Head and Neck Imaging

Petrous apex

1. Anatomy

part of the petrous bone situated anteriorly to the internal auditory canal. The petrous apex may be completely ossified, showing a cortical margin surrounding a medullary cavity, or be partially or completely pneumatized. Due to varying degrees of pneumatization, the appearance of the petrous apices may be asymmetrical. On MR images, asymmetrical signal intensities observed in the petrous apices may mimic a pathological condition, particularly cholesterol granuloma. Awareness of this variant and the absence of any mass effect usually leaves no doubt, but if necessary an additional CT study resolves the matter (Fig.1).

2. Pathology

  • Congenital cholesteatoma presents as an expansile lesion in the petrous apex, mostly well demarcated. It cannot be distinguished from cholesterol granuloma on CT images. On MR, it shows low to intermediate signal intensity on T1-weighted and relative high signal intensity on T2-weighted images (see cholesteatoma (VI:2), Fig. 5).

  • Inflammation. When pneumatized, a similar spectrum of inflammatory disease can be seen in the petrous apex as in the middle ear and mastoid. Active infection may cause acute apical petrositis; when associated with osteomyelitis, this may cause Gradenigos syndrome. In the case of osteomyelitis, CT shows destructive bone changes in the petrous apex; on MRI, meningeal enhancement will be visible (Fig.2). Further complications include thrombosis of the cavernous sinus or other intracranial venous sinuses, or formation of an intracranial abscess.

    Inflammation in the petrous apex may trigger the formation of a cholesterol granuloma.

  • Trauma. The petrous apex may be involved by a transverse fracture temporal bone; this may be associated with lesions of the adjacent cranial nerves (facial, trigeminal and abducens nerve).

  • Tumour. A variety of benign and malignant tumoral lesions may be encountered, either originating in the petrous apex or secondarily growing into this structure (Table 1).

    Petrous apex, Table 1. Petrous apex tumours (adapted from J.D. Swartz, H. Ric Harnsberger: Imaging of the temporal bone. 3 Ed., Thieme, New York, 1998, p. 453).

    Originating in petrous apexSecondarily invading petrous apex
    metastasisschwannoma acoustic
    giant cell tumour, giant cell tumourglomus tumour
    aneurysmal bone cystmeningioma
    Langerhans histiocytosis Xchordoma
    rhabdomyosarcoma head and neckchondrosarcoma head and neck
    plasmacytomanasopharynx cancer
    haemangioma head and neckpituitary adenoma

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    Fig.1

    Axial T1-weighted spin-echo image of temporal bones. The left petrous apex appears hyperintense (arrow) as it is nonpneumatized and contains fatty bone marrow; the right petrous apex is pneumatized and generates no signal.
    Petrous apex, Fig.1
    Petrous apex, Fig.2