Method and control unit for compensation of eddy current induced magnetic fields in magnetic resonance imaging

Patent metadata

Field	Value
Publication number	US-12055609-B2
Application number	US-201916598056-A
Country	US
Kind code	B2
Filing date	Oct 10, 2019
Priority date	Oct 16, 2018
Publication date	Aug 6, 2024
Grant date	Aug 6, 2024

Abstract

Eddy current induced magnetic fields (MF) are compensated in a magnetic resonance imaging system. An MR-sequence (M) includes a number of gradients. A dataset includes values of an amplitude and a time constant of eddy current fields of a number of gradients on at least one gradient axis. A number of points in time within the time period of the MR-sequence are defined. A number of constant currents are calculated for a number of coils of the magnetic resonance imaging system based on the dataset. The number of constant currents is designed to compensate at least at the one defined point in time (PT 1 , PT 2 ). The calculated number of constant currents are applied on the related coils prior or during the application of the MR-sequence or a section of the MR-sequence.

First claim

The invention claimed is:

1. A method for compensation of eddy current induced magnetic fields in a magnetic resonance imaging system, the method comprising:
providing an MR-sequence comprising gradients, the gradients varying over time,
providing a dataset comprising values to generate constant currents designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence with a constant offset of the eddy currents,
defining at least one point in time within a time period of the MR-sequence on at least one gradient axis,
calculating the constant currents for coils of the magnetic resonance imaging system based on the dataset, wherein the constant currents are constant over the time during which the gradients vary and are designed to compensate magnetic fields induced by eddy currents of the gradients of the MR-sequence at the at least one defined point in time,
applying the calculated constant currents on the coils prior or during the application of the MR-sequence or a section of the MR-sequence.

2. The method according to claim 1 , wherein the dataset comprises values of amplitudes of the magnetic fields induced by the eddy currents of the gradients on at least one gradient axis, wherein the calculation of the constant currents comprises:
calculating amplitudes of the magnetic fields induced by the eddy currents of the gradients of the MR-sequence, based on the dataset, and
calculating the constant currents for the coils of the magnetic resonance imaging system.

3. The method according to claim 2 wherein the dataset comprises a time constant, and wherein calculating the constant currents comprises calculating each constant current applicable for one of the coils, wherein the constant currents are designed to compensate the calculated amplitudes of the magnetic fields induced by the eddy currents of the gradients of the MR-sequence at the at least one defined point in time.

4. The method according to claim 1 , wherein the dataset comprises values of spatial orders of amplitudes of the magnetic fields induced by the eddy currents of a number of spatially separated gradients on at least two gradient axes.

5. The method according to claim 4 wherein the amplitudes of the magnetic fields induced by the eddy currents are calculated, based on the spatial orders of the amplitude of the magnetic fields induced by the eddy currents for spatial separated gradients on gradient axes, wherein the coils comprise shim coils, and wherein the constant currents for a number of the shim coils are calculated based on the spatial orders of the amplitude of the magnetic fields induced by the eddy currents, for the spatial separated gradients on gradient axes, and wherein the calculated constant currents are applied to the shim coils designed to compensate the respective spatial order.

6. The method according to claim 1 , wherein one of the at least one point in time is selected from the group comprising the point in time of a fat saturation pulse, the point in time of a water saturation or excitation pulse and the point in time of a k-space center.

7. The method according to claim 2 , wherein the calculation of the amplitudes of the magnetic fields induced by eddy currents comprises actual parametrization of a measurement protocol for a number of slices, a thickness of the slices, an orientation of the slices, the field of view, and/or the resolution.

8. The method according to claim 1 , wherein the at least one point in time comprises multiple points in time, wherein the calculation of the constant currents for the coils is designed to compensate the magnetic fields induced by the eddy currents at different ones of the multiple points in time, wherein an average value for each constant current is chosen concerning the compensation at every chosen of the multiple points in time.

9. The method according to claim 8 , wherein two or more of the multiple points in time are chosen and the constant currents for the coils are calculated such that for one point of the two or more of the multiple points in time not a best compensation is chosen, but in total a best compensation for all the two or more of the multiple points in time.

10. The method according to claim 9 , wherein a quality of measurements is calculated for each point of the multiple points in time in dependence of a degree of compensation at this point of the multiple points in time and after that the compensation is designed such that an average or weighted average of the constant currents is applied so that the quality of measurements for each of the multiple points point in time meets at the average value for each constant current.

11. The method according to claim 1 , wherein after the application of the calculated constant currents on the coils, there is included a predefined stabilizing time until the MR-sequence is applied.

12. The method according to claim 1 , wherein the at least one point in time comprises multiple points in time, wherein the MR-sequence comprises two or more sections, wherein different sections comprise different gradients and wherein different ones of the multiple points in time are chosen in different sections, and wherein the calculation and application of the constant currents is done separately for respective gradients and the points in time of different sections of the MR-sequence.

13. The method according to claim 1 , wherein the method is repeated for two or more MR-sequences or sections of an MR-sequence, wherein values of the calculated constant currents for the two or more MR-sequences or sections are stored and the stored values are applied without again calculating the constant currents in the case the respective MR-sequence or section is applied again.

14. The method according to claim 13 , wherein between two different ones of the MR-sequences or sections of the MR-sequence, a pause or dummy measurements is applied:
at a time of changing the constant current for the coils, and/or
at a time of changing an acquisition of lesser important parts of the dataset are performed.

15. A control system comprising a compensation processor, the control system comprising
a data interface for receiving an MR-sequence comprising gradients,
a data interface for receiving and/or a memory for storing a dataset comprising values of an amplitude and a time constant of eddy current fields of the gradients on at least one gradient axis, or comprising values of constant currents configured to compensate magnetic fields induced by the eddy currents of the gradients of the MR-sequence at least at one defined point in time,
a processor configured to calculate the constant currents for coils of a magnetic resonance imaging system based on the dataset, wherein the constant currents are configured to be constant over time during which the gradients vary and compensate the magnetic fields induced by the eddy currents of the gradients of the MR-sequence at least at the one defined point in time,
a gradient system interface for applying the calculated constant currents on the coils prior or during the application of the MR-sequence or a section of the MR-sequence.

16. The control system of claim 15 , further comprising:
a magnetic resonance imaging system comprising the coils.

17. A non-transitory computer-readable medium on which is stored program instructions that can be read and executed by a computer, the instructions comprising:
providing an MR-sequence comprising gradients,
providing a dataset comprising values to generate constant currents designed to compensate magnetic fields induced by

Assignees

• Siemens Healthcare Gmbh
• Siemens Healthineers Ag

Inventors

• Hölscher Uvo
• Köhler Michael
• Niederlöhner Daniel
• Stemmer Alto

Classifications

• A61B5/055

  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging · CPC title

• G01R33/3875

  using correction coil assemblies, e.g. active shimming · CPC title

• G01R33/56518 Primary

  due to eddy currents, e.g. caused by switching of the gradient magnetic field · CPC title

• G01R33/56

  Image enhancement or correction, e.g. subtraction or averaging techniques {, e.g. improvement of signal-to-noise ratio and resolution} · CPC title

• G01R33/446

  Multifrequency selective RF pulses, e.g. multinuclear acquisition mode (spatially selective RF pulses G01R33/4833) · CPC title