Open MRI Magnet With Uniform Imaging Volume

The present invention relates generally to a superconductive magnet (such as, but not limited to, a helium-cooled and/or cryocooler-cooled superconductive magnet) used to generate a high magnetic field as part of a magnetic resonance imaging (MRI) system, and more particularly to such a magnet having an open design and having a uniform (i.e., homogeneous) magnetic field within its imaging volume.

MRI systems employing superconductive or other type magnets are used in various fields such as medical diagnostics. Known superconductive magnets include liquid-helium cooled and cryocooler-cooled superconductive magnets. Typically, for a helium-cooled magnet, the superconductive coil assembly includes a superconductive main coil which is at least partially immersed in liquid helium contained in a helium dewar which is surrounded by a dual thermal shield which is surrounded by a torroidal-shaped vacuum enclosure having a bore and a longitudinal axis. In a conventional cryocooler-cooled magnet, the superconductive main coil is surrounded by a thermal shield which is surrounded by a torroidal-shaped vacuum enclosure having a bore and a longitudinal axis, and the cryocooler coldhead is externally mounted to the vacuum enclosure with the coldhead's first stage in thermal contact with the thermal shield and with the coldhead's second stage in thermal contact with the superconductive main coil. Nb-Ti superconductive coils typically operate at a temperature of generally 4 Kelvin, and Nb-Sn superconductive coils typically operate at a temperature of generally 10 Kelvin.

A superconductive coil assembly of a conventional MRI system includes three pulsed (i.e., not time-constant) resistive gradient coils including a Z-axis coil which is coaxially aligned with the longitudinal axis and which carries a pulsed electric current which may be in a direction opposite to the current direction of the superconductive main coils. Such gradient coils are all located outside the vacuum enclosure (i.e., coil housing) in the bore. The superconductive coil assembly also includes several resistive radio-frequency coils all located outside the vacuum enclosure (i.e., coil housing) in the bore.

Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore and a longitudinal axis. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are thus designed to create a magnetic field of high uniformity within a spherical imaging volume centered within the magnet's bore where the object to be imaged is placed. Although the magnet is so designed to have a highly uniform magnetic field within the imaging volume, manufacturing tolerances in the magnet and magnetic field disturbances caused by the environment at the field site of the magnet usually require that the magnet be corrected at the field site for such minor irregularities in the magnetic field. Typically, the magnet is shimmed at the field site by using pieces of iron, or, for Nb-Ti superconductive magnets cooled by liquid helium, by using numerous Nb-Ti superconductive correction coils. The correction coils are placed within the superconductive coil assembly typically radially near and radially inward of the main coils. Each correction coil carries a different, but low, electric current in any required direction including a direction opposite to the direction of the electric current carried in the main coils. It is also known to shim a closed magnet by using numerous resistive DC shim coils all located outside the vacuum enclosure (i.e., coil housing) in the bore. The resistive DC shim coils each produce time-constant magnetic fields and may include a shim coil coaxially aligned with the longitudinal axis and carrying an electric current in a direction opposite to the current direction of the superconductive main coils to correct a harmonic of symmetrical inhomogeneity in the magnetic field within the imaging volume caused by manufacturing tolerances and/or site disturbances.

Open magnets typically employ two spaced-apart superconductive coil assemblies with the space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI 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. The literature is largely silent on how superconductive open magnets can be made to have a magnetic field of high uniformity within the imaging volume when the creation of the open space between the superconductive coil assemblies grossly distorts the magnetic field creating a magnetic field of low uniformity within the imaging volume.

What is needed is an open MRI magnet designed to have a highly uniform magnetic field within its imaging volume despite the gross magnetic field distortions created by the open space between the magnet's superconductive coil assemblies.

It is an object of the invention to provide an open superconductive MRI magnet designed to have a uniform magnetic field within its imaging volume.

The open MRI magnet of the invention includes a first superconductive coil assembly including a generally toroidal-shaped first coil housing and a generally annular-shaped first superconductive main coil. The first coil housing surrounds a first bore and has a generally longitudinal first axis. The first main coil is generally coaxially aligned with the first axis, disposed within the first coil housing, and carries a first main electric current in a first direction. The open MRI magnet also includes a generally annular-shaped first resistive coil generally coaxially aligned with the first axis, spaced radially inward and radially apart from the first superconductive main coil, and carrying an electric current in a direction opposite to the first direction. The open MRI magnet additionally includes a second superconductive coil assembly including a generally toroidal-shaped second coil housing and a generally annular-shaped second superconductive main coil. The second coil housing is longitudinally spaced apart from the first coil housing to create a cylindrical-shaped open space therebetween, surrounds a second bore, and has a generally longitudinal second axis which is generally coaxially aligned with the first axis. The second main coil is generally coaxially aligned with the second axis, disposed within the second coil housing, and carries a second main electric current in the first direction. The open MRI magnet further includes a generally annular-shaped second resistive coil generally coaxially aligned with the second axis, spaced radially inward and radially apart from the second superconductive main coil, and carrying an electric current in the opposite direction, wherein the first and second resistive coils together correct within an imaging volume for magnetic field inhomogeneities caused by the presence of the open space, the imaging volume being located in the open space near the first and second axes. In a preferred embodiment, the second superconductive coil assembly is a generally mirror image of the first superconductive coil assembly, and the second resistive coil is a generally mirror image of the first resistive coil.

Several benefits and advantages are derived from the invention. With Applicant's open MRI magnet design, the resistive coils are chosen by magnetic field analysis to overcome the gross magnetic field distortions within the imaging volume of the main coils (created by the open space between the magnet's superconductive coil assemblies) to produce a magnetic field of high uniformity within the imaging volume. Applicant's highly uniform magnetic field permits high quality MRI imaging. Applicant's open magnet design overcomes any claustrophobia feelings of patients. Applicant's design of an open magnet with a highly uniform magnetic field gives access to the patient by medical personnel for surgery or other medical procedures during high-quality MRI imaging.

The accompanying drawings illustrate two preferred embodiments of the present invention wherein:

FIG. 1 is a perspective view of a first preferred embodiment of the open MRI magnet of the invention having internal resistive coils;

FIG. 2 is a schematic cross-sectional side-elevational view of the MRI magnet of FIG. 1 with a magnet floor mount added;

FIG. 3 is a view, as in FIG. 2, but of a second preferred embodiment of the open MRI magnet of the invention having external resistive coils; and

FIG. 4 is a schematic cross-sectional view of the MRI magnet of FIG. 3 taken along the lines 4--4 of FIG. 3.

Referring now to the drawings, wherein like numerals represent like elements throughout, FIGS. 1-2 show a first preferred embodiment of the open magnetic resonance imaging (MRI) magnet 110 of the present invention. The magnet 110 includes a first superconductive coil assembly 112 with a generally toroidal-shaped first coil housing 114 which surrounds a first bore 116 and which has a generally longitudinal first axis 118. The magnet 110 also includes a second superconductive coil assembly 120 with a generally toroidal-shaped second coil housing 122 which surrounds a second bore 124 and which has a generally longitudinal second axis 126. The second coil housing 122 is longitudinally spaced apart from the first coil housing 114 by structural posts 128 to create a cylindrical-shaped open space 129 therebetween, and the second axis 126 is generally coaxially aligned with the first axis 118. It is preferred that the coil housings 114 and 122 alone or together (through a hollow structural post 128) define a vacuum enclosure. Preferably, the second superconductive coil assembly 120 is a generally mirror image of the first superconductive coil assembly 112 about a plane 130 (seen on edge as a dashed line in FIG. 2) oriented perpendicular to the first axis 118 and disposed longitudinally midway between the first and second coil housings 114 and 122.

The first coil housing 114 includes a first generally-circumferential outside surface 132 facing generally towards the first axis 118 and a second generally-circumferential outside surface 134 radially spaced apart from the first circumferential outside surface 132 and facing generally away from the first axis 118. The first coil housing 114 also includes a first generally-annular outside surface 136 facing generally towards the plane 130 and a second generally-annular outside surface 138 longitudinally spaced apart from the first annular outside surface 136 and facing generally away from the plane 130.

The first superconductive coil assembly 112 also includes a generally annular-shaped first superconductive main coil 140 and preferably includes generally annular-shaped additional superconductive main coils (not shown in the figures). The additional superconductive main coils may be needed to achieve a high magnetic field strength, within the magnet's imaging volume, without exceeding the critical current density of the superconductor being used in the coils, as is known to those skilled in the art. The first superconductive main coil 140 is conventionally supported on a coil form (not shown in the figures). The first superconductive main coil 140 is a DC (direct current) coil having a generally time-constant operating voltage. The first superconductive main coil 140 is generally coaxially aligned with the first axis 118, is disposed within the first coil housing 114, 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 118 with any slight longitudinal component of current direction being ignored. Hence, the first superconductive main coil 140 has a first magnetic field direction within the first bore 116 which is generally parallel to the first axis 118. The first superconductive main coil 140 typically would be a superconductive wire or superconductive tape wound such that the first superconductive main coil 140 has a longitudinal extension and a radial extension (i.e., radial thickness) far greater than the corresponding dimensions of the superconductive wire or superconductive tape.

The magnet 110 additionally includes a generally annular-shaped first resistive coil 144 which is a DC (direct current) coil having a generally time-constant operating voltage. The first resistive coil 144 is generally coaxially aligned with the first axis 118, is spaced radially inward and radially apart from the first superconductive main coil 140, and carries an electric current in a direction opposite to the first direction. Hence, the first resistive coil 144 produces a generally time-constant magnetic field and has a magnetic field direction within the first bore 116 which is generally opposite to the first magnetic field direction. The ampere-turns of the first resistive coil 144 is less than the ampere-turns of the first superconductive main coil 140. In the first preferred embodiment, as shown in FIG. 2, the first resistive coil 144 is disposed within the first coil housing 114 proximate the first circumferential outside surface 132 and proximate the first annular outside surface 136. In an exemplary construction, the first resistive coil 144 is a wound copper coil having insulated turns. Preferably, the ampere-turns of the first resistive coil 144 is at least equal to generally 5% of the ampere-turns of all of the superconductive main coils in the first coil housing 114.

As previously mentioned and as shown in FIGS. 1 and 2, the second superconductive coil assembly 120 is a generally mirror image of the first superconductive coil assembly 112 about the plane 130. Therefore, in addition to the second coil housing 122, the second superconductive coil assembly 120 also includes a generally annular-shaped second superconductive main coil 146 and preferably includes generally annular-shaped additional superconductive main coils (not shown in the figures). It is noted that the additional superconductive main coils would be needed by the second superconductive coil assembly 120 to balance any extra additional superconductive main coils of the first superconductive coil assembly 112, as can be appreciated by those skilled in the art. The second superconductive main coil 146 is conventionally supported on a coil form (not shown in the figures).

The second superconductive main coil 146 is a DC (direct current) coil having a generally time-constant operating voltage and is generally identical to the first superconductive main coil 140. The second superconductive main coil 146 is generally coaxially aligned with the second axis 126, is disposed within the second coil housing 122, and carries a second main electric current in the first direction (i.e., in the same direction as the electric current in the first superconductive main coil 140). Hence, the second superconductive main coil 146 has a second magnetic field direction within the second bore 124 which is generally identical to the first magnetic field direction.

The magnet 110 further includes a generally annular-shaped second resistive coil 158 which is a DC (direct current) coil having a generally time-constant operating voltage. The second resistive coil 158 is generally coaxially aligned with the second axis 126, is spaced radially inward and radially apart from the second superconductive main coil 146, and carries an electric current in a direction opposite to the first direction, wherein the first and second resistive coils 144 and 158 together correct within an imaging volume 160 for magnetic field inhomogeneities caused by (and preferably caused solely by) the presence of the open space 129, the imaging volume 160 being located in the open space 129 proximate the first and second axes 118 and 126. Hence, the second resistive coil 158 produces a generally time-constant magnetic field and has a magnetic field direction within the second bore 124 which is generally opposite to the second magnetic field direction. The ampere-turns of the second resistive coil 158 is less than the ampere-turns of the second superconductive main coil 146. In the first preferred embodiment, as shown in FIG. 2, the second resistive coil 158 is disposed within the second coil housing 122 proximate the circumferential outside surface which faces radially inward toward the second axis 126 and proximate the annular outside surface which faces longitudinally inward toward the plane 130. In an exemplary construction, the second resistive coil 158 is a wound copper coil having insulated turns. Preferably, the second resistive coil 158 is a generally mirror image of the first resistive coil 144 about the plane 130. In an exemplary embodiment, the second resistive coil 158 is generally identical to the first resistive coil 144. Hence, the electrical parameters of the first and second resistive coils 144 and 158, such as voltage, current, ampere-turns, etc. are generally identical, and the first and second resistive coils 144 and 158 produce generally identical magnetic fields.

It is noted that the magnet 110 of the invention may be described in other terms as including the previously-described first and second superconductive coil assemblies 112 and 120, the previously-described imaging volume 160, and the previously-described magnetic field inhomogeneities together with presently-described means for correcting the magnetic field inhomogeneities, such means including the previously-described first and second resistive coils 144 and 158. In an exemplary embodiment, such means consists essentially of (and preferably consists of) the previously-described first and second resistive coils 144 and 158.

The superconductive main coils 140 and 146 of the magnet 110 typically produce a generally spherical imaging volume 160 (shown as a dotted circle in FIG. 2) centered generally at the intersection of the plane 130 and the first axis 118. The effect of the open space 129 between the two superconductive coil assemblies 112 and 120 is to distort the uniformity of the magnetic field of the spherical imaging volume 160. As one moves longitudinally through the spherical imaging volume 160, the magnitude of the magnetic field decreases with decreasing distance from the center of the spherical imaging volume 160 because of the missing superconductive main coils which were removed to create the open space 129. The effect of the resistive coils 144 and 158 is to lower the magnitude of the magnetic field toward the longitudinal edges of the spherical imaging volume 160 in line with the lower magnitude at the center. The resistive coils 144 and 158 are designed, using the principles of the present invention, previously disclosed herein, together with conventional magnetic field analysis, as is within the skill of the artisan, to produce a highly homogeneous magnetic field within the spherical imaging volume 160 for improved MRI imaging. It is noted that the magnet 110 is supported on a conventional magnet floor mount 164.

Referring again to the drawings, FIGS. 3-4 show a second preferred embodiment of the open magnetic resonance imaging (MRI) magnet 210 of the present invention. Magnet 210 is identical to magnet 110 of the first preferred embodiment of the invention, with differences as hereinafter noted. The first resistive coil 244 is disposed outside (and not within) the first coil housing 214 in the first bore 216, and the second resistive coil 258 is disposed outside (and not within) the second coil housing 222 in the second bore 224. In a preferred construction, the first resistive coil 244 is the only resistive coil in the first bore 216 which is a DC (direct current) coil producing a generally time-constant magnetic field, and the second resistive coil 258 is the only resistive coil in the second bore 224 which is a DC (direct current) coil producing a generally time-constant magnetic field. Preferably, as shown in FIG. 3, the first resistive coil 244 is attached to the first coil housing 214, and the second resistive coil 258 is attached to the second coil housing 222.

It is noted that magnet cooling mechanisms do not form a part of the present invention and have been omitted from the figures. Any cryogenic cooling mechanism, such as, but not limited to, liquid helium (or other cryogenic fluid) cooling and/or cryocooler cooling may be employed in combination with the present invention.

The foregoing description of several preferred embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.