MRI stands for Magnetic Resonance Imaging. To produce the images, a person is placed inside a very powerful electromagnet. When the magnet is turned on, the magnetic field is strong enough to cause the protons in hydrogen atoms to align with the magnetic field.

Since the human body is mostly water, it has lots of hydrogen atoms. When the magnet is turned off, the hydrogen atoms return to their previous alignment, giving off energy in the form of radio waves.

That radio signal is picked up by sensors. Different tissues have different amounts of water, so the signal varies from one tissue to another. That makes the MRI very useful in seeing different types of tissue, as well as abnormalities such as tumors.