MRI Thermal Radiation Shield Weld Pattern for Resonance Control
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Solution Overview
Problem
Conventional magnetic resonance imaging systems experience mechanical oscillations due to interactions between the thermal radiation shield and oscillating gradient magnetic fields, leading to excessive heating and potential magnet quenching, which existing solutions like introducing 'forbidden' frequency bands or increasing shield material thickness do not fully address.
Innovation Solution
Introducing pseudo-random variations in the welding pattern between the thermal radiation shield's end walls and bore tube to break up geometric symmetry, reducing the likelihood of mechanical resonance and energy transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the thermal radiation shield is made from high conductivity material to reduce thermal radiation, then thermal protection is improved, but mechanical oscillations increase due to eddy currents generated by time varying magnetic fields
Solution Approach 1:
The patent introduces geometric asymmetry into the thermal radiation shield by varying the weld lengths between the end walls and bore tube. This asymmetric weld pattern breaks up the symmetry of the shield structure, which disrupts the formation of resonant modes and reduces mechanical oscillations caused by eddy currents, while maintaining the high conductivity material for thermal protection.
2Reliability
If gradient shield coils are located radially some distance from main gradient coils to be more effective, then magnetic field shielding is improved, but the stray gradient field interaction with the thermal radiation shield increases
Solution Approach 1:
The asymmetric weld pattern in the thermal radiation shield reduces its susceptibility to mechanical oscillations from stray gradient fields. By breaking the geometric symmetry, the shield becomes less prone to resonant excitation from the time-varying magnetic fields generated by the gradient coils, even when positioned at optimal distances for magnetic shielding.
3Ease of manufacture
If conventional symmetrical welding patterns are used to attach end walls to bore tube, then manufacturing simplicity is maintained, but mechanical resonance is excited by oscillating magnetic fields
Solution Approach 1:
The patent deliberately introduces asymmetry into the welding pattern by varying weld lengths around the circumference of the bore tube. This asymmetric configuration maintains reasonable manufacturing simplicity while effectively breaking the geometric symmetry that would otherwise allow mechanical resonance to be excited by oscillating magnetic fields.
4Object-affected harmful factors
If pseudo-random variations in weld pattern are introduced to break geometric symmetry, then mechanical resonance is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent applies parameter changes by varying the lengths of welds in a pseudo-random pattern around the circumference of the bore tube. This approach introduces sufficient asymmetry to break mechanical resonance modes while maintaining a relatively simple manufacturing process, as the variations can be implemented during standard welding operations without requiring complex jigs or fixtures.
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 resonance and associated heating, enhancing the stability of the superconducting magnet and allowing for higher power gradient coil systems without destabilizing the imaging volume.
Implementation Method 1
A thermal radiation shield 14 is interposed between the main magnet coils 10 and the OVC, to reduce the amount of thermal radiation from the OVC (at a temperature of approximately 300 K) which reaches the main magnet coils
Implementation Method 2
They usually comprise suitably-shaped coils of copper or aluminium wire embedded in thermosetting resin
Implementation Method 3
in which case large eddy currents will be generated if subjected to a time varying magnetic field
Implementation Method 4
The oscillating gradient magnetic fields interact with the electrically conductive thermal radiation shield to generate oscillating electrical currents within the thermal radiation shield. These oscillating electrical currents in turn interact with the static background magnetic field and generate mechanical oscillations in the system
Implementation Method 5
The assembly is essentially symmetrical about axis A-A. Directions parallel to axis A-A will be referred to herein as 'axial'
Implementation Method 6
Main magnet coils 10 produce a static magnetic field of high strength
Implementation Method 7
Main gradient coils 22 typically comprise a number of resistive coils, arranged to generate time-variant magnetic fields in three orthogonal planes
Implementation Method 8
They complement the main gradient coils 22 to produce the required pattern of time-variant gradient fields within the bore 18
Data Source
AI summary
A device for use in magnetic resonance imaging (MRI) systems may include a superconducting main magnet coil; and a thermal radiation shield that encloses the superconducting main magnet coil. The shield may include an inner cylindrical bore tube, an outer cylindrical wall, and annular end walls welded between the annular cylindrical bore tube and the outer cylindrical wall to form a closed, hollow cylindrical vessel. A distribution of a position and length of welds that affix the annular end walls to the inner cylindrical bore tube may include a predetermined arrangement of welds of varying lengths interspersed with gaps of varying lengths.