Magnets and Hydrogen

MRI sees what your eyes never can, using invisible radio waves to map the inside of a living body. It starts with water. You are roughly 60% water, and every water molecule carries two hydrogen atoms.

Normally, hydrogen proton spins are split between lower-energy and higher-energy orientations in ways that almost cancel. But an MRI's massive magnet - typically 1.5 to 3 Tesla, about 30,000 times stronger than Earth's field - creates a tiny excess pointing with the field. That small net magnetization, not most protons marching in one direction, is what the scanner manipulates and detects.