Ruptured Dermoid Cyst

ruptured dermoid PD axial

This axial PD-weighted image shows multiple small foci of altered signal in the ventricles and subarachnoid spaces. The signal alterations are notable in that they all show high signal anteriorly and low signal posteriorly. If the AP direction were phase-encoding, then motion such as pulsation could account for the appearance. In this case, the AP direction is frequency-encoding, meaning that the appearance is due to chemical shift. The presence of chemical shift artefact is usually associated with macroscopic fat. In this case, a ruptured dermoid cyst is the cause. This T1 coronal, and this T1 coronal fat-sat post-Gd, show that the high T1 signal in the cyst disappears with fat saturation, confirming the presence of fat.

Dermoid cysts, along with epidermoid cysts, are ectodermal inclusion cysts. The interior of the cyst is lined with squamous epithelium and dermal appendages. It is these dermal appendages that characterise a dermoid as distinct from epidermoid, and which secrete the fat so typical of the lesion. Calcification is common. The most common sites are lumbosacral spinal canal, parasellar, frontobasal, and posterior fossa. Dermoids most commonly present with seizure or headache if unruptured. Cyst rupture leads to a chemical meningitis, which may be complicated by seizure, vasospasm, infarction, or death. Cysts are typically very hypodense (-20 to -40HU) on CT, and hyperintense on T1-weighted MRI. T2 signal is variable. Fat-saturated sequences will confirm the presence of fat. The differential diagnosis is intracranial lipoma, which typically has a very homogeneous signal unless calcified.

Reference:
Osborn, AG. Diagnostic Neuroradiology. Mosby 1994

Credit: Dr Laughlin Dawes