MRI with superconducting coil

CROSS REFERENCE TO RELATED APPLICATIONS

THIS APPLICATION CLAIMS BENEFIT OF U.S. PROVISIONAL APPLICATION SERIAL NO. 60/094,520, FILED JUL. 29, 1998, THE DISCLOSURE OF WHICH IS HEREBY INCORPORATED BY REFERENCE HEREIN.

FIELD OF THE INVENTION

The present invention relates to superconducting magnetic resonance imaging (“MRI”) devices for producing images used in medical diagnostics.

BACKGROUND OF THE INVENTION

Medical magnetic resonance studies are typically carried out in strong magnetic fields. In an electromagnet employing normal conductors, a portion of the electrical power used to generate the magnetic field is consumed in heating of the resistive coil conductor. Thus, many kilowatts of power may be required to produce the magnetic field strength for a volume sufficient to develop a magnetic resonance image for medical diagnostics.

A coil of a conductive material, which is wound into a solenoid, generates the magnetic field for the magnet. The use of superconducting coils in MRI systems greatly reduces the amount of power consumed since a superconductor has a resistance which approaches zero and thus does not expend as much power to produce the same magnetic field. One of the controlling factors in superconductivity is the critical temperature of the conductive material of the coil. In a typical superconducting MRI magnet, a cryogenic fluid is used to cool a superconducting coil to a temperature at or below the critical temperature so that the coil exhibits superconducting properties.

One type of MRI magnet incorporates a frame formed from a ferromagnetic material, disclosed in U.S. Pat. Nos. 4,766,378 and 5,754,085, the disclosures of which are hereby incorporated by reference herein.

The MRI magnet disclosed in certain embodiments of U.S. Pat. No. 5,754,085 includes a first superconducting coil assembly supported on an upper support and a second superconducting coil assembly supported on a lower support. Superconducting coil assemblies typically include a container for the superconducting coil and the coil is disposed inside the container and immersed in a bath of liquid helium held in the container.

A substantial field is created by the primary magnet assembly. The net forces acting on the coils results in attraction of the coils to each other or attraction between the individual coils and the adjacent ferromagnetic components. The direction and magnitude of the force depends on the location of the coils with respect to the median plane of the magnet. The forces created by the field tends to spread the coil in a radial direction, tending to unravel the coil, or causes the coils to be attracted to the adjacent ferromagnetic structure. Thus, mounting and supporting the superconducting coils within an MRI magnet is a substantial problem. In addition, one of the issues which must be addressed is the tendency of the support structure for the superconducting coil to add heat to the coil by conduction from the outside environment.

The cryogenic fluid utilized to cool a superconducting coil is costly, which is a drawback as compared to a non-superconducting electromagnet. An MRI magnet including support structure for the superconducting coil which minimizes the amount of heat introduced into the superconducting coil, and which utilizes a minimum amount of cryogenic fluid is desirable for reasons of operating cost. Moreover, it is desirable to minimize the amount of cryogenic fluid in proximity to the coil for other reasons. A superconducting magnet coil can become normally-conducting in certain unusual circumstances known as a “quench”. When this happens, energy stored in the flowing current is dissipated rapidly, and the surrounding cryogenic liquid is converted to a gas. While a properly designed superconducting magnet should not quench, safety precautions should be provided to take into account possible quenching and gas evolution. With less cryogenic fluid in the superconducting coil assembly, this problem is minimized.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

A magnetic resonance imaging magnet for examining a patient comprises a first ferromagnetic pole piece and a second ferromagnetic pole piece, the pole pieces being disposed facing each other and defining a patient-receiving gap therebetween. The pole pieces are arranged for producing a magnetic field within the patient-receiving gap and have a central axis extending through the pole pieces. The magnet also comprises a ferromagnetic yoke for supporting the first pole piece and the second pole piece in their respective positions facing each other. The magnet includes a first superconducting coil assembly and a second superconducting coil assembly. Each assembly has an insulated housing and is disposed adjacent one of the pole pieces for producing a magnetic field which passes through the pole pieces and the patient-receiving gap. Each coil assembly includes at least one coil encircling the associated pole piece, at least one thermally conductive member mounted adjacent to the at least one coil, and means for maintaining a cryogenic fluid. The at least one thermally conductive member is in heat transfer relationship with the cryogenic fluid, thereby maintaining the at least one coil at a temperature at which the at least one coil exhibits superconducting properties.

The superconducting coil is cooled by conduction through the thermally conductive member which removes heat from the superconducting coil. The coil is not immersed in a cryogenic fluid such as helium so that the volume of helium required in the superconducting coil assembly is reduced.

The ferromagnetic yoke preferably comprises a magnetic flux conduit to minimize leakage magnetic fields.

The superconducting coil assembly of the magnet also preferably includes a plurality of suspension members connected to the coil assembly for restraining the superconducting coil assembly. The substantial forces acting on the superconducting coil assembly when the coils are generating a magnetic field are counteracted by the plurality of suspension members. The suspension members carry less heat to the superconducting coil assembly than a compression-type mounting having a greater cross-sectional area. However, a compression-type mounting may be used in some embodiments of the invention. Accordingly, the superconducting coil assembly may include a plurality of supporting legs connected to the ferromagnetic yoke for supporting the superconducting coil assembly within the magnet.

In preferred embodiments, the housing for the superconducting coil assembly includes at least one layer of superinsulation surrounding the at least one coil. The housing may also include at least one heat shield within the housing. The heat shield or shields must be optimized to reduce the heat transfer from the hot environment to the cold mass. In certain preferred embodiments, the housing may enclose an inner anti-buckling ring, an outer clamping ring, a top support ring and a bottom support ring for supporting the at least one coil and restraining the at least one coil in the coil assembly. The coil assembly preferably includes a housing having a toroidal shape and being arranged concentrically with the central axis.

In certain preferred embodiments, the magnet includes a first support for supporting and restraining the at least one coil adjacent the at least one thermally conductive member, the first support having a U-shaped cross section. The first support may have any number of cross-sectional shapes. A second housing is preferably included for mounting the at least one thermally conductive member adjacent the at least one coil, the second support having a U-shaped cross-section. The second support also may have a number of cross-sectional shapes.

In preferred embodiments, the means for maintaining the cryogenic fluid for the magnet includes a cryocooler. The cryocooler preferably includes a cryogenic fluid. The at least one thermally conductive member may include at least one tube, carrying a p-cryogenic fluid. The at least one tube may comprise three tubes, for example, depending on the size of the coil. The means for maintaining the cryogenic fluid preferably comprises a cryocooler, re-condensing the cryogenic fluid to a liquid phase, connected to the at least one tube. The cryogenic fluid preferably comprises liquid helium.

The at least one thermally conductive member, in certain preferred embodiments, comprises at least one strap of a thermally conductive metal, such as high purity aluminum. Preferably, the thermally conductive metal comprises aluminum having a purity of at least 99%.

A superconducting coil assembly for a magnetic resonance imaging magnet for examining a patient in accordance with the above is also provided in another aspect of the present invention. In a further aspect of the present invention, a superconducting coil assembly comprises an insulated vessel having a toroidal shape and a central axis extending therethrough, a coil subassembly having a first side and a second side and at least one superconducting coil. The superconducting coil assembly includes a plurality of suspension members extending parallel to the central axis, into the vessel, for supporting the superconducting coil assembly in a magnetic resonance imaging magnet. The suspension members are arranged in pairs so that one suspension member is connected to the subassembly at the first side and another suspension member is connected to the subassembly at the second side.

In preferred embodiments, the plurality of suspension members include suspension members extending through the housing from a top end to a bottom end of the housing and having termini connected to the bottom end.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and features of the invention will appear from the following description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:

FIG. 1

is a top plan view of an MRI magnet in accordance with a first embodiment of the present invention;

FIG. 2

is a schematic sectional view taken along line

2

—

2

in

FIG. 1

;

FIG. 3

is a sectional view taken along line

3

—

3

in

FIG. 2

;

FIG. 4

is a plan view, partially in section, of the coil assembly of the embodiment of

FIGS. 1-3

;

FIG. 5

is a sectional view, taken along line

5

—

5

in

FIG. 4

, showing a superconducting coil assembly in accordance with the embodiment of the invention in

FIGS. 1-4

;

FIG. 6

is a partial cross-sectional view showing an upper coil assembly and cryocooler in accordance with another embodiment of the invention;

FIG. 7

is a partial cross-sectional view of a thermally conductive strap and cryocooler in accordance with yet another embodiment of the invention; and

FIG. 8

is a sectional view showing a superconducting coil assembly in accordance the embodiment of FIG.

7

.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A magnetic resonance imaging (“MRI”) magnet for examining a patient in accordance with an embodiment of the invention is depicted in

FIGS. 1-5

. The MRI magnet includes a first ferromagnetic pole piece

11

and a second ferromagnetic pole piece

12

disposed facing each other and defining a patient-receiving gap

13

therebetween. The first pole piece

11

and second pole piece

12

have a common central axis

15

extending through the patient-receiving gap

13

. The MRI magnet also includes a first pole stem

19

connected to first pole piece

11

and a second pole stem

21

connected to second pole piece

12

. In preferred embodiments, the pole stems each comprise a plurality of smaller stem elements. The pole stem or pole stem elements are preferably cylindrical.

A ferromagnetic yoke

16

supports the first pole piece and second pole piece in their respective positions facing each other. As best seen in

FIGS. 1 and 2

, the ferromagnetic yoke

16

includes an upper support

18

to which pole stem

19

is connected and a lower support

20

to which pole stem

21

is connected. The upper support

18

and the lower support

20

comprise a cross-shaped member having radially extending extensions. A plurality of columns, preferably

4

columns, are equally spaced around the central axis

15

. The columns

22

,

23

,

24

and

25

are connected to extensions

29

,

14

,

17

and

28

, respectively, and radially spaced from the patient-receiving gap. The columns are connected to the lower support in a similar manner. The upper support

18

is connected to upper ends

27

of the columns

22

,

23

,

24

and

25

and the lower support

20

is connected to lower ends

26

of the columns

22

,

23

,

24

and

25

. Thus, the upper support

18

is connected to the pole stem

19

, which is connected to pole piece

11

, which is disposed above the patient-receiving gap. Lower support

20

is connected to pole stem

21

, which is connected to pole piece

12

, which is disposed below the patient-receiving gap.

A first coil assembly

40

is connected to the upper support

18

and a second coil assembly

42

is connected to the lower support

20

.

The ferromagnetic yoke is preferably constructed in accordance with U.S. Pat. No. 5,754,085, the disclosure of which is hereby incorporated by reference herein. The ferromagnetic yoke of U.S. Pat. No. 5,754,085 is constructed so as to control the magnetic flux within the supporting structure for the magnet to minimize leakage magnetic fields, and to create the strong magnetic field necessary for diagnostics imaging, and provide a comfortable space in which the patient can lie. Alternatively, the ferromagnetic frame of U.S. Pat. No. 4,766,378, the disclosure of which is hereby incorporated by reference herein, may also be used to support the first coil assembly

30

and the second coil assembly

37

.

As best seen in

FIG. 3

, first coil assembly

40

has a housing

32

having a toroidal shape surrounding pole stem

19

so that the housing

32

is concentrically arranged with pole stem

19

on the central axis

15

. The second coil assembly

42

has a similarly arranged housing adjacent to and concentrically arranged with pole stem

21

on central axis

15

.

As best seen in

FIGS. 4 and 5

, the superconducting coil assemblies are mounted adjacent the pole stems so that, when the coils are energized with an electric current, magnetic flux flows through the pole stems and into the pole pieces, creating a magnetic field between the pole pieces in the patient-receiving gap. The first superconducting coil assembly

40

includes a vacuum-tight vessel

41

, preferably formed from a non-magnetic metal, such as aluminum, or non-magnetic stainless steel. The vessel may be comprised of side plates

30

and

31

, top plate

33

and bottom plate

34

. The vessel

41

has a first end

43

including a flange for connecting to upper support

18

. A second superconducting coil assembly

42

is constructed in the same manner as first superconducting coil assembly

40

, except that second superconducting coil assembly

42

is connected to lower support

20

, as best seen in FIG.

2

.

The following description of the first superconducting coil assembly

40

applies equally to the second superconducting coil assembly

42

. The vacuum-vessel

41

of first superconducting coil assembly

40

contains, as best seen in

FIG. 5

, a layer of superinsulation and heat shields surrounding a cold mass

50

. A layer of superinsulation

45

, surrounds a heat shield

46

. The term “superinsulation”, commonly used in the field of cryogenics, comprises a multilayered insulative material having low emissivity, such as metallized, reflective layers of thin polymer film. Adjacent the heat shield

46

is a space

47

. The cold mass

50

is surrounded by another heat shield

48

, which is separated from the heat shield

46

by the space

47

. The heat shields must shield the cold mass

50

against heat up to an appropriate temperature. The heat shields will vary according to the particular application. In the particular embodiment of

FIGS. 1-5

, shield

46

is a 40° Kelvin shield and shield

48

is a 20° Kelvin heat shield. The spaces within the vacuum-vessel

41

are evacuated to a very low pressure to minimize any heat transfer due to convection. The cold mass

50

includes a “strongback”

51

, or a U-shaped housing, in which a number of tubes containing a cryogenic fluid are mounted, and a coil bobbin

52

, another U-shaped housing in which a superconducting coil

53

is wound and mounted. The strongback and coil bobbin may have a number of cross-sectional shapes. The strong back and coil bobbin are preferably comprised of stainless steel for strength. The coil bobbin

52

is adjacent the strongback

51

, with the coil bobbin

52

on the inside of the coil, closest to axis

15

. Thus, the coil encircles axis

15

. Also included, between the coil

53

and tubes, is a member or hoop strap

54

in contact with the coil

53

. The hoop strap

54

, restrains the coil in the radial direction toward central axis

15

and provides thermally conductive contact between the coil

53

and the cryogenic fluid tubes

55

. The coil bobbin

52

and strongback

51

may have a U-shaped cross section and may be arranged with their open sides facing each other, providing structural rigidity for the superconducting coil

53

. The superconducting coil

53

is comprised of a superconductor wound to form a solenoid having a toroidal shape corresponding to the shape of the coil assembly.

The superconducting coil is not immersed in the cryogenic fluid, as in prior art superconducting MRI magnets. The superconducting coil assembly in accordance with this embodiment is compact and requires a relatively smaller volume of cryogenic fluid within the coil assembly.

Strong magnetic fields are commonly used for MRI magnet diagnostics, typically at least 1 kilogauss, and in some cases up to about 10 Kilogauss (1 TESLA) or more. This large magnetic field acts on the other components in the MRI magnet. The magnetic field generated by the superconducting coils exert substantial magnetic forces on the coils and the structure of the magnet, tending to pull the coils towards each other and tending to unravel each coil radially outwardly. The strongback

51

, hoop strap

54

, and coil bobbin

52

restrain the coil and maintain the coil shape.

The vacuum vessel

41

is comprised of relatively strong, top and bottom plates which have a toroidal shape, in addition to curvilinear side plates. These plates form the boundary of the vacuum vessel and the superinsulation and heat shields separate the cold mass

50

from the vacuum vessel plates. Thus, the heat loss through the plates is greatly reduced.

The superconducting coil assemblies must be securely connected to their respective upper support

18

or lower support

20

and must be restrained against their tendency to move. As shown in

FIG. 5

, a series of suspension rods arranged in pairs, such as suspension rods

56

and

57

, are connected to the top and bottom plates of the vessel

41

. The suspension rods may be comprised of a number of materials, such as stainless steel, titanium, fiberglass, and others. The service port

49

is connected to the top or bottom plate of the vacuum vessel. At a second end

44

of the superconducting coil assembly, tensioning devices

58

are mounted in the vacuum-vessel

41

. The tensioning devices

58

, as best seen in

FIGS. 3

,

4

, and

5

, are accessible on the first end and second end of the vessel

41

for pre-tensioning and adjusting the tension in the tension rods. A high thermal resistant bumper assembly

59

in the vacuum vessel

41

allows the position of the coil

53

to be adjusted and centered.

The superconducting coil may be comprised of a fully stabilized niobium, titanium superconductor. The coil may be provided with a conventional start switch. In the embodiment shown in

FIG. 5

, three cryogenic tubes

55

, preferably carrying liquid helium, attached to the hoopstrap

54

of the cold mass

50

. The cryogenic tubes

55

may comprise three tubes of a thermally conductive material. A number of thermally conductive materials may be used, preferably a thin-walled pipe of a thermally conductive material, such as high purity aluminum. Aluminum having a purity of at least about 99% is preferred. A service port

49

is located adjacent first end

43

of the superconducting coil assembly. Superinsulation surrounds the service port

49

.

The MRI magnet also includes a cryocooler for re-condensing the cryogenic fluid to a liquid phase so that the coil may be cooled to a temperature at which the coil exhibits superconducting properties. As seen in

FIG. 1

, one type of cryocooler is included in the coil assembly and may include two cold heads

67

. If two cold heads are used, the cold heads

67

cool the helium in two stages. For example, when two heat shields are used, one cold head may cool the cryogenic fluid to 20° Kelvin and the other may cool the cryogenic fluid to 4° Kelvin. The 20° Kelvin cold head is in communication with space

47

and the 4° Kelvin cold head is in communication with the cryogenic fluid tubes

55

discussed above. In the alternative, a cryocooler according to

FIG. 6

may be used.

The superinsulation and heat shielding protect the cold mass from heat loads. Heat loads added to the cold mass

50

include those due to radiation and conduction between room temperature of the vacuum vessel plates and the cold mass

50

, and heat loads added to the system through the support system for the superconducting coil assembly, which must be minimized. The suspension rod system of

FIG. 5

is preferred to a compression mounting system such as that disclosed in U.S. Pat. No. 4,766,378, the disclosure of which is hereby incorporated by reference herein, because the suspension rod system utilizes a member passing into the cold space having a smaller cross-sectional area and longer in length than a comparable member in the compression mounting system. The suspension rods add less heat to the cold mass

50

.

In certain preferred embodiments, the MRI magnet includes the cryocooler depicted in FIG.

6

. The cryocooler

60

may comprise a Sumitomo RDK-415D cryocooler, which includes a closed cycle helium compressor, two high-pressure gas lines and a two-stage cold head.

The cryocooler

60

is included in the same housing as the coil assembly, but is separated from the superconducting coil assembly by an umbilical section and has a body

62

including a liquid cryogenic fluid vessel

63

mounted therein. A re-condensing cold head

64

is mounted at top end

65

of the body

62

and includes a vessel neck tube

68

for maintaining the cryogenic fluid. When the cryogenic fluid is re-condensed, the cryogenic fluid has a temperature approaching 4° Kelvin.

A bottom

69

of the vessel

63

is disposed adjacent a bottom end

66

of the body

62

. Bottom

69

includes an inlet

71

and an outlet

72

for the cryogenic fluid

70

. The outlet

72

is connected to a pipe

73

and inlet

71

is connected to return pipe

74

. Feed pipe

73

and return pipe

74

extend from vessel

63

to a thermally conductive member, such as the cryogenic fluid tubes

55

discussed above. The thermally conductive member is located in conductive contact with the superconducting coil

53

. The thermally conductive member cools the superconducting coil

53

to a temperature at which the superconducting coil exhibits superconducting properties.

The cryogenic fluid in the cryogenic fluid tubes is thereby heated and flows through return pipe

74

to vessel

63

to be cooled by the cold head

61

. Also illustrated in

FIG. 6

is a compression-type support

75

for the superconducting coil assembly

40

. Alternatively, the coil assembly may include the cryocooler discussed above. As seen in

FIG. 1

, one or more cold heads

67

may be provided.

A further embodiment of the invention is depicted in

FIG. 8

, showing the superconducting coil assemblies

140

and

142

, which are substantially the same as the superconducting coil assemblies

40

and

42

discussed above in connection with

FIG. 6

, except as follows. The strongback

151

houses a thermally conductive member comprising a strap

160

comprised of a material having a high conductivity. High purity aluminum is preferred, although a number of thermally conductive materials may be utilized. The strap

160

is connected to a cryocooler. The cryocooler

60

sustains the strap

160

at a temperature sufficient to maintain the superconducting coil

153

at a temperature at which the superconducting coil exhibits superconducting properties. The strap

160

may extend to the cryocooler

60

shown in FIG.

6

. The cryogenic fluid need not enter the superconducting coil assemblies

140

and

142

in this embodiment. The strap

160

may be immersed in cryogenic fluid in the cryocooler, as shown in FIG.

7

. The strap

160

is immersed in liquid helium at helium space

76

, at one point along the toroidal strap. The liquid helium space

76

is connected to cold head

61

, shown schematically in FIG.

7

. In the alternative, may be connected to a surface cooled by the cryogenic fluid.

Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other versions are possible. For example, the superconducting coil assemblies of

FIGS. 5 and 8

may include the compression-type mounting discussed above. In addition, other cryocoolers may be utilized to cool the thermally conductive members discussed above. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments discussed above.