Skull

Neurocranium

Bones

• Frontal
• Parietal (2)
• Temporal (2)
• Occipital
• Sphenoid
• Ethmoid

These bones form the calvarium and skull base, the latter being critical due to dense anatomy, cranial nerves, and vascular structures.

Skull Base

Anterior Cranial Fossa

Bones: Frontal, ethmoid, sphenoid
Key structures:

• Cribriform plate – olfactory nerve filaments (CN I)
• Crista galli
• Fovea ethmoidalis

Radiology relevance:

• CSF leaks (cribriform plate defects)
• Traumatic anosmia
• Tumour spread from sinonasal malignancy

Variants:

• Asymmetric olfactory fossae (Keros classification)
• Cribriform plate thinning

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Middle Cranial Fossa

Bones: Sphenoid, temporal
Key foramina:

• Optic canal – CN II, ophthalmic artery
• Superior orbital fissure – CN III, IV, V1, VI
• Foramen rotundum – CN V2
• Foramen ovale – CN V3
• Foramen spinosum – Middle meningeal artery

Radiology relevance:

• Perineural tumour spread
• Cavernous sinus pathology
• Skull base fractures → vascular injury

Variants:

• Absent or hypoplastic foramen spinosum
• Accessory foramen ovale
• Persistent trigeminal artery (vascular variant)

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Posterior Cranial Fossa

Bones: Occipital, temporal
Key structures:

• Internal auditory canal (IAC) – CN VII, VIII
• Jugular foramen – CN IX, X, XI
• Hypoglossal canal – CN XII
• Foramen magnum

Radiology relevance:

• Vestibular schwannoma
• Jugular foramen lesions (glomus tumours)
• Chiari malformations
• Occipital condyle fractures

Variants:

• Jugular bulb asymmetry or high-riding jugular bulb
• Enlarged mastoid emissary vein

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Temporal Bone (Dedicated Radiology Focus)

Components

• Squamous
• Mastoid
• Tympanic
• Petrous

Key structures:

• Ossicles (malleus, incus, stapes)
• Facial nerve canal (labyrinthine → tympanic → mastoid segments)
• Semicircular canals
• Cochlea

Radiology relevance:

• Cholesteatoma
• Temporal bone fractures (longitudinal vs transverse)
• Otosclerosis
• Facial nerve palsy

Variants:

• Dehiscent facial nerve canal
• High-riding jugular bulb
• Jugular bulb diverticulum
• Superior semicircular canal dehiscence
• Asymmetric mastoid pneumatization

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Viscerocranium (Facial Skeleton)

Major Bones

• Maxillae
• Zygomatic bones
• Nasal bones
• Lacrimal bones
• Palatine bones
• Inferior nasal conchae
• Mandible
• Vomer

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Orbits

Walls:

• Roof: Frontal
• Lateral: Zygomatic, sphenoid
• Medial: Ethmoid (lamina papyracea), lacrimal
• Floor: Maxilla

Key landmarks:

• Optic canal
• Inferior orbital fissure
• Superior orbital fissure

Variants:

• Infraorbital canal dehiscence
• Asymmetric orbital volume
• Accessory ethmoidal air cells (Haller cells)

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Paranasal Sinuses

Sinuses

• Frontal
• Ethmoid
• Maxillary
• Sphenoid

Radiology relevance:

• Drainage pathways (OMU)
• Surgical planning (FESS)
• Orbital/intracranial complications

Variants (very high-yield):

• Concha bullosa
• Agger nasi cells
• Haller cells
• Onodi cells (posterior ethmoid cell closely related to optic nerve)
• Hypoplastic or absent frontal sinus
• Septal deviation and spurs

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Sutures and Fontanelles

Major Sutures

• Coronal
• Sagittal
• Lambdoid
• Squamosal

Radiology relevance:

• Differentiating sutures from fractures
• Craniosynostosis

Variants:

• Wormian bones (sutural bones; common in lambdoid suture)
• Persistent metopic suture
• Asymmetric suture closure

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Diploë and Vascular Channels

• Diploic veins
• Middle meningeal artery grooves
• Venous sinuses

Radiology relevance:

• Mimics of fractures
• Pathways for infection or haemorrhage

Variants:

• Prominent arachnoid granulations
• Enlarged emissary veins
• Asymmetric venous sinus dominance

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Skull Variants That Commonly Mimic Pathology

• Arachnoid granulations vs lytic lesions
• Venous lakes vs fractures
• Persistent sutures vs fracture lines
• Pneumatized anterior clinoid process
• Hyperostosis frontalis interna
• Calvarial thinning in elderly patients

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Imaging Modality Considerations

CT

• Gold standard for:
  • Fractures
  • Skull base
  • Temporal bone
  • Sinuses

MRI

• Superior for:
  • Cranial nerves
  • Bone marrow
  • Soft tissue extension
  • Intracranial complications

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Reporting Tips for Radiologists

• Always assess symmetry
• Trace foramina and canals bilaterally
• Actively look for normal variants
• Correlate skull base findings with cranial nerve deficits
• Use thin-slice bone algorithms where appropriate