Force-compensated Gradient Coil for MRI Systems
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Solution Overview
Problem
In MRI systems, the mechanical vibrations induced by gradient coils' oscillating magnetic fields cause significant noise and heat due to eddy currents in conductive components, leading to cryogen loss and potential quenching, especially when resonant frequencies of bore tubes align with the oscillation frequency, interfering with imaging quality and patient comfort.
Innovation Solution
A secondary gradient coil assembly is positioned radially outside the primary gradient coil assembly, with cross-tubes linking inner and outer surfaces of the OVC and thermal radiation shield to increase stiffness, and currents flowing in the same direction in both assemblies to cancel out forces and magnetic fields, reducing mechanical oscillations and eddy currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If gradient coils generate rapidly oscillating magnetic fields for imaging, then imaging capability is improved, but mechanical vibrations and eddy currents increase causing noise and heat
Solution Approach 1:
A compensating gradient coil assembly is positioned radially outside the primary gradient coil assembly. When currents flow through both assemblies in the same direction, they generate opposing forces that cancel each other out, reducing mechanical vibrations and Lorentz forces acting on the gradient coil structure.
Solution Approach 2:
The system uses an asymmetric configuration with primary gradient coils inside the bore and compensating gradient coils outside the bore. This asymmetric arrangement allows the compensating coils to counteract the harmful effects of the primary coils without interfering with the imaging process.
2Object-affected harmful factors
If eddy currents are generated in conductive components by gradient coils, then thermal radiation shielding is improved, but Lorentz forces cause mechanical vibrations
Solution Approach 1:
The patent converts the harmful eddy currents and Lorentz forces into a beneficial effect by using compensating gradient coils to generate opposing forces. The eddy currents that would normally cause vibrations are counteracted by the compensating coils, transforming the harmful interaction into a force-cancellation mechanism.
3Stability of the object's composition
If gradient coil assembly is mechanically stiffened to reduce vibrations, then mechanical stability is improved, but resonant frequency increases may align with oscillation frequency
Solution Approach 1:
The compensating gradient coil assembly is positioned and configured to preemptively counteract the Lorentz forces and mechanical vibrations before they can cause significant disruption. By applying opposing forces through the compensating coils, the system prevents resonance interference rather than reacting to it after occurrence.
4Object-generated harmful factors
If secondary gradient coils are added for force compensation, then mechanical vibrations are reduced, but device complexity increases
Solution Approach 1:
The patent addresses the vibration problem by adding compensating coils in a different spatial dimension (radially outside the bore) rather than modifying the existing primary coil structure. This dimensional approach allows force compensation without significantly complicating the primary gradient coil assembly.
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 configuration significantly reduces mechanical vibrations and gradient coil-induced heating without altering the bore diameter or length, improving imaging quality and patient comfort by minimizing acoustic noise and cryogen consumption.
Implementation Method 1
a secondary gradient coil assembly (24) is provided, positioned radially outside of the primary gradient coil assembly (22)... which generates magnetic fields opposing those of the primary gradient coil assembly (22), thereby reducing forces on the thermal radiation shield and the OVC
Implementation Method 2
The eddy currents produced in the material of the OVC 14 will help to shield the thermal radiation shield 16 and cryogenically cooled components such as cryogen vessel bore tube 12b, magnet coils and magnet former 10 from stray fields from the gradient coils 20. However, because of the constant background magnetic field produced by the magnet, those eddy currents produce Lorentz forces, resulting in mechanical vibrations in the bore tube of the OVC.
Implementation Method 3
cross-tubes are provided linking radially inner and radially outer surfaces of the OVC and thermal radiation shield, which substantially increases the stiffness of the OVC or thermal radiation shield, as appropriate. This increased stiffness in turn reduces the tendency of the OVC or thermal radiation shield to mechanically oscillate
Implementation Method 4
Stray fields from the gradient coils generate eddy currents in the closest conductive surface, typically a bore tube 14b, of the OVC. As described below, this can in turn lead to induced eddy currents on other conductive surfaces
Implementation Method 5
The eddy currents produced in the material of the OVC 14 will help to shield the thermal radiation shield 16 and cryogenically cooled components such as cryogen vessel bore tube 12b, magnet coils and magnet former 10 from stray fields from the gradient coils 20. However, because of the constant background magnetic field produced by the magnet, those eddy currents produce Lorentz forces, resulting in mechanical vibrations in the bore tube of the OVC.
Data Source
AI summary
A cylindrical superconducting magnet system for use in magnetic resonance imaging has axially aligned primary superconducting coils that are situated within an outer vacuum chamber (OVC). A thermal radiation shield surrounds the primary superconducting coils within the OVC. A primary gradient coil assembly is axially aligned with the primary superconducting coils and is situated radially within the primary superconducting coils. The cylindrical superconducting magnetic system also includes a secondary gradient coil assembly, that is radially situated outside of the primary superconducting coils and that is mechanically attached to the primary gradient coil assembly.