PET scanning
In PET (positron emission tomography), a tracer containing a β⁺ emitter, often fluorine-18 in a glucose-like molecule, is absorbed by active tissue. Each emitted positron travels a millimetre or so and annihilates with an electron, producing two γ photons of 511 keV each, travelling in opposite directions (conserving momentum). A ring of detectors records pairs arriving at the same time; the line joining them and the tiny difference in arrival times locate the event. Many events build a map of tracer concentration.
≈ 35 min