MRI Signal Sources
CORRECT: Hydrogen atoms (but "the hydrogen nucleus" would be more specific). MRI relies on the nuclear magnetic resonance of the hydrogen nucleus (a single proton). The electron does not contribute to the signal in MRI, though it can undergo a similar process as the nucleus, called electron spin resonance.
The spin of a nucleus is a kind of aggregate of the spins of its 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.
CORRECT: Water molecules (but "the hydrogen nucleus" would be more specific—MRI relies on the nuclear magnetic resonance of the hydrogen nucleus (a single proton)). Additionally, this answer is not complete, because 1H nuclei are present in other molecules too. The hydrogen nucleus is abundant human tissue in lipid (CH2) as well as water (H2O).
A sample of nuclei used in NMR include 19F, 31P, 13C, 23Na, 17O and 1H.
CORRECT. MRI relies on the nuclear magnetic resonance of the hydrogen nucleus (a single proton).
The hydrogen nucleus is used for MRI because it is abundant in human tissue in water (H2O) and lipid (CH2) molecules.
In MRI literature, 1H nuclei are sometimes referred to as "the protons" or "the spins" of a sample, at the exclusion of other types of nuclei which can also undergo NMR. The same convention is adopted in these pages.
Further reading on this topic:
Books: MRI From Picture to Proton p135