Briefly applied magnetic field gradient.

See also Bipolar Gradient Pulse and Gradient Magnetic Field.

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.

The uniformity of the main magnetic field measured in ppm. In a defined volume it is the difference between maximum and minimum field strength and this multiplied by 1 million.
In MR, the homogeneity of the static magnetic field is an important criterion of the quality of the magnet. Homogeneity requirements for MR imaging are generally lower than the homogeneity requirements for NMR spectroscopy, but for most imaging techniques must be maintained over a large region.

1) Fidelity of response, e.g. of magnetic field gradients or the RF system, to input. The output of a linear system is directly proportional to its input.
2) Spatial uniformity of the magnetic field gradient over the imaging volume. Because of eddy current effects, static and dynamic linearity have to be distinguished. Both together with the magnet homogeneity determine the geometrical correctness of the images.

Advantages of low field imaging are the small-sized 5 Gauss fringe field and therefore the less static magnetic field exposure for the surrounding area, as well as less contraindications causing lower risks for the MRI safety by implemented metal and magnetic devices and equipment.
Low field systems are sometimes for restricted use, e.g. dedicated extremity scanner or open MRI devices. Open MRI devices equipped with permanent magnets are well-suited for MR guided interventions because these machines combine the lower magnetic fields of this type of magnets and the better patient access of open MRI scanner.
In some cases, the contrast of different tissues is better at lower field strength, depending on their T1 or T2 relaxation times. The disadvantage of the lower signal to noise ratio are a poor resolution and a longer scan time for a good image quality.

See also Claustrophobia, Contraindications and MRI Safety.

See also the related poll result: 'In 2010 your scanner will probably work with a field strength of'