Note: This case has been tagged as "legacy" as it no longer meets image preparation and/or other case publication guidelines.
From the case:
Tectal plate lipoma
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A focal region of fat is demonstrated in the quadrigeminal (tectal plate) cistern consistent with a lipoma.
Case Discussion
Lipomas are easily identified on CT but may also be mistaken for CSF or air depending on the windowing values. They typically have Hounsfield values between -20 to -100 HU (depending on size / partial voluming). CSF would have values close to 0 HU while air would be below -800 HU.