MRI Superconducting Magnet Heat Shield for Eddy Current and Quench Loads
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Solution Overview
Problem
Conventional superconducting magnets in MRI apparatuses face challenges in selecting a heat shield material that simultaneously meets the requirements for maintaining image quality during imaging and reducing the impact of quench events, as the electrical conductivity needed for each case is conflicting.
Innovation Solution
The heat shield is designed with a double-layer structure where the inner layer facing the gradient coil has higher electrical conductivity to minimize eddy current effects, and the outer layer has higher mechanical strength to withstand electromagnetic forces during quench events, using materials like aluminum and stainless steel or copper to optimize conductivity and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a metallic material with high electrical conductivity is used for the heat shield, then eddy current effects are minimized and image quality is maintained, but the heat shield becomes vulnerable to damage from electromagnetic forces during quench events
Solution Approach 1:
The heat shield is divided into multiple layers with different material properties. The inner layer uses high electrical conductivity material (aluminum or copper) to minimize eddy current effects and maintain image quality, while the outer layer uses high mechanical strength material (stainless steel) to withstand electromagnetic forces during quench events. This segmentation allows each layer to perform its specific function optimally.
Solution Approach 2:
The heat shield employs a composite structure combining different metallic materials with complementary properties. The inner layer consists of aluminum or copper for electrical conductivity, while the outer layer uses stainless steel for mechanical strength. This composite material approach resolves the contradiction by integrating materials that individually excel at different aspects of the heat shield's functional requirements.
2Strength
If a metallic material with high mechanical strength is used for the heat shield, then the heat shield can withstand electromagnetic forces during quench events, but eddy current effects increase and image quality deteriorates
Solution Approach 1:
The heat shield is segmented into functional layers where the outer layer provides mechanical strength using stainless steel, while the inner layer maintains image quality using high electrical conductivity materials. This segmentation prevents the entire structure from being compromised by the trade-off between strength and conductivity.
Solution Approach 2:
Different regions of the heat shield are assigned different material qualities based on local requirements. The outer surface facing the vacuum vessel receives high mechanical strength material for quench protection, while the inner surface facing the gradient coil receives high electrical conductivity material for eddy current suppression. This local quality differentiation resolves the global contradiction.
3Ease of manufacture
If a single metallic material is used for the heat shield, then the structure is simple and easy to manufacture, but it cannot simultaneously satisfy both image quality requirements and quench protection requirements
Solution Approach 1:
The heat shield uses a composite material structure with an inner layer of aluminum or copper and an outer layer of stainless steel. This composite approach maintains relative manufacturing simplicity through standard fabrication techniques while achieving the versatility needed to perform optimally under both imaging conditions (low eddy currents) and quench conditions (high mechanical strength).
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 reduces image deterioration from eddy currents and withstands electromagnetic forces during quench events, ensuring high-quality MR images and structural integrity.
Implementation Method 1
a heat shield (also called a radiation shield) configured to reduce heat intrusion into the helium vessel due to radiant heat
Implementation Method 2
a vacuum vessel configured to thermally insulate the inside of the superconducting magnet by vacuum
Implementation Method 3
during imaging by the MRI apparatus, eddy currents are generated in the heat shield due to the operation of the gradient coil. The eddy currents generated in the heat shield form eddy-current magnetic fields that affect the imaging space
Implementation Method 4
When a quench occurs, significantly large electromagnetic force acts within the heat shield due to the induced current generated in the heat shield
Data Source
AI summary
In one embodiment, a superconducting magnet configured to be used in an MRI apparatus that includes a gradient coil. The superconducting magnet comprising a helium vessel, a vacuum vessel, and a heat shield. Among components constituting the heat shield, at least a component of a portion facing the gradient coil is composed by superimposing two layers of metallic materials, the two layers being a first shield layer and a second shield layer. The first shield layer is disposed in such a manner that distance between the gradient coil and the first shield layer is shorter than distance between the gradient coil and the second shield layer, and a metallic material forming the first shield layer is higher in electrical conductivity than a metallic material forming the second shield layer.