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Imaging of temporal bone brain hernias: atypical appearances on magnetic resonance imaging.

Brain tissue with or without its dural covering may protrude into the temporal bone through a defect in the tegmen tympani. Infection or granulation tissue, with or without cholesteatoma may make diagnosis difficult. While computed tomography (CT) may demonstrate a bony defect, it is difficult to distinguish between herniated brain, cholesteatoma or granulation tissue. Magnetic resonance imaging (MRI) clearly demonstrates healthy herniated brain tissue, but ischaemic or necrotic elements may mimic other lesions. We present a series of five patients with brain hernia, highlighting the spectrum of preoperative radiological appearances, and some of the difficulties encountered in interpreting these images. MRI demonstrated herniation of healthy brain in two patients but in three, showed irregular soft tissue with signal characteristics consistent with cholesteatoma. Surgery in these three patients demonstrated necrotic brain with coexisting cholesteatoma in two. Brain hernias are often necrotic and may have morphological and signal characteristics similar to cholesteatoma with which they may coexist.