text: Let M(t) = (M(t), M(t), M(t)) be the nuclear magnetization. Then the Bloch equations read:
  where γ is the gyromagnetic ratio and B(t) = (B(t), B(t), B + ΔB(t)) is the magnetic field experienced by the nuclei.
  The z component of the magnetic field B is sometimes composed of two terms:
  *one, B, is constant in time,
  *the other one, ΔB(t), may be time dependent. It is present in magnetic resonance imaging and helps with the spatial decoding of the NMR signal.
  M(t) &times; B(t) is the cross product of these two vectors.
  M is the steady state nuclear magnetization (that is, for example, when t → ∞); it is in the z direction.