Imaging of the petrous apex: a pictorial review.

A wide variety of pathologies arise from the petrous apex. Such lesions may present with symptoms caused by mass effect or cranial nerve palsies, or may be detected during an investigation for an unrelated disease. CT and MRI are complementary in providing an appropriate differential diagnosis and in aiding surgical planning. This pictorial review appraises the anatomy and contents of the petrous apex. Benign (e.g. cholesterol granuloma, cholesteatoma), inflammatory (e.g.apical petrositis) and dysplastic (e.g. Pagets disease) lesions of the petrous apex are discussed and illustrated. Whilst it is more frequent for neoplastic lesions to extend from adjacent structures, we demonstrate a variety of aggressive tumours and tumour-like conditions (e.g. metastasis, rhabdomyosarcoma, Langerhan's cell histiocytosis, endolymphatic sac tumour) that directly involve the petrous apex. A range of normal radiological appearances are seen, some of which may be mistaken for significant pathology (e.g. asymmetric marrow space development, simple effusions, cephaloceles). An imaging algorithm to aid the formulation of a differential diagnosis is also presented.