Effects on MRI due to altered rf polarization near conductive implants or instruments

In magnetic resonance imaging near metal parts variations in radio frequency (rf)-amplitude and of receive sensitivity must be considered. For loop structures, e.g., vascular stents, B 1 produces rf eddy currents in accordance to Faraday’s law; the B 1 -related electrical rf field E 1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field B 1 , ind (induced B 1 ) is superimposed onto the rf field from the transmitter coil, which near the metal can dominate spin excitation. Geometry and arrangement of the parts determine the polarization of B 1 , ind . Components parallel to B 0 are of special interest. A copper sheet ( 100 mm × 15 mm , 3 mm thick) and a 27 cm long copper wire were examined in a water phantom using the spin-echo (SE) technique. In addition to rf-amplitude amplification, rf-phase shift due to z components of B 1 , ind could be detected near the metallic objects. Periodic rf-amplitude instabilities had an amplified effect for phase-shifted regions. Phase-encoding artifacts occurred as distinct ghosts ( TR = 200 ms ) or band-like smearing ( TR = 201 ms ) from affected spin ensembles. SE phase imaging can potentially be used in interventional magnetic resonance imaging for background-free localization of metallic markers.