Commercially available ⁸¹Rb and ⁴³K were evaluated objectively, that is, in terms of their effects on instrument imaging characteristics such as relative spatial resolution, plane source sensitivity and ability to demonstrate accurately various size spherical voids of radioactivity in the image plane. The rectilinear scanner was the instrument of choice because it is more readily adapted for counting the high energy photons of ⁸¹Rb and ⁴³K. The parameters evaluated for both radionuclides included various pulse height analyser energy windows and collimator types. The relative performance of the scanner for each radio-nuclide was used to determine the best energy window-collimator combination for imaging.