Magnetic field stability issues in magnetic resonance imaging

A description is presented of an iron-core electromagnet (Rx-4000) used in a magnetic resonance imaging (MRI) scanner. Periodic mapping of the Rx-4000 field has enabled the determination of shape stability over various time periods. With respect to field strength stability, computer simulations and analysis of imaging indicate that there are three relevant time scales. These time scales (1-100 ms; 100 ms-30 min; longer than 30 min) require progressively less stringent stability. Three techniques have been used for monitoring the field strength, one for each time frame. The design of the magnet is reviewed and field shape stability data is presented. Computer simulated images demonstrating the impact of field instability in two of the time frames are shown. The methods and results of the three field-monitoring techniques are presented. Human images acquired on the Rx-4000 MRI scanner are presented to illustrate the success of this iron-core electromagnet.