Cylindrical Thermal Shield Stiffener for MRI Eddy Current Heating
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Solution Overview
Problem
Gradient coil induced heating (GCIH) in MRI systems is a significant issue, particularly in 'low cryogen inventory' or 'dry' magnets, where the reduced cryogen volume makes them susceptible to overheating and quenching due to secondary and tertiary eddy currents induced by mechanical vibrations and magnetic fields, which existing solutions have not adequately addressed.
Innovation Solution
A hollow cylindrical thermal radiation shield with a stiffer inner tube, enhanced by a cylindrical stiffener that extends axially and is joined at intervals to improve mechanical rigidity, reducing the amplitude of mechanical vibrations and eddy currents without increasing the coil diameter or wire cost, allowing for thinner materials and potentially larger bore diameters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inner tube of the thermal radiation shield is made of highly conductive material to shield from high-frequency magnetic fields, then electromagnetic shielding effectiveness is improved, but eddy current heating increases
Solution Approach 1:
The inner tube is segmented into multiple axial sections that are electrically isolated from each other by non-conductive spacers. This segmentation breaks the continuous eddy current paths into smaller segments, significantly reducing the magnitude of eddy currents while maintaining electromagnetic shielding effectiveness at high frequencies.
Solution Approach 2:
Non-conductive spacers are introduced as intermediary elements between adjacent sections of the inner tube. These spacers serve as electrical insulators that prevent eddy current flow between sections while allowing the tube to maintain its structural integrity and electromagnetic shielding function.
2Stability of the object's composition
If the inner tube thickness is increased to reduce mechanical vibrations, then structural rigidity is improved, but device complexity and material usage increase
Solution Approach 1:
The inner tube is divided into discrete axial sections connected by spacers, creating a modular structure. This segmentation provides mechanical rigidity to reduce vibrations while avoiding the need for a single thick-walled tube, thereby reducing overall material usage and structural complexity.
Solution Approach 2:
The segmented tube structure with thin-walled sections and spacer connections creates a flexible yet rigid assembly. The thin walls reduce material usage while the segmented configuration with spacers provides the necessary mechanical stability to suppress vibrations.
3Quantity of substance
If the inner tube is made thinner to reduce material usage, then material cost is reduced, but mechanical rigidity and vibration resistance deteriorate
Solution Approach 1:
The tube is segmented into multiple thin-walled axial sections joined by rigid spacers. This configuration allows each section to use minimal material while the spacer connections provide the necessary mechanical rigidity to resist vibrations, achieving overall structural stability with reduced material consumption.
Solution Approach 2:
The structure combines thin-walled tubular sections with spacer materials to create a composite assembly. The thin walls reduce material usage while the spacers contribute structural rigidity, creating a composite structure that achieves both material efficiency and mechanical performance.
4Productivity
If gradient coil pulsing frequency is increased to improve imaging speed, then productivity is improved, but resonant heating increases
Solution Approach 1:
The segmented tube structure with electrical isolation between sections disrupts the formation of large-scale eddy currents that cause resonant heating. By breaking the continuous conductive path, the system can operate at higher gradient coil pulsing frequencies without experiencing excessive resonant heating, enabling faster imaging.
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 effectively separates the resonant frequencies of the inner tubes, reducing mechanical vibrations and eddy currents, thereby minimizing heating in cryogenically cooled components and maintaining the efficiency of MRI systems, especially in low cryogen inventory or dry magnets.
Implementation Method 1
The stiffener is of greater diameter than the inner cylindrical tube, and is joined at intervals to the inner cylindrical tube, thereby to improve the mechanical rigidity of the inner cylindrical tube
Implementation Method 2
Such time-variant magnetic fields will induce heating into conductive materials in the vicinity
Implementation Method 3
Relatively large eddy currents may be induced in the inner tube of the thermal radiation shield due to the pulsing of a magnetic field by the gradient coils
Implementation Method 4
A thermal radiation shield surrounds the cryogen vessel to shield it from radiated heat
Implementation Method 5
The coils, former, cryogen vessel and thermal radiation shield are cooled by a cryogenic refrigerator
Data Source
AI summary
A hollow cylindrical thermal shield for a tubular cryogenically cooled superconducting magnet, has a first axis, an inner cylindrical tube having an axis aligned with the first axis, an outer cylindrical tube of greater diameter than the diameter of the inner cylindrical tube, having an axis aligned with the first axis, and annular end pieces, joining the inner cylindrical tube and the outer cylindrical tube to form an enclosure. The hollow cylindrical thermal shield further has a cylindrical stiffener, extending axially at least part of the axial length of the inner cylindrical tube, the stiffener being joined at intervals to the inner cylindrical tube, thereby to improve the mechanical rigidity of the inner cylindrical tube.