A 59-year-old man with history of multiple sclerosis and residual sensory and motor dysfunction presented with progressive lower-extremity weakness, ataxic gait, and intermittent urinary incontinence. Brain MRI demonstrated volume loss with disproportionate ventricular dilatation, but no evidence of infarction or abnormal enhancement. Radionuclide cisternography showed early and persistent ventricular reflux, poor progression of radiopharmaceutical over convexities, and delayed clearance in a pattern consistent with normal pressure hydrocephalus. Asymmetric activity in the right parietal region was also identified. Fused SPECT/CT, as well as fusion of the SPECT with a previous brain MRI, demonstrated a communicating arachnoidal cyst.