Often asymmetric (R>L). Hypoplastic /atretic segment common. Tributaries from tentorium, cerebellum, inferiof temporal,

occipital lobes. Important tributary – Vein of Labbe.

Sigmoid sinuses: Anteroinferior continuation of TSs. Gentle S- shaped inferior curve. Terminate by becoming Internal jugular veins. Cavernous sinuses: Irregularly shaped, trabeculated venous compartment along

sides of sella turcica. Extends from superior orbital fissure anteriorly to clivus and

petrous apex posteriorly. Contains cavernous ICA, cranial nerves III, IV, V1, V2, VI. Tributaries include superior and inferior ophthalmic veins,

sphenoparietal sinus. Communicate inferiorly with pterygoid venous plexus, medially

with contralateral CS, posteriorly with superior/ inferior petrosal sinus.

Inconstantly visualized at DSA.

Superior petrosal sinus: Lie in the anterior margin of attached margin of tentorium along

upper border of petrous temporal bone. Drains cavernous sinus into TS. Inferior petrosal sinus: Lie in petro-occipital fissure. Drain CS into superior bulb of IJV. Sphenoparietal sinuses: Lie along posterior free margin of lesser wing of sphenoid bone. Drain into anterior part of CS. May receive frontal trunk of middle meningeal vein. Basilar plexus of veins: Lies over the clivus. Connects the two inferior petrosal sinuses and communicates

with internal vertebral venous plexus.

Imaging pitfalls• TSs often asymmetric, hypoplastic/ atretic segments

common. Should not be misdiagnosed as occlusion.• Jugular bulb flow often very asymmetric, turbulent

(pseudolesion).• Unopacified venous blood streaming into dural sinus on

DSA should not be mistaken for filling defect (thrombus).• “Giant” arachnoid granulations appear as round/ ovoid CSF-

equivalent filling defects in dural sinuses (esp. TSs).• Acute dural sinus/ cortical vein thrombi isointense with

brain on T1WI. So T2*(GRE) or T1 C+ imaging very helpful.• Subacute clot hyperintense on T1WI- should not be

mistaken for enhancement.

Superficial cerebral veins

o Located within subarachnoid space, highly variable in number and configuration.

o Organized into superior, middle and inferior groups.

o Superior group:• 6-12 in number, drain superolateral surface of

hemisphere.• Terminate in superior sagittal sinus.• Lateral DSA- arranged in spoke-like pattern. AP

DSA- step-ladder appearance.

• Middle group:• Superficial middle cerebral vein – drains the area

around posterior ramus of lateral sulcus.• Terminates into CS or at times into sphenoparietal

sinus.• Communicates with deep middle cerebral vein.• Inferior group:• Orbital veins terminate in Sup.SS. Tentorial veins

terminate in CS.• Basal vein of Rosenthal: begins near anterior

perforated substance, curves posteriorly around cerebral peduncles. Receives anterior cerebral veins, DMCV- drains into vein of Galen.

• BVR- looks like “frog leg” on DSA AP view.

Deep cerebral veinso Includes medullary, subependymal and deep paramedian

veins.o Medullary veins:• Small linear veins, originate 1-2 cm below cortex, course

toward ventricles.• DSA- “Dots” of contrast at subependymal/ medullary vein

junction define roof of lateral ventricle.o Subependymal veins• Septal veins course posteriorly along septum pellucidum.• Thalamostriate veins course anteriorly between caudate and

thalamus.

• TSV terminate at IV foramen by uniting with septal veins to form ICV.

• DSA AP view- TSVs define size and configuration of lateral ventricle, characteristic “double curve” appearance.

• Deep paramedian veins:• Internal cerebral veins - course posteriorly in tela choroidea of 3rd ventricle. - terminate in rostral quadrigeminal cistern by uniting with each other, BVRs to form great cerebral vein.

o Vein of Galen:• Short, U-shaped midline vein formed by union of

ICVs and BVRs.• Unites with inf.SS at falcotentorial apex to form

straight sinus.• DSA- VofG forms a prominent arc curving around

corpus callosum splenium.• Vein of Galen malformation: - primitive median promesencephalic vein persists as outlet for diencephalic, choroidal venous drainage. - persisting falcine sinus +/- absent/hypoplastic straight sinus.

Vein of Galen malformation

Posterior fossa veins Superior (Galenic) group: Drains up into vein of Galen. Includes precentral cerebellar vein, superior vermian vein,

anterior pontomesencephalic vein. Anterior (petrosal) group: Petrosal vein- prominent trunk in CPA, collects tributaries from

cerebellum, pons, medulla. Posterior (tentorial) group: Inferior vermian veins- paired paramedian structures, curve

posterosuperiorly under uvula of vermis. T1 C+ MR: APMV enhancement along pontine/ medullary surface

is normal. Should not be mistaken for meningitis.

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