Dynamic MR Shimming via Sub-Area Segmentation
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Solution Overview
Problem
Modern MR systems face challenges in fully compensating for local B0 field inhomogeneities, especially near susceptibility interfaces, due to limited shim channel orders and long settling times, which restrict dynamic shimming and result in residual artifacts in MR images.
Innovation Solution
The method involves dividing the excitation volume into sub-areas and determining distinct adjustment parameters, including RF center frequency and shim currents, for each sub-area to optimize MR image acquisition, allowing for better compensation of B0 inhomogeneities and reducing artifacts by acquiring and superimposing multiple MR images with different settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If dynamic shimming is performed with limited shim channels and long settling times, then measurement time is reduced, but B0 field homogeneity compensation is insufficient resulting in residual artifacts
Solution Approach 1:
The excitation volume is divided into multiple sub-areas, and separate adjustment parameters (shim currents and RF center frequency) are determined for each sub-area. This segmentation allows targeted optimization of B0 field homogeneity in different regions without requiring global re-adjustment, thereby reducing measurement time while improving compensation precision.
Solution Approach 2:
Different adjustment parameters are applied to different sub-areas based on their specific B0 field characteristics. Each sub-area receives customized shim currents and frequency settings optimized for its local field inhomogeneity pattern, enabling precise local compensation without affecting other regions.
2Manufacturing precision
If multiple adjustment parameters are determined for different sub-areas, then B0 field homogeneity is improved, but device complexity and measurement time increase
Solution Approach 1:
Adjustment parameters for all sub-areas are determined in advance before MR signal acquisition. The B0 field map is analyzed beforehand to calculate optimal shim currents and RF center frequency for each sub-area, so that during actual imaging, only pre-determined parameter sets need to be applied without real-time computation complexity.
Solution Approach 2:
The system uses pre-calculated adjustment parameter sets that can be stored and reused for similar imaging conditions. Once parameters are optimized for a particular sub-area configuration, they can be copied and applied to similar cases, reducing the need for repeated complex calculations.
3Strength
If shim coils with long settling times are used, then field strength is sufficient, but dynamic shimming capability is restricted
Solution Approach 1:
The system implements dynamic shimming by sequentially applying different pre-determined adjustment parameter sets to different sub-areas during the imaging sequence. Although individual shim coils have long settling times, the overall system achieves dynamic adaptation by switching between pre-optimized parameter configurations for different regions.
Solution Approach 2:
The imaging sequence is structured to periodically apply different adjustment parameter sets corresponding to different sub-areas. Each parameter set is applied for a sufficient duration to allow shim coil settling, then switched to the next set in a periodic manner that matches the imaging requirements.
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
This approach reduces residual artifacts in MR images by optimizing shim settings and RF frequencies for specific sub-areas, improving image quality and diagnostic accuracy by compensating for B0 inhomogeneities more effectively than traditional dynamic shimming methods.
Implementation Method 1
a magnetic resonance (MR) system... radiating radio-frequency pulses into the object (12)... switching magnetic field gradients... the magnetization of nuclear spins... is disturbed... the currents induced in receiver coils... as the excited spins relax to the equilibrium position are converted into magnetic resonance signals
Implementation Method 2
DC-offset currents for the three gradient coils and currents for special shim coils... are calculated... so that the local field distortions are optimally compensated
Implementation Method 3
the local B0 field is measured in the region to be investigated in order to generate a B0-map or basic field map... then DC-offset currents for the three gradient coils and currents for special shim coils of an order higher than the first order are calculated... so that the local field distortions are optimally compensated
Data Source
AI summary
In a method and magnetic resonance (MR) apparatus for performing an adjustment of the MR system, an examination object under is divided into at least one excitation volume. First adjustment parameters for the at least one excitation volume of the object, and second adjustment parameters for the at least one excitation volume of the object, which differ from the first adjustment parameters are determined. First MR signals are acquired from the at least one excitation volume using the first adjustment parameters. Second MR signals are acquired from an excitation volume using the second adjustment parameters. A first MR image of the at least one excitation volume is reconstructed using the first MR signal. A second MR image of the at least one excitation volume is reconstructed using the second MR signal.