Permanent conical magnet for interventional magnetic resonance imaging

We describe an open structure of permanently magnetized material for interventional magnetic resonance imaging of the brain. We transformed the ideal magnet, contained between two coaxial cones, into a practical device by a series of geometrical steps that minimize field perturbations. By taking advantage of the quasi-linear demagnetization characteristics of rare-earth materials, we could analyze the structure with an exact mathematical model. The magnet, built of material of remanence 1.38 T, generates a field of 0.45 T within its central gap 30 cm wide. Our numerical computations show a remarkable 0.25% field uniformity in the imaging region. The large opening makes the conical magnet suitable for interventional and surgical imaging.