To describe the CT and MR features of intraocular silicone oil which is used to treat complex retinal detachments in patients with acquired immune deficiency syndrome (AIDS). Seven male patients with AIDS were treated by pars plana vitrectomy and intraocular silicone oil injection for complex retinal detachments due to biopsy proven cytomegalovirus retinitis. Two patients had bilateral therapy. Silicone oil was hyperdense to muscle on CT with attenuation values of 106-139 HU (mean 115, SD 4.5). On MR, when compared with normal vitreous, intraocular silicone oil appeared hyperintense on T1-, proton density, and T2-weighted spin-echo sequences. A chemical shift artifact was seen on all MR images, being most marked on the T2-weighted images. The high attenuation value of silicone oil on CT and its hyperintensity on T1 weighted MR images my cause diagnostic confusion with haemorrhage. These entities can be distinguished at CT by directly measuring the attenuation number (silicone oil > 100 HU; blood < 90 HU), and at MR by the presence of a chemical shift artifact.

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