Ferromagnetic Marker Geometry for Low-Artefact MRI Guidance
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Solution Overview
Problem
Existing magnetic markers used for surgical guidance create significant MRI artefacts, compromising the assessment of tumour size during MRI monitoring, which is crucial for cancer management, especially for breast cancer where tumour size determines the type of surgery required.
Innovation Solution
Development of a magnetic marker with a high length-to-diameter ratio and low volume, using ferromagnetic materials with high initial relative permeability and low saturation induction, configured to minimize MRI artefacts while maintaining effective sensing performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a magnetic marker with high magnetic susceptibility is used for surgical guidance, then the sensing performance and detection distance are improved, but the MRI artefact size increases significantly
Solution Approach 1:
The marker is segmented into multiple ferromagnetic elements arranged in a specific geometric configuration (e.g., tetrahedral arrangement of 4 spheres, or cubic arrangement of 8 spheres). This segmentation reduces the volume of each individual ferromagnetic element while maintaining the overall magnetic susceptibility and sensing performance through collective interaction, thereby reducing MRI artefact size
Solution Approach 2:
The marker uses composite construction combining ferromagnetic materials with non-magnetic materials (such as titanium or polymer coatings). This composite approach allows the ferromagnetic components to provide necessary magnetic susceptibility for sensing while the non-magnetic components reduce overall magnetic susceptibility and minimize MRI artefact generation
2Object-generated harmful factors
If the marker volume is reduced to minimize MRI artefact, then the MRI image quality is improved, but the sensing signal strength decreases
Solution Approach 1:
Multiple small ferromagnetic elements are arranged in specific geometric configurations (tetrahedral, cubic, or linear arrangements) that optimize the magnetic moment per unit volume. The segmented structure allows each element to contribute to the overall magnetic signal while maintaining small individual volumes that minimize MRI artefact
Solution Approach 2:
The marker optimizes parameters such as the number of ferromagnetic elements, their individual volumes, spacing between elements, and material composition to achieve the optimal balance between sensing signal strength and MRI artefact reduction. The geometric arrangement parameters are specifically tuned to maximize magnetic susceptibility while minimizing artefact
3Length of stationary object
If ferromagnetic material is used to enhance magnetic detection capability, then the detection distance is increased, but the MRI artefact is enlarged
Solution Approach 1:
The ferromagnetic material is divided into multiple small elements distributed over a larger spatial volume. This segmentation extends the effective detection distance through increased spatial distribution of magnetic moment while each small element generates minimal MRI artefact, keeping the total artefact size acceptable
Solution Approach 2:
The marker transitions from a compact single-element design to a multi-element spatial distribution (e.g., tetrahedral or cubic arrangements). This dimensional expansion allows the magnetic moment to be distributed in three-dimensional space, extending detection distance while maintaining small individual element volumes that minimize MRI artefact
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 marker provides satisfactory sensing response with MRI artefacts smaller than 2 cm, allowing accurate tumour size assessment under MRI, enabling appropriate surgical planning and reducing the need for more invasive procedures.
Implementation Method 1
uses magnetic fields and a magnetic marker with high magnetic susceptibility. A handheld susceptometry probe generates an alternating field which excites a magnetically responsive marker, and detects the responding magnetic field
Implementation Method 2
Ferromagnetic materials are well known for creating MRI distortions... The artefact is predominantly generated by the component of the magnetic field generated by the ferromagnetic object (By) that is in the same direction as the main field produced by the MRI machine. The effect of By is to shift the local Larmor frequency of protons near the object
Data Source
AI summary
An implantable marker for imaging and surgical guidance by susceptometry comprising one or more pieces of a ferromagnetic material having a total length to diameter ratio of at least about 500, and a total volume of less than about 1×10−11 m3. The one or more pieces of ferromagnetic material may have a high initial relative permeability (μr,i)>about 1000. Also disclosed is a detection system for locating an implantable marker comprising such an implantable marker; at least one drive coil arranged to excite the marker with an alternating magnetic field, and at least one sense coil arranged to detect a signal received from the excited marker; a magnetic field generator arranged to drive an alternating magnetic field through the at least drive coil; and at least one detector arranged to receive the signal from the sense coil and detect one or more harmonics of the drive frequency in the received signal.