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 field B0 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.

In MRI systems, the gradient amplifier increases the energy of the signal before it reaches the gradient coils, in a way, that the field strength is intense enough to produce the variations in the main magnetic field for localization of the later received signal. Three gradient coils normally require three gradient amplifiers. As all amplifiers, gradient amplifiers produce heat, which requires cooling.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

MRI hardware includes the electrical and mechanical components of a scanning device.
The main hardware components for the MRI machine are:
The magnet establishing the B0 field to align the spins.
Within the magnet are the gradient coils for producing variations in B0 in the X, Y, and Z directions to make a localization of the received data possible.
Within the gradient coil or directly on the object being imaged is the radio frequency (RF) coil. This RF coil is used to establish the B1 magnetic field necessary to excite the spinning nuclei. The RF coil also detects the signal emitted from the spins within the object being imaged.
The RF amplifier increases the power of the pulses.
The analog to digital converter converts the received analog raw data into digital values.
Depending on the design of the device and the body part being imaged the patient is positioned inside the magnet (e.g. on a movable table or standing upright).
The MRI scan room is surrounded by a RF shield (Faraday cage).
In addition, a computer console, a display, and a film printer belong to the MRI equipment.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'

A Helmholtz pair consists of two identical circular magnetic coils that are placed symmetrically one on each side of the experimental area along a common axis, and separated by a distance equal to the radius of the coil. Actually, a slightly larger separation improves the field uniformity. Each coil carries an equal electrical current flowing in the same direction. A cylindrical region extending between the centers of the two coils and approximately 1/5 of their diameter will have a nearly spatially uniform magnetic field.
In MRI, the Helmholtz pair coils are used as z-gradient coils to produce linear variations in the main magnetic field along the z-axis. Also in use as z-gradient coils are the Maxwell coils (three coils in a slightly more complicated geometry than the Helmholtz configuration). These coils are only occasionally used as RF coils for imaging.

Hardware components from MRI systems. The radio frequency (RF) amplifier increases the power (measured in Watt) of the pulses from mW to kW and the gradient amplifier increases the power of the gradient pulses for the gradient coils.

See also Hardware.