Positron emission tomography (PET) scan of the brain of a person suffering from sub-cortical dementia, a loss of functioning brain cells. PET scans reveal details of tissue function, as opposed to the structural information given by CT (computed tomography) and MRI (magnetic resonance) images. PET images are made by mapping the radioactivity from tracers injected into the bloodstream. The type of tracer chosen depends on the abnormality & which function is being studied. Radioactive carbon dioxide, for example, has been used to measure how oxygen supply (blood flow) and demand (tissue consumption) are reduced in cases of dementia.