Proton Precession
In MRI we are imaging properties of hydrogen nuclei (protons) in tissue. Without getting into the characteristic of quantum mechanics, protons act like small magnets; they will tend to underscore up in the one and the same direction as any outermost magnetic field. Thus, the first stride in MRI is to set up a strapping magnetic field (called Bo) along the axis of the scanner. The tendency to align along the magnetic field is relatively weak, so arbitrary vibrations and collisions of protons within tissue will keep most of them from actually lining up with Bo. While magnetic resonance can petition to a large number of variance atoms (or even molecules), in clinical MRI we are looking at the magnetic moments of the hydrogen nuclei (protons), in the tissue. Hydrogen is used, once again, because it has a very high abundance in the body, among other characteristics.

Spins wobble (or process) about the axis of the BO field so as to describe a cone. This is called precession. Spinning protons are like dreidels spinning about their axis. Precession corresponds to the gyration of the rotating axis of a spinning body about an intersecting axis. The resonance frequency, called Larmor frequency (ω0) or precessional frequency, is proportional to the main magnetic field strength: ω0 = γ B0.

Left: Illustration of a proton precession in a magnetic field (along the green vertical axis). Note that the proton spins around its own axis (large arrow) as well as precession throughout the magnetic field.

Right: Simulation of many protons precession in a block of tissue, with Bo pointing up. Note that while each proton is aligned in a random direction, the bulk magnetization (green) points along Bo. (Because we are only looking at a small number of protons, the bulk magnetization is not exactly along Bo, and you can see it recessing slightly.) To reemphasize, within a block of tissue, the millions of protons will be precession in a disorganized fashion and point in different directions - in other words, they are not synchronized. The MRI scanner detects the sum total of these protons: the different directions of wobbling will cancel out, and all that will be left is a slight magnetization pointing along Bo with no wobbling.