Imaging description

The petrous apex (PA) is a part of the petrous bone that extends medial to the inner ear structures and contributes to the central skull base. It is not amenable to otoscopic examination, and the disease processes that involve the PA often generate symptoms that are very non-specific such as headache, tinnitus, and hearing loss, making imaging the most important part of diagnosis.

PA cells are normally pneumatized in a symmetric fashion, but asymmetric pneumatization occurs in about 30% of the population and may lead to diagnostic errors, particularly on MRI (see Case 56). The most common abnormality in the PA is fluid entrapment (a.k.a. effusion). Fluid entrapment does not require surgical intervention in most cases and should be differentiated from the “surgical” lesions of the PA such as cholesterol granulomas (CG) and cholesteatomas [1].

Fluid entrapment presents as opacified air cells on CT without expansion and preservation of bony septations between air cells. On MRI, increased T2 signal is seen almost invariably and mildly elevated T1 signal is not infrequent, which may lead to confusion with CG, which is always T1 bright (Fig. 64.1). On post-contrast imaging there is no significant enhancement associated with entrapped fluid other than mild lacy enhancement around the cells. Fluid entrapment is seen at least 10 times more commonly than the most frequent lesions such as CG.