text: A useful novel radiolabelled compound is one that is suitable either for medical imaging of certain body parts such as brain or tumors (injecting low doses of radioactivity) or for treating tumors (requiring injection of high doses of radioactivity). In both cases, the compound needs to accumulate in the target organ and any surplus compound present needs to clear the body rapidly. In medical diagnostic imaging, this then produces a clear diagnostic image (high image contrast), and in radiotherapy leads to an attack of the target (e.g. tumor) while minimizing side effects to non-target organs. Additional factors need to be evaluated in the development of a new diagnostic or therapeutic compound, including safety for humans. From an efficacy point of view, the biodistribution is an important aspect which can be measured by dissection or by imaging.