THE AAPM RSNA PHYSICS TUTORIAL FOR RESIDENTS - MR-IMAGING INSTRUMENTATION AND IMAGE ARTIFACTS

Current magnetic resonance (MR) imaging systems use four basic types o f magnets: permanent, resistive (electromagnets), hybrid (combines pri nciples of permanent and resistive magnets), and superconducting (resi stive-like magnets cooled to near absolute zero to achieve superconduc tivity). The first three have relatively low field strength (0.2-0.6 T ) compared with superconducting magnets (2.0 T), but they cost less, h ave limited fringe fields, and have minimal siting restrictions. Altho ugh current low-field-strength units have signal-to-noise ratios compe titive with those of high-field-strength superconducting units, the la tter still allow faster data acquisition. Use of shim coils improves t he uniformity of the primary magnetic field and thus image quality. Ho wever. distortions may be incurred depending on the magnetic susceptib ility of materials being imaged or chemical shift effect. Image nonuni formities can result from the interactions between the shim coils and gradient coils, which are used to create the high-performance linear g radients needed in many MR techniques (eg, gradient-echo and echo-plan ar 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 ca used by respiratory motion, cardiac motion, and blood flow can be redu ced by use of gating and special imaging techniques.