Asymmetric pneumatization of the petrous apex is a frequent normal anatomic variant resulting in an appearance that can be mistaken for pathology within the petrous apex; see petrous apex lesions (differential).
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Clinical presentation
As this is an anatomical variant it is entirely asymptomatic and only discovered incidentally when the base of skull is imaged for other reasons.
Pathology
Asymmetric pneumatization of the petrous apex results in the presence of bone marrow instead of gas. Normal marrow contains significant adipose tissue, and signal characteristics parallel those of scalp or orbital fat.
Radiographic features
MRI
The non-pneumatized petrous apex will show fatty marrow appearing hyperintense on routine T1- and T2-weighted sequences with no expansion of the bone.
Confirmation is made by observing the complete loss of signal with fat-saturation sequences.
Asymmetric fatty infiltration of the apex may be observed as a conspicuous asymmetric high signal on contrast-enhanced images and could be mistaken for an enhancing pathologic lesion. Correlation with unenhanced T1-weighted images and use of fat-saturation techniques are necessary to avoid this pitfall 1.
The pneumatized side can also cause confusion on imaging if the air-cell contains fluid (effusion) 3.
Differential diagnosis
Other lesions with a bright T1 signal such as a cholesterol granuloma should be considered. Another differential diagnosis is a petrous apex effusion, but this tends to have intermediate or high T1 signal 2.
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