Using Ionizing Radiation to Determine Trace Element Content in Living Human Subjects

The use of ionizing radiation for medical procedures is very familiar, both for diagnosis, such as in radiography or nuclear medicine, or for therapy, such as external beam radiotherapy or brachytherapy. There are also many instances in which samples are analyzed for their elemental content in order to help diagnose or monitor a medical condition or environmental exposure. The use of ionizing radiation to measure the elemental content within the human body directly and non invasively has been developed and is used in a number of laboratories around the world. The in vivo analysis of trace and minor elements in this way has been termed Occupational Nuclear Medicine. There are two general considerations that apply to Occupational Nuclear Medicine procedures. One is that the radiation dose to the subject must be carefully controlled and maintained within acceptable levels in all circumstances. The second is that both the probe that it used to excite a signal from the element of interest and the signal itself must be sufficiently penetrating for the signal to be detected with adequate sensitivity externally to the body. In practice, this means that photons or neutrons are used as the probe and that varieties of x-ray fluorescence and neutron activation are the techniques most commonly used. For in vivo measurements, the use of x-ray fluorescence is confined to elements with relatively high atomic number, because the K shell x-rays are higher in energy and therefore more likely to escape from the body and the K shell fluorescence yield also increases with increasing atomic number. This presentation will examine measurements of both lead (Pb) and strontium (Sr) in bone. Pb is toxic, whereas Sr may confer strength on bone, although it is also damaging at high concentrations. Cadmium (Cd) can be measured in liver and kidney by measuring -rays emitted promptly following neutron capture. Manganese (Mn) or aluminum (Al) can be measured in bone by counting induced radioactivity following neutron absorption. Although Mn is an essential element, it is the toxic effects of each of these elements, associated with chronic exposure and excessive accumulation in the body, that have motivated the development and application of these Occupational Nuclear Medicine techniques.