Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/387—Compensation of inhomogeneities
  • G01R33/3875—Compensation of inhomogeneities using correction coil assemblies, e.g. active shimming
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
  • G01R33/56572—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of a gradient magnetic field, e.g. non-linearity of a gradient magnetic field
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/3806—Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets

Definitions

• 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.
• a device for magnetic resonance imaging consists of four components.
• a magnet generates a static, homogeneous magnetic field across the measurement volume.
• magnets are embodied as a permanent magnet, an electromagnet or current-charged supra conducting coils.
• the homogeneity achieved by the magnet depends on the individual structure of the magnet.
• 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.
• 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 x X, G y y and G z z .
• 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.
• 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 B 0 + (G x X + G y y+ G z z) + (Ax + By + Cz) + (Dx 2 + Ey 2 + Fz 2 ) + ...
• B 0 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.
• x,y,z are spa- tial coordinates.
• the coefficients A, B, C etc contribute differently to the entire magnetic field.
• the coefficients may be real numbers.
• the magnetic field deviations described by the coefficients D, E, F etc may be generated by means of separate gradient coils.
• the separate gradient coils may generate the respective field characteristics described by the coefficients D, E, F etc.
• 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.
• 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.
• 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 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.
• different gradient fields specifically gradient fields of different orders, may be generated with the exact same coils.
• a provision of separate gradient coils for the im- aging gradients switched and for the magnetic field correction is therefore not required.
• one and the same coil may be required for generating both a certain linear gradient field and a certain second or higher order gra- dominant 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.
• 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 .
• 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 .
• 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
• 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
• Fig. 3 shows a set of coils for a gradient system according to the present invention.
• 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.
• 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.
• the gradient system 100 comprises a set of coils.
• the coils of the gradient system 100 may be arranged around the measurement volume 104.
• 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.
• 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.
• 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 B 0 + (G x X + G y y+ G z z) + (Ax + By + Cz) + (Dx 2 + Ey 2 + Fz 2 ) + ...
• 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 x x + G y y+ G z z) 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.
• the term (Dx 2 + Ey 2 + Fz 2 ) defines second order gradients describing a square deviation.
• Further second order gradients are defined by xy, yz, xz, each multiplied by a coefficient.
• 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.
• 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.
• 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.
• 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.
• two parameters may be individually selected, that is on the one hand the direction and on the other hand the intensity of the current.
• 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.
• 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.
• 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.
• 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.
• the controller may be configured to adjust the current intensities and/or current directions independently from one another.
• the controller may comprise switching means.
• 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.
• 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.
• 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.
• an arrangement of 3*3 identical individual coils SIa - S9a, SIb - S9b is located on each plate.
• a current to be given may flow both in a switched or permanent manner and both in the clockwise and anticlockwise directions.
• 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.
• 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.
• 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.
• the individual coils may overlap.
• 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.
• inventive gradient system indepen- dently of a conventional gradient system.
• the gradient system may be operated in addition to a conventional gradient system.
• 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.
• 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.
• controlling the coils of the gradient system is effected according to the certain adjustment specification so as to generate the certain gradient field.
• 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.

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Health & Medical Sciences (AREA)
• General Health & Medical Sciences (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Engineering & Computer Science (AREA)
• Signal Processing (AREA)
• High Energy & Nuclear Physics (AREA)
• Magnetic Resonance Imaging Apparatus (AREA)

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• 2008
  • 2008-04-10 DE DE200810018265 patent/DE102008018265B4/de active Active
• 2009
  • 2009-04-02 EP EP09729656A patent/EP2265969A1/de not_active Withdrawn
  • 2009-04-02 WO PCT/EP2009/053929 patent/WO2009124873A1/en active Application Filing

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