Rapidly restoring blood flow to the brain in patients with stroke caused by a blocked blood vessel in the brain is the key to reducing disability. Current treatments often leave small blocked arteries that cannot be safely opened with mechanical clot removal devices. Furthermore, stagnant flow limits access of clot-dissolving medication to the clot. This trial tests iron nanoparticles (similar to iron infused to replace low body stores but injected directly into the brain artery upstream of the blockage) combined with an external rotating magnet that draws the nanoparticles towards the clot, overcoming stagnant blood flow. The aim is to bring fresh blood which contains naturally-occurring clot-dissolving substances, and any clot-dissolving medication that may be circulating, to the surface of the clot with the aim of restoring blood flow to the brain. The trial will recruit up to 30 patients. All will receive injection of nanoparticles via an angiogram (small tube inserted into a leg or arm artery and fed up into the brain artery under Xray control). The procedure takes 30min and the degree of success in opening the artery at the end of procedure is the primary outcome, combined with an absence of symptomatic brain bleeding at 24h.