How does MR work? 
The water in your body is made up of trillions of atoms that have a magnetic moment, in other words they can act as small magnets.  When placed in a magnetic field, these atoms line up with the field, much like a compass points to the North Pole.  Radio waves of a specific frequency tip these tiny magnets away from the magnetic field.  As they tip, they gain energy.  When the radio waves are turned off, the atoms try to realign with the magnetic field, releasing the energy they gained as very weak radio signals.    
A special antenna picks up these signals from your brain and sends them to a computer, which reconstructs an image of your brain.

MRI in Alzheimer’s Disease
MRI is beneficial in ruling out other causeMRI is beneficial in ruling out other causes of dementia, such as tumors or strokes. It also may help to show the structural and functional changes in the brain that are associated with Alzheimer's disease.  One example of this would be a longitudinal study across a certain length of time which can show the amount of brain tissue that has been lost since the individual was first scanned with MRI.  By measuring one area of the brain and then again after a year, we can understand the rate of change to the brain in Alzheimer’s Disease, which can be used to show a treatment’s effectiveness (or lack of) on brain changes.

How does PET work? 
Scientists (called technologists) at the PET center make a special form of glucose that is labeled with radioactivity.  This is called fluorodeoxyglucose, or FDG.  The FDG is injected through a small needle into a vein in a person’s arm, and the PET scanner picks up this labeled.  Pictures are taken every few seconds during the time that a person is in the scanner, which shows how the brain is processing during that time period.

PET in AD
The PET scans being used in ADNI measure the brain's rate of glucose metabolism using the tracer [18F]Fluorodeoxyglucose.  In Alzheimer's disease, characteristic brain regions show decreased glucose metabolism, specifically regions in the temporal and parietal lobes.  We hope that following these changes over time in people with AD, people with normal aging changes, and those with mild cognitive impairment will enable us to track the disease progression as a potential biomarker for clinical trials.  PET could also become a potential biomarker used to predict who might later develop Alzheimer’s disease.