Diffusion-Weighted MRI Sequence Using Bipolar Gradients to Suppress Eddy Currents
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Solution Overview
Problem
Current diffusion-weighted MR imaging techniques suffer from significant geometric distortions, hardware loading issues, and interference artifacts due to unwanted signal coherence paths, particularly eddy current effects, which limit the quality of acquired images and signal-to-noise ratio.
Innovation Solution
The method employs a diffusion-weighted acquisition sequence with multiple diffusion coding gradients and readout gradients, using dephasing gradients to suppress unwanted signal coherence paths, and bipolar gradient switches to eliminate eddy current fields, thereby reducing artifacts and optimizing gradient system utilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If monopolar diffusion coding gradients are used, then diffusion-weighted imaging can be performed, but strong geometric distortions and significant hardware loading occur
Solution Approach 1:
The patent inverts the conventional monopolar diffusion coding approach by using bipolar gradient pulses with alternating polarities. This inversion cancels out eddy current fields and reduces geometric distortions while maintaining diffusion weighting capability, directly addressing the harmful effects of the monopolar scheme
Solution Approach 2:
The patent changes the polarity parameter of the diffusion coding gradients from monopolar to bipolar. This parameter change fundamentally alters the interaction with eddy currents and geometric distortions, eliminating the harmful effects while preserving the imaging function
2Object-affected harmful factors
If bipolar double spin echo scheme is used to eliminate eddy current fields, then geometric distortions are reduced, but echo time increases and signal-to-noise ratio decreases
Solution Approach 1:
The patent applies dephasing gradients before the diffusion coding gradients to pre-compensate for unwanted signal coherence paths. This preliminary action eliminates the need for longer echo times while maintaining artifact suppression, addressing the time loss issue
Solution Approach 2:
The patent extracts and eliminates unwanted signal coherence paths (FIDs, spin echoes, stimulated echoes) through dephasing gradients applied before the main diffusion coding sequence. This extraction removes harmful signals without requiring extended echo times, solving the contradiction between artifact reduction and time efficiency
3Object-generated harmful factors
If dephasing gradients are added to eliminate unwanted signal coherence paths, then interference artifacts are reduced, but echo time is extended and eddy current field compensation is undermined
Solution Approach 1:
The patent applies dephasing gradients before the diffusion coding gradients as a preliminary action. This timing ensures that unwanted signal coherence paths are suppressed before they can interfere with the main signal, eliminating interference artifacts without extending the echo time
Solution Approach 2:
The patent segments the gradient sequence into distinct dephasing gradient segments and diffusion coding gradient segments. This segmentation allows independent optimization of each function: dephasing gradients suppress unwanted paths while diffusion gradients provide the necessary weighting, avoiding the trade-off between artifact suppression and echo time extension
4Loss of time
If implicit spoiling is used to reduce echo time and eliminate interference artifacts, then signal-to-noise ratio improves, but eddy current artifact reduction is compromised
Solution Approach 1:
The patent applies dephasing gradients as a preliminary action before the diffusion coding sequence. This preliminary dephasing suppresses unwanted signal coherence paths that would otherwise generate eddy current artifacts, allowing short echo times while maintaining eddy current compensation
Solution Approach 2:
The dephasing gradients act as an intermediary mechanism that suppresses unwanted signal coherence paths before they can generate harmful eddy current fields. This intermediary action allows the main diffusion coding sequence to proceed with short echo times without compromising eddy current artifact reduction
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 results in artifact-free diffusion-weighted images with improved signal-to-noise ratio and minimal distortions, allowing for efficient acquisition of diffusion-weighted images while effectively managing eddy current artifacts.
Implementation Method 1
dephasing gradients being switched in order to reduce the acquisition of MR signal portions with unwanted coherence paths
Implementation Method 2
The gradient moments of the dephasing gradients for the reduction of the unwanted signal coherence paths are determined such that a dephasing gradient moment for each unwanted signal coherence path is greater than a threshold Mspoil
Implementation Method 3
the dynamic deformations (eddy current effects) depend on details of the gradient pulse timing
Implementation Method 4
bipolar gradient switches to eliminate eddy current fields
Implementation Method 5
diffusion coding gradients with high amplitude and long duration are combined with a suitable image acquisition module
Implementation Method 6
diffusion-weighted acquisition of magnetic resonance (MR) signals
Implementation Method 7
With the RF pulses of the image acquisition sequence, MR signal portions are generated with a desired signal coherence path and MR signals are generated with unwanted signal coherence paths
Data Source
AI summary
In a method and system for diffusion-weighted acquisition of MR signals with an image acquisition sequence that has multiple diffusion coding gradients and readout gradients to read out the MR signals, MR signal portions are generated with a desired signal coherence path and MR signal portions are generated with unwanted signal coherence paths, with predominantly the MR signal portions with the desired signal coherence path being acquired by the readout gradients by activating dephasing gradients that reduce the acquisition of MR signal portions with unwanted coherence paths. The dephasing gradients are determined under consideration of the diffusion gradients that are used and under consideration of the unwanted signal coherence paths, so that each has a dephasing gradient moment for each unwanted signal coherence path that is greater than a threshold.