A magnetic resonance imaging scanner includes a magnet (20) generating a temporally constant magnetic field, magnetic field gradient-generating structures (30) superimposing selected magnetic field gradients on the temporally constant magnetic field, and a radio frequency coil (32) producing a radio frequency field. A magnetic field-modifying structure (60) disposed inside a radio frequency shield (64) includes dispersed particles of magnetic material (701, 702, 703, 704) that enhance the temporally constant magnetic field. The particles are generally smaller in at least one dimension than a skin depth of the radio frequency field in the magnetic material. The magnetic field-modifying structure has a longitudinal demagnetization factor (Nz) parallel to the temporally constant magnetic field and a tangential demagnetization factor (NT) in a tangential direction transverse to the temporally constant magnetic field. The longitudinal demagnetization factor is larger than the tangential demagnetization factor to produce tangential flux guiding.