阅读说明：本技术 用于磁共振成像系统的磁体系统 (Magnet system for magnetic resonance imaging system ) 是由 赖纳·基尔施 斯特凡·波佩斯库 于 2021-04-15 设计创作，主要内容包括：本发明涉及用于磁共振成像系统的磁体系统。本发明描述了一种用于磁共振成像系统(1)的磁体系统,其包括基本场磁体(4)和梯度系统(6),其中,该梯度系统(6)的线圈位于所述基本场磁体(4)的预定义基本磁场(B0)的区域外部。本发明还描述了相应的梯度系统和具有这样的磁体系统的磁共振成像系统。(The invention relates to a magnet system for a magnetic resonance imaging system. The invention describes a magnet system for a magnetic resonance imaging system (1), comprising a basic field magnet (4) and a gradient system (6), wherein coils of the gradient system (6) are located outside the region of a predefined basic magnetic field (B0) of the basic field magnet (4). The invention also describes a corresponding gradient system and a magnetic resonance imaging system with such a magnet system.)

1. A magnet system for a magnetic resonance imaging system (1), comprising a basic field magnet (4) and a gradient system (6), wherein coils of the gradient system (6) are positioned outside the region of a predefined basic magnetic field (B0) of the basic field magnet (4).

2. Magnet system according to claim 1, wherein the coils located outside the region of the predefined basic magnetic field (B0) of the basic field magnet (4) are gradient coils (6x, 6z), in particular gradient coils (6x, 6z) for all three coordinate axes (x, y, z), and preferably shim coils and/or coils generating a non-linear encoding field and/or dynamic field cycling coils for multi-dimensional spatial signal encoding and acceleration signal acquisition.

3. Magnet system according to one of the preceding claims, wherein the basic field magnets (4) are C-shaped magnets and the coils of the gradient system (6) are arranged in the region of the pole shoes (4a) of the basic field magnets (4) or such that the pole shoes (4a) of the basic field magnets (4) are located between two coils of the gradient system (6).

4. Magnet system according to one of the preceding claims, wherein the basic field magnet (4) has an annular field distribution and preferably two or more examination zones (E), preferably wherein the basic field magnet coils (4b) are arranged in a star shape.

5. Magnet system according to one of the preceding claims, wherein the basic field magnet (4) comprises a yoke (4c) and the coils of the gradient system (6) are arranged such that their fields are coupled into the yoke (4c),

preferably, wherein the distance between the yoke (4c) and each respective coil of the gradient system (6) is less than 2cm, particularly preferably wherein the respective coil is in contact with the yoke (4 c).

6. Magnet system according to one of the preceding claims, wherein the coils of the gradient system (6) are planar coils, preferably arranged in parallel.

7. Magnet system according to one of the preceding claims, wherein the coils of the gradient system (6) are placed symmetrically with respect to an examination region (E) within a predefined basic magnetic field (B0) of the basic field magnet (4).

8. Magnet system according to one of the preceding claims, wherein the coils of the gradient system (6) are mechanically decoupled from the basic field magnet (4),

preferably, wherein the coil is movable relative to the basic field magnet (4), in particular by an actuator arm (20).

9. Magnet system according to one of the preceding claims, wherein the basic field magnets (4) are arranged between coils of the gradient system (6), wherein the distance between these coils is preferably larger than the size of the basic field magnets (4).

10. Magnet system according to one of the preceding claims, wherein the basic field magnet (4) is shaped such that it fits the target anatomy, preferably wherein the basic field magnet (4) is shaped like an open segment of a torus or cylinder for examining a limb, or like a bicycle saddle or saddle for prostate examination.

11. Magnet system according to one of the preceding claims, wherein the gradient system (6) comprises a cooling system which is also arranged outside the region of the predefined basic magnetic field (B0) of the basic field magnet (4).

12. Magnet system according to one of the preceding claims, wherein the coils of the gradient system (6) are pivotably arranged, in particular such that they can move like a door, preferably wherein the coils of the gradient system (6) are planar coils and in particular comprise a full-face cover, such that they are designed to act as louvers.

13. Gradient system (6) for a magnet system according to one of the preceding claims, comprising coils of the gradient system (6) which are designed to be arranged outside the region of a predefined basic magnetic field (B0) of a basic field magnet (4).

14. A magnetic resonance imaging system (1) comprising a magnet system according to one of claims 1 to 12.

15. The magnetic resonance imaging system is designed for examination of the head and/or prostate and/or the extremities and/or the animal and/or the neonate, preferably for dental imaging and/or for examination of a body part of a group comprising the brain, wrist, elbow, foot, ankle, knee, breast and prostate of a patient.

Technical Field

The present invention describes a magnet system for Magnetic Resonance Imaging (MRI), preferably a gradient system, in particular for dental and extremity MRI scanners.

Background

For over forty years, the principles of magnetic resonance imaging ("MRI") have been used for imaging and other measurements. Briefly, for this purpose, the examination subject is positioned in a Magnetic Resonance (MR) scanner in a relatively strong and homogeneous static magnetic field, also referred to as B0 field, having a field strength of 0.2T to 7T, such that the nuclear spins of the subject are oriented along the static magnetic field lines. For triggering nuclear spin resonance, a high-frequency excitation pulse (HF pulse) is irradiated toward the examination subject. The measured nuclear spin resonance is called k-space data, and it is used for reconstruction of MR images or calculation of spectral data.

For spatial encoding of the measurement data, dynamically switched magnetic gradient fields are superimposed on the static magnetic field. The recorded measurement data are digitized and stored as complex values in a k-space matrix. The associated MR image can be reconstructed from a k-space matrix populated with such values, for example by means of a multi-dimensional fourier transform.

Conventional MR scanners employ a solenoid-type superconducting magnet that positions the patient within the bore of the MR scanner during imaging. Such scanner designs limit the patient to a compact space and limit access to the patient's body by medical personnel, for example, to perform interventional or therapeutic procedures guided by real-time MR imaging.

Most prior art dental MR scanners employ solenoid-type magnets which tend to have a large outer diameter due to the need for an integrated external active shielding coil for safe operation in a dental clinic, as shown, for example, in US 2018/0199853a 1. Furthermore, since the imaging field of view is positioned deep into the central interior region of the solenoid, access of the patient's head into the magnet bore is also limited by the cylindrical gradient coil positioned inside the magnet bore. The net effect is that the magnet bore should be large enough to allow access to the patient's shoulders. This solution greatly increases the size of the scanner and therefore also the cost.

Other scanner configurations use a C-shaped basic field magnet with an examination region located between two magnetic shoes of the magnet.

More recently, a new scanner configuration has been introduced which includes a toroidal basic magnetic field and a plurality of examination zones between magnetic coils which generate the toroidal magnetic field.

Considering all these scanner architectures, they have in common that they comprise gradient coils which are always located inside the basic magnetic field. The functional structure of the known system can be compared with a russian nesting doll: in the middle, there is an examination region or patient "surrounded" by coils for transmitting and receiving Radio Frequency (RF). In the next "sphere", a gradient coil surrounded by basic field magnets is arranged.

With regard to the gradient coils, their shape is adapted to the basic magnetic field. Solenoids are present, but there are also planar coils which may be arranged in parallel or in a V-shape. However, the general shape of the basic field magnet severely limits the space available for assembling the gradient coils around the examination region. The achievable linearity of the gradient field over the extent of the imaging region creates problems, since the gradient coils are very close to the field of view (FOV), i.e. the region of the examination region, and the gradient coil size is limited by the need to keep the magnet size as small as possible.

Another serious drawback of known scanner architectures is the acoustic noise generated by the gradient coils during the measurement.

Disclosure of Invention

It is an object of the present invention to disclose a new design for a dedicated MR scanner structure, in particular for imaging a specific organ or body part, such as the dentition of a patient, which on the one hand provides a better gradient field and on the other hand generates a lower acoustic noise level.

This object is achieved by a magnet system, a gradient system and a magnetic resonance imaging system according to the invention.

The magnet system for a magnetic resonance imaging system according to the invention comprises a basic field magnet and a gradient system (with gradient coils). These gradient coils are preferably pairs of coils of a gradient system, which are used to generate gradient fields for the axes of the gradient system. It is important for the effect of the invention that the coils of the gradient system are located outside the region of the predefined basic magnetic field of the basic field magnet. It is also clear that the basic field magnet is designed to apply a predetermined basic magnetic field in a predetermined examination zone during an examination, and that the gradient coils are designed and positioned to apply a gradient field in this examination zone during this examination. However, not all coils of the gradient system have to be outside the region of the basic magnetic field, at least the gradient coils should be arranged outside.

The magnet system for a magnetic resonance imaging system according to the invention actually comprises a plurality of pairs of gradient coils. This means that there are two gradient coils for each gradient axis. Typically, the gradient system creates a gradient in the X, Y and Z directions to achieve a gradient profile with X, Y and Z components. Therefore, it is preferable that there are a pair of X gradient coils, a pair of Y gradient coils, and a pair of Z gradient coils. It is clear that two gradient coils designed to generate the same gradient (and arranged on opposite sides of the examination zone) form a pair. The gradient coils (i.e., X, Y and/or Z gradient coils) in each pair are arranged on opposite sides of the examination region. The examination region is the region within which the imaging process takes place, i.e. the volume surrounding the field of view (FOV) of the scanner.

It is certain that the gradient system may also comprise further components which are also comprised by prior art gradient systems for optimal operation. These components are, for example, dedicated gradient power amplifiers or holding structures for each axis GPAx, GPAy and GPAz. Furthermore, the gradient system may comprise further coils, such as shim coils, coils generating a non-linear encoding field or dynamic field cycling coils for multi-dimensional spatial signal encoding and accelerating signal acquisition. Although it is preferred that at least the gradient coils are arranged outside the basic magnetic field, it is also preferred that the other coils are also arranged outside the basic magnetic field, since their wires carry the strongest and fast switching currents.

The "predefined magnetic field" of the basic field magnet is the field used for measurement. Since the magnetic field always circulates in a closed path (and extends to infinity), it is not easy to define the inside and outside of the magnetic field if the magnetic field is observed carefully. However, a person skilled in the art of magnetic resonance imaging can easily divide the basic magnetic field into a main magnetic field which is located in the examination zone and is used for measurements and a stray field which is located outside the examination zone. The "main magnetic field" is the above-mentioned predefined basic magnetic field. The other part of the magnetic field will be designated as "stray field".

The expression "external" means that at least the parts of the basic magnetic field used for the measurement (the field lines that pass through the examination region/imaging volume) do not penetrate the corresponding gradient coil. The respective gradient coils are therefore arranged such that the field lines, which preferably directly relate to the measured basic magnetic field (main magnetic field), do not pass through these gradient coils. Depending on the shape of the basic field magnet, it can also be said that the corresponding gradient coil is not arranged in the space between the basic field magnet generator and the examination region. Alternatively, seen from the C-shaped basic field magnet, it can be said that at least one pole shoe in each polarity of the magnet is located within the gradient coil, or the pole shoes are located between the gradient coils (depending on the shape of the gradient coils). When observing the solenoid basic field magnet, the gradient coil is arranged outside the solenoid coil. Looking at the stray field, it can be said that the respective gradient coil is arranged in the stray field. Observing the absolute value of the field strength, "external" means that the respective gradient coil is arranged in a region in which the (predefined) field strength of the (stray) field applied by the basic field magnet generator is less than 10% of the (predefined) field strength in the examination region or 10%, preferably less than 1%, in particular less than 0%, of the maximum field strength of the basic field magnet.

The above-described arrangement of the coils of the gradient system has the advantage that the basic field magnet can be designed smaller, which has a positive effect on the homogeneity of the basic magnetic field and on the cost of the magnet. Furthermore, the level of acoustic noise generated during the examination is reduced. The level of noise occurring during the examination is proportional to the lorentz forces acting on the gradient coil conductors, which can be described by the formula F q · v × B. (F is the force on the conductors of the gradient coil, q · v is the current flowing through the conductors of the gradient coil, B is the strength where the conductors of the gradient coil are perpendicular to the magnetic field).

In the case of an open magnetic resonance scanner with pole pieces (e.g. C-shaped magnets), the basic magnetic field is located between the pole pieces. A solution to this problem (volume) is solved by mounting the gradient coils with an open magnetic resonance scanner outside the main magnetic field according to the invention. This also reduces the forces acting on the conductor and thereby reduces the mechanical vibrations and acoustic noise generated.

In order to keep the linearity of the magnetic gradient field as high as possible by minimizing cross product terms (maxwell terms) and higher order terms, it is necessary to increase the size of the gradient coils to at least twice the imaging volume and also to position these coils far from the imaging area and at a considerable distance. This can be achieved by the invention since the size of the gradient coils is not limited by the geometry of the basic field magnet.

In the case of gradient coils located within the magnet, as in the prior art, the enlargement of the gradient coils will result in an increase in the size of the magnet. Larger magnet sizes involve higher production costs, which are proportional to the 3 rd power of the magnet outer diameter. The larger magnet size results in an extended larger footprint of the stray field (this is the 5G safety profile). The larger footprint means higher installation costs at the customer site and limited clinical workflow due to safety regulations. Thus, the present invention has advantageous effects in many aspects of the MRI system.

Thus, the new architecture places the gradient coil pairs outside the basic field magnet, which has the main benefit that the linearity of the generated magnetic gradient field can be maximized at least inside the FOV, while the geometry of the magnet can be optimized to fit the target anatomy. Thereby, the size and the resulting cost of the main magnet (basic field magnet) can be reduced considerably for the same spatial extension of the imaging volume (examination zone). Since the prior art places the gradient coils inside the magnet ring, the size and cost of the magnet is increased. In addition, the present invention allows medical personnel to better access the patient for clinical intervention and treatment procedures.

The gradient system according to the invention for the magnet system according to the invention comprises coils of the gradient system which are designed to be arranged outside a predetermined region of the magnetic field of the basic field magnet.

The magnetic resonance imaging system according to the invention comprises a magnet system according to the invention. The preferred MR scanner architecture is particularly suitable for MR imaging of dedicated body parts or organs of a patient (human or animal). Since the gradient system is arranged outside the basic magnetic field, the size of the basic field magnet may be selected depending on the organ to be imaged with the system. The magnetic resonance imaging system is preferably designed for examination of the head, prostate, limbs, animals, neonates, preferably for dental imaging and/or for examination of body parts of a group comprising the brain, wrist, elbow, foot, ankle, knee, breast and prostate of a patient.

Preferably, the magnetic resonance imaging system comprises: at least one examination region, preferably two or more examination regions; and a gradient system according to the invention for at least one examination zone. A preferred MRI scanner of such a magnetic resonance imaging system comprises an inclined arrangement, e.g. a star arrangement, of basic field magnets. MRI scanners with a ring-shaped MR scanner architecture are particularly preferred. In a star arrangement of basic field magnets with a toroidal magnetic field, the front side of the gradient system should be directed outside the toroidal magnetic field.

Particularly advantageous embodiments and features of the invention are given by the dependent claims, as disclosed in the following description. Features from different claim categories may be combined as appropriate to give other embodiments not described herein.

According to a preferred magnet system, the coils located outside the region of the predetermined basic magnetic field of the basic field magnet are gradient coils, in particular for all three coordinate axes. Alternatively or additionally, the coils located outside the region of the predefined basic magnetic field of the basic field magnet are shim coils and/or coils generating a non-linear encoding field and/or dynamic field cycling coils for multi-dimensional spatial signal encoding and accelerating signal acquisition. Preferably, the additional coil is arranged parallel to the gradient coil, in particular wherein the gradient system comprises a coil block comprising the gradient coil and the further coil. Since the gradient coil is arranged outside the basic magnetic field between the two magnetic shoes, there will be a greatly reduced noise emission which may be reduced below the hearing threshold depending on the arrangement of the gradient coil outside the basic magnetic field.

According to a preferred magnet system, the basic field magnets are C-shaped magnets, and the coils of the gradient system, in particular the gradient coils, are arranged in the region of the pole shoes of the basic field magnets, or such that the pole shoes of the basic field magnets are located between two of these coils.

According to a preferred magnet system, the basic field magnet has an annular field distribution and preferably has two or more examination zones. Preferably, the basic field magnet coils are arranged in a star shape. One advantage of using a ring magnet system for an MRI scanner is that such a magnet configuration minimizes stray fields and eliminates the need for active shielding coils, which makes these magnets also more efficient and cost effective. This allows for a compact positioning of the MRI scanners, which can be directly in the doctor's office without having to install them in a separate examination room.

Thus, according to a preferred embodiment, the basic field magnet arrangement comprises at least one set of basic field magnet segments (coils or coil sets) arranged in a star around at least one spatial axis, wherein the side walls or edges of the respective basic field magnet segments point towards this central axis. This arrangement is preferably rotationally symmetrical, wherein a rotational symmetry of 360 °/N is particularly preferred for the N basic field magnet segments (of a group). In the case of six basic field magnet segments, the basic field magnet arrangement would, for example, resemble a hexagram. However, the star-shape may also comprise another (partial) regular arrangement of magnet segments, e.g. the basic field magnet segments are all regularly arranged within a semicircle. The arrangement of several of these partly regular stars around several central or spatial axes, for example two semi-circular arrangements slightly spaced apart, form a whole to generate the above-mentioned basic magnetic field in the form of a circular ring with an interposed straight channel.

According to a preferred magnet system, the basic field magnet comprises a (ferrous) yoke, and the coils of the gradient system are arranged such that their fields couple into the yoke. Therefore, both the basic field magnet and the coils of the gradient system preferably use the same yoke. Preferably, the distance between the yoke and each respective coil of the gradient system (arranged outside the basic magnetic field) is less than 2cm, particularly preferably wherein the respective coil is in contact with the yoke. Due to this small gap, the magnetic field coupling into the yoke is more efficient than if there were a large gap between the coil and the yoke.

According to a preferred magnet system, the coils of the gradient system are planar coils, preferably arranged in parallel. The parallel arrangement may be an advantageous architecture allowing a better linearity of the gradient field given relaxed design constraints, compared to a V-shaped arrangement. These degrees of freedom may further be used to minimize the peripheral nerve stimulation effect and the generation of undesirable eddy currents. According to a preferred gradient system, the gradient coils are biplane gradient coils. This has the advantage that the gradient system does not require much space. Preferably, the gradient system comprises two or three pairs of gradient coils (e.g. X, Y and Z gradient coils), wherein all pairs of gradient coils are arranged at the same angle to each other, i.e. the angle between the central planes is equal.

According to a preferred magnet system, the coils of the gradient system are placed symmetrically with respect to the examination region inside the predefined basic magnetic field of the basic field magnet. Further preferably, the individual coils are arranged at a distance from each other such that the object to be examined or the examination region fits between the individual coils. For example, in the particular case of dental imaging, such a solution allows the head of the patient to fit inside the space between the gradient coils.

According to a preferred magnet system, the coils of the gradient system are mechanically decoupled from the magnets, preferably wherein the coils are movable relative to the basic field magnet, in particular by an actuator arm. In a preferred geometry, the gradient coils are mounted such that they can be rotated through an angle about the Z-axis. This allows the gradient coils to be fitted closer to the head and avoids mechanical collisions with the patient's shoulders. For example, the gradient coils are mechanically decoupled from the magnets and can be independently raised by the actuator arms during non-imaging times and repositioned away from the patient's body. This approach provides medical personnel with better access to the patient, as surgical or non-surgical dental or orthopedic procedures may often require such access.

According to a preferred magnet system, the basic field magnets are arranged between coils of the gradient system, wherein the distance between these coils is preferably larger than the size of the basic field magnets. This allows for a minimum size of the basic field magnet.

According to a preferred magnet system, the basic field magnet is shaped such that it fits the target anatomy, preferably wherein the basic field magnet is shaped like an open segment of a torus or cylinder for examining a limb, or like a bicycle saddle or saddle for prostate examination. With a gradient system external to the basic field magnet, the scanner architecture can be easily modified to suit other body parts and internal organs, for example for imaging the wrist, elbow, knee, foot, ankle, female breast or male prostate. The new structure will also be suitable for scanners dedicated to imaging various small animals for veterinary applications. The scanner structure can also be used for imaging neonates, with the main advantage that only weak acoustic noise is generated by the operation of the gradient coils.

An advantage of all these various scanner configurations is that the magnet for generating the static magnetic field can be made as small as possible in size (and thus with the lowest possible cost and with the smallest footprint with respect to stray fields) and shaped so as to best fit the target anatomy, while the gradient coil system can be larger and designed so as to allow easy access to the patient's body for interventional procedures and to allow unrestricted positioning of the patient within the scanner.

According to a preferred magnet system, the gradient system comprises a cooling system which is also arranged outside the region of the predefined basic magnetic field of the basic field magnet. Furthermore, the invention provides the general advantage that the gradient coil does not have to be absolutely shielded, which makes the gradient coil more efficient and the heat dissipation lower. However, forced cooling of the gradient coils may be advantageous. For the above embodiments, the cooling scheme would neither mechanically interfere with the basic field magnet nor limit its geometry. With the invention, and in particular this embodiment, the heat generated in the gradient coil is largely not transferred to the magnet, resulting in a thermal drift of the magnetic field strength or its spatial distribution, while the heat source in the gradient coil will not be positioned close to or radiating towards the patient's body.

According to a preferred magnet system, the gradient coils are pivotably arranged, in particular such that they can be moved like a door, preferably wherein the gradient coils are planar coils, in particular comprising a full face cover, such that they are designed to act as louvers. This is particularly advantageous for a star arrangement of basic field magnets or other arrangements with a plurality of examination zones, since it may be necessary to arrange the coils of the gradient system for the first examination zone in front of the second examination zone. These coils may block the second examination region and should be openable like a door opening the second examination region. However, when the coils are used like a door, they may also act as louvers blocking the view into the second examination region. The star-shaped arrangement of the examination zone may be surrounded by coils of a gradient system, which all may serve as doors and/or blinds in addition to their technical use.

Since the coils of the gradient system are located outside the basic field magnet and their orientation may be limited by the mechanical constraints of the magnetic resonance imaging system, it may happen that their windings have to be adapted in order to generate an optimal gradient field.

According to a preferred gradient system, the gradient coil is formed by a plurality of conductor loops. It is obvious that preferably only one long conductor is wound into a plurality of loops, however, open loops (open loops) connecting to each other may also occur. In the following, the loops of the coil are designated as "set of loops", wherein any reference to the movement action is meant to be understood as a change of the subsequent loop. The following design is preferred (alternatively or additionally).

The loops of the gradient coil for the X-gradient comprise two sets of counter-rotating loops adjacent in the X-direction, preferably wherein the radius of one set of loops increases while the outer conductor in the X-direction (front and back) remains substantially at the side of the gradient coil. This means that the shape of such a coil is reminiscent of the shape of a butterfly.

The loops of the gradient coil for the Y-gradient comprise two sets of counter-rotating loops adjacent in the Y-direction, preferably wherein the radius of one set of loops increases while the outer conductor in the Y-direction remains substantially at the sides of the gradient coil (sides perpendicular to the front and back sides). This means that the coil can be seen as a coil for an X-gradient rotated only 90 °.

The loops of the gradient coil for the Z-gradient comprise an incremental set of loops, preferably wherein the center of the loops remains substantially at the center of the gradient coil. This means that the coils can be coaxial but become larger at least in the X-direction.

The specific design of the more preferred coil is described below. This special design produces a field that increases in the direction of the V-shaped mouth of the gradient system (to the front side).

With regard to the gradient coil for the X-gradient, the distance of the field-dependent conductors of a set of loops steadily decreases at least in the direction of the aperture of the V-shape of the gradient system (in the direction to the front side). The field dependent conductors are those parts of the loop of the magnetic field that determine the gradient.

With regard to the gradient coil for the Z-gradient, the distance of the field-dependent conductors of a set of loops steadily decreases at least in the direction of the aperture of the V-shape of the gradient system (in the direction to the front side).

With regard to the gradient coil for the Y-gradient, the radius of the set of loops increases in the X-direction as well as in the Y-direction, and the outer conductor at the V-shaped aperture remains substantially at the sides of the aperture (i.e., at the front side) as well as at the sides perpendicular to the front side.

Various hardware or software tools may be used to further fine-tune these lead patterns in order to satisfy some additional constraints (e.g., gradient linearity) to reduce the magnitude of stray fields, mechanical vibrations, and the level of acoustic noise or peripheral nerve stimulation.

For X and Z gradient coils, the particular loop shaping described can eliminate the nonlinear component along the X-axis by modifying the line spacing along the X-axis from a constant spacing to a more quadratic (quadratic) spacing, where the line density increases approximately quadratically with radial distance from the axis of symmetry (e.g., the axis of symmetry of the annular basic magnet). For a Y-gradient coil, the exemplary solution adds an additional wire distribution with constant spacing along the X-axis. This is similar to the wire pattern of the magnet coils used to generate the static magnetic field B0.

The preferred method has the additional advantageous effect that it can be used to compensate for stray magnetic fields in a magnetic resonance imaging system having two or more examination zones. The preferred method comprises the steps of:

providing values of a predefined gradient field to be applied in the first examination zone in addition to the basic magnetic field,

providing a predefined sequence of control pulses to be applied in a second examination region (in particular adjacent to the first examination region, since the effect is strongest in the adjacent region),

determining stray magnetic fields in a second examination zone with application of a gradient field in the first examination zone,

calculating a compensation sequence control pulse for the second examination zone from the predefined sequence control pulse and the determined stray magnetic field, wherein the compensation sequence control pulse is calculated such that measurements in the second examination zone can be performed irrespective of stray fields,

-applying a compensation sequence control pulse to the second examination region, and

preferably, these steps are repeated for other examination zones, in particular for all examination zones.

The values for the gradient field applied in the first examination zone are known. When applied in a first examination region, the gradient field generates a stray field in the other examination region.

The stray field influences the measurement in the second examination region. If the second examination zone is adjacent to the first examination zone (wherein this is preferred because the stray field is strongest in the adjacent zone), the stray magnetic field will disturb the measurement seriously. For the measurement, a predefined sequence of control pulses is applied in the second examination zone, wherein the predefined sequence of control pulses is preferably a predefined second gradient field. Since the stray field affects the measurement with the sequence of control pulses, the sequence of control pulses is adjusted to the stray field with the following steps.

The information defining the predefined sequence of control pulses is data about the strength and direction of the sequence of control pulses. Since there are defined gradient magnet coils in an MRI system, the data may include information about the signal amplitude or current and the antenna or coil applying the signal.

It should be noted that in all examination regions of the MRI system, the influence of stray magnetic fields should be compensated for. Thus, preferably, the values of the predefined sequence of control pulses of all examination regions should be provided and the method should be performed for all examination regions while considering any examination region as a first region and any other examination region as a second examination region.

Before, during or after any information on the predefined sequence of control pulses is provided, the stray magnetic fields in the second examination zone are determined, for example, their direction and their strength (magnetic field vector). This is the stray field of the gradient field. This step can be achieved by calculation or by measuring the stray magnetic field.

For example, a gradient field may be applied in a first examination region, and stray magnetic fields (e.g., different currents for an induced gradient field) may be measured in a second examination region. For the case of application of a gradient field (with a predefined current) in the first examination zone, this measurement value can be stored and used for determining the stray magnetic field in the second examination zone. However, if the properties of the MRI scanner are well known, the magnetic field may also be calculated (e.g. in simulations). Finally, for a set of identical MRI scanners, a set of stored values may be used to make the determination.

Using the determined stray magnetic field and the provided (predefined) sequence control pulse, a compensation sequence control pulse can be calculated for the second examination zone. The compensation sequence control pulse may be determined directly or a correction term may be calculated and added to/subtracted from the predefined sequence control pulse. Since the direction of the predefined sequence of control pulses and the stray field may be important, the resulting compensation vector is preferably calculated from the vector representing the predefined sequence of control pulses and the correction vector (based on the stray field).

Thereafter, a compensation sequence control pulse is applied to the second examination region. This application is well known and applies a compensating sequence control pulse rather than a predefined sequence control pulse.

The solution of the invention allows an active compensation of stray gradient fields at least in the first order. By this compensation, images in different examination zones can be acquired simultaneously and independently, wherein in each examination zone a dedicated triaxial gradient system can be operated. If the target field of view is not too large and the active shielding of the gradient coils is quite effective, the compensation of stray fields up to the first order is good enough. However, this method can be extended to correct for higher order stray fields. This would preferably require a set of dynamic higher order shimming coils and associated coil current amplifiers and correspondingly a larger sensitivity matrix for inversion. The higher order compensation is further described below.

Although the invention is very advantageous for a star-shaped magnet arrangement, it is also advantageous for other MRT systems having an arrangement, for example a linear arrangement of the examination zone or an arrangement of the "satellite examination zone" using the basic magnetic field of the central examination zone.

According to a preferred magnetic resonance imaging system, the magnetic resonance imaging system is designed such that the gradient systems in the examination zone (preferably in each examination zone) operate asynchronously and/or independently of the gradient systems in another examination zone of the magnetic resonance imaging system.

Preferably, the gradient system comprises a central control unit designed to coordinate all gradient activities, preferably the independent operation of the different gradient systems, in particular even the minimization and/or correction of cross interference terms between the gradient systems. The term "independently operated" means that the MR sequences running in the examination region are not necessarily identical or synchronized or interleaved.

The magnet structure according to the invention comprises the advantage that the gradient coil is acoustically quiet, in particular in combination with a ring magnet having a relatively weak stray magnetic field outside the ring surface. This is due to the fact that, unlike the prior art which places the gradient coils in the region of a strong magnetic field, the new solution exposes the wires of the gradient coils to a significantly weaker static magnetic field, i.e. to virtually only stray fields. The faraday forces acting on these wires operated by high current pulses are therefore significantly lower, with mechanical vibrations and the resulting acoustic noise having a much lower amplitude.

Furthermore, advantageously, the size of the coils of the gradient system is no longer limited, so that gradient coils for all three coordinate axes as well as further coils generating non-linear encoding fields or dynamic field cycling coils for multi-dimensional spatial signal encoding and accelerated signal acquisition can be realized.

Drawings

Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.

Figure 1 shows a simplified MRI system of the prior art.

Figure 2 shows a prior art simplified C-shaped MRI scanner.

Figure 3 shows an examination in a prior art C-shaped MRI scanner.

Fig. 4 shows the arrangement of an RF coil in a basic magnetic field according to the prior art.

FIG. 5 shows the fields of an exemplary pair of z-gradient coils.

Figure 6 shows an arrangement of gradient coils according to the invention.

Figure 7 shows an arrangement of gradient coils according to the invention.

Fig. 8 shows an arrangement of a gradient system with open basic field magnets according to the invention.

Fig. 9 shows an arrangement of a movable gradient system according to the invention.

Fig. 10 shows an arrangement of a gradient system according to the invention in combination with a yoke of a basic field magnet.

Fig. 11 shows a star-shaped basic field magnet arrangement with a gradient system according to the invention.

Fig. 12 shows the basic field magnet arrangement of the gradient system of fig. 11 seen from above.

In the drawings, like reference numerals refer to like elements throughout. The objects in the drawings are not necessarily to scale.

Detailed Description

Fig. 1 shows a schematic representation of a magnetic resonance imaging system 1 ("MRI system"). The MRI system 1 comprises a real magnetic resonance scanner (data acquisition unit) 2 with an examination space 3 or a patient tunnel, in which a patient or a test person is located on a drive couch 8 in the examination space 3 or patient tunnel, in which a real examination object O is located.

The magnetic resonance scanner 2 is typically equipped with a basic field magnet system 4, a gradient system 6, and an RF transmit antenna system 5 and an RF receive antenna system 7. In the exemplary embodiment shown, the RF transmit antenna system 5 is a whole-body coil (whole-body coil) which is permanently mounted in the magnetic resonance scanner 2, in contrast to the RF receive antenna system 7 which is formed as a local coil (here represented by only a single local coil) to be arranged on the patient or test object. In principle, however, the whole-body coil can also be used as an RF receiving antenna system, and the local coils can be switched to different operating modes, respectively.

The basic field magnet system is typically designed such that it generates a basic magnetic field in the longitudinal direction of the patient (i.e. along the longitudinal axis of the magnetic resonance scanner 2 running in the z direction). The gradient system 6 typically comprises individually controllable gradient coils 6x, 6y, 6z (see the following figures) to be able to switch (activate) the gradients independently of each other in the x-, y-or z-direction.

The MRI system 1 shown here is a whole-body system with a patient tunnel into which the patient can be introduced completely. In principle, however, the invention may also be used for other MRI systems, such as C-shaped housings with a lateral opening, and smaller magnetic resonance scanners in which only one body part can be placed.

Furthermore, the MRI system 1 has a central control device 13 for controlling the MRI system 1. The central control device 13 comprises a sequence control unit 14 for measurement sequence control. With the sequence control unit 14, the sequence of radio frequency pulses (RF pulses) and gradient pulses can be controlled depending on the selected pulse sequence.

For outputting the individual RF pulses of the pulse sequence, the central control device 13 has a radio frequency transmission device 15, which radio frequency transmission device 15 generates and amplifies the RF pulses and feeds them into the RF transmission antenna system 5 via a suitable interface (not shown in detail). For controlling the gradient coils of the gradient system 6, the control device 13 has a gradient system interface 16. The sequence control unit 14 communicates with the radio frequency transmission device 15 and the gradient system interface 16 in a suitable manner to transmit the pulse sequence.

Furthermore, the control device 13 has a radio frequency receiving device 17 (likewise in communication with the sequence control unit 14 in a suitable manner) in order to acquire magnetic resonance signals (i.e. raw data) for the respective measurements, which are received in a coordinated manner from the RF receiving antenna system 7 over the range of the pulse sequence.

A reconstruction unit 18 receives the acquired raw data and reconstructs magnetic resonance image data therefrom for measurement. The reconstruction is typically performed based on parameters that may be specified in the respective measurement protocol or control protocol. The image data may then be stored in the memory 19, for example.

The operation of the central control device 13 may be performed via a terminal 10 having an input unit and a display unit 9, and therefore, the entire MRI system 1 may also be operated by an operator via the terminal 10. The MR images may also be displayed at the display unit 9 and measurements may be planned and started by means of an input unit (which may be integrated with the display unit 9), and in particular a suitable control protocol may be selected (and may be modified) with a suitable series of pulse sequences PS as described above.

The MRI system 1 may have a number of additional components, not shown in detail but usually present in such systems, such as a network interface, in order to connect the entire system with a network and to be able to exchange raw data and/or image data or parameter maps, respectively, but also other data (e.g. patient-related data or control protocols).

The manner in which suitable raw data are obtained by transmission of RF pulses and generation of gradient fields and MR images are reconstructed from the raw data is known to the person skilled in the art and therefore need not be explained in detail herein.

Figure 2 shows a simplified C-shaped MRI scanner 2 of the prior art. The general setup of a corresponding MRI system is similar to that of fig. 1, except that the scanner 2 now comprises a C-shaped basic field magnet 4 as shown in the figure. A theoretical coordinate system is shown in which the z-axis points in the direction of the basic magnetic field B0, and the x-axis and the y-axis are perpendicular to each other and to the z-axis.

A body part of the patient (see e.g. fig. 3) is arranged in the gap between the two magnetic shoes 4A of the basic field magnet 4. The patient can lie on the bed 8 or stand upright as can be taken from fig. 3.

Fig. 3 shows an examination in a C-shaped basic field magnet 4 of a prior art MRI scanner 2 (see e.g. fig. 2). The scanner structure uses a planar V-shaped gradient system 6.

This solution severely limits the space for mounting the gradient system 6 within the toroid of the basic field magnet 4, which is in line with its magnet coils 4b, and the acoustic noise level is high.

Furthermore, the achievable linearity of the gradient field over the extension of the imaging area containing the dental arch creates problems, since the gradient system 6 is arranged very close to the field of view (FOV), and the size of the gradient system 6 is limited by the need to keep the magnet size as small as possible: or the maxilla or the mandible or both.

Fig. 4 shows the arrangement of an RF coil in a basic magnetic field B0 according to the prior art (see also fig. 1). The object O to be examined is surrounded by coils of an RF transmit antenna system 5, which is also used as an RF receive antenna system 7. The RF signal is defined by the sequence control unit 14 and applied by the radio frequency transmission device 15. The resulting signal to be measured is recorded by the radio frequency receiving device 17 and reconstructed by the reconstruction unit 18.

Fig. 5 shows the gradient field G of an exemplary pair of Z-gradient coils 6Z. The local magnetic field direction is shown by the arrows, while the field strength is indicated by the magnitude of these arrows. It is clear that maxwell's physical laws prohibit the realization of ideal parallel and linear gradient fields, especially in those spatial regions that are located very close to the gradient coil 6 z.

Fig. 6 shows an arrangement of gradient coils 6z according to the invention. A pair of gradient coils 6z are arranged around the magnetic shoes 4a (see, for example, fig. 2) of the C-shaped basic field magnet 4. Between the two gradient coils 6Z, a gradient field G is shown which is effective along the Z-axis Z, wherein direction and intensity are indicated by arrows. By observing the field more accurately, its field strength distribution will not be perfectly linear, but contain Maxwell terms and higher order nonlinear terms as depicted in FIG. 5.

Since the gradient coil 6z is arranged outside the basic magnetic field between the two magnetic shoes 4, there will be a greatly reduced noise emission which can be reduced below the hearing threshold depending on the arrangement of the gradient coil 6z outside the basic magnetic field.

Figure 7 shows an arrangement of gradient coils according to the invention. The gradient coils 6x are arranged outside the basic magnetic field, similarly to fig. 6, with the only difference that the two gradient coils 6z generate a gradient field G (indicated with arrows) effective along the x-axis x.

If the arrangement were to be rotated 90 deg. around the z-axis z, the gradient coils would generate a gradient field effective along the y-axis y.

Fig. 8 shows an arrangement of a gradient system 6 according to the invention with an open basic field magnet 4. The gradient coil pairs of the gradient system 6, in particular the gradient coil pairs for generating gradient fields of all three axes x, y, z, are arranged outside the basic magnetic field B0 of the basic field magnet 4. This has the main benefit that the linearity of the generated gradient magnetic field can be maximized at least within the examination zone E (respectively inside the FOV), while the geometry of the magnet can be optimized to fit the target anatomy. Thus, the size and the resulting cost of the basic field magnet 4 can be significantly reduced for the same spatial extension of the examination zone E. In addition, noise emissions are also greatly reduced.

The gradient system 6 uses a pair of planar gradient coils, preferably arranged in parallel. The parallel coil arrangement is here an advantageous architecture to achieve better linearity of the gradient field given relaxed design constraints, compared to a V-shaped arrangement. These degrees of freedom may further be used to minimize the peripheral nerve stimulation effect and the generation of undesirable eddy currents.

Fig. 9 shows an arrangement of a movable gradient system 6 according to the invention. The gradient system uses planar gradient coils, while the basic field magnets are not shown here due to the sharper image. An arrangement of basic field magnets 4 as shown in fig. 8 can be envisaged here.

The gradient coil pairs of the gradient system 6 are symmetrically placed with respect to the examination zone E and at a distance therefrom. In the particular case of dental imaging, this solution allows the head of the patient as the examination object O to fit in the space between the coils of the gradient system 6.

In the advantageous geometry shown, the gradient coils are held by a positioning unit 20, which positioning unit 20 comprises a fork-shaped actuator arm 21 with a motor 22 at the end of the fork, wherein the coils of the gradient system 6 are mounted such that they can be rotated through an angle about the z-axis z. Preferably, the coils of the gradient system can also be displaced from each other along the Z-axis Z. This allows adapting the gradient system 6 closer to the head and avoiding mechanical collisions with the patient's shoulders.

Preferably, the gradient coils are mechanically decoupled from the magnets and can be independently lifted out by the actuator arms 21 during non-imaging times and repositioned away from the patient's body. This approach further improves the access opportunities for medical personnel to the patient, which is often required for surgical or non-surgical dental or orthopedic procedures. The actuator arm (21) may be made of a soft magnetic material of high permeability to act as a yoke for the gradient field. This will increase the efficiency of the gradient coil, thereby reducing the peak current, thermal development and size and cost of the gradient system 6.

Fig. 10 shows the arrangement of the gradient system 6 according to the invention in combination with the yoke 4c of the basic field magnet 4. The basic field magnet 4 with the basic field magnet coil 4B in the middle of the C-shaped yoke 4C is equipped with a gradient system 6 outside the basic magnetic field B0. The gradient system 6, like the basic field magnet coil 4B, also uses the ferromagnetic yoke 4c to direct magnetic flux lines to the examination region E. The basic magnetic field B0 is generated by the current flowing into the basic field magnet coil 4B. The coils of the gradient system 6 reuse the same ferromagnetic yoke 4c to effectively direct and concentrate the gradient magnetic field into the examination region E.

Fig. 11 shows an arrangement of star-shaped basic field magnets 4, wherein the gradient system 6 is arranged outside the region in which the basic magnetic field is applied during the examination. To be more precise with respect to the expression "external", the coils of the gradient system 6 are arranged such that they are positioned in the region of a significantly reduced magnetic field (stray field region) such that they are technically arranged outside the basic magnetic field for examination.

The resulting magnetic resonance imaging system 1 comprises up to six examination zones E, each equipped with a gradient system 6. The star-shaped arrangement of the coils of the basic field magnet 4 generates a ring-shaped basic magnetic field.

Fig. 12 is a schematic diagram showing the apparatus of fig. 11 from above with two gradient fields G (dashed lines) applied.

For easy patient access, the gradient coils may be pivotably arranged such that they can be opened like a door to the examination area. This has the advantage that the gradient system 6 can also be used as a louver.

Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For the sake of clarity, it is to be understood that the use of "a" or "an" in this application does not exclude a plurality, and "comprising" does not exclude other steps or elements. Reference to "a unit" or "a device" does not preclude the use of more than one unit or device.