A semi-automatic system for determining volumes of interest (VOI) from positron emission tomography (PET) scans of brain is de- scribed. The VOIs surface extraction is based on user selectable thresh- old and three-dimensional target flood-fill. Contrast to anatomical vol- ume detection approaches, volumes are determined from functional PET images and the obtained objects are checked against anatomical images. The developed VOI program was evaluated with brain FDOPA-PET studies where the striatum was the object. The results were comparable to entirely manual method and the target extraction time is reduced to about one third of manual method.