Importance

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Abstract

An MRI scanner generates a temporally constant (B.sub.0) magnetic field through an examination region (10), as well as a surrounding fringe field. The fringe field tends to decrease in strength with distance from the examination region and includes a 5 Gauss line (70, 70') and a 1 Gauss line (74). In a vertical field magnet, the 5 Gauss line can extend more than 3 or 4 meters above and below upper and lower pole assemblies (12, 14). By placing permanent magnets (70, 76) above and below the upper and lower pole assemblies, respectively, with an opposing magnetic polarity, the fringe field is shaped and controlled reducing a distance (d) of the 5 Gauss line above the scanner to about 2 meters and reducing an amount of ferrous material in a ferrous flux return path (24).

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First Claim