Open magnet with floor mount

BACKGROUND OF THE INVENTION

The present invention relates generally to open magnets, and more particularly to an open magnet having a floor mount.

Magnets include resistive and 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, cryocooler-cooled, and hybrid-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet typically also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in solid conduction thermal contact with the thermal shield, and having its second stage in solid conduction thermal contact with the superconductive main coil. A liquid-helium-cooled magnet typically also includes a liquid-helium vessel surrounding the superconductive main coil with the thermal shield surrounding the liquid-helium vessel. A hybrid-cooled magnet uses both liquid helium (or other liquid or gaseous cryogen) and a cryocooler coldhead, and includes designs wherein the first stage of the cryocooler coldhead is in solid conduction thermal contact with the thermal shield and wherein the second stage of the cryocooler coldhead penetrates the liquid-helium vessel to recondense “boiled-off” helium.

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.

Open magnets, including “C” shape and support-post 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.

It is also known in open magnet designs to place an iron pole piece in the bore of a resistive or superconductive coil assembly. 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. Nonmagnetizable support posts are connected to the face of the pole pieces. It is additionally known in horizontally-aligned open magnets to support the magnet on the floor using two spaced-apart feet attached to each assembly, such feet raising the assemblies to provide room underneath the assemblies for necessary wires, pipes, etc.

The sharpness of an MRI image depends, in part, on the magnetic field in the imaging volume being time-constant and highly uniform, such magnetic field suffering time and spatial deformation caused by vibrations, especially vibrations imparted to the coil assemblies of an open magnet by the presence of a cryocooler coldhead. What is needed is a design for a superconductive open magnet which reduces vibrations and hence which improves the sharpness of an MRI image.

BRIEF SUMMARY OF THE INVENTION

In a first expression of an embodiment of the invention, a open magnet includes first and second assemblies, at least one support beam, and a support skirt. Each assembly includes a longitudinally-extending axis, at least one superconductive main coil generally coaxially aligned with the axis, and a vacuum enclosure enclosing the assembly's at least one main coil. The first axis of the first assembly is generally vertically aligned, the second assembly is positioned generally vertically below the first assembly, and the second axis of the second assembly is generally coaxially aligned with the first axis. The at least one support beam has a first end attached to the first assembly and has a second end attached to the second assembly. The support skirt is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis, which has a first longitudinal end attached to the second assembly, and which has a second longitudinal end which can be supported by a floor. In one example, the open magnet also includes a first cryocooler coldhead having a first housing attached to one of the first and second vacuum enclosures.

In a second expression of an embodiment of the invention, an open magnet includes first and second assemblies, nonmagnetizable first and second support beams, and a support skirt. Each assembly includes a longitudinally-extending axis, at least one superconductive main coil generally coaxially aligned with the axis, a vacuum enclosure enclosing the assembly's at least one main coil and surrounding a bore, and a magnet pole piece. The magnet pole piece is located inside the bore and outside the vacuum enclosure and is attached to the vacuum enclosure. The first axis of the first assembly is generally vertically aligned, the second assembly is positioned generally vertically below the first assembly, and the second axis of the second assembly is generally coaxially aligned with the first axis. The first and second support beams each are generally vertically aligned, each have a first longitudinal end attached to the first magnet pole piece of the first assembly, and each have a second longitudinal end attached to the second magnet pole piece of the second assembly. The support skirt is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis, which has a first longitudinal end attached to the second assembly, and which has a second longitudinal end which can be supported by a floor. In one example, the open magnet also includes a first cryocooler coldhead having a first housing attached to the first vacuum enclosure.

Several benefits and advantages are derived from the invention. Engineering analysis shows that, compared to using conventional feet found on horizontally-aligned open magnets, the support-skirt design for a vertically-aligned open magnet stiffens the support of the magnet thereby shifting the natural frequency of the open magnet to a higher value which reduces the susceptibility of the open magnet to vibrate at the dominant low-excitation-frequencies imparted to the magnet by the presence of a cryocooler coldhead attached to an assembly (such as attached to a pole piece of an assembly). Applicants found that cryocooler vibrations cause vibration of the superconductive main coils, cause unwanted eddy-currents generated by vibrations of the thermal shields, and cause unwanted movement of the superconductive coils relative to the pole pieces all contributing to MRI image degradation. It is noted that, in a vertically-aligned open magnet, when the support member(s) provide a “clam-shell” support for the assemblies, the superconductive coils of such assemblies are subject to significant vibration from the cryocooler coldhead(s). It is further noted that a “clam-shell” support is provided by having only two support members, especially when the two support members are not diametrically aligned. Such clam-shell support is a very open support providing ease of patient table access to the imaging volume and providing ease of patient positioning within the imaging volume. Engineering analysis shows the support-skirt design of the invention reduces magnet vibrations in a vertically-aligned open magnet having a “clam-shell” support for the assemblies, especially for vibrations caused by the presence of a cryocooler coldhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

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

FIG. 2

is a cross-sectional view of the open magnet of

FIG. 1

taken along lines

2

—

2

of

FIG. 1

wherein the cryocooler coldheads are shown without hatching for clarity.

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 present invention. In a first expression of the embodiment of

FIGS. 1-2

of the invention, an open magnet

10

includes a first assembly

12

, a second assembly

14

, at least one support beam

16

and

18

, and a support skirt

20

. The first assembly

12

has a longitudinally-extending and generally-vertically-aligned first axis

22

, at least one superconductive main coil

24

, and a first vacuum enclosure

26

. By “generally-vertically-aligned” is meant vertically aligned plus or minus twenty degrees. The at least one superconductive main coil

24

of the first assembly

12

is generally coaxially aligned with the first axis

22

and 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

22

with any slight longitudinal component of current direction being ignored. The first vacuum enclosure

26

encloses the at least one superconductive main coil

24

of the first assembly

12

. The second assembly

14

is longitudinally spaced apart from and disposed generally vertically below the first assembly

12

. The second assembly

14

has a longitudinally-extending second axis

28

, at least one superconductive main coil

30

, and a second vacuum enclosure

32

. The second axis

28

is generally coaxially aligned with the first axis

22

. The at least one superconductive main coil

30

of the second assembly

14

is generally coaxially aligned with the second axis

28

and carries a second main electric current in the previously-described first direction. The second vacuum enclosure

32

encloses the at least one superconductive main coil

30

of the second assembly

14

. The at least one support beam

16

and

18

has a first end

34

attached to the first assembly

12

and has a second end

36

attached to the second assembly

14

. The support skirt

20

is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis

28

, which has a first longitudinal end

38

attached to the second assembly

14

, and which has a second longitudinal end

40

supportable by a floor

42

.

In one design, the open magnet

10

includes a first cryocooler coldhead

44

having a first housing

46

attached to one of the first and second vacuum enclosures

26

and

32

. The first housing

46

is shown as horizontally-aligned and rigidly attached to the first vacuum enclosure

26

in the figures, but in another application (not shown) the first cryocooler is attached to the first vacuum enclosure by a flexible bellows attachment and receives total weight-bearing support from a separate and rigid support ceiling or floor mount, such attachments reducing cryocooler-induced vibration. Although omitted from the figures for clarity, typically the first cryocooler coldhead

44

includes first and second stages. The second stage is in solid conduction thermal contact with the at least one superconductive main coil, or the second stage acts as a recondenser of boiled-off liquid helium contained in a cryogenic vessel. The first stage is in solid conduction thermal contact with a thermal shield which generally surrounds the at least one superconductive main coil or which generally surrounds the cryogenic vessel containing the at least one superconductive main coil and which is itself surrounded by the first vacuum enclosure. In the application shown in the figures, there is added a second cryocooler coldhead

48

which is a single-stage cryocooler coldhead whose first stage (omitted from the figures for clarity) is in solid conduction thermal contact with the thermal shield of the first assembly. Here, the at least one superconductive main coil of the first assembly would be thermally connected to the at least one superconductive main coil of the second assembly, as can be appreciated by those skilled in the art.

In one example, the support skirt

20

provides the only weight-bearing support for the second assembly

14

. In this example, the support skirt

20

has an annularly-cylindrical wall

50

surrounding an interior space

52

, and the annularly-cylindrical wall

50

has at least one opening

54

allowing access to the interior space

52

for necessary wires, pipes, etc., as can be appreciated by the artisan. It is noted that although the annularly-cylindrical wall

50

has at least one opening

54

, to maintain structural integrity to support the weight of the open magnet

10

, the annularly-cylindrical wall

50

is never longitudinally split apart. Here, the support skirt

20

proximate its second longitudinal end

40

has a radially-outwardly-extending flange

56

having a plurality of generally vertically-extending bolt holes

58

. Bolts (not shown in the figures) would attach the flange

56

to the floor

42

or to a plate or other structure resting on the floor.

In one construction, the second vacuum enclosure

32

has generally horizontally-aligned and generally planar and annular-shaped first and second walls

60

and

62

generally coaxially aligned with the second axis

28

. The second vacuum enclosure

32

also has circumferentially-extending third and fourth walls

64

and

66

. The first wall

60

is spaced apart from and disposed vertically above the second wall

62

. The third wall

64

is spaced apart from and disposed radially inward from the fourth wall

66

. The third and fourth walls

64

and

66

are each hermetically attached to both the first and second walls

60

and

62

. In one design, the third wall

64

monolithically extends vertically below the second wall

62

, and the third wall

64

vertically below the second wall

62

defines the annularly-cylindrical wall

50

of the support skirt

20

.

In one application, the at least one support beam

16

and

18

is radially disposed proximate the support skirt

20

. One type of “clam-shell” support for the first and second assemblies

12

and

14

is provided when the at least one support beam

16

and

18

consists of diametrically-offset first and second support beams

16

and

18

. In one example, the first and second support beams

16

and

18

are radially disposed between generally 110 and 150 degrees apart.

In a second expression of the embodiment of

FIGS. 1-2

of the invention, an open magnet

10

includes a first assembly

12

, a second assembly

14

, nonmagnetizable first and second support beams

16

and

18

, and a support skirt

20

. A support member is said to be a nonmagnetizable support member if it includes at least a nonmagnetizable portion which blocks having a magnetizable path between its ends. Such nonmagnetizable portion would have a relative permeability of generally unity. Examples of nonmagnetizable materials include aluminum, copper, nonmagnetic stainless steel, plastic, wood, etc. The first assembly

12

has a longitudinally-extending and generally-vertically-aligned first axis

22

, at least one superconductive main coil

24

, a first vacuum enclosure

26

, and a first magnet pole piece

68

. The at least one superconductive main coil

24

of the first assembly

12

is generally coaxially aligned with the first axis

22

and carries a first main electric current in a first direction. The first vacuum enclosure

26

encloses the at least one superconductive main coil

24

of the first assembly

12

and surrounds a first bore

70

. The first magnet pole piece

68

is generally coaxially aligned with the first axis

22

, is disposed inside the first bore

70

and outside the first vacuum enclosure

26

, and is attached to the first vacuum enclosure

26

. The second assembly

14

is longitudinally spaced apart from and disposed generally vertically below the first assembly

12

. The second assembly

14

has a longitudinally-extending second axis

28

, at least one superconductive main coil

30

, a second vacuum enclosure

32

, and a second magnet pole piece

72

. The second axis

28

is generally coaxially aligned with the first axis

22

. The at least one superconductive main coil

30

of the second assembly

14

is generally coaxially aligned with the second axis

28

and carries a second main electric current in the previously-described first direction. The second vacuum enclosure

32

encloses the at least one superconductive main coil

30

of the second assembly

14

and surrounds a second bore

74

. The second magnet pole piece

72

is generally coaxially aligned with the second axis

28

, is disposed inside the second bore

74

and outside the second vacuum enclosure

32

, and is attached to the second vacuum enclosure

32

The first and second support beams

16

and

18

each are generally vertically aligned, each have a first end

34

attached to the first assembly

12

and each have a second end

36

attached to the second assembly

14

. The support skirt

20

is a generally longitudinally-extending, annularly-cylindrical support skirt which is generally coaxially aligned with the second axis

28

, which has a first longitudinal end

38

attached to the second assembly

14

, and which has a second longitudinal end

40

supportable by a floor

42

.

Options for the second expression of an embodiment of the open magnet

10

of the invention are the same as the optional designs, examples, constructions, and applications previously-described for the first expression of an embodiment of the open magnet

10

of the invention with differences hereinafter noted. In one design of the second expression of an embodiment of the open magnet

10

of the invention, the first cryocooler coldhead

44

has a first housing

46

which is attached to the first vacuum enclosure

26

. In one example of the second expression of an embodiment of the open magnet

10

of the invention, the flange

56

of the support skirt

20

is attached (such as by welding) to the annularly-cylindrical wall

50

of the support skirt

20

. In one construction, engineering calculations show an open magnet

10

weighing generally 18,000 pounds can have its second assembly

14

supported by a support skirt

20

wherein the third wall

64

of the second vacuum enclosure

32

(which below the second wall

62

defines the annularly-cylindrical wall

50

of the support skirt

20

) comprises nonmagnetic stainless steel having a diameter of generally four feet and having a plate thickness of generally 0.25 to 0.50 inch with the first, second, and fourth walls requiring a thickness of generally 0.20 inch. It is noted that the stiffness of the first and second support beams

16

and

18

and the stiffness of the support skirt

20

can be chosen by those skilled in the art to reduce magnet vibration, such as cryocooler-imparted vibration, while providing structural support against gravitational and electromagnetic forces.

Several benefits and advantages are derived from the invention. Engineering analysis shows that, compared to using conventional feet found on horizontally-aligned open magnets, the support-skirt design for a vertically-aligned open magnet stiffens the support of the magnet thereby shifting the natural frequency of the open magnet to a higher value which reduces the susceptibility of the open magnet to vibrate at the dominant low-excitation-frequencies imparted to the magnet by the presence of a cryocooler coldhead attached to an assembly (such as attached to a pole piece of an assembly). Applicants found that cryocooler vibrations cause vibration of the superconductive main coils, cause unwanted eddy-currents generated by vibrations of the thermal shields, and cause unwanted movement of the superconductive coils relative to the pole pieces all contributing to MRI image degradation (including “ghosting” in images). It is noted that, in a vertically-aligned open magnet, when the support member(s) provide a “clam-shell” support for the assemblies, the superconductive coils of such assemblies are subject to significant vibration from the cryocooler coldhead(s). It is further noted that a “clam-shell” support is provided by having only two support members, especially when the two support members are not diametrically aligned. Such clam-shell support is a very open support providing ease of patient table access to the imaging volume and providing ease of patient positioning within the imaging volume. Engineering analysis shows the support-skirt design of the invention reduces magnet vibrations in a vertically-aligned open magnet having a “clam-shell” support for the assemblies, especially for vibrations caused by the presence of a cryocooler coldhead.

It should be noted that additional superconductive main coils, superconductive shielding coils, superconductive correction coils, and magnetizable rings may be present, as is known to the artisan, but such coils and rings have been omitted from the figures for clarity. Likewise, coil forms (if needed) to support the superconductive main coils and spacers to position a thermal shield with respect to a cryogenic vessel and to position a thermal shield with respect to a vacuum enclosure have been omitted from the figures but are well known to those skilled in the art. In an example, the open magnet

10

is a 0.5 or higher Tesla magnet, and the cryocooler coldheads are Gifford McMahon cryocooler coldheads.

The foregoing description of several expressions of an embodiment 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.