Quick Overview

A disturbance of the field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the coil, the homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase. The same problem appears by scanning at a distance from the isocenter in left-right direction or too large field of view.
There are different types of bad image quality, the images are noisy, distorted or the fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the coil the same inversion time is sufficient to suppress water. Since the inversion time is not changed, the T1 recovery rates will increase.

Take a smaller imaging volume (and for fat suppression a volume shimming), take care that the imaged region is at the center of the coil and that no magnetic material is inside the imaging volume.

A feedback control used to maintain the static magnetic field at a constant strength, usually by monitoring the resonance frequency of a reference sample or line in the spectrum.

(FOV) Defined as the size of the two or three dimensional spatial encoding area of the image. Usually defined in units of mm². The FOV is the square image area that contains the object of interest to be measured. The smaller the FOV, the higher the resolution and the smaller the voxel size but the lower the measured signal. Useful for decreasing the scantime is a field of view different in the frequency and phase encoding directions (rectangular field of view - RFOV).
The magnetic field homogeneity decreases as more tissue is imaged (greater FOV). As a result the precessional frequencies change across the imaging volume. That can be a problem for fat suppression imaging. This fat is precessing at the expected frequency only in the center of the imaging volume. E.g. frequency specific fat saturation pulses become less effective when the field of view is increased. It is best to use smaller field of views when applying fat saturation pulses.

Smaller FOV required higher gradient strength and concludes low signal. Therefore you have to find a compromise between these factors. The right choice of the field of view is important for MR image quality. When utilizing small field of views and scanning at a distance from the isocenter (more problems with artifacts) it is obviously important to ensure that the region of interest is within the scanning volume.
A smaller FOV in one direction is available with the function rectangular field of view (RFOV).

See also Field Inhomogeneity Artifact.

In every MR examination, a large static magnetic field is applied. Field strengths for clinical equipment can vary between 0.2 and 3 T; experimental imaging units have a field strength of up to 11 T, depending on the MRI equipment used. In MRS, field strengths up to 12 T are currently used. The field strength of the magnet will influence the quality of the MR image regarding chemical shift artifacts, the signal to noise ratio (SNR), motion sensitivity and susceptibility artifacts.

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

The figure 8 coil consists of two loops, connected in the middle like the number eight. This type of coil is used in magnetic resonance imaging and transcranial magnetic stimulation (TMS).