Blood supply of the brain
Arterial supply of the brain, internal carotid artery and venous drainage
CT and MR angiography show the carotid and vertebrobasilar systems meeting at the circle of Willis, and every stroke on diffusion MRI is read by matching its shape to an arterial territory. CPSP asks about the segments and branches of the internal carotid, the territories and their deficits, common variants and aneurysm sites, and the dural sinuses and deep veins.
Internal carotid artery
| Segment | Course | Important point |
|---|---|---|
| Cervical | Bifurcation to skull base | No branches in the neck; low-resistance Doppler trace with continuous diastolic flow (the brain is a low-resistance bed); the ECA trace is high resistance |
| Petrous | Carotid canal, turning forwards | Crosses over the cartilage of the foramen lacerum |
| Cavernous | S-shaped siphon inside the cavernous sinus with VI | Tear gives carotid–cavernous fistula (pulsatile proptosis, dilated superior ophthalmic vein) |
| Ophthalmic (supraclinoid) | Leaves the roof of the cavernous sinus | Ophthalmic artery runs forward through the optic canal; an aneurysm here compresses the optic nerve |
| Communicating | Above the ophthalmic origin | Posterior communicating and anterior choroidal arteries |
| Terminal | Beside the anterior perforated substance | Divides into ACA and MCA |
- Vertebral arteries usually enter the C6 transverse foramen, climb to C1, cross the atlas, pierce the dura, give PICA, and join at the pontomedullary junction to form the basilar artery. The basilar gives AICA (usually the labyrinthine artery), pontine branches, the superior cerebellar arteries and ends as the two PCAs.
- III emerges between the PCA and the superior cerebellar artery, so basilar tip and posterior communicating aneurysms compress it (pupil dilated early).
Circle of Willis
- ACAs joined by the anterior communicating artery; each ICA joined to the PCA by a posterior communicating artery.
- Fetal PCA: the PCA fills mainly from a large posterior communicating artery with a thread-like P1, so a carotid stenosis can cause an occipital infarct.
- Berry aneurysms lie on the circle in the subarachnoid space: commonest at the anterior communicating artery, then posterior communicating (III palsy), MCA bifurcation and basilar tip.
- Moyamoya: narrowing of both terminal ICAs with a fine collateral network ("puff of smoke"); in children, hyperventilation (crying, blowing) lowers CO2 and triggers attacks.
Territories and their deficits
| Artery | Territory | Typical deficit |
|---|---|---|
| ACA | Medial frontal and parietal lobes, front four-fifths of the corpus callosum | Leg more than arm, incontinence, apathy |
| Recurrent artery of Heubner (ACA) | Caudate head, anterior limb of internal capsule | Face and arm weakness, leg spared |
| MCA | Lateral convexity, insula, speech areas | Face and arm more than leg, aphasia (dominant) or neglect |
| Lenticulostriate (MCA) | Basal ganglia, most of the internal capsule | Lacunes; hypertensive haemorrhage (putamen) |
| Anterior choroidal (ICA) | Lower posterior limb of the capsule, optic tract, lateral geniculate, choroid plexus | Hemiplegia, hemisensory loss, hemianopia without aphasia |
| PCA | Occipital lobe, inferior temporal lobe, thalamus, midbrain, splenium | Homonymous hemianopia with macular sparing (MCA collateral) |
| Artery of Percheron | Single trunk to both paramedian thalami and rostral midbrain | Drowsiness, vertical gaze palsy |
| PICA | Lateral medulla, inferior cerebellum | Lateral medullary syndrome |
| AICA | Lateral pons, inner ear, anterior cerebellum | Facial palsy with deafness |
| Superior cerebellar | Upper cerebellar surface | Limb and gait ataxia, slurred speech; no deafness |
- Axial and respiratory muscles have bilateral descending control, so a single hemisphere stroke spares breathing and coughing.
- Watershed (border zone) infarcts after hypotension lie between ACA and MCA or MCA and PCA territories; in preterm babies the border zone is periventricular.
Early stroke imaging
- Dense MCA sign: a bright linear MCA on non-contrast CT is fresh thrombus (a one-sided sign; a high haematocrit makes both look dense).
- Early CT signs: loss of the insular ribbon, blurred lentiform nucleus. Diffusion MRI shows restriction within minutes, when CT is still normal.
- Hypertensive haemorrhage sites: putamen, thalamus, pons, cerebellum; lobar haemorrhage in the elderly suggests amyloid angiopathy.
Dural venous sinuses
- Sinuses lie between the periosteal and meningeal layers of dura, have no valves, and drain to the IJV.
| Sinus | Course |
|---|---|
| Superior sagittal | Attached border of the falx to the confluence at the internal occipital protuberance; usually continues into the right transverse sinus |
| Inferior sagittal | Lower (unattached) border of the falx; joins the great cerebral vein to form the straight sinus |
| Straight | Along the falx–tentorium junction to the confluence |
| Transverse and sigmoid | To the jugular foramen; sigmoid sinus thrombosis follows mastoiditis |
| Cavernous | Beside the sella; receives ophthalmic veins (spread from the face); drains by the superior and inferior petrosal sinuses (sampled in Cushing disease) |
- A hypoplastic transverse sinus (often the left) is a normal variant that can mimic thrombosis on flow-based MR venography; check for actual clot.
- Deep veins: paired internal cerebral veins on the roof of the third ventricle and the basal veins join to form the great cerebral vein, which drains into the straight sinus.
Venous disease on imaging
| Lesion | Clue |
|---|---|
| Superior sagittal sinus thrombosis | Dense sinus on plain CT; empty delta (unfilled triangle) on contrast CT; haemorrhagic infarcts not matching an arterial territory; dehydration, pregnancy and the puerperium |
| Deep venous thrombosis | Dense internal cerebral veins with swollen bilateral thalami |
| Vein of Galen malformation | Newborn with high-output cardiac failure; midline cyst behind the third ventricle with turbulent Doppler flow |
| Developmental venous anomaly | Radiating medullary veins into one collector; often beside a cavernoma ("popcorn" core, dark haemosiderin rim); the anomaly is left alone at surgery |
Remember The ICA gives nothing in the neck; its first important intracranial branch is the ophthalmic artery.
Exam trap A posterior cerebral territory infarct can come from a carotid stenosis when the PCA is of fetal origin.
Exam trap Bilateral thalamic swelling may be venous (internal cerebral vein thrombosis), not only an artery of Percheron infarct.
Test yourself on this topic
5 of the 21 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 68-year-old man is brought to the emergency department 90 minutes after sudden right hemiplegia and loss of speech. Non-contrast CT shows no low density in the brain yet, but a bright linear density lies in the left Sylvian fissure along the course of a large artery. What does this bright density represent?
Answer: Fresh thrombus in the middle cerebral artery
Fresh clot has higher attenuation than flowing blood because red cells are packed together, so an occluded M1 segment of the middle cerebral artery appears as a bright line on plain CT (the hyperdense MCA sign). It often appears before any low density of infarction is visible. Subarachnoid blood spreads through the cistern and sulci rather than following one tubular vessel, and a non-contrast scan has no contrast to enhance vessels.
Hyperdense MCA sign
- Fresh clot is denser than flowing blood
- Bright line in the Sylvian fissure = M1 clot
- Seen before the infarct shows
- One-sided sign; compare with the other MCA
Remember Bright artery on plain CT: think clot.
Exam trap A high haematocrit or calcified wall can make both MCAs look dense; the true sign is one-sided.
Why the other options are wrong
- Calcified atheroma of the internal carotid Carotid calcification lies at the siphon, not as a long line in the Sylvian fissure.
- Acute subarachnoid blood in the Sylvian fissure Would spread through cisterns and sulci, usually with sudden headache.
- Contrast retained from an earlier angiogram This was a non-contrast scan.
- Normal flow enhancement within the artery Plain CT shows no flow enhancement; flowing blood is less dense than clot.
As the vertebral artery ascends from the subclavian artery, which transverse foramen does it usually enter first?
Answer: Transverse foramen of C6
The first part of the vertebral artery ascends between longus colli and scalenus anterior and usually enters the transverse foramen of C6; the C7 foramen normally transmits only veins. The second part rises through the foramina of C6 to C1, the third part curves behind the lateral mass of the atlas on its posterior arch, and the fourth part pierces the dura, passes through the foramen magnum and joins its partner at the lower border of the pons to form the basilar artery.
Vertebral artery course
- Enters the C6 transverse foramen
- Rises through C6 to C1
- Curves on the posterior arch of atlas
- Joins partner to form basilar at the pons
Remember C7 foramen: veins only, usually.
Exam trap Entry at C7, C5 or C4 occurs as a variant, so check the level before cervical spine surgery.
Why the other options are wrong
- Transverse foramen of C7 Usually carries only the vertebral vein.
- Transverse foramen of C5 Entry at C5 is an uncommon variant.
- Transverse foramen of C1 The artery leaves, not enters, through C1 at the top.
- Transverse foramen of T1 T1 has no transverse foramen.
At the internal occipital protuberance, into which sinus does the superior sagittal sinus usually continue?
Answer: Right transverse sinus
At the internal occipital protuberance the superior sagittal sinus usually turns to the right as the right transverse sinus, while the straight sinus, carrying deep venous blood, usually turns to the left. Because the superficial drainage is larger, the right transverse and sigmoid sinuses and the right jugular foramen and internal jugular vein are often bigger. A small left transverse sinus is therefore a common normal finding.
Transverse sinus dominance
- Superior sagittal sinus turns right
- Straight sinus usually turns left
- Right transverse sinus often larger
- Small left sinus is normal
Remember Hypoplastic sinus is not thrombosis.
Exam trap A hypoplastic left transverse sinus is a normal variant and must not be mistaken for sinus thrombosis.
Why the other options are wrong
- Left transverse sinus Usually receives the straight sinus and is often the smaller side.
- Straight sinus Carries deep venous blood from the great cerebral vein.
- Inferior sagittal sinus Runs in the free edge of the falx and joins the straight sinus.
- Occipital sinus A small sinus in the falx cerebelli draining to the confluence.
A 60-year-old man has sudden deafness and severe vertigo in the right ear from occlusion of the labyrinthine artery. From which artery does the labyrinthine artery most commonly arise?
Answer: Anterior inferior cerebellar artery
The labyrinthine (internal auditory) artery enters the internal acoustic meatus with the facial and vestibulocochlear nerves and supplies the cochlea and vestibular apparatus. It most often arises from the anterior inferior cerebellar artery, and less often directly from the basilar artery. It is an end artery, so its occlusion causes sudden deafness and vertigo. AICA infarcts therefore often combine ipsilateral deafness with facial weakness and ataxia.
Labyrinthine artery
- Usually from AICA
- Sometimes from the basilar
- Runs in the internal acoustic meatus
- End artery: sudden deafness, vertigo
Remember AICA stroke: deafness plus facial weakness.
Exam trap In a minority it arises straight from the basilar artery, so either answer can appear; AICA is the usual origin.
Why the other options are wrong
- Posterior inferior cerebellar artery Supplies the inferior cerebellum and lateral medulla.
- Superior cerebellar artery Arises at the top of the basilar and supplies the upper cerebellum.
- Vertebral artery Gives the PICA and spinal arteries, not the labyrinthine artery.
- Posterior cerebral artery Supplies the occipital lobe and thalamus.
A 64-year-old hypertensive man has sudden unsteadiness, slurred speech and clumsiness of the left arm and leg, with no facial weakness or deafness. The infarct involves the upper surface of the left cerebellar hemisphere and the upper vermis. Which artery has been occluded?
Answer: Left superior cerebellar artery
The superior cerebellar artery arises near the top of the basilar artery and supplies the superior (tentorial) surface of the cerebellar hemisphere, the upper vermis, the dentate nucleus and the superior cerebellar peduncle. Its infarction gives ipsilateral limb ataxia and dysarthria. The posterior inferior cerebellar artery supplies the inferior surface and lateral medulla, and the anterior inferior cerebellar artery supplies the anterior inferior cerebellum and lateral pons with facial palsy and deafness.
Cerebellar arteries
- SCA: upper surface and upper vermis
- PICA: inferior surface + lateral medulla
- AICA: anterior inferior + lateral pons
- Ataxia is ipsilateral
Remember Top of cerebellum: superior cerebellar artery.
Exam trap The SCA arises just below the PCA at the basilar tip, so both territories may be hit by a top-of-basilar clot.
Why the other options are wrong
- Left posterior inferior cerebellar artery Supplies the inferior cerebellum and lateral medulla.
- Left anterior inferior cerebellar artery Supplies the anterior inferior cerebellum; deafness and facial palsy.
- Left posterior cerebral artery Supplies the occipital lobe and thalamus.
- Anterior spinal artery Supplies the medial medulla and anterior cord.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Brain, Arterial supply of the brain, internal carotid artery and venous drainage