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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.