Bloch equations are used to calculate the nuclear magnetization M = (M, M, M) as a function of time when relaxation times T and T are present. Bloch equations are phenomenological equations that were introduced by Felix Bloch in 1946.
Where  is the cross-product, γ is the gyromagnetic ratio and B(t) = (B(t), B(t), B + B(t)) is the magnetic flux density experienced by the nuclei.
The z component of the magnetic flux density B is typically composed of two terms: one, B, is constant in time, the other one, B(t), is time dependent. It is present in magnetic resonance imaging and helps with the spatial decoding of the NMR signal.
The equation listed above in the section on T and T relaxation are those in the Bloch equations.