Equilibrium nuclear spin polarization (P) is governed by the Boltzmann distribution, and it can be shown that P ~DE/2kT or P ~ għB0/2 kT for spin ½ nuclei, where DE is the Zeeman levels’ energy difference, ħ is the Planck constant (divided by 2pi), B0 is the static magnetic field, k is the Boltzmann constant, T is the temperature in Kelvin, and g is the gyromagnetic ratio for the nuclear spin of a given isotope. Even when high (e.g. 1.5–21 T) magnetic fields are applied, P is generally 10^-6 to 10^-4 for most frequently employed spin ½ isotopes: 1H, 13C, 15N, 129Xe, etc. The NMR signal strength is directly proportional to P, and since P is generally very low, it results in relatively low sensitivity of all conventional NMR experiments including most notably NMR imaging (or MRI).