Conventional nuelear medieine proeedures study the dis- tribution of radiolabelled eompounds (radiopharmaeeutieals) in the body under physiologieal as well as under pathologieal eonditions. Beeause of their ability to visua- lise and to quantify the distribution of radiopharmaeeuti- eals within the body by means of external deteetors, nuelear medieine teehniques are basieally non invasive and funetion oriented. The spatial variation of the traeer distribution in the field of view, or the ehange in distribution during a time interval are interpreted as representing speeifie phy- siologie or...