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

Abstract

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, B1 produces rf eddy currents in accordance to Faraday's law; the B1-related electrical rf field E1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field Bl,ind (induced B1) 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 B0 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 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.

Metal artifact suppression at the hip: diagnostic performance at 3.0 T versus 1.5 Tesla

“…This is a well-known problem at 3 T due to the reduced wavelength of applied RF fields. In addition, the presence of metal implants can further increase the level of B1 transmission inhomogeneity [19,20]. To mitigate this effect, inversion pulses are typically applied using adiabatic excitation principles [19]; however, the broadband adiabatic inversion pulses required for metal artifact reduction such as those used in MAVRIC SL, often are limited by specific absorption rate and RF hardware limits at 3.0.…”

“…5). The increased blurring at 3.0 T can be partially explained by the reduced T2 of musculoskeletal tissues (ranging from 10-35 % reduction) at 3 T relative to 1.5 T. Proton-density-weighted MAVRIC SL images use an echo train length (ETL) between [16][17][18][19][20]. Echo trains of such length are susceptible to some blurring from T2 decay, which explains the increased observed blurring at 3.0 T. Our study has a number of limitations.…”

Quantification of MRI visibility and artifacts at 3T of liquid fiducial marker in a pancreas tissue‐mimicking phantom

“…Possible signal alterations due to eddy currents or RF shielding induced in the solid markers were not considered. Given the size and composition of fiducial markers tested, this effect is expected to be secondary relative to the susceptibility induced reduction effect . Eddy currents resulting from changing RF field are, however, very likely the main source of the Gold Anchor™ signal voids observed in SE sequences (as seen in Fig.…”

Ensuring safety of implanted devices under MRI using reversed RF polarization

“…The same wire currents that cause implant heating also produce a B 1 field near the wire, resulting in local RF enhancement or cancellation in MR images (15, 16). This phenomenon has been exploited for catheter visualization, wherein a resonant coil is intentionally coupled to the RF receiver to enhance its signal (17, 18).…”