Abstract: In a method for aligning a magnetic field-modifying structure (74) in a magnet bore (12) of a magnetic resonance imaging scanner (8), a reference magnetic field map of the magnet bore (12) is measured without the magnetic field-modifying structure (74) inserted. The magnetic field-modifying structure (74) is inserted into the magnet bore (12). A second magnetic field map of the magnetic bore (12) is measured with the magnetic field-modifying structure (74) inserted. At least one odd harmonic component of the first and second magnetic field maps is extracted. The magnetic field-modifying structure (74) is aligned in the magnet bore (12) based on a comparison of the odd harmonic component of the first and second magnetic field maps.

Abstract: In a method for aligning a magnetic field-modifying structure (74) in a magnet bore (12) of a magnetic resonance imaging scanner (8), a reference magnetic field map of the magnet bore (12) is measured without the magnetic field-modifying structure (74) inserted. The magnetic field-modifying structure (74) is inserted into the magnet bore (12). A second magnetic field map of the magnetic bore (12) is measured with the magnetic field-modifying structure (74) inserted. At least one odd harmonic component of the first and second magnetic field maps is extracted. The magnetic field-modifying structure (74) is aligned in the magnet bore (12) based on a comparison of the odd harmonic component of the first and second magnetic field maps.

Abstract: A pair of annular magnets (10) generate a vertical magnetic flux field through an imaging volume (12). The flux is focused by a pair of Rose rings (26) of high cobalt steel. A high order shim set includes a plurality of permanently magnetized or magnetized iron rings (32a, 32b, 32c, 32d) which cooperatively interact with the magnet assembly to optimize the homogeneity of the magnetic flux through the imaging volume. One of the permanent magnetic rings (32d), is mounted with an opposite polarity relative to the others. The magnetized rings are mounted in a non-ferrous, electrically insulating support structure (34) such that gradient coils (50) can be positioned behind the permanent magnet rings. A flux return path (14, 16, 18, 20, 22, 24) provides a low flux resistant path from adjacent the Rose ring at one side of the imaging volume remotely around the imaging volume to a position adjacent the Rose ring at the other side of the imaging volume.

Abstract: A region of interest of a subject (20) on a subject support (18) is positioned above a ferrous pedestal (16, 116) that is supported on a ferrous floor yoke portion (76, 176). A lower imaging coil assembly (50) including a lower gradient coil (52), a radio frequency coil (54), and a lower pole piece (58) are disposed between the pedestal and a region of interest of the subject. An upper imaging coil assembly (40) including an annular gradient coil (42, 142) and an upper annular pole piece (44, 144) is supported from a ceiling ferrous yoke member (74, 174). The upper imaging coil assembly is supported by supports (70, 170) which are moved by drives (72, 78, 172) to raise and lower the upper imaging coil assembly. A laser gauging system (80, 180) gauges the position of the upper imaging coil assembly such that, with a control circuit (82), the upper imaging coil assembly is accurately repositioned at preselected imaging positions.

Abstract: A magnetic resonance imaging scanner includes a pair of opposing pole pieces (20, 20') disposed symmetrically about an imaging volume (24) facing one another. The pair of opposing pole pieces (20, 20') includes a first ferrous pole piece (20) which has a front side (22) facing the imaging volume (24) and a back side (28). Also included is a second ferrous pole piece (20') which also has a front side (22) facing the imaging volume (24) and a back side (28). A magnetic flux return path (30) extends, remotely from the imaging volume (24), between a point adjacent the back side 28 of the first pole piece (20) and a point adjacent the back side 28' of the second pole piece (20)'. A source of magnetic flux generates a magnetic flux through the imaging volume (24), the pair of opposing pole pieces (20, 20'), and the magnetic flux return path (30). An array of annular hoops (40) and (40') are integrated with the pair of opposing pole pieces (20, 20') for homogenizing the magnetic flux through the imaging volume (24).

Abstract: A subject receiving bore (12) of a magnetic resonance apparatus has an axial length to diameter ratio of less than 1.75:1 and preferably about 1:1. The temporally constant magnetic field generated by superconducting magnets (10) surrounding the bore has various magnetic field harmonic distortions including a Z12 harmonic distortion which is described generally by the equation Z.sup.12. In long bore magnets with a length to diameter ratio of 2:1 or more, harmonic distortions above Z6 are not present, or if present, not measured. A magnetic field probe (80 ) measures the magnetic field in the bore at a sufficient number of axial locations to measure at least the Z12 harmonic, e.g., 24 axially displaced locations. The probes are sampled at a sufficient number of angular increments, e.g., 32-36, that any present transverse spherical harmonics are sampled. Ferrous shims (62) are mounted in the bore in the pockets (58) of shim trays (44) to compensate for the Z12 and other sampled harmonics above Z6.

Abstract: A subject receiving bore (12) of a magnetic resonance apparatus has an axial length to diameter ratio of less than 1.75:1 and preferably about 2:1. The temporally constant magnetic field generated by superconducting magnets (10) surrounding the bore have various magnetic field harmonic distortions generally in the Z1-Z18 range. Shim trays (80) are disposed longitudinally around the bore (12). Each shim tray contains a number of shim pockets (84) which receive ferrous shims for shimming the magnetic field in the bore (12). An initialization system (60) calibrates the initial magnetic field within the bore (12). An initial shim distribution is determined which shims the inhomogeneous magnetic field toward a target more homogeneous magnetic field. An optimization system (66) determines a residual magnetic field from a difference between the present inhomogeneous magnetic field of the bore and the target magnetic field.

Abstract: A magnet arrangement for producing a magnet field in a gap (3) between a pair of pole pieces (1), e.g. to provide the main magnetic field for application to a subject for magnetic resonance imaging, wherein adjacent the pole pieces the magnet core 15 has sections 17 of restricted cross-sectional area which serve to improve the homogeneity of the field in the gap.

Abstract: A coil arrangement for producing a magnetic field of high homogeneity, such as is required in magnetic resonance imaging, comprising a single pair of identical annular coils (17A, 17B) disposed coaxially in spaced relationship and a pair of annular members (19A, 19B) of ferromagnetic material disposed coaxially with the coils, symmetrically with respect to the plane which perpendicularly intersects the axis of the coils centrally between them. The coils and ferromagnetic members have dimensions and are relatively positioned so that, with the coils carrying equal energizing currents, the more significant spherical harmonic coefficients are eliminated.