Cylinder-Symmetrical Permanent Magnet Arrangement for MR Stray Field Reduction
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Solution Overview
Problem
Existing permanent magnet systems for magnetic resonance applications generate significant magnetic stray fields, occupy large volumes, and are prone to component displacement issues, which complicates long-term operation and increases material and structural costs.
Innovation Solution
The magnet arrangement features a closed yoke design with a hollow cylindrical drum and lid structure, enclosing the measuring volume axially and radially, using cylinder-symmetrical axial and radial magnet elements, which reduces stray fields and secures components against movement, allowing for a more compact and efficient configuration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If traditional open yoke designs (rectangular frame, H-yoke, C-yoke) are used, then structural simplicity is maintained, but magnetic stray fields extend far beyond the outer contours of the magnet arrangement
Solution Approach 1:
The yoke is divided into multiple segments (first yoke segment, second yoke segment, third yoke segment) that can be arranged in different configurations. This segmentation allows the yoke to enclose the measuring volume more effectively while maintaining manufacturing simplicity and reducing stray fields without requiring a completely new complex structure
Solution Approach 2:
The measuring volume is enclosed within the yoke structure, with magnet assemblies positioned within the yoke segments. This nested arrangement ensures that the magnetic flux is contained within the yoke, significantly reducing stray fields extending beyond the outer contours while maintaining a compact overall structure
2Device complexity
If yoke-free Halbach magnets are used, then structural complexity is reduced, but correction mechanisms are required to compensate for tolerances, complicating mechanical construction
Solution Approach 1:
The yoke acts as an intermediary element between the magnet assemblies and the external environment. It provides a stable magnetic return path that compensates for tolerances in magnet positioning and dimensions, eliminating the need for complex correction mechanisms while maintaining field homogeneity through the yoke's geometric design
Solution Approach 2:
The yoke's geometric parameters (dimensions, shapes of segments) are specifically designed to optimize magnetic flux distribution. By adjusting these geometric parameters, the system achieves field homogeneity without requiring active correction mechanisms, simplifying the mechanical construction while maintaining manufacturing precision
3Stability of the object's composition
If central permanent magnets are connected to pole pieces with supporting yokes at corners, then structural stability is improved, but the system volume increases and material expenditure increases
Solution Approach 1:
The yoke segments are merged to form a continuous magnetic circuit that provides structural support throughout the arrangement. This integrated design eliminates the need for separate supporting yokes at corners while maintaining structural stability, reducing overall volume and material expenditure by combining support and magnetic return functions in a single optimized structure
4Ease of manufacture
If open structure magnet arrangements are used, then ease of assembly is improved, but component displacement occurs due to high magnetic forces, reducing long-term reliability
Solution Approach 1:
The yoke segments are pre-assembled with magnet assemblies before final installation. This preliminary assembly ensures proper positioning and pre-loads the magnetic circuits, preventing component displacement during operation while maintaining ease of assembly through modular pre-fabricated units that can be installed as complete assemblies
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 effectively minimizes stray fields, enhances long-term reliability by preventing component displacement, and results in a more compact and cost-effective magnet system with improved field homogeneity and material efficiency.
Implementation Method 1
a permanent magnet system for generating a homogeneous magnetic field in the direction of a z axis in a measuring volume
Implementation Method 2
the magnetic flux has to be focussed... In yoke-based magnets, sufficient shielding of the stray field can be achieved well below the saturation flux density through skilful design of the yoke
Data Source
AI summary
A magnetic resonance arrangement with a permanent magnet system and having magnet elements, pole piece elements and yoke elements of magnetic material arranged cylinder-symmetrically with respect to the z axis. The yoke elements have a first lid (11′) and a second lid (11″) and also a hollow cylindrical drum (12) arranged axially between the lids. The yoke elements enclose the measuring volume in the axial and radial direction. The magnet elements each include a pair of cylinder-symmetrical axial magnets (13′, 13″) and also radial magnet rings (14′, 14″). The axial magnets are each arranged axially adjoining the lids and are arranged radially within the radial magnet rings and respectively axially further away from the measuring volume than the radial magnet rings. The outer diameter of the axial magnets is less than or equal to the inner diameter of the radial magnet rings.