In the areas of biology and medicine, nuclear tracers are usually radiopharmaceutical products whose molecules contain a radioactive element - a marker. The emissions of radiations by such atoms allow to follow the path or the metabolism of these tracers in the body. Tracers can be reduced to a marker itself : this is the case of krypton 81m or xenon 133 , noble gases used for lung scans. Markers can be also attached to objects other than molecules, such as albumin microspheres.