The biggest and most important component in an MRI system is the magnet. The magnet in an MRI system is rated using a unit of measure known as a Telsa. Another unit of measure commonly used with magnets is the gauss (1 Tesla = 10,000 gauss). The magnets in use today in MRI are in the 0.5-Tesla to 3.0-Tesla range, or 5,000 to 30,000 gauss. Extremely powerful magnets -- up to 60 Tesla -- are used in research. Compared with the Earth's 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are.

The magnetic force exerted on an object increases exponentially as it nears the magnet. Imagine standing 15 feet (4.6 m) away from the magnet with a large pipe wrench in your hand. You might feel a slight pull. Take a couple of steps closer and that pull is much stronger. When you get to within 3 feet (1 meter) of the magnet, the wrench likely is pulled from your grasp. The more mass an object has, the more dangerous it can be. The force with which it is attracted to the magnet is much stronger. Smaller objects can usually be pulled free of the magnet by hand. Large ones may have to be pulled away with a winch, or the magnetic field may even have to be shut down.

Magentic field of MRI