MRI Slice Selection Gradient Polarity for Metal Artifact Reduction
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Solution Overview
Problem
Magnetic resonance imaging (MRI) data acquisition is hindered by metallic objects due to significant image distortions caused by susceptibility differences, leading to unreliable diagnoses, especially with edge slices where off-resonant spins are not acquired, resulting in signal extinctions and prolonged measurement times.
Innovation Solution
The method involves analyzing the distortion in edge slices to select the polarity of the slice selection gradient, ensuring that the combination of susceptibility-induced interference fields and the slice selection gradient excites spins across the desired range, thereby reducing or eliminating signal extinctions by adjusting the polarity based on the primary direction of distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a standard slice selection gradient is used for MRI acquisition, then the measurement process is simple and fast, but signal extinctions occur in edge slices due to off-resonant spins not being excited
Solution Approach 1:
The slice selection gradient polarity is made dynamic and adaptable rather than fixed. The system determines the primary direction of distortion in edge slices and adjusts the gradient polarity accordingly, allowing the imaging system to adapt to local field conditions and eliminate signal extinctions without sacrificing measurement efficiency
Solution Approach 2:
The polarity of the slice selection gradient is changed based on the determined primary direction of distortion. By flipping the gradient polarity in edge slices where distortion causes signal loss, the system recovers excited spins that would otherwise fall outside the excitation bandwidth, eliminating signal extinctions while maintaining fast measurement
2Manufacturing precision
If additional phase-encoding steps are applied to correct slice distortion (SEMAC method), then spatial and spectral extensions of field distortions are covered, but the measuring time is significantly prolonged
Solution Approach 1:
The invention extracts and addresses only the critical problem affecting edge slices (signal extinctions due to distortion) rather than applying a comprehensive correction method to all slices. By focusing computational and measurement resources only on edge slices and using a simple polarity flip based on distortion direction, the system achieves practical improvement without the time penalty of full SEMAC correction
Solution Approach 2:
Different treatment is applied to different regions: edge slices receive adaptive polarity adjustment based on their specific distortion characteristics, while inner slices use the standard gradient. This localized approach corrects problems where they occur without unnecessarily complicating or prolonging the measurement of unaffected regions
3Ease of operation
If the slice selection gradient polarity is fixed, then the acquisition process is simple, but signal extinctions occur when distortion pushes spins outside the excitation bandwidth
Solution Approach 1:
The system performs a preliminary determination of the primary distortion direction in edge slices before final image acquisition. This preliminary analysis allows the system to pre-set the appropriate gradient polarity for edge slices, ensuring that spins are properly excited without signal extinctions while maintaining overall process simplicity
Solution Approach 2:
When distortion causes spins to shift outside the excitation bandwidth in a given direction, the invention inverts the gradient polarity in affected edge slices. This reversal compensates for the distortion-induced frequency shift, bringing spins back into the excitation bandwidth and eliminating signal loss while keeping the method simple
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 reduces signal losses and maintains or minimizes the measurement time by optimizing the slice selection gradient polarity, allowing for improved image quality and visualization of anatomy near metal objects, such as in hip replacements or other metal implants.
Implementation Method 1
a slice selection gradient rising in one direction is used to select a slice to be acquired... the overall magnetic field in the homogeneity region rises in one direction. The result is a change in the resonance frequency along the slice-selection direction.
Implementation Method 2
Radio-frequency excitations are used in order to deflect nuclear spins in an examination subject situated in a basic magnetic field (B0 field)
Implementation Method 3
metallic objects are present in the target area, for example metallic implants in a patient... the distortion of the static magnetic basic field (B0 field), which in turn is attributable to the great difference in the magnetic susceptibility between body tissue and metal
Implementation Method 4
the combination of susceptibility-induced interference fields and the slice selection gradient excites spins across the desired range... The resonance frequency deviation (often also designated the off-resonance frequency) corresponds to the frequency difference between the actual or theoretical resonance frequency
Data Source
AI summary
In a method and apparatus for acquiring a magnetic resonance (MR) data set from a target area of a patient containing at least one metal object that distorts the basic magnetic field due to susceptibility differences, a slice selection gradient that rises in one direction is used to select a slice from which MR data are to be acquired. At least for at least one outermost edge slice on one side of the slice stack from which the MR data are to be acquired, the polarity of the slice selection gradient is selected as a function of a primary direction of distortion in the edge slice.