Brain Anatomy - Structures, Vasculature and Variants

Overview

The brain comprises the cerebrum, brainstem, cerebellum, ventricular system, meninges, and associated vascular structures. For the RANZCR Part 1 examination, a thorough understanding of normal cross-sectional imaging appearances on CT and MRI, and their relationship to functional anatomy, is essential. This note covers grey and white matter organisation, functional cortical systems, cranial nerve anatomy, CSF pathways, pituitary anatomy, meningeal layers, and cerebrovascular supply.

Cerebrum: Cortex, White Matter, and Deep Grey Matter Nuclei

Cerebral Cortex and Gyri/Sulci

The cerebral hemispheres are surfaced by grey matter, the cerebral cortex, folded into gyri (ridges) and sulci (grooves), dramatically expanding neuronal surface area. Key named structures:

On CT, cortical grey matter is isodense to mildly hyperdense compared with subjacent white matter (~32-40 HU vs ~20-30 HU). On MRI, grey matter is T1-hypointense and T2-hyperintense relative to white matter.

White Matter Organisation

The cerebral white matter consists of myelinated axonal tracts in three categories:

The corpus callosum (rostrum, genu, body, splenium) is the largest commissure, readily identified on midsagittal MRI. Above the lateral ventricles, white matter is the centrum semiovale; below the ventricular roof it is the corona radiata, which condenses into the internal capsule (anterior limb, genu, posterior limb, retrolenticular and sublenticular portions).

White matter is hypodense on CT (lipid-rich myelin lowers HU), T1-hyperintense and T2-hypointense relative to grey matter. Diffusion tensor imaging (DTI)/tractography delineates individual tracts in vivo.

Deep Grey Matter Nuclei

The basal ganglia include the caudate nucleus and putamen (together = striatum) and globus pallidus. The thalamus and hypothalamus form the diencephalon. These structures are isodense to cortex on CT. On MRI they approximate cortical T1/T2 signal, except iron-laden structures (substantia nigra, globus pallidus, red nuclei, dentate nuclei) which are T2-hypointense.

Thalamic nuclei and their connections (exam-relevant):

The thalamus is supplied by branches of the posterior communicating and posterior cerebral arteries.

Hypothalamus

Located anteroinferior to the thalamus, forming the floor of the third ventricle, separated from the thalamus by the hypothalamic sulcus. Key nuclei and functions:

The hypothalamus connects to the posterior pituitary directly via the infundibulum, and to the anterior pituitary indirectly via the hypothalamic-hypophyseal portal system.

Functional Neuroanatomy

Motor System

The corticospinal (pyramidal) tract originates from the precentral gyrus and premotor cortex, descending: corona radiata → posterior limb of internal capsule → cerebral peduncles (crus cerebri) → basis pontis (dispersing into fascicles among pontocerebellar fibres) → pyramids of medulla (reforming) → pyramidal decussation at the cervicomedullary junction → lateral corticospinal tract (contralateral spinal cord). ~10-15% of fibres do not decussate, forming the ipsilateral anterior corticospinal tract.

Sensory Systems

Dorsal column-medial lemniscal pathway (vibration, proprioception, fine touch):

• First-order neuron → dorsal column (gracile = lower body; cuneate = upper body) → nucleus gracilis / cuneatus (medulla) → internal arcuate fibres decussate → ascend as medial lemniscus → VPL thalamus → postcentral gyrus

Spinothalamic (anterolateral) pathway (pain, temperature, crude touch):

• First-order neuron → posterior horn (via Lissauer's tract, laminae I and V) → decussate via anterior white commissure → contralateral anterolateral column → VPL thalamus → postcentral gyrus
• Also gives rise to spinoreticular and spinomesencephalic tracts

Speech

Broca's area (inferior frontal gyrus, dominant hemisphere) mediates motor speech production; Wernicke's area (posterior superior temporal gyrus) mediates comprehension. Connected by the arcuate fasciculus (superior longitudinal fasciculus). Broca's aphasia = non-fluent, intact comprehension; Wernicke's aphasia = fluent, impaired comprehension.

Visual System

Retina → optic nerve → optic chiasm (nasal fibres decussate; temporal fibres remain ipsilateral) → optic tracts → LGN of thalamus → optic radiations:

• Inferior fibres (Meyer's loop) curve through temporal lobe → lower calcarine cortex (upper visual field)
• Superior fibres course through parietal lobe → upper calcarine cortex (lower visual field)
• Primary visual cortex along calcarine fissure (BA 17), medial occipital lobe

Auditory System

Cochlear nerve → cochlear nuclei (pontomedullary junction) → superior olivary complex → lateral lemniscus → inferior colliculus (midbrain tectum) → MGN of thalamus → auditory radiations → primary auditory cortex (Heschl's gyrus, superior temporal plane). Pathway is partially bilateral above the cochlear nuclei, explaining why unilateral cortical lesions rarely cause complete deafness.

Limbic System

Components: hippocampus (short-term memory, memory consolidation), amygdala (emotion, fear conditioning), fornix, mammillary bodies, anterior thalamic nuclei, cingulate cortex, parahippocampal / entorhinal cortex. The mammillothalamic tract connects mammillary bodies to anterior thalamus.

Clinically relevant disorders: Alzheimer disease, mesial temporal sclerosis (MTS), Wernicke-Korsakoff syndrome (thiamine/B1 deficiency → mammillary body degeneration), Klüver-Bucy syndrome, LATE (Limbic-predominant Age-related TDP-43 Encephalopathy).

The hippocampus is best assessed on coronal T1 and T2/FLAIR MRI for atrophy or signal change (e.g. MTS).

Brainstem: Structure and Tracts

The brainstem extends from the posterior commissure (diencephalon-midbrain junction) to the pyramidal decussation at the cervicomedullary junction. It comprises the midbrain, pons, and medulla oblongata.

Key points:

• CN IV is the only cranial nerve to exit from the dorsal brainstem surface, crossing and exiting below the inferior colliculi, traversing the perimesencephalic cistern.
• CN V (trigeminal) exits the anterolateral pons, traverses the prepontine cistern, enters Meckel's cave (dural recess containing the Gasserian/trigeminal ganglion).
• Facial colliculi on the floor of the fourth ventricle mark CN VII fibres looping over the CN VI nucleus.
• The medial longitudinal fasciculus (MLF) runs paramedially throughout the brainstem, linking abducens nucleus (CN VI) to contralateral oculomotor nucleus (CN III) to coordinate conjugate horizontal gaze. Demyelination (MS) or infarction → internuclear ophthalmoplegia (INO). On MRI, the MLF is a paramedian dorsal structure, T2-hyperintense when demyelinated.
• Substantia nigra and red nuclei show characteristic T2 hypointensity (iron accumulation) on MRI.
• Pyramids are separated by the anterior median fissure, briefly interrupted at the pyramidal decussation.
• The quadrigeminal (tectal) plate with superior colliculi (visual reflex) and inferior colliculi (auditory relay) lies posterior to the cerebral aqueduct. Compression by pineal masses → Parinaud syndrome (vertical gaze palsy, eyelid retraction, convergence-retraction nystagmus).

Cerebellum

The cerebellum occupies the posterior fossa, posterior to the fourth ventricle and brainstem, separated from the occipital lobes by the tentorium cerebelli. It consists of two hemispheres joined by the midline vermis, surfaced by finely folded folia (finer than cerebral gyri). The cortex overlies white matter containing four pairs of deep cerebellar nuclei (lateral to medial): dentate (largest), emboliform, globose, and fastigial.

Functional Zones

Each cerebellar hemisphere coordinates movement on the ipsilateral side of the body.

Midline vermis lesions (e.g. chronic alcoholism) → truncal ataxia; hemispheric lesions → limb ataxia, dysmetria, intention tremor, dysdiadochokinesia.

Cerebellar Cortex Layers

Three layers (superficial to deep): molecular layer (parallel fibres, Purkinje cell dendrites), Purkinje cell layer, granule cell layer. Afferents arrive as mossy fibres (from pontine nuclei, spinal cord, vestibular nuclei, synapse on granule cells) or climbing fibres (from inferior olivary nucleus, synapse directly on Purkinje cells).

Cerebellar Peduncles and Connections

Corticopontocerebellar pathway: motor/premotor cortex → corticopontine fibres (in crus cerebri) → pontine nuclei → decussate → pontocerebellar fibres → MCP → contralateral cerebellar cortex.

Dentato-rubro-thalamo-cortical pathway: dentate nucleus → SCP (decussates) → contralateral red nucleus and VL thalamus → motor cortex. This is the primary cerebellar output pathway.

Spinal cord → ICP (spinocerebellar tracts, vestibular and reticular inputs) → ipsilateral cerebellum.

On MRI, dentate nuclei appear as T2-hypointense crescent-shaped structures (iron deposition) within the cerebellar white matter, readily identified on axial T2 sequences. Vermian lobules on sagittal MRI (anterior to posterior): lingula (I), central (II), culmen (III-IV), declive (VI), folium (VIIA), tuber (VIIB), pyramid (VIII), uvula (IX), nodule (X).

Ventricular System and CSF Cisterns

CSF is produced predominantly by choroid plexus (lateral and third ventricles supplied by anterior and posterior choroidal arteries; fourth ventricle choroid plexus supplied by PICA, AICA, and SCA).

CSF flows from fourth ventricle into basal cisterns, then over cerebral convexities for reabsorption via arachnoid granulations into dural venous sinuses. The glymphatic system describes para-arterial perivascular CSF flow washing brain interstitium, with efflux via perivenous spaces, activity increases during sleep and anaesthesia.

Major cisterns: suprasellar/chiasmatic, interpeduncular, prepontine, cerebellopontine angle (CPA), ambient, quadrigeminal (cistern of the great cerebral vein), cisterna magna.

On CT, CSF ~0 HU. On MRI: T1-hypointense, T2-hyperintense, FLAIR-suppressed (dark), distinguishes CSF spaces from pathological fluid (which may not fully suppress on FLAIR).

Pituitary Gland and Related Structures

The pituitary gland sits in the sella turcica of the sphenoid bone. It is one of the few CNS regions lacking a blood-brain barrier, enabling chemoregulation and hormone secretion.

On MRI:

• Posterior pituitary: normally T1-hyperintense ("bright spot"), vasopressin storage granules; absent in central diabetes insipidus
• Anterior pituitary: T1-isointense to grey matter; enhances avidly with gadolinium (no BBB)
• Normal gland height: ≤8 mm adults; ≤10-12 mm in adolescent females / pregnant or lactating women
• Infundibulum: midline structure passing through diaphragma sellae; should not deviate

Adjacent structures:

• Optic chiasm: immediately superior, suprasellar masses cause bitemporal hemianopia
• Cavernous sinuses: lateral to sella, contain CN III, IV, V1, V2 (in lateral dural wall) and CN VI and the cavernous ICA (within sinus lumen)
• Pineal gland: commonly calcifies (normal); historically used as radiographic midline landmark; masses may compress superior colliculi → Parinaud syndrome

Cranial Nerves and Their Nuclei

CN I and CN II are CNS white matter tracts covered by oligodendrocytes and meninges, they do not have Schwann cell sheaths and do not develop schwannomas. All other CNs can develop schwannomas; CN VIII (superior vestibular division) is most common.

Meninges and Associated Spaces

From outermost to innermost:

1. Dura mater: two layers, outer periosteal (endosteal) and inner meningeal layer. Dural reflections:

   • <strong>Falx cerebri</strong>: interhemispheric, double meningeal layer
   • <strong>Tentorium cerebelli</strong>: separates posterior fossa from supratentorial compartment
   • <strong>Diaphragma sellae</strong>: roof of sella turcica

2. Arachnoid mater: adherent to inner dural surface; avascular; joined to pia by trabeculae crossing the subarachnoid space. Arachnoid granulations project into dural sinuses for CSF reabsorption.

3. Pia mater: adherent to brain surface; accompanies vessels into parenchyma, forming perivascular (Virchow-Robin) spaces

Sources

• RANZCR position papers & standards
• Radiopaedia (open educational resource)