Functional magnetic resonance imaging (FMRI) is used to look at changes in brain activity based on where the blood is flowing in your brain. For example, if you are wiggling your fingers, the region in your brain that helps with motor control will have more blood flowing to it than when you are not wiggling your fingers. When you see pictures of fmri scans you’ll see spots that are lit up in different colors, showing where the changes to blood flow are occurring.
The fMRI scanner itself is essentially a very powerful magnet and radio wave pulses. Magnets have positive and negative charges that pull or push away other magnetic objects. Our bodies have positive and negative charges in each atom, making our bodies responsive to strong magnets.
The fMRI magnet aligns the positive and negative charges in your brain. Then radio waves knock the charges out of alignment, and a computer reads the signals that are produced from the charges getting knocked in and out of alignment.
If you are in the scanning room holding something magnetic, the object will be pulled toward the scanner due to how strong the fMRI magnet is. The dangerous part is that the magnetic field of the scanner changes throughout a test, so a little piece of metal can move around in any direction. This makes it very important to remove metal objects like belts, keys, and even certain types of shoes before you go into an fMRI test.