Answer:
Nuclear medicine or PET scanning (Positron Emission Tomography)
Explanation:
Not super sure what your question is looking for but I think it can be either. PET is a technology used in the nuclear medicine field, and nuclear medicine is a broad field of using the technology the question described.
Melting, boiling, and freezing are state changes!
Hope this helps! Any questions please just ask! Thank you so much!