The scanners used in nuclear medicine are, much like Geiger counters, radiation detectors; they emit no radiation.  Scanners record the photons being emitted from the patient, who becomes the source of radiation following the administration of the isotope.  The emissions can be quantified, localized, and converted into images.

Gamma cameras v PET scanners
Scanners can be grouped into two large, but quite distinct, categories:  gamma cameras and PET scanners.  They are both radiation (photon) detectors that convert the emissions into images. 

Gamma cameras count photons emitted, one at a time.  The most commonly used isotope in gamma cameras is technetium-99 metastable or (Tc-99m).  
PET scanners are used exclusively for studies performed with positron emitting isotopes.  (A positron is a beta particle with a positive charge).  After emission from a nucleus, it will travel a short distance and then undergo what is known as “annihilation” with an electron.  In the annihilation reaction, the mass of the positron and of the electron are converted completely into energy in the form of two photons traveling in opposite directions.  The scanner needs to detect both photons simultaneously for it to be counted.  The most commonly used isotope in PET scanners is fluorine-18 (F-18).