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Glossary • M

Mxy → Transverse magnetization.

Mz → Longitudinal magnetization.

M0: equilibrium value of the magnetization; directed along the direction of the static mag­netic field. Proportional to spin density, rho, ρ or N.

Macroscopic magnetic moment: → Macroscopic magnetization vector.

Macroscopic magnetization vector: net magnetic moment per unit volume (a vector quantity) of a sample in a given region, considered as the integrated effect of all the in­di­vi­du­al microscopic nuclear magnetic moments. Most NMR experiments actually deal with this.

Magnetic dipole: north and south magnetic poles separated by a finite distance. An elec­tric current loop, including the effective current of a spinning nucleon or nucleus, can create an equivalent dipole.

Magnetic field (H): the region surrounding a magnet (or current carrying conductor) is endowed with certain properties. One is that a small magnet in such a region experiences a torque that tends to align it in a given direction. Magnetic field is a vector quantity; the direction of the field is defined as the direction that the north pole of the small magnet points when in equilibrium. A magnetic field produces a magnetizing force on a body with­in it. Formally, the forces experienced by moving charged particles, current carrying wires, and small magnets in the vicinity of a magnet are due to magnetic induction (B), which includes the effect of magnetization, while the magnetic field (H) is defined so as not to include magnetization. However, both B and H are often loosely used to denote mag­ne­tic fields. For definition of field strength see Chapter 3.

Magnetic field gradient → Gradient magnetic field.

Magnetic induction (B): also called 'magnetic flux density'. The net magnetic effect from an externally applied magnetic field and the resulting magnetization. B is pro­por­tio­nal to H, with the SI unit being the Tesla (T).

Magnetic moment: a measure of the net magnetic properties of an object or particle. A nucleus with an intrinsic spin will have an associated magnetic dipole moment, so that it will interact with a magnetic field (as if it were a tiny bar magnet).

Magnetic resonance → Nuclear magnetic resonance (NMR). Another magnetic re­so­nan­ce phenomenon is electron spin resonance (ESR).

Magnetic resonance imaging: earlier also: NMR ima­ging, magnetic resonance ima­ging, commonly used term: MR imaging, MRI (→ zeugmatography). Creation of images of objects such as the human body by use of the nuclear magnetic resonance phe­no­me­non usually by the application of magnetic field gradients. The immediate practical ap­pli­ca­tion involves imaging the distribution of hydrogen nuclei (protons) in the body. The image contrast in a given region is usually dependent jointly on the spin density and the relaxation times, with their relative importance determined by the particular imaging tech­ni­que employed. Contrast is also affected by motion such as blood flow.

Magnetic susceptibility (χ): measure of the ability of a substance to become mag­ne­ti­zed.

Magnetization: the magnetic polarization of a material produced by a magnetic field (magnetic moment per unit volume); (→ Macroscopic magnetization vector).

Magnetogyric ratio → Gyromagnetic ratio.

Magnitude: also 'absolute value' or 'modulus'; referring to the signal intensity of an MR image which can be proportional to the amplitude of the transverse magnetization which is a non-negative number.

MAST → Motion compensation.

Matrix image matrix; grid of columns and rows, for instance 256×256, with a total num­ber of 64,536 pixels.

Maxwell coil a particular kind of gradient coil, commonly used to create gradient mag­ne­tic fields along the direction of the main magnetic field.

Megahertz (MHz): unit of frequency, equal to one million Hertz.

Meiboom-Gill sequence → Carr-Purcell-Meiboom-Gill sequence.

MHz → Megahertz.

Modulus → Magnitude.

Motion compensation: modifying the field gradients used in a pulse sequence such that flow and acceleration do not induce any additional phase effects.

MRA: Magnetic Resonance Angiography.

MRI: Magnetic Resonance Imaging.

MRM: Magnetic Resonance Mammography.

MRR: Magnetic Resonance Relaxometry.

MRS: Magnetic Resonance Spectroscopy.

MRSI: Magnetic Resonance Spectroscopic Imaging.

Multiple line-scan imaging (MLSI): variation of sequential line imaging techniques that can be used if selective excitation methods which do not affect adjacent lines are em­ployed. Adjacent lines are imaged while waiting for relaxation of the first line toward equilibrium, which may result in decreased imaging time. A different type of MLSI uses simultaneous excitation of two or more lines with different phase encoding followed by suitable decoding. This can be extended to planar sequences.

Multiple slice imaging: variation of sequential plane imaging techniques that can be used with selective excitation techniques that do not affect adjacent planes. Adjacent pla­nes are imaged while waiting for relaxation of the first plane toward equilibrium, re­sul­ting in decreased imaging time.

Multiple sensitive point: sequential line imaging technique utilizing two orthogonal oscillating magnetic field gradients, an SSFP pulse sequence, and signal averaging to iso­la­te the sensitivity to a desired line in the body.

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