Polarized multichannel transmit MRI to reduce shading near metal implants

To investigate the benefit of a two-channel transmit system on shading close to total hip replacements and other elongated metal structures in parallel to the magnet bore.

An analytical model comprising a water cylinder and a metal rod is introduced to describe the B1 effects close to elongated metal structures and it is verified. The dependence of the optimal polarization, which induces minimum shading, on the position of the metal is analyzed. Furthermore, the optimal polarization for two patients is determined both on the basis of the model and experimentally and its benefit compared with circular polarization is investigated.

The cylindrical model approximates the modification of the B1 field due to the metal well, and the optimal polarization strongly depends on the position of the rod. In vivo, shading can be ameliorated by the use of the optimal polarization; for total hip replacements with shafts of titanium, this polarization can be determined on the basis of both the analytical model and the experimental data.

Parallel transmission offers the possibility of a substantial reduction of shading close to long metal structures in parallel to the magnet bore.