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, 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 X 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 re-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. (c) 2006 American Association of Physicists in Medicine.