Think of it like an x ray for rock. We expose a radioactive capsule and detect the reflected radiation. Since radiation can travel through solid objects we can use it to determine the relative density of rocks in a certain area which can help identify different minerals. Rocks are much denser than human flesh obviously. So the quick exposure we get from a medical x ray would be too short. Usually intervals of around 60 seconds are needed for the device to get a clear picture.

https://en.m.wikipedia.org/wiki/Nuclear_densitometry