Method and magnetic resonance apparatus for shimming the basic magnetic field by operation of a shim device

Patent metadata

Field	Value
Publication number	US-10429464-B2
Application number	US-201815902520-A
Country	US
Kind code	B2
Filing date	Feb 22, 2018
Priority date	Feb 22, 2017
Publication date	Oct 1, 2019
Grant date	Oct 1, 2019

Abstract

In a magnetic resonance apparatus having a scanner that generates a basic magnetic field in an imaging volume, and an operating method to acquire data from an entirety of a recording volume, wherein the scanner has a global shim coil acting on the entire imaging volume, and a local shim coil acting, with the global shim coil, on a sub-volume containing a region of interest, a first adjustment volume is established that contains the recording volume. A smaller, second adjustment volume is established containing the region of interest, and at most, the sub-volume. Using a field map of the basic magnetic field that covers the first adjustment volume, shim currents are respectively identified for the global shim unit, for homogenizing the first adjustment volume, and for the local shim unit, for homogenizing the second adjustment volume, accounting for the effect of the first shim currents on the second adjustment volume.

First claim

The invention claimed is:

1. A method for operating a magnetic resonance (MR) apparatus that comprises an MR data acquisition scanner having a basic field magnet that generates a basic magnetic field in an imaging volume of the scanner, and a global shim unit that acts on an entirety of said imaging volume in order to homogenize the basic magnetic field therein, and said scanner having a local shim unit that acts on a sub-volume comprising a region of interest, together with said global shim unit, said method comprising:
in a computer, establishing a first adjustment volume that comprises a recording volume, wherein MR data are to be acquired with said scanner in an entirety of said recording volume;
in said computer, establishing a second adjustment volume, which is smaller than said first adjustment volume, said second adjustment volume comprising at least said region of interest and, as a maximum, said sub-volume;
with said computer, operating said scanner in order to obtain a field map of said basic magnetic field at least within said first adjustment volume;
in said computer, using said field map to determine first shim currents for said global shim coils that cause homogenization of said basic magnetic field in said first adjustment volume, and to determine second shim currents for said local shim coils that cause homogenization of said basic magnetic field in said second adjustment volume, taking into account an effect of said first shim currents on said second adjustment volume; and
in said computer, generating control signals in order to operate respective amplifiers that feed said global shim coils and said local shim coils so as to supply said global shim coils with said first shim currents and to supply said local shim coils with said second shim currents, while operating said scanner in order to acquire MR data from said recording volume.

2. A method as claimed in claim 1 wherein said field map is a first field map, and wherein said method comprises:
in said computer, determining said first shim currents by executing a first optimization algorithm with regard to the homogeneity of the basic magnetic field in the first adjustment volume; and
in said computer, calculating a second field map for said second adjustment volume from said first field map, by also taking into account said first shim currents, and using said second field map to determine said second shim currents by executing a second optimization algorithm with regard to homogeneity of the basic magnetic field in the second adjustment volume.

3. A method as claimed in claim 1 comprising, in said computer, determining said first and second shim currents by executing a joint optimization algorithm, wherein optimization with regard to the homogeneity of the basic magnetic field in the first adjustment volume and optimization with regard to the homogeneity of the basic magnetic field in the second adjustment volume are weighted respectively as optimization objectives, with weighting values.

4. A method as claimed in claim 1 comprising manually entering inputs representing said weighting values into said computer.

5. A method as claimed in claim 1 comprising establishing said first adjustment volume to encompass an entirety of the imaging volume of the scanner.

6. A method as claimed in claim 1 comprising establishing said second adjustment volume dependent on a user entry that describes said region of interest.

7. A magnetic resonance (MR) apparatus comprising:
an MR data acquisition scanner comprising a basic field magnet that generates a basic magnetic field in an imaging volume of the scanner, a global shim unit that acts on an entirety of said imaging volume with at least one global shim coil, and a local shim unit that acts on a sub-volume, comprising a region of interest of a subject, with at least one local shim coil together with said global shim unit, said global shim unit comprising a global shim amplifier that supplies said at least one global shim coil and said local shim unit comprising a local shim amplifier that supplies said at least one local shim coil;
a computer configured to establish a first adjustment volume that comprises a recording volume, wherein MR data are to be acquired with said scanner in an entirety of said recording volume;
said computer being configured to establish a second adjustment volume, which is smaller than said first adjustment volume, said second adjustment volume comprising at least said region of interest and, as a maximum, said sub-volume;
said computer being configured to operate said scanner in order to obtain a field map of said basic magnetic field at least within said first adjustment volume;
said computer being configured to use said field map to determine first shim currents for said global shim coils that cause homogenization of said basic magnetic field in said first adjustment volume, and to determine second shim currents for said local shim coils that cause homogenization of said basic magnetic field in said second adjustment volume, taking into account an effect of said first shim currents on said second adjustment volume; and
said computer being configured to generate respective control signals in order to operate said global shim amplifier to feed said global shim coils and said local shim amplifier to feed said local shim coils, so as to supply said global shim coils with said first shim currents and to supply said local shim coils with said second shim currents, while operating said scanner in order to acquire MR data from said recording volume.

8. A magnetic resonance apparatus as claimed in claim 7 wherein said global shim unit is designed to compensate for inhomogeneities in said basic magnetic field of at least the first and second orders.

9. An MR apparatus as claimed in claim 7 wherein said scanner comprises a gradient coil arrangement and wherein said at least one global shim coil of said global shim unit is installed in said gradient coil arrangement.

10. An MR apparatus as claimed in claim 7 wherein said scanner comprises a local coil arrangement, and wherein said at least one local coil of said local shim unit is used as at least one coil of said local coil arrangement.

11. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer of a magnetic resonance (MR) apparatus that comprises an MR data acquisition scanner comprising a basic field magnet that generates a basic magnetic field in an imaging volume of the scanner, a global shim unit that acts on an entirety of said imaging volume with at least one global shim coil, and a local shim unit that acts on a sub-volume, comprising a region of interest of a subject, with at least one local shim coil together with said global shim unit, said global shim unit comprising a global shim amplifier that supplies said at least one global shim coil and said local shim unit comprising a local shim amplifier that supplies said at least one local shim coil, said programming instructions causing said computer to:
establish a first adjustment volume that comprises a recording volume, wherein MR data are to be acquired with said scanner in an entirety of said recording volume;
establish a second adjustment volume, which is smaller than said first adjustment volume, said second adjustment volume comprising at least said region of interest and, as a maximum, said sub-volume;
operate said scanner in order to obtain a field map of said basic magnetic field at least within said first adjustment volume;
use said field map to determine first shim currents for said global shim coils that cause homogenization of said basic magnetic field in

Assignees

• Siemens Healthcare Gmbh

Inventors

• Niederloehner Daniel
• Paul Dominik

Classifications

• G01R33/3875 Primary

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

• G01R33/387 Primary

  Compensation of inhomogeneities · CPC title

• A61B5/055

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