RF shield for RF coil contained within gradient coils of NMR imaging device

DETAILED DESCRIPTION OF THE INVENTION Referring initially to FIG.
1, a radio-frequency (RF) shield 10, in accordance with the present invention is a hollow cylindrical conductive member situated between a RF whole body coil 11 and a set of magnetic field gradient coils 12, within the bore 14 of a magnet (not shown) in a nuclear magnetic resonance (NMR) imaging device.
By convention, the static main magnetic field B.
sub.
0 of the main NMR system magnet (produced by the magnetic means formed about bore 14) is aligned with the Z axis of a Cartesian coordinate system.
The RF coil 11 forms an RF magnetic field B.
sub.
1, within the coil 11 bore responsive to a RF signal The NMR significant part of the B.
sub.
1 field is transverse to field B.
sub.
0 and therefore is typically in the X-Y plane.
A significant RF magnetic field is also present outside of coil 11, and, as is well known in the art, will impinge upon gradient coils 12 unless the RF shield 10 is present and effectively operates as an RF short circuit.
Shield 10 must be substantially transparent to the magnetic fields from gradient coils 12.
allowing those gradient magnetic fields to enter into the bore of RF coil 11 and place spatial-encoding information upon the volume therein.
A typical whole body RF coil 11 is shown in FIG.
1a; this high-pass "birdcage" coil, formed upon a cylindrical base 11a of insulative material, comprises first and second spaced-apart end rings 11b-1 and 11b-2, each having a plurality of conductive segments (here, 8 segments) joined to one another by capacitive elements 16.
Each of the end rings 11b is thus substantially in the X-Y plane, as is the B.
sub.
1 RF field produced.
A like number of axial conductors 11c (here, eight conductors 11c-1 through 11c-8) are used.
Each conductor extends in the Z direction between one of the conductive segments of first end ring 11b-1 and a like-positioned position conductive segment of the second conductive end ring 11b-2.
Thus, a first elongated conductive element 11c-1 is positioned at an angle