Asymmetric pneumatisation of petrous apex results in asymmetric fatty bone marrow within the petrous apex. It is a common incidental finding on brain and skull base MRI.
Asymmetric pneumatisation of the petrous apex results in the presence of bone marrow instead of air. Normal marrow contains significant adipose tissue, and signal characteristics parallel those of scalp or orbital fat.
The non-pneumatised petrous apex will show fatty marrow appearing hyperintense on routine T1- and T2-weighted sequences with no expansion of 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.
Other lesions with a bright T1 signal such as a cholesterol granuloma should be considered. Another differential diagnosis is a petrous air-cell effusion, but this tends to have intermediate or high T1 signal 2.
- 1. Chapman PR, Shah R, Curé JK, Bag AK. Petrous apex lesions: pictorial review. American Journal of Roentgenology. 2011 Mar;196(3_supplement):WS26-37.
- 2. Moore KR, Harnsberger HR, Shelton C, Davidson HC. 9Leave me alone9 lesions of the petrous apex. American journal of neuroradiology. 1998 Apr 1;19(4):733-8.