MRI Thermal Radiation Shield Asymmetry for Resonance Suppression
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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
Introducing random or pseudo-random variations in the thickness, stiffness, or local shape of the thermal radiation shield by adding or removing material, breaking up geometric symmetry to reduce mechanical resonance tendencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the thermal radiation shield is made of uniform thickness and symmetric geometry, then manufacturing is simple and cost-effective, but mechanical resonance occurs when excited by oscillating gradient magnetic fields
Solution Approach 1:
The patent applies asymmetry by introducing non-uniform thickness variations and asymmetric geometric features into the thermal radiation shield. Specifically, the shield includes regions of different thickness (first region with thickness t1, second region with thickness t2 where t1 ≠ t2) and asymmetric cutouts or protrusions that break the geometric symmetry. This asymmetric design shifts and disperses the mechanical resonance frequencies, preventing resonance at the gradient coil operating frequencies while maintaining manufacturing feasibility through standard fabrication techniques.
2Stability of the object's composition
If the 50K shield thickness is increased to reduce mechanical vibrations, then vibration amplitude decreases, but material cost and shield weight increase
Solution Approach 1:
The patent applies local quality by varying the thickness of the thermal radiation shield locally rather than uniformly throughout. The shield has first regions with thickness t1 and second regions with thickness t2, where the thicker regions provide enhanced mechanical stability and vibration damping, while the thinner regions reduce overall material consumption. This localized thickening strategy places additional material only where mechanically critical, optimizing the balance between vibration reduction and material efficiency.
3Stability of the object's composition
If forbidden frequency bands are introduced into gradient pulse sequences, then mechanical resonance is avoided, but pulse sequence flexibility and K-space sampling efficiency are reduced
Solution Approach 1:
The patent applies preliminary action by pre-modifying the thermal radiation shield's physical structure to eliminate resonance susceptibility before gradient pulse sequences are executed. The asymmetric thickness variations and geometric features are built into the shield during manufacturing, creating a passive mechanical filter that prevents resonance across a broad frequency range. This upfront structural modification eliminates the need for software-based frequency restrictions, allowing gradient pulse sequences to operate with full flexibility and optimal K-space sampling efficiency without encountering resonant conditions.
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 energy transfer to the magnet cold mass, minimizing heating and allowing for more flexible gradient pulse sequences while maintaining system stability.
Implementation Method 1
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 2
These oscillating electrical currents in turn interact with the static background magnetic field and generate mechanical oscillations in the system
Implementation Method 3
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
Data Source
AI summary
A method for reducing the tendency of a thermal radiation shield for a superconducting magnet of a magnetic resonance imaging system to vibrate, in particular to reduce mechanical oscillations caused by interaction of such thermal radiation shield with an oscillating gradient magnetic field. The mass per unit area of the material of the thermal radiation shield is locally modified in a random or pseudo- random pattern, such that the tendency for the development of mechanical oscillation resonances is reduced.