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Magnetic field gradients

Spatial localization is based on magnetic field gradients, applied successively along different axes.
Magnetic gradient causes the field strength to vary linearly with the distance from the center of the magnet.
These gradients are employed for slice selection, phase encoding and frequency encoding.

Magnetic gradient fields are generated using gradient coils. They operate in pairs in the magnet, with opposite polarities (one coil increases the static magnetic field, the other reduces it), resulting in a linear change of the magnetic field in the direction of the gradient.
Within a plane perpendicular to the direction of the gradient, the magnetic field does not vary.
The properties of magnetic gradients are their peak amplitude (strength), rise time, and slew rate.

Magnetic gradient fields increase or reduce linearly the static magnetic field B₀, resulting in a linear change of precessional frequency. The change in precessional frequency is directly proportional to the distance from the center of the magnet.

These gradients are switched at different times in different directions to perform spatial localization. Here is the Spin Echo sequence diagram we are going to use to explain slice selection, phase encoding and frequency encoding.