GB2284061A - Low-leakage hybrid MRI magnets - Google Patents

Abstract

A low leakage magnet with high field homogeneity comprises an assembly of permanent magnets, forming a closed magnetic circuit, in which at least one of the permanent magnets is replaced by a thin-walled coil acting as a current sheet surrounding the working volume. All of the permanent magnets, which may be of barium ferrite type, may be replaced (figure 12). Modifications to improve patient access may be provided (figures 7 and 8). The coils may be superconducting. <IMAGE>

Description

SYSTEMS FOR GENERATING MAGNETIC FILEDS WHICH ARE UNIFORM OVER A GIVEN WORKING SPACE AND ARE ZERI OUTSIDE THAT SPACE.

This invention relates to a means of generating magnetic fields (hereinafter abbreviated to fields) which,in the ideal case, are precisely uniform throughout a prescribed working space and identically zero external to that space.The invention is,in its most general form,a hybrid magnet consisting in part of permanent magnets (abreviated to magnet) and in part of a thin-walled current-carrying coil (abbreviated to coil with the presence of the current implied) or of a number of such coils.The ideal case referred to is that in which the magnets are perfectly uniformly magnetized and the coils are thin-walled to the extent that they can be considered to constitute current sheets of negligible thickness characterized by a surface current density 1 (A m-1) per unit length measured normal to the current vector.The first condition may be closely approached by recourse to modern materials and it will be seen that in the presence of the currents there are no demagnetizing fields within the magnetic material so that only irreversible demagnetization in the temporary absence of the currents is of significance.The second condition may be the more closely approached the greater the size of the asembly,since then the ratio of the well thickness divided by the coil diameter may be made small for a given current per unit cross-sectional area J.The walls of the coil may also be reduced in relative thickness by increasing J in pulse operation or by using superconductors but,in general, continuous operation with resistive windings will be assumed.In an extreme example of the invention it will be seen that all the magnets may be replaced by equivalent coils so that the hybrid system is replaced by one which consists of coils alone.

Uniform magnetic fields of a prescribed magnitude are the primary requirement for a wide range of scientific and technical instruments or devices.One example is magnetic resonance spectroscopy,for which a low-magnitude variable field may be superimposed on the pescribed large static field.An example of greatest importance and topicelity is (nuclear, magnetic resonance imaging or MRI,for which a smaller gradient field with controled variations in space and time is generally superimposed on the large uniform static field,the primary requirement here, for whole-body diagnostic MRI,being for fields which are uniform over large working volumes.When this is achieved by the use of superconducting solenoids or of large conventional electromagnets or simple permanent magnet assemblies,correspondingly large fields may extend over large regions around the equipment and these are disadantageous for several reasons.

According to the present application there is provided primarily a hybrid magnet system comprising an assembly of magnets and a thin-walled coil,constructed and assembled in the manner to be described and with a particular relation between the intensity of magnetization M of the magnets and the surface current density I of the coil,giving a field H or B = oH which is uniform within the working space enclosed by the coil with a sharp cut-off to zero field outside that space.In practical realizations of the invention the ideal specifiations,H perfectly uniform or identically zero, may be reasonably approached to the extent that the wall-thickness of the coil may be made substantially less than its diameter.The magnitude of the filed is simply equal to M (in the S1,or 4# M in the cgs system) in the simpler examples and may thus range up to about B = 1.0 Tesla or H = 800 000 A m or 10 000 Dersteds.More than one coil may be included in the assembly and in a particular example to be described the system becomes purely an assembly of coils.

Specific embodiments of the invention will now be described by way of examples with reference to that acompanying drawings.

Figure 1 shows an example in which the array of magnets all have the same magnetization,the directions of which are indicated by the arrows M. With the angles &alpha; = 45 ,ss = 90 ,the only net components of M across any interface appear at the top and bottm of the central cavity or working space S.The magnets alone produce a filed in S which is far from uniform and large fields arise throughout the magnet material and the surrounding space.The coil which is also shown in the Figure and separately in Fig 2,fits closely inside the space S with external dimensions those of S and a thickness which is idealy much less than the dimension of the coil in the plane containing the currents.The coil is uniformly wound and approximates a current sheet of density I Ampere metre-1 .With the condition I = M in this case and assuming the limits specified above, the filed inside the working space S,for the total assembly,is uniform,equal to M and directed vertically upward in the Figure while the field external to S is zero.That this is so can be demonstrated by numerical calculations or on the grounds of general magnetostatic principles.

Figure 2 shows the coil alone, for greater clarity.

Figure 3 shows,in section,an example indicative of the flexibility of the basic invention,the geometry here being such that most of the field energy is contributed by the magnets and little by the currents and,further ,deviations from the ideal specifications due to the finite coil wall thickness are less pronounced than in the example of Figure 1.

Figure 4 shows the cross-section of an example of the invention in which two different magnet materials with magnetizations M1 and M2 ,M2 greater than M1 ,are utilized so as to reduce the size and weight for a given working space,a consideration of importance in MRI where the space may be of the order of 1 m .The angle &alpha; is such that M1 cos &alpha; = M2 sin&alpha; ,the current density is I = M1 and the field magnitude is H = M1 (in the SI).M1 might be that appropriate to barium ferrite,giving B = o H = 0.5 Tesla approximately,with reasonably low current densities.

Figure 5 illustrates in cross-section a less symmetrical example of the invention,again indicating the flexibility of the basic design,the filed in the working space being uniform and equal to M1 if the current density is set equal to M1 .

Figure 6 illustrates a furtehr example of the invention wherein relatively low-weight and low-field specification,still adequate for MRI,are achieved without commissioning special low-M magnet materials,by assembling laminations or magnet frames with intervening air spaces.The current density required is now I = M 1/w with 1 the lamination thickness and w the width of the itervening spaces.If the period 1 + w is substantially less than the characteristic dimensions of the working space the field uniformity in the central region of the space is not greatly affected.This example may be considered environmentally advantageously in relation to whole-body MRI.

Figure 7 illustrate an example in which free access to the working space is achieved by forming part of the coil into a gate afforded with contacts to which the conductors are gathered.The conductors in the gate may constitute arrays of relatively heavy rods or strips without excessive disturbance of the field uniformity in regions some distance from the gates in an elongated assembly.

Figure 8 illustrates an example in which continuous access is afforded by substantial distortions of the current sheets at the ends of an elongated assembly,the fileds now being adequately uniform over the central regions only.

Figure 9 and the following Figures 10 to 12 show further examples illustrating the flexibility with which the basic invention may be modified while preserving the underlying magnetostatic principles.Figure 9 itself shows an assembly of four magnets giving zero field H both internally and externally since,with all principal angles either 45 or 90 there are so no net normal components of M at any interface.

Figure 10 shows a coil constructed with the shape of the lower long magnet in Fig 9,carrying a surface current of magnitude I = M on the top,bottom and vertical side faces,with the directions indicated or implied,but with I = M @#2 on the slanting end faces,as achieved automatically with uniform winding,this being termed the coil which is equivalent to the particular magnet.The specification of this equivalence is that the surface current density of the coil should be equal to the vector product of the magnetization vector and the outwardly-drawn unit vector normal to the appropriate surface of the magnet to which the coil is equivalent.When this is so the external fields of the magnet and of the equivalent coil are the same while the internal field of the coil differs from that in the equivalent magnet by the addition of a component which is everywhere equal to M.

Figure 11 shows an example of the invention in which the upper long magnet of an assembly similar to that in Fig 9 is replaced by its equivalent coil,giving a working space within the coil wherein the field H is uniform and equal to M'= M tan &alpha; with the direction indicated,whie all fields external to this space are zero.If the coil were a direct replacement of the upper magnet in Fig 9 as drawn,the requisite current on the top,bottm and vertical side faces would be I = M and the field within the coil would be H = M simply.However it has been chosen to draw Fig 11 with the angle &alpha; less than 45 .With the corresponding implicit adjustment of Fig 9 the condition of zero initial field is given by changing the magnatization of the upper long magnet to M'= M tan&alpha;.Thus the equivalent coil in Fig 11 carries a surface current of I = M'on the top,bottom and vertical sides with I = M'sin &alpha; on the slanting ends and the field within the coil is M'or M tan &alpha;.Thus,in this particular example,the field magnitude may be specified,by the choice of &alpha; ,between limits controlled in practice only by the rising current density as H increases and the expanding working space as H decreases,even with the same permanent magnet material.This constitues a further practicable magnet for MRI so long as one face of the coil is hinged or detatchable, while in a further modification one end magnet may also be replaced by its equivalent coil so as to facilitate access by detatching this end coil and one end face of the long coil.It is noted that the example of Fig 1 may also be regaredd as an assembly of magnetic giving zero field throughout in which the notional central magnet,occupying the space S with M directed vertically upwards,as been replaced by its equivalent coil.

Figure 12 shows what may be termed an extreme example of the invention in which all the magnets of an assembly such as that illustrated by Fig 9,but more generally any assembly having the feature that alone it generates no internal or external fields,are replaced by their equivalent coils.This example is not itself a hybrid magnet but is purely a system of coils or conductors,maintaining the essential feature that,since the basic principales of the design are preserved,the external fields are zero while th einternal fields are uniform within any one closed coil,in the limit of negligible coil wall thickness,although there are abrupt changes in direction from one coil to another.A particular feature of the invention as realized in this example is that,since all the coil currents may be simultaneously adjusted,the fields produced are readily variable and not fixed according to the magnetization of the magnets included in the hybrid examples.

Claims (10)

1. CLAIMS 1 A hydrid magnet system,in the general case,as illustrated primarily by Fig 1 and Fig 3,based on the princple of first devising a model assembly of permanent magnets which gives zero field throughout and replacing one or moe of those magnets by thin-walled coils having the same shape as the magnets and carrying currents with densities which render the coils equivalent to the magnets replaced,giving external fields identical to the magnets replaced,the fields in the working spaces which are the spaces enclosed by the coils then being ideally uniform in each coil while the fields external to the coils are identically zero,both specifications applying in the limit in which the magnetization of the permanent magnets is taken to be uniform and the wall thicknesse of the coils to be negligible.
2. 2 A magnet system as claimed in Claim 1 wherein permanent magnet materials having different intensities of magnetization are utilized in such a way,as illustrated by Fig 4,as to achieve reductions in size and weight for a given working space.
3. 3 A magnet system as claimed in Claims 1 and 2 wherein the permanent magnet assembly is subdivided to form laminations,as illustrated by Fig 6,so as to achieve further control over the weight of the system and of the magnetic field intensity.
4. 4 A magnet system as claimed in Claim 1 - 3 wherein parts of the coils take the form of detachable or hinged gates so as to provide access to the working space.
5. 5 A magnet system as claimed in claims 1 - 3 wherein parts of the coil are deformed in such a way,as illustrated by Fig 6,as to permit continuous access to the working space.
6. 6 A magnet system of the general type exemplified by Claim 1 based on the principle of devising an@ arra@ of permanent magnets such as that shown in Fig 9 which,in itself,gives zero field throughout all space and replacing one of these magnets by an equivalent coil,as defined in the foregoing,so as to give a uniform field in the space enclosed by the coil and zero external field.
7. 7 A magnet system as calimed in Claim @ wherein part of the coil is modified to give access to the working space.
8. 8 A magnet system as claimed in Claim 6 or Claim 7 wherein two or more of the magnets are replaced by their ac equivalent coils.
9. 9 A system as claimed in claim 8 wherein specifically all of the magnets. in the initial model assembly are replaced by their equivalent coils so as to give,ideally,uniform fields within each coil and zero field external to the coil assembly,in this limit which constitutes purely a system of coils.
10. 10 A magnet system consisting of permanent magnets and coil or purely of coils designed according to the magnetostatic principles specified and substantially as described herein with reference to Figures 1 - 12 of the accompannying drawings.