Half-life, 99mTc

De-excitation of 99mTc has specific features. This nuclide decays with a half-life of 6 hours, but its half-life varies slightly according to environmental conditions [30] or chemical states [31,32], Moreover, the emission probabilities of characteristic X-rays just after the isomeric transition 99mTc — "Tc are influenced by environmental factors [33] which result in a change of the K/VKoc X-ray intensity ratio [34],... 

The half-life of 99mTc is 6 hours. Calculate the percentage of 99mTc remaining after 4 hours. 

I (radioactive half-life 13 hours) has replaced 131I for diagnostic purposes however, for in vivo imaging meta-stable technetium-99 (99mTc) is often preferred, because of its lower radiation dose, availability, and cost. 

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