(I) Property of all nuclei related to the largest measurable component of the nuclear angular momentum. Non-zero values of nuclear angular momentum are quantized (fixed) as integral or half-integral multiples of (h/2p), where h is Planck's constant. The number of possible energy levels for a given nucleus in a fixed magnetic field is equal to 2I + 1. Similarly, an unpaired electron has a spin of 1/2 and two possible energy levels.

See Spin Quantum Number.

The NMR, MRI relevant nuclear spin is the rotational movement of a subatomic particle (proton or neutron) around its axis. Whether a nucleus has an overall spin, depends on its amount of protons and neutrons. Nuclei with an identical number of protons and neutrons cancel out their overall spins. Nuclei with an odd number of protons or an odd number of neutrons or both have an overall spin. This spin is measured with a nuclear spin quantum number (I). The nuclear spin quantum number of a nuclei depends on the protons/neutrons which are not paired, and is a positive integer multiple of 0.5. 1H, 19F, 13C, 31P and 15N are examples of nuclei with an nuclear spin quantum number of 0.5, 2H and 14N have a nuclear spin quantum number of 1.

See also Spin Quantum Number.

Excitation by a RF pulse can be considered as creating a transition (or 'coherence') between different energy levels. Formally, transitions are only allowed between states of the spin system differing in spin quantum number by one unit (single quantum coherence), but multiple RF pulses can act in cascade and produce multiple quantum coherence. Only single quantum coherence produces a directly observable signal, requiring indirect observation of multiple quantum frequencies.

A pattern of multiple resonance's (spectral lines) observed when the initially single Larmor frequency of a given nucleus in a spin system is split by interactions with neighboring spins through the scalar or spin spin interaction. The magnitude of this interaction is independent of the applied magnetic field and is referred to as J, the spin spin coupling constant. The specific pattern produced depends on the number of coupled nuclei and their spin quantum numbers.