Conductive shield for ultra low field magnetic resonance imaging: Theory and measurements of eddy currents.
Eddy currents induced by applied magnetic field pulses have been a common issue in ultra low field magnetic resonance imaging. In particular, a relatively large prepolarizing field applied before each signal acquisition sequence to increase the signal induces currents in the walls of the surrounding conductive shielded room. The magnetic field transient generated by the eddy currents may cause severe image distortions and signal loss, especially with the large prepolarizing coils designed for in vivo imaging. We derive a theory of eddy currents in thin conducting structures and enclosures to provide intuitive understanding and efficient computations. We present detailed measurements of the eddy current patterns and their time evolution in a previous generation shielded room. The analysis led to the design and construction of a new shielded room with symmetrically placed 1.6 mm thick aluminum sheets that were weakly coupled electrically. The currents flowing around the entire room were heavily damped, resulting in a decay time constant of about 6 ms for both the measured and computed field transients. The measured eddy current vector maps were in excellent agreement with predictions based on the theory, suggesting that both the experimental methods and the theory were successful and could be applied to a wide variety of thin conducting structures.
Journal of applied physics