MAVRIC SEMAC Gradient Polarity for MRI Metal Artifact Reduction
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Solution Overview
Problem
Current multi-spectral imaging techniques like MAVRIC and SEMAC suffer from susceptibility artefacts due to metal objects, leading to back-folded signals and inadequate spatial selectivity, as they are not effectively limited to restricted spatial and frequency ranges.
Innovation Solution
Adapting the acquisition schemes of MAVRIC and SEMAC by using different gradient strengths and polarities during refocusing and excitation to limit signal selection to a restricted spatial region and frequency band, ensuring spatial selectivity and avoiding back-folding of off-resonant signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multi-spectral imaging techniques (MAVRIC/SEMAC) are used to reduce susceptibility artefacts, then image quality near metal objects is improved, but back-folded signals occur due to lack of spatial selectivity
Solution Approach 1:
The patent applies local quality by making the gradient selection spatially dependent. Different gradient strengths are applied in different spatial regions: a first gradient strength is used for spins within a predefined distance from the metal object, while a second gradient strength is used for spins beyond that distance. This resolves the contradiction by locally optimizing the gradient selection to prevent back-folding in critical regions while maintaining image quality.
Solution Approach 2:
The patent changes the gradient strength parameter based on spatial location relative to metal objects. By dynamically adjusting the gradient strength parameter (first strength vs. second strength) depending on the distance from metal objects, the system prevents back-folded signals while maintaining effective suppression of susceptibility artefacts in the regions where metal objects are present.
2Measurement precision
If a small encoded volume is used in MAVRIC to improve spatial resolution, then detail imaging is improved, but back-folded signal occurs due to non-volume selective acquisition
Solution Approach 1:
The patent applies local quality by implementing volume selectivity through spatially dependent gradient selection. For regions where high spatial resolution is needed, the system applies appropriate gradient strengths to limit the encoded volume, preventing back-folded signals while maintaining the desired spatial resolution in the encoded region.
Solution Approach 2:
The patent segments the imaging space into different regions based on distance from metal objects. By dividing the space and applying different gradient strengths to different segments, the system achieves volume selectivity where needed while maintaining the ability to suppress artefacts in metal-affected regions.
3Device complexity
If SEMAC uses single frequency band selection with single gradient strength, then acquisition is simplified, but distant off-resonance signals are selected causing back-folding
Solution Approach 1:
The patent resolves this contradiction by applying local quality through spatially dependent gradient selection. Instead of using a uniform gradient strength throughout the entire imaging volume, the system applies a first gradient strength for spins within a predefined distance from metal objects and a second gradient strength for spins beyond that distance. This prevents back-folded off-resonance signals while maintaining relatively simple acquisition protocols.
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 enhances the spatial selectivity of MAVRIC and SEMAC, reducing the risk of back-folding distant off-resonant signals, making multi-spectral imaging more effective in the presence of metal objects, thereby improving diagnostic imaging capabilities, especially for patients with metal implants.
Implementation Method 1
The magnetic resonance imaging system comprises a magnet (1004) generating a magnetic field, magnetic field gradient coils (1010) connected to a magnetic field gradient coil power supply (1012)
Implementation Method 2
a radio frequency coil (1014) for selectively exciting and for selectively receiving magnetic resonance signals from magnetic spins within a restricted imaging region (1024) of the subject
Data Source
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AI summary
The invention relates to magnetic resonance imaging in the vicinity of a metallic object (like, for instance, a metal implant) where severe spatial perturbations of the static magnetic field occur. In order to suppress the back-folding of distant off-resonant signals into the region of interest, the imaging volume is spatially restricted by means of selection gradients applied concurrently with the excitation and the refocusing RF pulses in a spin echo sequence. The selection gradient applied during the excitation pulse has an amplitude and/or a polarity different from that of the selection gradient applied during the refocusing pulse so that the respectively selected slices in an off-resonance frequency versus spatial coordinate diagram become tilted with respect to one another. The applied imaging technique may of the SEMAC or MAVRIC type and may incorporate compressed sensing, parallel imaging, fat suppression and/or SVD-based noise reduction.