<u>PET scan (positron emission energy) is an imaging technique that is used to help in the diagnosis of a disease by observing metabolic processes in the body.</u>
PET scans, although very expensive, are of great utility in the fields of Oncology, Neurology and Cardiology.
<u>Isotopes are atoms that share the same number of protons and electrons but differ in the number of neutrons. This gives the isotopes different properties than regular atoms.</u> In PET, isotopes are radiotracers used to discern the biologic pathway of any compound.
Marsupials are actually defined by their ability to hold the young in the pouches, where they can be well protected. Some examples are Kangaroos and Koalas.
Other answers are wrong: for example, gymnosperms are plants, not animals.
