Go further · Chapter 2 · 2a · Real-world case
How does an MRI scanner work?
The short answer (tap to open)
An MRI scanner reads the spin of hydrogen nuclei in your body. A strong magnet gives each nucleus two energy levels. Radio waves of exactly the right frequency tip the spins, and as they settle back they send out faint radio signals. Different tissues settle at different speeds, and that difference becomes the image.
What does a magnet do to a proton?
Your body is mostly water, and every water molecule holds two hydrogen nuclei: single protons. Each proton has spin, which makes it act like a tiny magnet. Outside a scanner those tiny magnets point every which way.
Inside the scanner's huge magnet, each proton's spin can line up with the field or against it. Those are two energy levels, just like the rungs in chapter 1, with a small gap between them. A tiny majority, a few in every million, sits on the lower rung. With trillions of protons in every drop of water, that's plenty.
Why radio waves?
The gap between the two levels matches the energy of radio waves at one exact frequency, about 64 million cycles a second in a common 1.5-tesla scanner. Send a pulse at that frequency and the spins absorb it and tip over. Switch the pulse off and they settle back, sending out a faint radio signal of the same frequency. Coils around you pick it up.
Where does the picture come from?
Spins in different tissues settle back at different speeds. In fat they settle fast, in water-rich tissue slower, and tumours and inflammation change the timing. The scanner turns those timings into brightness. Extra coils make the field slightly stronger on one side than the other, so the frequency of each signal reveals where in the body it came from. That's why scanners clank: those coils switch on and off thousands of times.
Fat: the spins settle back fast, so fat shows up bright.
The picture builds from how far each tissue has settled. The timings are illustrative.
Each tissue, in words
- Fat: the spins settle back fast, so fat shows up bright.
- Muscle: the spins settle back more slowly, so muscle shows up grey.
- Fluid: the spins settle back slowest, so fluid shows up dark.
The picture builds from how far each tissue has settled. The timings are illustrative.
Why is it safe, and why so cold?
Radio waves and magnetic fields don't damage cells the way X-rays can, which is why doctors can use MRI for repeated scans. The magnet is a superconductor (the same physics as chapter 9's qubits), cooled by liquid helium to about 4 degrees above absolute zero.
Who made it?
Felix Bloch and Edward Purcell discovered nuclear magnetic resonance in 1946 and shared the 1952 Nobel Prize in Physics. Paul Lauterbur and Peter Mansfield, at the University of Nottingham, turned it into imaging in the 1970s and shared the 2003 Nobel Prize in Medicine. England's NHS alone ran 4.9 million MRI scans in 2024 to 2025.
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MRI is spin, made visible.
A magnet gives protons two levels, radio waves tip them, and the way they settle draws the picture.