7.7 : Atomic Nuclei: Magnetic Resonance

The number of nuclear spins aligned in the lower energy state is slightly greater than those in the higher energy state. In the presence of an external magnetic field, as the spins precess at the Larmor frequency, the excess population results in a net magnetization oriented along the z axis. When a pulse or a short burst of radio waves at the Larmor frequency is applied along the x axis, the coupling of frequencies causes resonance and flips the nuclear spins of the excess population from the lower to the higher energy state, This causes the net magnetization to shift from the z axis towards the y axis. On withdrawing the applied radiation, as the nuclear spins lose the absorbed energy and return to the spin-up state, the net magnetization vector returns to its orientation along the z axis, and equilibrium is established. All NMR-active nuclei exhibit nuclear magnetic resonance, which forms the basis of NMR spectroscopy and imaging.