Asymmetric gradient coils for magnetic resonance imaging

Abstract: A magnetic resonance imaging apparatus includes main field coils (10) for generating a temporally uniform magnetic field longitudinally through a central bore (12). A whole body gradient magnetic field coil (30) and radio frequency coil (36) are disposed around the bore. An insertable coil assembly (40) includes an insertable gradient coil (42) and a radio frequency coil (44). The insertable coil assembly places the gradient and radio frequency coil significantly closer to the region of interest to be imaged than the whole body coils. In one embodiment (FIGURE 2), the gradient coil assembly is a cylindrical coil assembly including x, y, and z-gradient coils which receive current pulses in order to generate linear, uniform, magnetic field gradients through a selected region adjacent a patient end of the gradient coil. Although the asymmetric coil design promotes linearity of the magnetic field gradients and improves patient access, the currents in the asymmetric coil interact with the main magnetic field to generate a net torque. Additional windings are added on the service end, opposite to the patient end, of the gradient coil to (1) contribute to the linearity and uniformity of the gradient magnetic field and (2) to generate counteracting torques. In another embodiment (Figure 4) planar gradient coils (80, 82) each tend to suffer a net torque due to the interaction of the current pulses flowing in each and the main magnetic field. Additional windings (92, 94) are incorporated in the planar gradient coils to zero the net torque in each. By eliminating the net torque, strong, rigid mechanical supports become unnecessary and coil vibration and the inherent image artifacts attributable to the vibration are eliminated.

Description: The invention relates to a magnetic field gradient coil assembly for disposition within the bore of a magnetic resonance apparatus for generating magnetic field gradients across a selected portion of a subject within a main magnetic field in the bore, the gradient coil assembly comprising: a cylindrical dielectric former, a first pair of distributed coil winding portions mounted opposite each other adjacent a first end of the dielectric former, the first pair of distributed coil winding portions receiving current pulses to induce first magnetic field gradient pulses across the selected portion of the subject displaced from a geometric centre of the cylindrical former toward the first end, which current pulses interact with the main magnetic field to cause a first torque on the cylindrical dielectric former about the geometric centre of the former; and a second pair of distributed coil winding portions mounted opposite each other on the cylindrical dielectric former adjacent the first pair of distributed coil winding portions, the second pair of distributed coil winding portions receiving current pulses to generate second magnetic field gradient pulses across the selected subject portion, the first and second magnetic field gradient pulses combining to generate a linear and uniform magnetic field gradient displaced from the geometric centre of the cylindrical former toward the first end, which current pulses applied to the second pair of distributed coil winding portions interact with the main magnetic field to generate a second torque on the cylindrical dielectric former about the geometric centre of the former, said second torque being equal and opposite to said first torque, the first and second torques thereby cancelling each other, wherein one coil winding portion of said first pair and one coil winding portion of said second pair are curved around one side of said cylindrical dielectric former and together comprise one half of an x or y gradient coil, and the other coil winding portion of said first pair and the other coil winding portion of said second pair are curved around the 180 degrees opposite side of said cylindrical dielectric former and together comprise the other mirror image half of said x or y gradient coil, each said half x or y gradient coil comprising a region of positive current distribution and an adjacent region of negative current distribution, in each half x or y gradient coil said regions meeting at a plane perpendicular to the axis of said cylindrical former. Magnetic resonance imaging apparatus commonly include a large diameter, whole body gradient coil which surrounds a patient receiving bore. Main field magnets, either superconducting or resistive, and radio frequency transmission/reception coils also surround the bore. Although the whole body gradient coils produce excellent linear magnetic field gradients, they have several drawbacks. With large diameter gradient coils, the slew rate is sufficiently slow that it is a limiting factor on the rate at which gradient magnetic fields can be induced and changed. Large diameter whole body gradient coils have relatively low gradient field per unit drive ampere for a given inductance, which limits their use for some of the highest speed magnetic resonance imaging techniques. The energy stored in gradient coils is generally proportional to greater than the fifth power of the radius. Hence, large diameter, whole body coils require large amounts of energy. Further, superconducting main magnets have cold shields disposed around the bore. The larger the diameter of the gradient coil, the closer it is to the cold shields and hence the more apt it is to produce eddy currents. More shielding is needed to prevent the whole body gradient coils from inducing eddy currents in the cold shields than would be necessary for smaller diameter coils. Due to these and other limitations in whole body coils, numerous insertable coils have been developed which are small enough to fit within the bore with the patient. Typically, the insertable coils are customized to a specific region of the body, such as a head coil, or a cardiac coil. Traditionally, head coils have been a cylinder sized to accommodate the human head easily, e.g. 28 cm in diameter, while cardiac coils have been biplanar sized to accommodate the human torso. Most brain examinations center around the portion of the brain that is substantially in the same plane as the eye sockets. In a symmetric coil, the magnetic and physical isocenters are both configured to be disposed in a common plane with the patient's eyes, or patient's heart. As a general rule, the longer the cylindrical head coil, the larger the region over which the gradient is linear and the more linear the region is. However, the patient's shoulders are a limiting factor on the length of a symmetric gradient coil. The shoulders limit the isocenter to about 20 cm at the patient end. Thus, symmetric head coils have heretofore been limited to about 40 cm in length. In order to achieve the beneficial effects of a longer head coil, head coils have been designed in which the magnetic isocenter is offset toward the patient from the physical geometric center of the coil. See, for example, U.S. Patent No. 5,278,504 of Patrick, et al. or U.S. Patent No. 5,177,442 of Roemer, et al. Although asymmetric head coils have beneficial effects on the linearity and the size of the linear region, the improvement is not without an offsetting difficulty. Within the main magnetic field, the asymmetric gradient coil is subject to mechanical torques from the main and gradient magnetic field interactions. In order to counteract these torques, the asymmetric head coils are mounted with rigid mechanical constraints. Even with substantial mechanical structures anchored to the main field magnet assembly, the torque still tends to cause at least mechanical vibration and noise. Not all insertable gradient coils are cylindrical. Some, for example, as mentioned previously, are planar or biplanar. See, for example, U.S. Patent No. 5,036,282 of Morich, et al. Planar and biplanar gradient coils also suffer from mechanical torques. Again, rigid mechanical constraints were used, but were generally insufficient to remove all mechanical vibration and noise. US-A-5,278,504 discloses a magnetic field gradient coil assembly having an examination region which is defined along a z-axis offset from a geometric center of the gradient coil assembly. Cylinders of a non-conductive, non-magnetic material support x, y, and z-gradient coils for causing orthogonal magnetic field gradients through the offset examination region. The z-gradient coil includes a plurality of distributed loop arrays with a winding pattern selected to cause a region of linear magnetic field gradients in the z-direction in a region offset toward a cylinder first end. The x- and y-gradient coils each include two pairs of oppositely disposed windings which include a pair of inner spirals offset towards the first end of the cylinder and an outer spiral extending therearound. The outer spiral bows in toward the cylinder first end and fans out toward the second end. In this manner, the x, y, and z-gradient coils create linear magnetic field gradients along x, y, and z-directions which have a region of primary linearity offset from their geometric center. In this manner the axial shifting of the linear volume of the coil assembly is facilitated, even on smaller diameter coils. A magnetic field gradient coil assembly of the kind mentioned in the opening paragraph is known from the article "Torque Free Asymmetric Gradient Coils for Echo Planar Imaging", published in Magnetic Resonance in Medicine, vol. 31, no. 4, April 1994, pages 450 - 453. In this article, a torque-free asymmetric gradient coil is presented that is capable of generating high quality axial, sagittal, and coronal echo planar images of the human head. The gradient coil has excellent shoulder-to-coil clearance and, as such, has the advantage of providing good patient access to the linear portion of the gradient while minimizing patient discomfort from claustrophobia. The net torque of the gradient coil is balanced by adding a second coil set in series with the asymmetric gradient in such a way that current flowing in the second coil set opposes the torque produced by the gradients allowing the net torque to be minimized. The present invention provides a magnetic field gradient coil assembly of the kind mentioned in the opening paragraph which overcomes the above-referenced problems. For this purpose a magnetic field gradient coil assembly according to the invention is characterized in that said plane, in which the regions of positive and negative current distribution of each half x or y gradient coil meet each other, is displaced from the geometric centre of the former towards said first end, the winding patterns of said x or y gradient coil being configured to optimise x or y gradient linearity in a generally spherical region centred on said plane. A magnetic field gradient coil assembly in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus comprising a magnetic field gradient coil assembly in accordance with the invention; Figure 2 is an enlarged diagrammatic illustration of the gradient coil assembly used in the apparatus of figure 1; Figure 3 is a diagrammatic illustration of the windings of an x-gradient coil for use in the apparatus of Figure 1; Referring to FIGURE 1, the apparatus includes a plurality of primary magnet coils 10 which generate a temporally constant magnetic field along a longitudinal or z-axis of a central bore 12. In a preferred superconducting embodiment, the primary magnet coils 10 are supported by a former 14 and received in a toroidal helium vessel or can 16. The vessel is filled with liquid helium to maintain the primary magnet coils 10 at superconducting temperatures. The can 16 is surrounded by a series of cold shields 18, 20 which are supported in a vacuum dewar 22. A whole body gradient coil assembly 30 is mounted along the bore 12. Preferably, the gradient coil assembly 30 is a self-shielded gradient coil assembly that includes primary x, y, and z-coil assemblies 32 potted in a dielectric former and a secondary gradient coil assembly 34 that is supported on a bore defining cylinder of the vacuum dewar 22. A whole body RF coil 36 is mounted inside the gradient coil assembly 30. A whole body RF shield 38, e.g. copper mesh, is mounted between RF coil 36 and the gradient coil assembly 34. An insertable gradient coil 40 is removably mounted in the center of the bore 12. The insertable coil assembly includes an insertable gradient coil assembly 42 supported by a dielectric former. An insertable RF coil 44 is mounted inside the dielectric former. An RF shield 46 is mounted between the insertable RF and gradient coils. An operator interface and control station 50 includes a human-readable display such as a video monitor 52 and an operator input means including a keyboard 54 and a mouse 56. A computer control and reconstruction module 58 includes computer hardware and software for controlling the radio frequency coils 36 and 44 and the gradient coils 30 and 42 to implement any of a multiplicity of conventional magnetic resonance imaging sequences, including echo-planar and echo-volume imaging sequences. Echo-planar imaging and echo-volume imaging sequences are characterized by short repetition rates and low flip angles. The processor 58 also includes a digital transmitter for providing RF excitation and resonance manipulation signals to the RF coils and a digital receiver for receiving and demodulating magnetic resonance signals. An array processor and associated software reconstruct the received magnetic resonance signals into an image representation which is stored in computer memory or on disk. A video processor selectively extracts portions of the stored reconstructed image representation and formats the data for display by the video monitor 52. In the embodiment of FIGURE 2, the active gradient coil windings of the insertable gradient coil assembly 40 are supported on a cylindrical, dielectric former 60. Preferably, the former is surrounded by an x-gradient coil 62, a y-gradient coil 64, and radially outermost, a z-gradient coil 66. The x, y, and z-gradient coils may be encased in an epoxy resin to become a unitary portion of the former or may be built on two or more discrete formers to provide cooling passages between the gradient coils. Preferably, the x, y, and z-gradient coils are distributed coils, although bunched windings may also be used. The distributed or fingerprint coils have an asymmetric winding pattern such that the sweet spot, i.e. the region of the coil with optimal linearity and uniformity, is disposed offset from the geometric center towards a patient end of the coil. The distributed winding pattern of FIGURE 3 is one half an x or y-gradient coil. The half is curved around one side of the dielectric cylinder and a mirror image half is mounted around the cylinder 180° opposite. A similar pair of gradient coil halves are mounted rotated 90° about the z-axis to function as the other of the x and y-gradient coils. The winding pattern is configured to optimize x or y-gradient linearity in a generally spherical region 68 within the coil centered on a plane 70. A further torque correcting winding pattern portion 72 is provided such that the generated magnetic field creates a zero net magnetic torque. That is, a primary winding pattern 74 and the torque neutralizing pattern 72 interact to (1) create a uniform gradient centered on the plane 70 and (2) create a zero net torque. Stated another way, the primary and torque balancing coil portions create equal and opposite torques relative to a geometric center point of the coil. Various techniques are contemplated for adjusting the relative torques. For example, the length of the gradient coil along the z-axis can be increased or decreased to modify the "lever arm" with which the torque correction coil acts. However, preferably the length of the z-gradient coil is fixed and the winding pattern of the primary and torque correction windings are designed with the length of the coil in the z-axis taken as a constraint. Asymmetrical cylindrical gradient coils have less stored energy and faster gradient rise times than their symmetrical cylindrical counterparts due in part to the lack of severe length constraints on the gradient coil structure associated with asymmetrical cylindrical gradient coils. The present technique eliminates the torque effects in an asymmetric cylindrical coil design. Due to the symmetry in the radial direction, it is the transverse gradient coil structures which experience torque effects. In particular, when symmetry in the longitudinal variable z is lost, a net torque results. Accordingly, the expressions of both components of the surface current density for the gradient coil are chosen such that they generate a transverse gradient field with the desired linearity and uniformity, yet eliminating the net torque which is generated by the interaction of the current density with the z-component of the main magnetic field of the magnetic resonance imager. For a finite size cylindrical coil, the expression for the two components of the current density are: Both quantities are zero for |z|>L/2 with k 1n =2mπ/L, k 2n =2(m-1)π/L, where L represents the total length of the cylinder. Expressing the magnetic field and the stored energy in terms of the current density j a and the Green function for cylindrical coordinates, the corresponding functional E has the form: where W is the stored energy, λ j are the Lagrange multipliers, B z ( r j ) is the calculated value of the magnetic field at the constraint points r j , B z sc( r j ) are the constraint values of the magnetic field at the constraint points. Minimizing E with respect to the current density, a matrix equation for j a 1n , and j a 2n , are obtained as follows: where: Determining the Lagrange multipliers λ j using the constraint equation for the magnetic field, the continuous current density distribution for the azimuthal J a  and the axial J a z components of the current density are obtained. By applying stream function techniques to the continuous current density, the positive and negative current patterns for the current density are generated for the creation of high quality transverse gradient fields. The magnetic field is reevaluated inside and outside the imaging volume by applying the Biot-Savart formula to the discrete current distribution. In FIGURE 3, the asymmetric cylindrical gradient coil has a positive current distribution 76 in which each of the 20 turns carries a current of 325 amps and a discrete negative current distribution 78 in which each of 17 turns carries 325 amps of current. Using constraints of an x-gradient coil which is 0.85 m long and 0.36 m in diameter and four constraint point to generate a 40 mT/m gradient field strength with 5% on-axis linearity and 10% off-axis uniformity at the borders of a 29 cm diameter spherical volume, one obtains the current distribution of FIGURE 3. Alternatively, the current patterns can be changed to produce either better linearity at the price of coil efficiency or greater coil efficiency at the price of linearity. Analogously, a greater offset of the center of the gradient imaging volume can be obtained at the price of linearity or efficiency or a combination of the two. Similar design constraints can be applied to actively shielded gradient coils such that eddy current effects are further eliminated. The active shield coils are designed using the above-described techniques.

Claims: A magnetic field gradient coil assembly (40) for disposition within the bore (12) of a magnetic resonance apparatus for generating magnetic field gradients across a selected portion of a subject within a main magnetic field in the bore (12), the gradient coil assembly (40) comprising: a cylindrical dielectric former (60); a first pair of distributed coil winding portions (74) mounted opposite each other adjacent a first end of the dielectric former (60), the first pair of distributed coil winding portions (74) receiving current pulses to induce first magnetic field gradient pulses across the selected portion of the subject displaced from a geometric centre of the cylindrical former (60) toward the first end, which current pulses interact with the main magnetic field to cause a first torque on the cylindrical dielectric former (60) about the geometric centre of the former (60); and a second pair of distributed coil winding portions (72) mounted opposite each other on the cylindrical dielectric former (60) adjacent the first pair of distributed coil winding portions (74), the second pair of distributed coil winding portions (72) receiving current pulses to generate second magnetic field gradient pulses across the selected subject portion, the first and second magnetic field gradient pulses combining to generate a linear and uniform magnetic field gradient displaced from the geometric centre of the cylindrical former (60) toward the first end, which current pulses applied to the second pair of distributed coil winding portions (72) interact with the main magnetic field to generate a second torque on the cylindrical dielectric former (60) about the geometric centre of the former (60), said second torque being equal and opposite to said first torque, the first and second torques thereby cancelling each other, wherein one coil winding portion of said first pair (74) and one coil winding portion of said second pair (72) are curved around one side of said cylindrical dielectric former (60) and together comprise one half of an x or y gradient coil, and the other coil winding portion of said first pair (74) and the other coil winding portion of said second pair (72) are curved around the 180 degrees opposite side of said cylindrical dielectric former (60) and together comprise the other mirror image half of said x or y gradient coil, each said half x or y gradient coil comprising a region (76) of positive current distribution and an adjacent region (78) of negative current distribution, in each half x or y gradient coil said regions (76, 78) meeting at a plane (70) perpendicular to the axis of said cylindrical former (60), characterized in that said plane (70) is displaced from the geometric centre of the former (60) towards said first end, the winding patterns of said x or y gradient coil being configured to optimise x or y gradient linearity in a generally spherical region (68) centred on said plane (70).

Inventors: MORICH MICHAEL A, PATRICK JOHN L, PETROPOULOS LABROS, MORICH, MICHAEL A., PATRICK, JOHN L., PETROPOULOS, LABROS
Applicants: PHILIPS MED SYST INC, PHILIPS MEDICAL SYSTEMS (CLEVELAND), INC.
Application year: 1995
Main classification: G01R33
IPC codes: G01R33, A61B5