MRI stands for “magnetic resonance imaging” and MRI machines use magnets to produce images of tissue. How MRI machines do this is rather amazing.
Humans, like all things, are made up of atoms and atoms have magnetic properties. Protons in the nucleus of atoms are like tiny bar magnets – they have a north and south magnetic pole. Different types of atoms have varying magnetic properties due to having varying numbers of protons.
Unless we are in range of a powerful magnetic field, the magnetic alignment of the protons in our bodies is random – they point in all directions. When our bodies are placed within a strong magnetic field – like an MRI machine – the magnetic axes all line up just like how a compass needle points north. The two images below illustrate this.
“The nuclear magnetic fields have their lowest energy if they point in the same direction as the outside magnet, and they’ll have their highest energy if they are aligned in the exact opposite of the magnet’s direction.” Source. Once all the atomic magnetic fields are lined up, the MRI changes the magnetic direction so that atomic magnetic fields flip 180 degrees. The amount of energy to flip the magnetic fields differs by element because each element has different magnetic properties. Thus, by precisely altering the strength of the magnetic fields on each side of the body, the MRI machine can make different elements exist in varying magnetic directions.
The MRI machine transmits radio waves through the body being examined while it is altering the magnetic field directions of the atoms. When the atoms of various types are in different magnetic directions the radio wave frequencies are deflected differently. Receivers in the MRI machine record which radio waves are deflected at various levels of magnetic power and thus can determine the concentration of various types of atoms in the body. MRI machines focus in on the concentration of hydrogen atoms. Different body tissues have different concentrations of hydrogen atoms. Using a ton of computing power the MRI machine creates images based on the concentration of atoms detected and corresponding tissues with those hydrogen concentrations. Wow. Crazy. Amazing.