In a magnetic resonance imaging system, for example, a radio frequency magnetic field unit, or coil, is positioned in the bore of a magnet. The object to be imaged is placed within the magnetic field unit. The magnetic field unit is driven by an excitation signal that stimulates a nuclear induction (free induction decay) signal in the object, which, in turn, is received by a radio frequency coil. The nuclear induction signal includes information characteristic of the object being imaged. The information in the induction signal can be used to identify chemicals and to diagnose diseases.
Different radio frequency magnetic field units are used to image different portions of a patient depending on such variables as, for example, the patient size and shape and the biomedical region of interest. Thus, for any particular imaging application, the magnetic field unit selected is typically a compromise between performance, size, cost and availability. Consequently, the images resulting from the use of a particular radio frequency magnetic field unit may be inadequate for their intended purpose.