Positron emission tomography (PET) provides unbiased in vivo measurement of local tracer activity at very high sensitivity. This is a unique property unmatched by other imaging modalities. When PETwas introduced into medicine more than 25years ago, the first organ of major interest was the brain. Since then, PET has flourished as an extremely powerful and versatile tool in scientific brain studies, whereas its use as a diagnostic tool in clinical neurology remains limited. This is in contrast to its use in otherapplications, particularlyoncology, where its value in clinical diagnosis is more...