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• MRI Terms

- G -

• GADOLINIUM (Gd)

Gadolinium is a non-toxic paramagnetic contrast enhancement agent utilized in MR imaging. When injected during the scan, gadolinium will tend to change signal intensities by shortening T1 in its surroundings.



• GATING

Timing the acquisition of MR data to physiological motion in order to minimize motion artifacts (e.g., cardiac gating, respiratory gating).



• GAUSS

A unit of magnetic field strength that is approximately the strength of the earth's magnetic field at its surface (the earth's field is about 0.5 to 1G). The value of 1 gauss is defined as 1 line of flux per cm2. As larger magnetic fields have become commonplace, the unit gauss (G) has been largely replaced by the more practical unit tesla (T), where 1 T = 10, 000 G. GHOSTING - an image artifact primarily associated with the phase direction.



• GRADIENT COILS

Three paired orthogonal current-carrying coils located within the magnet which are designed to produce desired gradient magnetic fields which collectively and sequentially are superimposed on the main magnetic field (Bo) so that selective spatial excitation of the imaging volume can occur. Gradients are also used to apply reversal pulses in some fast imaging techniques.



• GRADIENT ECHO (GE)

See Field Echo.



• GRADIENT MAGNETIC FIELD

A small linear magnetic field applied in addition to (superimposed on) the large static magnetic field in an MRI scanner. The strength (amplitude) and direction of the gradient fields change during the scan, which allows each small volume element (voxel) within the imaging volume to resonate at a different frequency. In this way, spatial encoding may be performed.



• GYROMAGNETIC RATIO (g)

A constant for any given nucleus that relates the nuclear MR frequency and the strength of the external magnetic field. It represents the ratio of the magnetic moment (field strength) to the angular momentum (frequency) of a particle. The value of the gyromagnetic ratio for hydrogen (1H) is 4, 258 Hz/Gauss (42.58 MHz/Tesla).