U.S. patent application number 09/738838 was filed with the patent office on 2001-06-28 for mold for bonding mri pole piece tiles and method of making the mold.
Invention is credited to Aksel, Bulent, Barber, William D., Laskaris, Evangelos T., Ogle, Michele D., Thompson, Paul S., van Oort, Johannes M..
Application Number | 20010005165 09/738838 |
Document ID | / |
Family ID | 22733655 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005165 |
Kind Code |
A1 |
Laskaris, Evangelos T. ; et
al. |
June 28, 2001 |
Mold for bonding MRI pole piece tiles and method of making the
mold
Abstract
A laminate tile pole piece for an MRI, a method and a mold for
manufacturing laminate tile metal pole pieces for an MRI. Each
laminate tile has a trapezoidal or annular sector shape. The
trapezoidal shape allows the tiles to be attached side by side to
form a multiple concentric annular array pole piece without using
oddly shaped edge filler tiles needed to fill a circular pole piece
with square tiles. The pole piece is made by placing a plurality of
tiles into a mold and filling the mold with an adhesive substance
to bind the plurality of tiles into a unitary tile body. The
unitary tile body is then removed from the mold and attached to a
pole piece base to form the pole piece. The mold cavity surface
preferably has a non-uniform contour. The bottom surface of the
unitary tile body forms a substantially inverse contour of the mold
cavity surface contour.
Inventors: |
Laskaris, Evangelos T.;
(Niskayuna, NY) ; Barber, William D.; (Ballsto
Lake, NY) ; van Oort, Johannes M.; (Niskayuna,
NY) ; Aksel, Bulent; (Clifton Park, NY) ;
Thompson, Paul S.; (Stephentown, NY) ; Ogle, Michele
D.; (Burnt Hills, NY) |
Correspondence
Address: |
Jack L. Lahr
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Family ID: |
22733655 |
Appl. No.: |
09/738838 |
Filed: |
December 18, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09738838 |
Dec 18, 2000 |
|
|
|
09198507 |
Nov 24, 1998 |
|
|
|
Current U.S.
Class: |
335/299 |
Current CPC
Class: |
Y10T 29/49078 20150115;
Y10T 29/49075 20150115; H01F 7/0278 20130101; Y10S 29/001 20130101;
G01R 33/383 20130101; Y10T 29/4902 20150115 |
Class at
Publication: |
335/299 |
International
Class: |
H01F 005/00 |
Claims
What is claimed is:
1. A pole piece for a magnetic resonance imaging (MRI) system,
comprising a plurality of trapezoid or annular sector shaped tiles
arranged in a plurality of concentric annular arrays.
2. The pole piece of claim 1, wherein the tiles comprise a
plurality of laminated layers.
3. The pole piece of claim 1, wherein the height of a first
concentric annular array is greater than a height of a second
concentric annular array.
4. The pole piece of claim 3, wherein the surface contour of the
plurality of concentric annular arrays forms a substantially
inverse contour of a non-uniform mold cavity surface.
5. The pole piece of claim 1, wherein the plurality of concentric
annular arrays comprise a unitary tile body.
6. The pole piece of claim 1, wherein a first tile is separated
from a second tile by an adhesive layer.
7. The pole piece of claim 2, wherein the laminating direction of
the tile layers is parallel to a plane of each concentric annular
array.
8. The pole piece of claim 2, wherein the laminating direction of
the tile layers perpendicular to a plane of each concentric annular
array.
9. The pole piece of claim 2, further comprising a first tile
containing laminated layers attached over a second tile containing
laminated layers to form a combined laminate tile.
10. The pole piece of claim 9, wherein the layer laminating
direction of the first tile is perpendicular to the layer
laminating direction of the second tile.
11. The pole piece of claim 1, wherein at least one tile contains a
cavity in the tile face.
12. The pole piece of claim 11, wherein at least one cavity is
filled by a shim or an auxiliary magnet.
13. The pole piece of claim 1, further comprising: a yoke; a first
magnet containing a first side attached to a first portion of the
yoke and a second side attached to the laminate tile pole piece; a
second magnet attached to a second portion of the yoke; and a
second pole piece comprising a plurality of second tiles attached
to the second magnet and facing the laminate tile pole piece.
14. A method of making a pole piece comprising the steps of:
placing a plurality of tiles into a mold cavity; filling the mold
cavity with an adhesive substance to bind the plurality of tiles
into a unitary body; removing the unitary body from the mold
cavity; and attaching a second surface of the unitary body to a
pole piece base to form a first pole piece.
15. The method of claim 14, wherein: the mold contains a
non-uniform cavity surface contour; and a first surface of the
unitary body forms a substantially inverse contour of the
non-uniform mold cavity surface.
16. The method of claim 15, further comprising the steps of:
attaching the first pole piece to a first surface of a first
magnet; attaching the second surface of the first magnet to a first
portion of a yoke; attaching a second surface of a second magnet to
a second portion of the yoke; and attaching a second pole piece to
a first surface of the second magnet.
17. The method of claim 16, further comprising the steps of:
performing a simulation of magnetic flux density between the first
magnet and the second magnet; determining an optimum contour of the
first surface of the pole piece based on an optimum value of the
magnetic flux density between the first magnet and the second
magnet; and forming the mold cavity surface contour as a
substantial inverse of the contour of the first surface of the pole
piece.
18. The method of claim 17, wherein: the non-uniform mold cavity
surface comprises a plurality of spacers of different height; and
wherein the top surface of the spacers forms the non-uniform mold
cavity surface contour.
19. The method of claim 18, wherein the spacers comprise
cylindrical spacers.
20. The method of claim 18, further comprising the steps of:
pressing the unitary body against the spacers; and forming a
plurality of cavities in the unitary body.
21. The method of claim 14, further comprising the step of
arranging the tiles in a concentric annular array in the mold.
22. The method of claim 21, wherein the tiles comprise laminate
tiles.
23. The method of claim 14, further comprising the step of covering
the mold with a cover plate prior to filling the mold with
adhesive.
24. The method of claim 23, further comprising the step of placing
a plurality of tiles into the mold until a top surface of the tiles
is substantially even with a top of the mold prior to covering the
mold with the cover plate.
25. The method of claim 14, further comprising the steps of:
placing a first side of an adhesive tape in contact with a portion
of a first tile; and placing a second side of the adhesive tape in
contact with the mold cavity surface.
26. The method of claim 14, wherein the adhesive substance
comprises epoxy.
27. A mold containing an non-uniform cavity surface for forming a
laminate tile pole piece for an MRI system, made by the method of:
performing a simulation of magnetic flux density between a first
magnet of the MRI system and a second magnet of the MRI system;
determining an optimum contour of a first surface of the pole piece
based on an optimum value of the magnetic flux density between the
first magnet and the second magnet; and forming the mold cavity
surface contour as a substantial inverse of the optimum contour of
the first surface of the pole piece.
28. The mold of claim 27, wherein the non-uniform cavity surface
comprises a plurality of cylindrical spacers of non-uniform
height.
29. A method of making a mold containing an non-uniform cavity
surface, for forming a laminate tile pole piece for an MRI system,
comprising: performing a simulation of magnetic flux density
between a first magnet of the MRI system and a second magnet of the
MRI system; determining an optimum contour of a first surface of
the pole piece based on an optimum value of the magnetic flux
density between the first magnet and the second magnet; and forming
the mold cavity surface contour as a substantial inverse of the
optimum contour of the first surface of the pole piece.
30. The mold of claim 29, wherein forming the bottom mold surface
comprises placing a plurality of cylindrical spacers of non-uniform
height into the mold.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a laminate tile pole
piece for an MRI system, a method manufacturing the pole piece and
a mold used for bonding a pole piece tiles.
BACKGROUND OF THE INVENTION
[0002] In recent years, a so-called laminate tile pole piece has
been developed for an MRI. In view of such development, a plan view
of the laminate tile pole piece is shown in FIG. 1A and a side view
is shown in FIG. 1B. The pole piece 10 comprises a soft iron
circular base plate 11, a soft iron ring 12 around the
circumference of the base 11 for directing the magnetic flux into
the gap between magnets, soft ferrite laminate tiles 13 and 14 and
a soft iron core 15 for mounting a gradient magnetic coil. The
laminate tiles 13, 14 and the core 15 comprise the pole piece face.
The laminate tiles 14 in the center of the base plate 11 have a
greater thickness than laminate tiles 13 at the periphery of the
base plate 11 to form a convex protrusion 16. The convex protrusion
16 improves the uniformity of the magnetic field.
[0003] However, the prior art laminate tile pole piece has several
disadvantages. First, most laminate tiles 13, 14 have a square or
rectangular shape. However, the base 11 and the ring 12 have a
circular shape. Therefore, in order to fit square or rectangular
tiles into a circular opening, edge filler tiles 13A are required.
As shown in FIG. 1A, each edge filler tile 13A has a unique, odd
shape to allow the peripheral tiles 13 to completely fill the
circular base 11 and ring 12. Each edge filler tile 13A must be
formed separately from other tiles 13 to create its unique shape.
This increases process costs and complexity.
[0004] Second, the protrusion 16 also has a circular shape, as
shown in FIG. 1A. Therefore, in order to arrange the square or
rectangular central tiles 14 in a circle, edge filler tiles 14A are
required, as shown in FIGS. 1A and 1B. The edge filler tiles 14A
also have a unique, odd shape to allow central tiles 14 to form a
circular protrusion 16. Furthermore, in order to allow central
tiles 14 to fit with the peripheral tiles 13 without leaving gaps,
edge filler tiles 14A also must have two different thicknesses, as
shown in FIG. 1B. Each uniquely shaped edge filler tile 14A must
also be formed separately from other central tiles 14. This further
increases process costs.
[0005] Third, the prior art methods of attaching individual
laminate tiles 13, 14 to the base 11 involve placing the individual
tiles onto the base and then poring epoxy over the tiles. However,
the epoxy may flow out of the base and coat portions of the pole
piece not intended to be coated by epoxy. Some tiles may also be
insufficiently coated with the epoxy because the epoxy is not
supplied under pressure. These tiles may become delaminated during
MRI use. Furthermore, it becomes very difficult to achieve the
optimum height for the protrusion 16 by manually stacking tiles 14
onto a base 11 because of human error. Therefore, different pole
pieces manufactured by the prior art method suffer from poor
reproducibility and have different performance characteristics due
to a variance in the height of the protrusion.
SUMMARY OF THE INVENTION
[0006] In view of the foregoing, it would be desirable to decrease
the manufacturing process cost and complexity by forming a
laminated tile pole piece that does not contain oddly shaped edge
filler tiles. The present invention provides a pole piece for a
magnetic resonance imaging (MRI) system. The pole piece comprises a
plurality of trapezoid or annular sector shaped tiles arranged in a
plurality of concentric annular arrays.
[0007] It would also be desirable to obtain a reproducible and
accurate laminate tile pole piece manufacturing process. The
present invention provides a method of making a pole piece. The
method comprises placing a plurality of tiles into a mold cavity,
filling the mold cavity with an adhesive substance to bind the
plurality of tiles into a unitary body, removing the unitary body
from the mold cavity and attaching a second surface of the unitary
body to a pole piece base to form a first pole.
[0008] The present invention also provides a mold containing an
non-uniform cavity surface for forming a laminate tile pole piece
for an MRI system. The mold is made by performing a simulation of
magnetic flux density between a first magnet of the MRI system and
a second magnet of the MRI system, determining an optimum contour
of a first surface of the pole piece based on an optimum value of
the magnetic flux density between the first magnet and the second
magnet and forming the mold cavity surface contour as a substantial
inverse of the optimum contour of the first surface of the pole
piece.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1A is a plan view of a prior art pole piece.
[0010] FIG. 1B is a side cross sectional view of a prior art pole
piece across line X-X' in FIG. 1A.
[0011] FIG. 2A is a perspective view of a laminate tile arrangement
according to the first embodiment of the current invention.
[0012] FIG. 2B is a plan view of a laminate tile arrangement
according to the first embodiment of the current invention.
[0013] FIG. 2C is a side cross sectional view taken along line C-C'
in FIG. 2B of a laminate tile arrangement according to the first
embodiment of the current invention.
[0014] FIGS. 3A and 3B are side cross sectional views of MRI
systems.
[0015] FIG. 4A is a plan view of a mold according to the present
invention.
[0016] FIG. 4B is a side cross sectional view of the mold along
line A-A' in FIG. 4B.
[0017] FIG. 4C is a schematic of the pole piece and mold cavity
surface contours.
[0018] FIG. 4D is a side cross sectional view of a mold according
to an alternative embodiment of the present invention.
[0019] FIG. 5A is a side cross sectional view of the mold filled
with laminate tiles according to the present invention.
[0020] FIG. 5B is a close up side cross sectional view of a section
of FIG. 5A.
[0021] FIG. 5C is a side cross sectional view of a laminate tile
according to another embodiment of the present invention.
[0022] FIG. 6A is side cross sectional view of a laminate tile pole
piece according the present invention.
[0023] FIG. 6B is a close up side cross sectional view of a section
of FIG. 6A.
[0024] FIG. 7A is a plan view of a laminate tile arrangement
according to the second embodiment of the current invention.
[0025] FIG. 7B is a side cross sectional view of a section of FIG.
7A.
[0026] FIG. 7C is a plan view of a laminate tile according to the
second embodiment of the current invention.
[0027] FIG. 7D is a side cross sectional view of a laminate tile
according to the second embodiment of the current invention.
[0028] FIG. 8 is a perspective view of a laminate tile according to
the third embodiment of the current invention.
[0029] FIGS. 9A and 9B are side cross sectional views of MRI
systems according the third embodiment of the current
invention.
[0030] FIGS. 10A and 10B are perspective views of laminate tiles
according to the fourth embodiment of the current invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] FIG. 2A shows a perspective view of a cut away portion of a
pole piece 50 comprising a pole piece base 51 having a first
surface 40 and a second surface 41. Pole piece laminate tiles 42
are attached to the first surface 40 of the base 51. The tiles 42
may be attached to the base 51 by epoxy, glue and/or bolts. Each
laminate tile comprises interposed plural metal layers, shown as
21, 22, 23, and adhesive layers, shown as 31, 32. Each laminate
tile 42 actually contains 100 to 10,000 metal layers, where each
metal layer is preferably less than 5.times.10.sup.-3 inches thick
(i.e. less than 5 mils thick). Each tile is 0.1 to 10 inches thick
and 1 to 30 inches wide. For example, each tile is about 8 inches
wide in its middle and 1 inch thick, and contains 1000 1 mil thick
metal layers. However, the tiles and the metal layers may have
other dimensions based on the required end use. Furthermore, each
tile may alternatively comprise a solid metal block or bar instead
of metal layer laminates.
[0032] The laminate tiles 42 are preferably fabricated from
amorphous metal layers. However, the metal does not have to be
amorphous and may have a crystalline structure. The metal may
comprise steel, iron, silicon steel or iron (i.e. non-oriented
silicon steel), nickel steel or iron, permendur (FeCoV), nickel
chromium steel or iron, aluminum steel or iron, aluminum chromium
steel or iron or any other low coercivity material. Furthermore,
the pole pieces 50 according to the current invention may comprise
laminate tiles 42 made from different metals listed above. In other
words, adjacent tiles may comprise different metals.
[0033] The laminate tiles 42 preferably have a trapezoidal shape.
However, laminate tiles may have an annular sector shape. An
annular sector is a trapezoid that has a concave top or short side
43 and a convex bottom or long side 44. The trapezoidal laminate
tiles 42 may be manufactured by adhering plural metal sheets with
an adhesive and subsequently cutting the laminate sheets into
trapezoidal tiles. Another method of making laminate tiles 42 is
disclosed in a copending application Ser. No. 09/______ (GE RD
26570, Attorney Docket Number 70191/139) to E. Trifon Laskaris et
al., filed on the same date the current application, and
incorporated herein in its entirety. This method comprises
unwinding a metal ribbon, guiding the ribbon through an adhesive
bath, winding the ribbon on a polygonal bobbin, such as a
rectangular bobbin, to form a coil with at least one flat side,
removing the coil from the bobbin, cutting the coil into laminate
bars and shaping the laminate bars into trapezoidal or annular
sector shaped laminate tiles.
[0034] As shown in FIGS. 2B and 2C, the circular pole piece base 51
contains a support ring 52 for containing laminate tiles 42 and for
directing the magnetic flux into a gap between magnets. The entire
circular base 51 and the ring 52 are filled in with trapezoidal
laminate tiles 42. The base 51 and ring 52 are sometimes called a
"pole shoe." Alternatively, the ring 52 is sometimes called an
"edge shim." FIG. 2B shows a plan view of the pole piece 50, while
FIG. 2C shows a cross sectional view taken along line C-C' in FIG.
2B. The laminate tiles are arranged in concentric annular arrays or
rings 53 to 62. The advantage of the trapezoidal or annular sector
shape of the laminate tiles 42 becomes apparent from FIG. 2B. All
laminate tiles may have the same size and shape. Therefore, no
oddly shaped edge filler tiles are necessary to fill the base 51
and the ring 52. The cost and complexity of the method of
assembling the laminate tile pole piece is thus reduced.
[0035] For example, the concentric tile annular arrays 53-57 near
the center of the base 51 may have a larger thickness (i.e. height
as measured from the base 51) than concentric tile annular arrays
58, 60 and 61 near the periphery of the base 51 to form a
protrusion near the center of the base 51. The protrusion also does
not require oddly shaped edge filler tiles. Optionally, the
peripheral concentric annular arrays 59 and 62 may also have a
larger thickness than peripheral annular arrays 58, 60 and 61. Of
course other annular array thickness and configurations are
possible. For example, there may be more or less than 10 concentric
annular arrays. All the annular arrays may have the same thickness
or different thickness. The number of annular arrays and the
particular annular array thickness should be determined by a
computer simulation of magnetic field flow between MRI system
magnets through the pole piece 50. Alternatively, the central
annular array 53 may be an iron core for mounting a gradient
magnetic coil.
[0036] Furthermore, the annular arrays may be formed by stacking
plural laminate tiles 42 on each other. The thicker concentric
annular arrays may comprise more stacked laminate tiles than the
thinner annular arrays. The space between the top of the pole piece
support ring 52 and the laminate tiles may optionally be filled by
passive shims.
[0037] Embodiments of magnetic field generating devices used for
magnetic resonance imaging, MRI, ("MRI system") according to the
present invention are shown in FIGS. 3A and 3B. The MRI system
shown in FIG. 3A has two plate yokes 71A and 71B and at least two,
and preferably four columnar yokes 71C and 71D. Alternatively, an
MRI system with a single "C" shaped yoke 71 may be used as shown in
FIG. 3B. The MRI systems contain magnets 72, 72' secured to yoke
surfaces, pole piece bases 51, 51' and support rings 52, 52'
secured to the magnets 72, 72' and laminate tile pole pieces 74,
74' secured to the pole piece bases and support rings. A gap 73 is
formed between the pole pieces. A body part to be imaged is
inserted into the gap 73.
[0038] The magnets 72, 72' may comprise permanent magnets such as
RFeB, RCoFeB or SmCo magnets, or electromagnetic magnets, such as a
conductive or superconductive coil wrapped around a core. The MRI
systems may also optionally contain gradient coils or shims shown
as 75, 75' in FIGS. 6A and 6B. Furthermore, the MRI systems may
optionally contain an insulating, low magnetic permeability layer,
such as Bakelite, synthetic resin, wood, or ceramic, between the
base and the laminate tiles to reduce the remnant magnetism in the
pole pieces.
[0039] The MRI systems also may contain electronics 76 and a
display 77. The electronics 76 may comprise a control system, a
transmitter, a receiver, an imager and/or a memory.
[0040] The optimum contour of the laminate tile pole pieces is
determined by a simulation of the magnetic flux between the top
magnet 72 and bottom magnet 73. For example, the simulation may
comprise a conventional finite element analysis method. The optimum
height for each concentric annular array pole piece array 53-62 is
determined from the simulation.
[0041] The laminate tile pole piece 50 containing the concentric
annular arrays is preferably manufactured using a mold and a
molding method of the present invention. An embodiment of the mold
100 is shown in FIGS. 4A and 4B. FIG. 4B is a cross sectional view
taken along line A-A' in FIG. 4A. The mold contains a bottom
surface 101, a side surface 102 and a cover plate 103. The mold
further contains one or more epoxy inlet openings 104 and one or
more air outlet openings 105. The opening(s) 104 is preferably made
in the bottom mold surface 101 and the opening(s) 105 is preferably
made in the cover plate 103. The bottom mold surface 101 and cover
plate 103 are preferably attached to the side wall 102 by bolts
106. However, the bottom surface 101 and the side surface 102 may
alternatively comprise a unitary body and the cover plate 103 may
be attached to the side wall 102 by other ways, such as a latch.
The mold 100 has optional handles 107.
[0042] The mold preferably contains a non-uniform cavity surface
contour. Preferably, the non-uniform contour is established by
attaching spacers to the mold cavity bottom surface 101.
Preferably, the spacers form a plurality of concentric annular
arrays 153-162 around the circular bottom mold surface 101. The
spacers 153-162 may be attached to the mold cavity surface 101 by
screws 108 or by glue. Preferably the spacers have a cylindrical
shape. However, the spacers may have any other shape.
[0043] As shown in FIG. 4B, spacers in different concentric annular
arrays 153-162 have a different height or thickness. Preferably
there are as many spacers 153-162 as there are laminate tiles 42 in
the pole piece. Each spacer corresponds to a particular pole piece
tile. The spacer surface in the mold forms a substantially inverse
contour of the pole piece concentric annular tile array contour. In
other words, if the pole piece annular array, such as tile array
62, has a large height or thickness, then the corresponding spacer
array in the mold, such as spacer array 162 has a small height or
thickness. If the pole piece annular array, such as tile array 61,
has a small height or thickness, then the corresponding spacer
array in the mold, such as spacer array 161 has a large height or
thickness. "Substantially inverse" means that the spacer contour
may differ from the tile contour. For example, the tiles are
attached to each other by an epoxy adhesive, while there may be
gaps 109 between the spacers. Thus, the spacer contour also
contains the gaps 109, while the tile contour does not contain the
thin protrusions that would correspond to the gaps. Furthermore,
there may be other slight vertical and horizontal variations in the
contours.
[0044] Therefore, the contour of the non-uniform mold cavity
surface 110 is an inverse of a laminate tile pole piece contour
114, as shown in FIG. 4C. The contour of the laminate tile pole
piece is determined by performing simulation of a magnetic flux
density between the MRI system magnets for different tile contours
and then choosing the tile contour 114 which produces the optimum
magnetic flux between the MRI system magnets. The magnetic flux
lines from a finite element simulation of a field between two
hypothetical MRI magnets are superimposed on the plan view of the
mold in FIG. 4A.
[0045] Alternatively, the non-uniform mold cavity surface contour
may be made without using spacers 153-162, as shown in FIG. 4D. In
FIG. 4D, the mold cavity surface itself is irregularly shaped to
form a non-uniform contour 110. The contour 110 comprises
protrusions 111 and recesses 112. The protrusions 111 form plural
concentric annular arrays whose contour is the substantial inverse
of the pole piece tile contour. As with the mold shown in FIG. 4B,
each protrusion 111 should correspond to an individual tile 42 of
the pole piece.
[0046] A method of making the laminate tile pole piece according to
the present invention is shown in FIGS. 5 and 6. The mold cavity
and the spacers are first coated with a release agent. Laminate
tiles 42 are then placed into the mold cavity in concentric annular
tile arrays 53-62, as shown in FIG. 5A. The tiles are stacked on
top of the corresponding concentric annular spacer arrays 153-162.
Of course, the spacers may be replaced by the protrusions of FIG.
4C. Each tile should overlie one spacer, as shown in FIG. 5B. The
height of each tile and spacer stack should equal to the height of
the mold cavity, such that the top surface of the tile arrays 53-62
is level with the top of the mold cavity. All variations as a
result of tile height tolerances are taken as a small gap near the
top of the mold cover plate 103. Alternatively, each tile may be
attached to its respective spacer with adhesive tape 123, as shown
in FIG. 5C.
[0047] The mold is then covered with the cover plate 103 and an
adhesive substance is introduced into the mold through the inlet
opening 104. The adhesive substance is preferably a synthetic epoxy
resin. The epoxy does not becomes attached to the mold cavity and
the spacers because they are coated with the release agent. The
epoxy permeates between the individual tiles and forces out any air
trapped in the mold through outlet opening(s) 105. The epoxy 113
binds the individual tiles into a unitary body 120 comprising
concentric annular tile arrays 53-62 of different height.
Alternatively, the epoxy may be introduced through the top opening
105 or through both top and bottom openings.
[0048] The mold cover plate 103 is taken off the mold and the
unitary tile body 120 is removed from the mold 100. The unitary
body 120 is then attached with its flat (top) side to the pole
piece base 51 and ring 52, as shown in FIGS. 6A and 6B. The base
51, ring 52 and the unitary tile body comprise the pole piece 50.
The unitary tile body 120 may be attached to the base 51 by epoxy,
glue and/or bolts.
[0049] The second embodiment of the present invention is shown in
FIGS. 7A to 7D. In the second embodiment, at least one tile 42
contains a cavity 121 in its face, as shown in FIGS. 7A and 7C. The
cavity may be formed by introducing the epoxy 113 into the mold 110
at high pressure. The high pressure epoxy flows over the concentric
annular tile arrays 53-62 and presses the tile arrays against the
cylindrical spacers 153-162 or protrusions 111 in the mold cavity.
Each spacer or protrusion has a smaller surface area than the area
of the corresponding tile. Therefore, pressure of the comparably
softer tiles against the spacers or protrusions forms cavities 121
in the tiles, as shown in FIGS. 7A-7B. The cavities may be filled
by passive shims or small permanent magnets 122 as shown in FIG. 7D
and described in copending application as disclosed in application
Ser. No. 09/______ (GE RD 26,591) to Johannes M. van Oort, filed
Oct. 23, 1998, hereby incorporated by reference in its
entirety.
[0050] In FIG. 2A, the laminate layers are laminated along the
height or thickness direction of the laminate tile 42. However, in
a third embodiment of the present invention, the laminate layers
91, 92, 93, 94 are stacked or laminated along the width of the
laminate tile 42", as shown in FIG. 8. Laminate tile 42" may be
produced by forming a thick stack or coil of epoxy bound metal
layers, cutting a tile from the stack or coil and turning the tile
on its side.
[0051] Laminate tile 42" is mounted on the pole piece base 51 with
the laminating direction perpendicular to the direction of the
magnetic flux (i.e. perpendicular to an imaginary line between the
bottom magnet 72' and the top magnet 72) as shown in FIG. 9A. In
other words, the laminating direction is parallel to the plane of
the concentric annular tile arrays. The advantage of the this
embodiment is increased stability of the magnetic field and a
decrease in eddy currents and hysteresis effects. Alternatively,
the laminate tile 42" may be mounted on the edge of another pole
piece member 90 to reduce sideways magnetic flux leakage, as shown
in FIG. 9B. Member 90 may itself comprise multiple laminate tiles
42 with layers laminated in a direction parallel to the direction
of the magnetic flux. In other words, the laminating direction is
perpendicular to the laminating direction of tiles 42" and
perpendicular to the plane of the concentric annular tile
arrays).
[0052] In a fourth embodiment of the present invention, laminate
tiles whose laminating directions are different by 90 degrees may
be attached to each other. Such an arrangement improves the
uniformity of the magnetic field in the gap 73. For example, a
laminate tile 42 may be attached to laminate tile 42" to form a
combined tile 91 as shown in FIG. 10A. Alternatively, two tiles 42"
may be attached to form a combined tile 91' as shown in FIG. 10B.
Of course two tiles 42 may also be attached with their laminating
directions inclined by 90 degrees to each other. The combined tiles
91 and 91' may be attached to the pole piece base 51 with any
surface facing the MRI system gap 73. Combined tiles 91 and 91' may
also comprise individual tiles made from different metals listed
above.
[0053] The laminate tiles were described as being suitable for an
MRI system pole piece. However, other uses for the laminate tiles
and the laminate tile fabrication method are within the scope of
the current invention. The mold may also be used to manufacture
unitary bodies for uses other than an MRI system pole piece.
Furthermore, in some applications, it may be advantageous to use
laminate bars instead of trapezoidal tiles. In this case, the
laminate bars may be considered "laminate tiles" for the purposes
of this invention.
[0054] While the invention has been described in detail and with
reference to specific embodiments thereof, it will be apparent to
one skilled in the art that various changes and modifications can
be made therein without departing from the spirit and scope of the
invention.
* * * * *