WO2009124873A1

WO2009124873A1 - Gradient system for a magnetic resonance apparatus

Info

Publication number: WO2009124873A1
Application number: PCT/EP2009/053929
Authority: WO
Inventors: Florian Fidler; Stefan Wintzheimer; Michael Ledwig
Original assignee: Mrb Forschungszentrum Für Magnet – Resonanz - Bayern E.V.
Priority date: 2008-04-10
Filing date: 2009-04-02
Publication date: 2009-10-15
Also published as: EP2265969A1; DE102008018265A1; DE102008018265B4

Abstract

The present invention relates to a gradient system (100) for a magnetic resonance apparatus. The gradient system (100) comprises a set of coils (S1a - S9a, S1b - S9b) configured to generate, according to different adjustment specifications, both a linear gradient field and a higher order gradient field, wherein at least one of the coils employable for generating the linear gradient field is also employable for generating the higher order gradient field.

Description

GRADIENT SYSTEM FOR A MAGNETIC RESONANCE APPARATUS

Description

The present invention relates to a gradient system and a method for generating a gradient field, which may specifically be employed in a magnetic resonance apparatus.

As a rule, a device for magnetic resonance imaging consists of four components.

A magnet generates a static, homogeneous magnetic field across the measurement volume. In typical devices, such magnets are embodied as a permanent magnet, an electromagnet or current-charged supra conducting coils. To an essential degree, the homogeneity achieved by the magnet depends on the individual structure of the magnet. In general, the direction of a stationary magnetic field is designated as the z component in an orthogonal coordinate system. The other components are referred to as x and y.

A radio frequency transceiver generates a magnetic field, which rotates at radio frequency, across the sample and may thus excite the sample by selecting an appropriate frequency. Equally, it may detect the rotating magnetic fields generated by the sample.

A gradient system generates a switchable magnetic field, the direction of which corresponds to that of the static magnetic field. The intensity of the magnetic field switched varies across the measurement volume. As a rule, this alteration is a linear alteration over the location along a space axis. The gradient system serves for generating main magnetic field gradient fields, the component in the z direction of which depends on the three coordinates G_xX, G_yy and G_zz . Here, G_x, G_y und G_z are given constants, which is why the reference here is to linear gradients. The independent combination of three gradients changing their intensities along three orthogonal space axes allows the generation of an arbitrary gradient direction as a combination. With the help of these gradients, a spatial allocation of the signal received may be effected.

A central control unit conducts the time flow of the measurement and the processing of the signals received.

Magnetic resonance tomographs generate a homogeneous magnetic field across the measurement volume. The quality of this homogeneity is crucial for the quality of the image recording. The homogeneity is on the one hand influenced by the structure of the magnet itself but on the other hand also by the magnetic properties of the sample itself. Such stationary magnetic field gradients generally have an adverse effect on the image recording.

Basically, the amplitude B of the entire magnetic field consisting of the stationary magnetic field and the magnetic field gradients may be represented mathematically by the following series:

B = B₀ + (G_xX + G_yy+ G_zz) + (Ax + By + Cz) + (Dx² + Ey² + Fz²) + ...

Here, B₀ is the amplitude of the stationary magnetic field presumed ideal. The coefficients A, B, etc are constants and describe the deviation of the real magnetic field from the magnetic field that is presumed ideally homogeneous. Here, x,y,z are spa- tial coordinates. Depending on the spatial location, the coefficients A, B, C etc contribute differently to the entire magnetic field. The coefficients may be real numbers.

Typical magnetic resonance tomographs dispose of a coil system, which is to be considered separate and consists of an assembly of three coils, each of which generates a linear gradient field in one of the orthogonal directions. Same serve to generate the linear gradient fields switched that are necessary for the image recording. If a selection of coefficient G_x = -A, coefficient G_y = - B and coefficient G_z = -C is made, this linear gradient system may serve to balance a respective linear deviation of the real magnetic field from the magnetic field presumed ideal. Thus, by a continuous deviation to the desired value of G_x from -A, G_y from - B and G_z from -C, a balancing of the stationary deviation of the magnetic field may be effected.

The magnetic field deviations described by the coefficients D, E, F etc may be generated by means of separate gradient coils. Here, the separate gradient coils may generate the respective field characteristics described by the coefficients D, E, F etc. Customarily, a system of a multitude of individual coils is employed, wherein a separate gradient coil is provided for each of the coefficients D, E, F etc.

For some of the gradient coils, which describe a very high order of the magnetic field correction, a specific enhancement may be found. Here, with the help of sub elements in an appropriate com- bination, a relatively large number of corrections may be made. In this manner, a total of 18 coefficients may be balanced by means of 15 correction coils, for example.

The patent specification DE 697 35 617 T2 presents an additional coil system, which may generate magnetic fields in addition to the linear gradient system, the magnetic fields satisfying certain equations and correcting higher than linear orders. The gradient corrections described may additionally be switched, in dependence on the time and in a manner synchronized with the linear gradients switched.

Other methods, such as the method known from the patent specification US 4,591,789, serve to correct an image distortion resulting from the magnetic field deviation, wherein a correction of the magnetic field itself is not effected.

Known approaches have in common that they exhibit substantially separate gradient coils for the imaging gradients switched and the magnetic field correction.

The correction of the magnetic field deviations is effected by means of additional coils, the designs of which each follow indi- vidual magnetic field deviations, which may be described by individual coefficients of a mathematical series expansion.

The result is that the accomplishable number of orders that can be corrected will lie in the order of magnitude of the number of additional correction coils.

This fact results in crucial drawbacks for the linear magnetic field gradients switched. Their linearity may be gained from the designs of the individual linear coils only. Deviations from the linearity are not corrected by the known approaches.

The necessity of each individual coil generating a given, often complex, field course results in drawbacks regarding controlling. This includes the linear gradients. As a result of the generally extensive line lengths and the size of the areas enclosed by the coils, there is a high inductive resistance of the individual gradient coils. Especially in the case of rapidly switched controlling by means of an electronic circuit, serious errors may occur in the magnetic field generated during the switching procedure .

It is the object of the present invention to provide an improved gradient system, an improved magnetic resonance apparatus and an improved method for generating a gradient field.

This object is achieved by a device according to claim 1, a magnetic resonance apparatus according to claim 22 and a method according to claim 24.

The present invention is based on the finding that an arbitrary gradient field may be generated by combining a plurality of single magnetic fields, each single magnetic field, taken alone, making a minor contribution to the gradient field. By means of different combinations of the single magnetic fields, different gradient fields, specifically gradient fields of different orders, may be generated with the exact same coils. According to the invention, a provision of separate gradient coils for the im- aging gradients switched and for the magnetic field correction is therefore not required. Instead, according to the invention, one and the same coil may be required for generating both a certain linear gradient field and a certain second or higher order gra- dient field. The inventive coils may be of a simple structure as they are not required to image any specific field courses, e.g. in correspondence with a magnetic field deviation.

Therefore, the inventive approach provides for a magnetic reson- ance tomograph and specifically a gradient system advantageously providing a suitable possibility of correction for the deviations described by the coefficients A, B, etc and generating the linear gradients switched necessary for the image recording and described by the coefficients G_x, G_y und G_z . Advantageously, the in- ventively employed coils may exhibit minimum inductance in a maximum generatable magnetic field.

The present invention provides a gradient system for a magnetic resonance apparatus, comprising:

a set of coils configured to generate, according to different adjustment specifications, both a linear gradient field and a higher order gradient field, wherein at least one of the coils employable for generating the linear gradient field is also employ- able for generating the higher order gradient field.

The present invention further provides a magnetic resonance apparatus with at least one gradient system according to the present invention .

Furthermore, the present invention provides a method for generating a certain gradient field for a magnetic resonance apparatus comprising a set of coils configured to generate, in combination, both a linear gradient field and a higher order gradient field, wherein at least one of the coils employable for generating the linear gradient field is also employable for generating the higher order gradient field, the method comprising: selecting a certain adjustment specification from different adjustment specifications; and

controlling the coils according to the certain adjustment speci- fication in order to generate the certain gradient field.

In the following, preferred embodiments of the present invention exemplarily explained in detail referring to the accompanying drawings, in which:

Fig. 1 shows a magnetic resonance apparatus with a gradient system according to the present invention;

Fig. 2 shows a coil assembly for a gradient system according to the present invention; and

Fig. 3 shows a set of coils for a gradient system according to the present invention.

In the subsequent description of the preferred embodiments of the present invention, like or similar reference numbers are used for the elements represented in the figures, which have a similar effect, with a repeated description of these elements being dispensed with

Fig. 1 shows a schematic representation of a magnetic resonance apparatus with a gradient system 100 according to an embodiment of the present invention.

The magnetic resonance apparatus comprises a magnet 102 for generating a static homogeneous magnetic field. The stationary homogeneous magnetic field is oriented along a z component of an orthogonal coordinate system. In a measurement volume 104, a sample to be examined may be disposed. Radio frequency transceiver means 106 may excite the sample by selection of a suitable frequency and detect magnetic fields generated by the sample. A control unit 108 is coupled to the gradient system 100 and the radio frequency transceiver means 106. The control unit 108 is configured to control the gradient system 100 and the radio frequency transceiver means 106.

The gradient system 100 is configured to generate a switchable magnetic field, the direction of which corresponds to that of the static magnetic field. For generating the switchable magnetic field, the gradient system 100 comprises a set of coils. The coils of the gradient system 100 may be arranged around the measurement volume 104. For example, the coils may be disposed on both sides of the measurement volume 104.

The arrangement of the coils of the gradient system 100 is configured to generate, in accordance with the different adjustment specifications, both a linear gradient field and a higher order gradient field. Here, each gradient field may be generated by a combination of the magnetic fields of a plurality of the coils only. A magnetic field generatable by a single one of the coils is not sufficient for generating a gradient field utilizable for the magnetic resonance apparatus. The coils are arranged such that linear gradient fields, second or higher order gradient fields, stationary magnetic fields and combinations of these fields may be generated by the exact same coils. The generatable gradient fields may exhibit different directions.

For example, the gradient system 100 may be configured to generate, by means of the set of coils, a magnetic field that may be described by the equation

B = B₀ + (G_xX + G_yy+ G_zz) + (Ax + By + Cz) + (Dx² + Ey² + Fz²) + ...

Here, BO is an amplitude of a stationary magnetic field, which may be superimposed by the stationary homogeneous magnetic field generated by the magnet 102. Thereby, a lifting or lowering of the homogeneous magnetic field may be achieved. The term (G_xx + G_yy+ G_zz) defines linear gradients for the image recording, and the term (Ax + By + Cz) defines linear gradients describing linear deviations of the magnetic field from a magnetic field presumed ideal. Non-linear deviations of the magnetic field may be balanced by higher order gradient fields. For example, the term (Dx² + Ey² + Fz²) defines second order gradients describing a square deviation. Further second order gradients are defined by xy, yz, xz, each multiplied by a coefficient. Depending on the circumstances, the inventive gradient system may be configured to generate the further gradients as exclusive or additional gradients so that arbitrary gradient fields may be generated. Depending on the circumstances, the magnetic field generatable by the gradient system 100 may, next to the second order gradients, also generate third order gradients and/or higher order gradients. In addition, the gradient system 100 may also generate a magnetic field, which may be described by individual terms or a combination of a plurality of the terms of the above equation.

In order to control the set of coils of the gradient system 100 according to the different adjustment specifications, the control unit 108 may comprise a controller configured to control the coils according to the different adjustment specifications. The different adjustment specifications may establish which of the coils of the gradient system are to be operated in which manner in order to generate the desired gradient field. According to this embodiment, for each coil, two parameters may be individually selected, that is on the one hand the direction and on the other hand the intensity of the current. Accordingly, the con- troller may be configured to adjust a direction of current flow and/or a current intensity in at least one of the coils, according to the different adjustment specifications. For example, the controller may be configured to adjust, according to a first adjustment specification, a first direction of current flow and current intensity in a first group of the coils and a second direction of current flow and current intensity in a second group of the coils. According to a second adjustment specification, the controller may be configured to adjust the second direction of current flow and current intensity in at least some of the coils of the first group and the first direction of current flow and current intensity in at least some of the coils of the second group. Thus, different gradient fields may be generated by the same coils without it being necessary that individual ones of the coils provide a specific single magnetic field that is adapted to a certain gradient field. Here, the controller may be configured to adjust the current intensities and/or current directions independently from one another. Specifically, according to the dif- ferent adjustment specifications, different current intensities and/or current directions may be adjusted in different coils. In order to be able to temporally switch the current flow and/or the current intensity in the coils, the controller may comprise switching means.

In general, the gradient system 100 may comprise a set of coils generating a magnetic field, the coils being arranged such that they are capable of generating the linear gradient fields required for the image recording. For this purpose, a current may flow, in a certain combination of the direction of current flow and the current intensity, in the coils such that they will generate the respective linear gradient. These combinations may also be used for the correction of linear magnetic field deviations.

All other combinations of directions of current flow and current intensities may be drawn upon for the correction of a deviation other than linear. This also includes, if provided for, a general lifting or lowering of the magnetic field uniformly across the measurement volume.

Fig. 2 shows an assembly of coils Sl - S9 of a gradient system 100 according to an embodiment of the present invention. The coils Sl - S9 may each be formed of an individual conductor loop, and the conductor loops may be encapsulated, for example.

The coils Sl - S9 may be arranged such in the magnetic resonance apparatus shown in Fig. 1 that every single one of the coils Sl - S9 may generate a single magnetic field each, which is oriented predominantly in parallel to the direction z of the stationary magnetic field. In an orthogonal coordinate system with the coordinates x, y, z, the coils Sl - S9 may each be arranged at a distance from one another in the x and y directions. Fig. 3 shows a technical embodiment of a planar gradient system 100 according to an embodiment of the present invention. Here, two plates with coils as shown in Fig. 2 are arranged opposite to each another. Between the plates, the measurement volume shown in Fig. 1 may be arranged. A first one of the plates includes nine coils SIa - S9a generating magnetic fields. The direction of the stationary magnetic field z may be oriented in a manner perpendicular or approximately perpendicular to the plates. Such a structure of a gradient system 100 is particularly suitable for the most commonly employed magnet designs using permanent magnets.

According to this embodiment, an arrangement of 3*3 identical individual coils SIa - S9a, SIb - S9b is located on each plate. In each of these coils, a current to be given may flow both in a switched or permanent manner and both in the clockwise and anticlockwise directions. In addition, a combination of a portion of a permanent current flow and a portion of a temporally switched current flow is also possible. The temporally switched portion may be switched on as an offset.

If an equal current flows through the coils SIb - S9b of the front side in the clockwise direction and an equal current flows through the coils SIa - S9a of the back side in the anticlockwise direction, a linear gradient in the z direction may be generated.

If an equal current flows through the coils SIa, S4a, S7a, SIb, S4b, S7b of the front and back sides in the clockwise direction and an equal current flows through the coils S3a, S6a, S9a, S3b, S6b, S9b of the front and back sides in the anticlockwise direc- tion, a linear gradient in the x direction may be generated.

If an equal current flows through the coils SIa, S2a, S3a, SIb, S2b, S3b of the front and back sides in the clockwise direction and an equal current flows through the coils S7a, S8a, S9a, S7b, S8b, S9b of the front and back sides in the anticlockwise direction, a linear gradient in the y direction may be generated. By means of individually adapting the discrete currents flowing through a single coil, the achievable linearity of the gradient field may be correspondingly optimized.

All other combinations of current intensities and directions of flow may generate linear field courses other than the ones in the x and y and z directions stated.

The number of the coils SIa - S9a, SIb - S9b shown in Fig. 3 and the arrangement thereof in the rows and columns of a matrix is exemplary only. Depending on the circumstances, a number and arrangement of the coils and accordingly adapted controlling of the coils may be chosen at will. The number of the coils used may be individually chosen and is not limited to 18 individual coils as it is shown in the embodiment. For example, more or less coils may be arranged in the columns or rows of the matrices. Alternatively, the coils may be arranged in another suitable form. A larger number of coils generally provides more possibilities.

Geometries other than the planar geometry shown in Fig. 3 may also be realized. A cylindrical embodiment is also possible, for example. Therewith, the inventive gradient system may be optimally adapted to magnet structures having a cylindrical assembly dimension for the measurement volume.

The size and shape of the discrete coils may be varied individually. In principle, there is no need for all of them to be of an identical structure. This permits individual optimization of the gradients generated.

The structure of the individual coils may be freely chosen and is not limited to one conductor loop as it is shown in the embodiment. A conductor loop may have the shape of an eight, for example. It is possible to use multiply wound conductor loops. In general, the wound conductor loops may enclose areas of different sizes. It is also possible to use other structures such as e.g. current-carrying spirals. The spirals may be embodied in two or three dimensions. For example, a spiral may be fabricated by etching several windings out of a copper foil. The coils may be arranged on a circuit board. The coils may exhibit a rectangular or any other arbitrarily shaped coil cross-section. In addition, the individual coils may overlap.

For generating the linear gradients, a combination of current intensities and directions of current flow of all individual coils may be used. Besides, the current in each individual coil may be temporally switched.

Magnetic resonance measurements not aimed at imaging may be advantageously performed by means of this coil system.

It is possible to operate the inventive gradient system indepen- dently of a conventional gradient system. Alternatively, the gradient system may be operated in addition to a conventional gradient system. It is also possible to replace gradient coils, which, in a conventional gradient system, are each adapted to specific gradient fields, by a respective inventive gradient sys- tem.

The controlling of the inventive gradient system may be configured to execute a method for generating a certain gradient field. The certain gradient field may be selected in dependence on the circumstances of the magnetic resonance apparatus as well as a measurement procedure to be effected. In a first method step, selecting a certain adjustment specification corresponding to the certain gradient field to be generated is effected. The certain adjustment specification may be selected from different adjustment specifications by the controller or may be provided to the controller. In a second method step, controlling the coils of the gradient system is effected according to the certain adjustment specification so as to generate the certain gradient field.

The embodiments described are only exemplary and may be combined with one another. In this manner, the inventive gradient system and the inventive method may be adapted to the respective circumstances . The inventive method may be implemented in hardware or in software. The implementation may be effected on a digital storage medium with electronically readable control signals, which may coo- perate such with a programmable computer system that the inventive method is effected. The intervention therefore also consists in a computer program product with a program code stored on a machine-readable carrier for performing the inventive method when the computer program product runs on a computer. Therefore, the invention may also be realized as a computer program with a program code for performing the inventive method when the computer program runs on a computer.

Claims

1. Gradient system (100) for a magnetic resonance apparatus, comprising:

a set of coils (Sl - S9; SIa - S9a, SIb - S9b) configured to generate, according to different adjustment specifications, both a linear gradient field and a higher order gradient field, wherein at least one of the coils employable for generating the linear gradient field is also employable for generating the higher order gradient field.

2. Gradient system according to claim 1, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are configured to generate a stationary magnetic field, wherein at least one of the coils employable for generating the stationary magnetic field is also employable for generating the linear gradient field or the higher order gradient field.

3. Gradient system according to any one of the preceding claims, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are configured to generate a gradient field comprising both a linear portion and at least one higher order portion.

4. Gradient system according to any one of the preceding claims, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are configured to generate a magnetic field satisfying the following equation:

B = B₀ + (G_xX + G_yy+ G_zz) + (Ax + By + Cz) + (Dx² + Ey² + Fz²) + ...

with

BO: amplitude of a stationary magnetic field,

G_x, Gy, G_z : coefficients of an image recording,

A, B, C, D, E, F, ...: constants of a magnetic field deviation.

5. Gradient system according to any one of the preceding claims, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are arranged such that the coils each generate a single magnetic field, which is oriented predominantly in parallel to a direction (z) of the stationary magnetic field of the magnetic resonance apparatus.

6. Gradient system according to any one of the preceding claims, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are arranged in a manner offset from one another with respect to the direction (z) of the stationary magnetic field.

7. Gradient system according to any one of the preceding claims, in which a first amount of the coils (SIa - S9a) and a second amount of the coils (SIb - S9b) are arranged on opposite sides each of a measurement volume (104) of the magnetic resonance apparatus .

8. Gradient system according to claim 7, with a first plate and a second plate, the first plate comprising the first amount of the coils (SIa - S9a) and the second plate comprising the second amount of the coils (SIb - S9b) .

9. Gradient system according to claims 7 or 8, in which the first amount and the second amount of the coils (SIa - S9a, SIb - S9b) are each arranged in a plurality of rows and columns of a matrix.

10. Gradient system according to any one of claims 7 to 9, in which, according to an adjustment specification for generating a linear gradient field in a first direction, the first amount of the coils (SIa - S9a) exhibits a first direction of current flow and the second amount of the coils (SIb - S9b) exhibits a second direction of current flow, the first direction of current flow being opposite to the second direction of current flow.

11. Gradient system according to claim 9, in which, according to an adjustment specification for generating a linear gradient field in a second direction, the coils (SIa, S4a, S7a, SIb, S4b, S7b) of a first column of the respective matrix exhibit the first direction of current flow and the coils (S3a, S6a, S9a, S3b, S6b, S9b) of a last column of the respective matrix exhibit the second direction of current flow, and in which, according to an adjust- ment specification for generating a linear gradient field in a third direction, the coils (SIa, S2a, S3a, SIb, S2b, S3b) of a first row of the respective matrix exhibit the first direction of current flow and the coils (S7a, S8a, S9a, S7b, S8b, S9b) of a last row of the respective matrix exhibit the second direction of current flow.

12. Gradient system according to any one of claims 1 to 7, in which the coils are arranged cylindrically .

13. Gradient system according to any one of the preceding claims, in which the coils (Sl - S9; SIa - S9a, SIb - S9b) are identically formed.

14. Gradient system according to any one of the preceding claims, in which at least one of the coils (Sl - S9; SIa - S9a, SIb - S9b) is formed as a single conductor loop.

15. Gradient system according to any one of the preceding claims, in which at least one of the coils (Sl - S9; SIa - S9a, SIb -

S9b) is formed as a conductor loop with more than one winding.

16. Gradient system according to any one of the preceding claims, in which at least one of the coils (Sl - S9; SIa - S9a, SIb - S9b) is formed as a current-carrying spiral.

17. Gradient system according to any one of the preceding claims, wherein the set of coils comprises at least four coils.

18. Gradient system according to any one of the preceding claims, with a controller (108) configured to control the coils (Sl - S9; SIa - S9a, SIb - S9b) according to the different adjustment specifications in order to generate the different gradient fields.

19. Gradient system according to claim 18, in which the controller (108) is configured to adjust a direction of current flow in at least one of the coils (Sl - S9; SIa - S9a, SIb - S9b) according to the different adjustment specifications.

20. Gradient system according to claims 18 or 19, wherein the controller (108) is configured to adjust a current intensity in at least one of the coils (Sl - S9; SIa - S9a, SIb - S9b) according to the different adjustment specifications.

21. Gradient system according to any one of claims 18 to 20, wherein the controller (108) is configured to temporally switch the current flow in the coils (Sl - S9; SIa - S9a, SIb - S9b) .

22. Magnetic resonance apparatus, with at least one gradient system (100) according to any one of the preceding claims.

23. Magnetic resonance apparatus according to claim 22, with at least one additional coil system configured to generate a specific, predefined gradient field.

24. Method for generating a certain gradient field for a magnetic resonance apparatus comprising a set of coils (Sl - S9; SIa - S9a, SIb - S9b) configured to generate, in combination, both a linear gradient field and a higher order gradient field, wherein at least one of the coils employable for generating the linear gradient field is also employable for generating the higher order gradient field, the method comprising:

selecting a certain adjustment specification from different adjustment specifications; and

25. Computer program with a program code for performing the method according to claim 24 when the computer program runs on a computer .