Comparison of Superconductors and Permanent Magnets for Small-scale Magnetic Resonance Imaging Devices

In this feasibility study, we use finite element method simulations to compare superconductors (SC) coils and permanent magnets (PM) assemblies as main field source for a small-scale magnetic resonance imaging scanner. The motivation behind this investigation is that for certain tissues, e.g., knee ligament, diagnosis can be performed with equipment operating with relatively weak magnetic fields. It is then interesting to assess whether the relaxed field requirement enables alternative technologies, which present some advantages over the well-established helium-cooled low-temperature superconductors. As prototypical magnetic systems, we consider the Helmholtz coil and the Halbach cylinder for the SC and PM cases, respectively. Each possibility is simulated over a wide range of combinations of the fundamental design parameters. We estimate field strength, field homogeneity, and total cost over the service life of the device. From these figures of merit, we evaluate the optimal combinations of design parameters. Finally, we compare the SC and PM systems and we establish the range of field requirement values for which one alternative is more suitable than the other one. The result of the present investigation provides the starting point for more detailed studies where more specific design considerations are taken into account.