AIDS dementia. Coloured Positron Emission Tomo- graphy (PET) scan of an axial section through the brain of an AIDS patient, suffering dementia. At top is the affected brain showing cerebral (red) atrophy, a shrinking of the brain associated with loss of function and symptoms of dementia. At bottom is a healthy brain for comparison. The on- set of dementia in AIDS (Acquired Immunodeficiency Syndrome) is often subtle, with short-term memory loss. The HIV virus causing AIDS can be found in spinal fluid. PET scanning uses a radioactive tracer (for example, oxygen-15) injected into the blood, and detected by a PET scanner.