Shimming procedure that includes determination of the target field by optimization in a parameter space of reduced dimensionality

• US 20150102809A1
• Filed: 09/23/2014
• Published: 04/16/2015
• Est. Priority Date: 10/16/2013
• Status: Active Grant

• Current Assignee: Bruker Switzerland AG (Bruker Corporation)
• Original Assignee: Bruker Biospin AG (Bruker Corporation)
• Inventors: Speck, Thomas, van Beek, Jacco Douwe
• Granted Patent: US 9,817,097 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/3875 using correction coil assemblies, e.g. active shimming
• CPC Class Codes: G01R 33/443 Assessment of an electric or a magnetic field, e.g. spatial mapping, determination of a B0 drift or dosimetry
• CPC Class Codes: G01R 33/46 NMR spectroscopy
• CPC Class Codes: G01R 33/58 Calibration of imaging systems, e.g. using test probes , Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material

Abstract

A method for homogenizing the static magnetic field with a distribution B0(r) in the active volume of a magnetic resonance apparatus having a number N of shim coils defines a target field distribution B0T(r) using a filter method in which a norm of the shim currents is influenced by means of filter factors. An optimization procedure works in a parameter space having M control parameters, wherein 2≦M<N. One of the control parameters is used as a weighting parameter for modification of a spatial weighting function and another control parameter is used to control the filter factors. Using this method the hardware limitations can be taken into account when determining the target field distribution, without a significant increase in the computational effort to determine the target field distribution during optimization.

Claims

1. A method for homogenizing a static magnetic field having a distribution B0(r) in an active volume of a magnetic resonance apparatus, the magnetic resonance apparatus having a number N of shim coils, the method comprising the steps of:

a) mapping the magnetic field distribution B0(r) of the static magnetic field;

b) defining a target field distribution B0T(r) using an optimization procedure to optimize a numerical quality criterion for the target field distribution B0T(r), the optimization procedure supplying values for currents through the N shim coils, wherein a spatial weighting function is used in the optimization procedure, the optimization procedure further comprising a filter method in which a norm of the currents in the shim coils is influenced by means of filter factors, wherein the optimization procedure works in a parameter space having M control parameters with 2≦

M<

N, one of the control parameters being used as a weighting parameter for modification of a spatial weighting function, wherein an additional control parameter controls the filter factors; and

c) generating, following step b), the target field distribution B0T(r) in the active volume by setting the currents in the shim coils.

2. The method of claim 1, wherein an NMR spectrometer, an MRI scanner, an EPR instrument or an ion cyclotron resonance apparatus is used as the magnetic resonance apparatus.

3. The method of claim 2, wherein the magnetic resonance apparatus is an NMR spectrometer in which a sample is rotated about one or more axes.

4. The method of claim 3, wherein the axes are inclined with respect to a direction of the static magnetic field.

5. The method of claim 1, wherein in step (a), a gradient-echo method or a spin-echo method is used for mapping the magnetic field distribution B0(r) of the static magnetic field.

6. The method of claim 1, wherein, in defining the target field distribution in step (b), an adjustment range of currents in the shim coils and power consumed by all the shim coils are taken into account.

7. The method of claim 1, wherein the filter method used in step (b) includes one of Tikhonov regularization, Tikhonov-Phillips regularization, truncated singular value decomposition and damped singular value decomposition.

8. The method of claim 7, wherein the optimization procedure optimizes a numerical quality criterion in step (b) and a calculation method is used to calculate the quality criterion, wherein the calculation method uses as input the static magnetic field B0(r), the influence of the currents in the shim coils on the magnetic field, a weighting parameter and a regularization parameter, the calculation method outputting the quality criterion and a list of current settings, wherein the optimization procedure comprises the steps of:

i) selecting a list of values for a first control parameter, wherein the first control parameter is the weighting parameter;

(ii) selecting a list of values for a second control parameter, wherein the second control parameter is the regularization parameter;

(iii) forming pairs of weighting parameters and regularization parameters from the list of values from (i) and the list of values from (ii) and calculating the quality criterion with input of those pairs of weighting parameters and regularization parameters;

(iv) evaluating whether a list of current settings comprises feasible current settings; and

(v) selecting an optimum pair of weighting parameters and regularization parameters on a basis of the quality criterion, while excluding non-feasible current settings.

9. The method of claim 8, wherein a simulated magnetic resonance spectrum is used in calculation of the quality criterion.

10. An electronically readable data medium on which a computer program is stored, that computer program containing instructions for executing the method of claim 1.