A Target Field Approach to Open MRI Magnet Design

The design of an fMRI magnet for the study of the human motor cortex poses a number of challenges due to the necessity of maintaining the subject in a natural, erect position, with free access to the environment. This paper presents the design of an asymmetric superconducting magnet composed of racetrack coils. The coil configuration is derived from the one presented in [1] from which the non-planar coils have been eliminated, with significant simplifications of the manufacturing process. The position of the coils is defined along the principles described in [1] and the current in the coils is determined by means of an approach similar to the Inverse Boundary Method [2-4] in which the goal is established as a set of spherical harmonic coefficients. The major challenge in the design lies in limiting the coil currents to sustainable values without compromising the field homogeneity. The current limit requires an increase in the number of winding layers that, in turn, induces a change in the physical dimensions, making the system nonlinear. It is shown how this can be counteracted by means of an iterative procedure that converges to acceptable results. In the final configuration the magnet accommodates a sitting individual and produces a field intensity of 1.4 T. It consists of planar coils only with current densities and maximum field intensities compatible with currently available superconductors.