Quick Overview
Please note that there are different common names for this artifact.

During frequency encoding, fat protons precess slower than water protons in the same slice because of their magnetic shielding. Through the difference in resonance frequency between water and fat, protons at the same location are misregistrated (dislocated) by the Fourier transformation, when converting MRI signals from frequency to spatial domain. This chemical shift misregistration cause accentuation of any fat-water interfaces along the frequency axis and may be mistaken for pathology. Where fat and water are in the same location, this artifact can be seen as a bright or dark band at the edge of the anatomy.
Protons in fat and water molecules are separated by a chemical shift of about 3.5 ppm. The actual shift in Hertz (Hz) depends on the magnetic field strength of the magnet being used. Higher field strength increases the misregistration, while in contrast a higher gradient strength has a positive effect. For a 0.3 T system operating at 12.8 MHz the shift will be 44.8 Hz compared with a 223.6 Hz shift for a 1.5 T system operating at 63.9 MHz.

For artifact reduction helps a smaller water fat shift (higher bandwidth), a higher matrix, an in phase TE or a spin echo technique. Since the misregistration offset is present in the read out axis the patient may be rescanned with this axis parallel to the fat-water interface. Steeper gradient may be employed to reduce the chemical shift offset in mm. Another strategy is to employ specialized pulse sequences such as fat saturation or inversion recovery imaging. Fat suppression techniques eliminate chemical shift artifacts caused by the lack of fat signal.

See also Black Boundary Artifact and Magnetic Resonance Spectroscopy.

The magnetic field outside the magnet, which is also called the stray field. Its strength depends on the magnet type and the field strength. The higher the field strength, the larger the fringe field.

See Magnetic Fringe Field.

The principal advantage of MRI at high field is the increase in signal to noise ratio. This can be used to improve anatomic and/or temporal resolution and reduce scan time while preserving image quality. MRI devices for whole body imaging for human use are available up to 3 tesla (3T). Functional MRI (fMRI) and MR spectroscopy (MRS) benefit significantly. In addition, 3T machines have a great utility in applications such as TOF MRA and DTI. Higher field strengths are used for imaging of small parts of the body or scientific animal experiments. Higher contrast may permit reduction of gadolinium doses and, in some cases, earlier detection of disease.
Using high field MRI//MRS, the RF-wavelength and the dimension of the human body complicating the development of MR coils. The absorption of RF power causes heating of the tissue. The energy deposited in the patient's tissues is fourfold higher at 3T than at 1.5T. The specific absorption rate (SAR) induced temperature changes of the human body are the most important safety issue of high field MRI//MRS.
Susceptibility and chemical shift dispersion increase like T1, therefore high field MRI occasionally exhibits imaging artifacts. Most are obvious and easily recognized but some are subtle and mimic diseases. A thorough understanding of these artifacts is important to avoid potential pitfalls. Some imaging techniques or procedures can be utilized to remove or identify artifacts.

See also Diffusion Tensor Imaging.

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

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'

From Siemens Medical Systems;
the MAGNETOM Harmony system is a high-performance MR imaging system based on 1.0 T field strength for best clinical routine MRI. It combines patient-friendly design features with the benefits of short imaging time, high anatomical resolution, and highest productivity.