Patent Description:
Magnetic resonance tomography represents a prominent imaging method for acquiring images of an interior of the examination object. For carrying out a magnetic resonance measurement, the examination object is usually positioned in a strong and homogeneous static magnetic field (B0 field) of a magnetic resonance imaging device. The static magnetic field may comprise magnetic field strengths of <NUM> Tesla to <NUM> Tesla, thus aligning nuclear spins inside the examination object along the static magnetic field. For triggering so-called nuclear spin resonances, radiofrequency excitation pulses are emitted into the examination subject. Each radiofrequency excitation pulse causes a magnetization of nuclear spins within the examination object to deviate from the static magnetic field by an amount which is known as the flip angle. A radiofrequency excitation pulse may comprise an alternating (electro-)magnetic field with a frequency which corresponds to the Larmor frequency at the respective static magnetic field strength. Excited nuclear spins may exhibit a rotating and decaying magnetization (nuclear magnetic resonance), which can be detected using dedicated radiofrequency antennas. For spatial encoding of measured data, rapidly switched magnetic gradient fields are superimposed on the static magnetic field.

The received nuclear magnetic resonances are typically digitized and stored as complex values in a k-space matrix. This k-space matrix can be used as a basis for a reconstruction of magnetic resonance images and for determining spectroscopic data. A magnetic resonance image is typically reconstructed by means of a multi-dimensional Fourier transformation of the k-space matrix.

During an imaging examination, the patient is typically enclosed in a bore (e. in solenoid scanners) or between a pair of magnets (e. in C-type scanners) of the magnetic resonance imaging device. Due to spatial restrictions within the bore or a space between the pair of magnets, magnetic resonance imaging provides limited use for surgical procedures or biopsies. As the patient typically needs to be positioned inside the magnetic resonance imaging device, a size of the bore as well as other dimensions of the magnetic resonance imaging device are dictated by the size of the human body. From a cost and/or space utilization perspective, this may be unsatisfactory, especially if the examination is restricted to a body region of the patient, which is significantly smaller than an imaging volume provided by the magnetic resonance imaging device. Furthermore, patients with a claustrophobic condition or children may not tolerate elongated examination times usually associated with a magnetic resonance measurement.

In addition, a field generation of the magnetic resonance imaging device must be built to ensure a fixed relative position of included magnets, e. to counteract occuring magnetic forces. As differently sized patients have to fit inside the magnetic resonance imaging device without physically interfering with any hardware components, the magnets are usually positioned in quite a distance to an imaging volume of the magnetic resonance imaging device. This may decrease an efficiency of the field generation unit - e. characterized by a ratio of a volume of required magnetic material and a usable magnet field strength in the imaging volume - as well as an efficiency of electric and/or cooling infrastructure of the magnetic resonance imaging device. Even dedicated scanners adapted to specific body regions of a patient may encounter this problem, when conventional solenoidal or C-type scanners are used.

<CIT> discloses a magnetic field generating device configured to reduce magnetic flux leaking from pole pieces fixed to permanent magnets of the magnetic field generating device.

In <CIT>, an open magnet having a frustoconical shaped inner bore for magnetic resonance imaging is described.

<CIT> discloses a remotely positionable device including a magnet which is juxtaposable against an object to be imaged.

In <CIT>, a nuclear magnetic resonance imaging system comprising a semi-ring permanent magnet arranged on an iron yoke fixing seat is disclosed.

The application <CIT> discloses an apparatus for magnetic resonance imaging having a casing with an inner bore for receiving a subject and employing dual access.

<CIT> pertains to a magnetic resonance device comprising a main field magnet at a gradient coil configured to provide more free space for patients and treatment providers.

It is therefore an object of the invention to increase an efficiency of a field generation unit of a magnetic resonance imaging device.

This object is achieved by a magnetic resonance imaging device according to independent claims <NUM> and <NUM>. Further advantageous embodiments are specified in the dependent claims.

The inventive magnetic resonance imaging device comprises a field generation unit configured to provide a magnetic field in an imaging volume of the magnetic resonance imaging device, wherein the field generation unit comprises at least one magnet, wherein the at least one magnet confines the imaging volume in at least one spatial direction.

A field generation unit may comprise a plurality of magnets required for performing a magnetic resonance measurement. For example, the field generation unit may comprise at least one magnet configured to generate a static magnetic field inside an imaging volume of the magnetic resonance imaging device. A static magnetic field may be a B0-field of the magnetic resonance imaging device comprising either an essentially homogeneous magnet field or a predetermined magnetic gradient field. In a further example, the field generation unit comprises at least one gradient coil for generating a temporary magnetic gradient field inside the imaging volume. It is also conceivable, that the field generation unit includes at least one radiofrequency antenna for emitting radiofrequency excitation pulses in a power and frequency range of a magnetic resonance measurement into an image acquisition region of the magnetic resonance imaging device. The at least one radiofrequency antenna may also be configured to receive magnetic resonance signals (nuclear magnetic resonance) from the image acquisition region.

An imaging volume may be characterized by a predefined magnetic field direction and/or magnetic field strength. For example, the imaging volume may comprise a volume with a substantially uniform magnetic field direction and/or a uniform magnetic field strength. Such a volume may be an isocenter of the magnetic resonance imaging device. It is also conceivable, that the imaging volume comprises a predefined magnetic gradient field. Such a magnetic gradient field may be used for spatial encoding of magnetic resonance signals acquired from an examination object positioned within the imaging volume.

The at least one magnet confines the imaging volume in at least one spatial direction in such a way, that accessing or entering the imaging volume from a direction substantially corresponding to the at least one spatial direction is impractical or impossible. For example, the at least one magnet may bar or block an access to the imaging volume in any direction essentially coinciding with the at least one direction.

The at least one magnet is curved in such a way, that a perpendicular distance between a line oriented along a direction of access to the imaging volume and a surface directed towards the imaging volume of the at least one magnet varies in the direction of access to the imaging volume.

The at least one magnet may be configured as a main magnet for generating a static magnetic field inside the imaging volume. However, the at least one magnet may also correspond to a gradient coil, a set of gradient coils and/or a radiofrequency antenna of the magnetic resonance imaging device. In one example, the at least one magnet is curved in such a way, that the surface directed towards the imaging volume of the at least one magnet comprises a non-planar shape. In a preferred embodiment, the surface directed towards the imaging volume of the at least one magnet comprises the shape of a cone or a frustum. However, other shapes, such as a hemisphere, or any complex three-dimensional shape tapering in a direction towards the imaging volume is also conceivable. In providing at least one magnet with a curved surface directed towards the imaging volume or tapering in a direction towards the imaging volume, a clearance may favorably be provided for accommodating body region of a patient. For example, a conically shaped surface directed towards the imaging volume of the at least one magnet may favorably accommodate for a shoulder or an abdominal area of a patient when positioning an eye region, a jaw region or a heart of the patient inside the imaging volume.

The direction of access to the imaging volume may be characterized by a substantially straight line leading from an environment or an arbitrary point within an examination room to the imaging volume. The line is oriented along the direction of access to the imaging volume. Preferably, a trajectory defined by the line is unobstructed by components of the magnetic resonance imaging device. The direction of access relates to a trajectory along which a patient can enter the imaging volume.

A perpendicular distance between the line and the surface directed towards the imaging volume of the at least one magnet may vary along the direction of access to the imaging volume in such a way, that the perpendicular distance decreases in a direction towards the imaging volume. For example, the perpendicular distance between the line and the surface directed towards the imaging volume of the at least one magnet may decrease in a linear, a non-linear, a hyperbolical, a parabolical, an exponential or a similar fashion in the direction of the imaging volume. A perpendicular distance may be interpreted as a distance between an arbitrarily chosen point on the line and a nearest point on the surface directed towards the imaging volume of the at least one magnet. Particularly, the perpendicular distance may be characterized by a length of a normal vector of the line connecting the arbitrarily chosen point on the line and the nearest point on the surface directed towards the imaging volume of the at least one magnet.

In providing at least one magnet with a curved surface directed towards the imaging volume, a mean distance between a center of the imaging volume and the at least one magnet may be reduced while still providing sufficient clearance for accommodating a body and/or a body region of a patient inside the image acquisition region of the magnetic resonance imaging device. Thus, an efficiency of the field generation unit may favorably be increased in comparison to conventional solenoid and/or C-type magnetic resonance imaging devices with a comparable magnetic field strength.

According to an embodiment of the inventive magnetic resonance imaging device according to claim <NUM> the field generation unit comprises a second magnet, wherein the at least one magnet and the second magnet are arranged in such a way to provide an access to the imaging volume along at least two perpendicular spatial directions.

For example, the at least one magnet may confine the imaging volume in a first spatial direction and the second magnet may confine the imaging volume in a second spatial direction which is different from the first spatial direction. The first spatial direction and the second spatial direction may be oriented in parallel. Particularly, the first spatial direction and the second spatial direction may be oriented in opposite directions at an angle of <NUM>°. However, the two spatial directions may also be oriented at an angle below <NUM>° or above <NUM>°. The at least one magnet and the second magnet may be carried and/or kept apart by a supporting structure of the magnetic resonance imaging device and the imaging volume may be positioned in a free volume or gap between the at least one magnet and the second magnet. It is conceivable, that an access to the imaging volume is provided along any unobstructed line connecting an arbitrarily chosen position in the examination room with the center of the imaging volume. The examination room may be a room wherein the magnet resonance imaging device is positioned and/or wherein a magnetic resonance measurement may be performed.

The at least one magnet and/or the second magnet may comprise an axis of rotational symmetry. In one embodiment, a first direction of access to the imaging volume may be oriented essentially perpendicular to an axis of rotational symmetry of the at least one magnet and/or the second magnet. A second direction of access may be oriented essentially perpendicular to the first direction of access and the axis of rotational symmetry of the at least one magnet and/or the second magnet. However, the first direction of access and/or the second direction of access may also comprise an angle with respect to the axis of rotational symmetry of the at least one magnet and/or the second magnet. An axis of rotational symmetry may be characterized in that the at least one magnet and/or the second magnet look the same after turning the at least one magnet and/or the second magnet around the axis of rotational symmetry at least a part of a turn. It is also conceivable, that the at least one magnet and/or the second magnet comprise a discrete rotational symmetry of a higher order of n > <NUM>. This may signify, that a rotation of the at least one magnet and/or the second magnet about the axis of rotational symmetry by an angle of <NUM>°/n does not change the at least one magnet and/or the second magnet.

Preferably, the imaging volume may be accessed via a plurality of directions of access. In one embodiment, the imaging volume is accessible via at least two essentially perpendicular spatial directions. However, the imaging volume may also be accessible via three essentially perpendicular spatial directions. The three perpendicular spatial directions may correspond to the axes of a cartesian coordinate system, an origin of the cartesian coordinate system being positioned in the center of the imaging volume.

The direction of access to the imaging volume along each of the at least two perpendicular spatial directions is angled with respect to a main direction of magnetic field lines within the imaging volume.

Preferably, an angle between each of the at least two perpendicular spatial directions and the main direction of magnetic field lines within the imaging volume is a non-zero angle. For example, the angle between each of the at least two perpendicular spatial directions and the main direction of magnetic field lines within the imaging volume may comprise a value in the range of <NUM>° to <NUM>°, <NUM>° to <NUM>° or <NUM>° to <NUM>°.

However, any angle differing from a substantially parallel orientation with respect to the main direction of magnetic field lines within the imaging volume is conceivable.

In one embodiment, the at least one magnet and the second magnet may be separated by a shield coil. The shield coil may be configured to shield the at least one magnet and/or the second magnet from electromagnetic interference and/or magnetic fields provided by other magnets (e. the at least one magnet may be shielded from the second magnet and/or electromagnetic interference in an environment). For example, the shield coil may be positioned adjacent to the surface directed away from the imaging volume of the at least one magnet. However, the shield coil may also be positioned adjacent to a surface directed towards the imaging volume of the at least one magnet. In a preferred embodiment, the at least one magnet comprises a first shield coil positioned adjacent to the at least one magnet. Likewise, the second magnet may comprise a second shield coil positioned adjacent to the second magnet.

In arranging the at least one magnet and the second magnet in such a way to provide an access to the imaging volume along at least two perpendicular spatial directions, an accessibility of the imaging volume may favorably be enhanced while maintaining or decreasing a distance between the at least one magnet and/or the second magnet and the imaging volume. Thus, a better compromise between the efficiency of the field generation unit and the accessibility of the imaging volume may advantageously be provided in comparison to conventional magnetic resonance imaging devices.

According to an embodiment of the inventive magnetic resonance imaging device according to claim <NUM>, a surface directed towards the imaging volume of the second magnet is curved in such a way, that a perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the second magnet varies in the direction of access to the imaging volume.

In an example outside the scope of the present invention, the curvature of the surface directed towards the imaging volume of the second magnet may correspond to the curvature of the at least one magnet directed towards the imaging volume. For example, the at least one magnet and the second magnet may be arranged symmetrically with respect to the imaging volume. In this case, a first distance between the center of the imaging volume and a nearest point on the surface of the at least one magnet may correspond to a second distance between the center of the imaging volume and a nearest point on the surface of the second magnet. It is also conceivable, that the at least one magnet and the second magnet are arranged symmetrically with respect to a symmetry plane running through the center of the imaging volume. In this embodiment, a shape of the at least one magnet may essentially correspond to a shape of the second magnet. However, the at least one magnet and the second magnet may also comprise different shapes. For example, the curvature of the surface directed towards the imaging volume of the at least one magnet may comprise a different shape, slope and/or surface area than the surface directed towards the imaging volume of the second magnet. According to the invention, the curvature of the second magnet is different from the curvature of the at least one magnet.

Particularly, the at least one magnet and the second may be arranged asymmetrically with respect to the imaging volume. In one embodiment, the first distance may differ from the second distance. It is conceivable, that the at least one magnet and the second magnet confine the imaging volume from two opposing sides.

In providing a second magnet with a curved surface directed towards the imaging volume, an image acquisition region of the magnetic resonance imaging device may favorably be adapted to better accommodate a patient and/or match a contour of a body region of the patient (such as a belly, a shoulder and/or a neck of the patient) from more than one side. Thus, dedicated scanners with a high efficiency of the field generation unit and increased patient comfort can be provided.

In a further embodiment of the magnetic resonance imaging device according to claim <NUM>, a surface directed towards the imaging volume of the second magnet is shaped in such a way, that a perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the second magnet is constant in the direction of access to the imaging volume.

For example, the surface directed towards the imaging volume of the second magnet may be essentially flat or planar. In this case the direction of access to the imaging volume may be oriented in parallel to the flat or planar surface of the second magnet. It is conceivable, that a second surface area of the surface directed towards the imaging volume of the second magnet differs from a first surface area of the surface directed towards the imaging volume of the at least one magnet. The second surface area may exceed the first surface area or be smaller than the first surface area. However, it is also conceivable that the first surface area essentially matches the second surface area.

In providing a second magnet with an essentially planar or flat surface directed towards the imaging volume, manufacturing costs of the field generation unit may favorably be decreased while still providing at least one magnet with a curved surface directed towards the imaging volume for increasing the efficiency of the field generation unit. In one example, the flat or planar surface directed towards the imaging volume of the second magnet may favorably be used to accommodate a back of the patient, which typically features a topology which is more level or even than a front side of the patient.

In a further embodiment of the inventive magnetic resonance imaging device, the at least one magnet and/or the second magnet are configured to generate a static magnetic field in the imaging volume, wherein the static magnetic field is homogenous throughout the imaging volume or wherein the static magnetic field comprises a predefined magnetic field gradient across the imaging volume.

The at least one magnet and/or the second magnet may represent main magnets of the field generation unit configured to provide a B0 magnetic field. It is conceivable, that the B0 magnetic field provided via the at least one magnet and/or the second magnet is a static magnetic field with particularly high magnetic homogeneity. However, the B0 magnetic field provided via the at least one magnet may also comprise a predefined magnetic field gradient. This predefined magnetic field gradient may be used for spatial encoding of magnetic resonance signals acquired from the image acquisition region. In an embodiment, the at least one magnet and the second magnet are arranged on two opposing sides of the imaging volume and represent two magnetic poles or segments of a combined magnet for providing a homogenous B0 magnetic field. For this purpose, the magnetic field strengths of the at least one magnet and the second magnet may substantially coincide. However, the magnetic field strengths of the at least one magnet and the second magnet may also differ, thus providing the predefined magnetic field gradient across the imaging volume. According to an embodiment of the invention according to claim <NUM>, the at least one magnet is configured as a solenoid circumferentially enclosing the imaging volume. In this case, a second magnet may be omitted.

In providing at least one magnet with a curved surface directed towards the imaging volume for generating a B0 magnetic field, a major component of the field generation unit may be shaped in such a way to better accommodate a specific body region of the patient and/or increase the efficiency of the magnetic resonance imaging device. Furthermore, the curved surface directed towards the imaging volume of the at least one magnet may enhance bundling of magnetic field lines within the imaging volume, thus providing stronger magnetic gradient fields.

In a preferred embodiment, the at least one magnet comprises a set of gradient coils, wherein the set of gradient coils is configured to generate a magnetic gradient field in the imaging volume.

The at least one magnet be configured as a gradient magnet comprising a set of gradient coils. A set of gradient coils may comprise one or more gradient coils. For example, the set of gradient coils may comprise one, two, three or more gradient coils. The gradient coils may consist of wires coiled in such a way to provide an essentially planar shape, a curved shape or a tubular shape. An axial cross-section of a tubular gradient coil may be circular, oval or polygonal. In one embodiment, the set of gradient coils of the at least one magnet may comprise one gradient coil, circumferentially enclosing the imaging volume along the direction of access to the imaging volume. The gradient coil may be configured as a tubular solenoid and a diameter of the tubular solenoid may vary along the direction of access to the imaging volume.

However, the set of gradient coils may also comprise at least two gradient coils, positioned on opposite sides of the imaging volume. The set of gradient coils may be positioned in such a way, that the direction of access to the imaging volume is oriented essentially perpendicular to an axis of rotational symmetry of the set of gradient coils. It is also conceivable, that the set of gradient coils is positioned on one side of the imaging volume. For example, a first gradient coil and a second gradient coil of the set of gradient coils may be positioned adjacent to one another and confine the imaging volume in essentially one spatial direction.

In a further embodiment of the inventive magnetic resonance imaging device, the second magnet comprises a second set of gradient coils.

The second set of gradient coils may comprise one or more gradient coils. The second set of gradient coils may be arranged along an axis of rotational symmetry of the at least one magnet and/or the second magnet. Particularly, an axis of rotational symmetry of the second set of gradient coils may coincide with the axis of rotational symmetry of the at least one magnet. In one embodiment, the axis of rotational symmetry of the at least one magnet coincides with a direction of access to the imaging volume. However, according to another embodiment, the axis of rotational symmetry of the at least one magnet is oriented essentially orthogonal to the direction of access to the imaging volume. In the latter case, the set of gradient coils and the second set of gradient coils may be positioned on opposing sides of the imaging volume. However, it is also conceivable, that a first gradient coil and a second gradient coil of the at least one magnet are positioned on opposing sides of the imaging volume and that a first gradient coil and a second gradient coil of the second magnet are positioned on opposing sides of the imaging volume. The first gradient coil of the at least one magnet may be positioned adjacent to the first gradient coil of the second magnet and/or the second gradient coil of the second magnet.

The second set of gradient coils is configured to generate a second magnetic gradient field in the imaging volume, wherein the second magnetic gradient field is oriented perpendicular to the magnetic gradient field provided via the at least one magnet.

In a further embodiment, the field generation unit of the inventive magnetic resonance imaging device comprises a third magnet, wherein the third magnet comprises a third set of gradient coils and wherein the third set of gradient coils is configured to generate a third magnetic gradient field in the imaging volume, wherein the third magnetic gradient field is oriented perpendicular to the magnetic gradient field provided via the at least one magnet. It is also conceivable, that the gradient fields of the first magnet, the second magnet and the third magnet are oriented essentially perpendicular to one another. Preferably, the third magnet comprises a curved surfaced similar or different to the curved surface of the first magnet and/or the second magnet.

In providing a set of gradient coils with a curved surface according to the invention, an efficiency of a gradient system can favorably be improved in comparison to conventional magnetic resonance imaging devices. A possible advantage of the enhanced efficiency of the gradient system may be an increased slew rate of a gradient coil or a set of gradient coils in comparison to conventional magnet resonance imaging devices.

In the inventive magnetic resonance imaging device according to claim <NUM>, the second magnet is curved in such a way, that a perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the second magnet varies in the direction of access to the imaging volume, wherein a curvature of the second magnet is different from a curvature of the at least one magnet.

As described above, the at least one magnet and/or the second magnet may represent a main magnet or a gradient magnet of the magnetic resonance imaging device. The curvature of the surface directed towards the imaging volume of the second magnet differs from the curvature of the surface directed towards the imaging volume of the at least one magnet. For example, the curvature of the surface directed towards the imaging volume of the at least one magnet may comprise a different shape, slope and/or surface area than the surface directed towards the imaging volume of the second magnet. The at least one magnet and the second magnet may be positioned adjacent to one another. In this case, the at least one magnet and the second magnet may favorably be arranged in such a way to facilitate an access of a specific body region of the patient to the imaging volume and/or accommodate the specific body region of the patient positioned within the imaging volume. In an alternative embodiment, the at least one magnet and the second magnet confine the imaging volume in substantially two opposing spatial directions.

The different curvatures of the surfaces directed towards the imaging volume of the at least one magnet and the second magnet may favorably enable reducing a distance between at least a part of the at least one magnet and the imaging volume and/or at least a part of the second magnet and the imaging volume, thus increasing the efficiency of the field generation unit.

Alternatively, in the magnetic resonance imaging device according to claim <NUM>, the surface directed towards the imaging volume of the second magnet is shaped in such a way, that the perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the second magnet is constant in the direction of access to the imaging volume.

For example, the surface directed towards the imaging volume of the second magnet may be substantially planar or flat. It is also conceivable, that the surface directed towards the imaging volume of the second magnet is oriented in parallel to the direction of access to the imaging volume. Furthermore, the at least one magnet and the second magnet may be positioned adjacent to one another. Thus, a particularly compact or space-efficient arrangement of the at least one magnet and the second magnet may favorably be provided. In this case, the at least one magnet preferably is positioned closer to the imaging volume than the second magnet. However, the at least one magnet and the second magnet may also be arranged in such a way, that the imaging volume is confined by the at least one magnet and the second magnet in essentially two opposite spatial directions.

In providing a second magnet with a substantially flat or planar surface directed towards the imaging volume, manufacturing costs and/or an effort for assembly of the field generation unit may favorably be reduced while still enabling the at least one magnet to be positioned in proximity to the imaging volume. Thus, a cost-to-efficiency ratio of the magnetic resonance imaging device may favorably be adapted according to a requirement of a specific body region of a patient.

In the inventive magnetic resonance imaging device according to claim <NUM>, the at least one magnet is configured to enclose the imaging volume along at least a part of the line oriented along the direction of access to the imaging volume, wherein the direction of access to the imaging volume is oriented in parallel to a main direction of magnetic field lines within the imaging volume.

The at least one magnet may represent a main magnet or a gradient magnet comprising a set of gradient coils. According to an example outside the scope of the present invention, the at least one magnet may enclose at least a part of a surface of an imaginary cylinder whose axis of rotational symmetry coincides with the line oriented along the direction of access to the imaging volume. Particularly, the at least one magnet may enclose an arc of the imaginary cylinder along the direction of the imaging volume. For example, an angle defining the arc may exceed <NUM>°, <NUM>°, <NUM>°, <NUM>° or <NUM>°.

In the inventive MRI device according to claim <NUM>, the at least one magnet is shaped in such a way, that a minimum perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the at least one magnet is located at a center of the imaging volume. For example, a first perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the at least one magnet is located at a first distance from the imaging volume and a second perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the at least one magnet is located at a second distance from the imaging volume. The at least one magnet may be curved in such a way, that the first perpendicular distance exceeds the second perpendicular distance and the first distance exceeds the second distance. It is also conceivable, that the perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the at least one magnet increases in a direction facing away from the center of the imaging volume along the line oriented along the direction of access to the imaging volume. An overall shape of the at least one magnet may essentially correspond to an hourglass, a double-cone, a double-frustum or the like.

According to one embodiment of the inventive magnetic resonance imaging device, the field generation unit comprises a second magnet, wherein the second magnet is configured to enclose at least a part of the imaging volume along at least a part of the line oriented along the direction of access to the imaging volume, wherein the second magnet is curved in such a way, that a perpendicular distance between the line oriented along the direction of access to the imaging volume and the surface directed towards the imaging volume of the second magnet varies in the direction of access to the imaging volume.

A slope of the curvature of the surface directed towards the imaging volume of the second magnet may correspond to a slope of the curvature of the surface directed towards the imaging volume of the at least one magnet at one or more cross-sections of the field generation unit along the direction of access to the imaging volume. However, the slopes of the curvatures of the at least one magnet and the second magnet may also differ at one or more cross-sections of the field generation unit along the direction of access to the imaging volume. In a preferred embodiment, the at least one magnet and the second magnet comprise equal shapes but different dimensions. For example, a diameter and/or a length of the at least one magnet may differ from a diameter and/or a length of the second magnet. However, the second magnet may also be shaped differently than the second magnet.

Analogous to the at least one magnet, the second magnet may represent a main magnet or a gradient magnet comprising a second set of gradient coils. The second magnet may enclose at least a part of the surface area of the imaginary cylinder whose axis of rotational symmetry coincides with the line oriented along the direction of access to the imaging volume. Particularly, the second magnet may enclose at least an arc of the imaginary cylinder along the direction of the imaging volume. For example, an angle defining the arc may exceed <NUM>°, <NUM>°, <NUM>°, <NUM>° or <NUM>°.

In providing a magnet enclosing at least a part of the imaging volume along at least a part of the line oriented along the direction of access to the imaging volume, an enclosure of the imaging volume with magnetic material may favorably be enhanced. Thus, a homogeneity and/or a magnetic field strength within the imaging volume may be increased in comparison to conventional C-type or dipole magnetic resonance imaging devices.

In a preferred embodiment, the second magnet circumferentially encloses the imaging volume along the line oriented along the direction of access to the imaging volume.

In the inventive MRI device according to claim <NUM>, the at least one magnet is configured as a solenoidal magnet comprising the shape of a tube. The at least one magnet may comprise an arbitrary cross-section, such as a circular, an oval, an elliptical or a polygonal cross-section. The center of the imaging volume may be positioned along an axis of rotational symmetry of the tube, particularly a geometric center of the tube. In one example, the at least one magnet may be a main magnet providing a static magnetic field within the imaging volume. In another example, the at least one magnet may be a gradient magnet comprising a set of gradient coils circumferentially enclosing the imaging volume along the line oriented along the direction of access to the imaging volume.

Likewise, the second magnet may be configured as a solenoid magnet circumferentially enclosing the imaging volume along the line oriented along the direction of access to the imaging volume. Analogous to the at least one magnet, the second magnet may comprise a tubular shape with an arbitrarily shaped cross-section. The second magnet may enclose at least a section of the at least one magnet along at least a part of the line oriented along the direction of access to the imaging volume. However, in an alternative embodiment, the second magnet may also be at least partially enclosed by the at least one magnet along at least a part of the line oriented along the direction of access to the imaging volume.

A diameter of the at least one magnet and/or the second magnet may vary in the direction of access to the imaging volume. In one embodiment, a cross-section of the at least one magnet and/or the second magnet with a minimum diameter essentially coincides with a position of the center of the imaging volume along the line oriented along the direction of access to the imaging volume. In a further example, a first diameter of the at least one magnet and/or the second magnet located at a first distance from the imaging volume exceeds a second diameter of the at least one magnet and/or the second magnet located at a second distance to the imaging volume, wherein the first distance exceeds the second distance. In a preferred embodiment, the diameter of the at least one magnet and/or the second magnet increases in a direction facing away from the center of the imaging volume along the line oriented along the direction of access to the imaging volume.

In providing a solenoidal magnet with a varying diameter along the line oriented along the direction of access to the imaging volume, an efficiency of the field generation unit may favorably be enhanced in comparison to conventional solenoidal magnetic resonance imaging devices. Furthermore, an increased diameter of the magnet at an opening or an end of the magnet may favorably facilitate an access of a patient or a body region of the patient to the imaging volume in comparison to existing solenoidal magnetic resonance imaging devices.

According to one embodiment of the inventive magnetic resonance imaging device, the at least one magnet and/or the second magnet comprise at least one of.

The at least one magnet and/or the second magnet may consist of coils and/or tubular magnet segments. The coils and/or tubular magnet segments may comprise a wire made from a material with superconducting properties. The wire may be connected to a cryostat to keep a temperature of the wire below a predefined value (e.g. a superconducting temperature). In case of a low-temperature superconducting magnet, liquid helium may be used as a coolant, ensuring a temperature of the wire below <NUM>. Low-temperature superconducting magnets may comprise materials such as niobium-titanium, niobium-tin, niobium-germanium and alloys thereof. In contrast, high-temperature superconducting magnets exhibit superconducting properties at temperatures in the range of <NUM> to <NUM>. High-temperature superconducting magnets may comprise materials such as barium-copper-oxide, calcium-copper-oxide, magnesium diboride, but also fullerides and the like. The wire may be wound and/or arranged in a variety of shapes, such as a solenoid, as a substantially planar loop or a tubular magnet segment. Preferably, the wires of the high-temperature superconducting magnet and/or the low-temperature superconducting magnet are embedded within a matrix made of an electrical conductor, such as copper, aluminum, gold and the like.

By using a high-temperature superconducting magnet and/or a low-temperature superconducting magnet, the magnetic field strength provided via the at least one magnet and/or the second magnet may favorably be increased in comparison to a permanent magnet or an electromagnet of comparable size.

A permanent magnet may consist of any suitable magnetic material such as aluminum-nickel-cobalt, neodymium-iron-boron or samarium-cobalt alloys. Furthermore, the surface directed towards the imaging volume of the permanent magnet may comprise the shape of a cone or a frustum. In one embodiment, the permanent magnet may be composed of smaller, stacked permanent magnets. Permanent magnets advantageously provide a low-cost solution for generating a magnetic field within the imaging volume. The use of a permanent magnet may favorably avoid costs and space required for cooling equipment usually associated with superconducting magnets and electromagnets.

An electromagnet may be a non-superconducting magnet. Particularly, an electromagnet may comprise an electrical conductor wound in a predefined pattern. The electrical conductor may be wound around a magnetic core made of, for example, a ferromagnetic or ferrimagnetic material. The magnetic core of the electromagnet may comprise a cylindrical shape, a cuboid shape, a prism shape or any other desirable shape. By using an electromagnet, the magnetic field strength can be favorably increased in comparison to a permanent magnet of comparable size. Higher magnetic field strengths can advantageously enhance a quality and/or a signal-to-noise ratio of a magnetic resonance image acquired via the magnetic resonance imaging device.

According to one embodiment, the at least one magnet and the second magnet confine the imaging volume in two spatial directions oriented at an angle of <NUM>° to one another, wherein a smallest distance between the at least one magnet and a center of the imaging volume is equal or unequal to a smallest distance between the second magnet and the center of the imaging volume.

In case the smallest distance between the center of the imaging volume and the at least one magnet is equal to the smallest distance between the center of the imaging volume and the second magnet, the at least one magnet and the second magnet may be arranged symmetrically with respect to the imaging volume. In a symmetrical arrangement of the at least one magnet and the second magnet with respect to the imaging volume, a volume, a dimension, a shape and/or a curvature of the surfaces directed towards the imaging volume of the at least one magnet and the second magnet may substantially coincide. In providing a symmetrical arrangement of the at least one magnet and the second magnet with respect to the imaging volume, an engineering and/or manufacturing effort for providing different magnets for the field generation unit may favorably be avoided.

In case the smallest distance between the center of the imaging volume and the at least one magnet is unequal to the smallest distance between the center of the imaging volume and the second magnet, the at least one magnet and the second magnet are arranged asymmetrically with respect to the imaging volume. An asymmetrical arrangement of the at least one magnet and the second magnet with respect to the imaging volume may be characterized by different volumes, dimensions, shapes and/or curvatures of the surfaces directed towards the imaging volume of the at least one magnet and the second magnet. It is also conceivable, that the at least one magnet and the second magnet comprise different magnetic field strengths and/or different magnetic materials. In this case, different volumes of magnetic material and/or different shapes may be required to match the at least one magnet and the second magnet. Thus, the smallest distance between the center of the imaging volume and the at least one magnet may be equal to the smallest distance between the center of the imaging volume and the second magnet. However, the at least one magnet and the second magnet may be arranged asymmetrically with respect to the imaging volume due to differences in volume, dimension, shape and/or curvature of the surface directed towards the imaging volume of the at least one magnet and the second magnet.

In providing an asymmetrical arrangement of the at least one magnet and the second magnet with respect to the imaging volume, the curvatures of the surfaces directed towards the imaging volume of the at least one magnet and the second magnet may individually be matched with a geometry of a specific body region of a patient. Thus, a dimension of a clearance between the at least one magnet and the second magnet and/or a direction of access to the imaging volume may be optimized for imaging of the specific body region. Furthermore, by taking into account the geometry of the specific body region, both the at least one magnet and the second magnet may favorably be positioned closer to the imaging volume, thus enhancing the efficiency of the field generation unit.

Further advantages and details of the present invention may be recognized from the embodiments described below as well as the drawings. The figures show:.

<FIG> depicts a schematic representation of an inventive magnetic resonance imaging device <NUM> configured to perform a magnetic resonance measurement of a jaw region and/or an eye region of a patient <NUM>. An application of the magnetic resonance imaging device <NUM> for imaging of the jaw region and/or the eye region of the patient <NUM> is to be understood as an example. The inventive magnetic resonance imaging device <NUM> may also be configured to perform cardiac imaging, mammography imaging, neurological imaging, urological imaging, orthopedics imaging, prostate imaging or imaging of other body regions of the patient <NUM>.

In the depicted embodiment, the magnetic resonance imaging device <NUM> comprises a field generation unit <NUM> with a first magnet <NUM> and a second magnet <NUM>. The first magnet <NUM> and the second magnet <NUM> are carried by a supporting structure <NUM> which maintains a predefined distance between the first magnet <NUM> and the second magnet <NUM>. The supporting structure <NUM> may be implemented as an iron yoke <NUM>. A free volume between the first magnet <NUM> and the second magnet <NUM> represents an image acquisition region <NUM> configured to receive an examination object <NUM>, e. a body region of the patient <NUM>. The image acquisition region <NUM> is confined by the first magnet <NUM> and the second magnet <NUM> in two opposing spatial directions.

The patient <NUM> may be positioned within the image acquisition region <NUM> in an upright or standing position. However, the patient <NUM> may also be positioned in the image acquisition region <NUM> in a sitting or lying position. In the latter case, the patient <NUM> may be positioned by means of a dedicated patient positioning device (not shown). However, the magnetic resonance imaging device <NUM> may also comprise a positioning unit <NUM> for adjusting a position and/or an orientation of the field generation unit <NUM> with respect to the patient <NUM>. For example, the positioning unit <NUM> may comprise a swivel joint configured to rotate the field generation unit <NUM> along a rotation direction WX and/or a rotation direction WY. A position of the field generation unit <NUM> along a Y-direction and/or a Z-direction may be adjusted via a suitable telescope system and/or rail system mechanically connected to the supporting structure <NUM>. Of course, other embodiments of the supporting structure <NUM> and/or the positioning unit <NUM> are conceivable. In particular, the positioning unit <NUM> may further be configured to position the field generation unit <NUM> along at least one spatial direction and/or rotate the field generation unit <NUM> in at least one rotation direction (not shown).

The surfaces directed towards the imaging volume <NUM> of the first magnet <NUM> and the second magnet <NUM> are curved in such a way, that a perpendicular distance between a line oriented along a direction of access <NUM> to the imaging volume <NUM> and a surface directed towards the imaging volume <NUM> of the first magnet <NUM> varies along the direction of access <NUM> to the imaging volume <NUM>. In the depicted embodiment, the first magnet <NUM> and the second magnet <NUM> are arranged symmetrically with respect to the imaging volume <NUM>. The surfaces directed towards the imaging volume <NUM> of the first magnet <NUM> and the second magnet <NUM> comprise the shape of a cone or a dome. Due to the curvature of the first magnet <NUM> and the second magnet <NUM>, a clearance is provided for the shoulders of the patient <NUM>. Thus, a minimum distance between the first magnet <NUM> and the imaging volume <NUM> as well as a minimum distance between the second magnet <NUM> and the imaging volume <NUM> may be reduced in comparison to conventional C-type magnetic resonance imaging devices while still providing sufficient clearance for the shoulders of the patient <NUM>.

In the shown example, the first magnet <NUM> and the second magnet <NUM> are main magnets configured to generate a static magnetic field in the image acquisition region <NUM>. The field generation unit <NUM> may further comprise a gradient field system <NUM> (see <FIG> and <FIG>) with at least one gradient coil <NUM> for generating a magnetic gradient field used for spatial encoding of magnetic resonance signals acquired during a magnetic resonance measurement. Preferably, the field generation unit <NUM> further comprises a radiofrequency system with at least one radiofrequency antenna (not shown) configured to emit a radiofrequency excitation pulse into the image acquisition region <NUM>. The at least one radiofrequency antenna may also be configured to receive magnetic resonance signals from the image acquisition region <NUM>, particularly the imaging volume <NUM>. In one embodiment, the at least one radiofrequency antenna may be configured as a local coil.

In order to control the field generation unit <NUM> as well as the at least one radiofrequency antenna, the magnetic resonance imaging device <NUM> comprises a control unit <NUM>. The control unit <NUM> is configured to control the magnetic resonance imaging device <NUM> to perform a magnetic resonance measurement of the body region of the patient <NUM> positioned within the imaging volume <NUM>. For this purpose, the control unit <NUM> may comprise a signal connection with a gradient control unit <NUM> and a radiofrequency antenna control unit <NUM>. It is also conceivable, that the gradient control unit <NUM> and the radiofrequency antenna control unit <NUM> are integrated within the control unit <NUM>. Furthermore, the control unit <NUM> may comprise a processing unit <NUM> configured to coordinate an acquisition of magnetic resonance image data and/or a reconstruction of magnetic resonance image data acquired from the imaging volume <NUM>. It is conceivable, that the processing unit <NUM> is also configured to evaluate and/or process data, such as magnetic resonance signals and/or magnetic resonance image data. The control unit <NUM> may comprise a controller, a microcontroller, an analog circuit, a logic unit and the like. The processing unit <NUM> may comprise a processor, such as a CPU, a GPU and the like. It is also conceivable, that the control unit <NUM> and/or the processing unit <NUM> comprise a memory and/or an internal storage, such as a RAM, a ROM, a PROM, an EPROM, an EEPROM, a flash memory, as well as an HDD, an SSD and the like.

Control information, such as imaging parameters and/or magnetic resonance image data, can be displayed on an output unit <NUM>. The output unit <NUM> may comprise at least one monitor configured to display control information and/or images acquired via the magnetic resonance imaging device <NUM> to an operator of the magnetic resonance imaging device <NUM>. The magnetic resonance imaging device <NUM> may further comprise an input unit <NUM> configured to receive information and/or parameters input by the operator during an imaging examination.

The illustrated magnetic resonance imaging device <NUM> may of course include further components that magnetic resonance imaging devices <NUM> usually comprise. The general mode of operation of a magnetic resonance imaging device <NUM> is well-known to the skilled person. Thus, a further description of the general components or a sequencing of a magnetic resonance measurement is not deemed necessary.

<FIG> depicts an embodiment of the inventive magnetic resonance imaging device <NUM>, wherein the first magnet <NUM> and the second magnet <NUM> are arranged asymmetrically with respect to the imaging volume <NUM>. In the shown example, a volume as well as a height of the first magnet <NUM> exceeds a volume and a height of the second magnet <NUM>. The first magnet <NUM> may further comprise a higher magnetic field strength than the second magnet <NUM>. Thus, the imaging volume <NUM> provided via the first magnet <NUM> and the second magnet <NUM> may be shifted towards the surface directed towards the imaging volume <NUM> of the first magnet <NUM>. Furthermore, a smallest distance between a center of the imaging volume <NUM> and the first magnet is unequal to the smallest distance between the center of the imaging volume <NUM> and the second magnet <NUM>.

In the depicted example, the surface directed towards the imaging volume <NUM> of the second magnet <NUM> is curved in such a way to accommodate a neck region as well as an upper back region of the patient <NUM>. In contrast, the surface directed towards the imaging volume <NUM> of the first magnet <NUM> is curved in such a way to provide clearance for a breast region of the patient <NUM>. As can be seen from <FIG>, enough clearance is provided to facilitate an access of the patient <NUM> to the imaging volume <NUM> along at least two perpendicular spatial directions. For example, the patient <NUM> may access the imaging volume <NUM> along the Y-direction as well as the X-direction. The two perpendicular directions of access to the imaging volume <NUM> are oriented perpendicular to a main direction of magnetic field lines in the imaging volume <NUM>, which substantially coincides with the Z-direction.

<FIG> shows a further embodiment of the inventive magnetic resonance imaging device <NUM>, wherein the surface directed towards the imaging volume <NUM> of the second magnet <NUM> is curved in such a way, that a perpendicular distance <NUM> (e.g. 37a, 37b and 37c) between a line oriented along the direction of access <NUM> to the imaging volume <NUM> and the surface directed towards the imaging volume <NUM> of the second magnet <NUM> is constant along the direction of access <NUM> to the imaging volume <NUM>. In comparison, the surface directed towards the imaging volume <NUM> of the first magnet <NUM> is curved in such a way, that the perpendicular distance <NUM> (e.g. 37d, 37e and 37f) between the line oriented along the direction of access <NUM> to the imaging volume <NUM> and the surface directed towards the imaging volume <NUM> of the first magnet <NUM> varies along the direction of access <NUM> to the imaging volume <NUM>. In the shown example, the first magnet <NUM> and the second magnet <NUM> represent main magnets of the field generation unit <NUM>, which are arranged asymmetrically with respect to the imaging volume <NUM>. However, it is also conceivable that the first magnet <NUM> and the second magnet <NUM> are gradient magnets of a gradient system. In this case, the first magnet <NUM> and the second magnet <NUM> may each comprise a set of gradient coils, which are configured to generate a magnetic gradient field in the imaging volume <NUM>.

It is conceivable, that the first magnet <NUM> and the second magnet <NUM> comprise an axis of rotational symmetry, which passes through the center of the imaging volume <NUM> and is oriented essentially in parallel to the Z-direction.

<FIG> shows a further embodiment of the inventive magnetic resonance imaging device <NUM> comprising a gradient system <NUM> including a set of gradient coils 28a and 28b. In an example outside the scope of the invention, a first magnet <NUM> comprises a first set of gradient coils 28a on one side of the imaging volume <NUM> and a first set of gradient coils 28b positioned in an essentially opposing direction on the other side of the imaging volume <NUM>. The first set of gradient coils 28a and the first set of gradient coils 28b may be configured as (mirror image) transverse gradient coils with a fingerprint winding pattern. However, other winding patterns are also conceivable. For example, the winding pattern of the first set of gradient coils 28a and the first set of gradient coils 28b may comprise an arrangement of essentially circular or ring-shaped wires according to the lines depicted in <FIG>. Particularly, the first magnet <NUM> according to the embodiment shown in <FIG> may be configured to generate a magnetic gradient field oriented along the Z-direction. In an alternative embodiment within the scope of the present invention, the element 28a is interpreted as a first magnet <NUM> and the element 28b is interpreted as a second magnet <NUM>.

In the embodiment depicted in <FIG>, the first set of gradient coils 28a and the shield coil 34a are positioned adjacent to one another. In a similar manner, the first set of gradient coils 28a and a second set of gradient coils (not shown) may be positioned adjacent to one another. In an alternative embodiment, the first set of gradient coils 28a may be positioned adjacent to the first set of gradient coils 28b. In a preferred embodiment, the first set of gradient coils 28a is positioned adjacent to a shield coil 34a and the first set of gradient coils 28b is positioned adjacent to a shield coil 34b. The shield coils 34a and 34b may be configured to shield the first set of gradient coils 28a and the first set of gradient coils 28b from electromagnetic interference and/or magnetic fields provided by further magnets (e. a magnet positioned adjacent to the first set of gradient coils 28a and/or the first set of gradient coils 28b). As depicted in <FIG>, the shield coil 34a may be positioned adjacent to a surface directed away from the imaging volume <NUM> of the first set of gradient coils 28a. Likewise, the shield coil 34b may be positioned adjacent to a surface directed away from the imaging volume <NUM> of the first set of gradient coils 28b. However, the shield coils 34a and 34b may also be positioned adjacent to the surfaces directed towards the imaging volume <NUM> of the first set of gradient coils 28a and the first set of gradient coils 28b.

According to an alternative embodiment, the magnetic resonance imaging device <NUM> depicted in <FIG> may comprise a second magnet <NUM> positioned adjacent to the first magnet <NUM>. For example, the second magnet <NUM> may comprise a second set of gradient coils 28c (see <FIG>) positioned adjacent to the first set of gradient coils 28a and a second set of gradient coils 28d positioned adjacent to the first set of gradient coils 28b. The second sets of gradient coils 28c and 28d may be positioned in proximity to the shield coils 34a and 34b and comprise a similar shape. Preferably, a shield coil 34a is positioned between a first set of gradient coils 28a and 28b and a second set of gradient coils 28c and 28d. However, the shield coils 34a may also be positioned adjacent to a surface directed away from the imaging volume of a second set of gradient coils 28c and 28d in order to shield the first magnet <NUM> and the second magnet <NUM> from a main magnet (not shown) generating a static magnetic field within the imaging volume <NUM>.

<FIG> shows a further example outside the scope of the present invention of a magnetic resonance imaging device <NUM> comprising a gradient system <NUM> with a first magnet <NUM> configured as a gradient magnet. The first magnet <NUM> comprises a first set of gradient coils 28a and a first set of gradient coils 28b. The first set of gradient coils 28a and the first set of gradient coils 28b are arranged in such a way, that they confine the imaging volume <NUM> in two essentially opposing spatial directions. In the depicted embodiment, the wiring pattern of the first set of gradient coils 28a and a first set of gradient coils 28b of the first magnet <NUM> may be configured to provide a magnetic gradient field oriented along the Y-direction.

According to one embodiment, the magnetic resonance imaging device <NUM> may further comprise a second magnet <NUM> including second sets of gradient coils 28c and 28d arranged at two opposing sides of the imaging volume <NUM>. The second sets of gradient coils 28c and 28d may be configured to provide a magnetic gradient field along the X-direction. For this purpose, a winding pattern of the second sets of gradient coils 28c and 28d may essentially correspond to the winding pattern of the first set of gradient coils 28a and the first set of gradient coils 28b but be rotated by an angle of <NUM>° or <NUM>°.

The magnetic resonance imaging device <NUM> may further comprise a third magnet (not shown). The third magnet may comprise third sets of gradient coils (not shown) with a winding pattern consisting of essentially circular or ring-shaped wires as depicted in <FIG> (e. the first set of gradient coils 28a). Particularly, the third magnet may represent a gradient magnet configured to provide a magnetic gradient field oriented along the Z-direction. The magnetic resonance imaging device <NUM> may also comprise shield coils 34a and 34b positioned adjacent to the first magnet <NUM>, the second magnet <NUM> and/or the third magnet. For example, the shield coils 34a and 34b may be positioned adjacent to the surfaces directed away from the imaging volume <NUM> of the first sets of gradient coils 28a and 28b, the second sets of gradient coils 28c and 28d and/or the sets of gradient coils of the third magnet. In a preferred embodiment, the shield coils 34a and 34b may be configured to shield the first magnet <NUM>, the second magnet <NUM> and/or the third magnet from a main magnet for generating a static magnetic field within the imaging volume <NUM>.

It is conceivable, that the curvature of the surface directed towards the imaging volume <NUM> of a first set of gradient coils 28a and 28b of the first magnet <NUM> differs from a curvature of a surface directed towards the imaging volume of a second set of gradient coils 28c and 28d of the second magnet <NUM>. For example, the curvature of the first set of gradient coils 28a may differ from the curvature of the second set of gradient coils 28c. However, the curvatures of the surfaces directed towards the imaging volume <NUM> of the first set of gradient coils 28a and 28b and the second set of gradient coils 28c and 28d may also be substantially identical.

<FIG> shows an example outside the scope of the present invention of a magnetic resonance imaging device <NUM>, wherein the first magnet <NUM> and the second magnet <NUM> are arranged symmetrically with respect to the imaging volume <NUM>. Thus, a minimum distance between the center of the imaging volume <NUM> and the surface directed towards the imaging volume <NUM> of the first magnet <NUM> may coincide with a minimum distance between the center of the imaging volume <NUM> and the surface directed towards the imaging volume <NUM> of the second magnet <NUM>. The first magnet <NUM> and the second magnet <NUM> may represent main magnets (or sections of a main magnet) of the field generation unit <NUM> configured to provide a static magnetic field in the imaging volume <NUM>. However, the first magnet <NUM> and the second magnet <NUM> may also represent gradient magnets, each comprising a set of gradient coils, which are configured to provide a magnetic gradient field within the imaging volume <NUM>.

In an example outside the scope of the present invention, <FIG> depicts a magnetic resonance imaging device <NUM>, wherein the first magnet <NUM> is configured to enclose only a part of the imaging volume <NUM> along at least a part of the direction of access <NUM> to the imaging volume <NUM> and wherein the direction of access <NUM> to the imaging volume <NUM> is oriented in parallel to a main direction of magnetic field lines within the imaging volume <NUM>. The first magnet <NUM> may be a gradient magnet including a set of gradient coils 28a enclosing an arc of an imaginary cylinder (not shown) whose cylinder axis corresponds to the line oriented along the direction of access <NUM> to the imaging volume <NUM>. Likewise, the second magnet <NUM> may be a gradient magnet including a set of gradient coils 28b enclosing an arc of the imaginary cylinder along the direction of access <NUM> to the imaging volume <NUM>. For example, an angle defining the arc of the imaginary cylinder enclosed by the first magnet <NUM> and/or the second magnet <NUM> may comprise about <NUM>° as depicted in <FIG>. However, the angle defining the arc of the imaginary cylinder may also comprise a value below <NUM>° or above <NUM>°. In a preferred embodiment, the first gradient magnet <NUM> and the second gradient magnet <NUM> circumferentially enclose the imaging volume <NUM> in such a way, that the angle defining the arc of the imaginary cylinder comprises a value of essentially <NUM>°. The set of gradient coils 28a of the first magnet <NUM> and the set of gradient coils 28b of the second magnet <NUM> may be configured to provide substantially perpendicular magnetic gradient fields within the imaging volume <NUM>. The first gradient magnet <NUM> and/or the second gradient magnet <NUM> may comprise fingerprint-like winding patterns oriented in a transverse direction.

In an example within the scope of the present invention, the first magnet <NUM> and the second magnet <NUM> depicted in <FIG> are configured as tubular solenoidal magnets circumferentially enclosing the imaging volume <NUM> along at least a part of the direction of access <NUM> to the imaging volume <NUM>. For example, the set of gradient coils 28a and the set of gradient coils 28b circumferentially encloses the imaging volume <NUM> and/or the patient <NUM> along the line oriented along the direction of access <NUM> to the imaging volume <NUM>. The diameter <NUM> of the set of gradient coils 28a and/or the set of gradient coils 28b varies along the direction of access <NUM> to the imaging volume <NUM>, and a minimum diameter <NUM> of the set of gradient coils 28a and/or the set of gradient coils 28b is located at the center of the imaging volume <NUM>.

In an alternative embodiment, the second magnet <NUM> may be configured as a conventional solenoid with a cylindrical shape circumferentially enclosing the first magnet <NUM> along the direction of access <NUM> to the imaging volume <NUM>. The inventive magnetic resonance imaging device <NUM> may also comprise a main magnet (not shown) for generating a static magnetic field within the imaging volume <NUM>. The main magnet may comprise a conventional cylindrical shape or a tube-shape according to the first magnet <NUM>. As described above, the inventive magnetic resonance imaging device <NUM> may also comprise a shield coil <NUM> positioned adjacent to the first magnet <NUM> and/or the second magnet <NUM>.

In a further embodiment, the elements <NUM> and <NUM> represent sections of a main magnet configured to provide a static magnetic field within the imaging volume <NUM>. For example, the elements <NUM> and <NUM> may represent an outer surface <NUM> and an inner surface <NUM> of the main magnet. It is also conceivable, that the first magnet <NUM> comprises a shield coil <NUM> positioned adjacent to the surface directed away from the imaging volume <NUM> or directed towards the imaging volume <NUM> of the first magnet <NUM>.

The diagram in <FIG> depicts a dependency between a slew rate of a gradient magnet with a curved surface directed towards the imaging volume <NUM> according to an embodiment of the inventive magnetic imaging device <NUM> as a function of a gap for a patient bore. The gap for the patient bore may relate to a minimum distance between the surfaces directed towards the imaging volume <NUM> of first magnet <NUM> and the second magnet <NUM> as depicted in <FIG>. According to <FIG>, the slew rate of a gradient system increases when the distance between the first magnet <NUM> and the second magnet <NUM> at the imaging volume <NUM> decreases. Thus, an efficiency of the field generation unit <NUM> (as represented by the slew rate) increases when using a curved surface directed towards the imaging volume <NUM> according to an embodiment described above. In one example, the gap for a patient bore of a conventional magnetic resonance imaging device may be about <NUM>, whereas the gap for the patient bore according to an embodiment of the inventive magnetic resonance imaging device <NUM> may be around <NUM>, thus increasing the slew rate from about <NUM> mT/m/ms to over <NUM> mT/m/ms.

Claim 1:
Magnetic resonance imaging device (<NUM>) comprising a field generation unit (<NUM>) configured to provide a magnetic field in an imaging volume (<NUM>) of the magnetic resonance imaging device (<NUM>), wherein the field generation unit (<NUM>) comprises at least one magnet (<NUM>) and a second magnet (<NUM>, wherein the at least one magnet (<NUM>) confines the imaging volume (<NUM>) in at least one spatial direction, wherein the at least one magnet (<NUM>) and the second magnet (<NUM>) are arranged in such a way to provide an access to the imaging volume (<NUM>) along at least two perpendicular spatial directions and wherein the direction of access (<NUM>) to the imaging volume (<NUM>) along each of the at least two perpendicular spatial directions is angled with respect to a main direction of magnetic field lines within the imaging volume (<NUM>), wherein the at least one magnet (<NUM>) is curved in such a way, that a perpendicular distance between a line oriented along a direction of access (<NUM>) to the imaging volume (<NUM>) and a surface directed towards the imaging volume (<NUM>) of the at least one magnet (<NUM>) varies in the direction of access (<NUM>) to the imaging volume (<NUM>), characterised in that
• the second magnet (<NUM>) is curved in such a way, that a perpendicular distance between the line oriented along the direction of access (<NUM>) to the imaging volume (<NUM>) and a surface directed towards the imaging volume (<NUM>) of the second magnet (<NUM>) varies in the direction of access (<NUM>) to the imaging volume (<NUM>), wherein a curvature of the second magnet (<NUM>) is different from a curvature of the at least one magnet (<NUM>) or wherein
the surface directed towards the imaging volume (<NUM>) of the second magnet (<NUM>) is shaped in such a way, that the perpendicular distance between the line oriented along the direction of access (<NUM>) to the imaging volume (<NUM>) and the surface directed towards the imaging volume (<NUM>) of the second magnet (<NUM>) is constant in the direction of access (<NUM>) to the imaging volume (<NUM>).