FIG. 5 is a plot of magnetic field lines generated by magnet array 400 of FIG. 4, according to an embodiment of the present invention. Uniformity is not evident by uniform density of the lines (as lines were drawn denser in the imaging zone for better details) rather by z-axis alignment of the lines. MPMR array 400 can achieve, at a same array weight, a more uniform magnetic field along the z-axis, compared with, for example, that achieved with arrays 100 and 200. Moreover, the uniform field extends radially almost to the rings. Such lightweight MPMR arrays may be therefore particularly useful for mobile MRI applications, such as an MRI ambulance.

FIGS. 4 and 5 show an exemplary array 400 containing ten MPMRs. It will be appreciated that the array may contain more MPMRs (e.g., several tens or hundreds of MPMRs) which are all optimized as described above with many of them phase dissimilar. The more rings contained in the array, the better magnet performance can be achieved (e.g., higher uniformity level, larger magnetic field or larger imaging volume). The improved performance comes with the drawback of increased complexity and production cost of the array due to the large number of elements. Thus, a practitioner skilled in the art should consider the required number of MPMRs according to the specific application.

FIG. 6 is a perspective drawing of an exemplary MPMR 600 according to an embodiment of the invention. The ring consists of six repeating segments 610, each of which has four elements: 620a, 620b, 620c, and 620d.