WO1992007278A1

WO1992007278A1 - Magnetic system presenting a highly accessible homogeneous magnetic field

Info

Publication number: WO1992007278A1
Application number: PCT/FR1991/000796
Authority: WO
Inventors: Jacques Taquin; Michel Sauzade
Original assignee: Sopha Imaging
Priority date: 1990-10-12
Filing date: 1991-10-14
Publication date: 1992-04-30
Also published as: FR2667948A1; FR2667948B1

Abstract

Homogeneous magnetic field system comprising means for generating a magnetic field in the form of two substantially identical annular elements (1, 2) arranged coaxially to one another with a centre distance (L) less than the radii (r) of said annular elements, an element (3) for compensating the defects of the magnetic field, made of a tubular-shaped (3) ferromagnetic material and arranged coaxially to the annular elements (1, 2) generating the magnetic field (B).

Description

Homogeneous field magnetic system with great accessibility.

The present invention relates to a magnetic system for generating a homogeneous magnetic field in a defined area of space.

In many technical fields, it is often necessary to create a magnetic field whose homogeneity is essential to make precise measurements. This is the case, for example, for the technique of nuclear magnetic resonance imaging. According to this technique, in fact, the quality of the images obtained from an examined part of a subject, in a useful area under a so-called main magnetic field, is directly linked to the homogeneity of the magnetic field in the useful area.

Concretely, a nuclear magnetic resonance imaging device comprises magnetic or electromagnetic means in annular form and arranged coaxially with one another so as to generate a main magnetic field B substantially parallel to the axis Z common to said magnetic or electromagnetic means. In general, the means generating the main magnetic field are arranged symmetrically with respect to a plane which is called "median plane" perpendicular to the axis Z. If we choose the point of intersection between the median plane and the Z axis as the origin of a reference, the main magnetic field B can be expressed according to the following formula:

B (z) = B ₀ + B ₂ z ² + B ₄ z ⁴ + B ₆ z ⁶ + ... + B2 _n z ²ⁿ + ...

with B (z) the main magnetic field at distance z from the origin of the coordinate system along the Z axis; B2, B4, ... B2 _n coefficients which depend on the position and the shape of the magnetic field generating means. The main magnetic field is homogeneous if it is invariant or varies negligibly in the useful area. To do this, it is necessary to cancel the coefficients B2, B4 or even Bg so that the main magnetic field B (z) is equal to a constant BQ the order coefficients higher B, B * LQ ... being small enough to be negligible.

In practice, the coefficients B2, B and Bg are canceled, using appropriate means to compensate for the defects of the main magnetic field. In the current state of the art, nuclear magnetic resonance imaging devices include field compensation elements making it possible to obtain a homogeneous magnetic field in a useful zone inside the space where the magnetic field prevails. main B. Only the useful area is chosen to make good quality images. For example, to examine the head of a patient, it is necessary to place the head of the patient in the useful zone in order to obtain, by nuclear magnetic resonance, images representative of the section planes of the head.

However, although having resolved the problem of homogeneity of the main magnetic field, the structural design of currently known imaging devices does not allow easy access to the useful area where the main field is homogeneous.

Indeed, the most widespread imaging devices on the market use superconductive magnets as means generating the main magnetic field. In this type of device, the superconducting magnet is produced in an elongated cylindrical form at the ends of which are coaxially mounted magnetic field compensation coils. The useful area is located, in a long and narrow tunnel inside the superconducting magnet and the field compensation coils, at a distance of about 1 meter from each end of the cylinder. This particular geometry of imaging devices with superconductive magnet has a number of drawbacks:

^• - some patients placed in the tunnel experience strong anxiety due to claustrophobia and refuse the examination (approximately 5% of the population examined);

- the images of the parts of the body examined, located at the limit of the useful area, often present a phenomenon of distortion and the use, for these parts, of local antennas of emission and reception necessary for the imaging is often difficult because of the narrowness of the tunnel;

- it is necessary to introduce the whole body into the tunnel itself to obtain images of the arms or legs of a patient, therefore to build large-size superconducting magnets which are very expensive while the volume to be examined, from much more modest dimensions, does not require a large useful area of homogeneous field.

Another type of known imaging device is resistive magnets without coil iron. The main magnetic field generating means in this case consist, for example, of two pairs of resistive coils traversed by electric currents. The coils are arranged coaxially with each other. The spacing between the coils and the diameter of the coils are such that they are arranged on a spherical surface. This type of imaging device has the same accessibility disadvantages as devices with superconductive magnets, since the useful area is in the center of a long and narrow tunnel for examining patients.

The same is true for known imaging devices of the permanent magnet type. In order to make nuclear magnetic resonance imaging devices more suitable for users, especially in the medical field for the precision and speed of patient examinations, it is not only important to have a homogeneous main magnetic field in the useful area, but also to offer easy access to the useful area for the exam.

British patent application GB 2 184 243 discloses a different theoretical approach compared to conventional systems for compensating for defects in the main magnetic field. According to this document, the means generating the main magnetic field consist of a pair of identical annular coils traversed by an electric current. A pair of annular elements made of ferromagnetic material is arranged coaxially with the annular coils and symmetrically with respect to the median plane of said coils. In this way, the conventional coaxial and external compensation coils of the field generating means are replaced by the ferromagnetic annular elements arranged coaxially. inside the field generating means. The accessibility of the useful area is thus improved.

However, the configuration suggested by the aforementioned patent application requires a separation of the two annular coils, generating the main magnetic field, greater than the radius of said coils in order to be able to compensate for the defects of main magnetic field using the pair d 'annular ferromagnetic elements. This means that, in addition to the technical constraints and difficulties, an imaging device produced according to the document would have a useful area whose accessibility is limited by the spacing / diameter ratio of the coils generating the main magnetic field.

The object of the present invention is to provide a magnetic system making it possible, on the one hand, to produce a homogeneous main magnetic field in a useful area and, on the other hand, to offer great accessibility to the useful area of homogeneous field.

Another object of the present invention is to minimize the leakage of the magnetic field created by the field generating means. The object of the invention is also to produce compact magnetic systems, suitable for obtaining images of the limbs such as the arms, legs or the head of a patient without requiring the introduction of the entire coips of the patient into the system.

The magnetic system according to the invention comprises magnetic field generating means in the form of two substantially identical annular elements arranged coaxially with one another. The annular elements, also called magnets, can be constituted by coils of conductive or superconductive wires, traversed by an electric current, or produced using a permanent magnetic material or permanent magnet.

The system of the invention comprises an element made of ferromagnetic material in tabular form and arranged coaxially inside or outside the annular elements generating the magnetic field. The spacing between the two annular field-generating magnets is less than the radius of said magnets. Of preferably, the ferromagnetic tube is arranged symmetrically with respect to the median plane between the two annular field-generating magnets, said median plane being perpendicular to the axis of the ferromagnetic tube. Advantageously, the ferromagnetic tube consists of a plurality of sheets of ferromagnetic material superimposed on each other and electrically insulated from each other.

The ferromagnetic tube can consist of several tubular elements arranged end to end with optionally air gaps filled or not with an insulating material.

According to one embodiment of the invention, the ferromagnetic tube is placed inside the annular field-generating magnets. A cylinder made of ferromagnetic material, called a "shielding cylinder", is arranged coaxially outside the annular field-generating magnets, for the purpose of channeling magnetic field lines outside the field-generating magnets and thus reduce the leakage of the magnetic field. The shielding cylinder can be completed on both sides by two ferromagnetic annular discs to externally enclose the annular magnets and thus further reduce the leakage of the magnetic field. This results in an increase in the main magnetic field inside the ferromagnetic tube.

It is possible to carry out additional field compensation by adding at least one secondary ring of ferromagnetic material or permanent magnet, arranged coaxially inside the ferromagnetic compensation tube and symmetrically with respect to the median plane between the two annular magnets. It is also possible to increase the compensating effect of the secondary ferromagnetic ring by providing it with a toroidal winding traversed by an electric current of adjustable intensity.

The invention will be better understood on studying the detailed description of an exemplary embodiment taken by way of non-limiting example and illustrated by the appended drawings, in which: FIG. 1 is a schematic representation of an axial section of the magnetic system of the invention; and Figure 2 is a perspective view of a ferromagnetic secondary ring provided with a toroidal winding according to the invention.

As illustrated in FIG. 1, the magnetic system of the invention comprises two annular elements 1, 2 generators of main magnetic field B represented by an arrow which is parallel to a horizontal axis Z. The annular elements 1, 2 also called Annular magnets are each constituted, in this example, by four coil rollers separated by a certain distance from each other to facilitate cooling. The magnets 1 and 2 can be of the resistive or superconductive type. The magnets 1 and 2 are arranged coaxially to each other with their common axis coinciding with the horizontal axis Z. The spacing L between the annular magnets 1 and 2 corresponds to the axial distance along the axis Z, between the respective centers of radial section of the two rings. The radius R of the annular magnet 2 is equal to that of the annular magnet 1 and corresponds to the distance between the axis Z and the center of a radial section of the ring.

According to the invention, the axial spacing L between the annular magnets 1 and 2 is less than the radius R of the annular magnets 1 or 2. For example, the radius R can be of the order of 50 cm while the spacing L can be around 40 cm. As a result, the annular magnets 1, 2 constitute a "flat" assembly in the axial direction.

Between the annular magnets 1 and 2, a vertical median plane V is defined perpendicular to the axis Z. All the constituent elements of the magnetic system of the invention preferably have symmetry with respect to the median plane V.

Coaxially and internally with the annular magnets 1 and 2 is a tubular element 3 made of ferromagnetic material. The ferromagnetic tube 3 may have a length slightly greater than the distance between the two radial planes externally delimiting the assembly formed by the annular magnets 1 and 2. The presence of the ferromagnetic tube 3 plays a preponderant role in compensating for the defects of the magnetic field B created by magnets 1 and 2. Outside of magnets 1 and 2 and coaxially with them finds a shielding cylinder 4 also made of ferromagnetic material. As its name suggests, the shielding cylinder 4 makes it possible to center the magnetic field lines outside the magnets 1 and 2 so as to limit the leakage of the field to the outside. Preferably, the length of the shielding cylinder 4 is of the same order of magnitude as the length of the ferromagnetic compensation tube 3.

At the ends of the shielding cylinder 4 can be assembled two annular discs 5 and 6 also made of ferromagnetic material which surrounds the annular magnets 1 and 2 externally and axially, which makes it possible to limit the leakage of the magnetic field even more effectively.

Optionally, two secondary compensation rings 7, 8 are added inside and coaxially to the compensating ferromagnetic tube 3 and symmetrically with respect to the median plane V. The secondary rings 7, 8 contribute to finer compensation for defects in the magnetic field principal B.

Preferably, the secondary compensation rings 7, 8 are produced using a ferromagnetic material. It is possible to provide an adjustment of the compensating effect of the secondary rings 7, 8 using a toroidal coil 9 around said secondary ferromagnetic rings (see FIG. 2). By passing an electric current through the winding 9 and by adjusting the direction and the intensity of the current, the compensating effect of the secondary rings 7, 8 is actively adjusted. The number and position of the secondary rings can be varied according to practical cases.

The magnetic system thus formed makes it possible to create a very homogeneous main magnetic field B in a useful area U being centered at the point of insertion between the median plane V and the axis Z. 'It may be noted that the shielding elements 4, 5, 6 made of ferromagnetic material, while minimizing the leakage of the magnetic field to the outside, thus make it possible to increase the magnetic field B inside the useful area U.

To minimize the harmful effects due to the eddy currents generated during the transient phase of the energization of the coils 1, 2 generating the main magnetic field for example, the compensating tube 3 is advantageously made using sheets of ferromagnetic material superimposed and electrically isolated from each other.

In summary, thanks to its particular structure, the magnetic system of the invention makes it possible to produce a very homogeneous magnetic field in a useful area that is much more accessible than compared to existing systems. Obviously, the system of the invention can find its application in numerous technical fields which require very strict conditions for the homogeneity of the magnetic field. In its particular application to nuclear magnetic resonance imaging devices, the advantages immediately flow from the very design of the system:

- the device is much flatter axially with an examination tunnel of short length compared to the diameter. By way of example, the ferromagnetic tube 3 field compensator may have a length of the order of 65 cm and an internal diameter of the order of 70 to 80 cm. As a result, the discomforts felt by certain participants during the examination can be minimized, since they are less enclosed compared to conventional systems; - the possibility of changing the positioning of the patient in the radial plane to produce images of the parts of the body located at the edge of the useful area thanks to the large diameter of the examination tunnel, which makes it possible to eliminate the inevitable image distortions in conventional systems for these parts, for example the shoulders. the possibility also of producing more compact or even portable imaging devices while respecting the dimensioning ratios of the constituent elements of the system, to allow local examination of an arm, a leg or the head for example, without requiring the introduction of the whole body into the device unlike conventional systems. This latter possibility opens up an important field of application for further development, in particular for providing imaging devices adapted to specific needs.

Claims

1. Homogeneous field magnetic system comprising magnetic field generating means and magnetic field compensating means, characterized in that the magnetic field generating means are constituted by two annular elements (1, 2) substantially identical and arranged coaxially l '' to one another with a center distance (L) smaller than their radius (R), and that the magnetic field compensating means comprise a tubular element (3) made of ferromagnetic material arranged coaxially inside the annular generating elements magnetic field (B) and symmetrically with respect to the median plane (V) between said annular elements.

2. Magnetic system according to claim 1, characterized in that the tubular element (3) field compensator is constituted by a plurality of sheets of ferromagnetic material superimposed and electrically insulated from each other.

3. Magnetic system according to claim 1 or 2, characterized in that it further comprises a shielding cylinder (4) made of ferromagnetic material arranged coaxially and externally to the annular elements (1, 2) generating magnetic field and symmetrically by relation to the median plane (V).

4. Magnetic system according to claim 3, characterized in that it further comprises two annular discs (5, 6) of ferromagnetic material assembled at the two ends of the shielding cylinder (4) so as to enclose the elements externally and axially annular (1, 2) magnetic field generators.

5. Magnetic system according to claim 4, characterized in that the shielding cylinder (4) and the end discs (5, 6) are constituted by sheets of ferromagnetic material superimposed and electrically insulated from each other.

6. Magnetic system according to one of the preceding claims, characterized in that it comprises at least one secondary compensation ring made of ferromagnetic material disposed inside the compensating tube (3) coaxially with the latter and symmetrically with respect to in the median plane (V).

7. Magnetic system according to claim 6, characterized in that the secondary compensation ring (7, 8) of ferromagnetic material is provided with a toroidal coil (9) traversed by an electric current of adjustable intensity.

8. Magnetic system according to one of claims 1 to 5, characterized in that it comprises at least one secondary compensation ring (7, 8) in permanent magnet disposed inside the compensator tube (3) coaxially with this and symmetrically with respect to the median plane (V).

9. Magnetic system according to one of the preceding claims, characterized in that the ferromagnetic compensating tube (3) consists of several rings arranged end to end axially with optionally air gaps which may be constituted by an insulating material.

10. Magnetic system according to one of the preceding claims, characterized in that the annular elements (1, 2) generating the magnetic field are of the resistive or superconductive or permanent magnet type.