MRI Thermal Bus Shield Structure for Eddy Current Stability
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Solution Overview
Problem
Magnetic stray fields from gradient coils in MRI devices cause eddy currents, leading to Ohmic heating and magnetic field inefficiencies, and conventional electromagnetic shields fail to adequately address temperature variations and resulting imaging artifacts.
Innovation Solution
A magnetic resonance imaging device with a thermal bus structure and electromagnetic shield, comprising spaced shield elements and a cryocooler, to manage heat and equalize eddy currents, using thermal bus elements with varying heat transfer properties and a control unit for dynamic temperature control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If active electromagnetic shields are used to reduce magnetic stray fields, then the impact of stray fields on the main magnet is significantly decreased, but additional cooling space and distance between main magnet and imaging region center are required, reducing generation efficiency
Solution Approach 1:
The patent extracts the electromagnetic shielding function from a separate active shield structure and integrates it into the main magnet assembly itself. The shield elements are positioned within the main magnet structure, allowing the main magnet to provide both its primary function and electromagnetic shielding simultaneously, thereby eliminating the need for additional cooling space and reducing the distance between the main magnet and imaging region center.
Solution Approach 2:
The patent merges the electromagnetic shielding function with the main magnet structure by integrating shield elements into the main magnet assembly. This combination allows the main magnet to serve dual purposes: generating the static magnetic field and providing electromagnetic shielding against stray fields from gradient coils, thus eliminating the need for separate active shield structures and their associated cooling systems.
2Area of stationary object
If disjoint electromagnetic shield structures are used to reduce footprint, then the shielding coverage is reduced, but temperature variations cause varying eddy currents that create imaging artifacts
Solution Approach 1:
The patent applies local quality by providing individualized thermal management for each shield element through dedicated thermal bus elements. Each shield element can be independently thermally coupled to the cold head, allowing localized temperature control that compensates for varying heat loads from gradient coils during different imaging sequences, thereby maintaining stable eddy currents and image quality while using a compact shield structure.
3Quantity of substance
If shield elements are spaced apart to reduce material usage, then the electromagnetic shielding effectiveness is reduced, but thermal connection is insufficient to stabilize eddy currents
Solution Approach 1:
The patent introduces thermal bus elements as intermediary thermal conduction paths between the shield elements and the cold head. These thermal bus elements act as mediators that efficiently transfer heat from the shield elements to the cooling system, enabling effective thermal management even when shield elements are spaced apart to reduce material usage while maintaining temperature stability for consistent eddy current behavior.
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
Reduces the impact of stray fields on the main magnet by at least 60-80%, minimizing temperature variations and preventing imaging artifacts, thereby improving MRI image quality.
Implementation Method 1
a thermal bus structure (31) comprising a plurality of thermal bus elements (31i) configured to provide a thermal connection between the plurality of spaced shield elements (12i) and a cold head (32a) of the cryocooler (32)
Implementation Method 2
an electromagnetic shield (12) arranged between the gradient system (19) and the main magnet (17)... configured to provide an electromagnetic shielding of the main magnet (17) from a magnetic field generated by the at least one gradient coil (11i)
Implementation Method 3
a cryocooler (32) having a cold head (32a)... configured to cool the electromagnetic shield (12) and the thermal bus structure (31)
Implementation Method 4
A magnetic resonance imaging device with a thermal bus structure and an electromagnetic shield comprising spaced shield elements made of high thermal and electrical conductivity materials
Data Source
AI summary
A magnetic resonance imaging device including a main magnet, a gradient system with at least one gradient coil, a cryocooler, a thermal bus structure, and an electromagnetic shield arranged between the gradient system and the main magnet. The electromagnetic shield includes spaced shield elements. The electromagnetic shield is configured to provide an electromagnetic shielding of the main magnet from a magnetic field generated by the at least one gradient coil. The thermal bus structure includes thermal bus elements configured to provide a thermal connection between the plurality of spaced shield elements and a cold head of the cryocooler. At least two thermal bus elements of thermal bus elements include different heat transfer properties to provide individualized temperature control of the spaced shield elements.