Got some really good answers on this one. The most common by far was vitreous hemorrhage/Coat’s disease. This would certainly be in the differential, but I think the key is to look at a) how dense the globe it (ie = to the lens) and b) the lack of displaced retina (ie the V-shaped appearance given it’s fixation at the optic disc). Thus, this density must involve the vitreous cavity, and it fills it completely and uniformly.

This is actually a case of intraocular silicone injection for treatment of retinal detachment (confirmed by patient’s history). If you could measure the density of the vitreous it would be >100 HU, and thus can not be blood.

nice reference: http://radiographics.rsna.org/content/28/6/1741.full