Apparatus and method for a superconductive magnet with pole piece

This application is a division of application Ser. No. 09/385,407, filed Aug. 31, 1999 now U.S. Pat. No. 6,172,588 which is hereby incorporated by reference in its entirety.

This application claims priority of a Provisional Application entitled “High Field Open Magnet” by Evangelos T. Laskaris et al., Serial No. 60/130,885 filed Apr. 23, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to a superconductive magnet used to generate a uniform magnetic field, and more particularly to such a magnet having a pole piece.

Magnets include resistive and superconductive magnets which are part of a magnetic resonance imaging (MRI) system used in various applications such as medical diagnostics and procedures. 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 vessel. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum vessel, 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 typically also includes a liquid-helium dewar surrounding the superconductive main coil and a second thermal shield which surrounds the first thermal shield which surrounds the liquid-helium dewar.

Known resistive and superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped resistive or superconductive coil assembly having a bore. The coil assembly includes several radially-aligned and longitudinally spaced-apart resistive or superconductive main coils each carrying a large, identical electric current in the same direction. The 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 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 resistive or 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 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 magnet designs having shielding include those wherein each coil assembly has an open bore and contains a resistive or superconductive shielding coil positioned longitudinally and radially outward from the resistive or superconductive main coil(s). In the case of a superconductive magnet, 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 resistive or superconductive coil assembly which lacks a shielding coil. The iron pole piece enhances the strength of the magnetic field and, by shaping the surface of the pole piece, magnetically shims the magnet improving 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. In the case of a superconductive magnet, 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 a superconductive magnet design which is physically more compact and which provides greater magnetic field homogeneity within the magnet's imaging volume than known designs.

BRIEF SUMMARY OF THE INVENTION

In a first expression of an embodiment of the invention, a superconductive magnet includes a longitudinally-extending axis and a first assembly having a superconductive main coil and a magnetizable pole piece. The main coil is generally coaxially aligned with the axis, carries a first main electric current in a first direction, and is positioned a first radial distance from the axis. The pole piece is generally coaxially aligned with the axis and is spaced apart from the main coil of said first assembly. Most of the pole piece of said first assembly is disposed radially inward of the main coil of said first assembly, and the pole piece of said first assembly extends from the axis radially outward a distance equal to at least 75 percent of the first radial distance. During operation of the magnet, the pole piece of the first assembly has a temperature equal generally to that of the main coil of the first assembly.

In a second expression of an embodiment of the invention, a superconductive magnet includes a longitudinally-extending axis and includes a first assembly having a superconductive main coil, a magnetizable pole piece, and a cryogenic-fluid dewar. The main coil is generally coaxially aligned with the axis and caries a first main electric current in a first direction. The pole piece is generally coaxially aligned with the axis, is spaced apart from the main coil of the first assembly, and has a surface portion. Most of the pole piece of the first assembly is located radially inward of the main coil of the first assembly. The dewar encloses the main coil of the first assembly and has an interior surface defined in part by the surface portion of the pole piece of the first assembly.

In a third expression of an embodiment of the invention, a superconductive open magnet includes a longitudinally-extending axis and longitudinally spaced-apart first and second assemblies each having a superconductive main coil, a superconductive shielding coil, a magnetizable and generally cylindrical-shaped pole piece, and a cryogenic-fluid dewar. Each main coil is generally coaxially aligned with the axis and carries a first main electric current in the same first direction. Each pole piece is generally coaxially aligned with and intersects the axis, is spaced apart from its associated main coil, and has a surface portion. Most of each pole piece is located radially inward of its associated main coil. Each dewar encloses its associated main and shielding coils and has an interior surface defined in part by the surface portion of its associated pole piece.

In one construction, the open magnet also includes spaced-apart and generally-nonmagnetizable first and second support posts each having a first end structurally attached to the pole piece of the first assembly, each having a second end structurally attached to the pole piece of the second assembly, and each having a surface portion. In this construction, the open magnet further includes first and second dewar conduits each in fluid communication with the dewar of the first assembly and the dewar of the second assembly. Here, the first dewar conduit has an interior surface defined in part by the surface portion of the first support post, and the second dewar conduit has an interior surface defined in part by the surface portion of the second support post.

A first method of the invention includes steps a) and b) and provides a homogeneous magnetic resonance imaging volume for a superconductive magnet having a magnetizable pole piece and a superconductive main coil, wherein the main coil has a critical temperature. Step a) includes cooling the main coil to a temperature equal to or less than the critical temperature. Step b) includes cooling the pole piece to a temperature equal to generally the temperature of the main coil.

A second method of the invention includes steps a) through d) and provides both physical compactness and a homogeneous imaging volume for a superconductive magnet having a magnetizable pole piece and a superconductive main coil. Step a) includes obtaining a generally nonmagnetizable coil support. Step b) includes attaching the coil support to the pole piece. Step c) includes supporting the main coil with the coil support. Step d) includes constructing and positioning a cryogenic-fluid dewar to surround the main coil and to have an interior surface defined in part by a surface portion of the pole piece.

A third method of the invention includes steps a) through j) and provides both physical compactness and a homogeneous imaging volume for a superconductive open magnet having a longitudinally-extending axis and having longitudinally spaced-apart and generally coaxially-aligned first and second assemblies each including a magnetizable and generally cylindrical-shaped pole piece intersecting the axis, a superconductive main coil, and a superconductive shielding coil. Step a) includes obtaining generally nonmagnetizable first coil supports. Step b) includes attaching the first coil supports to the pole piece of the first assembly. Step c) includes supporting the main and shielding coils of the first assembly with the first coil supports. Step d) includes constructing and positioning a cryogenic-fluid dewar to surround the main and shielding coils of the first assembly and to have an interior surface defined in part by a surface portion of the pole piece of the first assembly. Step e) includes obtaining generally nonmagnetizable second coil supports. Step f) includes attaching the second coil supports to the pole piece of the second assembly. Step g) includes supporting the main and shielding coils of the second assembly with the second coil supports. Step h) includes constructing and positioning a cryogenic-fluid dewar to surround the main and shielding coils of the second assembly and to have an interior surface defined in part by a surface portion of the pole piece of the second assembly. Step i) includes attaching a first end of a generally-nonmagnetizable support port to the pole piece of the first assembly and attaches a second end of the support post to the pole piece of the second assembly. Step j) includes constructing and positioning a dewar conduit in fluid communication with the dewar of the first assembly and the dewar of the second assembly, wherein the dewar conduit has an interior surface defined in part by a surface portion of the support post.

Several benefits and advantages are derived from the invention. Making the pole piece a cryogenically-cold pole piece provides greater magnetic field homogeneity within the magnet's imaging volume by eliminating magnetic field inhomogeneities caused by temperature changes of conventional room-temperature pole piece designs caused by changes in room temperature. Making the pole piece a part of the dewar provides physical compactness by eliminating the extra space otherwise required for the cryogenically-cold pole piece of the invention to be completely surrounded by a cryogenic-fluid dewar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a schematic perspective view of an embodiment of the magnet of the invention; and

FIG. 2

is a schematic cross sectional view of the magnet of

FIG. 1

taken along lines

2

—

2

of FIG.

1

.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like numerals represent like elements throughout,

FIGS. 1-2

show an embodiment of the magnet

10

of the present invention. In one application, magnet

10

provides the static magnetic field for a magnetic resonance imaging (MRI) system (not shown) used in medical diagnostics. It is noted that in describing the invention, when a magnet is said to include a component such as a coil, a pole piece, or a dewar, etc., it is understood to mean that the magnet includes at least one coil, at least one pole piece, or at least one dewar, etc.

In a first expression of the invention, a superconductive magnet

10

includes a longitudinally-extending axis

12

and a first assembly

14

. The first assembly

14

includes a superconductive main coil

16

and a magnetizable pole piece

18

. The main coil

16

is generally coaxially aligned with the axis

12

, carries a first main electric current in a first direction, and is disposed a first radial distance from the axis

12

. The first direction is defined to be either a clockwise or a counterclockwise circumferential direction about the axis

12

with any slight longitudinal component of current direction being ignored. The pole piece

18

is generally coaxially aligned with the axis

12

, and is spaced apart from the main coil

16

of the first assembly

14

. Most of the pole piece

18

of the first assembly

14

is disposed radially inward of the main coil

16

of the first assembly

14

. The pole piece

18

of the first assembly

14

extends from the axis

12

radially outward a distance equal to at least 75 percent of the first radial distance. During operation of the magnet

10

, the pole piece

18

of the first assembly

14

has a temperature equal generally to that of the main coil

16

of the first assembly

14

. It is noted that the first assembly

14

may be used alone as a table magnet (not shown) or may be one of two assemblies of an open magnet (as shown in the figures). During operation of the magnet

10

, the main coil

16

and the pole piece

18

of the first assembly

14

are cooled by a cryocooler coldhead (not shown), and/or by a cryogenic fluid, or the like.

In a second expression of the invention, a superconductive magnet

10

includes a longitudinally-extending axis

12

and a first assembly

14

. The first assembly

14

includes a superconductive main coil

16

, a magnetizable pole piece

18

, and a cryogenic-fluid dewar

20

. The superconductive main coil

16

is generally coaxially aligned with the axis

12

and carries a first main electric current in a first direction. The pole piece

18

is generally coaxially aligned with the axis

12

, is spaced apart from the main coil

16

, and has a surface portion

22

. Most of the pole piece

18

is disposed radially inward of the main coil

16

. The dewar

20

encloses the main coil

16

and has an interior surface

24

defined in part by the surface portion

22

of the pole piece

18

.

In particular magnet designs, additional superconductive main coils (not shown) may be needed in the first assembly

14

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. An example of a superconductor for the superconductive main coil

16

is niobium-titanium. An example of a material for the pole piece

18

is iron.

In one example, the magnet

10

also includes a second assembly

26

longitudinally spaced apart from the first assembly

14

. The second assembly

26

includes a superconductive main coil

28

, a magnetizable pole piece

30

, and a cryogenic-fluid dewar

32

. The superconductive main coil

28

is generally coaxially aligned with the axis

12

and carries a first main electric current in the previously-described first direction. The pole piece

30

is generally coaxially aligned with the axis

12

, is spaced apart from the main coil

28

, and has a surface portion

34

. Most of the pole piece

30

is disposed radially inward of the main coil

28

. The dewar

32

encloses the main coil

28

and has an interior surface

36

defined in part by the surface portion

34

of the pole piece

30

. In the example shown in

FIGS. 1 and 2

, the pole piece

18

includes another surface portion

23

which does not help define the interior surface

24

of the dewar

20

, and the pole piece

30

includes another surface portion

35

which does not help define the interior surface

36

of the dewar

32

.

In one construction, the magnet

10

also includes a generally-nonmagnetizable coil support

38

attached to the pole piece

18

and supporting the main coil

16

of the first assembly

14

and further includes a generally-nonmagnetizable coil support

40

attached to the pole piece

30

and supporting the main coil

28

of the second assembly

26

. By “nonmagnetizable” is meant being able to be magnetized no better than nonmagnetic stainless steel. An example of a material for the coil supports

38

and

40

is nonmagnetic stainless steel or fiberglass.

In one magnet design, the magnet

10

also includes a generally-nonmagnetizable (first) support post

42

having a first end structurally attached (e.g., welded) to the pole piece

18

of the first assembly

14

, having a second end structurally attached (e.g., welded) to the pole piece

30

of the second assembly

26

, and having a surface portion

44

. An example of a material for the (first) support post

42

is nonmagnetic stainless steel. In this design, the magnet

10

further includes a (first) dewar conduit

46

in fluid communication with the dewar

20

of the first assembly

14

and the dewar

32

of the second assembly

26

. The (first) dewar conduit

46

has an interior surface

48

defined in part by the surface portion

44

of the (first) support post

42

. Here, a plate assembly

50

has an interior surface including a first portion

52

defining in part the interior surface of the dewar

20

of the first assembly

14

, a second portion

54

defining in part the interior surface of the dewar

32

of the second assembly

26

, and a third portion

56

defining in part the interior surface of the (first) dewar conduit

46

. In this example, the magnet

10

additionally includes a thermal shield

58

and a vacuum vessel

60

. The thermal shield

58

is spaced apart from and generally encloses the pole piece

18

and

30

and the dewar

20

and

32

of the first and second assemblies

14

and

26

, the (first) support post

42

, and the (first) dewar conduit

46

. The vacuum vessel

60

is spaced apart from and hermetically encloses the thermal shield

58

. An example of a material for the plate assembly

50

, the thermal shield

58

, and the vacuum vessel

60

is nonmagnetic stainless steel. It is noted that, in this example, the previously-mentioned “spacing apart” is accomplished by using conventional spacers

62

.

In operation, the magnet

10

would include cryogenic fluid

64

disposed in the dewar

20

and

32

of the first and second assemblies

14

and

26

and in the (first) dewar conduit

46

. An example of a cryogenic fluid is liquid helium. A cryocooler coldhead (not shown) may be used to recondense evaporated liquid helium by having the first stage of the coldhead be in contact with the thermal shield

58

and by having the second stage of the coldhead penetrate into the dewar void volume near the highest point of a dewar

20

and

32

In another embodiment (not shown) of the magnet of the invention, the first and second assemblies

14

and

26

each would have a self-contained dewar, thermal shield, and vacuum vessel wherein support posts would interconnect the vacuum vessels or wherein the two assemblies

14

and

26

would be supported in spaced-apart relationship by a “C”-shaped arm, by being bolted to a floor and/or walls, or by other means. In the embodiment not shown, the cryogenic fluid

64

would be disposed only in the dewar

20

and

32

of the first and second assemblies

14

and

26

since there would be no (first) dewar conduit

46

. In the embodiment shown in

FIGS. 1 and 2

, the magnet

10

also includes a magnetic resonance imaging volume

66

having a center located generally on the axis

12

longitudinally equidistant between the first and second assemblies

14

and

26

. One shape of the imaging volume

66

is a sphere. It is noted that typically the second assembly

26

is a general mirror image of the first assembly

14

about a plane (not sown) which is perpendicular to the axis

12

and which is disposed generally equidistant between the first and second assemblies

14

and

26

.

In a third expression of the invention, a superconductive open magnet

10

includes a longitudinally-extending axis

12

, a first assembly

14

, and a second assembly

26

longitudinally spaced apart from the first assembly

14

. The first assembly

14

includes a superconductive main coil

16

, a superconductive shielding coil

68

, a magnetizable and generally cylindrical-shaped pole piece

18

, and a cryogenic-fluid dewar

20

. The superconductive main coil

16

is generally coaxially aligned with the axis

12

and carries a first main electric current in a first direction. The superconductive shielding coil

68

is generally coaxially aligned with the axis

12

, is disposed longitudinally outward from the main coil

16

, and carries a first shielding electric current in an opposite direction to the previously-described first direction. The pole piece

18

is generally coaxially aligned with and intersects the axis

12

, is spaced apart from the main and shielding coils

16

and

68

, and has a surface portion

22

. Most of the pole piece

18

is disposed longitudinally between and radially inward of the main and shielding coils

16

and

68

. The dewar

20

encloses the main and shielding coils

16

and

68

and has an interior surface

24

defined in part by the surface portion

22

of the pole piece

18

. The second assembly

26

includes a superconductive main coil

28

, a superconductive shielding coil

70

, a magnetizable and generally cylindrical-shaped pole piece

30

, and a cryogenic-fluid dewar

32

. The superconductive main coil

28

is generally coaxially aligned with the axis

12

and carries a second main electric current in the previously-described first direction. The superconductive shielding coil

70

is generally coaxially aligned with the axis

12

, is disposed longitudinally outward from the main coil

28

, and carries a second shielding electric current in the previously-described opposite direction. The pole piece

30

is generally coaxially aligned with and intersects the axis

12

, is spaced apart from the main and shielding coils

28

and

70

, and has a surface portion

34

. Most of the pole piece

30

is disposed longitudinally between and radially inward of the main and shielding coils

28

and

70

. The dewar

32

encloses the main and shielding coils

28

and

70

and has an interior surface

36

defined in part by the surface portion

34

of the pole piece

30

.

In one construction, the open magnet

10

also includes generally-nonmagnetizable coil supports

38

and

72

attached to the pole piece

18

and supporting the main and shielding coils

16

and

68

of the first assembly

14

and further includes generally-nonmagnetizable coil supports

40

and

74

attached to the pole piece

30

and supporting the main and shielding coils

28

and

70

of the second assembly

26

. In one magnet design, the open magnet

10

also includes generally-nonmagnetizable first

42

and second (not shown but identical with the first

42

) support posts each having a first end structurally attached to the pole piece

18

of the first assembly

14

, each having a second end structurally attached to the pole piece

30

of the second assembly

26

, and each having a surface portion

44

. In this design, the open magnet

10

further includes first

46

and second (not shown but identical with the first

46

) dewar conduits each in fluid communication with the dewar

20

of the first assembly

14

and the dewar

32

of the second assembly

26

. The first dewar conduit

46

has an interior surface

48

defined in part by the surface portion

44

of the first support post

42

, and the second dewar conduit has an interior surface defined in part by the surface portion of the second support post. In this example, the open magnet

10

additionally includes a thermal shield

58

and a vacuum vessel

60

. The thermal shield

58

is spaced apart from and generally encloses the pole piece

18

and

30

and the dewar

20

and

32

of the first and second assemblies

14

and

26

, the first

42

and second support posts, and the first

46

and second dewar conduits. The vacuum vessel

60

is spaced apart from and hermetically encloses the thermal shield

58

. It is noted that the first support post

42

and the first dewar conduit

46

are disposed inside a first portion

76

of the vacuum vessel

60

, that the second support post and the second dewar conduit are disposed inside a second portion

78

of the vacuum vessel, and that such first and second portions

76

and

78

of the vacuum vessel

60

are shown in FIG.

1

. In operation, the magnet

10

would include the previously-described cryogenic fluid

64

and magnetic resonance imaging volume (also known as just “imaging volume”)

66

. In one construction, the first

42

and second support posts (as seen from the enclosing first and second portions

76

and

78

of the vacuum vessel

60

shown in

FIG. 1

) are angularly spaced apart between generally 110 and 150 degrees about the axis

12

and disposed radially outward from the imaging volume

66

. In one example an angular spacing of generally 130 degrees is provided for convenient placement of the patient (not shown) in the imaging volume

66

.

In one application, the open magnet

10

has a magnetic field within its imaging volume

66

of generally 1.4 to 1.5 Tesla. In one orientation of the open magnet

10

, the first and second portions

76

and

78

of the vacuum vessel

60

are horizontally aligned (as shown in FIG.

1

), and the patient would typically be in a standing position within the imaging volume

66

. In another orientation (not shown) of the open magnet

10

, the first and second portions

76

and

78

of the vacuum vessel

60

are vertically aligned, and the patient would typically be lying on a patient table within the imaging volume

66

. It is noted that the pole pieces

18

and

30

provide the main structural support of the magnet

10

including the coils

16

,

28

,

68

, and

70

and the dewars

20

and

32

, and that the pole pieces

18

and

30

are shaped (e.g., have ring steps) to provide a more uniform magnetic field within the imaging volume

66

. Any further correction of magnetic field inhomogeneities may be accomplished by active shimming, as is within the skill of the artisan. It is further noted that in the example shown in the figures, magnet

10

is designed for each assembly

14

and

26

to have a recess

80

in the vacuum vessel

60

facing the imaging volume

66

for a split pair of flat shielded gradient/RF coils, wherein the pole faces of the pole pieces

18

and

30

are not laminated, as can be appreciated by the artisan.

A first method of the invention includes steps a) and b) and is a method for providing a homogeneous magnetic resonance imaging volume

66

for a superconductive magnet

10

having a magnetizable pole piece

18

and a superconductive main coil

16

, wherein the main coil

16

has a critical temperature (i.e., a temperature at which, and below which, superconductivity occurs). Step a) includes cooling the main coil

16

to a temperature equal to or less than the critical temperature. Step b) includes cooling the pole piece

18

to a temperature equal to generally the temperature of the main coil

16

. During operation of the magnet

10

, the main coil

16

and the pole piece

18

of the first assembly

14

are cooled by a cryocooler coldhead (not shown), and/or by a cryogenic fluid, or the like.

A second method of the invention includes steps a) through d) and is a method for providing both physical compactness and a homogeneous magnetic resonance imaging volume

66

for a superconductive magnet

10

having a magnetizable pole piece

18

and a superconductive main coil

16

. Step a) includes obtaining a generally nonmagnetizable coil support

38

. Step b) includes attaching the coil support

38

to the pole piece

18

. Step c) includes supporting the main coil

16

with the coil support

38

. Step d) includes constructing and disposing a cryogenic-fluid dewar

20

to surround the main coil

16

and to have an interior surface

24

defined in part by a surface portion

22

of the pole piece

18

.

In one implementation of the method, a step is added to dispose a thermal shield

58

to be spaced apart from and to generally enclose the pole piece

18

and the dewar

20

. In this implementation, another step is added to dispose a vacuum vessel

60

to be spaced apart and to hermetically enclose the thermal shield

58

. Here, a further step is added to dispose a cryogenic fluid

64

in the dewar

20

.

In one application of the method, step d) constructs the dewar

20

such that the surface portion

22

of the pole piece

18

is between generally 40 percent and generally 60 percent of the total surface area of the pole piece

18

. In the same or another application, step d) constructs the dewar

20

to have a void volume wherein at least generally sixty percent of the void volume is located longitudinally outward of the pole piece

18

.

A third method of the invention includes steps a) through j) and is a method for providing both physical compactness and a homogeneous magnetic resonance imaging volume

66

for a superconductive open magnet

10

having a longitudinally-extending axis

12

and having longitudinally spaced-apart and generally coaxially-aligned first and second assemblies

14

and

26

each including a magnetizable and generally cylindrical-shaped pole piece

18

and

30

intersecting the axis

12

, a superconductive main coil

16

and

28

, and a superconductive shielding coil

68

and

70

. Step a) includes obtaining generally nonmagnetizable first coil supports

38

and

72

. Step b) includes attaching the first coil supports

38

and

72

to the pole piece

18

of the first assembly

14

. Step c) includes supporting the main and shielding coils

16

and

68

with the first coil supports

38

and

72

of the first assembly

14

. Step d) includes constructing and disposing a cryogenic-fluid dewar

20

to surround the main and shielding coils

16

and

68

of the first assembly

14

and to have an interior surface

24

defined in part by a surface portion

22

of the pole piece

18

. Step e) includes obtaining generally nonmagnetizable second coil supports

40

and

74

. Step f) includes attaching the second coil supports

40

and

74

to the pole piece

30

of the second assembly

26

. Step g) includes supporting the main and shielding coils

28

and

70

with the second coil supports

40

and

74

of the second assembly

26

. Step h) includes constructing and disposing a cryogenic-fluid dewar

32

to surround the main and shielding coils

28

and

70

of the second assembly

26

and to have an interior surface

36

defined in part by a surface portion

34

of the pole piece

30

. Step i) includes attaching a first end of a generally-nonmagnetizable (first) support post

42

to the pole piece

18

of the first assembly

14

and attaching a second end of the (first) support post

42

to the pole piece

30

of the second assembly

26

. Step j) includes constructing and disposing a (first) dewar conduit

46

in fluid communication with the dewar

20

of the first assembly

14

and the dewar

32

of the second assembly

26

wherein the (first) dewar conduit

46

has an interior surface

48

defined in part by a surface portion

44

of the (first) support post

42

.

In one implementation of the method, a step is added to dispose a thermal shield

58

to be spaced apart from and to generally enclose the pole piece

18

and

30

and the dewar

20

and

32

of the first and second assemblies

14

and

26

, the (first) support post

42

, and the (first) dewar conduit

46

. In this implementation, another step is added to dispose a vacuum vessel

60

to be spaced apart and to hermetically enclose the thermal shield

58

. Here, a further step is added to dispose a cryogenic fluid

64

in the dewar

20

and

32

of the first and second assemblies

14

and

26

and in the (first) dewar conduit

46

.

In one application of the method, step d) constructs the dewar of the first assembly

14

such that the surface portion

22

of the pole piece

18

is between generally 40 percent and generally 60 percent of the total surface area of the pole piece

18

, and step h) constructs the dewar

32

of the second assembly

26

such that the surface portion

34

of the pole piece

30

is between generally

40

percent and generally

60

percent of the total surface area of the pole piece

30

. In the same or another application, step d) constructs the dewar

20

of the first assembly

14

to have a void volume wherein at least generally sixty percent of the void volume of the dewar

20

is located longitudinally outward of the pole piece

18

, and step h) constructs the dewar

32

of the second assembly

26

to have a void volume wherein at least generally sixty percent of the void volume of the dewar

32

is located longitudinally outward of the pole piece

30

. In one example, step j) constructs the (first) dewar conduit

46

to have a void volume located entirely radially outward of the (first) support post

42

.

Several benefits and advantages are derived from the invention. Making the pole piece a cryogenically-cold pole piece provides greater magnetic field homogeneity within the magnet's imaging volume by eliminating magnetic field inhomogeneities caused by temperature changes of conventional room-temperature pole piece designs caused by changes in room temperature. Making the pole piece a part of the dewar provides physical compactness by eliminating the extra space otherwise required for the cryogenically-cold pole piece of the invention to be completely surrounded by a cryogenic-fluid dewar.

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 a shielded superconductive open magnet

10

of a desired magnetic field strength, a desired level of magnetic field inhomogeneity, and a desired level of shielding (i.e., a desired position of the 5 Gauss stray field from the center of the imaging volume

66

of the superconductive open magnet). The pole piece enhances the strength of the magnetic field so less superconductor is needed in the main coil. The radially-outermost portion of the pole piece 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 radially-outermost portion of the pole piece also magnetically decouples the shielding coil from the main coil so that the magnetic flux lines from the shielding coil are captured by the radially-outermost portion of the pole piece 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 radially-outermost portion of the pole piece.

Computer simulations show that a 1.4 Tesla MRI (magnetic resonance imaging) magnet, as shown in the figures and having a 35 centimeter-diameter spherical imaging volume, would weigh about 30,000 pounds and have a 5 Gauss stray field that extends 4.5 meters vertically and 5.5 meters horizontally from the center of the imaging volume

66

. The 5 Gauss stray field can be contained to 2.5 meters vertically and 3.5 meters horizontally by the use of a 12,000 pound room shield. The magnet would fit inside a 180 centimeter cube (i.e., a cube having length=width=height=180 centimeters).

The foregoing description of several expressions of an embodiment of the magnet of the invention and several methods relating thereto have 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.