Diffusion Weighted MRI Preparatory Sequence for Motion Artifact Reduction
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Solution Overview
Problem
Diffusion-weighted imaging (DWI) methods are exquisitely sensitive to patient motion, leading to severe image artifacts such as ghosting and blurring due to phase misregistrations, which limits their clinical use, especially in multi-shot methods that require multiple excitations and acquisitions.
Innovation Solution
A preparatory pulse sequence that diffusion weights the longitudinal spin magnetization, nulling first moment phase shifts caused by bulk patient motion, while being sensitive to acceleration and higher order motions, thereby reducing phase misregistrations and image artifacts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional diffusion-weighted imaging pulse sequences are used, then diffusion weighting is achieved, but severe image artifacts such as ghosting and blurring occur due to sensitivity to patient motion
Solution Approach 1:
The pulse sequence is divided into two distinct parts: a preparatory diffusion-weighted sequence that weights the longitudinal magnetization, and a separate imaging sequence that acquires the signal. This segmentation allows the diffusion weighting to be applied without the full sequence being sensitive to motion-induced phase misregistrations, thereby resolving the contradiction between achieving diffusion weighting and maintaining image quality.
Solution Approach 2:
The preparatory pulse sequence performs diffusion weighting on the longitudinal spin magnetization before the main imaging sequence is executed. By applying the diffusion gradients and weighting the magnetization in advance, the main imaging sequence can be made insensitive to bulk patient motion, thus preventing ghosting and blurring artifacts while preserving diffusion weighting accuracy.
2Manufacturing precision
If multi-shot acquisition methods are used to improve spatial resolution, then more k-space lines can be acquired, but phase misregistrations due to patient motion increase
Solution Approach 1:
By separating the diffusion weighting function from the imaging acquisition function, the patent allows multi-shot k-space sampling to proceed without the phase misregistration problems that would otherwise plague such sequences. The preparatory sequence establishes diffusion-weighted longitudinal magnetization that is insensitive to bulk motion, enabling accurate phase registration across multiple shots.
Solution Approach 2:
The longitudinal spin magnetization serves as an intermediary that carries the diffusion weighting information. By weighting this intermediate state rather than the transverse magnetization used in conventional sequences, the system decouples the diffusion weighting from motion sensitivity, allowing multi-shot acquisitions to maintain both high spatial resolution and phase accuracy.
3Measurement precision
If diffusion gradients with large amplitude are applied to increase diffusion weighting, then sensitivity to diffusion is improved, but sensitivity to bulk patient motion increases
Solution Approach 1:
The preparatory pulse sequence applies the large amplitude diffusion gradients and establishes the weighted longitudinal magnetization before the imaging sequence begins. This preliminary application of strong gradients achieves the desired diffusion sensitivity while the subsequent imaging sequence remains insensitive to bulk motion, thereby resolving the contradiction between diffusion sensitivity and motion sensitivity.
Solution Approach 2:
The harmful motion sensitivity is extracted from the diffusion weighting function. By applying diffusion gradients in the preparatory sequence and transferring the weighting to longitudinal magnetization, the harmful phase shifts associated with bulk motion are separated from the useful diffusion weighting, allowing large gradient amplitudes to be used without proportionally increasing motion artifacts.
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
The solution enables the acquisition of high-quality DWI images that are not sensitive to bulk patient motion, reducing ghosting and blurring, and allowing for improved spatial resolution and clinical applicability of DWI techniques.
Implementation Method 1
a magnetic field gradient known as a diffusion gradient that sensitizes the MR signal to spin motion
Implementation Method 2
nuclear magnetic resonance imaging (MRI) methods and systems
Implementation Method 3
A 180° RF refocusing pulse is produced in the presence of a slice select gradient pulse to refocus the transverse magnetization
Implementation Method 4
nulling first moment phase shifts caused by bulk patient motion
Data Source
AI summary
A preparatory pulse sequence is applied prior to an imaging pulse sequence during a diffusion-weighted MRI scan. The preparatory pulse sequence diffusion weights the longitudinal magnetization using a gradient waveform that is first moment nulled to reduce image artifacts caused by patient motion.