Decay chain or decay series is a series of decays that certain radioisotopes go through before reaching a stable form. For example, the production of technetium-99m (daughter nuclide with a half-life of 6 hours) from molybdenum-99 (parent nuclide with a half-life of 66 hours) in a generator is part of a decay chain.

See also Decay and Generator.

The decay constant is inversely proportional to the radioactive half-life time. It is the fraction of a number of atoms of a radionuclide that disintegrates in a unit of time.

See also Half-Life, Decay.

Radiotoxicity refers to radioactive materials that are toxic to living cells or tissues. Radiotoxicity results from the type of radiation, the radioactive half-life of the used radionuclide, the biological half-life in the tissue and the radioactivity absorbed in the organ. Radiotoxic substances can be collected following ingestion, inhalation and absorption.

A gas ventilation scintigraphy is a diagnostic imaging test of lung ventilation with radioactive noble gases during breathing maneuvers, e.g. with krypton (81mKr) or xenon (133Xe).
The radioactive gas is administered by a mask and requires a special delivery and trapping system (gas trap). The radioactivity in the lungs is measured with a gamma camera and is subsequently evaluated.
The use of krypton or xenon gases involves problems like the relatively short half-lives (about 15-30 seconds) and relatively high costs of xenon and krypton. The short half-life requires that the scan is performed directly after administration of the gas. In addition, the gaseous radiopharmaceutical is expelled from the body almost quantitatively within a few minutes of completing the study.
A ventilation scintigraphy combined with a pulmonary perfusion scintigraphy is highly sensitive for the detection of pulmonary embolism.
Radioactive noble gases are widely used as a ventilation agent to diagnose pulmonary embolism. However, 81mKr and 133Xe are rare and expensive, which limits their continuous availability. Tc99m-Technegas can be an alternative ventilation agent with the advantage of being less expensive and available daily.

See also Inhalation Scintigraphy.

Lifetime is the mean life of a particle or in case of a radioactive nucleus equal to its decay time.

See also Half-Life, Decay.