Cascaded Gradient Amplifier Control for MRI Common-Mode Noise
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Solution Overview
Problem
Magnetic resonance imaging (MRI) systems face significant degradation in image quality due to high frequency harmonic contents generated by gradient drivers, particularly common mode (CM) electromagnetic noise, which contaminates radio frequency (RF) signals.
Innovation Solution
A switching amplifier with cascaded elements and a control device that uses space vector modulation (SVM) to control the common mode voltage within a predetermined range, reducing electromagnetic interference (EMI) by ensuring the change of the CM voltage is minimal, thereby minimizing the CM noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If PWM method is used to control power semiconductor devices in gradient driver, then electrical power control is achieved, but high frequency harmonic contents are generated which contaminate RF signals and degrade MR image quality
Solution Approach 1:
The gradient amplifier is divided into multiple cascaded stages (first gradient amplifier, second gradient amplifier, third gradient amplifier) with distributed capacitances. Each stage processes a portion of the voltage, and the total common mode voltage is the sum of individual stage voltages. This segmentation allows independent control of each stage's common mode voltage to achieve minimal overall change.
Solution Approach 2:
The invention changes the control parameter from simple PWM switching to space vector modulation (SVM) that specifically controls the common mode voltage parameter. By using SVM, the common mode voltage is maintained within a predetermined range, minimizing its change and thereby reducing electromagnetic noise while maintaining electrical power control capability.
2Speed
If fast voltage or current transient is generated in gradient driver, then response speed is improved, but electromagnetic noise is increased which degrades RF signal quality
Solution Approach 1:
The invention dynamically adjusts the switching patterns of power semiconductor devices in each cascaded stage to maintain common mode voltage within a predetermined range. The control device monitors and adjusts switching timing and duration to minimize common mode voltage change, thereby reducing electromagnetic noise while maintaining fast response capability through coordinated switching of multiple stages.
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 solution effectively reduces CM electromagnetic noise, leading to improved MRI image quality by minimizing the interference that degrades RF signals.
Implementation Method 1
A switching amplifier with cascaded elements and a control device that uses space vector modulation (SVM) to control the common mode voltage within a predetermined range
Implementation Method 2
a gradient coil coupled to the power stage and configured to produce a magnetic field proportional to a coil current signal supplied by the power stage
Data Source
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AI summary
A method is used for operating a switching amplifier, the switching amplifier includes a plurality of cascade elements. The method includes: coupling the cascade elements in series between two terminals of a load; providing two leg circuits each comprised of switches in each of the cascade elements; and controlling all of the switches comprised in the switching amplifier using space vector modulation (SVM), such that a change of a common mode (CM) voltage generated by the switching amplifier is in a predetermined range.