Larmina

Meet Larmina, a spinning proton who wobbles around a magnetic field at a beat set by its strength.

A proton spins and acts like a tiny bar magnet. Put it in a magnetic field and the field tries to twist it into line. Because the proton is spinning, it does not simply flip. Instead its spin axis sweeps around the field direction in a cone, the way a tilted spinning top wobbles around the vertical.

The wobble frequency is f = γB/(2π), where γ is the gyromagnetic ratio. For a proton γ/(2π) ≈ 42.58 MHz/T. In a 1.5 T hospital scanner, f ≈ 64 MHz, and in a 3 T scanner it is about 128 MHz.

A stronger field gives a faster wobble. Other nuclei have different γ values, so each wobbles at its own frequency.

MRI works by sending radio waves at exactly this frequency. The protons in the water of your body absorb them, then send signals back that are used to build the image.