Dixon MR Imaging Fat Shift Correction
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Solution Overview
Problem
Dixon-type MR imaging techniques using bipolar readout magnetic field gradients face limitations due to phase errors and geometric distortions, particularly incomplete fat suppression at tissue interfaces, which existing correction methods fail to fully address.
Innovation Solution
The method involves acquiring two echo signals with opposed polarity readout gradients and modifying single-echo images to compensate for chemical shift-induced distortions, estimating phase errors based on both original and modified images to accurately register fat and water signals, and iteratively correcting for main field inhomogeneities to improve water-fat separation accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If Dixon-type MR imaging uses bipolar readout magnetic field gradients, then acquisition speed and signal-to-noise ratio are improved, but phase errors and geometric distortions occur leading to incomplete fat suppression at tissue interfaces
Solution Approach 1:
The patent segments the image registration process into two distinct paths: one for water signal registration and one for fat signal registration. By separating the correction procedures for different signal types, the method can apply appropriate corrections to each, resolving the phase errors and geometric distortions that affect water-fat separation accuracy while maintaining the fast bipolar gradient acquisition
Solution Approach 2:
The patent applies different correction strategies to different parts of the image based on local signal characteristics. Water signals and fat signals receive different registration treatments depending on their respective phase error profiles and geometric distortion patterns at tissue interfaces, allowing localized optimization of water-fat separation accuracy in different regions
2Reliability
If existing correction methods are applied to bipolar gradient echoes, then some phase errors are reduced, but incomplete fat suppression persists at tissue interfaces due to uncorrected geometric distortions
Solution Approach 1:
The patent performs preliminary geometric distortion correction on the bipolar gradient echoes before applying water-fat separation. By pre-correcting the geometric distortions in the raw data, the subsequent water-fat separation algorithm receives already-aligned signals, enabling complete fat suppression at tissue interfaces rather than leaving residual artifacts
Solution Approach 2:
The patent introduces an intermediate geometric distortion correction step that acts as a mediator between the bipolar gradient acquisition and the water-fat separation process. This intermediate correction layer transforms the distorted bipolar echoes into a format suitable for accurate water-fat separation, bridging the gap between fast acquisition and precise separation
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 effectively corrects phase errors and geometric distortions, enhancing the accuracy of water-fat separation and reducing artifacts in MR images, applicable to any n-point Dixon technique, and is compatible with existing MR devices with appropriate programming.
Implementation Method 1
The magnetic field B0 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 2
the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle
Implementation Method 3
constant magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field B0, leading to a linear spatial dependency of the spin resonance frequency
Implementation Method 4
these types of experiments are often referred to as Dixon-type of measurements. By means of Dixon MR imaging or Dixon water/fat MR imaging, a water/fat separation is achieved by calculating contributions of water and fat from two or more corresponding echoes, acquired at different echo times. In general such a separation is possible because there is a known precessional frequency difference of hydrogen in fat and water
Data Source
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AI summary
The invention relates to a method of Dixon-type MR imaging. It is an object of the invention to provide an MR imaging technique using bipolar readout magnetic field gradients with an improved estimation of the main field inhomogeneity to eliminate residual artifacts. In accordance with the invention, a method of MR imaging of an object placed in a main magnetic field within an examination volume of a MR device is proposed, wherein the method comprises the steps of: - subjecting the object (10) to an imaging sequence to generate at least two sets of echo signals at two or more different echo times using bipolar pairs of readout magnetic field gradients, one set of echo signals being generated at a first echo time (TE1) and the other set of echo signals being generated at a second echo time (TE2), - acquiring the echo signals from the object (10), - reconstructing a first image from the echo signals attributed to the first echo time (TE1) and a second image from the echo signals attributed to the second echo time (TE2), - computing modified first and second images by compensating for a fat shift in the reconstructed first and second images respectively, - estimating phase errors in the acquired echo signals on the basis of the first and second images and the modified first and second images using a signal model including the resonance spectra of fat and water and the spatial variation of the main magnetic field, and - reconstructing a water image and/or a fat image by separating the signal contributions of fat and water to the acquired echo signals using the estimated phase errors. Moreover, the invention relates to a MR device (1) and to a computer program to be run on a MR device (1).