RF Shimming Method for MRI B1 Homogeneity and SAR Reduction
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Solution Overview
Problem
In high-field MRI systems, the Specific Absorption Ratio (SAR) increases with higher magnetic fields, and existing RF shimming methods do not adequately account for the varying contributions to SAR across channels, leading to suboptimal B1 distribution homogeneity.
Innovation Solution
An RF shimming method that adjusts the amplitudes and phases of high-frequency signals transmitted by a multi-channel transmission coil, determining objective function parameter values based on the contribution to SAR for each channel to reduce SAR while homogenizing the B1 distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the magnetic field strength is increased to improve SNR, then the image signal quality is improved, but the SAR increases and B1 distribution becomes inhomogeneous
Solution Approach 1:
The patent changes the parameters of the RF transmission system by adjusting the amplitude and phase of RF signals for each channel individually. This allows optimization of the B1 distribution homogeneity while controlling SAR levels, resolving the contradiction between achieving good image quality and minimizing harmful effects.
Solution Approach 2:
The patent divides the RF transmission coil into multiple independent channels, each with independently controllable amplitude and phase. This segmentation enables precise control of the electromagnetic field distribution, allowing homogenization of B1 while managing SAR contributions from different channels.
2Measurement precision
If the magnetic field strength is increased to improve SNR, then the image signal quality is improved, but the B1 distribution homogeneity deteriorates
Solution Approach 1:
The patent adjusts the amplitude and phase parameters of RF signals for each channel to compensate for B1 inhomogeneity. By changing these parameters, the system achieves homogeneous B1 distribution across the imaging region while maintaining the high magnetic field strength needed for good SNR.
Solution Approach 2:
The patent employs a feedback mechanism where B1 maps are measured and used to calculate optimal amplitude and phase settings for each channel. This feedback loop enables continuous optimization of B1 homogeneity while maintaining high-field operation for improved SNR.
3Stability of the object's composition
If existing RF shimming methods are used to homogenize B1 distribution, then B1 homogeneity is improved, but SAR is not adequately reduced because channel contributions are not considered
Solution Approach 1:
The patent applies local quality by determining SAR contribution values for each individual channel and using these local characteristics to weight the objective function. This allows the system to achieve B1 homogeneity while preferentially reducing SAR from channels that contribute more to the total SAR, rather than treating all channels equally.
Solution Approach 2:
The patent modifies the RF shimming approach by introducing SAR contribution-based weighting parameters in the objective function. This parameter change enables simultaneous optimization of B1 homogeneity and SAR reduction, going beyond conventional methods that only focus on B1 distribution.
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 SAR while maintaining B1 distribution homogeneity, improving image quality by optimizing the high-frequency magnetic field conditions across the imaging region.
Implementation Method 1
A radio frequency (hereinafter, referred to as RF) wave that is a type of electromagnetic wave is transmitted to the object to excite spins of the atomic nuclei in the object
Implementation Method 2
causes nuclear magnetic resonance to atomic nuclei in an arbitrary cross section across an object to acquire a tomographic image in the cross section from nuclear magnetic resonance signals to be generated
Data Source
AI summary
The present invention provides an RF shimming method for a nuclear magnetic resonance imaging apparatus comprising: a transmission coil having a plurality of channels that respectively transmit high frequencies to an object and a calculation unit performing RF shimming calculation that determines at least one of amplitudes and phases of the high frequencies to be transmitted respectively to a plurality of the channels so as to improve homogeneity of a high-frequency magnetic field distribution generated by the transmission coil and reduce a specific absorption ratio of the object. Objective function parameters for setting the objective function are determined according to contribution to the SAR for each of the channels during the RF shimming calculation based on the objective function and the restriction condition.