Unipolar FSE MRI Gradient Sequence for Residual Magnetization Artifacts
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Solution Overview
Problem
Fast Spin Echo (FSE) MRI techniques in permanent magnet systems suffer from artifacts due to inaccurate phase gradient areas caused by imperfections in the gradient power amplifier and residual magnetization, which lead to inconsistent magnetic field gradients.
Innovation Solution
The proposed FSE pulse sequence includes a residual magnetization gradient pulse before the 90° RF excitation, followed by a series of encoding gradient pulses of the same sign, with a single preparation gradient pulse and a final gradient pulse to maintain a constant residual magnetization and compensate for amplifier imperfections, ensuring all gradient pulses have the same sign to eliminate inverted pulses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional FSE pulse sequence with inverted gradient pulses is used, then phase encoding can be achieved, but residual magnetization and gradient amplifier imperfections cause inaccurate phase gradient areas and artifacts
Solution Approach 1:
The patent inverts the conventional approach by eliminating gradient pulse sign inversions. Instead of using alternating positive and negative gradient pulses, the patent applies gradient pulses of consistent sign throughout the echo train, fundamentally reversing the conventional FSE gradient strategy to avoid RM-related artifacts
Solution Approach 2:
The patent changes the gradient pulse parameters by maintaining constant sign throughout the sequence. A constant polarity gradient is applied during the entire echo train, and the phase encoding is achieved through variable gradient duration rather than sign inversion, thereby changing the fundamental parameters of gradient application
2Ease of operation
If gradient pulses of alternating sign are used in FSE, then phase encoding is achieved, but gradient amplifier imperfections during sign switching cause inaccurate gradient shapes
Solution Approach 1:
The patent inverts the conventional gradient approach by eliminating sign switching entirely. Gradient pulses maintain consistent polarity throughout the sequence, removing the source of amplifier-related distortion while preserving phase encoding functionality through alternative means
Solution Approach 2:
The patent extracts and removes the problematic sign-inversion operation from the gradient pulse sequence. By taking out the alternating polarity mechanism and replacing it with unipolar gradients, the patent eliminates the harmful interaction between gradient reversal and amplifier imperfections
3Adaptability or versatility
If residual magnetization is allowed to vary during the scan, then gradient flexibility is maintained, but phase consistency between 180° pulses cannot be ensured
Solution Approach 1:
The patent applies preliminary gradient pulses of constant sign before and during the echo train to establish a stable RM state. By preparing the gradient field in advance with consistent polarity, the patent ensures that RM develops in a predictable manner, maintaining phase consistency throughout the sequence
Solution Approach 2:
The patent introduces homogeneity by maintaining uniform gradient sign throughout the sequence. This homogeneous gradient application pattern ensures consistent RM behavior and stable phase relationships between successive 180° pulses, eliminating the phase variations caused by alternating gradient directions
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 reduces artifacts by maintaining a constant residual magnetization and improving the accuracy of gradient amplifier performance, ensuring consistent phase encoding and constant phase between 180° pulses, thereby enhancing image quality in MRI scans.
Implementation Method 1
The magnetic field gradients produced during the scan can induce a mirror field on the magnet pole pieces that remains for a significant amount of time (at least several minutes) after the gradients are turned off. This RM is approximately linear in space and adapts the field lines of the field gradient.
Implementation Method 2
The gradient power amplifier can produce inaccurate gradient shapes while switching the sign of the current.
Data Source
AI summary
A method of reducing artifacts produced during Fast Spin Echo measurements made using permanent magnet NMR instruments. The method includes applying encoding gradients that do not switch signs throughout the experiment. Prior to the 90° RF pulse, a strong RM gradient pulse is given to produce a dominant and constant residual magnetization. The encoding is done through the combination of encoding gradients with the aid of the 180° RF pulses of the echo train. A first constant encoding gradient is given before the first 180 pulse. Then two variable encoding gradients are provided after each 180 pulse; one applied prior to and one applied subsequent to each acquisition in the echo train.