Radiographic differential diagnosis of petrous apex lesions.

As a consequence of improved diagnostic imaging modalities, otologists have encountered a steadily increasing number of petrous apex lesions in recent years. Contemporary imaging techniques not only provide precise anatomic localization of the lesion, but also are able to suggest specific tissue diagnoses in the majority of cases. Computed tomography (CT), by virtue of its sensitivity and low false-positive rate, is the screening examination of choice in a patient suspected of having a petrous apex lesion. Once a lesion is identified, it is often necessary to obtain a combination of CT and magnetic resonance imaging (MRI). Computed tomography is important in the detection of osseous erosion as well as in the evaluation of the extent of pneumatization and marrow formation. It also provides important details about potential surgical routes to this relatively inaccessible region. Magnetic resonance imaging provides information about the composition of the lesion that cannot be readily discerned on CT scans. In the great majority of cases, it is capable of differentiating between petrous apicitis, cholesterol granuloma, osteomyelitis, cholesteatoma, and neoplasms such as schwannoma, meningioma, chondroma, and chordoma. In the interpretation of MRI scans, a familiarity with the typical appearance of the lesions that affect the petrous apex on T1-weighted, T2-weighted, and gadolinium-enhanced images is essential. A combination of MRI and CT scanning is also necessary to evaluate normal anatomic variations, such as giant air cells and asymmetric bone marrow, which may at times, on MRI alone, simulate pathologic conditions.