What does

Positron Emission Tomography (PET) uses the anhillation of electrons and positrons to show what cannot be seen in normal x-rays or MRI.  That discloses what is where and what its atomic mass is. 

When a positron meets up with an electron it shoots out a powerful gamma ray.  If we hit a target with enough of them, it will emit alpha particles or neutrons, which we detect with a mass spectrometer and scintillation spectrometer. 

That MS/SS is in turn hooked up to a computer that has imaging software.  We can "see" what is deep within nearly any structure by the particles we released and when they bounce back to the detector.  It can be a little traumatic to some tissues.

Newer technology exists, and is far less damaging than this.