Dixon MRI Water-Fat Separation Using B0 Field Maps
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Solution Overview
Problem
Conventional Dixon-type MR imaging techniques fail to accurately separate water and fat signals in regions with large magnetic field imperfections, leading to incorrect signal contributions and restricted field-of-view, especially at high B0 fields where global shimming cannot fully compensate for local field variations.
Innovation Solution
Incorporating prior knowledge of the spatial variation of the main magnetic field B0 into the signal separation process, using methods such as simulation, measurements, or B0 mapping, to predict phase evolution and demodulate MR signals, allowing for reliable separation of chemical species even far from the main magnet's iso-center.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional Dixon-type MR imaging techniques are used, then water and fat signals can be separated in regions with small magnetic field imperfections, but the technique fails in regions with large magnetic field imperfections leading to incorrect signal contributions and restricted field-of-view
Solution Approach 1:
The patent applies preliminary action by acquiring prior knowledge about the spatial variation of the main magnetic field B0 before performing the Dixon-type separation. This prior knowledge (obtained through measurements, simulations, or B0 mapping) is used to predict and compensate for phase evolution in regions with large magnetic field imperfections, enabling accurate water-fat separation across the entire field-of-view including areas far from the iso-center where conventional techniques fail
2Stability of the object's composition
If global shimming is applied at high B0 fields, then some magnetic field uniformity is achieved, but local field variations cannot be fully compensated leading to remaining imperfections
Solution Approach 1:
The patent applies local quality by transitioning from global shimming to local field characterization. Instead of attempting to make the entire magnetic field uniformly homogeneous through global shimming, the method acquires prior knowledge about the specific spatial variation of B0 in each local region. This allows the separation algorithm to adapt to local field conditions, making the signal separation reliable even in regions with significant local variations that global shimming cannot correct
3Area of stationary object
If the field-of-view is extended to include regions far from the main magnet's iso-center, then more of the body can be imaged, but magnetic field imperfections increase causing separation failure
Solution Approach 1:
The patent introduces an intermediary element - the prior knowledge of B0 spatial variation - that mediates between the extended field-of-view and the separation accuracy. This intermediary information acts as a bridge, allowing the separation algorithm to correctly interpret signals from regions far from the iso-center by compensating for the known magnetic field imperfections in those regions, thus maintaining separation accuracy across the extended field-of-view
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 enables accurate separation of water and fat signals across a larger field-of-view without increasing noise susceptibility, improving image quality and reducing artifacts caused by magnetic field imperfections, making it suitable for applications like whole-body imaging and first-pass contrast-enhanced MR angiography.
Implementation Method 1
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 (so-called Larmor frequency, or MR frequency)
Implementation Method 2
the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle
Implementation Method 3
In order to realize spatial resolution in the body, constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B0, leading to a linear spatial dependency of the spin resonance frequency
Data Source
AI summary
A method and an apparatus for MR imaging of at least two chemical species having different MR spectra enables Dixon water/fat separation in cases in which a large field-of-view is required. The method includes the steps of: a) generating at least one echo signal by subjecting a body placed in the examination volume of a MR device to an imaging sequence of RF pulses and switched magnetic field gradients; b) acquiring the at least one echo signal; c) separating signal contributions of the at least two chemical species to the at least one acquired echo signal on the basis of a spectral model and prior knowledge about the spatial variation of the main magnetic field Bo in the examination volume; and d) reconstructing a MR image from the signal contributions of at least one of the chemical species.