What is a nuclear imaging of the heart?
This probably refers to an MRI scan ("Magnetic Resonance Imaging scan", commonly used in hospitals) of the heart. It works on the same principle as an NMR spectroscopy machine used in Chemistry.
"NMR" stands for "Nuclear Magnetic Resonance", and an MRI scanner should really be called an "NMRI scanner" ("Nuclear Magnetic Resonance Imaging scanner"). However, hospitals thought that it would be best to drop the word "nuclear" from the name, since people might think it was somehow related to nuclear bombs,power plants, our radioactivity. It's not related to any of these, and is perfectly safe.