RF Shimming for Homogeneous Flip Angle in MRI
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Solution Overview
Problem
High-field MRI systems face challenges in achieving homogeneous flip angle distribution due to inhomogeneous B1+ fields, which complicates clinical diagnosis by introducing spatially dependent T1 contrast, especially when imaging objects approximating the human head.
Innovation Solution
The method combines RF shimming with linear and nonlinear spatial encoding magnetic fields to remap the B1+ map into a lower dimension coordinate system, allowing for effective RF excitation with spatial selectivity and achieving a homogeneous flip angle distribution without the complexity of parallel transmission techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a volume RF coil is used for RF excitation in high-field MRI, then the signal-to-noise ratio is improved, but the flip angle distribution becomes inhomogeneous
Solution Approach 1:
The patent applies local quality by making the RF excitation properties spatially varying through B1+ field manipulation. Different regions of the imaging volume receive different RF field strengths, allowing the flip angle to be locally optimized for uniformity across the field of view while maintaining high overall SNR through the volume coil configuration
Solution Approach 2:
The patent changes the B1+ field distribution parameter through dedicated coil design and RF shimming techniques. By adjusting the amplitude and phase of RF signals across multiple coil elements, the B1+ field is shaped to compensate for dielectric inhomogeneities, transforming the spatial distribution of flip angles from inhomogeneous to homogeneous
2Manufacturing precision
If spatially selective RF excitation is used to correct flip angle inhomogeneity, then the flip angle distribution becomes more homogeneous, but the RF electronics and coil construction complexity increases
Solution Approach 1:
The patent makes a single volume RF coil perform multiple functions: it provides both the primary excitation field for high SNR and, through B1+ manipulation via RF shimming, acts as a spatially selective excitation system for homogeneity correction. This eliminates the need for separate dedicated coils for different anatomical regions
Solution Approach 2:
The volume RF coil system performs self-correction of its own inhomogeneities through RF shimming. By adjusting the relative phases and amplitudes of signals to individual coil elements, the system automatically compensates for dielectric effects and B1+ inhomogeneity without requiring external correction mechanisms
3Adaptability or versatility
If parallel RF transmission is used to achieve homogeneous flip angle distribution, then the degree of freedom in RF pulse design is increased, but the specific absorption rate management becomes more challenging
Solution Approach 1:
The patent applies partial action by using RF shimming with a single coil rather than full parallel transmission across multiple independently controlled coils. This provides sufficient degrees of freedom for B1+ manipulation and flip angle homogeneity while avoiding the excessive SAR accumulation that would result from simultaneous high-power transmission from multiple coils
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 results in a more homogeneous flip angle distribution, improving image quality and clinical diagnosis by reducing the relative standard deviation of flip angle distributions, as demonstrated by simulations and experimental data, while also simplifying RF excitation and reducing the need for complex RF electronics.
Implementation Method 1
the plurality of RF coils excite a B1+ field jointly, the ratio of signal amplitudes of the plurality of RF coils is kept invariant during the excitation process, and the phase relationship of signals from the plurality of RF coils is kept invariant during the excitation process
Implementation Method 2
incorporates the usage of one or a plurality of spatial encoding magnetic fields and the usage of a plurality of RF coils to generate a specific spatial distribution of flip angle
Data Source
AI summary
The present invention provides a method and apparatus for generating a specific flip angle distribution in magnetic resonance imaging; the method uses a plurality of RF transmission coils combined with linear and nonlinear spatial encoding magnetic fields to generate a homogeneous flip angle distribution.