1H Signal in MRI
There are a few different elements from which we can produce a nuclear magnetic resonance (NMR) signal, and we choose hydrogen simply because there's loads of it in your body. (60-80% of your body tissues is water: H2O !) This means more signal and better images.
Nucleons possess a property which we call spin (it is described by the same mathematics used to describe a spinning object). The spin of nuclei is a kind* of aggregate of the spins of the nucleons. Nuclei which have unpaired protons (that is, simply an odd number of protons in any orbital, leaving one left over without a notional partner) also have spin. This means they have a net magnetic field. In nuclei with all-paired protons, the magnetisations resulting from the spins of the nucleons cancel out. In actual fact there is a net magnetisation if a nucleus has an odd mass number (protons plus neutrons), or if there is an odd number of protons and an odd number of neutrons. Other nuclei used in NMR include 19F, 31P, 13C, 23Na and 17O. A hydrogen nucleus is just a single proton, which is why we sometimes refer to "the protons" of a sample. In MRI, we really mean just the hydrogen protons. We sometimes also refer to those protons as "the spins" of a sample.
*The way spins of nucleons add to form the spin of a nucleus is not obvious. This is to do with the quantum numbers of the nucleons. In our discussion here, we are hinting at quantum mechanics without really getting into it. Spin is an inherent property of the particle, and not actual rotation of a particle. For example, at absolute zero temperature, when all motion ceases, a particle still has "spin".