Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1995-01-10
1996-06-25
O'Shea, Sandra L.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324322, 335299, 336225, G01V 314, H01F 502, H01F 2728
Patent
active
055303552
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The field of this invention is electromagnetic coils for the purpose of efficiently generating transverse gradients, especially in magnetic resonance imaging (MRI) and other gradient techniques employing a superconducting magnet.
BACKGROUND OF THE INVENTION
A large number of gradient-based MRI techniques have been developed since its independent invention by Paul Lauterbur and Peter Mansfield in 1973, following the invention of the static technique of Damadian in 1972. Most modern MRI systems utilize a superconducting solenoid to establish a uniform B.sub.0 (or B.sub.Z) over the imaging volume. This results in the magnetic field being colinear with the path available for sample access. Coils are then required to produce monotonic (preferably linear) gradients in B.sub.Z with respect to x, y, and z over the sample region during precisely determined pulse sequences. The transverse gradients (.delta.B.sub.Z /.delta.x, .delta.B.sub.Z /.delta.y) in the prior art have generally been established by symmetrically located sets of saddle coils, similar to those first described by Golay in U.S. Pat. No. 3,569,823 or by related planar coils as disclosed by Roemer, U.S. Pat. No. 4,926,125 and Morich et al., U.S. Pat. No. 5,036,282. Maxwell pairs are universally used to generate the axial gradient, notwithstanding the incorrect usage of the word "toroidal" by Frese and Siebold in U.S. Pat. No. 4,468,622. The instant invention achieves order-of-magnitude improvements in several critical parameters for transverse gradient coils: acoustic noise, DC gradient efficiency, and high-speed switching efficiency.
The closest prior art to the instant invention, in terms of magnetic field configuration, appears to be the trapezium loops for use with an electromagnet, as disclosed in the article "Magnetic Field Gradient Coils for NMR Imaging" by Bangert and Mansfield in Journal Physics, E, 15, 235 (1982). The semi-cylindrical concept depicted by Mansfield in U.S. Pat. No. 4,165,479, while having some similar features, is not closely related, as its major field component is orthogonal to B.sub.Z.
The gradient pulses induce eddy currents and vibrations in nearby conducting structures (especially in flimsy shields, in the cryostat, and in light-weight rf coils) which perturb the field homogeneity following the pulses with time and spatial dependencies that are not easily characterized. Active shielding coils, were first publicly disclosed by Mansfield in 2/1986 at approximately the same time that Roemer filed the patent application which resulted in U.S. Pat. No. 4,737,716. Prior independent work was underway at Doty Scientific, who shipped the first such commercially available coils in 1/1987. FIG. 1 approximately depicts typical shielded Golay coils to generate .delta.B.sub.Z /.delta.x in the sample in the vicinity of the origin. A similar set of concentric coils, rotated 90.degree., is used to generate .delta.B.sub.Z /.delta.y. FIG. 2 shows second-order shielding of the Maxwell pair, or anti-Helmholtz coils, as used to generate linear .delta.B.sub.Z /.delta.z near the origin. Gradient coils 201, 202 at mean location z.apprxeq..+-.r.sub.f have about 2.5 times the amp-turns of shield coils 203, 204 when s.apprxeq.0.3 r.sub.f. Axial shield coils 205, 206 have about one-tenth the amp-turns of the gradient coils. Gradient linearity of .+-.20% is achieved over a sphere of radius 0.7 r.sub.f, and leakage flux through a cylinder of radius 1.4 r.sub.f is reduced by a order of magnitude compared to the unshielded case. Higher-order shielding achieves another order-of-magnitude reduction in leakage, but shielding techniques have never fully measured up to original expectations because of motion-related artifacts, especially ghosting in the phase-encoding direction and battle-zone levels of acoustic noise.
Recovery time is often found to increase quadratically with pulse amplitude, indicating it is related more to motion than to eddy currents. It is in fact arguable that eddy currents per se are no longer a significant
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Doty Scientific Inc.
O'Shea Sandra L.
Phillips Roger
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