Magnetic resonance apparatus

Abstract: In magnetic resonance apparatus harmonic distortion components above Z6 in the temporally constant magnetic field generated by magnets (10) surrounding the patient receiving bore (12) are reduced by shimming. A magnetic field probe (80) measures the magnetic field in the bore (12) at a sufficient number of axial locations to measure the harmonics, e.g., at 24 axially displaced locations so as to measure up to the Z12 harmonic. 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 pockets (58) of shim trays (44) to compensate for the Z12 and other sampled harmonics above Z6. The shim trays have a sufficient number of pockets axially along the bore to compensate for the Z12 harmonic, e.g., 24 pockets. Optionally, segmented ferrous rings (92) are mounted in the bore at locations theoretically calculated to minimize higher order harmonic distortion and the shim trays are used to adjust the correction. In a preferred embodiment, the shim trays and the segmented rings are mounted to a cylindrical former (34) that supports a primary gradient coil assembly (32) to position the ferrous shim material close to the patient receiving bore.

Description: This invention relates to magnetic resonance apparatus. It finds particular application in conjunction with magnetic resonance imaging apparatus having self-shielded gradient coil assemblies and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with other magnetic resonance apparatus in which a polarizing magnetic field or a magnetic field with a predetermined contour are advantageous. Magnetic resonance imagers commonly include a bore having a diameter of 90 cm or more for receiving the body of an examined patient. The bore is surrounded by a series of annular superconducting magnets for creating a substantially uniform magnetic field longitudinally through the patient receiving bore. The more axially spaced the annular magnets, i.e., the longer the magnet, the more uniform the primary magnetic field within the patient receiving bore tends to be and the longer the axial dimension over which such magnetic field uniformity exists. Typically, the length of the bore is about twice the diameter. Such "long" bore magnets are designed such that there is substantially no measurable degradation from harmonics below twelfth order, commonly denoted Z12. The lower order harmonics which occur as a result of construction inaccuracies are typically shimmed by placing pieces of iron in the bore. A single magnetic field sensor is moved manually to different locations including positions on 12 axial planes. In each plane the detector is rotated in 12 equal angular increments and the magnetic field is measured. In this manner, 12 measurements are made in each of 12 parallel planes on the surface of a spherical volume of interest centered at the isocenter of the magnet. Shim trays are removably mounted at a number of equiangular locations around the magnetic bore, e.g., 32 to 36. The shim trays each extend the length of the bore and have 12-14 pockets for receiving plates or shims of iron or steel. The shim trays are removed from the bore and the iron or steel shims are mounted in the pockets. The shim trays are replaced and the magnetic field is measured again to assure that the measured harmonics Z1→Z6 have been minimized. Typically, higher order harmonics require significantly more steel to effect a correction than do lower order harmonics. One of the disadvantages of long bore magnets is that the region of interest is often inaccessible to medical personnel. If a procedure is to be performed based on the image, the patient must be removed from the bore before the procedure can be performed. Moving the patient increases the risk of misregistration problems between the image and the patient. Other disadvantages of the long bore magnets is that they tend to be user-unfriendly and claustrophobic, larger magnets are more expensive than smaller magnets, and the like. One way to improve access to the patient is to shorten the length of the magnet and the patient receiving bore. If the magnet and the bore were shortened to about 1 meter or roughly the diameter of the patient bore, patient access would be much improved. Although the size of the uniform magnetic field compresses from a sphere toward a more disk-like shape, the area of substantial uniformity is still sufficient for a series of 10-20 contiguous slice images. However, shortening the bore of the magnet has its difficulties. First, the magnetic field tends to become non-uniform with the presence of higher order harmonics Z10, Z12, and the like. Second, the inventors herein have measured significant higher order harmonic distortions in such short bore magnets. The Z12 harmonic is found to be relatively strong. When monitoring harmonics with 12 probes, the 12 probes fall midway between Z12 harmonics. Because the 12 probes are measuring magnetic fields at the midpoint of the Z12 harmonics, even significant Z12 harmonics disappear. The inventors herein have found that higher order harmonics require more steel which is positioned with higher positional accuracy. US-A-5003266 discloses an MR system wherein shim holding, independently accessible, open cavities are attached to the magnet bore, for the purpose of increasing the efficiency of shimming to improve the homogeneity of the MR system's magnets. The cavities are arranged in rows and columns to provide readily identifiable locations. US-A-5349297 discloses a magnetic field assembly of a magnetic resonance imaging device including an annular superconducting magnet which is mounted within a toroidal vacuum vessel. A cylindrical member defines a central bore through which the superconducting magnets generate a uniform, static magnetic field. A cylindrical, dielectric former is mounted in the bore displaced by an annular gap from the cylindrical member. A shimset for shimming the uniformity of the magnetic field is mounted in the gap. A radio frequency coil is mounted within the cylindrical member defining a patient receiving examination region. An RF shield is mounted around the exterior peripheral surface of the former. Primary gradient coils are mounted around and potted to the exterior of the dielectric former around the RF shield. Gradient shield or secondary coils are potted around an exterior of the cylindrical member within the vacuum chamber. WO-A-8808126 discloses a magnetic field generating apparatus comprising a magnetic field generator for generating a magnetic field in a working volume in which a component of the magnetic field (Hz) along an axis and with respect to an origin defined by the magnetic field generator is defined in terms of a field expansion formula, where r is the radius of the sphere of interest, r 0 is the radius of a reference sphere upon which the components of the field expansion (C m / n ,S m / n ) are known, ,  are spherical polar coordinates defining the position of the point of interest upon the surface of the sphere, P m / n are Associated Legendre polynomials of order n and degree m, and C m / n , S m / n are the values of the field distortions as measured on a reference sphere radius r 0 . A magnetic field modification system comprising a number of ferromagnetic members is arranged to cancel at least one of the non-zero order terms and at least one of the non-zero degree order terms in the field expansion formula. It is an object of the present invention to provide magnetic resonance methods and apparatus wherein the above problems are alleviated. In accordance with a first aspect the invention provides a magnetic resonance apparatus comprising: annular main magnets surrounding a subject receiving bore, the annular magnets generating a temporally constant magnetic field through the bore, which magnetic field has spherical harmonic distortion including distortion components above Z6 including Z12; a plurality of ferrous rings fixedly disposed around the bore and arranged to provide at least partial Z12 shimming; a multiplicity of shim receiving pockets disposed circumferentially around the bore, longitudinally along the bore, and radially outward from the ferrous rings; shims disposed in said shim receiving pockets and arranged to further shim the Z12 distortion component, and to shim the other distortion components above Z6. In accordance with a second aspect the invention provides a method of shimming a magnetic resonance apparatus which includes a bore having a diameter and an axial length, an annular magnet surrounding the bore to generate a temporally constant magnetic field axially through the bore, and a radio frequency coil disposed to broadcast radio frequency signals into the bore and receive radio frequency signals from the bore, the method comprising: (a) measuring magnetic field uniformity including measuring for harmonic distortion components above Z6 including Z12; (b) placing ferrous rings around the bore to make an initial coarse compensation at least for the measured Z12 harmonic distortion component; (c) measuring the magnetic field at least at twenty-four axial positions along a surface of an imaging volume and at a plurality of rotationally displaced positions around the imaging volume; and (d) mounting ferrous plates selectively in an array of shim receiving pockets of shim trays which are disposed axially along and circumferentially around the bore and radially outward from the ferrous rings in accordance with the measurement of step (c) to reinforce the compensation for the Z12 harmonic distortion component and to shim the other harmonic distortion components above Z6. One advantage of the present invention is that it enables magnets to have shorter bores while maintaining uniformity comparable to longer bore magnets. Another advantage of the present invention is that it promotes patient access while maintaining image quality. Another advantage of the present invention is that it can shim not only for axial gradients, but also for higher order transverse gradients. One magnetic resonance apparatus in accordance with the invention forming part of a medical diagnostic imaging system and a method of shimming the apparatus 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 the imaging system and a 24 probe magnetic field sensor used in the shimming method; Figure 2 is a cross-sectional view illustrating the relationship of shim trays of the apparatus relative to RF coils and gradient primary and secondary coils; Figure 3 is a side-sectional view through section 3-3 of Figure 2; and, Figure 4 is a perspective view of one of the shim trays. Referring to FIGURES 1, 2, and 3, the system incorporates a magnetic resonance apparatus including a plurality of primary magnet coils 10 which generate a temporally constant magnetic field along a longitudinal or z-axis of a central patient receiving bore 12. The bore has a length to diameter ratio of 1.75:1 or less, preferably 1.67:1. However, still lower ratios such as 1:1 are also contemplated. In a preferred superconducting embodiment, the primary magnetic coils 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 at superconducting temperatures. The can is surrounded by one or more cold shields 18 which are supported in a vacuum dewar 20. The apparatus further includes a whole body gradient coil assembly 30 which includes x, y, and z-gradient coils mounted around the bore 12. In the preferred embodiment, the gradient coil assembly is a self-shielded gradient coil assembly that includes primary x, y, and z-gradient coil assemblies 32 potted in a dielectric former 34 and a secondary or shield gradient coil assembly 36 that is supported on a bore defining cylinder 38 of the vacuum dewar 20. The dielectric former 34 with the potted primary gradient coils can function as a bore liner or a cosmetic bore liner can be inserted into it. The primary and secondary gradient coil assemblies are mounted in a spaced relationship to define an annular shim receiving cavity 40 therebetween. Mechanical supports 42 extend through the shim receiving cavity 40 to support the primary gradient coil assembly. The supports have a minimum width in a circumferential direction to enable a plurality of shim trays 44 to be positioned substantially side by side. In the illustrated embodiment, thirty-six shim trays are mounted to an outer surface of the primary gradient coil assembly cylindrical former 34. With reference to FIGURE 4, each of the shim trays 44 in the preferred embodiment includes a dielectric strip member 50 having a lower surface 52 for accommodating the radius of curvature of the primary gradient coil assembly. Edge surfaces 54 are configured such that the trays are mountable substantially contiguous on the rounded primary gradient coil assembly. The tray defines a central region 56 which carries pocket indicia to provide appropriate labels for the pockets to facilitate identifying each one. To either side of the central indicia, the tray includes twenty-four pockets 58. Each pocket has an aperture 60 in the base. A multiplicity of thin ferrous shims 62 are provided, which shims are selectively placed in the pockets. The shims may be all of the same magnetic material and thickness such that each provides the same amount of shimming, or shims of different shim strengths, e.g., shims of double or quadruple thickness, may be provided. After the pockets have been filled with the appropriate amounts of shimming material, a dielectric spacer or spring is inserted to hold the shims securely in place. Thereafter, a cover member 64 is fastened over the shims. Apertures or bores 66 for receiving fasteners 68 are provided as selected intervals to facilitate mounting the trays securely to the primary gradient coil. With reference again to FIGURES 1-3, a radio frequency coil 70 is mounted inside the primary gradient coil. A radio frequency shield 72, e.g., a layer of copper mesh, is mounted between the radio frequency coil and the gradient coil assembly 30. With particular reference to FIGURE 1, a magnetic field sensor probe 80 includes a central shaft 82 which is securely, but removably mountable along a central axis of the bore 12. A means 84 is provided for selectively rotating the shaft, preferably in 10°-12° increments. The shaft carries twenty-four magnetic field sensor probes 88. The probes are positioned at selected locations to sample a set of spherical harmonics. The half volume is selected such that the probes lie along the surface of an imaging region within which the magnetic field is to be optimized by shimming. A spherical sampling volume is preferred to simplify the use of spherical coordinates for field error plot calculations. An operator control terminal 90 is connected with the means 84 and the probes 88 to rotate the probe assembly in the selected angular increments and read the output of the probes at each position. The operator station generates magnetic field measures to determine a deviation from uniformity from which the numbers of shims to be placed in each pocket of the shim strips 50 is determined, either iteratively, by operator experience, or by computer program. With twenty-four probes, sampling theory suggests that harmonics at least through Z12 can be measured accurately. For a sine wave, two measuring points per cycle facilitate accurate sampling. With only one point per cycle, it can be difficult to differentiate between a sine wave and a straight line. Although twenty-four probes produce optimal sampling through the Z12, i.e., Z 12 , component, the Z18 component is a sampled rate of 1.5 samples per cycle. The twenty-four probe sampling system, in fact, produces relatively accurate sampling through Z18. Typical harmonic distortions are as follows: AXIAL TERMS BEFORE SHIMMING (PPM) AFTER PASSIVE SHIMMING (PPM) Z1 187.5 1.0 Z2 474.9 0.7 Z3 -136.4 - 0.1 Z4 - 25.6 1.9 Z5 19.5 0.3 Z6 1.54 2.4 Z7 - 2.54 - 1.8 Z8 - 2.8 2.6 Z9 0.3 0.9 Z10 1.50 - 6.6 Z11 2.6 1.1 Z12 - 29.4 -15.5 Z13 1.7 - 0.4 Z14 16.9 5.8 Z15 0.2 0.1 Z16 - 5.2 0.2 Z17 0.2 0.2 Z18 2.1 - 0.4 In a traditional long bore magnet, the axial terms above Z6 would normally be zero or close to zero. However, higher order terms and corrections commonly cause aliasing back into lower order terms. Although it might be noted that with a twelve probe magnetic field sampling system, the sampling density would be too thin to detect higher order harmonics, such as Z12, Z14, and Z16. Based on the magnetic field variation numbers, the shim trays 44 are loaded and mounted to the exterior of the primary gradient coil assembly 32. The magnetic fields are again sampled analogously to determine how accurately the shimming has corrected the magnetic field harmonic aberrations. Additional shims may be added or subtracted from the shim trays and the procedure repeated iteratively until an acceptable level of magnetic field error is achieved. The inventors herein have determined that when correcting for Z12, Z14, and Z16 components, the metal shims lie primarily in rings symmetrically about the isocenter of the circle, typically about three or four rings on either side of isocenter. Moreover, the inventors herein have found that correcting for Z12, Z14, and to a limited extent for Z16 accounts for about 90% of the steel used for correcting for the Z1-Z14 axial terms. With reference again to FIGURE 2, these annular rings around the bore are at generally the same distance from isocenter for all like-designed magnets. However, each ring requires so much steel that any inaccuracy in the axial positioning of the rings can be detrimental. In the embodiment, shimming is facilitated by placing segmented rings of steel 92 around the primary magnetic field former. In the illustrated embodiment, the primary gradient magnetic coil former 34 is grooved and the grooves contain segments of iron or steel. The segments, such as a plurality of thin plates each covered with an insulating coating, extend around the grooves in one or more layers. In another embodiment, the steel platelets may be placed side to side extending radially and again separated by insulating layers or spacers. Various other segmentations of the ferrous material may be used in order to divide the material into sufficiently small pieces that significant eddy currents are not induced by the application of the radio frequency fields or the gradient magnetic fields. The segmented rings are installed in the gradient coil assembly during manufacture. Thus, these metal rings are already in place when the initial magnetic field reading is taken. The shimming arrangement in the shim trays is then calculated so as to correct and optimize the first order correction achieved by the segmented metal rings. Rather than placing the shim trays on the primary gradient coil, the shim trays can also be mounted on the dielectric former 38 of the secondary gradient coil. However, placing the shim trays on the primary gradient coil assembly is preferred because the closer the steel shims are placed to the imaging volume, the stronger their effect. With Z12, Z14, and Z16 gradients which require much higher volumes of steel than Z1 through Z6 axial terms, moving these ferrous materials closer to the imaging volume provides a significant reduction in steel. The operator interface and control station 90 includes a human-readable display such as a video monitor 100 and an operator input means including a keyboard 102 and a mouse 104. Trackballs, light pens, and other operator input devices are also contemplated. Computer racks 106 hold a magnetic resonance sequence memory and controller, a reconstruction processor, and other computer hardware and software for controlling the radio frequency coil 70 and the gradient coil assembly 30 to implement any of a multiplicity of conventional magnetic resonance imaging sequences, including echo-planar, echo-volume, spin-echo, and others. Echo-planar and echo-volume imaging sequences are characterized by a short data acquisition time and high gradient strengths and slew rates. The computer racks 106 also hold a digital transmitter for providing radio frequency excitation and resonance manipulation signals to the RF coil 70 and a digital receiver for receiving and demodulating magnetic resonance signals from the radio frequency coil or insertable coils (not shown). An array processor and associated software reconstruct the received magnetic resonance signals into an image representation which is stored in a computer memory, on disk, or other recording media. A video processor selectively extracts portions of the stored reconstructed image representation and formats the data for display on the video monitor 100. An image printer, optionally, provides film copies of selected images. In the preferred embodiment, the length and diameter of the bore have a size ratio of about 1.67:1. However, it is to be appreciated that the invention is also applicable to magnets with other geometries, particularly magnets with limited magnetic field uniformity. Typically, one might expect magnets with a bore length to diameter ratio of 1.75:1 or less to be candidates for Z12 harmonic correction. However, the present invention will also be applicable to magnetic resonance coils with longer bores in which there is a sufficient magnetic field non-uniformity in the higher order harmonics above Z6.

Claims: A magnetic resonance apparatus comprising: annular main magnets (10) surrounding a subject receiving bore (12), the annular magnets (10) generating a temporally constant magnetic field through the bore (12), which magnetic field has spherical harmonic distortion including distortion components above Z6 including Z12; a plurality of ferrous rings (92) fixedly disposed around the bore (12) and arranged to provide at least partial Z12 shimming; a multiplicity of shim receiving pockets (58) disposed circumferentially around the bore (12), longitudinally along the bore (12), and radially outward from the ferrous rings (92); shims (62) disposed in said shim receiving pockets (58) and arranged to further shim the Z12 distortion component, and to shim the other distortion components above Z6. A magnetic resonance apparatus according to Claim 1 wherein at least twenty-four pockets (58) are disposed along the bore (12) such that the shims (62) disposed in the pockets (58) can emulate at least twenty-four annular rings. A magnetic resonance apparatus according to claim 1 or claim 2 wherein said multiplicity of shim receiving pockets (58) are formed in shim trays (44), and said apparatus includes a self-shielded gradient coil assembly (30) including a primary gradient coil (32) and a larger diameter shield gradient coil (36), the primary gradient coil (32) and shield gradient coil (36) defining an annular shim tray receiving aperture (40) therebetween, said shim trays (44) being mounted to the primary gradient coil (32) in the shim tray receiving aperture (40). A magnetic resonance apparatus according to any preceding claim wherein the bore has a length to diameter ratio of less than 1.75:1. A method of shimming a magnetic resonance apparatus which includes a bore (12) having a diameter and an axial length, an annular magnet (10) surrounding the bore (12) to generate a temporally constant magnetic field axially through the bore (12), and a radio frequency coil (70) disposed to broadcast radio frequency signals into the bore (12) and receive radio frequency signals from the bore (12), the method comprising: (a) measuring magnetic field uniformity including measuring for harmonic distortion components above Z6 including Z12; (b) placing ferrous rings (92) around the bore (12) to make an initial coarse compensation at least for the measured Z12 harmonic distortion component; (c) measuring the magnetic field at least at twenty-four axial positions along a surface of an imaging volume and at a plurality of rotationally displaced positions around the imaging volume; and (d) mounting ferrous plates (62) selectively in an array of shim receiving pockets (58) of shim trays (44) which are disposed axially along and circumferentially around the bore and radially outward from the ferrous rings (92) (12) in accordance with the measurement of step (c) to reinforce the compensation for the Z12 harmonic distortion component and to shim the other harmonic distortion components above Z6. A method according to Claim 5 wherein the imaging volume is a sphere.

Inventors: DEMEESTER GORDON D, MCGINLEY JOHN V M, SRIVASTAVA VISHNU C, DEMEESTER, GORDON D., MCGINLEY, JOHN V.M., SRIVASTAVA, VISHNU C.
Applicants: MARCONI MEDICAL SYS INC, MARCONI MEDICAL SYSTEMS, INC.
Application year: 1995
Main classification: G01R33
IPC codes: A61B5, G01R33