rf enhancement and shielding in MRI caused by conductive implants: Dependence on electrical parameters for a tube model

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 diameter d(0), 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 field B-1,B-app. Tube axis was assumed parallel to B-1,B-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 X=R/Yapproximate to2. A rapid drop occurs with the reduction of X, whereas a further increase of X causes only a small effect. With xsimilar to1/d(0) (Ysimilar tod(0)(2), Rsimilar tod(0)), conditions for rf alteration clearly depend on the diameter of the tube. Inside tubes 0 with smaller diameter, rf shielding is less pronounced. rf alteration increases in good approximation with the square root of the strength of the static field B-0. The following experiments were carried out: Tubes of similar diameter (d(0) approximate to 8 mm) made of material of different conductivity (Cu, Nitinol, carbon fiber reinforced plastic with three different fiber structures) were examined at B-0=0.2 and 1.5 T in water phantoms. Tube axis was aligned perpendicular to B-0 and 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 with d(0) approximate to 8 mm and inside a smaller tube with d(0) = 3.3 mm was compared. Both tubes showed the same wall structure and thickness (d(0) = 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. (C) 2005 American Association of Physicists in Medicine.