A universal method for determining a region of surface source image obtained with an imaging plate (IP) was developed in order to quantify radioactivity contained in the source by means of an IP system. A source image was regarded as a set of images of an infinitesimal point source. The image of a 32P point source was characterized by using a point spread function (PSF) and a full width at half maximum (FWHM). The PSFs were fitted best by composite functions of Gausssian and Lorentzian. The FWHM of the infinitesimal point source image was 0.42 mm. The image region of any surface source was defined as the inside of the boundary determined by the radius ( Rin) of the infinitesimal point source image from the source's edge. The sources should be arranged at intervals of more than 2 Rin to avoid interference. The radius Rin determined by using PSF (I) and the BG count for 11 min was 5.4 mm.