Cross-sectional data have shown an association between cerebral blood flow and established neuroimaging measures of Alzheimer's disease.[1] A new study has identified that pericyte cells around capillaries in the brain shrink under the influence of the amyloid beta protein, leading to narrowing of blood vessels and halving of blood flow.[1][6] This reduced blood flow is the first clinically detectable sign of Alzheimer's disease.[1] In the early stages, there is a mismatch between brain glucose and oxygen metabolism and blood flow, particularly in areas of learning and memory.[4][7][8] Drugs that remove amyloid beta have been less successful in the later stages, but new directions are targeting the early stage.[1] A blood test measuring p-tau217 protein predicts the onset of symptoms with accuracy 3-4 years in advance.[5]