99mTc
technetium 99m has a half-life of 6 hours. It is formed when another radionuclide, molybdenum-99 (99Mo, popularly called "Molly") emits radioactivity. Molly has a half-life of 66 hours and is easily bound to a type of cartridge. As Molly gradually disintegrates, 99mTc is formed. Since 99mTc does not bind as well to the cartridge, it can be washed out as needed with sterile salt water. The cartridge is delivered to the customers in a lead container which protects against radiation exposure. This is called a technetium generator, also known as "technetium cow", as it can be "milked" repeatedly as needed. 99mTc binds easily to various carrier molecules, who transport it to the desired site of the body to be imaged.
GE Healthcare Glossary