# Glossary

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**N**

**N (= rho, ρ)** → spin density.
**Navigator echoes**: additional spin or gradient echoes used to monitor changes in the position of the sample during the scan time. (→ Retrospective gating).
**NMR** → Nuclear magnetic resonance.
**NMR imaging** → Magnetic resonance imaging.
**NMR signal**: electromagnetic signal in the radiofrequency range produced by the precession of the transverse magnetization of the spins. The rotation of the transverse magnetization induces a voltage in a coil, which is amplified and demodulated by the receiver.
**Nuclear magnetic resonance (NMR)**: the absorption or emission of electromagnetic energy by nuclei in a static magnetic field, after excitation by a suitable RF magnetic field. The peak resonance frequency is proportional to the magnetic field, and is given by the Larmor equation. Only nuclei with a non-zero spin exhibit NMR.
**Nuclear spin (see also Spin)**: an intrinsic property of certain nuclei that gives them an associated characteristic angular momentum and magnetic moment.
**Nuclear 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/2π), where h is Planck’s constant. The number of possible energy states for a given nucleus in a fixed magnetic field is equal to 2I +1.
**Nucleon**: generic term for a neutron or proton.
**Nutation**: a displacement of the axis of a spinning body away from the simple coneshaped figure which would be traced by the axis during precession. In the rotating frame of reference, the nutation caused by an RF pulse appears as a simple precession, although the motion is more complex in the stationary frame of reference.
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