Bipolar Diffusion Gradients for Motion-Insensitive MR Imaging
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Solution Overview
Problem
Current diffusion-weighted imaging (DWI) techniques are vulnerable to motion artifacts, leading to distorted images and limited spatial resolution, especially in whole-body applications, due to sensitivity to macroscopic physiological motion and unbalanced gradient moments that result in lost signal coherence and strong motion sensitivity.
Innovation Solution
A dual echo steady state (DESS) imaging sequence using bipolar diffusion gradients is employed, balancing gradient moments to preserve signal coherence and reduce motion sensitivity, allowing for high-quality, distortion-free DWI with improved signal-to-noise ratio efficiency by applying pairs of diffusion weighting gradient waveforms with equal phase integral and opposed polarity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional DWI techniques with unipolar diffusion gradients are used, then diffusion weighting is achieved, but motion artifacts increase and image quality deteriorates
Solution Approach 1:
The patent applies bipolar diffusion gradients instead of conventional unipolar gradients. The gradient waveform is inverted in polarity during the second half of the diffusion encoding period, creating equal and opposite gradient moments. This inversion approach makes the sequence insensitive to constant velocity motion while preserving diffusion weighting, directly resolving the contradiction between measurement precision and motion artifact susceptibility.
Solution Approach 2:
The patent changes the gradient moment parameter from non-zero (conventional) to zero (bipolar). By designing the diffusion gradient waveform such that the area under the gradient curve equals zero (equal positive and negative lobes), the sequence becomes motion-insensitive. This parameter change allows maintaining diffusion measurement capability while eliminating motion-induced phase errors.
2Productivity
If single-shot echo-planar imaging is used for DWI, then acquisition speed is improved, but spatial resolution decreases and geometric distortions increase
Solution Approach 1:
The patent segments the single-shot EPI acquisition into multiple separate shot acquisitions. Instead of acquiring all k-space lines in a single shot, the diffusion-weighted imaging is performed across multiple shots with bipolar gradients. This segmentation allows for higher spatial resolution and reduced geometric distortions while maintaining reasonable acquisition speed through efficient parallel imaging techniques.
Solution Approach 2:
The patent employs periodic bipolar diffusion gradient pairs applied across multiple repetition intervals. The diffusion encoding is performed periodically across different TR intervals, with each period containing a bipolar gradient pair. This periodic application allows for multiple signal averages and improved signal-to-noise ratio while maintaining motion insensitivity and high spatial resolution.
3Reliability
If bipolar diffusion gradients with maximized gradient moments are applied, then signal gain increases and motion sensitivity decreases, but dark band artifacts increase
Solution Approach 1:
The patent optimizes the gradient moment parameter by setting it to zero through bipolar waveform design. The gradient moments are maximized in magnitude but balanced to sum to zero, creating strong diffusion weighting without motion sensitivity. This parameter optimization allows achieving high signal gain and motion insensitivity while controlling dark band artifacts through balanced gradient design.
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 method achieves robust, high-quality diffusion-weighted MR images with reduced motion artifacts and increased signal gain, enabling precise diffusion property measurement by maximizing gradient moments and minimizing dark band artefacts within voxel sizes, thus enhancing clinical applicability.
Implementation Method 1
MR imaging is sensitive to diffusion. Known diffusion weighted imaging (DWI) techniques are commonly performed by using imaging sequences comprising diffusion gradients, wherein the diffusion of protons (of water molecules) along the direction of the diffusion gradient reduces the amplitude of the acquired MR signals.
Implementation Method 2
This produces an echo signal (spin echo) in the receiving coils.
Implementation Method 3
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency).
Implementation Method 4
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency).
Data Source
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AI summary
The invention relates to a method of MR imaging of an object (10) placed in an examination volume of a MR device (1). It is an object of the invention to enable distortion-free high-quality diffusion weighted imaging (DWI) with minimization of artefacts caused by motion. The method of the invention comprises the following steps: - subjecting the object (10) to a dual echo steady state imaging sequence, a free induction decay signal (FID) and an echo signal (ECHO) being generated in each interval between two successive RF pulses, wherein a pair of diffusion gradient waveforms (GDIF) of equal phase integral and opposed polarity is applied in the interval between the FID signal and the echo signal; - acquiring the FID signals and the echo signals in a number of repetitions of the imaging sequence with varying phase encoding; and - reconstructing a diffusion weighted MR image from the acquired FID signals and echo signals. Moreover, the invention relates to a MR device for carrying out this method as well as to a computer program to be run on a MR device.