Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-12-23
2001-11-20
Ptidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S322000
Reexamination Certificate
active
06320382
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to devices and methods for establishing magnetic field gradients, and in particular to etched Z-axis magnetic field gradient coils for magnetic resonance applications such as nuclear magnetic resonance spectroscopy and magnetic resonance imaging.
Nuclear Magnetic Resonance (NMR) and magnetic resonance imaging (MRI) devices often employ a superconducting magnet for generating a static magnetic field B
0
along the z-axis, and a special-purpose z-axis gradient coil for generating a magnetic field gradient along the z-axis. It is common notation to label the other two cartesian axes the x- and y-axes, and the angle measured from the x-axis in the x-y plane.
In one prior art approach, a z-axis gradient coil is made by winding wire coils on a cylindrical support, with a predetermined current distribution chosen to generate the desired magnetic field gradient. The predominant direction of the windings is then in the transverse (x-y) plane, with a relatively small component along the z-axis needed to make connections between adjacent turns. The magnetic field generated by the z-axis component of the windings can be canceled out by winding a second layer of turns in the same angular direction but opposite z-direction. While wire coils are capable of generating satisfactory magnetic field gradients, such coils are often relatively expensive to fabricate.
In U.S. Pat. No. 4,910,462, Roemer et al. describe a z-axis gradient coil made by an etched-circuit technique: a conductive pattern is etched onto a flat, flexible sheet and the sheet is rolled into a cylinder. In one suggested design approach (shown in FIG. 2 of Roemer et al.), the coil comprises a two-layer interconnector section that bends back sharply to lie along the cylindrical part of the coil. The sharp bend can generate stresses in the coil material.
Moreover, the interconnector section forms a bulge on the outside of the coil. In another design approach (illustrated in FIGS. 3, 3
a
and 3
b
) of Roemer et al.), a two-layer interconnector section lies along the cylindrical part of the coil, without bending sharply. In this approach, however, the z-axis current components in the two interconnector layers have unequal magnitudes, and unwanted current components in the interconnector section are not completely canceled.
In U.S. Pat. No. 5,623,208, Hasegawa describes a z-axis etched-circuit gradient coil formed by winding multiple turns of a flexible substrate on a cylindrical bobbin. As shown in FIG. 2 of Hasegawa, the coil includes a sharp 180° bend along a marginal interconnector region.
SUMMARY OF THE INVENTION
The present invention provides a magnetic field gradient coil for a magnetic resonance probe. The coil comprises an etched double-sided flexible sheet having a conductive pattern defined thereon. The conductive pattern comprises two subpatterns formed on opposite sides of the sheet. At least one via running through the sheet electrically interconnects the two conductive subpatterns.
In the preferred embodiment, the coil is a z-axis magnetic field gradient coil. The conductive pattern comprises transverse leads for generating the z-axis gradient, and longitudinal (z-axis) leads for interconnecting the transverse leads. Each z-axis current lead is adjacent to another z-axis current lead oriented in the opposite direction, such that the net magnetic effect of z-axis current flow through the conductive pattern in negligible. The use of a double-sided sheet with via interconnections allows canceling undesired z-axis currents without sharply bending the sheet.
REFERENCES:
patent: 4910462 (1990-03-01), Roemer et al.
patent: 5623208 (1997-04-01), Hasegawa
Berkowitz Edward H.
Ptidar Jay
Shrivastav Brij B.
Varian Inc.
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