B0 Mapping via Phase Error Correction and Fat-Fraction Refinement
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Solution Overview
Problem
Current methods for estimating and mapping the main magnetic field (B0) in magnetic resonance imaging are hampered by B0 inhomogeneity and chemical shift effects, leading to inaccurate B0 maps and fat/water swaps, especially when true B0 inhomogeneity is outside the range of conventional multi-point Dixon techniques.
Innovation Solution
The method involves generating an initial B0 map by unwrapping pseudo-in-phase complex data from selected echoes of multi-echo gradient echo image data, correcting global and local phase errors, and using a bias frequency shift to produce a corrected fat-fraction map, which is then employed to generate a final B0 map, thereby improving the accuracy and robustness of B0 mapping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional multi-point Dixon techniques are used for B0 mapping, then fat/water separation can be achieved, but phase errors and chemical shift effects cause inaccurate B0 maps and fat/water swaps
Solution Approach 1:
The patent segments the B0 mapping process into distinct phases: initial B0 map generation from multi-echo data, global phase error correction, local phase error correction, and final B0 map refinement. This segmentation allows each phase to be optimized independently, improving overall accuracy while maintaining reliability in fat/water separation.
Solution Approach 2:
The patent applies preliminary global phase error correction before local phase error correction. By correcting global phase errors first using a water mask, the subsequent local corrections operate on a pre-corrected basis, preventing error propagation and improving the reliability of fat/water separation in the final B0 map.
2Measurement precision
If phase unwrapping is performed on complex data to obtain B0 map, then B0 inhomogeneity can be measured, but chemical shift effects introduce additional phase errors
Solution Approach 1:
The patent extracts and removes the chemical shift phase component from the total phase measurement. By calculating the expected chemical shift phase based on known fat-water frequency differences and echo times, this component is separated from the B0-induced phase, allowing accurate B0 mapping without chemical shift contamination.
Solution Approach 2:
The patent introduces a water mask as an intermediary element that facilitates phase error correction. The water mask identifies pure water regions where only B0 phase effects are present, serving as a reference for calculating and removing global phase errors from the entire image, thereby preserving accurate phase information for B0 measurement.
3Measurement precision
If iterative multi-parameter fitting is used for fat-fraction quantification, then fat-fraction maps can be generated, but the process is computationally intensive and time-consuming
Solution Approach 1:
The patent performs preliminary B0 mapping and phase error correction before fat-fraction quantification. By obtaining an accurate B0 map in advance, the subsequent fat-fraction fitting process can use this pre-computed information, reducing the computational burden and iteration requirements while maintaining quantification accuracy.
Solution Approach 2:
The patent changes the approach from simultaneous multi-parameter fitting to a sequential process where B0 map parameters are determined first, then used to guide fat-fraction estimation. This parameter separation reduces the dimensionality of the optimization problem at each stage, significantly decreasing processing time while preserving measurement precision.
Data Source
AI summary
Various embodiments of the present disclosure present unwrapping based B0 mapping systems and methods. An initial B0 map is obtained by unwrapping a pseudo-in-phase complex data that is generated from selected echoes of multi-echo gradient echo image data. Global and local phase errors that are present in the initial B0 map are corrected to produce a phase-error-corrected B0 map. A bias frequency shift is employed to produce a corrected fat-fraction map based on the phase-error-corrected B0 map. The corrected fat-fraction map is employed to generate a final B0 map.