Spectrally-Resolved 3D MRI Without Frequency-Encoding Gradients
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Solution Overview
Problem
Magnetic resonance imaging (MRI) in the presence of metal objects is challenging due to off-resonance, signal loss, and signal pile-up artifacts caused by frequency-encoding gradients, which limit the ability to eliminate in-plane signal loss and pile-up errors.
Innovation Solution
A method for spectrally-resolved three-dimensional MRI that uses phase-encoding gradients along three orthogonal directions without frequency-encoding gradients, allowing for spectral encoding of magnetic resonance signals and measurement of local magnetic field inhomogeneities, chemical species separation, and relaxation rates of transverse magnetization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If frequency-encoding gradients are used for spatial encoding, then imaging speed and spatial resolution are improved, but in-plane signal loss and pile-up artifacts occur near metal objects
Solution Approach 1:
The patent removes the frequency-encoding gradient from the imaging sequence, extracting the harmful element that causes signal loss and pile-up artifacts near metal objects. By eliminating this gradient, the method achieves distortion-free imaging in regions with magnetic field inhomogeneities while maintaining spatial encoding through alternative phase-encoding mechanisms.
Solution Approach 2:
Instead of using frequency-encoding gradients to achieve rapid spatial encoding, the patent inverts the approach by using phase-encoding gradients in all three spatial dimensions. This reversal of the conventional encoding strategy eliminates the source of artifacts while maintaining the ability to reconstruct high-resolution images through Fourier transformation of the phase-encoded data.
2Reliability
If phase-encoding gradients are used in all three directions without frequency-encoding, then signal-to-noise ratio is improved and artifacts are reduced, but imaging time increases
Solution Approach 1:
The patent applies partial phase-encoding by encoding only the dimensions necessary to resolve the metal artifact problem (typically two phase-encoding directions plus one frequency-encoding direction without the problematic gradient), rather than fully phase-encoding all three dimensions. This partial application reduces imaging time while still achieving the desired artifact reduction and signal-to-noise ratio improvement.
Solution Approach 2:
The patent modifies the encoding parameters by changing the gradient application pattern - using phase-encoding gradients in directions where they provide maximum benefit for artifact reduction while minimizing total encoding steps. Parameter optimization in the phase-encoding amplitudes and durations allows faster acquisition compared to conventional full phase-encoding approaches.
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 eliminates frequency-encoding-related shift artifacts, achieves a significant gain in signal-to-noise ratio, and enables distortion-free imaging near metal objects, allowing for accurate chemical species separation and relaxation rate measurement.
Implementation Method 1
establish a first phase-encoding gradient along a first direction, establish a second phase-encoding gradient along a second direction that is orthogonal to the first direction, and establish a third phase-encoding gradient along a third direction that is orthogonal to the first direction and the second direction
Implementation Method 2
sampling a magnetic resonance signal at a plurality of time points during a period of time in which no magnetic field gradients are established by the MRI system
Implementation Method 3
produce a radio frequency ("RF") pulse that rotates net magnetization about an axis
Implementation Method 4
spectral encoding of magnetic resonance signals, such that local magnetic field inhomogeneities can be measured, chemical species signal separation can be performed, and the relaxation rate of transverse magnetization, R*2, can be measured
Implementation Method 5
a plurality of gradient coils configured to establish at least one magnetic gradient field to the polarizing magnetic field
Data Source
AI summary
A system and method for acquiring spectrally-resolved three-dimensional data with a magnetic resonance imaging (“MRI”) system without frequency-encoding gradients are provided. An MRI system is directed to produce a radio frequency (“RF”) pulse that rotates net magnetization about an axis, after which a first phase-encoding gradient is established along a first direction, a second phase-encoding gradient is established along a second direction that is orthogonal to the first direction, and a third phase-encoding gradient is established along a third direction that is orthogonal to the first and second directions. Spectrally-resolved data are acquired at a point in k-space that is defined by the first, second, and third phase-encoding gradients, and is acquired by sampling a magnetic resonance signal at a plurality of time points during a period of time in which no magnetic field gradients are established by the MRI system.