How MRI Works: Atoms

The human body is made up of untold billions of atoms, the fundamental building blocks of all matter. The nucleus of an atom spins, or precesses, on an axis. You can think of the nucleus of an atom as a top spinning somewhere off its vertical axis.

A top that is spinning slightly off the vertical axis is precessing about the vertical axis.

A hydrogen atom precesses about a magnetic field. Imagine billions of nuclei all randomly spinning or precessing in every direction. There are many different types of atoms in the body, but for the purposes of MRI, we are only concerned with the hydrogen atom. It is an ideal atom for MRI because its nucleus has a single proton and a large magnetic moment. The large magnetic moment means that, when placed in a magnetic field, the hydrogen atom has a strong tendency to line up with the direction of the magnetic field. Inside the bore of the scanner, the magnetic field runs straight down the center of the tube in which we place the patient. This means that if a patient is lying on his or her back in the scanner, the hydrogen protons in his or her body will line up in the direction of either the feet or the head. The vast majority of these protons will cancel each other out-- that is, for each one lined up toward the feet, one toward the head will cancel it out. Only a couple of protons out of every million are not cancelled out. This doesn't sound like much, but the sheer number of hydrogen atoms in the body gives us what we need to create wonderful images.

All of the hydrogen protons will align with the magnetic field in one direction or the other. The vast majority cancel each other out, but, as shown here, in any sample there is one or two "extra" protons.

Inside the magnetic field, these billions of extra protons are lined up and ready to go. What now?