MR imaging instrumentation and image artifacts.

Current magnetic resonance (MR) imaging systems use four basic types of magnets: permanent, resistive (electromagnets), hybrid (combines principles of permanent and resistive magnets), and superconducting (resistive-like magnets cooled to near absolute zero to achieve superconductivity). The first three have relatively low field strength (0.2-0.6 T) compared with superconducting magnets (2.0 T), but they cost less, have limited fringe fields, and have minimal sitting restrictions. Although current low-field-strength units have signal-to-noise ratios competitive with those of high-field-strength superconducting units, the latter still allow faster data acquisition. Use of shim coils improves the uniformity of the primary magnetic field and thus image quality. However, distortions may be incurred depending on the magnetic susceptibility of materials being imaged or chemical shift effect. Image nonuniformities can result from the interactions between the shim coils and gradient coils, which are used to create the high-performance linear gradients needed in many MR techniques (eg, gradient-echo and echo-planar imaging). The gradient coils must be shielded to prevent these eddy current effects. The imager must also be shielded from external radio-frequency signals, which can cause interference. Image degradation caused by respiratory motion, cardiac motion, and blood flow can be reduced by use of gating and special imaging techniques.