title: Conductive shield for ultra-low-field magnetic resonance imaging: Theory and measurements of eddy currents.
abstract: 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 forin vivoimaging. 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.
authors.forename: Koos C J
authors.initials: KC
authors.lastname: Zevenhoven
authors.forename: Sarah
authors.initials: S
authors.lastname: Busch
authors.forename: Michael
authors.initials: M
authors.lastname: Hatridge
authors.forename: Fredrik
authors.initials: F
authors.lastname: Oisjöen
authors.forename: Risto J
authors.initials: RJ
authors.lastname: Ilmoniemi
authors.forename: John
authors.initials: J
authors.lastname: Clarke
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