Johan Overweg, Philips Research, Hamburg, Germany
MRI scanners come in various shapes and they are based on several distinctly different technologies to create the static background field (B0). Whatever the shape and the technology, all B0 magnets for MRI have in common that they generate a strong field, which has, within narrow tolerances, the same value over a large volume and is very stable in time. The typical size of the imaging volume is a ellipsoid with 300-500 mm axis length, over which the field variations are less than a few ppm. The amplitude of time-dependent field variations is typically (far) less than 100 nT. This paper presents an overview of the technologies used for generating the main magnetic field in whole-body MRI scanners.