B1 Mapping and Shimming for MRI Field Homogeneity
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Solution Overview
Problem
Current B1-shimming methods in MRI systems are inadequate for high B0 fields, leading to non-uniform RF field excitation and potential misdiagnosis due to dielectric properties of the body, requiring increased transmit power and resulting in higher Specific Absorption Rate (SAR) values, which is impractical for clinical settings.
Innovation Solution
A volumetric B1 mapping procedure is performed pre-scan to acquire spatial sensitivity distributions of RF antennas, allowing for 3D B1 mapping and shimming across the entire volume, incorporating fast imaging techniques and parallel imaging to minimize SAR and achieve uniform B1 field homogeneity, using a modified cost function with a regularization parameter to balance RF power and B1 field homogeneity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the B0 field strength is increased to improve spatial resolution and contrast resolution, then diagnostic image quality is improved, but B1 field inhomogeneity worsens due to shortened wavelength and RF standing waves
Solution Approach 1:
The patent divides the B1 shimming process into multiple stages: initial B1 mapping to identify inhomogeneity patterns, followed by iterative optimization cycles that segment the adjustment of complex amplitudes for different transmit channels. This segmented approach allows systematic correction of B1 inhomogeneity across the imaging volume while maintaining high B0 field strength
Solution Approach 2:
The patent changes the parameters of the RF transmit system by optimizing the complex amplitudes (magnitude and phase) of each transmit channel based on measured B1 field maps. This parameter optimization compensates for the wavelength shortening effects at high B0 fields, restoring B1 field uniformity while maintaining the benefits of high-field imaging
2Stability of the object's composition
If B1-shimming is performed to correct B1 inhomogeneity, then B1 field uniformity is improved, but transmit power increases leading to higher SAR values
Solution Approach 1:
The patent implements a feedback mechanism where B1 field maps are measured during the imaging sequence, and the complex amplitudes of transmit channels are iteratively adjusted based on these measurements. This feedback loop optimizes B1 uniformity while monitoring and controlling SAR levels, allowing the system to find the optimal balance between field homogeneity and power consumption
Solution Approach 2:
The patent applies partial B1 shimming by optimizing complex amplitudes only for regions where B1 inhomogeneity significantly impacts image quality, rather than attempting uniform correction across the entire imaging volume. This selective approach reduces the overall transmit power requirement and SAR while maintaining adequate B1 uniformity in critical imaging regions
3Stability of the object's composition
If individual volume B1-shimming is performed for each imaged volume, then local B1 uniformity is improved, but B1 inhomogeneity in other volumes remains and total scan time increases
Solution Approach 1:
The patent performs a preliminary B1 mapping procedure that covers a large volume encompassing multiple imaging regions before the actual imaging sequences. This preliminary measurement provides B1 field information that can be used to optimize transmit parameters for subsequent imaging of multiple volumes, eliminating the need for repeated B1 mapping and reducing total scan time
Solution Approach 2:
The patent creates a universal B1 field map of a large volume that serves multiple imaging sequences and target regions. This single B1 mapping procedure provides the basis for optimizing B1 shimming across different volumes and imaging protocols, making the B1 correction strategy universally applicable to multiple imaging tasks without requiring separate mapping for each volume
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 enables efficient and flexible B1-shimming across larger volumes, reducing SAR and improving image quality by optimizing B1 field homogeneity, allowing for more comprehensive and time-efficient MRI protocols.
Implementation Method 1
Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field
Implementation Method 2
The problem is that dielectric properties of the body cause the wavelength λ of the transmitted RF field (so-called B1 field) to become shorter
Implementation Method 3
RF signals emitted by the nuclear spins are detected by a receiver coil
Data Source
AI summary
The invention relates to a method of acquiring MRI image data comprising the following steps: performing a 3-dimensional B1 mapping of a first volume using a first voxel size, selecting an MRI protocol, performing the B1-shim in accordance with the MRI protocol, performing the MRI protocol to acquire MRI imaging data of a second volume using a second voxel size, wherein the first voxel size is larger than the second voxel size, wherein the first volume is larger than the second volume, and wherein the second volume is contained within the first volume.