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Top: rf coil (black) on , metallic tube as passive shield (cut away) on , and -gradient coil (gray) on . Bottom: calculated flow lines for (solid) and (dashed) induced on tube.
Construction of the Faraday bracelet. The full shield (50.0 cm long) was sequentially segmented along incisions A, B, C, and D (i.e., the flow lines of in Fig. 1) and then shortened (to 29.4 cm) by excision of end pieces E.
Temporal evolution of gradient fields in the ROI. In order of decreasing rise time, curves correspond to the full shield; the shield following modifications A, B, and C; and no shield. Results following modifications D and E (not shown) exhibit no appreciable change relative to C.
Magnitude of the rf field 1 cm outside the shield. Left: rf field through the full shield as a function of frequency. Right: rf field as a function of the intentional azimuthal misalignment of the Faraday bracelet. Horizontal (vertical) dashed lines indicate corresponding magnitudes (frequencies) from full-shield measurements.
Frequency dependence of the magnitude (left) and phase (right) of the rf field at . The dashed line gives the theoretical shielding efficiency expected from Eq. (21) of Ref. 7. The upper abscissa ( and ) highlight the condition for the onset of shielding (Ref. 11).
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