MRI
Definition
Magnetic resonance imaging — the hospital scanner that takes detailed pictures of soft tissue inside your body using strong magnets and radio waves, with no X-rays involved.
Technical definition
An imaging technique exploiting nuclear magnetic resonance: a strong magnetic field aligns hydrogen nuclei spins, radio-frequency pulses tip them, and the emitted signals are spatially encoded with gradient fields and reconstructed into images. Contrast reflects water content and tissue environment.
Example
A 1.5-tesla MRI magnet is about 30,000 times stronger than Earth's magnetic field — strong enough to yank a wrench across the room, which is why metal objects are strictly banned near the scanner.
MRI uses no ionizing radiation, unlike X-rays and CT scans — the 'radiation' in its original name (NMR) was dropped partly to avoid that confusion. The radio waves involved are low-energy and non-ionizing; the main hazards are the powerful magnet and the loud knocking sounds, not radiation exposure.