RF Transmit Coil Pulse Sequence Optimization for MR Homogeneity
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Solution Overview
Problem
Conventional magnetic resonance (MR) apparatuses face challenges in maintaining homogeneous deflection fields due to eddy currents produced by patients, especially at higher magnetic field strengths, leading to increased SAR exposure and hotspot formation, which existing methods fail to adequately address for large deflection angles and non-linear limitations.
Innovation Solution
A method that solves a non-linear equation system to determine feed parameters for transmit channels, incorporating additional equations for SAR exposure and power loss, allowing for large-angle excitations and optimized patient exposure by specifying a feed sequence for gradient coils and using a non-linear least square method to calculate pulse sequences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher frequencies are used to achieve higher magnetic field strengths, then the deflection field homogeneity is improved, but eddy currents are produced in the patient that destroy the homogeneity
Solution Approach 1:
The patent applies dynamic adjustment of phase and amplitude for each transmit channel during the excitation process. Instead of static parameters, the system dynamically optimizes feed parameters to compensate for eddy current effects, allowing the deflection field homogeneity to be maintained despite the presence of eddy currents at higher frequencies.
Solution Approach 2:
The system changes multiple parameters simultaneously including phase, amplitude, and timing for each transmit channel. By adjusting these parameters dynamically during excitation, the system compensates for the harmful eddy current effects while maintaining the benefits of higher frequency operation.
2Adaptability or versatility
If gradient coils are used during RF excitation to select particular regions, then local deflection of spin is achieved, but SAR exposure increases
Solution Approach 1:
The patent applies partial action by selectively activating only the necessary transmit channels and gradient coil combinations for the desired local region. This avoids the excessive SAR exposure that would result from using all channels and gradients simultaneously, while still achieving the desired local spin deflection.
Solution Approach 2:
The system dynamically adjusts the combination of active transmit channels and gradient coils based on the specific excitation requirements. This dynamic optimization allows local deflection capability while minimizing SAR exposure by avoiding unnecessary RF power application.
3Device complexity
If conventional linearized methods are used to solve the equation system, then computational complexity is reduced, but large deflection angles cannot be achieved
Solution Approach 1:
The patent changes the fundamental approach from linearized approximation to non-linear optimization. By using non-linear least squares optimization, the system can handle large deflection angles while managing computational complexity through efficient numerical methods and iterative solving procedures.
Solution Approach 2:
The patent replaces the mechanical linearization approximation with a non-linear optimization framework. This substitution allows the system to accurately model and achieve large deflection angles by using non-linear least squares optimization instead of simplified linear equations.
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 the calculation of ideal excitation pulses for arbitrary magnetizations and deflection angles, minimizing patient exposure and preventing hotspot formation while maintaining high homogeneity, by incorporating non-linear equations that account for SAR limitations and power constraints.
Implementation Method 1
radio-frequency transmit coil having one transmit channel, or a number of transmit channels
Implementation Method 2
eddy currents are produced in the patient that in turn produce interference fields
Implementation Method 3
a k-space trajectory for the gradient coils is specified that selects the partial volumes
Implementation Method 4
optimization with respect to the SAR exposure of the patient
Data Source
AI summary
In a magnetic resonance apparatus having an RF radiating coil and gradient coils, and in a method for operating such a magnetic resonance apparatus, a pulse sequence, composed of multiple time steps, is specified for operating the gradient coils to time-dependently select regions of a selected slice of a selected volume of a subject. A non-linear equation system is then solved to obtain feed parameters for individual channels of the transmit coil for each time step, with specification of a desired target magnetization, and dependent on the pulse sequence specified for the gradient coils. The non-linear equation system is based on discrete values for time and space variable and, in addition to equations resulting from the Bloch equation, which are non-linear in their feed parameters, includes at least one additional equation that describes boundary conditions for the examination of the subject.