Magnetic Resonance Imaging (MRI)

In MRI, magnetic fields and radio wave pulses combine to get a unique, and medically beneficial, response from your body's hydrogen atoms. Take a peek in this tutorial.

MRI machines are awesome diagnostic tools, powered by strong superconducting magnets, that save countless lives with their ability to pinpoint tumors and other abnormalities. Tens of millions of scans are made worldwide every year. Let's see how they work.

You’re made up mostly of water, which means a large number of the atoms inside your body are hydrogen atoms. Hydrogen atoms are built in such a fashion that they react in a very useful way to an MRI's main magnetic field, as well as to the radio waves it emits.

In the nucleus of every hydrogen atom is a positively-charged proton that spins (or precesses) around an axis, much in the same way as a child’s top. This spinning generates its own tiny magnetic field, giving the proton its own north and south poles.

Under normal circumstances, these hydrogen protons spin about willy-nilly, on randomly oriented axes, as shown below. Let's take a look at this tutorial and find out what happens to these atoms inside an MRI.

In this tutorial, you will be able to observe what happens to the protons when the MRI's main Magnetic Field is on by clicking in the box to activate the field. When the field is on, you will also be able send a radio frequency pulse by clicking on the RF Pulse button, at which point you'll see RF waves (depicted in red) being emitted from the RF Coil. We'll walk you through this process.

When you turn on the magnetic field, running from north to south, the axes realign with the more powerful magnetic field. Half of them face in the direction of the field, the other half in the opposite direction. Well, not exactly half. A few more atoms (represented in blue) line up in the low-energy configuration (the proton's south pole facing toward the magnetic field's north) than in the opposite configuration, which requires a bit more energy. Those few “leftover” protons are the ones your MRI scanner will be using.

Every MRI patient has an RF coil placed near the part of the body being scanned. This coil is a radio transceiver that can communicate with your hydrogen atoms via radio frequency (RF) waves. The technologist uses that coil to send RF pulses at the body part under examination. The pulses are precisely timed to achieve the effect we’re about to describe, known as resonance.

The unmatched protons (in blue) absorb the pulse's energy, which causes them to flip on their axes – still in line with the magnetic field, but now in the opposite direction, in the high-energy configuration. Click on the RF pulse button (while the magnetic field is on, of course) to watch.

When the RF pulse stops, the protons release that absorbed energy, return to their previous alignments and, in so doing, emit a signal back to the coil. Click the RF Pulse button again for another look. The signal gets turned into an electric current, which the scanner digitizes. The lower the water content in an area, the fewer hydrogen protons there will be emitting signals back to the RF coils. The varying signal strengths gets translated into varying shades of grey, which radiologists recognize as different types of bone and tissue.

For a more complete explanation of how MRI works, we invite you to read MRI: A Guided Tour.

Last modified on 13 March 2015