GB2262611A - Side access mri magnet with external and internal shims - Google Patents

Abstract

Each toroidal element 4 of a side access MR cryostat system has both internal 15 and external 13 passive shims to homogenise the static magnetic field in the sensing volume 16. This allows similar field homogenisation performance to that achieved in a conventional axial access cylindrical magnet with internal shims extending through the bore (dashed extension of 14), but having no external shims. <IMAGE>

Description

PASSIVE SHIMS FOR SIDE-ACCESS MR (MRT) MAGNETS Background of the Invention Field of the Invention This invention relates to passive shims for side-access magnetic resonance MR magnets. Such structures of this type, generally, produce a larger number of magnetic field shapes than a conventional set of passive shims which increases the potential adjustability of the magnet and allows the passive shim set to bring a wider range of possible magnetic fields within the homogeneity specification.

Descnption of the Related Art It is known that prior MR magnets require some method of field correction in order to meet the stringent field homogeneity requirements, typically, on the order of a few parts per million (ppm) over the volume of a human body, which are necessary to produce an adequate MR image. Options are basically limited to two modes active shimming with electromagnetic coils and passive shimming with strategically placed pieces of some ferromagnetic material. Active shims may either be superconducting or resistive in naturc.

Traditionally, MR magnets have been made using niobium-titanium superconductors operating in liquid helium. Such magnet technology is ideally suited to shimming with superconducting correction coils, as they can be placed in the liquid helium dewar and operated in a persistent mode. With this technology, the magnet can be shimmed once and left for long periods without adjustment. For refrigerated magnets, the use of superconducting correction coils can be problematic, as the technology for producing reliable persistent current operation in niobium tin superconductors is not yet mature.If a correction coil current drifts in time, the field inhomogeneity will, in tum, go out of spedficadon with it Resistive correction coils are an option, but the need for many (up to 18) permanently connected power supplies and water cooling to remove the resistively generated heat makes this alternative unattractive.

Passive shimming can be performed with fewer parts, less costs and potentially vcry little increase in shimming time over superconducting or resistive correction coils. A type of internal passive shimming is set forth in U.S. Patent No. 4,698,611 ('611) to M.E.

Venilyea entided "Passive Shimming Assembly for MR Magnet" which patent is assigned to the same assignee as the present invention and incorporated herein be reference, has been proven on several magnet designs, but only with fllll length cylindrical bores.In the '611 patent several drawers, which are arc shaped fiberglass panels cut from a cylinder along the entire length of the magnet bore, to which the shim packets are attached can be retracted from the magnet bore for shim repositioning as the shimming iterations are performed The shims are arcuate pieces of low carbon steel of a fixed axial length (typically about one inch) and circumferential extent (typically about 20 degrees) and various thicknesses, from which an arbitrary thickness of the shim can be built up at any allowable axial and azimuthal location. The shimming procedure, typically, consists of measuring the field, predicting the thickness of the shims at each allowable location, placing the shims in the bore, remeasuring the magnetic field, and predicting thickness changes at each location (typically, small fractions of the existing shim thickness). This procedure is iterated until the magnetic field is within specification.

Unfortunately, several magnetic fields which can be created by reasonable and expected perturbations of the ideal main coil positions cannot be shimmed to within the required specification with the internal passive shims alone. The reason for this is that the magnetic field shapes which the internal passive shims can create are limited and the field correction requires magnetic field shapes which can only be generated by internal shims if they lie in the space between the cryostats. The space, typically, is created by a side-access MR magnets It is apparent from the above that there exists a need in the art for a passive shimming assembly for a side-access MR (MRT) magnet which will provide an increased number of magnetic field shapes while avoiding the use of the region between the cryostat halve.It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.

Summary of the Invention Generally speaking, this invention fulfills these needs by providing a passively shimmed side-access imaging magnet, comprising a main field coil, a cryostat having at least two toroids with each said toroid located at a predetermined distance away from each other and each said toroid having first and second sides, and a plurality of passive shims substantially located on said first and second side of each of said toroids.

In certain preferred embodiments, the first and second sides are the bore and the outer diameter of the cryostats, respectively. Also, the external passive shims, preferably, are rigidly attached to the outer diameter. Finally, the internal passive shims are capable of being removed from the bore.

In another further preferred embodiment, the present invention allows the shimming of the side-access magnets without resorting to correction coils as a backup method while increasing the number of magnetic field shapes achieved.

The preferred passively shimmed magnet, according to this invention, offers the following advantages: easy assembly and repair, increased magnetic field shapes; good economy; good stability; good durability; decreased inhomogeneity of the imaging volume; and high strength for safety. In fact, in many of the preferred embodiments, these factors of magnetic field shapes and inhomogeneity are optimized to an extent considerably higher than heretofore achieved in prior, known shimmed assemblies for MR magnets.

Brief Description of the Drawings The above and other features of the present invention which will become more apparent as the description proceeds are best understood by considering the following detailed description in conjunction with the accompanying drawings wherein like characters represent like parts throughout the several views and in which: Figure 1 is a schematic illustration of a MRT magnet showing available and unavailable internal passive shim locations and external passive shim locations, according to the present invention; Figure 2 is a schematic illustration of the magnetization of, and magnetic fields produced by, the internal and extemal passive shims, according to the present invention; and Figure 3 is an isometric schematic view of one cryostat showing internal and external shims with the internal shims located on drawers.

Detailed Description of the Invention With reference first to Figure 1, there is illustrated a passively shimmed MRT magnet 2. Magnet 2 includes a conventional side-access cryostat 4 having conventional superconducting coils 6,8,10, a main field 5, a set of extemal passive shim locations 12, a set of internal passive shim locations 14 and imaging volume 16. Shim locations 12 and 14, preferably, indicate where conventional passive shims 13 and 15, respectively, can be placed on cryostat 4. Shims 13 and 15, preferably, are constructed the same as the passive shims in the '611 patent.

The key to the present invention is the addition of locations on the outer diameter of the cryostats 4 (extemal passive shims 13) to those in the bore (internal passive shims 15).

The magnetic field shapes 18 produced by external shims 13 are significantly different from the magnetic field shapes 20 produced by internal shims 15, as is shown schematically in Figure 2. The fact that the external shims 13 are magnetized in the axial direction along the direction of arrows A and in opposite direction to that of the main field S along the direction of arrow C and, therefore, produce axial magnetic fields 20 which are along the direction of arrows B and in opposite directions to those produced by the internal shims 15 provides a set of shim locations with essentially "negative" strengths, the lack of which has historically been a drawback to prior passive shim assemblies.It is to be understood that for correction coils, the currents can be reversed, but for passive shims 13,15, the magnetization is in a fixed direction which is dictated by the field of the main coils. The advantage of allowing shims 15 to be placed on the outer diameter of cryostat 4 lies in the fact that the magnetization of shims 15 is in the opposite axial direction to the magnetization oftheshims 13 in the bore.

With respect to Figure 3, a half of cryostat 4 which is in the shape of a toroid is illustrated. In particular, cryostat 4 includes external shims 13 and internal shims 15.

Shims 13, preferably, include ferromagnetic strips 22 rigidly attached to fiberglass plates 24. Shims 13, may be slidebly or rigidly attached to the outside of cryostat 4 depending on the forces experienced by shims 13. Shims 15 are constructed substantially the same as shims 13 and include ferromagnetic strips 26 and fiberglass plates 28. Shims 15 are slidably attached to the core of cryostat 4 in substantially the same manner as the internal shims of the '611 patent.

Proof of the efficacy of this approach lies in the results of test runs of conventional passive shimming software, which uses linear programming (lop) to determine the optimal shim thicknesses for a given set of allowable shim locations and a given initial field to reduce the field inhomogeneity to given level. Such conventional software is set forth in U.S. Patent No. 4,771,244 ('244) to M.E. Vermilyea entitled "Method of Passively Shimming Magnetic Resonance Magnets", assigned to the same assignee as the present invention and hereby incorporated by reference. In determining the possibility of shimming the MRT magnet 2 to the required homogeneity level with either internal only or both internal and external passive shims 15 and 13, respectively, several approaches are used.

First, several magnetic fields which might be created by magnet 2 are given as initial ("virgin") fields, and the program, preferably, as set forth in the '244 patent is allowed to use only the internal shim locations 14 in the bore of the cryostat of MRT magnet 2. A typical requested inhomogeneity of 3 ppm on a 314 point magnetic field map covering the surface of a 30 cm diameter sphere is given to the program. In actuality, it is desired to shim this volume to 7 ppm but, preferably, a "safety" factor of 2 or more is required to ensure that magnet 2 will actually shim to the desired level when all nonidealities have been accounted for.

The LP program was applied to measured magnetic fields from several conventional unshielded magnets, as well as, those from several conventional shielded magnets. It was found that all unshielded fields were shimmable, but none of the shielded magnetic fields could be made to reach the requested inhomogeneity with the truncated set of internal shims. When the full set of passive shims along the entire bore was made available to shim the prior shielded magnet, the requested inhomogeneity was attained. This gave rise to some doubt about the ability of truncated internal shims alone to reach the 3 ppm target for the present invention.

While using actual measured magnetic fields from other MR magnet types is somewhat useful, the magnet main coils 6,8,10 are in radically different spatial positions than those in the prior unshielded and shielded magnets which are both six coil, cylindrical cryostat magnets. The magnetic field produced by the present invention has a different shape than those from the prior, known magnets, so some investigation using the magnetic fields of the present invention is indicated.The ideal magnetic field of main coils 6,8,10 has a homogeneity well below the 3 ppm target, so a conventional perturbation analysis is applied to the main coil locations wherein main coils 6,8,10 were assumed to be deformed or spatially displaced in various modes which are likely to actually occur in the manufacturing process, and the resulting field errors determined and superimposed on the ideal field as a sample virgin field requiring shimming.

The conventional main coil perturbations which were applied included axial and radial offset of a single main coil (maintaining axisymmetry, and producing only axisymmetric field harmonics as terrors); transverse displacement of a single main coil, maintaining its circularity; and ovalizing of a single main coil, maintaining its axis on the magnet centerline. The magnetic fields produced by these conventional perturbations were found to be unshimmable by the intemal passive shims 15 alone - with a 3 ppm requested field inhomogeneity from initial inhomogeneities ranging from 12-300 ppm (depending on the mode and magnitude of the deformation), the achievable homogeneities ranged from 7 to 100 ppm.

With the addition of several external shim locations 12 on the outer diameter of each of the two cryostats 4, these same perturbed fields were shimmable to the 3 ppm specification in every instance. This modification to the allowable shim set was also tested on the conventional shielded magnet fields which were unshimmable with the truncated internal set, and these fields were also found to be shimmable to the 3 ppm specification.

The number of allowable axial shim locations which was used for these runs was: (7) internal shim location 14 on each cryostat half; (6) external shim locations 12 on the z+ cryostat half, and (5) external shim locations 12 on the z- cryostat half. The number of allowable azimuthal locations was 12 (see Figure 3). The reason for the asymmetry in the number of axial locations 12 for the external shims was simply that the LP program is, preferably, set up for 25 allowable shim locations, 11 of which were excluded by the space between the cryostat halves. This space, typically, being created by a side-access MR magnet These 11 shim locations were, therefore, allocated to the external shim locations 12, with six for one cryostat half and 5 for the other. The addition of the sixth shim location for the z- cryostat half would probably have a small positive effect on the attainable homogeneity.

Once given the above disclosure, many other features, modifications and improvements will become apparent to the skilled artisan. Such features, modifications and improvements are, therefore, considered to be a part of this invention

Claims (10)

1. A passive shimmed side-access imaging magnet, said magnet comprised of: amain field coil; a cryostat having at least two toroids located at a predetermined distance away from each other and each of said toroids having first and second sides; and a plurality of passive shims locations substantially located on said first and second sides of each of said toroids.

2. The magnet, according to claim 1, wherein said first side of said toroid is further comprised of: a cryostat bore.

3. The magnet, according to claim 1, wherein said plurality of passive shims locations are further comprised of: internal shims located in a predetermined number of said shim locations.

4. The magnet, according to claim 1, wherein said plurality of passive shims locations are further comprised of: external shims located in a predetermined number of said shim locations.

5. The magnet, according to claim 3, wherein said internal shims are slidably located on said first side of each of said toroids.

6. The magnet, according to claim 4, wherein said external shims are rigidly attached to said second side of each of said toroids.

7. The magnet, according to claim 1, wherein said magnet is further comprised of: magnet field shapes such that said field shapes vary between said passive shim locations.

8. The magnet, according to claim 3, wherein said internal shims are magnetized in a first axial direction.

9. The magnet, according to claim 4, wherein said external shims are magnetized in a second axial direction

10. A magnet substantially as hereinbefore described with reference to the accompanying drawings.