Radiation dose is affected by all the emanations of a radionuclide,not just the desirable ones,thus constricting the choice of nuclide further.There can be no alpha radiation used in diagnosis; the use of materials with primary beta radiation should be avoided because the beta radiation confers a radiation dose without adding to the information being gained.

Detectors must be made from materials that exhibit some detectable change when ionizing radiation is absorbed and that are of a high enough atomic number and density to make possible stopping large percentages of those gamma rays emanating (high sensitivity).In addition,because the primary gamma rays are not the only rays emanating from the source — a human body and therefore a distributed source accompanied by an absorber — there must be energy discrimination in the instrument to prevent the formation of an image of the scattered radiation (cheap medical equipment).To achieve pulse size proportional to energy,and therefore to achieve identification of the energy and source of the energy,the detector must be a proportional detector.This means that Geiger-Muller detection,operating in an all-or-none fashion,is not acceptable.

Gaseous detectors are not practical because their density is not great enough.Liquid detectors (in which any component is liquid) are not practical because the liquid can spill when the detector is positioned; this problem can be compensated for if absolutely necessary,but it is better to consider it from the outset.Another property of a good detector is its ability to detect large numbers of gamma rays per time unit.With detection capabilities to separate 100,000 counts per second or a dead time of 2 msec,the system is still only detecting perhaps 1,000 counts per square centimeter per second over a 10 × 10 cm area.The precision of the information is governed by Poisson statistics,so the imprecision in information collected Nuclear Medicine.