Blood supply to the brain

Internal carotid artery and anterior circulation

The carotid system supplies blood to the anterior regions of the brain. During digital subtraction angiography (DSA) of the internal carotid artery (ICA), segmental branches are clearly visualized.

• Middle cerebral artery (MCA / MCA): divided into segments from M1 to M4.
• Anterior cerebral artery (ACA / ACA): includes segments from A1 to A3.

A number of functionally significant additional branches arise from the carotid system. These include the anterior choroidal artery (AChA), lenticulostriate arteries (LS), and ophthalmic artery (OphA). The pericallosal and callosomarginal arteries run along the corpus callosum.

It is interesting to note the features of the external carotid artery (ECA / ECA), whose branches are usually named after the region they supply (e.g., facial, lingual, occipital). They are easily differentiated on angiography: the vessels supplying the scalp (superficial temporal artery) have a tortuous, "corkscrew" course. In contrast, meningeal branches such as the middle meningeal artery (MMA — the first branch of the maxillary artery) have a significantly straighter course.

Vertebrobasilar system

The posterior circulation begins with the vertebral arteries (VA / VA). The largest branch of the VA is the posterior inferior cerebellar artery (PICA / PICA), which is responsible for supplying the brainstem, the inferior surface of the cerebellar hemisphere, the cerebellar vermis, and the choroid plexus.

Merging, the vertebral arteries form the basilar artery (BA / BA), which ascends along the anterior surface of the pons and ends in a bifurcation just above the sella turcica.

Branches of the basilar artery:

1. Anterior inferior cerebellar arteries (AICA / AICA): enter the cerebellopontine angles. They give off the labyrinthine (internal auditory) arteries (supplying the nerves of the internal acoustic meatus) and form anastomoses with branches of the PICA.
2. Superior cerebellar arteries (SCA / SCA): arise a few millimeters below the terminal branches of the BA, separated from them by the tentorium cerebelli (tentorium cerebelli). They wind around the brainstem and head toward the upper surface of the cerebellar hemispheres.
3. Perforating branches: provide local blood supply to the brainstem structures.

Posterior cerebral artery (PCA)

The terminal branches of the basilar artery are the posterior cerebral arteries (PCA). Each PCA is anatomically divided into four segments:

• P1 (precommunicating segment): runs to the junction with the posterior communicating artery (PCoA / PCoA). Perforators (thalamoperforating and thalamogeniculate arteries) arise from P1 and the PCoA, entering the posterior perforated substance.
• P2 (ambient segment): wraps around the cerebral peduncles. It gives rise to the medial and lateral posterior choroidal arteries, as well as cortical temporal branches.
• P3 (quadrigeminal segment): passes in the region of the corpora quadrigemina.
• P4 (terminal segment): includes branches supplying the inferior surface of the temporal lobe and the visual cortex on the medial surface of the occipital lobe.

Important feature: there is an inverse relationship between the diameter of the P1 segment and the PCoA. In 20–30% of cases, a fetal origin of the PCA is found. In this condition, the P1 segment is hypoplastic or completely absent, and blood flow to the PCA comes directly from the internal carotid artery.

Collateral pathways

Collaterals are divided into three main categories, which are vitally important in strokes and endovascular interventions:

1. Extracranial-intracranial anastomoses: connections between branches of the ECA, ICA, and vertebral arteries.
2. Circle of Willis: a critically important pathway, though it is complete in only 40% of people. Often the A1 segment or PCA is hypoplastic, rendering these collaterals ineffective.
3. Leptomeningeal (pial) collaterals: highly variable end-to-end anastomoses between the distal branches of intracerebral vessels. They provide blood flow across watershed zones.

In addition, there are persistent embryonic anastomoses — persistent connections between the carotid and vertebrobasilar circulations. The most common of these is the trigeminal artery (less than 1% of cases). The hypoglossal artery (passing through the hypoglossal canal) is extremely rare, while the otic artery is considered in most cases to be an anatomically unconfirmed fiction.

Intracranial veins and sinuses

The venous system is divided into dural venous sinuses and the veins of the brain itself (deep and superficial).

Sinuses run within the dural folds:

• Superior and inferior sagittal sinuses lie along the margins of the falx cerebri (falx cerebri).
• The inferior sagittal sinus joins the great cerebral vein of Galen to form the straight sinus, which descends obliquely toward the confluence of sinuses.
• Transverse sinuses (the right one is usually dominant) continue as the sigmoid sinuses, which drain into the internal jugular veins (IJVs).
• The cavernous sinuses connect to the sigmoid sinuses via the superior petrosal sinuses, and to the IJVs via the inferior petrosal sinuses.

The deep venous system is anatomically very consistent. The veins of the septum pellucidum join the thalamostriate veins at the foramen of Monro, forming the venous angle. From there, the internal cerebral veins originate, running along the roof of the third ventricle. By merging with the basal veins of Rosenthal, they form the unpaired great vein of Galen.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Segmental anatomy of the middle (M1-M4) and anterior (A1-A3) cerebral arteries
• Features of collateral blood flow in watershed zones
• Formation of the venous angle and its significance in neuroimaging
• Persistent embryonic anastomoses (proatlantal artery types 1 and 2)

Other topics in the Neuroimaging section

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