Open MRI Magnetic Shielding for Eddy Current Suppression
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Solution Overview
Problem
Open-type MRI apparatuses face challenges in suppressing leaked magnetic fields, leading to eddy currents and image quality deterioration, with existing solutions either failing to completely shield the field or increasing manufacturing costs and magnetic energy requirements.
Innovation Solution
The use of a pair of static magnetic field magnets with discoid and annular poles, combined with Z coils and correction coils, effectively shields the magnetic flux and reduces eddy currents without dividing the magnetic poles, thereby improving image quality and reducing magnetic energy requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a gradient magnetic field coil of a self-shielded type (provided with an active shield) is used to suppress leaked magnetic field, then eddy current generation is reduced, but the static magnetic field magnet must be located away from the gradient magnetic field coil by an amount equivalent to the thickness of the shield, requiring an increase in magnetic energy and resulting in an expansive MRI apparatus
Solution Approach 1:
A magnetic shield made of high-permeability magnetic material is introduced as an intermediary between the gradient magnetic field coil and the static magnetic field magnet. This shield intercepts and guides the leaked magnetic flux through its high-permeability path, preventing it from reaching the static magnetic field magnet and causing eddy currents, while allowing the magnet to be positioned closer than would otherwise be required
Solution Approach 2:
The magnetic shield is divided into multiple segments (first magnetic shield and second magnetic shield) positioned at different locations. This segmentation allows the shield to effectively intercept magnetic flux from different regions of the gradient coil without requiring a single thick continuous shield, thereby reducing the overall distance requirement between the gradient coil and static magnetic field magnet
2Object-affected harmful factors
If a gradient magnetic field coil of a self-shielded type is used to suppress leaked magnetic field, then eddy current generation is reduced, but the MRI apparatus becomes expansive
Solution Approach 1:
The magnetic shield acts as an intermediary that captures and redirects leaked magnetic flux, allowing the static magnetic field magnet to be positioned closer to the gradient magnetic field coil without risking eddy current generation. This reduces the overall apparatus volume while maintaining effective eddy current suppression
Solution Approach 2:
By dividing the magnetic shield into multiple segments positioned strategically, the system achieves effective flux interception with a more compact configuration than a single thick continuous shield would require, thereby reducing apparatus size
3Object-affected harmful factors
If a gradient magnetic field coil with active shield is used, then leaked magnetic field is suppressed, but the coil has large inductance and requires a high-output drive power supply, making the MRI apparatus expensive
Solution Approach 1:
The magnetic shield serves as a passive intermediary that reduces the strength of leaked magnetic flux reaching the static magnetic field magnet. This allows the gradient magnetic field coil to operate with lower inductance and less demanding power supply requirements, as the shield provides additional flux containment without requiring the coil itself to be overly complex or high-powered
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 configuration enhances MRI image quality by suppressing eddy currents and achieving desired magnetic field intensity with lower magnetic energy, resulting in a cost-effective open-type MRI apparatus without the need for high processing or assembling accuracy.
Implementation Method 1
a magnetic shield configured to shield the static magnetic field magnet from the magnetic flux generated from the gradient magnetic field coil
Implementation Method 2
The gradient magnetic field coil generates a magnetic field in the form of a pulse, which has an intensity that is spatially gradient... The gradient magnetic field coil also generates a variable magnetic field (a leaked magnetic field) not necessary for imaging outside the imaging region
Implementation Method 3
The leaked magnetic field generates an eddy current on a metallic structure, such as the static magnetic field magnet, which constitutes the MRI apparatus. The eddy current generates the variable magnetic field in the imaging region, thus affecting distribution of the static magnetic field as well as the gradient magnetic field
Implementation Method 4
In an open-type MRI apparatus, a magnetic pole made of iron is frequently used for the static magnetic field magnet. The variable magnetic field caused by the leaked magnetic field also affects magnetization of the iron magnetic pole that has a hysteresis property
Data Source
AI summary
An open-type MRI apparatus includes a pair of static magnetic field magnets and a pair of gradient magnetic field coils. Each static magnetic field magnet includes a discoid magnetic pole configured to generate a static magnetic field in a Z axis direction in which the pair of static magnetic field magnets are opposed each other, and an annular magnetic pole configured to generate a static magnetic field on an X-Y plane perpendicular to the Z axis direction. Each gradient magnetic field coil includes a Z coil configured to provide a magnetic field being gradient in the Z axis direction in the imaging region, a magnetic material block configured to shield the discoid magnetic pole from a magnetic flux generated from the Z coil, and a correction coil configured to shield the annular magnetic pole from the magnetic flux generated from the Z coil.