Cryostat Annular Segmentation for MR-Linac Beam Attenuation
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Solution Overview
Problem
Combining MR imaging with radiation therapy is challenging due to the attenuation of radiotherapy beams by the cryogenic fluid and varying magnetic field strengths in the cryostat, making it difficult to accurately control the radiation energy delivery to the target area, especially at different angular positions.
Innovation Solution
The design of a magnet assembly with a cryostat featuring annular sections of varying internal volumes and cryogenic fluid depths allows the radiotherapy beam to pass through a thinner, more stable cryogenic fluid layer while maintaining a large volume for cooling, ensuring consistent energy delivery and structural integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a large volume of cryogenic fluid is used in the cryostat, then the cooling capacity and structural integrity are improved, but the attenuation of the radiotherapy beam increases and becomes variable with angular position
Solution Approach 1:
The cryostat inner chamber is divided into multiple annular sections (first, second, and third annular sections) with different internal volumes. The first and second annular sections have larger volumes for cooling, while the third annular section has a smaller volume in the beam path. This segmentation allows the system to maintain both large overall cooling capacity and reduced beam attenuation.
Solution Approach 2:
Different regions of the cryostat are given different properties: the first and second annular sections have larger internal volumes to provide cooling capacity, while the third annular section has a smaller internal volume specifically in the region where the radiotherapy beam passes through. This local differentiation optimizes both cooling and minimizes beam attenuation.
2Temperature
If the cryostat is completely filled with liquid helium, then the cooling is maximized, but the radiotherapy beam attenuation becomes highly variable with angular position and difficult to control
Solution Approach 1:
The inner chamber is segmented into multiple annular sections with controlled fill levels. The cryogenic fluid is maintained at a level that does not completely fill the cryostat, specifically creating a thinner layer in the third annular section where the beam passes. This allows cooling while maintaining constant beam attenuation.
Solution Approach 2:
The design changes the physical parameter of cryogenic fluid depth/level to optimize both cooling and beam delivery. By controlling the fluid level to create varying depths in different annular sections, the system achieves adequate cooling while minimizing and stabilizing beam attenuation to less than 1% variation.
3Ease of manufacture
If a conventional uniform cryostat design is used, then the manufacturing is simpler, but the radiotherapy beam attenuation varies significantly with angular position
Solution Approach 1:
The inner chamber is divided into multiple annular sections with different internal volumes. This segmentation creates a non-uniform configuration that stabilizes beam attenuation while remaining manufacturable using standard cryostat fabrication techniques.
Solution Approach 2:
The cryostat inner chamber employs an asymmetric design where different annular sections have different internal volumes. The third annular section has a smaller internal volume compared to the first and second sections, creating an asymmetric configuration that optimizes beam attenuation stability.
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 configuration stabilizes the attenuation of the radiotherapy beam, allowing for precise control of radiation energy delivery to the target area, reducing damage to healthy tissue and improving treatment accuracy by minimizing the impact of cryogenic fluid level changes and angular position variations.
Implementation Method 1
An MR imager or scanner typically employs a superconducting magnet to generate the large magnetic fields which it requires for operation. To realize superconductivity, a magnet is maintained in a cryogenic environment at a temperature near absolute zero.
Implementation Method 2
The magnet includes one or more electrically conductive coils which are disposed in a cryostat and through which an electrical current circulates to create the magnetic field.
Implementation Method 3
a large cryogenic bath (e.g., liquid helium) disposed in a cryostat
Implementation Method 4
A cooling arrangement comprising cooling tubes is arranged in thermal contact with the superconducting coils and arranged to receive a cryogen flowing through the cooling tubes.
Implementation Method 5
the radiation beam should pass through the MR imager or scanner in a controlled and known manner so that the magnitude and location of energy delivered by the radiotherapy beam can be accurately controlled.
Data Source
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AI summary
A chamber (422) for a cryostat (420) includes: first and second annular sections (4221, 4222) separated and spaced apart from each other along a first direction, and a third annular section (4223) extending in the first direction between the first and second annular sections and connecting the first and second annular sections to each other. The first and second annular sections define corresponding first and second internal volumes, the third annular section defines a third internal volume, and the third internal volume is substantially less than the first internal volume and substantially less than the second internal volume.