U.S. patent application number 12/670781 was filed with the patent office on 2010-09-02 for magnet assembly.
This patent application is currently assigned to Emscan Limited. Invention is credited to Bowden Colin Besant, Mario Vincent John McGinley, Mihailo Ristic, Robert Ian Young.
Application Number | 20100219833 12/670781 |
Document ID | / |
Family ID | 39768962 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100219833 |
Kind Code |
A1 |
McGinley; Mario Vincent John ;
et al. |
September 2, 2010 |
MAGNET ASSEMBLY
Abstract
An electromagnet comprising a ferromagnetic yoke which comprises
a yoke. Mutually opposing first and second pole pieces are
provided. The first pole piece is provided with a planar coil
having a first side facing the yoke and a second side facing the
yoke. A balancing member is arranged on the second side of the
planar coil to counterbalance the attractive force between the
planar coil and the yoke. The other pole piece may also be provided
with a corresponding balancing member.
Inventors: |
McGinley; Mario Vincent John;
(London, GB) ; Ristic; Mihailo; (London, GB)
; Besant; Bowden Colin; (Buckinghamshire, GB) ;
Young; Robert Ian; (Wiltshire, GB) |
Correspondence
Address: |
Kaplan Gilman & Pergament LLP
1480 Route 9 North
Woodbridge
NJ
07095
US
|
Assignee: |
Emscan Limited
Kent
GB
|
Family ID: |
39768962 |
Appl. No.: |
12/670781 |
Filed: |
July 22, 2008 |
PCT Filed: |
July 22, 2008 |
PCT NO: |
PCT/GB08/02496 |
371 Date: |
May 5, 2010 |
Current U.S.
Class: |
324/318 ;
335/296 |
Current CPC
Class: |
G01R 33/383 20130101;
H01F 3/10 20130101; H01F 6/04 20130101; H01F 7/06 20130101 |
Class at
Publication: |
324/318 ;
335/296 |
International
Class: |
G01R 33/34 20060101
G01R033/34; H01F 7/06 20060101 H01F007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 26, 2007 |
GB |
0714600.4 |
Jun 10, 2008 |
GB |
0810607.2 |
Claims
1. An electromagnet comprising a yoke and a pair of mutually facing
pole pieces, one or both of which is surrounded by a respective
coil, the coil or coils being supported by a supporting member
enclosed in a cryostat assembly and also being provided with a
respective balancing member which at least partially
counterbalances the attractive force between the coil or coils and
the yoke.
2. The electromagnet of claim 1, wherein the yoke is a
ferromagnetic yoke.
3. The electromagnet of claim 2, wherein the ferromagnetic yoke
comprises first and second arms linked by a spine, wherein the
first and second arms are provided respectively with said pole
pieces which are first and second pole pieces, the first pole piece
being provided with said at least one coil in the form of a planar
coil having a first side facing the first arm and a second side
facing the second arm, the balancing member being a ferromagnetic
balancing member arranged on the second side of the planar
coil.
4. The electromagnet of claim 3, wherein the second pole piece is
provided with a second one of said coils in the form of a planar
coil having a first side facing the second arm and a second side
facing the first arm, a second of said at least one balancing
members being a ferromagnetic balancing member arranged on the
second side of the second coil.
5. The electromagnet of claim 3, wherein at least part of either or
both of said at least one balancing members has a higher magnetic
permeability than that of its respective associated arm and/or a
higher magnetic saturation than that of the respective associated
arm.
6. The electromagnet of claim 3, wherein either or both of said at
least one balancing members counterbalances the attractive force
between the respective coil and arm by virtue of its position.
7. The electromagnet of claim 3, wherein either or both of said at
least one balancing members is non-ferromagnetically separated from
its respective pole piece and yoke arm.
8. The electromagnet of claim 3, wherein the yoke is C-shaped, or H
shaped and the arms and spine are straight or curved.
9. The electromagnet of claim 1, wherein the or each balancing
member is constituted by part of its associated pole piece.
10. The electromagnet of claim 9, wherein each pole piece is
generally annular and extends toward the other pole piece thereby
defining an outer wall in which is provided with a circumferential
recess, the balancing member being that part of the pole piece
extending beyond and at least partially below the second side of
the coil.
11. The electromagnet of claim 9, wherein the yoke is provided with
a recess, near or in which the associated coil is at least
partially situated.
12. The electromagnet of claim 9, wherein the pole piece is
constructed as a composite comprising two or more constituent
members, the balancing member comprising at least two of said
constituent members.
13. The electromagnet of claim 12, wherein at least one of the
constituent members has a substantially square or substantially
rectangular profile in cross section and wherein at least one
corner of the substantially square or substantially rectangular
profile is chamfered.
14. The electromagnet of claim 13, wherein the chamfering is on a
corner facing the coil.
15. The electromagnet of claim 12, wherein the constituent members
are respectively composed of different materials.
16. The electromagnet of claim 12, wherein one or more of the
constituent members of the pole piece is substantially annular such
that each pole piece has azimuthally-varying non-uniform height for
correcting components of field inhomogeneity which are not axially
symmetric.
17. The electromagnet of claim 11, wherein the pole piece is
generally annular and inside one of the annular pole pieces is
situated one or more ferromagnetic or permanent magnet annular
field tuning rings.
18. An electromagnet comprising a ferromagnetic yoke being provided
with at least one pair of mutually facing planar coils, each coil
in said at least one pair being supported by a supporting member
enclosed in a cryostat assembly and having a first side facing the
yoke and a second side facing the other coil, wherein a respective
ferromagnetic balancing member is arranged on the second side of
each coil to counterbalance the attractive force between the coil
and the yoke.
19. The electromagnet of claim 18, wherein each coil is situated
closer to its associated balancing member than to the yoke.
20. The electromagnet of claim 18, wherein at least part of each
balancing member has a higher magnetic permeability than that of
the yoke.
21. The electromagnet of claim 18, wherein at least part of each
balancing member has a higher saturation magnetisation than that of
the yoke.
22. The electromagnet of claim 18, wherein each balancing member is
constituted by part of a respective pole piece.
23. The electromagnet of claim 22, wherein each pole piece is
generally annular and extends toward the other pole piece thereby
defining an outer wall in which is provided with a circumferential
recess, near or in which the associated coil is at least partially
situated, the balancing member being that part of the pole piece
extending beyond and at least partially below the second side of
the coil.
24. The electromagnet of claim 22, wherein the yoke is provided
with a recess, near or in which the associated coil is at least
partially situated.
25. The electromagnet of claim 22, wherein each pole piece is
constructed as a composite comprising two or more constituent
members, each balancing member comprising at least two of said
constituent members.
26. The electromagnet of claim 25, wherein at least one of the
constituent members has a substantially square or substantially
rectangular profile in cross section and wherein at least one
corner of the substantially square or substantially rectangular
profile is chamfered.
27. The electromagnet of claim 26, wherein the chamfering is on a
corner facing the coil.
28. The electromagnet of claim 24, wherein the constituent members
are respectively composed of different materials.
29. The electromagnet of claim 24, wherein one or more of the
constituent members of each pole piece is substantially annular
such that each pole piece has azimuthally-varying non-uniform
height for correcting components of field inhomogeneity which are
not axially symmetric.
30. The electromagnet of claim 22, wherein both pole pieces are
generally annular and inside one or both generally annular pole
pieces is situated one or more ferromagnetic or permanent magnet
annular field tuning rings.
31. The electromagnet of claim 1, wherein each coil of said at
least one pair is supported on a respective yoke member, the yoke
members being joined by a support member, the coils being
superconducting coils sharing a common cryo-cooling system.
32. An electromagnet comprising a ferromagnetic yoke which
comprises first and second arms linked by a spine, wherein the
first and second arms are provided respectively with mutually
opposing first and second pole pieces, the first pole piece being
surrounded by a planar coil supported by a supporting member
enclosed in a cryostat assembly and also having a first side facing
the first arm and a second side facing the second arm, a
ferromagnetic balancing member being arranged on the second side of
the planar coil to counterbalance the attractive force between the
planar coil and the first arm.
33. An electromagnet comprising a ferromagnetic yoke which
comprises first and second arms linked by a spine, wherein the
first and second arms are provided with mutually opposing first and
second pole pieces, the first pole piece being provided with a
planar coil having a first side facing the first arm and a second
side facing the second arm, a ferromagnetic balancing member being
arrange arranged on the second side of the planar coil, at least
part of the balancing member having a higher magnetic permeability
than that of the first arm and/or at least part of the balancing
member having a higher saturation magnetisation than that of the
first arm.
34. An electromagnet comprising a ferromagnetic yoke which
comprises first and second arms linked by a spine, wherein the
first and second arms are provided with mutually opposing first and
second pole pieces, the first pole piece being provided with a
planar coil having a first side facing the first arm and a second
side facing the second arm, a ferromagnetic balancing member being
arranged on the second side of the planar coil, wherein the
ferromagnetic balancing member is non-ferromagnetically separated
from the first pole piece and the first arm.
35. The electromagnet of claim 32, wherein the or each coil is
situated closer to its associated balancing member than to the
yoke.
36. The electromagnet of claim 32, wherein at least part of the or
each balancing member has a higher magnetic permeability than that
of the yoke.
37. The electromagnet of claim 32, wherein at least part of each
balancing member has a higher saturation magnetisation than that of
the yoke.
38. An MRI, NMR or ESR machine comprising an electromagnet which
comprises a yoke and a pair of mutually facing pole pieces, one or
both of which is surrounded by a respective coil, the coil or coils
being supported by a supporting member enclosed in a cryostat
assembly and also being provided with a respective balancing member
which at least partially counterbalances the attractive force
between the coil or coils and the yoke.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an assembly for an
electromagnet, particularly an electromagnet of the kind intended
for producing a very high intensity magnetic field such as may be
used in a magnetic resonance imaging (MRI) system. However, it is
also applicable to other high field applications such as, but not
limited to, nuclear magnetic resonance spectrometry (NMR), electron
spin resonance spectroscopy (ESR) or general physics laboratory
research.
BACKGROUND OF THE INVENTION
[0002] Traditionally, MRI machines are large expensive devices
which have to be located in a specially constructed or adapted MRI
room and require the operator to have a high degree of skill. These
aspects preclude the use of MRI as a diagnostic tool in
applications where space is at a premium and where it would be
desirable for the machine to be operated by, for example, nursing
staff with only a limited degree of training. An example of such an
application would be in an accident and emergency (A & E)
unit.
[0003] To meet the aforementioned requirements, a magnet design
should ideally have one or more of the following attributes: [0004]
Open, to facilitate patient handling and to reduce and/or minimise
the risk of patient claustrophobia; [0005] Compact and light, for
ease of installation; [0006] Small fringe field, to reduce and/or
minimise the need for magnetic shielding of the room; [0007] Low
power consumption thus, low cryocooler power supply requirements;
[0008] Low cost; and [0009] Suitable for stand-by use, allowing
rapid field ramp-up and ramp-down.
[0010] An impediment to such a design is the strong attractive
force between the primary field coils and the magnet yoke which may
severely limit the achievable performance and practicality of the
magnet design.
[0011] The present invention addresses a solution to this problem
by provision of a counterbalancing member which extends beyond the
plane of a coil. It is known for pole pieces in MRI machines to be
formed with a "lip" which extends over the outer surface of the
coils, such as disclosed in GB-A-2 282 451 but conventionally, this
lip is not fabricated or configured so as to counterbalance the
aforementioned forces.
SUMMARY OF THE INVENTION
[0012] The present invention is aimed at provision of a compact,
open and low-cost magnet for a magnetic resonance imaging (MRI)
system. The magnet can offer a substantial imaging field of
typically around 0.5 Tesla at moderate weight and fringe field
which would facilitate its location in a wide range of
environments.
[0013] In accordance with at least one aspect, the present
invention provides an electromagnet comprising a yoke and a pair of
mutually facing pole pieces, one or both of which is provided with
a respective coil, the coil or coils being provided with a
respective balancing member which at least partially
counterbalances the attractive force between the coil or coils and
the yoke. Generally speaking, the or each balancing member may be
considered to be positioned closest to a side of the relevant coil
which is opposite to that side of the coil closest to the part of
the yoke which carries the associated pole piece. Thus, along the
axis of a coil, the coil may be considered to be situated between
that part of the yoke which supports the associated pole piece and
the balancing member (or a radial plane through the balancing
member, substantially parallel to the coil axis). At least part of
the balancing member or members may, for example, have a higher
magnetic permeability and/or a higher saturation magnetisation
value than the yoke or, at least, than that part of the yoke
carrying the associate pole piece(s).
[0014] Thus, a first aspect of the present invention may now
provide an electromagnet comprising a ferromagnetic yoke which
comprises first and second arms linked by a spine, wherein the
first and second arms may be provided respectively with mutually
opposing first and second pole pieces, the first pole piece being
provided with a planar coil having a first side facing the first
arm and a second side facing the second arm, a ferromagnetic
balancing member being arranged on the second side of the planar
coil to counterbalance the attractive force between the planar coil
and the first arm.
[0015] A second aspect of the present invention may provide an
electromagnet comprising a ferromagnetic yoke which comprises first
and second arms linked by a spine, wherein the first and second
arms may be provided with mutually opposing first and second pole
pieces, the first pole piece being provided with a planar coil
having a first side facing the first arm and a second side facing
the second arm, a ferromagnetic balancing member being arranged on
the second side of the planar coil, at least part of the balancing
member having a higher magnetic permeability than that of the first
arm and/or at least part of the balancing member having a higher
saturation magnetisation than that of the first arm.
[0016] A third aspect of the present invention may provide an
electromagnet comprising a ferromagnetic yoke which comprises first
and second arms linked by a spine, wherein the first and second
arms may be provided with mutually opposing first and second pole
pieces, the first pole piece being provided with a planar coil
having a first side facing the first arm and a second side facing
the second arm, a ferromagnetic balancing member being arranged on
the second side of the planar coil, wherein the ferromagnetic
balancing member is non-ferromagnetically separated from the first
pole piece and the first arm.
[0017] In respect of the third aspect of the present invention, the
ferromagnetic balancing member is preferably non-ferromagnetically
separated from the entire yoke and when present, so is a further
ferromagnetic balancing member. Non-ferromagnetic separation of one
member from another means that the two members are not physically
connected by ferromagnetic material, e.g., by virtue of being
separated by an air gap or being joined by a material which is not
ferromagnetic, etc.
[0018] In one or more embodiments, the second pole piece which is
associated with the second arm may not be provided with a
corresponding coil but may be configured such that between the pole
pieces, a substantially homogeneous magnetic field may be generated
in a subject examination region when the planar coil associated
with the first pole piece is energised.
[0019] In one or more additional embodiments, the second pole piece
which is associated with the second arm may also be provided with a
further planar coil having a first side facing the second arm and a
second side facing the first, a further balancing member being
arranged on the second side of the further planer coil to
counterbalance the attractive force between the further planar coil
and the second arm, e.g., (i) the further balancing member having a
higher magnetic permeability than that of the second arm and/or at
least part of the further balancing member having a higher
saturation magnetism than that of the second arm; and/or (ii) the
further ferromagnetic balancing member being non-ferromagnetically
separated from the second pole piece and the second arm.
[0020] A magnet assembly according to a fourth aspect of the
present invention may provide an electromagnet comprising a
ferromagnetic yoke supporting at least one pair of mutually facing
planar coils, each coil in said at least one pair having a first
side facing the yoke and a second side facing the other coil,
wherein a respective ferromagnetic balancing member is arranged on
the second side of each coil to counterbalance the attractive force
between the coil and the yoke.
[0021] Any electromagnet or electromagnet assembly according to any
single aspect of the present invention may incorporate any one or
more preferred and/or specifically described features of any
electromagnet or electromagnet assembly according to any one or
more of the other aspects of the invention. The present invention
also extends to a machine, in particular an MRI, NMR or ESR machine
comprising an electromagnet or electromagnet assembly according to
the invention. These machines may also comprise the requisite r.f.
coils (transmitter and receiver coils) or the r.f. coils may be
part of a free-standing separate unit. An MRI machine will normally
also include gradient coils. As used herein, the term MRI may
include fMRI (functional magnetic resonance imaging).
[0022] Depending on the particular aspect or aspects of the
invention, it may be preferable for at least part of any or each
balancing member to have a higher magnetic permeability than that
of its associated yoke arm and/or for at least part of the
balancing member to have a higher saturation magnetisation than
that of the relevant arm. Typically, the yoke may be made of low
carbon steel, preferably with the pole regions being laminated, the
balancing member to be composed of steel with a significant cobalt
and/or nickel content.
[0023] The electromagnetic assembly according to any of the first
to third aspects of the present invention may employ at least one
pair of pole pieces. Some embodiments may comprise a second pair of
pole pieces, e.g., with an axis of symmetry orthogonal to that of
the first pair pole pieces. The second and any further pairs of
pole pieces need not have the same geometry as the first pair of
pole pieces, or of each other and need not conform to the
definition of the present invention, although preferably they will
also meet this definition.
[0024] The electromagnetic assembly according to the fourth aspect
of the present invention may employ at least one pair of planar
coils. Some embodiments may comprise a second pair of planar coils,
e.g., with an axis of symmetry orthogonal to that of the first pair
of coils. The second and any further pairs of coils need not have
the same geometry as the first pair of coils, or of each other and
need not conform to the definition of the present invention,
although preferably they will also meet this definition.
[0025] By `planar` coils is meant coils which are generally annular
and although having a finite thickness (height), the windings
generally lie within a single plane, as opposed to having the
structure of an elongate (cylindrical) winding such as used in a
solenoid coil. Thus, considering the plane through the maximum
circumference of the annulus, such a coil can be considered to have
two sides.
[0026] Thus, preferably the coil or coils for producing the main
field, i.e., the coil(s) associated with the pole pieces is, or
are, planar, or rather "substantially planar". Obviously, any such
coil has a finite thickness. A "solenoidal coil" can be recognised
as having a diameter:height ratio of 1:1 or less, e.g., 1:2 or
less. A planar coil would normally be recognised as having a
diameter:height ratio of more than 1:1, e.g., at least 2:1. In the
case of the present invention, preferred planar coils have a
diameter:height ratio of least 5:1, more preferably at least 10:1
and still more preferably at least 15:1. In practice, the
diameter:height ratio is unlikely to exceed 50:1 and preferably, in
order to ensure that sufficient turns can be incorporated in the
windings, that ratio would not normally exceed 25:1.
[0027] The coil or coils are mounted on a yoke, substantially
facing each other. Therefore, preferably, the yoke is H-shaped or
C-shaped although to enable access of a subject in a range of
orientations, C-shaped is the preferred embodiment. "C-shaped"
includes a curved `C` configuration as well as a yoke configuration
which resembles three sides of a square or rectangle (i.e., a
substantially straight spine with respective substantially straight
arms extending substantially parallel to each other from either end
of the spine. However, even in the case of a continuously curved
C-shaped configuration, the yoke may be considered to comprise two
curved arms joined by a curved spine contiguous therewith.
Generally speaking, the pole pieces may extend and face inwardly,
extending towards each other from the ends of the arms.
[0028] The balancing member for any or each coil functions to
counterbalance, preferably to substantially totally counterbalance,
the attractive force between the coil and the yoke. To assist this,
a number of approaches may be employed, either singly or in
combination.
[0029] Thus, depending on the particular embodiment, the
electromagnet of the present invention may provide one or more of
the following advantageous constructional features: [0030] An open
magnet involving a yoke (C or H shaped) to offer naturally low
fringe field; [0031] Compact design derived from smaller diameter
coils and poles, made possible by the complexity of design of the
pole pieces; [0032] Balanced forces on the main coils resulting in
reduced load bearing requirements for the cold mass support; [0033]
A single cryostat, even in the case where two coils are used;
[0034] A low thermal load (in combination with a cryostat) owing to
the reduced cross section of cold mass support and hence, reduced
head load; and [0035] Reduced peak power owing to unshielded
gradients, can be made possible by the use of powder metal in
critical areas of the pole pieces (to reduce eddy currents in the
pole pieces due to rapidly switching gradient coils).
[0036] In one approach, it is preferred for any or each coil to be
situated closer to its associated balancing member than to the
yoke. It is also preferred for any or each balancing member to have
a higher magnetic permeability than that of the yoke. This means
that given the high flux generated by a coil suitable for use in
applications such as in an MRI machine, if the coil is sufficiently
close to the balancing member, it can achieve a higher degree of
magnetisation than the yoke, before saturation, bearing in mind the
need for the balancing member to have smaller mass/dimensions than
the body of the yoke. For similar reasons, it is also preferable
for at least part of any or each balancing member to have a higher
magnetic permeability than that of the yoke.
[0037] The balancing member may, for example, be in the form of one
or more independently supported ferromagnetic rings. However, in
one or more preferred embodiments, any or each balancing member is
preferably constituted by part of a respective pole piece, i.e., is
integral therewith. The pole pieces are ferromagnetic members
extending towards each other from the yoke in the vicinity of,
preferably passing through the coils. A particularly preferred
configuration is wherein any or each pole piece is generally
annular when viewed axially, although as will be explained in more
detail hereinbelow, the outer circumferential surface and/or inner
surface of the annulus may be provided with one or more projections
and/or indentations and/or irregularities, preferably continuously
around the annulus of the pole piece and/or in the yoke in the
vicinity of the pole piece, to provide advantageous properties, in
particular with regard to the functioning of the balancing member.
Provision of shims to fine-line the magnetic field is also
advantageous, e.g., to compensate for lack of axial symmetry in the
yoke.
[0038] A pole piece may be constructed as a composite, for example,
comprising two or more constituent members made from different
materials, e.g., two or more generally annular shaped pieces which
may be joined by any suitable means such as, but not limited to,
bolting. One or more of these individual members may constitute the
balancing member and therefore, may be formed of material(s) having
a higher magnetic permeability and/or higher saturation than the
yoke. In general, substantially each constituent member of the
composite may play a role in balancing the force. Members having a
higher permeability will contribute more than those having a
relatively lower permeability.
[0039] One or more of the constituent members are preferably
generally annular and most preferably, have a substantially square
or substantially rectangular profile when viewed in axial
cross-section. In one or more embodiments, it is especially
preferred for one or more of the corners of the substantially
square or substantially rectangular profile to be chamfered,
preferably on a corner facing the relevant coil. The constituent
members may be of such a configuration that each pole piece has
azimuthally-varying non-uniform height for correcting components of
field inhomogeneity which are not axially symmetric.
[0040] The pole pieces may be constructed with one or more other
advantageous features to ensure substantial homogeneity of the
generated field. For example, one or more ferromagnetic or
permanent magnet field-tuning rings may be provided, preferably
situated inside the inner radial profile of each pole piece. In
addition, where a pole piece comprises a plurality of substantially
annular constituent members made of different materials, these may
have non-uniform height (thickness) around the circumference to
correct fields in homogeneity in at least one direction, for
example in a linear direction substantially perpendicular to side
supporting member(s) in a C-shaped or H-shaped yoke, which will
generate non-axially symmetric components of magnetic field.
[0041] For MRI or similar high field applications, any or each coil
is preferably superconducting coils provided with a suitable
cooling means. Particularly preferred are coils made from one or
more high temperature superconducting materials. A `high
temperature superconducting material` may be a material which
demonstrates superconductivity at a temperature above 20.degree. K.
Alternatively, a `high temperature superconducting material` may
also be a material with a superconducting working temperature
operating as a magnet of above 10.degree. K. That is because for
many of these materials, the onset of superconductivity in the
characteristic curve of the superconductor is not very sharp.
Therefore, lower temperatures than the highest at which
superconductivity is first demonstrated are preferred for better
performance. A particularly preferred high temperature
superconductor is magnesium diboride (MgB.sub.2) or MgB.sub.2 doped
with another suitable material such as silicon carbide, hosted in a
matrix of copper. Other suitable high temperature superconductors
include niobium nitride, niobium carbide, niobium boride and
molybdenum diboride, although these materials require lower
temperatures than MgB.sub.2. Yet other high temperatures
superconductors which could be used include NbTi, NbSb, bismuth
strontium calcium copper oxide (BSCCO) and yttrium barium copper
oxide (YBCO) which have a higher critical temperature but are
generally less suited to high current density and long wire
applications, as well as being more expensive.
[0042] When a plurality of superconducting coils is mounted on a
generally C-shaped yoke, they would normally be fixed to a
respective arm member which is joined by a support member and can
share a common cryo-cooling system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] Without wishing to restrict the invention to the embodiments
shown here, the present invention will now be explained in more
detail by way of the following description of preferred embodiments
and with reference to the accompanying drawings in which:
[0044] FIG. 1 shows a three dimensional view of a first embodiment
of the overall structure of a magnet assembly according to the
present invention;
[0045] FIG. 2 shows a more detailed two dimensional cross-section
in the x-z plane of the assembly shown in FIG. 1;
[0046] FIG. 3 shows how the coils in the assembly of FIGS. 1 and 2
are joined by a supporting member;
[0047] FIG. 4 shows the first quadrant of the central region of the
assembly of FIGS. 1-3, showing further details;
[0048] FIGS. 5A through 5G show some basic alternative pole piece
balancing member geometries;
[0049] FIG. 6 shows a three-dimensional cross-section through a
second embodiment of the present invention; and
[0050] FIG. 7 shows a partial two dimensional cross-section through
the embodiment of FIG. 6, showing the lines of magnetic flux.
DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
[0051] Referring now to FIG. 1, a magnet assembly 1 comprises a
C-shaped steel yoke 3. The yoke itself comprises an upper arm 5,
and a lower arm 7 linked by a spine 9. Mounted on the inside
surfaces 11, 13 of the free ends 15, 17 of the arms 5, 7,
respectively, are upper and lower pole pieces 19, 21, surrounded by
respective planar annular drive coils 23, 25. In use, the coils
drive magnetic flux around the yoke to produce a substantially
uniform magnetic field in the central region between the poles.
[0052] The drive coil 23 has a first side 22 facing the upper yoke
arm 5 and a second side 24 facing the other coil 25. Similarly, the
drive coil 25 has a first side 26 facing the lower yoke arm 7 and a
second side 28 facing the other coil 23.
[0053] Referring to FIGS. 2 and 3, the pair of drive coils 23, 25
carry equal currents current i in the same sense and in series with
one another. The coil formers are joined by mechanical supporting
member 27 to one side of the coil in the +x direction. The coils
23, 25 and supporting member 27 are enclosed in a single cryostat
assembly (not shown) which is needed to cool the coils to
superconducting temperatures. This single-sided supporting member
is needed to provide free access to the imaging region of the
magnet assembly 1 from three sides. The supporting member 27 is on
the same side x of the magnet as the spine 9 of the yoke 3.
Attached to the supporting member 27 at the midpoint thereof is a
cold head 29 of a cryocooler system which provided the cooling
required for both coils through thermal conduction along the
support. The cryocooler can most conveniently be fed through a hole
in the centre of the spine 9 of the yoke 3, preventing any
interference to access on the open three sides.
[0054] In a conventional C-coil magnet the axial Lorentz magnetic
forces acting on a coil can be of the order of many tonnes. If this
were the case, then a single-sided support would not be practicable
since the moment of the forces would be too great for such a
support to bear.
[0055] However the yoke and pole design of this embodiment are such
that the axial forces on the coils are substantially balanced. The
coils 23, 25 are recessed into respective annular recesses 31, 33
in the side of the pole pieces 19, 21 so that the magnetic pull of
the coils 23, 25 onto the yoke 3 is substantially counterbalanced
by the force of the coils 23, 25 onto the pole pieces 19, 21.
Therefore, this single-sided support system may bear only the
weight of the coils and their respective formers, which need not be
excessive. There is a moderate static load on the force in the x
direction owing to the lack of axial symmetry of the yoke 3.
[0056] Any remaining unbalanced axial component of the force on
each coil is absorbed as a tension in the support member 27.
[0057] The net force on the combined coil system thus comprises the
combination of weight and static x load which can be made a
fraction of one tonne. This means that the supports for the coils
23, 25 may be made relatively light which in turn reduces the
thermal load into the system and hence the power needed for the
cryocooler.
[0058] FIG. 4 shows a cross section of the central radial region of
the magnet assembly 1 in one quadrant. There is mirror symmetry
about the z=0 axis. There is also rotational symmetry about the r
(radial) axis apart from the vertical portion of the yoke 3 which
is included for reference.
[0059] The winding cross section of the upper coil 23 is shown as
35. Such a winding tends to produce regions of high magnetic flux
density and stress at opposite corners of the winding
cross-section. Chamfers 37, 39 in the shape of the winding 35 may
be included in the design to eliminate such hot spots and help keep
the winding in conditions of magnetic field stress below their
critical levels and favourable for superconductivity.
[0060] The magnet winding 35 is formed of High Temperature
Superconductor (HTSC) wire fabricated from magnesium diboride
(MgB.sub.2). For operating fields between 0.5 T and 1 T, MgB.sub.2
has been proved to be usable in these conditions. The advantage is
that the operating temperature can be in the region of 20K rather
than the 4.2 K required for conventional superconductors. This
makes conductive cooling by a single cryocooler possible and at a
relatively low cooling power.
[0061] The top arm 5 of the yoke 3 is joined to the support member
9 on the +x side only.
[0062] The pole assembly comprises a set of concentric rings 41,
43, 45, 47 (four are shown here for example but this number could
vary) the purpose of which is to provide a varying inner and outer
radial profile for the pole. The outer radial profile is dominant
in determining the force balance in the coil, in particular the
outer diameters of rings 41 and 43. The coil is positioned such
that its inner radius is significantly less than the outer radius
of ring 45, thereby creating an annular recess for the coil 23. As
the coil 23 should preferably be positioned close to the outer
radius 44 of ring 43 and the outermost surface 46 of ring 45, one
alternative form of construction may be to incorporate these
surfaces into the composition of the room-temperature walls of the
coil cryostat.
[0063] A second feature of the outer radial profile of the pole
which helps balance the force is the recess 42, formed by the
radial step between the outer radii, between members 41 and 43 of
the pole assembly.
[0064] A third feature which helps to balance the force is the
recess 48 in the inside surface 11 of the upper arm 5 of the yoke
3, extending from the inner radial surface of concentric ring 41 to
just beyond the coil 23. This helps to reduce the outer axial force
on the coil.
[0065] A combination of two or all three of these measures may be
combined to achieve an optimal force balance.
[0066] The material of the rings may vary as is required to carry
the required flux. In particular rings 43 and 45 may be made of a
material of higher magnetic saturation such as, but not limited to,
a steel of high-cobalt content.
[0067] The lower rings 43, 45, 47 constitute a `balancing member`
of the pole piece. In the context of the present invention, it may
be understood that this arrangement is situated closer to the
second side 24 of the coil 23 than the first side 22 of the coil 23
is to the upper arm 5 of the yoke 3.
[0068] The inner radii of the pole piece rings 41, 43, 45, 47 are
designed mainly to shape the magnetic field for optimal
homogeneity. As well as the pole piece rings having the vertical
inner walls shown, they may also contain chamfers, tapers or more
general curved surfaces to this end. This includes the inner-facing
wall of the yoke top inside the ring 41. Ring 45 indicates a recess
with respect to ring 43 to allow additional space for the presence
of shim rings. The four or more shim rings 49 provide a means of
fine tuning the systematic magnet homogeneity design in combination
with the pole ring geometries.
[0069] The shim rings 49 may comprise a combination of high
permeability steel or permanently magnetized material such as, but
not limited to, neodymium iron boron in a general orientation of
magnetization including that opposite to the main field or in a
radial direction. It should be noted that the entire hollow space
inside the pole rings 41, 43, 45, 47 is available for shim rings as
required although they will tend to be more powerful nearer the
imaging volume as shown. The net result of the field optimization
is a region of high homogeneity suitable for MRI imaging inside the
spherical shell indicated by 48. For some applications of magnet
however it may be advantageous to optimize the homogeneity toward
an oblate or prolate spheroid instead of the sphere.
[0070] The asymmetry of the magnet due to the presence of the yoke
3 will produce a transverse (i.e. non-axially symmetric) variation
in field. To compensate for this, a set of azimuthal variations may
be employed to compensate for which there are several measures
which can be combined. These include tilt of the poles, selective
cutaways or chokes in the interfacing surfaces between the
interfaces between pole piece members 41, 43, 45, 47 and the inner
surface 11 of the upper yoke arm 5. Further sculpting of the cross
section of the support member 9 can also be employed.
[0071] The inner radius of the rose ring 47 is further recessed
with respect to ring 43 to accommodate three further assemblies
namely the shimset 51, the gradient set 53 and the rf transmit coil
55. The order of shimset 51 and gradient set 53 in the stack is
reversible in principle.
[0072] The shimset 51 comprises an array of passive steel or
permanent magnet dipoles which may be adjusted to compensate for
non-systematic variations in magnet homogeneity such as
manufacturing tolerances or the magnetic environment of the room.
The shims are adjusted according to a field map in the target
homogeneous volume 48.
[0073] The gradient set may be an actively shielded or
non-actively-shielded set. Should the non-actively shielded option
be chosen for maximum efficiency, the inward facing surfaces 14 of
the pole assembly 41, 43, 45, 47 may optionally be formed of
high-permeability powdered or laminated iron. The effect of this
arrangement is to carry gradient coil flux whilst inhibiting eddy
current flow when current levels in the gradient coils are
switched.
[0074] FIGS. 5A through 5G show different basic geometries of pole
piece with integral balancing member, to better explain the most
important features of, and some possible variations in the
embodiment shown in FIG. 4. In FIGS. 5A-5G, reference numeral 5
denotes the upper arm of the yoke as depicted in FIG. 4. Reference
numeral 61 shows an integral pole piece/balancing member as
referred to in the previous figures, which may be of composite form
and with relevant chamfers but for convenience, these are not
depicted here. Reference numeral 23 depicts the coil.
[0075] In FIG. 5A, numeral 61 depicts a conventional pole piece
without any configuration to act as a balancing member and is
therefore conventional, and not in accordance with the present
invention.
[0076] The geometry shown in FIG. 5B is like that in FIG. 5A, but a
recess 63 is formed in the side wall 65 of the pole piece, thereby
providing a laterally extending portion 67 to act as a balancing
member. The portion denoted 67 is made from cobalt steel whereas
the upper arm 5 of the yoke is made from laminated carbon
steel.
[0077] The configuration in FIG. 5C is the same as that in FIG. 5B,
except the coil 23 is situated to one side of the laterally
extending balancing member 67 which is of greater thickness than
that depicted in FIG. 5B.
[0078] In FIG. 5D, it can be seen that there is a recess 69 in the
upper arm 5 of the yoke.
[0079] The configuration in FIG. 5E can be seen to combine the
arrangements of FIGS. 5B and 5C and the configuration of FIG. 5F
combines the configurations of all of FIGS. 5B, 5C and 5D. FIG. 5G
shows the situation where there is an annular counterbalancing
member 64 not attached to the side wall 65. FIG. 5G is an example
of non-ferromagnetic separation.
[0080] FIG. 6 shows a second embodiment of an electromagnetic
assembly of an MRI machine in accordance with a second embodiment
of the present invention.
[0081] Reference numeral 71 is a circle depicting a notional
patient or subject examination area. In this Figure, reference
numerals which are the same as those used in FIGS. 1-5 denote
integers which are the same as in those latter Figures.
[0082] One difference between this second embodiment and the first
embodiment is that there is only one coil, namely the lower coil 25
surrounding the lower pole piece 21. An upper pole piece 73 is not
provided with a coil and is dimensioned so as to have a smaller
radial cross-sectional diameter than that of the lower pole piece
21. In addition, a `floating` balancing member 75 is separated by
an air gap 77 from the lower pole piece 71.
[0083] FIG. 7 shows a partial two-dimensional cross-section through
the view of FIG. 6, in which the feint lines show the lines of
magnetic flux. It can be seen that through the patient or subject
examination region 71, these are substantially homogeneous.
[0084] In the light of the described embodiments, modifications of
those embodiments, as well as other embodiments, for example as
defined by any one or more of the appended claims, will now become
apparent to persons skilled in the art. Although the invention
herein has been described with reference to particular embodiments,
it is to be understood that these embodiments are merely
illustrative of the principles and applications of the present
invention. It is therefore to be understood that numerous
modifications may be made to the illustrative embodiments and that
other arrangements may be devised without departing from the spirit
and scope of the present invention as defined by the appended
claims.
* * * * *