Open magnet with recessed field shaping coils

This application is a continuation in part of application Ser. No. 09/681,371, filed Mar. 26, 2001.

BACKGROUND OF THE INVENTION

The present invention generally relates to open magnets used in magnetic resonance imaging (MRI) systems and, more particularly, to such an open magnet with recessed field shaping coils.

As described in U.S. Pat. No. 5,999,075 to Laskaris et al. assigned to the assignee of the present invention, an open magnet of a MRI system typically includes two spaced-apart coil assemblies which are substantially mirror images of one another. The space between the coil assemblies contains an imaging volume and allows for positioning of a patient in that volume and access by medical personnel for surgery or other medical procedures during magnetic resonance imaging. Each coil assembly typically has an annular-shaped main coil with a longitudinal axis, an annular-shaped shielding coil coaxially aligned along the longitudinal axis and spaced longitudinally outward and apart from the main coil, and a cylindrical-shaped magnetizable pole piece disposed about the longitudinal axis between the main and shielding coils.

The magnetizable pole pieces of the coil assemblies enhance the strength of the magnetic field produced by the main coils. Further, by selectively configuring the inner surfaces of the pole pieces, the open magnet is magnetically shimmed so as to improve homogeneity of the magnetic field. The current practice is to form several annular steps on the inner surfaces of the pole pieces which protrude to different heights or distances into the space between the coil assemblies in order to control the magnetization distributions of the pole pieces and thus shape the magnetic field to create the homogeneous field volume in the space between the coil assemblies for MRI imaging. However, such magnetic field shaping annular steps occupy some of the space between the coil assemblies thus reducing the imaging volume of the MRI system and complicating the magnet cryogenic structure design. Moreover, the field shaping capacity of the step technique is limited and reduced as the main field increases since more load areas of the stepped pole pieces become magnetically saturated.

Accordingly, it is desirable to provide an open magnet which provides a highly uniform, high field, open field of view for MRI with a well-contained stray field.

BRIEF SUMMARY OF THE INVENTION

An actively shielded open magnet useful for MRI applications comprises magnetized pole pieces, superconducting main and bucking coils, and recessed field shaping coils to provide a highly homogeneous, high field, open field of view for MR imaging with a wellcontained stray field.

The magnetized pole pieces comprise a ferromagnetic material and have superconducting field-shaping coils situated in annular grooves formed in the pole face. The magnetized pole pieces with annular recessed field-shaping coils are placed inside a cryogenic helium vessel of the magnet. The uniform cryogenic temperature inside the cryogenic vessel avoids the field fluctuation that would otherwise result from temperature changes of the pole pieces. A pair of superconducting main coils generates the high magnet field in the imaging volume. Unlike many of the open MRI magnets with warm iron poles, the cold magnetized pole pieces do not form a return path for the magnetic flux. Instead, the stray field is contained by the superconducting bucking coils.

The recessed field shaping coils advantageously result in more usable space between the two coil assemblies of the open magnet, enable higher field and simplify the manufacturing of the pole pieces and the magnet cryostat structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1

is a schematic view of an open MRI magnet showing an imaging volume of the magnet.

FIG. 2

is a schematic cross-sectional view of a prior art open magnet having field shaping annular steps formed on the inner surfaces of the pole pieces thereof.

FIG. 3

is a plan view of one of the prior art pole pieces as seen along line

3

—

3

of FIG.

2

.

FIG. 4

is a schematic cross-sectional view of pole pieces in accordance with a preferred embodiment of the present invention for use in the open magnet of

FIG. 2

with the pole pieces having field shaping coils inserted in annular grooves recessed in the inner surfaces of the pole pieces.

FIG. 5

is a plan view of one of the pole pieces as seen along line

6

—

6

of FIG.

4

.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and particularly to

FIGS. 1 and 2

, there is illustrated a prior art open magnet, generally designated

10

. The prior art open magnet

10

includes a pair of spaced-apart coil assemblies

12

which are substantially mirror images of one another and form a space

14

therebetween that contains an imaging volume

16

. Each coil assembly

12

typically has an annular-shaped main coil

18

with a longitudinal axis

20

, an annular-shaped shielding, or bucking, coil

22

coaxially aligned along the longitudinal axis

20

and spaced longitudinally outward and apart from the main coil

18

, and a cylindrical-shaped magnetizable pole piece

24

disposed about the longitudinal axis

20

between the main and shielding coils

18

,

22

. A support member

26

interconnects the two coil assemblies

12

and is made of a nonmagnetizable material, such as stainless steel. The main and shielding coils

18

,

22

are typically superconductive and thus cooled to a temperature below their critical temperature to achieve and sustain superconductivity by cryogenic cooling thereof. For achieving the cooling, the open magnet

10

also includes cryogenic vessels

28

containing a liquid cryogen, such as liquid helium, and surrounding the main and shielding coils

18

,

22

. The pole pieces

24

are disposed outside of and spaced apart from the cryogenic vessels

28

and are made of a ferromagnetic material, such as iron.

The open magnet

10

is not limited to a superconductive magnet but rather can be a resistive magnet or a combination resistive and superconductive magnet. Likewise, the main and shielding coils

18

,

22

are not limited to superconductive coils but rather can be resistive or a combination of resistive and superconductive coils.

Referring to

FIGS. 2 and 3

, the magnetizable pole pieces

24

of the spaced coil assemblies

12

enhance the strength of the magnetic field produced by the main coils

18

. The pole pieces

24

comprise magnetizable bodies

30

made of ferromagnetic material and of desired configurations, such as generally cylindrical configurations, with inner surfaces

32

facing toward one another. The inner surfaces

32

of the pole pieces

24

have several annular steps

34

,

36

,

38

formed thereon which shape the magnetic field of the open magnet

10

. As mentioned above, a problem with pole pieces

24

is that they protrude into the space

14

between the coil assemblies

12

and thus reduce the usable volume of the space

14

. In addition, the field shaping capacity of the steps technique decreases as the main magnet fields increase, thus tending to produce an unsatisfactory (i.e., non-homogeneous) imaging field for high field open MRI magnets. Another problem is the field fluctuation in the imaging volume due to temperature changes. As room temperature changes, the magnetization of the pole pieces changes, resulting in field fluctuation in the imaging volume and image quality issues.

Referring to

FIGS. 4 and 5

, there is illustrated the pole pieces

40

according to a preferred embodiment of the present invention for use in open magnet

10

. Like the prior art pole pieces

24

, the pole pieces

40

of

FIGS. 4 and 5

have generally cylindrical-shaped magnetizable bodies

42

with respective outer surfaces

44

and inner surfaces

46

facing in a direction generally opposite that of the outer surfaces

44

such that the inner surfaces

46

face toward one another. However, unlike the pole pieces

24

, pole pieces

40

of

FIGS. 4 and 5

have at least one annular-shaped groove

48

recessed in inner surfaces

46

, and an annular-shaped coil

50

, either superconductive or resistive, inserted in groove

48

. Alternatively, a plurality of annular-shaped, concentrically-arranged, radially-spaced grooves

48

are recessed in inner surfaces

46

, and a plurality of annular-shaped coils

50

, either superconductive or resistive, are inserted into the respective grooves

48

, so as to shape the magnetic field of the open magnet

10

. Coils

50

can be pre-wound prior to their insertion into the grooves

48

and then retained in the grooves

48

by means generally indicated at

52

, such as by mechanical fastening elements, an interference fit, or an epoxy bond with the magnetizable bodies

42

. Current passed through coils

50

redistribute the magnetization of the pole pieces

40

so as to create a homogeneous field in the image volume

16

of the space

14

. Pole pieces

24

are placed inside the cryogenic helium vessels

28

, thus forming an integrated magnet assembly with the superconducting main coils

18

and bucking coils

22

. Such assembly provides a complete electro-magnetic design to satisfy all MRI requirements, including a highly uniform high-field volume for imaging and the containment of the stray field for siting. The advantages of the present invention include a larger unoccupied volume in the space

14

between the coil assemblies

12

and simpler manufacturing of the pole pieces

40

and the magnet cryostat structure. Moreover, temperature induced field fluctuation is avoided by placing pole pieces

40

inside helium vessel

28

in order to operate at a constant liquid helium temperature. Since the pole pieces

40

, and the main and bucking coils

18

,

22

are structurally connected to form an integrated cold magnet assembly, it is easier to position precisely all the electro-magnetic components, advantageously resulting in a highly uniform field in the image volume

16

. Furthermore, axial corrections coils (not shown) can be easily co-wound with the field shaping coils to shim inhomogeneity due to manufacturing tolerances.

While only certain preferred features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.