(eng): When a patient is examined by positron emission tomography, radiotracer dose amount has to be determined. However, the rules used nowadays do not correspond with practical experience. Slim patients are given unnecessary amount of radiotracer and obese patients would need more activity to produce images of sufficient quality. We have built a model of a particular PET scanner and approximated human trunk, which is our region of interest, by a cylindrical model with segments of liver, outer adipose tissue and the rest. We have performed Monte Carlo simulations of PET imaging using the GATE simulation package. Under reasonably simplifying assumptions and for special parameters, we have developed curves, which recommend amount of injected activity based on body parameters to give PET images of constant quality. The dependence qualitatively differs from the rules used in clinical practice nowadays and the results indicate potential for improvement