Shielding Coil for Reducing Eddy Currents in Prepolarized MRI
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Solution Overview
Problem
In ultra-low-field MRI, prepolarized magnetic measurements face challenges with stray magnetic fields induced by eddy currents, which degrade image quality and interfere with simultaneous MEG recordings, due to the strong transient fields caused by the switching of prepolarizing coils in magnetically shielded rooms.
Innovation Solution
A novel coil arrangement involving a prepolarizing coil and a shielding coil, where the shielding coil is pulsed to counteract the transient fields, either by being connected in series with the prepolarizing coil or functioning as a separate dynamic shielding coil, to minimize eddy currents and maintain image quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the prepolarizing field strength is increased to improve signal-to-noise ratio, then image quality improves, but eddy currents are induced in the magnetically shielded room which generate unwanted transient fields and interfere with measurements
Solution Approach 1:
A shielding coil is introduced that generates a magnetic field opposite to the prepolarizing field. By controlling the shielding coil current, the unwanted transient fields and eddy currents are counteracted before they can significantly interfere with the measurement, allowing higher prepolarizing field strengths to be used safely
Solution Approach 2:
The shielding coil acts as an intermediary element between the prepolarizing coil and the magnetically shielded room. It mediates the interaction by generating compensating fields that prevent the direct coupling between the prepolarizing field and the conductive structures, thereby reducing eddy currents while maintaining measurement precision
2Productivity
If the prepolarizing field is switched off rapidly to reduce imaging time, then productivity improves, but strong transient fields are generated that interfere with MEG recordings and spin dynamics
Solution Approach 1:
The shielding coil is activated in synchronization with the prepolarizing field switching. When the prepolarizing field is rapidly switched off, the shielding coil generates a compensating field that counteracts the transient fields, allowing rapid switching without interfering with MEG recordings or spin dynamics
Solution Approach 2:
The shielding coil current is pulsed periodically in coordination with the prepolarizing field pulses. Each pulse of the shielding coil corresponds to the switching events of the prepolarizing field, providing continuous compensation during the dynamic phases while remaining inactive during stable measurement phases
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 approach significantly reduces unwanted transient fields, allowing for higher prepolarization field strengths, shorter imaging times, and improved patient comfort, while maintaining signal integrity and compatibility with MEG recordings.
Implementation Method 1
Changing magnetic fields induce eddy currents in conducting structures nearby. The shielding coil and second pulsing scheme are arranged to reduce unwanted transient fields, such as those caused by induced eddy currents
Implementation Method 2
the sample to be imaged needs to be prepolarized in a stronger magnetic field Bp, typically of the order of 10-200 mT, before the weaker, typically homogeneous, field B0 and the gradient fields are applied
Data Source
AI summary
The invention relates to a prepolarizing magnetic resonance- or relaxation-based measurement system, comprising a prepolarizing coil for producing a prepolarizing field at the target zone, means for pulsing the prepolarizing field according to a first pulsing scheme, and means for measuring magnetization of a target placed in the target zone. According to the invention, the system further comprises a shielding coil for producing a shielding field and means for pulsing the shielding field according to a second pulsing scheme, whereby the shielding coil and the second pulsing scheme are arranged to reduce the formation of unwanted transient fields caused by the coupling of the prepolarizing coil to conducting or magnetic structures in the surroundings of the system. The invention also relates to a corresponding method of measurement and a process of designing the pulsing schemes for the system or process. By means of the invention, the formation of unwanted eddy currents, for example, in the surroundings of the measurement system, can be reduced.