Our bodies are made up mostly of water (chemically known as H2O), which means a large number of the atoms inside your body are hydrogen (H) atoms. In the nucleus of every hydrogen atom is a positively charged proton that spins around an axis. This spinning generates its own magnetic field, giving the proton its own north and south poles.
Under normal circumstances, these hydrogen protons spin on randomly oriented axes. When the protons are placed in a strong magnetic field, however, they align with the powerful magnetic field and spin at a specific resonance frequency that depends on the strength of the magnetic field. Roughly half of the protons align with the direction of the magnetic field and the other half orients in the opposite direction of the magnetic field. Few extra protons, though, are aligned in opposition to the magnetic field and lack a match with the aligned protons. These are exactly the protons that are targeted by the MRI scanning instrument inside the cells of our body.
It takes more than a magnetic field to make the MRI imaging possible. The other key component is a radio frequency (RF) wave. When the RF coil sends out a wave at the same resonance frequency of the protons, most of the protons stay in the same position. The unmatched protons, however, which are aligned in opposition to the magnetic field, respond to the wave and flip on their axis. These turning protons reverse their direction momentarily as they absorb energy from the radio wave. When the RF wave stops, the protons release that energy and return to their original orientation. As they do, they emit their own little wave.
The resulting radio wave acts like a flashlight in a dark forest. When you shine the light outward, the little reflections that bounce back to you tell you where things are. The detectors in the MRI machine translate those little reflected waves into an electrical signal, which is digitized to create an image. The varying signal strengths gets translated into varying shades of grey, which radiologists recognize as different types of bone and tissue.
For a more complete explanation of how MRI works, we invite you to read MRI: A Guided Tour and watch a video about How MRI Machines Work.