Lesions Containing Fat

Case A: CT demonstrates a fat density lesion in the right quadrigeminal plate cistern. The lesion measured –96 HU with a standard deviation of 9 HU. The lesion is markedly hyperintense on T1-weighted imaging. The lesion also is hyperintense on T2-weighted imaging, and chemical shift artifact is noted.

Case B: CT demonstrates a fat density lesion in the left quadrigeminal plate cistern. The lesion measured –106 HU with a standard deviation of 4 HU. The lesion is markedly hyperintense on T1-weighted imaging and hyperintense on T2-weighted imaging, with chemical shift artifact. The lesion does not enhance and is hypointense on the fat-saturated T1 postcontrast sequence. No restricted diffusion is noted within the lesion.

Case C: A markedly T1 hyperintense lesion abuts the posterior aspect of the cerebellar vermis. The lesion demonstrates heterogeneous signal on T2-weighted imaging, with areas of both hypointensity and hyperintensity and subtle chemical shift artifact. Susceptibility artifact is associated with the lesion on the gradient echo sequence. On follow-up imaging for a severe headache 9 years later, multiple foci of T1 hyperintense signal are now seen in the subarachnoid space adjacent to the posterior right vermis. The T2 signal within these foci is now more homogeneous and isointense to the adjacent parenchyma. Subtle FLAIR hyperintense signal is noted within these foci.

Case D: A fat density lesion abuts the inferior vermis. This lesion had a density of approximately –35 HU with a standard deviation of 11 HU. On MRI, this lesion is heterogeneously T1 hyperintense and is predominantly hyperintense on the T2 sequence, with linear areas of hypointensity. A head CT scan obtained 8 years later for an episode of severe pain demonstrates fat density droplets in the subarachnoid space and within the ventricles anteriorly. The patient underwent surgery for resection, and a postoperative MRI study demonstrates the residual T1 hyperintense fat droplets throughout the subarachnoid space and ventricles. The T2 sequence demonstrates the chemical shift artifact associated with these foci of fat.