MRI Water-Fat Separation Gradient Waveform Optimization
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Solution Overview
Problem
Current MRI water/fat separation techniques using the two-point Dixon method suffer from artifacts caused by high-amplitude rephasing gradients, leading to issues like fuzziness and ghosts in images, particularly in low-field scanners with longer echo chains.
Innovation Solution
An improved FSE sequence is implemented where each refocusing RF pulse corresponds to two readout gradients of the same polarity and one rephasing gradient of opposite polarity, with the readout gradients divided into asymmetric parts, and the rephasing gradient's amplitude reduced by adjusting its duration and rate of change, followed by FFT and partial Fourier transforms to reconstruct pure water and fat images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-amplitude rephasing gradients are used in FSE sequence, then the echo signals can be properly refocused, but eddy currents and accompanying fields are generated causing artifacts in images
Solution Approach 1:
The patent changes the parameters of the rephasing gradient by reducing its amplitude and adjusting its duration and rate of change. This modification allows the gradient to still achieve proper echo refocusing while generating fewer eddy currents and accompanying fields, thereby reducing artifacts in the images.
Solution Approach 2:
The patent introduces dynamic adjustment of the rephasing gradient parameters, specifically its duration and rate of change. By making these parameters adjustable rather than fixed, the system can optimize the gradient waveform to minimize harmful effects while maintaining the necessary refocusing function.
2Productivity
If longer echo chains are used in FSE sequence, then more echoes can be acquired per excitation pulse cycle, but artifacts are more pronounced in low-field scanners
Solution Approach 1:
The patent modifies the rephasing gradient parameters (amplitude, duration, rate of change) to reduce the generation of eddy currents and accompanying fields. This allows longer echo chains to be used without the artifacts becoming more pronounced, thereby maintaining high productivity while reducing harmful effects in low-field scanners.
3Object-affected harmful factors
If rephasing gradient amplitude is reduced, then eddy currents and accompanying fields are minimized, but the gradient may not be strong enough to achieve proper refocusing
Solution Approach 1:
The patent uses dynamic adjustment of the rephasing gradient duration and rate of change to compensate for the reduced amplitude. By extending the duration and optimizing the rate of change, the system maintains sufficient refocusing capability even with lower gradient amplitude, thereby reducing eddy currents while preserving the refocusing function.
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 mitigates the effects of eddy currents and accompanying fields, resulting in clearer, artifact-free water/fat separated images without the need for new hardware, enhancing the imaging quality in low-field MRI systems.
Implementation Method 1
each refocusing RF pulse corresponds to two readout gradients of the same polarity and one rephasing gradient of opposite polarity
Implementation Method 2
multiple echoes can be acquired between two adjacent refocusing pulses (i.e. in one echo interval)
Implementation Method 3
the difference between the resonant frequencies of fat and water is represented by fΔ with units of Hertz (Hz)
Data Source
AI summary
In an imaging method and device for water/fat separation in MRI using a two-point Dixon FSE sequence, each refocusing RF pulse corresponds to two readout gradients of the same polarity, each being center-divided into a smaller rear part and a larger front part, and one rephasing gradient of opposite polarity. In running the FSE sequence, each echo signal acquired is subjected to an FFT, to reconstruct an image with water and fat in phase and an image with water and fat in opposed phases. Data of each echo signal are subjected to a partial Fourier transform; and the in-phase water/fat image and the opposite-phase water/fat image are subjected to a water/fat separation algorithm, to obtain a pure water image and a pure fat image.