Reduction of eddy currents in fingerprint gradient coils for MRI

Abstract: A magnetic resonance imaging apparatus includes a main field magnet (12) for generating a temporally constant magnetic field through an examination region. A radio frequency transmitter (84) excites and manipulates magnetic resonance in selected dipoles in the examination region. A receiver (90) demodulates magnetic resonance signals received from the examination region, a processor (74) reconstructs the demodulated resonance signals into an image representation. A plurality of fingerprint gradient magnetic field coils (24, 26) induce gradient magnetic fields across the temporally constant magnetic field. Each of the fingerprint gradient coils (24, 26) includes a generally spiral winding (32A-32D) having a first cross-sectional dimension perpendicular to the temporally constant magnetic field which is at least twice a second cross-sectional dimension.

Description: The invention relates to a magnetic resonance apparatus including a main field magnet for generating a temporally constant magnetic field through an examination region, a radio frequency transmitter for exciting and manipulating magnetic resonance in selected dipoles in the examination region, a receiver for demodulating magnetic resonance signals received from the examination region, a processor for reconstructing the demodulated resonance signals into an image or spectroscopic representation, and a gradient coil assembly including a cylindrical former having a plurality of fingerprint gradient magnetic field coils mounted thereto for inducing gradient magnetic fields across the temporally constant magnetic field, the cylindrical former being positioned within a bore of the magnetic resonance apparatus, wherein a central longitudinal axis of the cylindrical former is parallel to a central axis of the bore, and wherein a generally spiral winding of each fingerprint gradient coil comprises a plurality of radially oriented conductive layers laminated together, each conductive layer having a height cross-sectional dimension in a radial direction relative to the central axis of the cylindrical former, which height dimension is at least twice the width cross-sectional dimension which is defined perpendicular to the height dimension and perpendicular to the spiraling path followed by the winding. In magnetic resonance imaging applications, a 3-axis gradient coil set is used to provide spatial encoding of an NMR signal. Traditionally, X and Y-axis gradient coils are etched or machined from 1-2 mm thick copper sheet. These gradient coils are generally spiral in pattern, and are often referred to as "fingerprint" coils. The width of the individual windings that form the coils can vary, but is typically made to be as wide as possible so as to reduce the total resistance of the coil, thereby reducing thermal problems. Unfortunately, the wide windings support intra-coil eddy currents which cause ghosting and other artifact problems. U.S. Patent No. 5,177,442 of Roemer and U.S. Patent No. 4,840,700 of Edelstein describe gradient coils for use in magnetic resonance imaging applications. In particular, Roemer describes how the fingerprint pattern is mathematically generated. In their patents, Roemer and Edelstein specify a constant width of conductor material to be removed, thus generating the coil pattern. The disadvantage of the gradient sheet coils disclosed by Edelstein or Roemer is that no attempt is made to control internal or intra-coil eddy currents. Thus, using their methods, the eddy currents are reduced by reducing the total thickness of the copper. Although this is somewhat effective, a reduction in copper thickness causes a corresponding linear increase in the resistance of the coil. With modern day high current/high duty cycle scans, thin copper sheet coils generate an unacceptable amount of heat. U.S. Patent No. 5,424,643 of Morich, Patrick and DeMeester describe gradient sheet coils which minimize intra-coil eddy currents and heating by varying the width of the cut lines. In areas where the turns are bunched together, narrow cut lines are used to maximize the conductor widths. In other areas, two cut lines are used so that the turns are no wider than a specified maximum width. The islands defined by the two cut lines are then either segmented or removed to reduce eddy currents. The proposal of Morich, Patrick and DeMeester is advantageous over that of Edelstein and Roemer in that heating is reduced in the high current density areas, heating is made more uniform in the low current density areas, and the power loss of the coil is reduced. Although successful in reducing eddy currents, the Morich, Patrick and DeMeester method does not eliminate intra-coil eddy currents completely. JP-A-04180732 discloses a magnetic resonance apparatus of the general kind mentioned in the opening paragraph but with planarly arranged gradient coils. The gradient magnetic field coils of this apparatus comprise windings composed of a plurality of copper wires fixed into a groove provided on an electric insulating member. The individual copper wires each have a width within the range from 0,8 - 1,6 mm and a height within the range from 3 - 6 mm. As a result the electric resistance of the gradient magnetic field coil is low. An object of the present invention is to provide a magnetic resonance apparatus of the kind mentioned in the opening paragraph in which the intra-coil eddy currents in insert gradient coils are reduced, and in which the interception of flux lines from a gradient field is minimized. In order to achieve this object a magnetic resonance apparatus in accordance with the invention comprises conductive layers which are insulated from each other. Apparatus for magnetic resonance imaging in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic illustration of a magnetic resonance imaging system in accordance with the present invention; Fig. 2 is a diagrammatic illustration of an exemplary X- or Y-gradient coil laid out flat; Fig. 3 is a sectional view through an example of a X- or Y-gradient coil, not covered by the invention, taken along the line 3-3 in Fig. 2; and Fig. 4 is a sectional view through an embodiment of the X- or Y-gradient coil covered by the invention, taken along the line 3-3 in Fig. 2. With reference to Figure 1, a magnet assembly 10 generates a temporally constant magnetic field through an examination region. In the preferred embodiment, the magnet 10 is superconducting and has toroidal coils 12 disposed in a vacuum dewar 14. The examination region is defined in a central patient receiving bore 16 of the vacuum dewar 14. A circularly-cylindrical, whole-body gradient and radio frequency (RF) coil assembly 20 extends peripherally around the patient receiving bore 16. An insertable gradient and radio frequency (RF) coil assembly 22 is received within the bore 16 . The insert gradient coil assembly 22 includes a cylindrical former having an X-gradient coil 24 , Y-gradient coil 26 , Z-gradient coil 28 , and RF coil 30 mounted thereto. The former is formed into the shape of a cylinder from a dielectric material, such as epoxy. When inserted in the patient receiving bore 16 , a central or longitudinal axis of the cylindrical former is parallel with a central axis of the patient receiving bore 16. With reference to Figure 2, the X and Y-gradient coils 24 , 26 each include four (4) electric conductor quadrant windings 32A-32D having a fingerprint pattern as shown in Figure 2. The X and Y-gradient coils 24 , 26 are substantially equivalent to each other, but one of the gradient coils 24, 26 is circumferentially offset from the other gradient coil by approximately 90° relative to the central axis of the former. That is, the four quadrant winding construction of one of the gradient coils, (e.g. the X-gradient coil 24 ) , is mounted around an RF shield (not shown) and the Z-gradient coil 28 so that the edges 34 thereof are positioned adjacent to each other around the former. For the X-gradient coil, the abutting edges 34 and abutting edges 36 lie in a vertical plane, i.e., the (Y,Z) plane. The resulting assembly is potted in epoxy or other dielectric material. The four quadrant winding construction of the other gradient coil, e.g. the Y-gradient coil 26, is then mounted around the X-gradient coil 24 so that (1) the edges 34 thereof are positioned adjacent each other, and (2) the Y-gradient coil 26 is circumferentially offset from the X-gradient coil 24 by 90 degrees. For the Y-gradient coil, the edges 34 and 36 lie in a horizontal plane, i.e., the (X, Z) plane. The resulting assembly is potted in epoxy or other dielectric material. The exemplary X- and Y-gradient coils shown in cross-section in Figure 3 are not covered by the present invention but provide the same advantages as the X- and Y-gradient coils of the invention shown in Figure 4. Figure 3 is only used to form a reference for the description of Figure 4. With reference to Figure 3, the X- and Y-gradient coils 24, 26 are constructed with electrical conductor windings 32A-32D each having a cross-sectional profile which is substantially higher than it is wide. More particularly, the electrical conductor windings 32A-32D have a cross-sectional profile wherein the height (H1) of the electrical conductor windings 32A-32D is at least twice as great as the width (W1) of the electrical conductor windings 32A-32D, i.e. the electrical conductor windings 32A-32D satisfy: H x >=2W x In the example being described, the electrical conductor windings 32A-32D have a width (W 1 ) in the range of about 2.5 - 3.5 mm, and preferably about 3.0 mm. Depending upon the desired coil resistance and current, the electrical conductor windings 32A-32D have a height (H 1 ) in the range of about 5.5 - 6.5 mm, and preferably about 6.0 mm. The electrical conductor windings 32A-32D are oriented such that the height (H 1 ) dimension of the windings 32A-32D extends radially outwardly from the central axis of the former (and the central Z axis of the bore 16 when the insertable gradient and RF coil assembly 22 is positioned in the bore 16 ). Thus, the height (H 1 ) of the electrical conductor windings 32A-32D is oriented in a direction radially outwardly from the central axis of the former. The width (W 1 ) of the electrical conductor windings 32A-32D is defined parallel to the cylindrical surface of the former perpendicular to the spiralling path followed by the windings, that is, the width (W 1 ) is oriented in a direction parallel to the central axis of the former where the sectional view 3-3 is taken. It should be appreciated that intra-coil eddy currents are inherently non-uniform in space because gradient sheet coils typically do not have symmetrical windings (i.e., windings at the centre of the thumb print tend to bunched together while the outer windings are spaced apart). Eddy currents which are spatially dependent cause a significant problem for NMR imaging applications because eddy current compensation can only be performed at one point in space. The problem is compounded when a small diameter insert gradient coil is employed. In such cases, the imaging volume is much closer to the gradient sheet coils. Thus, the effects of the intra-coil eddy currents are much more apparent. The intra-coil eddy current problem is compounded by the use of wider copper electrical conductor windings which more readily support intra-coil eddy currents than narrow copper windings. The eddy current loops are primarily induced in planes tangential to the surface of the cylindrical former. However, wider electrical conductor windings advantageously provide reduced heat generation during high current/high duty cycle scans. Narrowing the width (W 1 ) of the electrical conductor windings 32A-32D reduces the interception of flux lines from the gradient field and the windings inherent ability to support eddy currents, thereby reducing the intensity of the induced intra-winding eddy currents. Increasing the height (H 1 ) of the electrical conductor windings 32A-32D reduces the resistance of the electrical conductor windings 32A-32D thus reducing the amount of heat generated during high current/high duty cycle scans. The electrical conductor windings 32A-32D are formed by cutting or machining away conductor material from 6.0 mm or thicker copper plating to form the windings 32A-32D . The electrical conductor windings 32A-32D can also be formed by milling, etching, punching, abrasive water jet cutting, etc. of copper plating. The fingerprint pattern can also be formed by bending copper strips, filling a pattern, building up multiple layers, wrapping solid or stranded wire about a form, or the like. Referring now to Figure 4, electrical conductor windings 32A'-32D' are formed by vertically laminating together several radially oriented layers of conductive material 40 . Each layer of conductive material 40 satisfies equation (1) above (e.g. H x ≥ 2W x ). The height (H 2 ) of each laminate or layer of conductive material 40 is preferably about 6.0 mm. The width (W 2 ) of each layer 40 is preferably about 3.0 mm. The ratio of height (H 3 ) to width (W 3 ) of the resulting laminated conductor windings 32A'-32D' is not critical. That is, the height (H 3 ) and width (W 3 ) of the resulting laminated conductor windings 32A'-32D' does not have to satisfy equation (1) above. The resulting laminated conductor windings 32A'-32D' may have a cross-sectional profile wherein the height (H 3 ) of the resulting laminated conductor windings is less that the width (W 3 ) of the resulting laminated conductor windings, i.e. the resulting laminated conductor windings satisfy: H x < W x The laminated conductor windings 32A'-32D' are formed by insulating adjacent layers or strips of thin conductive material (e.g. copper foil) from each other so that current flow between each of the conductive layers 40 forming the laminated conductor windings 32A'-32D' is minimised. The conductive layers 40 are joined together at each end of the respective laminated conductor windings 32A'-32D' to allow gradient current to flow through all of the conductive layers forming the laminated conductor windings 32A'-32D'. For a given conductor profile (W to H ratio), the decrease in intra-coil eddy current levels over the non-laminated electrical conductor windings 32A-32D is proportional to the thickness and to the number of laminate layers 40 forming the conductor windings 32A'-32D' . The electrical conductor windings 32A'-32D' are oriented so that the windings 32A'-32D' extend radially outwardly from a central longitudinal axis of the bore 16 . That is, the height (H 2 ) of the layers 40 forming the electrical conductor windings 32A'-32D' is oriented in a direction radially outwardly from the central axis of the former (and the central Z axis of the bore 16 when the insertable gradient and RF coil assembly 22 is positioned in the bore 16 ). The width (W 2 ) of the layers 40 forming the electrical conductor windings 32A'-32D' is oriented in a direction parallel to the central axis of the former where the sectional view 3-3 is taken. Thus, the electrical conductor windings 32A-32D, 32A'-32D' are provided with a sectional profile which reduces electrical conductor winding resistance. It should be appreciated that electrical conductor winding resistance disadvantageously facilitates the generation of I 2 R heat. The sectional profile of the electrical conductor windings 32A-32D, 32A'-32D' also reduces-the generation of deleterious intra-coil eddy currents which are responsible for undesirable imaging performance characteristics such as low slew rates. The reduction of intra-coil eddy currents is attributable to providing a conductor profile which is about 3.0 mm in width (W), and to providing an enlarged gap between adjacent electrical conductor windings 32A-32D, 32A'-32D' . The reduction of intra-coil eddy currents is also attributable to providing an electrical conductor comprising a number of layers or strands of conductive material which are laminated together so as to orient a height (H) dimension in a radial direction. With reference again to Figure 1, in operation, an operator using a keyboard 70 and/or a pointing device 72, such as a mouse, controls a workstation computer 74 . The computer 74 controls a magnetic resonance sequence controller 76 which controls the implementation of a selected one of a multiplicity of magnetic resonance imaging sequences. The sequence controller 76 causes a gradient coil control 78 to control an array of gradient or current amplifiers 80. The amplifiers 80 supply appropriate current pulses to either the X, Y, and Z-gradient coils 24-28 of the insertable gradient and RF coil assembly 22 or to the gradient coils of the whole-body gradient and RF coil assembly 20. A switch 82 selectively disconnects the current amplifiers 80 from the whole-body gradient and RF coil assembly 20 when the insertable gradient and RF coil assembly 22 is inserted into the bore 16. The X, Y, and Z-gradient coils 24-28 of the insert gradient coil assembly 22 or the whole body gradient coil assembly 20 generate magnetic field gradient pulses of the selected magnetic resonance sequence in response to current pulses received from the amplifiers 80. A digital transmitter 84 generates radio frequency pulses under the control of the sequence controller 76 to cause either the RF coil associated with the whole-body gradient and RF coil assembly 20 , or the RF coil 30 associated with the insertable gradient and RF coil assembly 22 to emit radio frequency magnetic resonance excitation and manipulation signals. In particular, the transmitter 84 generates radio frequency signals which are conveyed by a switch 86 through a quadrature splitter/combiner 88 to one of (1) the RF coil associated with the whole-body gradient and RF coil assembly 20 and (2) the insert RF coil 30 . The switch 86 also connects the selected one of the RF coils with a receiver 90 to supply received magnetic resonance signals thereto. The receiver 90 , preferably a digital receiver, demodulates the magnetic resonance signals emanating from the examination region. An analog-to-digital (A/D) converter 92 generates digital, raw, complex magnetic resonance signals. An array processor (2DFT) 94 performs a two or three-dimensional inverse Fourier transform on the digital resonance signals, thus generating a two-dimensional complex image. The complex image has an array or grid of complex vector data values. Each data value has a magnitude value and a phase angle value, i.e. real and imaginary components for each pixel or voxel of the field of view which is loaded into an image memory 96 . An operator using the keyboard 70 and mouse 72 controls the computer 74 which causes a video board 98 to withdraw selected images, such as magnitude images, planes of image data, three dimensional renderings, create cut plane images, and the like, for human-readable display on a video monitor 100 .

Claims: Magnetic resonance apparatus (10) including a main field magnet (12) for generating a temporally constant magnetic field through an examination region, a radio frequency transmitter (84) for exciting and manipulating magnetic resonance in selected dipoles in the examination region, a receiver (90) for demodulating magnetic resonance signals received from the examination region, a processor (94) for reconstructing the demodulated resonance signals into an image or spectroscopic representation, and a gradient coil assembly (22) including a cylindrical former having a plurality of fingerprint gradient magnetic field coils (24, 26) mounted thereto for inducing gradient magnetic fields across the temporally constant magnetic field, the cylindrical former being positioned within a bore (16) of the magnetic resonance apparatus (10), wherein a central longitudinal axis of the cylindrical former is parallel to a central axis of the bore (16), and wherein a generally spiral winding (32A'-32D') of each fingerprint gradient coil (24, 26) comprises a plurality of radially oriented conductive layers (40) laminated together, each conductive layer having a height cross-sectional dimension (H 2 ) in a radial direction relative to the central axis of the cylindrical former, which height dimension is at least twice the width cross-sectional dimension (W 2 ) which is defined perpendicular to the height dimension and perpendicular to the spiraling path followed by the winding (32A'-32D'), the conductive layers (40) of the winding being insulated from each other. Apparatus as claimed in claim 1, characterized in that an X-gradient magnetic field coil (24) is mounted to the cylindrical former around a Z-gradient magnetic field coil (28), and a Y-gradient magnetic field coil (26) is mounted to the cylindrical former around the X-gradient coil (24) and is circumferentially offset there from. Apparatus as claimed in any one of the preceding claims, characterized in that the height cross-sectional dimension (H 1 , H 2 ) is approximately 6.0 mm and the width cross-sectional dimension (W 1 , W 2 ) is approximately 3.0 mm.

Inventors: LAMPMAN DAVID A, MASTANDREA JR NICHOLAS J, RICHARD MARK A, LAMPMAN, DAVID A., MASTANDREA JR.,NICHOLAS J., RICHARD, MARK A.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1998
Main classification: G01R33
IPC codes: G01R33