Protons and Spins: The Origin of the MR Signal

Abstract

The MR signal originates from hydrogen nuclei within water and fat within the patient’s tissue. Hydrogen nuclei, (protons) possess an intrinsic property known as Nuclear Spin. This combined with the proton’s positive charge gives rise to a small magnetic field, known as a magnetic moment. In the presence of the externally applied B_o field, the magnetic moments tend to align either with or against the externally applied magnetic field. An equilibrium state is quickly attained where there is a small excess aligned with the field, generating a net magnetisation. The greater the applied magnetic field strength, the greater is the size of the Net Magnetisation. The value of the Net Magnetisation determines the maximum available MR signal.