In magnetic resonance imaging (MRI) the spin angular momentum of the proton is used. The most available source for protons in the human body is represented by hydrogen atoms in water. A strong magnetic field B applied to water causes the appearance of two different energy levels for spin angular momentum, + γ γ ℏ ℏ B / 2 and − − γ γ ℏ ℏ B / 2 , using E = − − μ μ ⋅ ⋅ B .