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A metastable state of an isomer is defined as an excited state that exists for greater than 10-6 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 10-15 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.