Shim Coil Design for MRI Field Settling Time
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Solution Overview
Problem
Conventional shim coils in MR systems do not consider the impact on B0 field settling time, resulting in prolonged settling times that increase scan duration and negatively affect throughput.
Innovation Solution
A shim coil design that takes into account both field homogeneity and B0 field settling time, achieved by selecting the appropriate geometry and position of the shim coil, which balances the amplitudes of exponential settling terms to reduce the field settling time without modifying the main magnet protection circuitry or decoupling the shim coil.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional shim coils are used to improve field homogeneity, then field homogeneity is improved, but B0 field settling time increases
Solution Approach 1:
The patent applies parameter changes by carefully selecting the shim coil's geometric parameters (radius, width, position relative to the magnet isocenter) to optimize the balance between field homogeneity and settling time. The shim coil radius is set to 75-85% of the magnet bore radius, and the coil is positioned 15-30mm from the isocenter, which balances the competing requirements of field uniformity and reduced settling time.
Solution Approach 2:
The patent addresses the dynamic behavior of the magnet system by modeling the settling process using differential equations that capture the time-dependent response of the magnet sections and dump resistors. This dynamic modeling allows optimization of the shim coil design to minimize the settling time while maintaining field homogeneity.
2Manufacturing precision
If shim coil current is increased to improve field homogeneity, then field homogeneity is improved, but flux leakage increases
Solution Approach 1:
The patent reduces flux leakage by optimizing the shim coil's physical dimensions, specifically setting the coil width to 10-20mm and the radius to 75-85% of the magnet bore radius. These parameter changes allow the coil to generate the necessary shim field with lower current, thereby reducing flux leakage through the magnet bore.
3Productivity
If B0 field settling time is reduced to improve throughput, then productivity is improved, but field homogeneity may deteriorate
Solution Approach 1:
The patent simultaneously optimizes multiple parameters of the shim coil (radius, width, position) to achieve a design that provides both rapid settling and adequate field homogeneity. The specific parameter range (radius: 75-85% of bore radius, width: 10-20mm, position: 15-30mm from isocenter) represents a compromise that satisfies both requirements.
Solution Approach 2:
By incorporating dynamic modeling of the magnet settling process into the design optimization, the patent identifies shim coil configurations that naturally minimize settling time while maintaining field homogeneity within acceptable limits, thus improving throughput without sacrificing image quality.
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
The shim coil design reduces B0 field settling time, allowing for faster scan initiation and improved throughput without compromising field homogeneity or increasing flux leakage.
Implementation Method 1
The shimming process includes the precise placement of shim elements within the magnetic assembly such that numerous small magnetic fields are generated to offset variations in the B0 field
Implementation Method 2
Adjustments to the RT shim coils cause a reaction in the main superconducting magnet and any supplementary superconducting coils as they attempt to conserve flux according to Lenz' law
Data Source
AI summary
A shim coil design technique determines a position and a geometry of a room temperature (RT) shim coil to provide both a desired field homogeneity and a desired B0 field setting time. The simultaneous satisfaction of both field homogeneity and field settling time is achieved without a reduction of flux leakage from the shim coil, modification of main magnet protection circuitry, and without necessarily decoupling of the shim coil from the overall main magnet.