Maxwell Correction for Simultaneous Multi-Slice MRI
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Solution Overview
Problem
Simultaneous multi-slice (SMS) magnetic resonance imaging acquisitions suffer from signal degradation due to concomitant fields, especially at low magnetic fields and high b-values, where conventional correction methods are not applicable for slices with large spatial separation, leading to reduced image quality and increased g-factor penalty.
Innovation Solution
The solution involves reducing the spatial distance between simultaneously acquired slices and calculating Maxwell correction gradient moments at an average position between slices, which provides a relatively accurate correction for each individual slice, either as an exact or weighted average, to mitigate signal degradation caused by concomitant fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If slices are separated by large spatial distance in SMS acquisitions, then g-factor penalty is reduced, but signal degradation from concomitant fields increases
Solution Approach 1:
The patent applies preliminary action by calculating and applying Maxwell correction gradient moments before the actual diffusion imaging data acquisition. The correction gradients are pre-calculated based on the anticipated slice positions and applied during the scan preparation phase, eliminating the need for post-processing corrections and ensuring accurate signal measurement from the outset.
Solution Approach 2:
The patent changes the spatial parameter of slice positioning by reducing the distance between simultaneously acquired slices. By optimizing the slice spacing parameter, the system achieves a balance where slices are close enough to allow accurate Maxwell term calculation at average positions, yet sufficiently separated to maintain acceptable g-factor performance in the reconstruction.
2Measurement precision
If conventional Maxwell correction methods are applied to slices with large spatial separation, then correction accuracy is improved, but the method becomes inapplicable
Solution Approach 1:
The patent introduces an intermediary approach by calculating Maxwell correction gradient moments at the average position between simultaneously acquired slices rather than at each individual slice position. This intermediary position serves as a representative reference point that provides sufficiently accurate correction for all slices in the group, enabling the method to work across varying slice configurations and separations.
Solution Approach 2:
The patent achieves universality by developing a correction method that works across different slice arrangements and separations. The approach of using average position calculations and providing correction options (with or without relaxation of the gradient moment constraint) makes the method adaptable to various SMS acquisition configurations, magnetic field strengths, and diffusion weighting parameters.
3Object-affected harmful factors
If slice distance is reduced for Maxwell correction, then concomitant field degradation is reduced, but g-factor penalty increases
Solution Approach 1:
The patent optimizes the slice spacing parameter to achieve a balance between two competing requirements. By carefully selecting the distance between simultaneously acquired slices, the system minimizes concomitant field degradation while maintaining acceptable g-factor performance. This parameter optimization is performed based on the specific acquisition conditions including magnetic field strength and diffusion weighting.
Solution Approach 2:
The patent applies preliminary Maxwell correction to compensate for concomitant field effects before they can degrade the signal. By calculating and applying the correction gradients in advance, the system prevents signal loss rather than attempting to recover it later, thereby maintaining better signal-to-noise ratio despite reduced slice spacing.
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 effectively reduces signal noise ratio (SNR) degradation from concomitant fields while minimizing the g-factor penalty, resulting in improved image quality, as demonstrated by higher signal intensities and reduced artifacts in diffusion-weighted imaging, particularly at low magnetic fields.
Implementation Method 1
The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals
Implementation Method 2
the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner
Implementation Method 3
The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object
Implementation Method 4
As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals)
Data Source
AI summary
In a method and apparatus for acquiring magnetic resonance (MR) data, MR signals are acquired simultaneously from S slices, of a total of N slices of a subject, with S being an SMS factor. The N slices are respectively at different positions from an isocenter of the data acquisition scanner, thereby causing said MR signals to be affected differently by Maxwell terms of magnetic fields that give said MR signals respective signal dephasings that are dependent on the distance of a respective slice from the isocenter. The SMS MR data acquisition sequence is executed with a spacing between each pair of adjacent slices being less than N/S. Maxwell correction gradient moments are calculated at an average position between the S slices, thereby generating corrected k-space data wherein the signal dephasing of the MR signals from the S slices is reduced.