The present invention relates to magnet coil assemblies, and more
particularly to driver coils and cooling techniques for use in resistive electromagnets.
The present invention finds particular application in the field of magnetic resonance
MRI is widely used for imaging anatomical and other structures. A
commonly used type of magnet for MRI systems is the superconducting type, which
is capable of generating field strengths in the range of 0.5-2 Tesla (T). Superconducting
magnets are, however, relatively expensive to manufacture and operate.
At lower fields, for example in the range of 0.1 to 0.3 T, resistive
magnets are commonly used to produce magnetic fields for MRI applications. Resistive
magnets typically include one more magnet driver coils operating in cooperation
with a suitable magnet structure, the magnetic field strength being generally proportional
to the current through the coils. A limitation on magnet performance, however,
is the heat generated in the resistance of the driver coils. As a result, cooling
systems have been used to remove this excess heat.
To provide the necessary cooling, annular cooling flanges or discs
fabricated from a material such as aluminium have been placed in thermal contact
with the magnet coils (JP-A-01 276 707). Channels having a size and depth sufficient
to accommodate and retain copper tubing having a round cross section have been
included on one side of the cooling flange, which has had a thickness greater than
that of the tubing. The channels and tubing are arranged in a bifilar wound pattern
so that the inlet and outlet of the tubing are both accessible from the outer radius
of the cooling flange. The cooling flanges have then been placed against the driver
coils, with the tubing preferably on the side of the flange facing away from the
magnet coil. In operation, a coolant such as water has been caused to flow through
In practice, however, neither the surface of the driver coil nor surface
of the cooling flange are perfectly flat. The resultant gaps degrade the thermal
conductivity between the coil and flange. There have also been radial gaps between
the channels and hence the turns of tubing, also reducing thermal efficiency. A
cooling flange with a flat cooling surface has been proposed for cooling the windings
of a transformer using a plate heat exchanger (WO-A-90/08390).
In magnet systems having two driver coils on each pole of the magnet
it has been necessary to use three cooling flanges. The structure associated with
each pole has thus included a first cooling flange, a driver coil having its first
side electrically insulated from but in thermal contact with the first cooling
flange, a second cooling flange located between the second side of the first driver
coil and first side of a second driver coil (the second cooling flange being in
thermal contact with but electrically insulated from the driver coils), the second
driver coil, and a third cooling flange electrically insulated from but in thermal
contact with the second side of the second driver coil.
Driver coils have also included many turns of a conductor such as
aluminium arranged in a generally planar, disc-shaped coil. The conductor has had
a rectangular cross-section, with the conductor wound or coiled from an inner to
an outer radius in the form of an annulus or disc. To insulate between the multiple
conductor turns, an anodized aluminium conductor has been used. A disadvantage
of anodized aluminium, however, is its cost. A further disadvantage is that defects
in the anodization may result in short circuits between coil turns, with a corresponding
deleterious effect on magnet performance.
In accordance with a first aspect of the invention, a magnet coil
assembly comprises a first generally planar driver coil having at least a first
surface, heat being generated in the first driver coil in response to the flow
of electrical current; and tubing for containing the flow of a coolant therethrough
for removing heat generated in the first driver coil, the tubing including a flat
exterior cross-section portion facing and in thermal communication with the first
Improved thermal communication between the driver coil and the tubing
for removing heat generated in the driver coil, is provided.
According to another more limited aspect of the invention, the tubing
is wound to define a plurality of generally planar turns. A layer of thermally
insulating material having a thickness less than that of the tubing may be located
between the turns.
According to yet another more limited aspect of the invention, the
tubing is bifilar wound.
According to another more limited aspect, the driver coil and the
and the tubing are separated by a layer of electrical insulation which defines
a plurality of holes. According to a still more limited aspect, an epoxy is located
between the insulation and the driver coil.
According to another aspect of the invention, a magnet for use in
MRI includes two pole pieces in an opposed relationship which define an imaging
region. A driver coil and tubing as defined above are associated with the first
pole piece. The tubing has a flat exterior cross-section portion facing and in
thermal communication with the driver coil. A driver coil and additional tubing
are also associated with the second pole piece. The additional tubing has a flat
exterior cross-section portion facing and in thermal communication with the driver
coil associated with the second pole piece.
According to a more limited aspect of the invention, the driver coil
associated with the first pole piece includes a conductor which has a generally
rectangular cross section. An edge of the conductor which is adjacent to the material
is beveled. According to a still more limited aspect, the conductor is wound to
define a plurality of generally planar turns. The turns are separated by a layer
of electrical insulation. According to a still more limited aspect, the electrical
insulation does not extend past the edge of the conductor facing the material in
thermal communication therewith.
According to another aspect of the invention, a magnet coil assembly
for use in magnetic resonance imaging includes a first driver coil and tubing for
containing the flow of a coolant therethrough, the tubing including a flat exterior
cross-section portion which faces and is in thermal communication with the first
surface of the first driver coil. A second surface of a second driver coil is adjacent
the second surface of the first driver coil. Tubing for containing the flow of
a coolant therethrough, the tubing including a flat exterior cross-section planar
portion faces and is in thermal communication with the first surface of the first
One way of carrying out the invention will now be described in greater
detail, by way of example, with reference to the accompanying drawings, in which:
- Figure 1 depicts an MRI apparatus according to the present invention;
- Figure 2 is an exploded view of a magnet coil assembly;
- Figure 3 is a top view of a cooling member;
- Figure 4 is a top view of a perforated insulating layer, and
- Figure 5 is a sectional view of a portion of the windings of a driver coil along
line 5-5 of Figure 2.
With reference to Figure 1, an MRI apparatus which produces images
of the anatomy of patient 1 includes a generally C-shaped magnet body 3. The patient
1 is placed in an imaging region located between the pole pieces 23. Current flowing
in the driver coils contained in coil assemblies 2a, 2b generates a magnetic field
Bo in the imaging region. Necks 4 connect the pole pieces 23 to the body 3 of the
magnet, thereby providing a return path for the body of the magnet.
Gradient coils 6 generate time-varying gradient magnetic fields, preferably
in three orthogonal directions (e.g., x, y, z). As known in the art, the MRI apparatus
100 also includes RF transmit and receive coils (not shown) for exciting magnetic
resonance of materials within the imaging region and detecting signals excited
thereby. As is also conventional in the art, associated signal processing and computer
apparatus generates and displays images of the internal anatomy of the patient
on a CRT or other suitable monitor.
Figure 2 depicts the various components of the lower magnet coil assembly
2b, it being understood that lower magnet coil assembly 2b is also representative
of the upper magnet coil assembly 2a.
With reference to Figure 1, 2, and 3, the magnet coil assembly 2b
includes a pair of cooling members 10, 12, a pair of magnet coils 14, 16, and electrical
insulation layers 18, 20, 22. Each cooling member 10 is fabricated from tubing
wound in a generally planar annular configuration. The tubing is preferably copper
tubing having an 8 x 12 mm rectangular exterior cross section and is bifilar wound,
with the 12 mm dimension of the tube running in the radial direction. Other materials
and cross sections may also he used. provided that tubing is wound so that a substantially
flat portion of the tubing cross section may be placed facing and in thermal contact
with the magnet coil 14. Placed between each of the adjacent layers of tubing is
thermal insulation such as PVC having a thickness of approximately 2 mm. Typical
conductivity for a polymer such as PVC is approximately 0.3 w/mK. The tubing is
wound so that adjacent turns of tubing are substantially adjacent, though separated
by thermal insulation 28. The member has an inlet 24 through which a coolant such
as water is introduced, and an outlet 26 from which the coolant exits after having
flowed through the tubing. Mechanical spacers 29a, 29b arc placed in the inner
layers of the winding to account for spaces caused by the bend of the bifilar wound
tubing and thus maintain the circularity of the cooling member 10.
An electrical insulation layer 18 is located between the cooling member
10 and the driver coil 14. The insulation layer 18 should provide a desired degree
of electrical isolation consistent with good thermal communication between the
cooling spiral 10 and the driver coil 14. With reference to Figure 4, the insulation
layer 18 is of an annular shape and contains a plurality of perforations or holes
. A uniform layer of epoxy adhesive is used to fasten the cooling member 10 to
the driver coil 14. The epoxy preferably provides a high degree of thermal conductivity,
which in practice means a high filler content, and a desired degree of electrical
isolation. Because the insulation layer 18 contains numerous perforations, the
epoxy layer joins the cooling member 10 and the driver coil 14 over a substantial
portion of their surface. The epoxy preferably has a minimum of voids so as to
maximize thermal communication between the cooling member 10 and the driver coil
14. To improve electrical isolation, the cooling member 10 may also be coated with
a layer of lacquer.
With reference to Figures 2 and 5, a generally planar driver coil
14 contains a plurality of turns of a spiral-wound electrical conductor 30 such
as aluminium. An electrical connection is made at one end of the conductor 30 at
the inside of the spiral and at the other end at the outside of the spiral. A cross
section of a portion of the driver coil 14 showing a representative portion of
the coil windings is shown in Figure 5. While gaps are shown between the windings
for ease of illustration, it will be appreciated that in practice the windings
are substantially adjacent.
The conductor 30 is characterized by a generally rectangular cross
section having beveled edges 32a and 32b. An electrical insulation layer 34 such
as Mylar is placed between the turns of the conductor 30, thus preventing the turns
from making electrical contact. The beveled surfaces prevent electrical contact
near the upper and lower edges of the conductor 30 in the event that the vertical
dimensions of the insulating layer 34 or conductor 30 should vary or if the insulating
layer 34 is not precisely positioned. In practice, the nominal dimensions of the
bevels 32a, 32b and the height of the insulating layer 34 are chosen so that electrical
insulation between the conductor 30 turns is achieved despite variations in the
material and assembly techniques while preventing or minimizing protrusion of the
insulating layer 34 beyond the vertical extent or edges of the conductor 30. This
in turn facilitates thermal communication between the conductor and adjacent layers
or structures such as the cooling member 10. In an arrangement where the conductor
has a nominal height of 100 mm and a nominal thickness of 0.5 mm, satisfactory
results have been achieved with bevels 32a, 32b having a height of 1 mm and a depth
of 0.02 mm and an insulation layer 34 having a height of 100 mm.
An electrical insulation layer 20 is placed between the driver coils
14, 16. The insulation layer 18 is selected to provide a desired degree of electrical
isolation between the driver coils 14, 16. The coils 14, 16 are bonded to the insulation
layer using an epoxy. Of course, other arrangements, such as those described above
in regard to insulation layer 18, may be used if improved thermal communication
between driver coils 14, 16 is desired.
While the foregoing description has been directed primarily to cooling
member 10, insulating layer 18, and driver coil 14, it will be appreciated that
it applies equally to driver coil 16, insulating layer 22, and cooling member 12.
After assembly, the entire structure is hermetically sealed using epoxy, a glass
fibre laminate, or like technique.
In operation, a current source provides an electrical current to the
magnet assemblies 2a, 2b so that a desired magnetic field Bo is generated, and
coolant such as water is caused to flow through passages defined by the material
of the cooling members 10, 12. Being relatively flexible, the cooling members may
be placed in good thermal contact with the driver coils 14, 16 during the manufacturing
process, for example by applying pressure during assembly. Thus, the system is
relatively tolerant of variations in the surfaces of the cooling member and driver
coils. Because protrusions of the relatively thermally insulating Mylar beyond
the surface of the driver coils are minimized, thermal communication between the
cooling members and the driver coils 14, 16 is further enhanced.
Although the cooling member has been described in terms of bifilar
wound tubing, other configurations are possible. Thus, for example, coolant may
be introduced to the cooling members through headers or manifolds, each feeding
a plurality of cooling passages. In one embodiment, each cooling member includes
a first inlet manifold and first exit manifold which are associated with a first
plurality of cooling passages, and a second inlet manifold and second exit manifold
which are associated with a second plurality of passages. The first and second
passages are interleaved, with the direction of coolant flow in opposite directions.
As will be appreciated, coolant entering the inlet side 24 of the
cooling member is cooler than that exiting through the outlet 26. A particular
advantage of the bifilar winding of the cooling member is that variation in temperature
of the cooling member in the radial direction are minimized. This in turn minimizes
variation in temperature of the driver coils 14, 16 in the radial direction. Effective
thermal insulation between the individual turns in the cooling member also improves
the thermal efficiency of the cooling member.
The invention has been described in relation to a C-shaped magnet
apparatus. It will be appreciated that the invention can be used with other magnet
configurations, such as the so-called four-poster type, the so-called H-form, or
other configurations which provide a return path for the magnet flux.
A first advantage of the described embodiment is that improved thermal
performance in a magnet system is provided while minimizing cost and complexity,
including that of the cooling system. Another advantage is that improved thermal
communication between the cooling member and the driver coil is provided. Not only
can absolute temperature of the magnet coils be reduced, but also temperature gradients
within the coils themselves can be minimized. Yet another advantage is that a separate
cooling flange may be eliminated. Another advantage is that the turns of the magnet
coil are insulated using a technique which avoids the disadvantages of the anodized
approach but which does not degrade the performance of the cooling system.
The invention has been described with reference to the preferred embodiment.
Obviously, modifications and alterations will occur to others upon reading and
understanding the preceding description. It is intended that the invention be construed
as including all such modifications an alterations insofar as they come within
the scope of the appended claims.