PET Scanning

PET, or Positron Emission Tomography, is a technique using γ-radiation and positrons. The radiopharmaceuticals used emit positrons - β+.

For example -  
  Fluorine-18 (18/9 F) decays into 0xygen-18 (18/8 O) along with β+, V and γ.

When the positron is emitted, it can collide with an electron - which is known as Positron-Electron Annihilation - and two γ-photons are emitted. These photons are emitted at 180 degrees to each other.

  By surrounding the patient with γ detectors, the γ-rays are detected, and the time difference between the two rays helps determine where they were emitted. This can eventually build up a 3D image of the area.

On this image you can see that there would be Gamma detectors surrounding the patient.