Arsenic-74
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.