MAGNET WITH IMPROVED ACCESS

Abstract: The present invention provides a magnet assembly (30a, 30b) comprising a pair of drive coils (31a, 31b; 32a, 32b; 33a, 33b) symmetrically disposed with respect to a reference plane. The reference plane is located approximately equidistant between the drive coils. Each of the drive coils has a current flowing through it which is in the opposite sense to the current flowing through the other drive coil, such that a primary magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane. The assembly may comprise further compensating coils (44a, 45a, 46a) which are configured to generate a magnetic field to improve homogeneity of the primary magnetic field. The drive coils and the compensating coils are generally in the shape of a racetrack and are preferably superconducting coils.

Description: MAGNET WITH IMPROVED ACCESS The present invention relates to a magnet with improved access. More specifically, the present invention relates to a magnet for use in a magnetic resonance imaging (MRI) system which allows improved access to the patient. A magnetic field with a high degree of homogeneity is essential for the successful application of a number of analytical techniques, in particular MRI. These fields can be generated by a number of devices, such as coil magnets or permanent magnets, or a combination of the two. Ferromagnetic material is known to be used, in addition to the above mentioned magnets, to increase the field strength, improve field homogeneity, and to limit stray magnetic fields. Most known MRI magnets consist of an assembly of coaxial superconducting coils. These coils are positioned in such a way that the required field strength and homogeneity is achieved. The patient is positioned inside the coils with the head-feet axis in line with the axis of the coils. Figure 1 is an example of this type of MRI system, which is known in the art as the solenoid configuration. In figure 1 a cross section through the solenoid type MRI system 10 is shown comprising a pair of drive coils 12a, 12b, which function to generate the primary magnetic field B along the central line Z of the system. A plurality of compensating coils 13a, 13b, 13c, 13d are used to improve the homogeneity of the primary magnetic field. The patient 15 is located along the centre line Z of the system. A disadvantage of the solenoid configuration shown in figure 1 is that the patient has to be located inside a tube. This can cause the patient stress and make him feel trapped. Furthermore, access to the patient once he is inside the tube is limited. An object of the present invention is to provide a magnet which generates a magnetic field suitable for use in an MRI system and which allows improved access to the patient and which causes the patient less stress. According to the present invention there is provided a magnet assembly for use in an MRI system comprising a pair of drive coils, each of said pair of drive coils being substantially the same size and shape and symmetrically disposed with respect to a reference plane, the reference plane being located between said pair of drive coils, each of said pair of drive coils being arranged such that a current flow there through, the current through the first drive coil of said pair being in an opposite sense to the current through the second drive coil of said pair, such that a primary magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane. According to an aspect of the present invention, the magnet assembly further comprises a pair of compensating coils, each of said pair of compensating coils being substantially the same size and shape and arranged such that a current flows there through, the current through the first compensating coil of said pair of compensating coils being in the opposite sense to the current through the first drive coil, and the current through the second compensating coil of said pair of compensating coils being in the opposite sense to the current through the second drive coil, such that a compensating magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane and which functions to compensate for inhomogeneity in the primary magnet field. According to a further aspect, the first drive coil and the first compensating coil are disposed in a first plane, and the second drive coil and the second compensating coil are disposed in a second plane. According to yet a further aspect, the reference plane is approximately equidistant between the first and the second planes. According to yet a further aspect, the pair of drive coils and the pair of compensating coils are superconducting coils. According to yet a further aspect, the assembly comprises a plurality of pairs of drive coils. According to yet a further aspect, the assembly comprises a plurality of pairs of compensating coils. While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of the preferred embodiment, presented by way of example only, in which; FIGURE 2 is a cross section through a hypothetical two dimensional magnet, FIGURE 3 is a side view of a pair of improved access magnetic coil assemblies according to the present invention, FIGURE 4 is a plane view of an alternative embodiment of the improved access magnetic coil assembly shown in figure 3, and FIGURE 5 is a MRI system incorporating the improved access magnetic coil assemblies shown in figure 3. In figure 2 a cross section through a hypothetical two dimensional magnet 20 is shown comprising conductors 21a and 21b, which are symmetrically disposed with respect to a reference plane X-Z. The reference plane in located approximately equidistant between the conductors. Theoretically, conductors which are infinitely long would generate a primary magnetic field B in a plane corresponding to the reference plane and perpendicular to the head-foot axis of patient 15. However, conductors with a finite length and disposed with the same vertical spacing Y between them can generate a magnetic field with the same imaging quality as can theoretically be achieved with the infinitely long conductors. This results in a MRI system with much-improved openness. A good approximation of the ideal, infinitely long two-dimensional magnet shown in figure 2 can be achieved by truncating the length of the conductors and providing a return path for the currents that is substantially away from the imaging region of the system. In figure 3 a pair of improved access magnetic coil assemblies 30a, 30b are shown. Each assembly consists of three pairs of racetrack coils 31a & 3 lb, 32a & 32b, and 33a & 33b. The term racetrack coil is well known in the art and refers to coils that contain both arc and straight sections and are generally rectangular or oval shaped. However, as will be appreciated by those skilled in the art, other shaped coils could be used. A patient 15 is located between the pair of magnet assemblies 30a, 30b. Each racetrack coils 31a, 32a, 33a in magnet assembly 30a above the patient is matched by a racetrack coil 3 lb, 32b, 33b in the magnet assembly 30b below the patient. Coils 31a, 32a, 33a are substantially the same size and shape as coils 3 lb, 32b, 33b. Coils 31a, 32a, 33a have a current running through them which is in the opposite sense to a current running through corresponding coils 3 lb, 32b, 33b. Thus for example, racetrack coil 31a has a current running in the direction indicated by arrow 34a and the corresponding racetrack coil 3 lb has a current running in the opposite sense as indicated by arrow 34b. Each corresponding pair of racetrack coils: 3 la & 3 lb, 32a & 32b, and 33a & 33b have substantially the same cross sectional area. Each coil in an assembly has a current running through it which is in an opposite sense to the current running through the coil immediately next to it. For example, in assembly 30a, coil 31a has a current running through it in the opposite direction a current running through coil 33a. Coil 33a has a current running through it in the opposite direction a current running through coil 32a. The direction of current flow in coils 31a, 33a, 32a is indicated by arrows 34a, 36a, 35a respectively. Similarly, in assembly 30b, the currents running through coil 31b is opposite to the current in coil 33b, which is opposite to the current in coil 32b. The direction of current flow in coils 3 lb, 33b, 32b is indicated by arrows 34b, 36b, 35b respectively. The coil assemblies 30a, 30b are symmetrically disposed with respect to a reference plane X-Z. The reference plane in located approximately equidistant between the coil assemblies. The coil assemblies generate a primary magnetic field B in the area between the coil assemblies. The primary magnetic field B is in a plane corresponding to the reference plane and perpendicular to the head-foot axis of patient 15. The coil assemblies also generate secondary magnet fields which are also parallel to and substantially coincident with the reference plane. The secondary fields function to compensate for inhomogeneity in the primary magnetic field B . As will be appreciated, further pairs of coils may be included in the magnetic coil assemblies, which would further improve the homogeneity of the primary magnetic field. Advantageously the configuration of coils in figure 3 generates a stronger homogeneous magnetic field along the length of the patient 15 than is possible with known solenoid type systems. This allows for imaging along a greater length of the patient. For example, the entire spine of the patient may be imaged in a single scan. In figure 4 a single magnetic coil assembly 40a is shown comprising primary racetrack coils 41a, 42a, 43a and compensating racetrack coils 44a, 45a, 46a. The primary drive coils generate the primary magnetic field and secondary fields for compensating for inhomogeneity in the primary field. The compensating coils generate further fields which function to further improve the homogeneity of the primary magnetic field. The area in which the primary magnetic field is suitable for imaging is indicated by the sphere 48. The magnetic fields generated by the Xj arm of each racetrack coil are of primary importance to the imaging sphere 48. The return path of each racetrack coil, the X 2 arms, have an adverse effect on the homogeneity of the primary magnetic field and are thus disposed at a great distance as possible from the imaging sphere. The Z] and the Z 2 arms of each racetrack coil do not contribute significantly to the primary magnetic field. As will be appreciated by those skilled in the art, the distance at which the X 2 arms of the coils can be located from the Xi arms will effect the overall size of the MRI system. Thus it is desirable to keep this distance to a minimum, while still maintaining a suitably homogenous primary magnet field. In an alternative embodiment, the X 2 arms of each coil in the top coil assembly are closer to the corresponding X 2 arms in the bottom coil assembly than the Xj arms of the top and bottom coil assemblies. Advantageously, this further reduces the size of the MRI system. Furthermore, the diameter and shape of each racetrack coil will effect the homogeneity of the primary magnetic field and thus can be varied in order to provide the most optimum configuration. In a preferred embodiment of the present invention, the racetrack coils are superconducting coils and are housed within a cryostat. Figure 5 shows a MRI system 50 incorporating the racetrack magnetic coil assemblies shown in figure 3. The pair of magnet assemblies 30a, 30b are disposed with cryostat structure 52. The patient 15 is positioned inside the MRI system 50. The primary magnetic field B is parallel to an substantially coincident with the reference plane X-Z and perpendicular to the head-foot axis of the patient. Advantageously, the MRI system shown in figure 5 allows for the patient to be more readily positioned within the system. Furthermore, access to the patient is greatly increased allowing for the possibility of performing various medical procedures on the patient while he is still located within the MRI system. As can be seen in figure 5, three sides of the MRI system are open. This results in less stress to the patient due to the claustrophic nature of known solenoid type systems. Furthermore, the arrangement shown in figure 5 allows for the possibility of locating the pair of magnet assemblies 30a, 30b closer together in the vertical direction Y. As is well known in the art, the closer the drive coils are to each other, the more the overall size of each coil and the amount of conductor needed to make the coil can be reduced without adversely effecting the homogeneity of the primary magnetic field. Any reduction in overall coil size will advantageously reduce the size and cost of the MRI system. As will be appreciated by those skilled in the art, various modifications may be made to the embodiment hereinbefore described without departing from the scope of the present invention.

Claims: CLAIMS 1. A magnet assembly for use in an MRI system comprising a pair of drive coils, each of said pair of drive coils being substantially the same size and shape and symmetrically disposed with respect to a reference plane, the reference plane being located between said pair of drive coils, each of said pair of drive coils being aπanged such that a current flows there through, the current through the first drive coil of said pair being in an opposite sense to the current through the second drive coil of said pair, such that a primary magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane. 2. A magnet assembly as claimed in Claim 1, wherein the magnet assembly further comprises a pair of compensating coils, each of said pair of compensating coils being substantially the same size and shape and arranged such that a current flows there through, the current through the first compensating coil of said pair of compensating coils being in the opposite sense to the current through the first drive coil, and the current through the second compensating coil of said pair of compensating coils being in the opposite sense to the current through the second drive coil, such that a compensating magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane and which functions to compensate for inhomogeneity in the primary magnet field. 3. A magnetic assembly as claimed in Claim 2, wherein the first drive coil and the first compensating coil are disposed in a first plane, and the second drive coil and the second compensating coil are disposed in a second plane. 4. A magnetic assembly as claimed in Claim 3 , wherein the reference plane is disposed equidistant between the first and the second planes. 5. A magnetic assembly as claimed in any preceding Claim, wherein said pair of drive coils are racetrack shaped. 6. A magnetic assembly as claimed in any of Claims 2-5, wherein said pair of compensating coils are racetrack shaped. 7. A magnetic assembly as claimed in any preceding Claim, wherein a first section of the first drive coil and a corresponding first section of the second drive coil define an imaging space, the imaging space including the reference plane. 8. A magnetic assembly as claimed in Claim 7, wherein a second section of the first drive coil opposite to the first section of the first drive coil and a corresponding second section of the second drive coil opposite to the first section of the second drive coil are disposed at a sufficient distance from the first sections of the drive coils such that the magnetic fields generated by the second sections does not adversely effect the homogeneity of the primary magnetic field in the imaging space. 9. A magnet assembly as claimed in any preceding Claims, wherein said pair of drive coils and said pair of compensating coils are superconducting coils. 10. A magnet assembly as claimed in any preceding Claims, wherein the assembly comprises a plurality of said pair of drive coils. 11. A magnet assembly as claimed in any preceding Claims, wherein the assembly comprises a plurality of said pair of compensating coils. 12. An MRI system including a magnetic assembly as claimed in any of preceding Claims 1-11. 13. A magnet assembly as hereinbefore described with reference to the accompanying figures 2-5.

Inventors: KRUIP MARCEL JAN MARIE, KRUIP, MARCEL, JAN, MARIE
Applicants: KRUIP MARCEL JAN MARIE, OXFORD MAGNET TECH, KRUIP, MARCEL, JAN, MARIE, OXFORD MAGNET TECHNOLOGY LIMITED
Application year: 2000
Main classification: G01R33
IPC codes: A61B5, G01R33, H01F7