MRI Physics — Seeing Inside with Magnets
MRI sees soft tissue better than any other scan — using no ionizing radiation at all. It lines up the quantum spins of hydrogen nuclei with a giant magnet, tickles them with radio waves, and listens to their reply.
Spin alignment in a giant magnet
Hydrogen nuclei (protons) are tiny quantum magnets. Inside the scanner's 1.5-3 tesla field — tens of thousands of times Earth's field — slightly more protons align with the field than against it, creating a net magnetization to work with.
Resonance: the radio-frequency kick
A pulse of radio waves at exactly the protons' Larmor precession frequency tips their magnetization sideways — resonance, like pushing a swing at its natural rhythm. Different tissues precess identically but relax back at different rates, which is what creates image contrast.
Relaxation and image contrast
After the pulse, protons relax back via two processes: T1 (realigning with the field) and T2 (losing sync with each other). Fat, water, muscle and tumors have distinct T1/T2 fingerprints, so the scanner can paint them in different shades.
Gradients locate the signal
Extra gradient coils make the magnetic field vary slightly across the body, so protons in each slice precess at a slightly different frequency. The scanner decodes position from frequency — a Fourier transform turns the raw radio signal into the image.
Try this yourself
- Notice MRI safety signs banning metal: the magnet is always on, and ferromagnetic objects become projectiles — a vivid force-on-magnetic-materials demo.
- Compare an MRI image with an X-ray of the same region: MRI wins on soft tissue, X-ray on bone — different physics, different strengths.
- Hum near a running scanner's knocking sound: those are the gradient coils switching kiloamp currents in milliseconds.