The radioactive signatures allow the PET scan to detect the presence of the inhaled gas in the brain or organs.  MRI stands for Magnetic Resonance Imaging.  It is a non-invasive brain scan that shows the structure of the brain, like an x-ray in high definition.  MRI machines cost millions of dollars.  Finally, the fMRI scan, or functional MRI, allows researchers to detect changes to the blood flow in the brain, allowing a very accurate view of the brain and neural activation.