Frequency Sweep RF Saturation for Fat Water Separation in Low Field MRI
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Solution Overview
Problem
Conventional fat suppression methods in magnetic resonance imaging (MRI) are ineffective at low magnetic field strengths, leading to challenges such as longer scan times and image artifacts due to reduced chemical shift between water and fat protons, as well as sensitivity to magnetic field inhomogeneities.
Innovation Solution
The implementation of a frequency sweep RF saturation technique, where RF saturation pulses with varying frequency offsets are used to generate imaging data, allowing for the reconstruction of signal-response curves that enable accurate separation and quantification of fat and water content, even in the presence of field inhomogeneities, using compressed sensing and radial sampling to maintain scan time efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional fat suppression methods are used at low magnetic field strengths, then the chemical shift between water and fat protons is reduced, but this leads to ineffective fat suppression and longer scan times
Solution Approach 1:
The patent applies dynamics by implementing a frequency sweep across multiple offsets rather than using a fixed frequency saturation pulse. The frequency offset is varied dynamically to compensate for the reduced chemical shift at low field strengths, allowing the saturation pulse to effectively target fat protons despite the smaller frequency separation between water and fat resonances.
Solution Approach 2:
The patent changes the frequency offset parameter of the saturation pulse across multiple acquisitions. By sweeping through different frequency offsets and combining the data, the method effectively increases the spectral selectivity without requiring longer individual pulse durations, thus maintaining scan time efficiency while achieving effective fat suppression at low field strengths.
2Measurement precision
If spectrally selective RF pulses are used to saturate fat protons, then fat signal can be suppressed, but the long pulse duration required for narrow spectral selectivity increases scan time
Solution Approach 1:
Instead of using a single long pulse with narrow spectral width, the patent uses multiple shorter pulses with varying frequency offsets. The dynamic sweeping of frequency across multiple acquisitions achieves the equivalent of a narrow spectral width without requiring any individual pulse to be long, thus avoiding the time penalty.
Solution Approach 2:
The patent adds the frequency offset dimension by acquiring data at multiple different frequency offsets and combining them. This dimensional approach allows the system to achieve narrow spectral selectivity in the frequency domain without increasing the time duration of individual pulses, effectively trading spectral precision for temporal efficiency.
3Measurement precision
If frequency-selective inversion recovery pulses are used for fat suppression, then fat signal can be suppressed, but the method is sensitive to magnetic field inhomogeneities
Solution Approach 1:
The patent segments the frequency spectrum by acquiring data at multiple discrete frequency offsets. This segmentation allows the system to sample different portions of the spectral profile, and by combining these segments, achieve robust fat suppression that is less sensitive to local field variations, as the multi-point sampling averages out the effects of inhomogeneities.
Solution Approach 2:
The patent implements a form of feedback by using the signal response at different frequency offsets to determine the optimal saturation frequency. The system effectively uses the measured signal characteristics to identify and suppress the fat resonance, adapting to local field conditions and reducing sensitivity to inhomogeneities.
4Measurement precision
If water-selective excitation with narrow spectral profile is used, then fat protons can be avoided, but the long pulse duration significantly increases scan time
Solution Approach 1:
Instead of trying to excite only water protons by using a narrow water-selective pulse, the patent takes out the fat signal by selectively saturating fat protons at their resonance frequency. This extraction approach removes the unwanted fat contribution without requiring a long water-selective pulse, thus avoiding the time penalty while achieving the desired spectral selectivity.
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 separates fat and water signals at low field strengths, reducing scan time and improving image quality by utilizing multiple frequency offsets and undersampling strategies, while being robust to magnetic field inhomogeneities and motion artifacts.
Implementation Method 1
The frequency difference between water and fat protons is called 'chemical shift'.
Implementation Method 2
the frequency of the RF excitation pulse can be set to the protons' Larmor frequency ω_H: ω_H=γ_H·B
Implementation Method 3
RF saturation pulses with varying frequency offsets are used to generate imaging data, allowing for the reconstruction of signal-response curves that enable accurate separation and quantification of fat and water content
Data Source
AI summary
Exemplary systems, methods and computer-accessible medium according to exemplary embodiments of the present disclosure can separate fat and water in magnetic resonance imaging using frequency sweep radiofrequency saturation pulses. In an exemplary procedure, periodic RF saturation pulses with varying frequency offset from a water resonance frequency with at least two different offsets are emanated. In another exemplary procedure, the signal response to saturation at different frequencies on a voxel-by-voxel basis can be analyzed.