Open MRI High Frequency Shield Vibration Reduction
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Solution Overview
Problem
In MRI apparatus with an open structure, the high frequency shield experiences vibration due to eddy currents induced by the gradient magnetic field, which compromises the magnetic field shielding capability and generates noise, whereas in tunnel structures, this issue is mitigated by the orientation of magnetic flux relative to the static field.
Innovation Solution
The implementation of a high frequency shield with enhanced rigidity, supported by a structure that deviates the natural frequency of the support member from the gradient magnetic field switching frequency, and designed to have lower resistivity to high-frequency magnetic fields while maintaining high resistivity to gradient magnetic field frequencies, using a configuration of conductive sheets and dielectric sheets to reduce eddy currents and prevent resonance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high frequency shield is disposed between the high frequency coil and the gradient magnetic field coil, then magnetic field shielding capability is improved, but vibration and noise are generated due to eddy currents
Solution Approach 1:
The high frequency shield is divided into multiple segments or sections along its length. This segmentation interrupts the continuous conductive path, thereby reducing eddy current loops and the associated vibration and noise, while still maintaining effective magnetic field shielding capability between the high frequency coil and gradient magnetic field coil.
Solution Approach 2:
Different portions of the high frequency shield have different properties - specifically, certain regions have modified electrical conductivity or magnetic permeability to locally suppress eddy currents. This allows the shield to maintain overall shielding effectiveness while reducing harmful vibrations and noise in critical areas.
2Strength
If the high frequency shield is made more rigid to reduce vibration, then vibration resistance is improved, but eddy current effects are intensified
Solution Approach 1:
The high frequency shield is constructed using composite materials that combine high mechanical strength and rigidity with low electrical conductivity. This allows the shield to resist vibration effectively while simultaneously reducing eddy current generation, as the composite structure provides structural integrity without creating large continuous conductive paths.
3Stability of the object's composition
If the interval between the high frequency coil and gradient magnetic field coil is increased, then magnetic field uniformity is improved, but the open structure space is reduced
Solution Approach 1:
The high frequency shield acts as an intermediary component between the high frequency coil and gradient magnetic field coil. It provides electromagnetic isolation that improves magnetic field uniformity without requiring a large physical interval, thus preserving the open structure space while still achieving the desired field stability.
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 solution effectively reduces vibration and noise in the high frequency shield while maintaining the magnetic field shielding capability, preventing the high frequency shield from becoming a noise source, even during high-speed gradient magnetic field switching, and ensures stable imaging with improved spatial resolution.
Implementation Method 1
a high frequency shield is disposed between each gradient magnetic field generator and each high frequency generator to shield electromagnetic field generated by the high frequency generator
Implementation Method 2
the high frequency shield has a structure of suppressing vibration occurring in the high frequency shield due to switching of gradient magnetic field generated by the gradient magnetic field generator
Implementation Method 3
eddy current is induced in the high frequency shield by the magnetic flux of the gradient magnetic field
Implementation Method 4
supported by a structure that deviates the natural frequency of the support member from the gradient magnetic field switching frequency
Data Source
AI summary
In an MRI apparatus having an open structure, vibration caused by eddy current is reduced with keeping the magnetic field shielding capability of a high frequency shield disposed between a gradient magnetic field coil and a high frequency coil. The high frequency shield comprises two conductive sheets 501, 503 which are divided into plural parts at different positions, and a dielectric sheet 502 sandwiched therebetween. Accordingly, in the high frequency shield, the electric resistivity to high frequency magnetic field is lower than the electric resistivity to the switching frequency of gradient magnetic field, whereby the high frequency shield can act as a uniform conductor to the high frequency magnetic field, exercise the shielding capability of the high frequency magnetic field, reduce eddy current occurring due to passage of gradient magnetic field therethrough and suppress vibration.