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Glossary • A

Absorption (electromagnetic): reversible or irreversible transfer of energy from an electromagnetic field to tissue.

Acquisition matrix → Image acquisition matrix.

Acquisition time → Image acquisition time.

ADC → Analog to digital converter.

Adiabatic fast passage (AFP): technique of producing rotation of the macroscopic magnetization vector by sweeping the frequency of an irradiating RF wave (or the strength of the magnetic field) through resonance (the Larmor frequency) in a time short compared to the relaxation times. Particularly used for inversion of the spins. A con­ti­nu­ous wave NMR technique.

Adiabatic rapid passage → Adiabatic fast passage.

Adiabatic pulses: a category of non-selective RF pulses which can produce uniform flip angles across non-uniform RF fields. Widely used in conjunction with surface coils.

AFP → Adiabatic fast passage.

Analog to digital converter (ADC): part of the interface that converts ordinary (ana­log) voltages, such as the detected NMR signal, into digital (number) form, that can be read by the computer.

Angiography: depiction of vessels using special pulse sequences, e.g. fast imaging se­que­nces, see also MRA.

Angular frequency (ω): frequency of oscillation or rotation (measured, e.g. in ra­di­ans/second) commonly designated by ω: ω = 2 × π ν, where ν is the frequency (e.g. in Hertz [Hz]).

Angular momentum: a vector quantity given by the vector product of the momentum of a particle and its position vector. In the absence of external forces, the angular mom­en­tum remains constant, with the result that any rotating body tends to maintain the same axis of rotation. When a torque is applied to a rotating body, the resulting change in angular momentum results in precession. Atomic nuclei posses an intrinsic angular momentum referred to as spin, measured in multiples of Planck’s constant.

Antenna: device to send or receive electromagnetic radiation. Electromagnetic ra­di­a­tion per se is not relevant to NMR, as it is the magnetic vector alone that couples the spins and the coils, and the term coil should be used instead.

Array processor: a component of computer systems specially designed to speed up numerical calculations, such as the FT used in MR imaging.

Artifacts: false features in the image produced by the imaging process (sometimes also artefacts in British English).

Averaging: summation of signals to improve the signal-to-noise ratio.

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