= pyramid-shaped structure formed by medial portion of temporal bone

Borders:

• lateral: inner ear
• medial: petro-occipital fissure
• anterior: petrosphenoidal fissure + internal auditory canal
• posterior: posterior cranial fossa

Surfaces: superiorly: middle cranial fossa + Meckel cave + ICA inferiorly: jugular bulb + inferior petrosal sinus

Division:

1. large anterior portion (bone marrow):
   • filled with marrow (60%)
   • pneumatized (33%) with communication to mastoid and middle ear cavity; asymmetric (5–10%)
     N.B.: with asymmetry high signal intensity fatty marrow may be mistaken for a cholesterol granuloma!
   • sclerotic (7%)
2. small posterior portion (otic capsule)
   N.B.: with asymmetry high–signal-intensity marrow in nonsclerotic apex can mimic a lesion!

Vascular & neural channels:

1. Petrous carotid canal in anterior portion of petrous apex (= horizontal portion of petrous segment of ICA), passes over foramen lacerum
2. Internal auditory canal (IAC) in midportion of petrous apex for vestibulocochlear + facial nn., anterolateral from cerebellopontine angle cistern
3. Dorello canal extends through posteromedial portion of petrous apex, contains abducens n. (CN VI)
4. Subarcuate canal = petromastoid canal between crura of superior semicircular canal, contains subarcuate a. (blood supply to bony labyrinth + facial canal + mastoid antrum)
5. Singular canal extends from posterior margin of IAC to junction of ampulla of posterior semicircular canal and vestibule, contains singular n. (= division of inferior vestibular n. that innervates ampulla of posterior semicircular canal)
6. Meckel cave = dura-lined diverticulum creating smooth depression along anterior aspect of petrous apex, contains trigeminal (gasserian / semilunar) ganglion + rootlets of trigeminal n. (CN V)