MRI Thermal Radiation Shield Mass Patterning Against Resonance
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Solution Overview
Problem
Conventional magnetic resonance imaging systems experience mechanical oscillations due to interactions between thermal radiation shields and oscillating gradient magnetic fields, leading to excessive heating and potential magnet quenching, which existing solutions either limit pulse sequence flexibility, increase costs, or fail to entirely eliminate resonant frequencies.
Innovation Solution
A thermal radiation shield with random or pseudo-random variations in mass per unit area, thickness, stiffness, or local shape is introduced to break up geometric symmetry, reducing the likelihood of mechanical resonance and energy transfer to the magnet cold mass.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional cylindrical thermal radiation shield with uniform geometry is used, then the shield effectively reduces thermal radiation from the vacuum vessel to the magnet cold mass, but the shield experiences mechanical resonance when excited by oscillating gradient magnetic fields, leading to excessive heating and potential magnet quenching
Solution Approach 1:
The patent applies asymmetry by introducing non-uniform geometric features to the thermal radiation shield. Specifically, the shield includes axial variations in radius and/or thickness, creating an asymmetric mass distribution along its length. This asymmetric geometry disrupts the formation of mechanical resonance modes that would otherwise be excited by oscillating gradient magnetic fields, thereby reducing mechanical oscillations and associated heating while maintaining thermal shielding effectiveness.
Solution Approach 2:
The patent implements local quality by creating regions with different mass-per-unit-area along the shield's length. The mass distribution is deliberately varied, with some axial regions having greater mass than others. This local variation in mass quality prevents uniform resonance throughout the structure, as different segments respond differently to external magnetic field excitations, thereby suppressing overall mechanical oscillation amplitude.
2Object-generated harmful factors
If the thermal radiation shield is made thicker to reduce mechanical resonance, then the amplitude of mechanical vibrations is reduced, but the cost increases due to increased material consumption and weight
Solution Approach 1:
Instead of uniformly increasing the shield thickness, the patent applies local quality by varying the mass distribution only in specific axial regions. This targeted approach provides the necessary vibration suppression through asymmetric mass distribution while minimizing overall material consumption. The shield maintains its base thickness but incorporates localized mass variations that achieve the desired mechanical damping effect at lower cost and weight.
Solution Approach 2:
The patent changes geometric parameters (radius, thickness) selectively along the axial direction rather than uniformly increasing all dimensions. By modifying parameters locally to create mass variations, the shield achieves reduced mechanical resonance with minimal additional material, avoiding the proportional increase in cost and weight that would result from uniform thickening.
3Object-generated harmful factors
If forbidden frequency bands are introduced into the gradient pulse sequence to avoid mechanical resonance, then resonant conditions are avoided, but the flexibility of pulse sequence programming is limited and K-space access efficiency is reduced
Solution Approach 1:
The patent extracts the resonance problem from the control software domain and relocates it to the physical structure domain. By incorporating asymmetric mass distribution directly into the shield's geometry, the resonance characteristics are passively modified at the hardware level. This eliminates the need for software-based frequency restrictions, allowing full flexibility in gradient pulse sequence programming while inherently preventing resonant conditions through the shield's asymmetric mass distribution.
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 approach effectively reduces mechanical oscillations and heating of the magnet cold mass, allowing for more flexible gradient pulse sequences and reduced cooling demands, while avoiding the limitations of previous methods.
Implementation Method 1
To reduce radiant thermal influx from the vacuum vessel to the magnet cold mass, it is typical to have a thermally conductive thermal radiation shield interposed between the vacuum vessel and the magnet cold mass
Implementation Method 2
The oscillating gradient magnetic fields interact with the electrically conductive thermal radiation shield to generate oscillating electrical currents within the thermal radiation shield
Implementation Method 3
These oscillating electrical currents in turn interact with the static background magnetic field and generate mechanical oscillations in the system
Implementation Method 4
The mechanical vibration of the 50 K shield in turn produces oscillating magnetic fields which reach the cold mass, in turn inducing eddy currents in the magnet cold mass
Data Source
AI summary
A method for reducing a tendency of a thermal radiation shield for a superconducting magnet of a magnetic resonance imaging system to vibrate. A mass per unit area of the material of the thermal radiation shield is locally modified in a random or pseudo-random pattern.