Defining Terms

Gyromagnetic ratio γ : An intrinsic property of a nucleus. It determines the Larmor frequency through the relation ω₀ = −γ B₀ .

k-space: The reciprocal of object space, k-space describes MRI data acquisition in the spatial Fourier transform coordinate system.

Larmor frequencyω₀: The frequency of precession of the spins. It depends on the product of the applied flux density B₀ and on the gyromagnetic ratio γ . The Larmor frequency is ω₀ = −γ B₀.

Magnetization M: The macroscopic ensemble of nuclear moments. The moments are induced by an applied magnetic field. At body temperatures, the amount of magnetization is linearly proportional (M₀ = 4 × 10⁻⁹H₀) to the applied magnetic field.

Precession: The term used to describe the motion of the magnetization about an applied magnetic field. The vector locus traverses a cone. The precession frequency is the frequency of the magnetization components perpendicular to the applied field. The precession frequency is also called the Larmor frequency ω₀.

Spin echo: The transverse magnetization response to a π RF pulse. The effects of field inhomogeneity are refocused at the middle of the spin echo.

Spin–lattice relaxation T₁: The exponential rate constant describing the decay of the z component of magnetization toward the equilibrium magnetization. Typical values in the body are between 300 and 3000 msec.

Spin–spin relaxation T₂: The exponential rate constant describing the decay of the transverse components of magnetization (M_x and M_y ).

Spins M: Another name for magnetization.

2D FT: A rectilinear trajectory through k-space. This popular acquisition scheme requires several (usually 128 to 256) excitations separated by a time TR, which is determined by the desired contrast, RF flip angle, and the T1 of the desired components of the image.