Page 234 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN008M-395 June 29, 2001 15:52

Magnetic Resonance in Medicine

John F. Schenck
GE Corporate Research and Development Center

I. Magnetic Resonance as a Physical Phenomenon
II. Nuclear Magnetic Resonance Scanners
as Medical Instruments
III. Imaging Techniques
IV. Clinical Applications of Magnetic
Resonance Imaging

GLOSSARY special gradient coils and are used to create a position-
dependent resonance frequency.
Bold contrast Blood-oxygen-level-dependent contrast. Gyromagnetic ratio (γ) Ratio of the magnetic moment
Contrast mechanism dependent on the level of deoxy- to the angular momentum of a nucleus or electron.
genated hemoglobin in the blood. This mechanism is Larmorfrequency(ω 0 ) Rateofprecessionofthenuclear
the basis of functional MRI. magnetization in an applied magnetic field B; ω 0 = γB
Chemical shift Slight shift in the Larmor resonance fre- or f 0 = γB/2π. This is the frequency at which mag-
quency produced by the magnetic field of the orbiting netic resonance occurs.
electrons and varying from one chemical compound to Magnetic field (B) Condition of the space in the vicinity
another. ofanelectriccurrentoramagnetizedmaterialthatleads
Fourier transform Mathematical process that can be to magnetic forces on moving charges. The SI unit is
used to analyze a function of time into the individual the tesla (T). An older unit still widely used is the gauss
frequency components that it contains. (G) (1 T = 10000 G).
Free induction decay (FID) Signal observed from the Magneticmoment(m) Measureoftheabilityofacurrent
precessing transverse magnetization after an excitation loop or a spinning charge to produce a magnetic field.
pulse. Magnetization (M) Vector quantity that measures the
Functional magnetic resonance imaging (fMRI) An degree to which a substance is magnetized. It is the
imaging technique sensitive to variations in regional total magnetic moment per volume. Longitudinal mag-
blood flow in the brain. This mechanism provides im- netization is parallel and transverse magnetization is
ages that demonstrate local brain activation during perpendicular to the applied static magnetic field.
thought processes and sensory stimulation. Nuclear magnetic resonance (NMR) Technique for de-
Gradient field Magnetic field that varies linearly in tecting nuclear magnetism, utilizes a strong, static field
some prescribed direction. Such fields are produced by B 0 and a weak, oscillating field B 1 at right angles to it.

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