The word gradient (from grade) means the inclination of a surface along a given direction. In MRI, gradient stands for gradient field and/or gradient coil. Inside the main magnet are three gradient coils located, which produce the desired gradient (magnetic) fields. These fields are used to alter (collectively and sequentially) the influence of the static magnetic fieldB0 on the imaged object by inc- or decreasing the field strength and changing the direction.
Through this influence selective spatial excitation and spatial encoding (each voxel resonate at a different frequency) is possible. Gradients are also utilized in another way for fast imaging sequences. See also Slew Rate and Duty Cycle.
Current carrying coils designed to produce a desired magnetic field gradient (so that the magnetic field will be stronger in some locations than others).
Proper design of the size and configuration of the coils is necessary to produce a controlled and uniform gradient. Three paired orthogonal current-carrying coils located within the magnet that are designed to produce desired gradient magnetic fields, which collectively and sequentially are superimposed on the main magnetic field (B0) 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 coils in general vary the main magnetic field, so that each signal can be related to an exact location. The gradientcoil configuration for the z-axis consists of e.g., Helmholtz pair coils, and of paired saddle coils for the x- and y-axis.