The MRI signal essentially comes from the protons in water and fat molecules. Protons can also be bound to macromolecules (proteins, membranes…) but their signal is less significant, as their T2 is very short, giving a very wide frequency band.

molecule_en

Spectrum of free (water) and bound (macromolecule) hydrogen nuclei

The hydrogen nuclei bound to macromolecules have a very short relaxation time T2 corresponding to a wide frequency band. The decrease in their transverse magnetization is too fast to be recorded, so they do not participate in the signal.