The NMR (Nuclear Magnetic Resonance) signal can be described classically by considering the motion of the net magnetisation (the vector sum of magnetic moments of individual nuclei). By considering individual isochromats – i.e. subsets of the spins that are behaving identically– we can visualise how the received signal will decay away due to T1, T2 and T2* relaxation. By additionally considering the effects of magnetic field gradients, we can determine the spatial location of the signal, producing images. All these effects can be described by the Bloch equations, which give complete classical description of the behaviour of magnetisation.