Metastable state

A metastable state of an isomer is defined as an excited state that exists for greater than 10 seconds. In chemical notation, metastable species are identified by the letter 'm'.

Typically, excited nuclei will instantaneously decay to a more stable energy state (within 10 seconds), emitting radiation in the process.

However, some excited nuclei can remain at a high energy state for a measurable period of time. This has important ramifications in the selection of an appropriate radiopharmaceutical for nuclear medicine imaging, where radionuclides with a sufficiently long enough half-life to allow tracer uptake is required.

An example of a common metastable tracer used in SPECT imaging is Tc-99m. It has a half-life of 6 hours, after which it decays to its daughter product Tc-99, emitting 2 Gamma rays in the process.