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rf enhancement and shielding in MRI caused by conductive implants: Dependence on electrical parameters for a tube model
Author(s) -
Graf Hansjörg,
Steidle Günter,
Lauer Ulrike A.,
Schick Fritz
Publication year - 2005
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 - 10.1118/1.1843351
Subject(s) - electromagnetic shielding , materials science , eddy current , electrical conductor , radio frequency , electromagnetic coil , magnetic field , amplitude , nuclear magnetic resonance , perpendicular , composite material , optics , electrical engineering , physics , geometry , mathematics , quantum mechanics , engineering
Radio frequency (rf) eddy‐currents induced in implants made of conductive material might cause significant image artifacts in magnetic resonance imaging (MRI) such as shielding of the lumen of vascular stents. rf alteration near metal parts was assessed theoretically in the approximation of alternating current electrodynamics: The implant was modeled as tube with diameterd o , resistance R , and reactance Y , constituting the secondary winding of a transformer. The transmitter coil of the scanner acted as primary winding and generated the linearly polarized rf fieldB 1 , app. Tube axis was assumed parallel toB 1 , app. The results of the calculations were as follows: Ninety percent of the applied rf‐field amplitude is reached in the lumen at a ratio χ = R ∕ Y ≈ 2 . A rapid drop occurs with the reduction of χ , whereas a further increase of χ causes only a small effect. With χ ∼ 1 ∕ d o ( Y ∼ d o 2 , R ∼ d o ) , conditions for rf alteration clearly depend on the diameter of the tube. Inside tubes with smaller diameter, rf shielding is less pronounced. rf alteration increases in good approximation with the square root of the strength of the static fieldB 0 . The following experiments were carried out: Tubes of similar diameter ( d o ≈ 8 mm ) made of material of different conductivity (Cu, Nitinol, carbon fiber reinforced plastic with three different fiber structures) were examined atB 0 = 0.2 and 1.5 T in water phantoms. Tube axis was aligned perpendicular toB 0and spin‐echo technique was applied. Local rf enhancement near the outer surface of the metal tubes was detected applying manual reduction of the transmitter amplitude. Shielding inside a carbon fiber tube withd o ≈ 8 mm and inside a smaller tube withd o = 3.3 mm was compared. Both tubes showed the same wall structure and thickness ( d w = 0.4 mm ) . All measurements confirmed the theoretical results. Consequences for the construction of vascular stents are discussed, as well as problems with image artifacts due to rf enhancement near solid conductive implants.