FIGURE 4:

Halbach cylinder designs of potential interest for low-field MRI. In the Halbach cylinder, a nearly uniform transverse field is produced inside the cylinder if the magnetic moment of the magnetized material is phased from 0 to 4π azimuthally. Note that this is similar to the phase relationship for a birdcage coil where a 0 to 2π azimuthal phase relationship is used. Top row shows an ideal cylinder with continuous magnetization and a more practical approximation comprising keystone-shaped sections. Far top right shows a simpler to construct configuration using only identical rectangular blocks and with all the magnetization vectors normal to a face. The phase relationship comes only from rotations of the blocks. The bottom row shows further optimizations allowing degrees of freedom to be adjusted to achieve a target field pattern (typically either a uniform field or a monotonic gradient) despite the imperfections of the array (eg, finite cylinder and sparse population). One option is to maintain linear rungs of material but vary the material. A second approach is to maintain rings of material but allow varying diameters.