Series of twelve PET scans (positron emission tomography) of axial sections through a normal human brain, illustrating the gradual incorporation of a radioactive tracer in protein synthesis. The images were taken at intervals after the introduction of radioactive methionine (an amino-acid building block of proteins) into the circulation. The first image (top left) was taken 30 seconds after injecting the tracer & shows methionine (red areas on image) in the vascular spaces only (bottom); successive images show its presence developing in the cerebral cortex, extending to the inner white matter and illustrate the extent of active protein synthesis. Image at bottom is control; shows blood volume (l'image de volume sanguin).

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