MRI Static Field Homogeneity Correction via Dipole Optimization
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Solution Overview
Problem
Existing methods for correcting inhomogeneities in the static magnetic field of nuclear magnetic resonance imaging machines are inefficient in minimizing the number of dipoles and total magnetic moment required for compensation, often resulting in unsatisfactory compensation due to combinations of dipoles with reverse polarities and similar magnetic moments.
Innovation Solution
A method that minimizes the total magnetic moment of compensation dipoles by optimizing the placement and magnetic moment of dipoles on a grid, using a series expansion of orthogonal functions to describe the magnetic field and dipole effects, and iteratively adjusting the positions and moments to achieve the best compensation while limiting magnetic moment values to discrete steps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing methods are used to correct magnetic field inhomogeneities, then compensation is achieved, but the number of dipoles and total magnetic moment required is excessive
Solution Approach 1:
The patent changes the optimization parameter from merely minimizing field inhomogeneity to minimizing a cost function that includes both field homogeneity and the total magnetic moment of dipoles. This is achieved by introducing weighting factors in the cost function J = ||E - Ed||² + λ||m||², where the second term penalizes large magnetic moments, thereby reducing the quantity of magnetic material required while maintaining compensation effectiveness.
Solution Approach 2:
The patent applies local quality by allowing different regions of the magnetic field to be compensated with different dipole configurations. The grid-based approach divides the spatial volume into discrete positioning points, enabling localized optimization where dipoles are strategically placed only where needed to correct local inhomogeneities, rather than using a uniform dense distribution throughout the entire volume.
2Reliability
If dipoles with reverse polarities and similar magnetic moments are combined, then field compensation is attempted, but the compensation is unsatisfactory due to partial suppression of effects
Solution Approach 1:
The patent replaces the trial-and-error mechanical adjustment of dipole configurations with a mathematical optimization system. The cost function J provides a quantitative metric that guides the selection of dipole positions, orientations, and magnetic moments, eliminating the need for complex manual balancing of opposing dipole effects and providing a systematic approach to achieving satisfactory compensation.
Solution Approach 2:
The patent implements feedback through the cost function evaluation process. The actual magnetic field E is measured, compared with the desired field, and the difference drives the optimization of dipole configuration. This feedback loop ensures that dipole combinations producing partial suppression or unsatisfactory compensation are identified and corrected by adjusting the configuration to minimize the cost function.
3Ease of manufacture
If the number of dipoles is reduced, then manufacturing and placement is simplified, but field compensation precision may deteriorate
Solution Approach 1:
The patent changes the optimization objective to minimize the cost function that balances both compensation precision and the number/magnitude of dipoles. By adjusting the weighting factor λ in the cost function, the system finds the optimal trade-off point where sufficient precision is achieved with the minimum necessary number of dipoles, thereby simplifying manufacturing and placement without sacrificing essential compensation accuracy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively reduces the number of dipoles and total magnetic moment, improving the homogeneity of the magnetic field within acceptable thresholds, reducing peak-to-peak oscillations, and simplifying the manufacturing and placement of compensation dipoles.
Implementation Method 1
describing the components of the magnetic field along at least one direction by a series expansion based on an orthogonal set of functions fn truncated at a predetermined finite maximum order
Data Source
AI summary
An exemplary method for correcting inhomogeneities of the static magnetic field generated by the magnetic structure of a machine for acquiring nuclear magnetic resonance images includes determining the number, the position and the magnetic moment of dipoles compensating inhomogeneities of the magnetic field to be positioned on a positioning grid such that a function is minimized. The total magnetic moment of compensation dipoles for compensating the magnetic field inhomogeneity can be minimized in addition to minimizing a norm of the difference between an inhomogeneity vector of each of the magnetic field components and the linear combination of effect vectors corresponding to the magnetic field component of compensation dipoles.