Brain tumors tend to concentrate certain ions (charged atoms or
molecules). When these ions are gamma-ray emitters, it is possible to
take advantage of the penetrating power of their gamma rays to locate
the tumor with a scanning device located outside the skull.
Arsenic-74 and copper-64 are isotopes emitting _positrons_,[8] which
have one peculiar property. Immediately after a positron is emitted from
a nucleus it decays, producing two gamma rays that travel in exactly
opposite directions. The scanning device has two detectors called
scintillation counters, one mounted on each side of the patient’s head.
The electrical circuitry in the scanner is such that only those gamma
rays are counted that impinge simultaneously on both counters. This
procedure eliminates most of the “noise”, or scattered and background
radiation.