A strong magnet helps to make high quality pictures of the brain.  A strong magnetic field, combined with radio frequency waves (picture radio waves as they are detected by your FM band radio), create detectable "echoes" from the water molecules within brain tissue.  High resolution images are then made directly from these water "echoes."  This technique is called Magnetic Resonance Imaging (MRI) and over the past decade MRI has become an invaluable tool for medical diagnosis and associated research.

Next, think of the brain as a collection of nerve cells, called neurons.  These neurons do all of the work we associate with the brain. For example, seeing, feeling, and thinking all depend on these neurons.  To work properly, these neurons need a constant fuel and oxygen supply.  Blood is used to move fuel and oxygen to all of the brain cells.

When the brain is working on a task, a set of neurons that is associated with a particular task, work harder than the rest of the brain cells.  Functional MRI (fMRI) can identify and characterize the set of neurons because the blood around them is altered in two ways.  First, the amount of blood in the area increases.  Second, the magnetic properties of the surrounding blood is altered as fuel and oxygen move from the blood into the cells.

For example, fMRI techniques are often used to characterize memory activities.  If someone is told a phone number to dial, a specific area of the brain begins working harder to hole it in memory until the dialing task is complete.  Specific changes occur in the blood within this area, and the scientist uses fMRI to image the location and extent of the changes.

Adapted from the work of J. Pekar and  P. Barker (1999)