Standard compound used as a standard reference spectral line for defining chemical shifts for a given nucleus. As recommended by the ASTM, for 1H it is tetramethylsilane (TMS) and for 31P it is phosphoric acid, although for practical biological applications water and PCr have been used as secondary references for hydrogen and phosphorus spectroscopy, respectively. The reference compound can be in a capsule outside of the subject (external) or can be in the subject (internal); internal references are generally preferable where possible, as external references may be subject to different conditions.

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.

(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.