Normal Anatomy on CT and MRI

Misreading anatomy is the commonest cause of misinterpretation in neuroimaging. This page is a structured walk through the landmarks you need to identify reliably on every brain CT and MRI — gray vs white matter, deep gray nuclei, internal capsule, ventricles, brainstem levels, cerebellum, and the major vascular territories. Master these before pattern recognition.

Cortical Anatomy (Lobes and Sulci)

Frontal Lobe

• Anterior to central sulcus; superior to Sylvian fissure.
• Precentral gyrus = primary motor cortex (M1). Homunculus: medial = leg/foot, lateral = face/hand.
• Superior frontal gyrus, middle frontal gyrus, inferior frontal gyrus (pars opercularis, triangularis, orbitalis — Broca’s area on the dominant side, BA 44/45).
• Supplementary motor area (medial superior frontal gyrus, BA 6).
• Frontal eye field (BA 8).

Parietal Lobe

• Posterior to central sulcus; superior to Sylvian fissure; anterior to parieto-occipital fissure.
• Postcentral gyrus = primary sensory cortex (S1).
• Superior parietal lobule + inferior parietal lobule (supramarginal + angular gyri — Gerstmann’s tetrad localizes here on the dominant side).

Temporal Lobe

• Inferior to Sylvian fissure.
• Superior temporal gyrus (Heschl’s gyrus = primary auditory cortex), middle, inferior temporal gyri.
• Wernicke’s area (dominant superior temporal/posterior, BA 22).
• Mesial temporal lobe — hippocampus, amygdala, parahippocampal gyrus — the epilepsy substrate.

Occipital Lobe

• Posterior to parieto-occipital fissure.
• Calcarine cortex = primary visual cortex (V1, BA 17). Cuneus above, lingual gyrus below.

Insula

• Hidden under the Sylvian fissure. Critical landmark — the “insular ribbon” is a key early-stroke sign on CT.

Deep Gray Nuclei

Basal Ganglia

• Caudate nucleus: head (anterior, indents lateral ventricle), body, tail. Sits medial to anterior limb of internal capsule.
• Lentiform nucleus = putamen (lateral) + globus pallidus (medial). Sits lateral to internal capsule.
• Globus pallidus: external (lateral) + internal (medial) segments. High iron content — predictable low T2 / SWI hypointensity with age.
• Claustrum: thin gray sheet lateral to putamen, separated from it by external capsule.

Thalamus

• Paired oval nuclei flanking the third ventricle.
• Subdivisions: anterior, medial (DM), lateral (VL, VP), posterior (pulvinar), intralaminar (CM), reticular.
• Pulvinar — the posterior thalamic nucleus — is the “pulvinar sign” location in variant CJD and Fabry.

Subthalamic Nucleus + Substantia Nigra

• STN: lens-shaped nucleus inferior to thalamus, lateral to red nucleus — the DBS target for PD.
• SN: in the midbrain (cerebral peduncles); pars compacta degenerates in PD (“loss of swallow-tail sign” on SWI).

White Matter Tracts and the Internal Capsule

Internal Capsule

• Anterior limb: between caudate head + lentiform; carries frontopontine and thalamocortical fibers.
• Genu: between anterior and posterior limbs; corticobulbar fibers.
• Posterior limb: between thalamus + lentiform; corticospinal tract (somatotopic — face anteromedial, arm middle, leg posterolateral).
• Retrolenticular part: optic radiations (geniculocalcarine), auditory radiations.
• Sublenticular part: auditory radiations (Heschl’s gyrus).

Corpus Callosum

• Rostrum → genu → body → splenium. Interhemispheric commissure.
• Genu = frontal connections; body = mostly somatosensory + premotor; splenium = posterior parietal, temporal, occipital.
• Splenium is the classic site for MERS (mild encephalopathy with reversible splenial lesion), CADASIL, and ischemic injury.

Other Major White Matter Bundles

• Cingulum: above corpus callosum, wraps around it.
• Superior longitudinal fasciculus / arcuate fasciculus: arches around Sylvian fissure; arcuate connects Broca and Wernicke.
• Inferior longitudinal fasciculus: occipital ↔ temporal.
• Inferior frontal-occipital fasciculus: orbitofrontal ↔ occipital, through external/extreme capsule.
• Uncinate fasciculus: orbitofrontal ↔ anterior temporal.
• Optic radiations: lateral geniculate ↔ V1 (calcarine cortex). Meyer’s loop swings forward into temporal lobe — vulnerable in anterior temporal lobectomy.

Ventricular System

• Lateral ventricles: frontal (anterior) horn → body → atrium (trigone) → temporal horn → occipital horn.
• Foramen of Monro: lateral ventricle → third ventricle.
• Third ventricle: midline, between thalami. Optic recess + infundibular recess (pituitary stalk) + suprapineal recess.
• Cerebral aqueduct (of Sylvius): third → fourth ventricle through midbrain.
• Fourth ventricle: between pons/medulla and cerebellum.
• Outflow: foramen of Magendie (median) + foramina of Luschka (paired lateral) → subarachnoid space.
• Choroid plexus: produces CSF; located in lateral ventricles (trigone + temporal horns), third ventricle (roof), fourth ventricle. Calcifies with age (normal). Asymmetric prominence raises tumor (papilloma, carcinoma) or metastasis.

Brainstem (Rostral to Caudal)

Midbrain

• Cerebral peduncles (crus cerebri): ventral; carry corticospinal + corticobulbar fibers.
• Tegmentum: dorsal; contains red nucleus, substantia nigra, periaqueductal gray, CN III nucleus (medial) + CN IV nucleus.
• Tectum (quadrigeminal plate): superior colliculi (vision/eye movement) + inferior colliculi (audition).
• Aqueduct: through midline.
• Key signs: red nucleus low T2 (iron); substantia nigra low T2; “Mickey Mouse” cross-section appearance; PSP causes midbrain atrophy (“hummingbird” / “morning glory” signs).

Pons

• Basis pontis (ventral): corticospinal + corticopontine + pontocerebellar fibers; transverse pontine fibers cross.
• Tegmentum (dorsal): CN nuclei (V motor + sensory; VI; VII; VIII), MLF, medial lemniscus, reticular formation.
• Middle cerebellar peduncles: emerge laterally.
• Key signs: pontine micro-infarcts (lacunar pattern); central pontine myelinolysis (trident); “hot cross bun” in MSA-cerebellar.

Medulla

• Pyramids (ventral): corticospinal tract; decussate at the cervicomedullary junction.
• Olives (lateral): inferior olivary nuclei.
• Tegmentum: CN nuclei (IX, X, XI, XII), nucleus solitarius, nucleus ambiguus, MLF, reticular formation.
• Inferior cerebellar peduncles: posterolateral.

Cerebellum

• Hemispheres: lateral; movement coordination + planning.
• Vermis: midline; truncal coordination, gait.
• Flocculonodular lobe: vestibular; eye movements + balance.
• Deep nuclei: dentate (lateral, “dentate-rubral-thalamic” pathway), interposed (emboliform + globose), fastigial.
• Peduncles: superior (cerebellum → midbrain), middle (pons → cerebellum), inferior (medulla → cerebellum).
• Key signs: dentate T2 hyperintensity in CTX; tonsillar herniation (>5 mm below foramen magnum); cerebellar atrophy patterns drive SCA differential.

Major Vascular Territories

Anterior Circulation (Carotid)

• Ophthalmic artery: first ICA branch after cavernous sinus.
• Anterior choroidal artery: small but critical — posterior limb internal capsule, lateral geniculate, optic tract, medial temporal.
• Posterior communicating artery: connects ICA to PCA; common aneurysm site.
• Anterior cerebral artery (ACA): medial frontal + parietal cortex; A1 → AcomA → A2 (pericallosal + callosomarginal). Lenticulostriate (recurrent artery of Heubner) supplies anterior caudate + anterior limb IC.
• Middle cerebral artery (MCA): lateral hemispheric cortex; M1 (lenticulostriate perforators to deep gray + posterior limb IC) → M2 (Sylvian) → M3 (operculum) → M4 (cortical).

Posterior Circulation (Vertebrobasilar)

• Vertebral artery: V1–V4; gives off PICA (lateral medulla — Wallenberg) and anterior spinal artery.
• Basilar artery: paired pontine perforators (medial + lateral) → AICA → SCA → terminates as paired PCAs.
• AICA: lateral pons + middle cerebellar peduncle + anterior cerebellum + labyrinthine artery (inner ear).
• SCA: superior cerebellum + dorsolateral pons.
• PCA: occipital + medial temporal + posterior thalamus + splenium. Thalamoperforators (P1) supply paramedian thalamus (artery of Percheron variant supplies bilateral paramedian thalami + rostral midbrain — classic embolic pattern).

Watershed Zones

• External / cortical watershed: ACA–MCA (frontal vertex) and MCA–PCA (parieto-occipital) cortical junctions. Patterned with global hypoperfusion.
• Internal / deep watershed: at the lateral ventricle border zone (centrum semiovale, corona radiata). Confluent string of round/oval foci (“string of beads”). Specific for hemodynamic insufficiency.

Other Critical Landmarks

• Circle of Willis: posterior comm + AcomA complete the polygon; many anatomic variants (fetal PCA, A1 hypoplasia, accessory branches).
• Cavernous sinus: paired; contains CN III, IV, V1, V2, VI (VI runs free; others in wall) + ICA.
• Pituitary gland: anterior + posterior lobes; sella turcica; infundibular stalk through diaphragma sellae.
• Pineal gland: midline behind third ventricle. Calcifies with age (normal); shift indicates mass effect.
• Falx cerebri + tentorium cerebelli: dural reflections defining supratentorial / infratentorial compartments; herniation pathways.
• Cisterns: perimesencephalic, suprasellar (5-pointed star), prepontine, cerebellomedullary (cisterna magna), interhemispheric. SAH blood layers in these.

Pitfalls and Pearls

• Always identify the internal capsule first on supratentorial slices — it’s the anatomic spine that orients everything else.
• Choroid plexus calcification, pineal calcification, basal ganglia mineralization (especially globus pallidus) are normal in adults — don’t call them pathologic unless they fit a syndrome.
• Globus pallidus iron increases with age → predictably low T2 / SWI dark. Beyond normal degree, think Fahr (calcium) or NBIA (PANK2 “eye of the tiger”).
• Don’t miss bilateral paramedian thalamic infarcts — artery of Percheron variant → impaired arousal + memory + vertical gaze palsy.
• Watershed patterns are specific for hemodynamic disease — find them, then look for ICA stenosis / dissection / cardiac arrest.
• Splenium lesions: cytotoxic in MERS, ischemia, AED toxicity, status epilepticus.
• Dentate T2 hyperintensity: classic for CTX, Wernicke, methotrexate toxicity.
• The “I never look at this” landmarks miss things: sphenoid sinus, mastoids (mastoiditis), orbits (vasogenic edema in IIH or thyroid eye disease), scalp (subgaleal/cephalohematoma).

References

1. Naidich TP, Castillo M, Cha S, Smirniotopoulos JG. Imaging of the Brain. Elsevier; 2013.
2. Tatu L, Moulin T, Bogousslavsky J, Duvernoy H. Arterial territories of human brain: brainstem and cerebellum. Neurology. 1996;47(5):1125-1135.
3. Tatu L, Moulin T, Bogousslavsky J, Duvernoy H. Arterial territories of the human brain: cerebral hemispheres. Neurology. 1998;50(6):1699-1708.
4. Mesulam MM. Principles of Behavioral and Cognitive Neurology. 2nd ed. Oxford; 2000.