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Effects on MRI due to altered rf polarization near conductive implants or instruments
Author(s) -
Graf Hansjörg,
Steidle Günter,
Martirosian Petros,
Lauer Ulrike A.,
Schick Fritz
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2132571
Subject(s) - radio frequency , amplitude , materials science , electromagnetic coil , nuclear magnetic resonance , magnetic field , magnetic resonance imaging , radiofrequency coil , imaging phantom , polarization (electrochemistry) , faraday cage , eddy current , optics , excitation , electrical conductor , physics , electrical engineering , chemistry , engineering , medicine , quantum mechanics , composite material , radiology
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 , indcould 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.