The present invention provides an apparatus for reducing acoustic noise in
a magnetic resonance imaging device including passive shielding located
outside the actively shielded gradient winding elements in order to
reduce the magnitude of fields that spread outside the gradient coil
assembly in unwanted directions and interact with the magnet cryostat or
other metallic magnet parts, inducing eddy currents that cause consequent
acoustic noise. The passive shielding elements are conducting layers
located on the outer radius of the cylindrical gradient coil assembly in
a cylindrical magnet system, conducting layers located at the ends of the
gradient coil assembly in a cylindrical magnet system, and conducting
layers located inside the actively shielded gradient winding inner
elements in a cylindrical magnet system. The passive shielding could also
be located on separate structures that are vibrationally isolated from
the magnet cryostat. The actively shielded gradient winding can also be
extended to portions at the ends of the actively shielded gradient
winding and further to portions inside the inner radius of the inner
portion of the actively shielded gradient winding. The actively shielded
gradient windings and passive shielding should be designed concurrently
in order to substantially optimize the gradient linearity and reduce the
eddy currents generated in metallic parts of the magnetic resonance
imaging system.