Imaging description

Radiologists rely on symmetry of the paired anatomic structures when interpreting images of the brain and skull base. Pneumatization of the paranasal sinuses and mastoids is not always symmetric, however. While asymmetrically pneumatized cells are easy to characterize on CT, they may lead to diagnostic errors on MRI due to decreased conspicuity of bony landmarks (Figs. 56.1, 56.2). When a part of the skull base is not pneumatized it usually contains normal marrow (and surrounding cortical bone), which appears on MRI as a T1 hyperintense structure that may mimic an enhancing mass. Completely pneumatized cells are extremely hypointense on all pulse sequences and may mimic the flow void of an aneurysm when in close proximity to the vessels (Figs. 56.1, 56.3). Alternatively, a pneumatized air cell may develop mucosal inflammation that can create diagnostic confusion when asymmetric (Figs. 56.4, 56.5).

Importance

Erroneous calls made on MRI as a result of asymmetrically pneumatized cells are usually reversed by additional work-up. There is usually no harm inflicted other than unnecessary anxiety and cost. The radiologist’s reputation may be damaged irreversibly, however.

Typical clinical scenario

Since asymmetric pneumatization of air cells is merely an anatomic variation, no specific clinical presentation can be ascribed. These are frequently misinterpreted as aneurysms or mass lesions.

Differential diagnosis

The primary role of the radiologist in these cases is to make the diagnosis of a normal anatomic variant and not invoke a diagnostic work-up. The common locations for asymmetric pneumatization include the anterior clinoid and pterygoid processes of the sphenoid bone, and the petrous apices of the temporal bone. Non-pneumatized bone contains fatty marrow, which can be positively characterized by comparing the unenhanced T1-weighted images to the contrast-enhanced ones and making sure that the “lesion” follows the fat signal on all pulse sequences. Fat-suppressed images are not essential but very helpful in confirming the presence of fat. In differentiating an anterior clinoid process air cell from an aneurysm, knowledge of the precise location of this cell is important. When there is a true aneurysmone can also identify the anterior clinoid process next to it whether it is pneumatized or not.