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Fig. 8 —Physical basis of radiofrequency shielding. In step A, rapidly
varying magnetic field (black arrows) induces a circulating electric
field (white arrows). In presence of a conductive medium (step B),
circulating electric field leads to a circulating current (gray
arrows). In step C, circulating current acts as electromagnet to
produce magnetic field in opposite direction; and in step D, amplitude of
overall magnetic field is reduced. Note that steps B and C require a
conductive medium and that effect is stronger in more conductive medium. In
imaging, this effect can be noticed in patients with ascites or fetal imaging,
in which circulating currents can be established in relatively large regions
of highly conductive fluid. Resulting artifacts are generally more visible in
sequences that use a large number of radiofrequency pulses to generate
contrast.
