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Glossary • 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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