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'

The subacute risks and side effects of magnetic and RF fields (for patients and staff) have been intensively examined for a long time, but there have been no long-term studies following persons who have been exposed to the static magnetic fields used in MRI. However, no permanent hazardous effects of a static magnetic field exposure upon human beings have yet been demonstrated.
Temporary possible side effects of high magnetic and RF fields:
The results of some animal and cellular studies suggest the possibility that electromagnetic fields may act as co-carcinogens or tumor promoters, but the data are inconclusive. Up to 45 tesla, no important effects on enzyme systems have been observed. Neither changes in enzyme kinetics, nor orientation changes in macromolecules have been conclusively demonstrated.
There are some publications associating an increase in the incidence of leukemia with the location of buildings close to high-current power lines with extremely low-frequency (ELF) electromagnetic radiation of 50-60 Hz, and industrial exposure to electric and magnetic fields but a transposition of such effects to MRI or MRS seems unlikely.
Under consideration of the MRI safety guidelines, real dangers or risks of an exposure with common MRI field strengths up to 3 tesla as well as the RF exposure during the MRI scan, are not to be expected.

For more MRI safety information see also Nerve Conductivity, Contraindications, Pregnancy and Specific Absorption Rate.

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

(MR) Resonance phenomenon resulting in the absorption and/or emission of electromagnetic energy by nuclei (for that reason also nuclear magnetic resonance) or electrons in a static magnetic field, after excitation by a suitable RF magnetic field.
The peak resonance frequency is proportional to the magnetic field, and is given by the Larmor equation. Only unpaired electrons or nuclei with a spin exhibit magnetic resonance. The absorption or emission of energy by atomic nuclei in an external magnetic field after the application of RF excitation pulses using frequencies, which satisfy the conditions of the Larmor equation.
The magnetic resonance phenomenon may be used in one of these ways:
By manipulation of the external field (application of gradient fields), the resonance frequency can become dependent on spatial location, and hence images may be generated (MRI).
The effect of the electron cloud in any atom or molecule is to slightly shield the nucleus from the external field, thus giving any chemical species a characteristic frequency. This gives rise to 'spectra' where nuclei in a molecule give rise to specific signals, thus facilitating the detection of individual chemicals by means of their frequency spectra (MRS)

(RF) Radio frequency refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.
The RF pulses used in MRI are commonly in the 1-100 megahertz range, and their effect upon a body is potential heating of tissues and foreign bodies, such as metallic implants, mainly at the surface.
This is a principal area of concern for MRI safety limits caused by absorption (see specific absorption rate) of the applied pulses of RF energy.

The higher the frequency, the larger will be the amount of heat developed.
The more ionic the biochemical environment in the tissue, the more energy that will be deposited as heat.
This effect is well known for homogeneous model systems, but the complex structure of various human tissues makes detailed theoretical calculations very difficult, if not impossible. By scanning problems, it is important to verify the transmission frequency. If the RF transmitted into the patient was, e.g. 5000 Hz lower than the resonance frequency of the protons, no protons was excited, and no signal returns.

The Larmor equation is important because it is the frequency at which the nucleus will absorb energy. The absorption of that energy will cause the proton to alter its alignment and ranges from 1-100 MHz in MRI. The equation states that the frequency of precession of the nuclear magnetic moment is directly proportional to the product of the magnetic field strength (B0) and the gyromagnetic ratio (g). This is stated mathematically as w = g B0.