Cryostat Thermal Radiation Shield Design for MRI Systems
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Solution Overview
Problem
In superconducting magnet systems, particularly in MRI and combined imaging-radiation therapy systems, gradient coil-induced heating and eddy currents in thermal radiation shields and outer vacuum chambers cause heat load and magnetic field fluctuations, leading to reduced imaging quality and increased cryogen boil-off in 'wet' systems, with conventional solutions either reducing patient space or increasing magnet weight and cost.
Innovation Solution
The thermal radiation shield is shaped with recesses and altered profiles to move conductive surfaces away from the imaging region, reducing magnetic field interactions and eddy currents, and mechanical support structures are used to manage mutual attractive forces between magnet halves, allowing for improved access and reduced heating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If gradient shield coils are added to reduce gradient coil induced heating, then eddy currents and heating in thermal radiation shield are reduced, but the diameter available for patient is reduced and/or larger diameter coils are required
Solution Approach 1:
The invention extracts and removes the thermal radiation shield from the imaging region (central bore area) while maintaining it in peripheral regions. This eliminates the source of eddy currents in the imaging region without requiring gradient shield coils that would occupy space in the bore, thus resolving the contradiction between reducing gradient coil induced heating and maintaining patient space.
Solution Approach 2:
The thermal radiation shield is segmented into different regions: it is present in peripheral regions surrounding the imaging region but absent from the central imaging region itself. This segmentation allows the shield to continue providing thermal protection and reducing eddy currents in non-imaging areas while leaving the imaging region clear for patient access and avoiding gradient coil induced heating issues.
2Temperature
If conventional thermal radiation shield is used, then thermal protection is provided, but eddy currents are induced by oscillating magnetic fields causing heat load and magnetic field fluctuations
Solution Approach 1:
The thermal radiation shield is extracted from the imaging region where it would generate harmful eddy currents, while being maintained in peripheral regions where it provides necessary thermal protection. This spatial extraction resolves the contradiction between providing thermal protection and avoiding eddy current generation in the imaging region.
Solution Approach 2:
The thermal radiation shield is applied selectively to different regions with different requirements: it is present in peripheral regions where thermal protection is needed and absent from the central imaging region where eddy current generation would interfere with imaging. This local differentiation resolves the contradiction by providing thermal protection only where necessary without causing harmful effects.
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 design reduces eddy current-induced heating and magnetic field fluctuations by up to a factor of two, enhancing imaging quality and reducing the need for cryogenic cooling in 'wet' systems, while maintaining access for radiation therapy and other diagnostic equipment.
Implementation Method 1
the magnet coils 12 and the cryogen vessel 14 (if any) are surrounded by at least one thermal radiation shield 16
Implementation Method 2
the oscillating magnetic fields of the gradient coils may induce oscillating electrical currents in the material of the thermal radiation shield(s) 16
Implementation Method 3
These oscillating electrical currents will in turn generate heat, so-called 'gradient coil induced heating'
Implementation Method 4
the magnet coils 12 are partially immersed in liquid cryogen
Implementation Method 5
power dissipation caused by electrical currents due to gradient coil induced heating causes further boil-off of liquid cryogen
Data Source
AI summary
A split cylindrical superconducting magnet system including two half magnets, each half magnet comprising superconducting magnet coils retained in an outer vacuum chamber, having a thermal radiation shield located between the magnet coils and the outer vacuum chamber, wherein the thermal radiation shield is shaped such that the axial spacing between thermal radiation shields of respective half magnets is greater at their internal diameter than at their outer diameter.