CALCULATING CURRENT DENSITIES AND FIELDS PRODUCED BY SHIELDED MAGNETIC-RESONANCE-IMAGING PROBES

A method is presented for computing the fields produced by radio frequ ency probes of the type used in magnetic resonance imaging. The effect s of surrounding the probe with a shielding coil, intended to eliminat e stray fields produced outside the probe, are included. An essential feature of these devices is the fact that the conducting rungs of the probe are of finite width relative to the coil radius, and it is there fore necessary to find the distribution of current within the conducto rs as part of the solution process. This is done here using a numerica l method based on the inverse finite Hilbert transform, applied iterat ively to the entire structure including its shielding coils. It is obs erved that the fields are influenced substantially by the width of the conducting rungs of the probe, since induced eddy currents within the rungs become more pronounced as their width is increased. The shield is also shown to have a significant effect on both the primary current density and the resultant fields. Quality factors are computed for th ese probes and compared with values measured experimentally.