This invention provides a method for determining whether a subject is afflicted with Alzheimer's disease by comparing the metabolic activity of the subject's hippocampal entorhinal cortex with that of a second region of the subject's brain. This invention further provides a method for determining whether a subject is afflicted with Alzheimer's disease by comparing the metabolic activity of the subject's hippocampal entorhinal cortex determined at a first and second time point. Finally, this invention provides a method for determining the amount of blood in a volume of cerebral tissue in vivo, wherein the volume of tissue is 1 mm.sup.3 or less.