MRI Shim Iron Slit Patterns Reduce Eddy Current Heating
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Shim irons in MRI scanners experience heating due to eddy currents, leading to thermal drift and potential detrimental effects on image quality and geometric accuracy, as they are temperature-sensitive and prone to heating when the bore heats up during gradient-intensive sequences.
Innovation Solution
A shim iron is designed with a stack of shim plates, where at least two plates have slits forming different slit patterns that are not congruent when stacked, preventing constructive superposition of eddy currents and reducing heating by ensuring slits do not coincide, even with rotational symmetry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If shim irons are placed near the gradient field to correct magnetic inhomogeneities, then field homogeneity is improved, but heating due to eddy currents increases
Solution Approach 1:
The shim iron is divided into multiple shim plates stacked together. Each plate contains slits that interrupt eddy current paths. By segmenting the continuous shim iron structure into discrete plates with slits, the eddy current loops are broken into smaller segments, reducing the overall heating effect while maintaining the magnetic field correction function.
Solution Approach 2:
Consecutive shim plates are designed with differently oriented slits rather than identical patterns. This asymmetric arrangement prevents constructive superposition of eddy currents between adjacent plates. The non-uniform slit orientation across the stack ensures that eddy current paths do not align, thereby minimizing cumulative heating while preserving shimming effectiveness.
2Ease of operation
If shim irons are mechanically connected to the gradient-generating unit for positioning, then ease of installation is improved, but thermal drift increases due to vibration and heating
Solution Approach 1:
The shim iron is constructed as a stack of separate shim plates rather than a single monolithic piece. This segmentation allows each plate to be independently positioned and secured within the shim rail, providing mechanical stability while accommodating thermal expansion and vibration without compromising the overall shimming configuration.
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 reduces heating caused by eddy currents, making the shim iron less sensitive to temperature fluctuations and maintaining magnetic field homogeneity, thus improving image quality and geometric accuracy.
Implementation Method 1
Heating of the shim plates is mainly caused by eddy currents. A time variant magnetic flux (the gradient field) induces currents in the shim plates which heat the shim plates.
Implementation Method 2
A time variant magnetic flux (the gradient field) induces currents in the shim plates
Data Source
AI summary
The invention relates to a shim iron (130) for use with an magnetic resonance (MR) apparatus (10), wherein the shim iron (130) is comprised of a stack of shim plates (131, 132, 133, 134, 135), wherein at least two of the shim plates (131, 132, 133, 134, 135) comprise slits, the slits forming a respective slit pattern of the slit shim plates (131, 132, 133, 134, 135), and wherein the slit patterns, when viewed from the same viewing direction, are comprised of at least two different slit patterns which may not be brought into congruent coverage with each other. In this way, a shim iron (130) is provided which does not heat up to high temperatures due to eddy currents.