The invention relates to a magnetic field generating assembly.
A wide variety of magnetic field generating assemblies have been
designed in the past for use in different applications, all of which require the
generation of a magnetic field in a working volume within which tasks are to be
performed. Examples of such applications include magnetic resonance imaging (MRI)
and nuclear magnetic resonance spectroscopy. These particular applications are
complicated further by the fact that in order to achieve the high field strength
required, superconducting magnets must be used with the consequent need for bulky
cryogenic equipment and the like. In the past, the working volume has been provided
within the bore of a superconducting coil although more recently certain proposals
have been made for projecting at least a homogeneous region of the magnetic field
outwardly of the bore to increase the ease of access. However, these constructions
are still bulky and expensive.
GB-A-2174248 describes a magnetohydrodynamic device. This is, however,
unsuitable for performing a nuclear magnetic resonance experiment on the human
In accordance with the present invention, the magnetic field generating
assembly comprises a magnetic field generator positioned between and spaced from
opposed pole pieces which are mounted in a wall of magnetic material surrounding
the magnetic field generator, the wall providing a substantially closed magnetic
flux path, and the arrangement being such that forces on the generator due to magnetic
flux in the wall are substantially balanced, characterized in that the magnetic
field generator comprises at least two counter-running, nested coils; in that the
gap between the magnetic field generator and at least one of the pole pieces is
sufficient to accommodate a human body; and in that the opposed pole pieces are
non-planar, the arrangement being such that when working currents flow in the
coils a magnetic field is generated in a working region situated in the gap between
the magnetic field generator and the at least one pole piece having a homogeneity
suitable for performing a nuclear magnetic resonance experiment.
This design leads to a new concept in magnetic field generating assemblies
since by causing the wall of magnetic material to have a significant influence,
a single, short magnetic field generator can be used to provide a source of magnetomotive
force leading to the use of a very cheap cryostat. Furthermore, the net force
on the cryostat will be substantially zero due to the balancing affect of the wall.
In addition, the wall provides magnetic shielding externally of the assembly.
Preferably, the balancing of forces is achieved by constructing the
assembly in a symmetrical form with the magnetic field generator symmetrically
positioned within the wall. Alternatively, it may be possible to achieve the same
balancing affect with a non-symmetrical arrangement by creating a pseudo-pole on
one side of the generator relatively closer to the generator than the other pole.
Typically, the wall will be made from iron or some other ferro-magnetic
In some arrangements, the homogeneity of the magnetic field in the
working region between the magnetic field generator and at least one of the pole
pieces will be sufficient for the purpose to which the assembly is to be put.
The homogeneity can be controlled during the design of the assembly by considering
the effect of the wall and the magnetic field generator together using finite
element methods. In particular, the wall itself may be contoured to achieve homogeneity
within the working region. For example, the wall is preferably rectangular or
square with the magnetic field generator placed substantially parallel with opposed
sides of the wall. In this case, preferably the internal surfaces of the sides
of the wall extending from the generator to the opposed walls taper towards the
generator. In some cases, however, the homogeneity produced by the generator and
wall alone may not be high enough in which case additional magnetic shims could
be provided. Such shims could comprise shaped iron, permanent magnets or coils
either on the wall or within the generator.
One major advantage of this system when applied to MRI is that the
space between the magnetic field generator and the at least one pole piece is sufficiently
large to allow a human patient to sit or stand within the space thus considerably
reducing the claustrophobic feeling which a patient presently suffers when inserted
in to the bore of a coil. Indeed it is envisaged that a patient could be sufficiently
free to move in the working region as to exercise in cardiac studies. Furthermore,
where the generator is symmetrically positioned between the two pole pieces, a
pair of homogeneous working regions can be provided on each side of the generator
allowing, in the case of MRI, two patients to be processed at once.
The pole pieces may be defined by parts of the wall itself or by
additional elements mounted to the wall. Typically they are contoured so as to
present a generally planar central face to the magnetic field generator but with
a circular recess substantially coaxial with the coils positioned radially outwardly
of the central face.
Examples of a magnet assembly for MRI in accordance with the present
invention will now be described with reference to the accompanying drawings, in
Figure 1 is a side elevation of one example of the assembly;
Figure 2 is a plan of the Figure 1 assembly;
Figure 3 is an end elevation of the Figure 1 assembly;
Figure 4 illustrates the magnetic flux distribution within the Figure 1 assembly;
Figure 5 illustrates a quadrant of a second assembly with the outer wall omitted;
Figure 6 illustrates the second assembly with a portion of the outer wall;
Figures 7A and 7B are a cross-section and view on X-X respectively of a third
The assembly shown in Figure 1 comprises a pair of nested, counter-running
coils 1 of superconducting wire mounted within a cryostat 2. Facing axially outwardly
from the cryostat 2 are a pair of shaped, iron pole pieces 3, 4. The cryostat
2 is mounted within an open sided box of iron defined by a substantially square,
iron wail 5. The coils 1 are arranged with their axes orthogonal to opposed end
sections 6, 7 of the wall 5, the sections 6, 7 carrying respective shaped iron
pole pieces 8, 9 coaxial with the coils 1.
In the past, where a magnet assembly suitable for MRI was required,
coils having a relatively short axial length, as shown in Figures 1 and 2, would
not be appropriate due to the considerable inhomogeneities in the magnetic field
which such a coil would produce. This has been overcome at least partly by providing
the iron wall 5 which, due to the magnetic flux which passes through the wall,
contributes to the magnetic field in the regions 10, 11 defined between the opposed
pairs of pole pieces 3, 8 and 4, 9 respectively. However, in general the degree
of homogeneity achieved is still not sufficient for MRI and additional shim pieces
12-15 are provided mounted on the inner surfaces of the wall 5. The size, position
and strength of these permanent magnet shims is determined empirically during design
of the assembly.
Figure 4 illustrates the flux distribution in one quadrant of an
assembly similar to Figure 1 but with three coils and with a contoured pole face
on the passive side only and without side shims on the iron frame. The quadrant
shown will be mirrored in the other three quadrants. For this computer simulation
the effect of the shaped pole pieces 8, 9 and shims 12-15 has been ignored. In
this case, the vertical axis in Figure 4 corresponds to the axis of the coil 1
while the horizontal axis in Figure 4 extends in the vertical direction of Figure
1. It can be seen from Figure 4 that the magnetic flux lines are concentrated within
the iron wall 5 with very little flux leaking beyond the wall indicating that
the wall acts as a shield as well as providing a return path for a majority of
the magnetic flux. Figure 4 also illustrates the 5 Gauss and 1 Gauss contours
outside the wall 5 illustrating the shielding achieved considering the bone field
has a magnitude of 5338.11 Gauss. The Figure also shows the location of the volume
over which the field varies by ± 1000ppm and it can be seen that this lies at a
radius of about 50 cm along the coil axis and at a radius of about 50 cm along
the coil radius.
In use, each of the regions 10, 11 can be used for a separate MRI
experiment with different patients being positioned simultaneously in each region.
Typically, one patient could be made ready in one region while another patient
was being imaged in another. This would mean that only one computer system would
be required since it could be shared between the two regions. The size of the
regions 10, 11 is such that a patient could be examined lying down or standing
up or in some intermediate position such as sitting or exercising to achieve particular
responses to, for example, drug therapy or diseased condition. Indeed the patient
could simply walk into the appropriate region.
The symmetrical arrangement of the coil 1 and wall 5 should be noted
since it is this which provides substantial symmetry of forces on the coil 1 thus
avoiding the need for complex and costly support structures. Furthermore, the
use of a thin coil results in a relatively small cryostat volume so that heat
leakage to the system will be small and there will be a very low cryogen demand.
In order to reduce the need for shims and the like, a more complex
coil system can be used for the magnetic field generator. For example, a nested
arrangement of counter-running coils could be employed as described, for example,
in EP-A-0160350. Part of such an arrangement is illustrated in Figure 5 in which
a 4 coil system is provided comprising coils 20-23 arranged concentrically and
with the relative numbers of turns in each coil being illustrated diagrammatically
by the thickness in the radial direction of the coils. It will be noted from the
arrows shown in Figure 5 that the working currents in coils 21 and 23 run in the
opposite direction (clockwise) to the current direction in the coil 22 (anti-clockwise).
In addition, in this case, a ring shaped iron shim 24 is provided (only a quadrant
of which is shown in Figure 5) positioned between the coils 21,22 to act as a shim
and further homogenize the magnetic field in the working region. The nested coils
can be arranged to result in projecting the centre of the homogeneious region out
of the volume defined by the coils. This is again decribed in the European Specification
mentioned above. Preferably, the centre of the homogeneious region can be projected
into the gap or space between the coils and the remote or passive pole piece and
thus optimize the region suitable for an MRI experiment.
By performing the finite element method mentioned above in order
to determine the form of the coils and iron wall required, it has been found that
a particuarly suitable geometry for the iron wall has a taper extending radially
inwardly from the position of the coil to the facing wall section. This is illustrated
in Figure 6 inwhich the coils 20-23 of Figure 5 are shown together with part of
the iron wall. As can be seen, an upper section of the iron wall 25 has an inner
surface 26 which tapers towards a vertical section 27 of the wall carrying a pole
piece 28. As can be seen in Figure 6, the pole 28 is shaped to have a generally
circular, outer section 29 with, a recessed, planar central portion 30.
A further example is shown in Figures 7A and 7B. In this example,
the magnetic field generator comprises a set of three coaxial, superconducting
coils 31-33 mounted in respective formers within a helium vessel 34 of a cryostat
35. The Table below sets out an indication of the position of the coils 31-33 relotive
to an axis 45 and the number of amp-terms.
The cryostat 35 has a helium vessel 34, as mentioned above, and a
generally conventional form including a nitrogen vessel surrounding the helium
vessel and an evacuated outer casing. The cryostat defines a bore 36 having a
diameter of 64.7 cm.
The cryostat 35 and coils 31-33 are positioned in an iron wall 37
which defines an open ended box having upright end walls 38,39 and generally horizontal
top and bottom walls 40,41. The walls 38-41 have a thickness of about 20 cm and
the walls, 38,39 have a height of about 560 cm. The overall width of the walls
between the open ends (as seen in Figure 7B) is about 420 cm while the length
of the walls 40,41 is about 230 cm.
A pair of iron pole pieces 42,43 are mounted on the sides of the
walls 38,39 facing the coils 31-33. It will be seen that they are contoured so
as to have a raised, circular central portion 44 coaxial with the axis 45 of the
coils, a radially outer recess 46 coaxial with the axis 45 and a further, circular
raised portion 47 coaxial with the axis 45. The exact form of the pole faces 42,43
is determined by calculation so that the magnetic field generated by the coils
31-33 combined with the effect of the iron box and pole faces causes a substantially
homogeneous field to be generated within working regions 48,49 between the coils
31-33 and respective pole pieces 42,43.
Typically, the coil configuration shown in Figure 7A and 7B will
produce magnetic fields within the working regions 48,49 having a homogeneity ±
100ppm within a sphere having a diameter of 26 cm. The working regions are centred
between the plane of the coils 31-33 (indicated at 50) and the plane of the pole
The gap between the side faces of the cryostat 35 and the adjacent
pole pieces is about 57.5 cm and the working regions 48,49 can be arranged to be
centred at the centre of these gaps.
Anordnung zur Erzeugung eines magnetischen Feldes, die eine Magnetfelderzeugungseinrichtung,
die zwischen und beabstandet von einander gegenüberliegenden Polstücken (8, 9)
angeordnet ist, die an einer Wand (5) aus magnetischen Material angebracht sind,
die die Magnetfelderzeugungseinrichtung umgibt, umfaßt, wobei die Wand (5) einen
im wesentlichen geschlossenen magnetischen Flußweg bildet und die Anordnung so
ist, daß die aufgrund des magnetischen Flusses in der Wand (5) auf die Erzeugungseinrichtung
wirkenden Kräfte im wesentlichen ausgeglichen werden, dadurch
gekennzeichnet, daß die Magnetfelderzeugungseinrichtung wenigstens zwei
gegenläufige, ineinandergeschachtelte Spulen (1) umfaßt; und dadurch, daß der Spalt
zwischen der Magnetfelderzeugungseinrichtung und wenigstens einem der Polstücke
ausreicht, um einen menschlichen Körper aufzunehmen; und dadurch, daß die einander
gegenüberliegenden Polstücke (8, 9) nicht eben sind, wobei die Anordnung so ist,
daß, wenn in den Spulen (1) Arbeitsströme fließen, ein magnetisches Feld in einem
Arbeitsbereich (10, 11) erzeugt wird, der sich in dem Spalt zwischen der Magnetfelderzeugungseinrichtung
und dem wenigstens einen Polstück (8, 9) befindet, dessen Homogenität zur Ausführung
eines nuklearen Magnetresonanzversuchs geeignet ist.
Anordnung nach Anspruch 1, wobei die Magnetfelderzeugungseinrichtung symmetrisch
innerhalb der Wand (5) angeordnet ist.
Anordnung nach Anspruch 1 oder Anspruch 2, wobei die Wand (5) aus einem ferromagnetischen
Anordnung nach einem der vorangehenden Ansprüche, wobei jedes Polstück (8,
9) eine im wesentlichen ebene Mittelfläche hat, die der Erzeugungseinrichtung zugewandt
ist, und wobei eine runde Aussparung (46) radial außerhalb der Mittelfläche koaxial
zur Spulenposition ist.
Anordnung nach einem der vorangehenden Ansprüche, wobei die Innenflächen (26)
der Seiten der Wand (5), die von der Erzeugungseinrichtung zu den einander gegenüberliegenden
Wänden verlaufen, in Richtung der Erzeugungseinrichtung aufeinanderzulaufen.
Anordnung nach einem der vorangehenden Ansprüche, die weiterhin ein oder mehrere
Beilagestücke (12 - 15) umfaßt, die an Innenflächen der Wand (5) angebracht sind,
um die Homogenität des magnetischen Feldes innerhalb des Arbeitsbereiches (10,
11) zu steuern.
Anordnung nach Anspruch 2, wobei ein Paar Arbeitsbereiche (10, 11) an jeder
Seite der Erzeugungseinrichtung vorhanden sind.
A magnetic field generating assembly comprising a magnetic field generator
positioned between and spaced from opposed pole pieces (8,9) which are mounted
in a wall (5) of magnetic material surrounding the magnetic field generator, the
wall (5) providing a substantially closed magnetic flux path, and the arrangement
being such that forces on the generator due to magnetic flux in the wall (5) are
substantially balanced, characterized in that the magnetic field generator comprises
at least two counter-running, nested coils (1); in that the gap between the magnetic
field generator and at least one of the pole pieces is sufficient to accommodate
a human body; and in that the opposed pole pieces (8,9) are non-planar, the arrangement
being such that when working currents flow in the coils (1) a magnetic field is
generated in a working region (10,11) situated in the gap between the magnetic
field generator and the at least one pole piece (8,9) having a homogeneity suitable
for performing a nuclear magnetic resonance experiment.
An assembly according to claim 1, wherein the magnetic field generator is positioned
symmetrically within the wall (5).
An assembly according to claim 1 or claim 2, wherein the wall (5) is made from
a ferromagnetic material.
An assembly according to any one of the preceeding claims, wherein each pole
piece (8,9) has a generally planar central face presented towards the generator
with a circular recess (46) coaxial with the coils position radially outwardly
of the central face.
An assembly according to any one of the preceeding claims, wherein the internal
surfaces (26) of the sides of the wall (5) is extending from the generator to the
opposed walls taper towards the generator.
An assembly according to any one of the preceeding claims, further comprising
one or more shim pieces (12-15) mounted to internal surfaces of the wall (5) for
controlling the homogeneity of the magnetic field within the working region (10,11).
An assembly according to claim 2, wherein a pair of working regions (10,11)
are provided on each side of the generator.
Ensemble générateur de champ magnétique comprenant un générateur de champ magnétique
placé entre, et espacé par rapport à, des pièces polaires (8, 9) opposées qui
sont montées dans une paroi (5) de matière magnétique entourant le générateur de
champ magnétique, la paroi (5) procurant un trajet de flux magnétique sensiblement
fermé, et l'agencement étant tel que les forces appliquées au générateur, en raison
du flux magnétique dans la paroi (5), sont sensiblement équilibrées, caractérisé
en ce que le générateur de champ magnétique comprend au moins deux bobines emboîtées
(1), bobinées en sens contraire ; en ce que l'espace entre le générateur de champ
magnétique et au moins l'une des pièces polaires est suffisant pour loger un corps
humain ; et en ce que les pièces polaires (8, 9) opposées ne sont pas planes,
l'agencement étant tel que, lorsque des courants de travail s'écoulent dans les
bobines (1), un champ magnétique, ayant une homogénéité appropriée pour réaliser
une expérience de résonance magnétique nucléaire, est produit dans une région de
travail (10, 11) située dans l'espace entre le générateur de champ magnétique
et ladite au moins une pièce polaire (8, 9).
Ensemble selon la revendication 1, dans lequel le générateur de champ magnétique
est placé symétriquement à l'intérieur de la paroi (5).
Ensemble selon la revendication 1 ou la revendication 2, dans lequel la paroi
(5) est faite d'une matière ferromagnétique.
Ensemble selon l'une quelconque des revendications précédentes, dans lequel
chaque pièce polaire (8, 9) a une face centrale globalement plane dirigée vers
l'extérieur du générateur avec un évidement circulaire (46) coaxial, les bobines
étant disposées radialement vers l'extérieur de la face centrale.
Ensemble selon l'une quelconque des revendications précédentes, dans lequel
les surfaces internes (26) des côtés des parois (5) s'étendant à partir du générateur
vers les parois opposées sont en biseau en direction du générateur.
Ensemble selon l'une quelconque des revendications précédentes, comprenant
en outre une ou plusieurs pièces de compensation (12 à 15) montées sur les surfaces
internes de la paroi (5) pour commander l'homogénéité du champ magnétique à l'intérieur
de la région de travail (10, 11).
Ensemble selon la revendication 2, dans lequel deux régions de travail (10,
11) sont disposées une de chaque côté du générateur.