# 词汇表

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**L**

**LAO**: left anterior oblique (projection through the human body).
**Larmor equation**: states that the frequency of precession of the nuclear magnetic moment is proportional to the magnetic field.

ω_{0} = -γ × B_{0} (radians per second); or

ν_{0} = -γ × B_{0}/2π (Hertz),

where ω_{0} and ν_{0}, respectively, are the frequency, γ is the gyromagnetic ratio, and B_{0} is the magnetic induction field. The negative sign indicates the direction of rotation.
**Larmor frequency (ω _{0} or ν_{0})**: the frequency at which magnetic resonance can be excited; given by the Larmor equation. By varying the magnetic field across the body with a gradient magnetic field, the corresponding variation of the Larmor frequency can be used to encode position. For protons (hydrogen nuclei), the Larmor frequency is 42.58 MHz/Tesla.

**Lattice**: the magnetic and thermal environment with which nuclei exchange energy in longitudinal relaxation.

**Line imaging**→ Sequential line imaging.

**Line scanning**→ Sequential line imaging.

**Line width**: width of line in spectrum; related to the reciprocal of the transverse relaxation time (T2* in practical systems). Measured in units of frequency, generally at the half-maximum points.

**Localized spectroscopy**: the acquisition of spectra from a region within a sample. The region is usually defined using selective RF pulses and field gradients.

**Longitudinal magnetization (M**: component of the macroscopic magnetization vector along the static magnetic field. Following excitation by RF pulse, M

_{z})_{z}will approach its equilibrium value M

_{0}, with a characteristic time constant T1.

**Longitudinal relaxation**: return of longitudinal magnetization to its equilibrium value after excitation; requires exchange of energy between the nuclear spins and the lattice.

**Longitudinal relaxation time**→ T1.

**Lorentzian line**: usual shape of the lines in an NMR spectrum, characterized by a central peak with long tails; proportional to 1/[(1/T2)

^{2}+ (ν-ν

_{0})

^{2}], where ν is frequency and ν

_{0}is the frequency of the peak (i.e. central resonance frequency).

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