MRI is a method that produces very clear pictures of your brain, without the use of X-rays or other invasive methods.  MRI is an advanced medical technique that uses a large magnet, radio waves, antenna, and a computer to produce these images.

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 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.