Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

15.7 INSTRUMENTATIONMagnetThe magnet is the heart of the MR system. For any particular magnet type, performancecriteria include field strength, temporal stability, and field homogeneity. Theseparameters are affected by the magnet design. Air core magnets are made of wirewrappedcylinders of I-m diameter and greater, where the magnetic field is producedby an electric current in the wires. The main magnetic field of air core magnets runsparallel to the long axis of the cylinder. Typically, the magnetic field is horizontal andruns along the cranial-caudal axis of the patient lying supine (Fig. 15-39A). Solid coremagnets are constructed from permanent magnets, a wire wrapped iron core "electromagnet,"or a hybrid combination. In these solid core designs, the magnetic fieldruns between the poles of the magnet, most often in a vertical direction (Fig. 15-39B).Resistive electromagnets are constructed in either an air core or solid core configuration.Most resistive magnets are now of the solid core design and have a vertical magneticfield with contained fringe fields. These systems use continuous electric power to producethe magnetic field (must overcome the electric resistance of the coil wires), producea significant amount of heat, and often require additional cooling subsystems. Themagnetic field of resistive systems ranges from 0.1 T to about 0.3 T. An advantage of aFIGURE 15-39. A: Air core magnets typically have a horizontal main field produced inthe bore of the electric windings, with the z-axis (80) along the bore axis. Fringe fieldsfor the air core systems are extensive and are increased for larger bore diameters andhigher field strengths. B: The solid core magnet usually has a vertical field, producedbetween the metal poles of a permanent or wire-wrapped electromagnet. Fringe fieldsare confined with this design. The main field is parallel to the z-axis of the Cartesiancoordinate system.