Magnetic field homogeneity correction for superconducting magnet

Magnetic field homogeneity correction system including coils for a superconducting magnet assembly are wound on the same coil forms as the axially spaced main magnet coils such that they are superimposed radially to simplify the system including design requirements for associated passive shimming and reduce transverse magnetic forces therebetween, and improve magnetic field homogeneity.

BACKGROUND OF INVENTION 
This invention relates to superconducting magnet assembly for a magnetic 
resonance imaging system (hereinafter called "MRI"), and more particularly 
to an improved and simplified arrangement for improving magnetic field 
homogeneity in such an assembly. 
As is well known, a superconducting magnet can be made superconducting by 
placing it in an extremely cold environment, such as by enclosing it in a 
cryostat or pressure vessel containing liquid helium or other cryogen. The 
extreme cold ensures that the magnet coils are made superconducting, such 
that when a power source is initially connected to the coil (for a period, 
for example, of up to one hour) to introduce a current flow through the 
coils, the current will continue to flow through the coils even after 
power is removed due to the absence of resistance, thereby maintaining a 
strong magnetic field. Superconducting magnets find wide application in 
the field of MRI. 
However, MRI requires very strong or large magnetic fields in the imaging 
bore with a very high degree of uniformity or homogeneity. Typically this 
homogeneity requirement, on the order of 10 parts per million (ppm) on 40 
to 50 cm diameter spherical volume (DSV), cannot be achieved by 
controlling manufacturing tolerances. In practice shim systems which may 
be extra coils, typically called correction coils, small pieces of iron, 
typically called passive shims, or some combination of the two are 
provided to correct or improve the magnetic field homogeneity. This shim 
system allows reasonable manufacturing tolerances. Typical magnet 
homogeneities after magnet manufacture are on the order of a few hundred 
ppm. Use of such shim systems can reduce this homogeneity to the required 
10-15 ppm. 
Considerable effort has been directed at systems which provide the required 
homogeneity yet which are uncomplex in structure and adjustment. Current 
correction coil designs utilize correction coil formers or supports, 
typically of fiber reinforced epoxy plastic (FRP) material. Wire coils are 
wound on the correction coil former with the correction coil system 
consisting of a plurality of correction coils added to the interior of the 
superconducting magnet assembly. These correction coils are typically 
designed to create single harmonics as purely as possible. One axial 
correction coil system has 14 coils to create or provide "6 orders of 
shim". In addition, a transverse correction coil system is typically added 
to the shim system. Such systems are complex and costly and require 
considerable space in the MRI magnet assembly. 
Current passive shim systems also utilize another FRP drum that is inserted 
in the warm space inside the magnet bore. Shims or pieces of magnetic 
material are mounted on drawers that slide into slots on the drum. The 
drawers must be removed and reinserted into the drum while the magnet is 
at field (in superconducting operation). However, the strong magnetic 
field created by the superconducting main magnet coils exert forces on the 
drawers and on the internal correction coils. In the case of, for example, 
a 1.5 Tesla magnet these forces can be very large. In addition, as the 
ambient temperature surrounding the passive shims increases, the 
homogeneity of the magnet is degraded. Minimizing the total amount of shim 
will reduce this undesirable effect. 
One design of such shim drums is shown in U.S. Pat. No. 5,389,909 of 
Timothy J. Havens, entitled "Open Architecture Magnetic Resonance Imaging 
Passively Shimmed Superconducting Magnet Assembly", which is assigned to 
the same assignee as the present invention. 
Such existing shimming systems tend to be relatively complex, costly and 
difficult and time consuming to adjust to obtain the required field 
homogeneity. The structure provided to support the shims must be 
mechanically strong enough and rigid enough to resist the strong magnetic 
forces exerted on the shimming structures by the strong main magnetic 
field. In addition, such support structures add complexity and weight to 
the superconducting magnet. Still further, passive correction systems tend 
to be large and transverse correction coils are costly such that a more 
efficient overall field homogeneity system also lowers cost and size. 
OBJECTS AND SUMMARY OF INVENTION 
It is an object of the present invention to provide an improved 
superconducting magnet coil assembly for an MRI magnet which provides 
desired field homogeneity with a simplified shimming system. 
It is another object of the present invention to provide an improved 
superconducting magnet coil assembly in which inhomogeneities in the 
magnetic field are minimized while minimizing magnet cost and the effect 
of magnetic forces, and simplifying shimming complexity. 
It is a further object of the present invention to provide an uncomplex 
shimming arrangement in an MRI magnet which minimizes the magnetic forces 
on the shimming arrangement, minimizes temperature dependence of 
homogeneity, and simplifies attaining magnetic field homogeneity. 
In accordance with one form of the invention, a magnetic resonance imaging 
superconducting magnet includes a plurality of main magnet coil assemblies 
positioned on coil support forms within a cryogen vessel and axially 
spaced along the axis of the cryogen vessel. Each of the plurality of coil 
support forms includes pockets to support the main magnet coil which 
develops the strong magnetic field along the magnet axis and within the 
imaging bore formed by the cryogen vessel, and also includes means to 
support a shimming correction coil on the same form with the two coils 
superimposed upon one another and supported by the same coil form. The 
main magnet and correction coils are radially displaced with the 
correction coils improving magnetic field homogeneity within the axial 
bore and correcting for axial or radial displacement of the main magnetic 
coils which occurs due to normal manufacturing tolerances during 
manufacture and assembly of the superconducting magnet. The coil form is 
molded FRP with the main magnet coil extending axially approximately 1 to 
1.5 times the axial length of its associated correction coil.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION 
Referring first to FIGS. 1 and 3, a cryogen or helium pressure vessel 10 
extends along and around axis 12 of imaging bore 14 formed within the 
helium vessel of superconducting magnet 16. A plurality of main magnet 
coils 20, 22, 24, 26, 28 and 30 are positioned within helium vessel 10 
contiguous to and surrounding imaging bore 14, and axially spaced along 
axis 12. As is common in magnetic resonance imaging, the axial length of 
main magnet coils 20, 22, 24; and of 26, 28, and 30, respectively, are 
different. One or more shielding coils such as those shown by 32 and 34 
are included within helium vessel 10 to reduce the magnetic stray field 
and minimize siting and installation costs. 
However, normal manufacturing tolerances in the axial and radial 
positioning of main magnet coils 20, 22, 24, 26, 28, 30, 32 and 34 result 
in inhomogeneities of the magnetic field produced within imaging bore 14 
which must be reduced in order to provide the desired imaging quality of 
the MRI equipment in which superconducting magnet 16 is incorporated. One 
or more shim drawers, shown generally as 33, utilizing ferromagnetic 
members in accordance with the aforementioned U.S. Pat. No. 5,389,909 are 
provided to enable adjustment and improvement of the magnetic field 
homogeneity within imaging bore 14. 
However, a need remains to further compensate for manufacturing tolerances 
and hence variations, for example, in the axial and radial positioning of 
main magnet coils 20, 22, 24, 26, 28 and 30, each of which are wound in a 
support or pocket 21 molded within FRP coil form 35. Also molded within 
coil form 35 is a support means or pocket 41 to receive a correction coil 
40 such that the main magnet coils such as coil 20 and their associated 
correction coil such as coil 40 are superimposed, concentric, and radially 
displaced about axis 12 of imaging bore 14. 
As shown in FIG. 1, correction coils 40, 42, 44, 46, 48 and 50 are each 
associated with a main magnet coil 20, 22, 24, 26, 28 and 30, 
respectively. The coils pairs such as correction coil 41 and main magnet 
coil 21 need only be separated by insulating tape 39 enabling them to be 
wound consecutively within pockets 41 and 21, respectively formed within 
coil form 35. Electrical leads such as 54 and 56 for the coils are brought 
outside helium vessel 10 to enable a desired current flow to be provided 
prior to commencing superconducting operation of superconducting magnet 
16. 
It is to be noted that the correction coils such as 41 need not be 
displaced axially from main magnet coil such as 21 such that the 
correction coils not only do not require their own coil form and 
structure, the axial forces that would otherwise be exerted against the 
correction coils by the main magnet coils are minimized and, more 
important, a correction coil such as 41 is in close proximity to the main 
magnet coil such as 21 for which it is providing magnetic field 
compensation or correction. The correction coils such as 41 are optimally 
placed for correction of the axial or radial displacement of the main 
magnet coils such as 21 and hence are in close proximity to the source of 
the error. It is not necessary in some installations to include a 
correction coil, such as 41, in association with all of the main magnet 
coils, such as 21. It is possible to eliminate some of the correction 
coils 40, 42, 44, 46, 48 and 50. The current flow through each of 
correction coils 40, 42, 44, 46, 48, and 15 is adjusted to provide the 
degree of magnetic field homogeneity correction provided by that coil as 
is common in the art for correction coils. 
It has been found that such a simplified correction coil arrangement when 
coupled with a passive shim system such as that shown in simplified form 
by magnetic shim 33 are sufficient to reduce magnetic forces on the shim 
drawers to an acceptable level and to provide acceptable levels of 
magnetic fields homogeneity within imaging bore 14. Complexity and cost of 
the overall system is reduced. 
FIG. 2 shows an arrangement in which correction coils 40, 42, 44, 46, 48 
and 50 are positioned closer to imaging bore 14 than the main magnet coils 
20, 22, 24, 26, 28 and 30. 
In one embodiment of the invention, the axial length of main magnet coils 
20, 22 and 24, and 30, 28, and 26, respectively, extend axially 
approximately 1 to 1.5 times that of the correction coils 40, 42, 44, 46, 
48 and 50. 
While only certain features of the invention have been illustrated and 
described herein, many modifications and changes will occur to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and changes as fall 
within the true spirit of the invention.