Differentiating Petrous Apex Lesions on MRI

Cholesterol granulomas, the most common primary petrous apex lesion, are bright on both T1 and T2 — a reflection of methemoglobin and cholesterol from recurrent hemorrhage.

This distinguishes them from epidermoids (congenital cholesteatomas), which follow CSF signal on T1/T2 but show restricted diffusion on DWI due to their packed keratin content.

Simple petrous apex effusions are T1 dark, T2 bright, and do not restrict.