Parallel MRI B0 Distortion Correction with Regularised SENSE
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Solution Overview
Problem
Current MR imaging techniques, particularly SENSE, face challenges in achieving high image quality due to sub-sampling artefacts caused by B0 field inhomogeneities, especially in EPI scans, leading to geometry distortions and incomplete elimination of artefacts.
Innovation Solution
The method involves acquiring sensitivity maps using EPI or 3D FFE sequences, correcting for geometry distortions, and employing a multi-point Dixon technique for B0 mapping to separate water and fat signals, allowing for accurate adaptation of sensitivity maps and simultaneous reconstruction of water and fat images using regularized SENSE with separate sensitivity maps and regularisation maps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If parallel acquisition with sub-sampled k-space data is used to accelerate MR imaging, then acquisition speed is improved, but sub-sampling artefacts and aliasing occur in the reconstructed images
Solution Approach 1:
The patent introduces sensitivity maps as an intermediary element that mediates between the sub-sampled k-space data and the final image reconstruction. These sensitivity maps, derived from calibration scans, serve as weighting functions that enable the reconstruction algorithm to distinguish and separate aliased signals from different spatial locations, thereby eliminating sub-sampling artefacts while maintaining accelerated acquisition
Solution Approach 2:
The patent changes the parameter of spatial sampling density in k-space by using sub-sampled data acquisition. By acquiring fewer k-space lines and applying sensitivity-based reconstruction, the system achieves faster acquisition while recovering full image quality through mathematical modeling of coil sensitivities
2Loss of time
If sensitivity maps are acquired at low resolution to enable parallel imaging, then acquisition time is reduced, but accuracy of sensitivity profiles decreases leading to residual artefacts
Solution Approach 1:
The patent applies partial action by acquiring sensitivity maps at a resolution that is sufficient for the intended parallel imaging application but not necessarily the maximum possible resolution. The calibration scan uses reduced matrix size or fewer phases, providing adequate sensitivity information for artifact suppression while accepting that the sensitivity maps are not perfectly accurate at all spatial frequencies
Solution Approach 2:
The patent makes the sensitivity map resolution dynamic and adaptive to the specific imaging requirements. The calibration scan resolution can be adjusted based on the desired acceleration factor and the specific anatomical region being imaged, optimizing the trade-off between pre-scan time and reconstruction accuracy for each application
3Productivity
If EPI sequence is used for fast imaging, then acquisition speed is improved, but B0 inhomogeneity causes geometry distortions and water-fat shift
Solution Approach 1:
The patent acquires additional reference data (B0 field maps and/or water-fat separation maps) that serve as copies or representations of the underlying physical properties. These reference maps are then used to correct the distorted EPI images by warping them back to the correct geometry or by separating water and fat signals that have been spatially shifted due to B0 inhomogeneity
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 significantly reduces residual sub-sampling artefacts, improves image quality, and enhances the signal-to-noise ratio by accurately representing coil sensitivities and using Dixon maps for improved conditioning of the reconstruction problem.
Implementation Method 1
Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images
Implementation Method 2
The magnetic field B0 produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency
Implementation Method 3
The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis
Data Source
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Figure 4(a)~5(c)
AI summary
The invention relates to a method of MR imaging of an object (10) positioned in an examination volume of a MR device (1). The method comprises the steps of: acquiring reference MR signal data from the object (10); deriving a Bo map from the reference MR signal data; adapting sensitivity maps according to the B0 map, which sensitivity maps indicate spatial sensitivity profiles of one or more RF receiving coils (11, 12, 13), to correct for geometric distortions of the sensitivity maps; acquiring imaging MR signal data from the object (10) via the one or more receiving coils (11, 12, 13) with sub-sampling of k-space; and reconstructing a MR image from the imaging MR signal data, wherein sub- sampling artefacts are eliminated using the adapted sensitivity maps. In a preferred embodiment, the reference MR signal data are acquired using a multi-point Dixon technique, wherein a water map and a fat map are derived from the reference MR signal data. A water image and a fat image are reconstructed from the imaging MR signal data using separate water and fat sensitivity maps. The water and fat images are preferably reconstructed using regularised SENSE, wherein a water regularisation map and a fat regularisation map are derived from the multi-point Dixon reference MR signal data. Moreover, the invention relates to a MR device (1) for carrying out the method, and to a computer program to be run on a MR device (1).