Open and shielded superconductive magnet

An open superconductive magnet useful in magnetic resonance imaging (MRI) applications. The magnet has two spaced apart assemblies, wherein each assembly has a magnetizable pole piece and a magnetizable ring radially spaced outwardly apart from, and at least partially longitudinally overlapping, the pole piece. A superconductive main coil is positioned radially between the pole piece and the ring. A superconductive shielding coil is positioned radially outward from the superconductive main coil and longitudinally outward from the longitudinally outer end of the ring.

FIELD OF THE INVENTION 
The present invention relates generally to an open superconductive magnet 
used to generate a uniform magnetic field, and more particularly to such a 
magnet having shielding to protect the area around the magnet from stray 
magnetic fields originating from the magnet. 
BACKGROUND OF THE INVENTION 
Superconductive magnets include those superconductive magnets which are 
part of a magnetic resonance imaging (MRI) system used in various 
applications such as medical diagnostics. Known superconductive magnets 
include liquid-helium-cooled and cryocooler-cooled superconductive 
magnets. Typically, the superconductive coil assembly includes a 
superconductive main coil surrounded by a first thermal shield surrounded 
by a vacuum enclosure. A cryocooler-cooled magnet preferably also includes 
a cryocooler coldhead externally mounted to the vacuum enclosure, having 
its first cold stage in thermal contact with the thermal shield, and 
having its second cold stage in thermal contact with the superconductive 
main coil. A liquid-helium-cooled magnet preferably also includes a 
liquid-helium vessel surrounding the superconductive main coil and a 
second thermal shield which surrounds the first thermal shield which 
surrounds the liquid-helium vessel. 
Known superconductive magnet designs include closed magnets and open 
magnets. Closed magnets typically have a single, tubular-shaped 
superconductive coil assembly having a bore. The superconductive coil 
assembly includes several radially-aligned and longitudinally spaced-apart 
superconductive main coils each carrying a large, identical electric 
current in the same direction. The superconductive main coils are thus 
designed to create a magnetic field of high uniformity within a typically 
spherical imaging volume centered within the magnet's bore where the 
object to be imaged is placed. A single, tubular-shaped superconductive 
shielding assembly may also be used to prevent the high magnetic field 
created by and surrounding the main coils from adversely interacting with 
electronic equipment in the vicinity of the magnet. Such shielding 
assembly includes several radially-aligned and longitudinally spaced-apart 
superconductive shielding coils carrying electric currents of generally 
equal amperage, but in an opposite direction, to the electric current 
carried in the main coils and positioned radially outward of the main 
coils. 
Open magnets, including "C" shape magnets, typically employ two 
spaced-apart superconductive coil assemblies with the space between the 
assemblies containing the imaging volume and allowing for access by 
medical personnel for surgery or other medical procedures during magnetic 
resonance imaging. The patient may be positioned in that space or also in 
the bore of the toroidal-shaped coil assemblies. The open space helps the 
patient overcome any feelings of claustrophobia that may be experienced in 
a closed magnet design. Known open and shielded superconductive magnet 
designs include those wherein each superconductive coil assembly has an 
open bore and contains a superconductive shielding coil positioned 
longitudinally and radially outward from the superconductive main coil(s). 
A large amount of expensive superconductor is needed in the main coil to 
overcome the magnetic field subtracting effects of the shielding coil. 
Calculations show that for a 0.75 Tesla magnet, generally 2,300 pounds of 
superconductor are needed yielding an expensive magnet weighing generally 
12,000 pounds. The modest weight makes this a viable magnet design. 
It is also known in open magnet designs to place an iron pole piece in the 
bore of a superconductive coil assembly which lacks a superconductive 
shielding coil. The iron pole piece enhances the strength of the magnetic 
field and, by shaping the surface of the pole piece, improves the 
homogeneity of the magnetic field. An iron return path is used to connect 
the two iron pole pieces. It is noted that the iron pole piece also acts 
to shield the magnet. However, a large amount of iron is needed in the 
iron pole piece to achieve shielding in strong magnets. Calculations show 
that for a 0.75 Tesla magnet, only generally 200 pounds of superconductor 
are needed yielding a magnet weighing over 70,000 pounds which is too 
heavy to be used in medical facilities such as hospitals. The weight does 
not make this a viable magnet design. 
What is needed is an open and shielded superconductive magnet design which 
is light enough to be used in medical facilities and which is less 
expensive than known designs. 
SUMMARY OF THE INVENTION 
The open superconductive magnet of the invention includes a first assembly 
and a second assembly. The first assembly includes a magnetizable and 
generally cylindrical-shaped first pole piece, a magnetizable and 
generally annular-shaped first ring, a generally annular-shaped first 
superconductive main coil, and a generally annular-shaped first 
superconductive shielding coil. The first pole piece has a generally 
longitudinal first axis. The first ring is generally coaxially aligned 
with the first axis and is radially spaced outwardly apart from and at 
least partially longitudinally overlaps the first pole piece. The first 
superconductive main coil is generally coaxially aligned with the first 
axis, positioned radially between and spaced apart from the first pole 
piece and the first ring, and carries a first main electric current in a 
first direction. The first superconductive shielding coil is generally 
coaxially aligned with the first axis, positioned radially outward and 
spaced apart from the first superconductive main coil, positioned 
longitudinally outward and spaced apart from the longitudinally outer end 
of the first ring, and carries a first shielding electric current in a 
direction opposite to the first direction. 
The second assembly includes a magnetizable and generally 
cylindrical-shaped second pole piece, a magnetizable and generally 
annular-shaped second ring, a generally annular-shaped second 
superconductive main coil, and a generally annular-shaped second 
superconductive shielding coil. The second pole piece is longitudinally 
spaced apart from the first pole piece and is without a magnetizable solid 
path to the first pole piece. The second pole piece has a generally 
longitudinal second axis generally coaxially aligned with the first axis. 
The second ring is generally coaxially aligned with the second axis and is 
radially spaced outwardly apart from and at least partially longitudinally 
overlaps the second pole piece. The second superconductive main coil is 
generally coaxially aligned with the second axis, positioned radially 
between and spaced apart from the second pole piece and the second ring, 
and carries a second main electric current in the previously-mentioned 
first direction. The second superconductive shielding coil is generally 
coaxially aligned with the second axis, positioned radially outward and 
spaced apart from the second superconductive main coil, positioned 
longitudinally outward and spaced apart from the longitudinally outer end 
of the second ring, and carries a second shielding electric current in the 
previously-mentioned opposite direction. In an exemplary construction, the 
rings and the pole pieces consist essentially of iron. 
Several benefits and advantages are derived from the invention. The pole 
piece and the ring enhance the strength of the magnetic field so less 
superconductor is needed in the main coil. The iron ring provides a 
partial magnetic flux return for the main coil which reduces the iron 
needed in the pole piece and which reduces the amount of superconductor 
needed in the main coil. The iron ring also magnetically decouples the 
shielding coil from the main coil so that the magnetic flux lines from the 
shielding coil are captured by the iron ring and do not reach the magnetic 
flux lines from the main coil. Thus the iron mass of the pole piece does 
not have to be increased, and the amount of the superconductor in the main 
coil does not have to be increased, to offset the field subtracting 
effects of the magnetic flux lines from the shielding coil, since they are 
blocked by the presence of the iron ring. Computer simulations show that a 
0.75 Tesla magnet of the present invention would use generally 500 pounds 
of superconductor yielding a magnet weighing generally 15,000 pounds 
(which is light enough to be installed in a medical facility) and costing 
only half of what a viable equivalent conventional magnet would cost.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, wherein like numerals represent like 
elements throughout, FIGS. 1-3 show a first preferred embodiment of the 
open superconductive magnet 110 of the present invention. Preferably, the 
magnet 110 is a 0.5 Tesla or higher magnet. The magnet 110 includes a 
first assembly 112. The first assembly 112 includes a magnetizable and 
generally cylindrical-shaped first pole piece 114 having a generally 
longitudinal first axis 116. Preferably the first pole piece 114 consists 
essentially of a ferromagnetic material. In a preferred construction, the 
first pole piece 114 consists essentially of iron. 
The first assembly 112 also includes a magnetizable and generally 
annular-shaped first ring 118 generally coaxially aligned with the first 
axis 116. The first ring 118 is radially spaced outwardly apart from, and 
at least partially longitudinally overlaps, the first pole piece 114. The 
first ring 118 has a longitudinally inner end 120 and a longitudinally 
outer end 122. Preferably the first ring 118 consists essentially of a 
ferromagnetic material. In a preferred construction, the first ring 118 
consists essentially of iron. 
The first assembly 112 additionally includes a generally annular-shaped 
first superconductive main coil 124 generally coaxially aligned with the 
first axis 116 and disposed radially between, and spaced apart from, the 
first pole piece 114 and the first ring 118. By "disposed radially 
between" is meant, in this instance, that the radial distance of the first 
superconductive main coil 124 from the first axis 116 falls between the 
radius of the first pole piece 114 and the inside radius of the first ring 
118, and that the first ring 118 does not have to longitudinally overlap, 
to any extent, the first superconductive main coil 124 and that the first 
superconductive main coil 124 does not have to longitudinally overlap, to 
any extent, the first pole piece 114. The first superconductive main coil 
124 carries a first main electric current in a first direction. The first 
direction is defined to be either a clockwise or a counterclockwise 
circumferential direction about the first axis 116 with any slight 
longitudinal component of current direction being ignored. It is noted 
that additional first superconductive main coils may be needed to achieve 
a high magnetic field strength, within the magnet's imaging volume, 
without exceeding the critical current density of the superconductor being 
used in the superconductive coils, as is known to those skilled in the 
art. A preferred superconductor for the first superconductive main coil 
124 is niobium-titanium. 
The first assembly 112 moreover includes a generally annular-shaped first 
superconductive shielding coil 126 generally coaxially aligned with the 
first axis 116. The first superconductive shielding coil 126 is disposed 
radially outward, and spaced apart, from the first superconductive main 
coil 124, and is disposed longitudinally outward, and spaced apart, from 
the longitudinally outer end 122 of the first ring 118. The first 
superconductive shielding coil 126 carries a first shielding electric 
current in a direction opposite to the previously-defined first direction. 
A preferred superconductor for the first superconductive shielding coil 
126 is niobium-titanium. 
The open superconductive magnet 110 also includes a second assembly 128. 
The second assembly 128 includes a magnetizable and generally 
cylindrical-shaped second pole piece 130 longitudinally spaced apart from, 
and without a magnetizable solid path to, the first pole piece 114. The 
second pole piece 130 has a generally longitudinal second axis 132 which 
is generally coaxially aligned with the first axis 116. Preferably the 
second pole piece 130 consists essentially of a ferromagnetic material. In 
a preferred construction, the second pole piece 130 consists essentially 
of iron. 
The second assembly 128 also includes a magnetizable and generally 
annular-shaped second ring 134 generally coaxially aligned with the second 
axis 132. The second ring 134 is radially spaced outwardly apart from, and 
at least partially longitudinally overlaps, the second pole piece 130. The 
second ring 134 has a longitudinally inner end 136 and a longitudinally 
outer end 138. The longitudinally inner ends 120 and 136 of the first and 
second rings 118 and 134 are longitudinally closer to each other than are 
the longitudinally outer ends 122 and 138 of the first and second rings 
118 and 134. Preferably the second ring 134 consists essentially of a 
ferromagnetic material. In a preferred construction, the second ring 134 
consists essentially of iron. 
The second assembly 128 additionally includes a generally annular-shaped 
second superconductive main coil 140 generally coaxially aligned with the 
second axis 132 and disposed radially between, and spaced apart from, the 
second pole piece 130 and the second ring 134. The second superconductive 
main coil 140 carries a second main electric current in the 
previously-defined first direction. It is noted that additional second 
superconductive main coils may be needed to balance any additional first 
superconductive main coils present in the first assembly, as is known to 
those skilled in the art. A preferred superconductor for the second 
superconductive main coil 140 is niobium-titanium. 
The second assembly 128 moreover includes a generally annular-shaped second 
superconductive shielding coil 142 generally coaxially aligned with the 
second axis 132. The second superconductive shielding coil 142 is disposed 
radially outward, and spaced part, from the second superconductive main 
coil 140, and is disposed longitudinally outward, and spaced apart, from 
the longitudinally outer end 138 of the second ring 134. The second 
superconductive shielding coil 142 carries a second shielding electric 
current in a direction opposite to the previously-defined first direction. 
A preferred superconductor for the second superconductive shielding coil 
142 is niobium-titanium. 
In an exemplary construction, the open superconductive magnet 110 includes 
only one support member 144 connecting the first and second assemblies 112 
and 128, wherein the support member 144 is a nonmagnetizable support 
member preferably consisting essentially of nonmagnetic stainless steel. 
Preferably, the support member 144 and the first and second assemblies 112 
and 128 together have a generally "C" shape when viewed in a cross section 
created by a cutting plane, wherein the first axis 116 lies completely in 
the cutting plane, and wherein the cutting plane generally bisects the 
support member 144. It is noted that the previously-defined cross section 
is the cross section shown in FIG. 3, with the "C" shape seen by rotating 
FIG. 3 ninety degrees counterclockwise. 
Preferably, the second assembly 128 is a general mirror image of the first 
assembly 112 about a plane 146 (seen on edge as a dashed line in FIG. 3) 
disposed longitudinally equidistant between the first and second 
assemblies 112 and 128 and oriented generally perpendicular to the first 
axis 116. When the magnet 110 is employed as an MRI (magnetic resonance 
imaging) magnet, the magnet 110 includes a magnetic resonance imaging 
volume 148 (seen as a dotted line in FIGS. 1-3) disposed generally 
longitudinally equidistant between the first and second assemblies 112 and 
128. It is preferred that the imaging volume is a generally spherical 
imaging volume having a center 150 lying generally on the first axis 116. 
A patient 151 is shown in an imaging position in FIGS. 1 and 2. 
It is noted that the superconductive coils 124, 126, 140, and 142 are 
cooled, to a temperature below their critical temperature to achieve and 
sustain superconductivity, preferably by liquid-helium (or other 
cryogenic) cooling, by cryocooler cooling, or by a combination thereof. In 
a first cooling option, illustrated in the first assembly 112, the magnet 
110 also includes a first cryogenic vessel 152 surrounding the first 
superconductive main and shielding coils 124 and 126, wherein the first 
pole piece 114 and the first ring 118 are disposed outside and spaced 
apart from the first cryogenic vessel 152. The first cryogenic vessel 152 
contains a liquid cryogen 154, such as liquid helium. Preferably, the 
first cryogenic vessel 152 consists essentially of aluminum or nonmagnetic 
stainless steel. Here, the magnet 110 further includes a plurality of 
plates 156, 158, 160, 162, 164, and 166 which, together with the first 
ring 118 and the first pole piece 114 define a first vacuum enclosure 168 
which surrounds and which is spaced apart from the first cryogenic vessel 
152. Preferably, the plates 156, 158, 160, 162, 164, and 166 consist 
essentially of nonmagnetic stainless steel. 
In a second cooling option, illustrated in the second assembly 128, the 
magnet 110 also includes a cryocooler coldhead 170 having a housing 172 
attached to the second vacuum enclosure 174 and having a cold stage 176 in 
solid thermal conduction contact with the second superconductive main and 
shielding coils 140 and 142. 
Other cooling options (not shown in the figures) include each assembly 
having its own cryogenic vessel, wherein the liquid cryogen in one 
cryogenic vessel is in fluid communication with the liquid cryogen in the 
other cryogenic vessel through conduits in the support member. 
Alternately, a solid thermal conduction path can be placed in the support 
member allowing the cryocooler coldhead on the second vacuum enclosure to 
also cool the superconductive main and shielding coil in the first vacuum 
enclosure. Typically one or more thermal shields (not shown in the 
figures) are spaced apart from, and surround, the superconductive main and 
shielding coils. For cryogenic-cooling, such thermal shields are located 
outside the cryogenic vessel. It is noted that the magnet 110 moreover 
includes, as needed, thermal spacers and coil forms, as is known to the 
artisan, for proper spacing and support of the magnet components, such 
spacers and coil forms being omitted from the figures for clarity. 
Referring again to the drawings, FIG. 4 shows a second preferred embodiment 
of the open superconductive magnet 210 of the present invention, such 
magnet 210 preferably being a generally 0.75 Tesla magnet. Here, the 
longitudinally inner and outer ends 220 and 222 of the first ring 218 are 
spaced a first longitudinal distance apart from each other, wherein the 
first superconductive main coil 224 is disposed a longitudinal distance, 
from the longitudinally inner end 220 of the first ring 218, which is less 
than generally one-half the first longitudinal distance. The first ring 
218 generally completely longitudinally overlaps the first superconductive 
main coil 224, and the first superconductive main coil 224 partially 
overlaps the first pole piece 214. It is noted that a first magnet 
component can "completely longitudinally overlap" a second magnet 
component only when the first component is longitudinally longer than, and 
surrounds, the second component. The first ring 218 generally completely 
longitudinally overlaps the first pole piece 214. The first 
superconductive shielding coil 226 is disposed a longitudinal distance, 
from the longitudinally outer end 222 of the first ring 218, which is less 
than generally one-half the first longitudinal distance. The first ring 
218 is disposed a first radial distance from the first pole piece 214, 
wherein the first superconductive shielding coil 226 is disposed a radial 
distance, from the first superconductive main coil 224, which is less than 
generally the first radial distance. 
It is noted that those skilled in the art, using computer simulations based 
on conventional magnetic field analysis techniques, and using the 
teachings of the present invention, can design an open and shielded magnet 
of a desired magnetic field strength, a desired level of magnetic field 
inhomogenity, and a desired level of shielding (i.e., a desired position 
of the 5 Gauss stray field from the center of the imaging volume of the 
open superconductive magnet). As previously mentioned, the pole piece and 
the ring enhance the strength of the magnetic field so less superconductor 
is needed in the main coil. The iron ring provides a partial magnetic flux 
return for the main coil which reduces the iron needed in the pole piece 
and which reduces the amount of superconductor needed in the main coil. 
The iron ring also magnetically decouples the shielding coil from the main 
coil so that the magnetic flux lines from the shielding coil are captured 
by the iron ring and do not reach the magnetic flux lines from the main 
coil. Thus the iron mass of the pole piece does not have to be increased, 
and the amount of the superconductor in the main coil does not have to be 
increased, to offset the field subtracting effects of the magnetic flux 
lines from the shielding coil, since they are blocked by the presence of 
the iron ring. Computer simulations show that a 0.75 Tesla magnet of the 
present invention would use generally 500 pounds of superconductor 
yielding a magnet weighing generally 15,000 pounds (which is light enough 
to be installed in a medical facility) and costing only half of what a 
viable equivalent conventional magnet would cost. 
The foregoing description of several preferred embodiments of the invention 
has been presented for purposes of illustration. It is not intended to be 
exhaustive or to limit the invention to the precise form disclosed, and 
obviously many modifications and variations are possible in light of the 
above teaching. It is intended that the scope of the invention be defined 
by the claims appended hereto.