Dixon MR Imaging Water-Fat Separation Bandwidth Segmentation
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Solution Overview
Problem
Conventional Dixon-type MR imaging techniques require higher receive bandwidths, leading to reduced signal-to-noise ratio (SNR) and increased scan times, and are prone to FID artifacts, making them inefficient for water-fat separation in TSE sequences.
Innovation Solution
A method combining unipolar single-echo readouts at low bandwidth with bipolar dual-echo readouts at high bandwidth, using unipolar and bipolar readout magnetic field gradients respectively, to maximize SNR and separate water and fat signal contributions, while also allowing for FID artifact cancellation without increasing scan time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If higher receive bandwidth is used for Dixon-type water-fat separation, then water and fat signals can be separated, but signal-to-noise ratio is reduced
Solution Approach 1:
The patent segments the receive bandwidth into two separate acquisition channels: a first receive bandwidth for acquiring in-phase echo signals and a second receive bandwidth for acquiring out-of-phase echo signals. This segmentation allows each channel to be optimized independently - the first channel uses lower bandwidth to maintain high SNR for in-phase signals, while the second channel uses higher bandwidth to achieve adequate frequency separation for Dixon water-fat separation.
Solution Approach 2:
Different receive bandwidths are applied to different signal acquisition processes based on their specific requirements. The in-phase signal acquisition uses a first receive bandwidth optimized for maximum SNR, while the out-of-phase signal acquisition uses a second receive bandwidth optimized for frequency separation. This local optimization of bandwidth for each specific signal type resolves the contradiction between separation accuracy and SNR.
2Measurement precision
If higher receive bandwidth is used for Dixon-type water-fat separation, then water and fat signals can be separated, but scan time is increased
Solution Approach 1:
The patent segments the echo acquisition into two separate TSE sequences: a first TSE sequence for in-phase echoes and a second TSE sequence for out-of-phase echoes. Each sequence can be independently optimized for its specific purpose, and the segmentation allows efficient utilization of available scan time by distributing acquisitions across multiple interleaved sequences rather than requiring all echoes to be acquired at high bandwidth.
Solution Approach 2:
The patent employs periodic interleaving of the first and second TSE sequences, where in-phase and out-of-phase echo acquisitions are alternated in a periodic manner. This periodic action allows both types of echoes to be acquired within the same overall scan time by distributing them across multiple repetition times, preventing the need to extend scan time while achieving both separation and maintaining temporal efficiency.
3Measurement precision
If conventional Dixon-type imaging is used, then water-fat separation is achieved, but FID artifacts occur
Solution Approach 1:
The patent extracts and separately processes in-phase and out-of-phase echo signals through distinct acquisition sequences. By taking out the in-phase signal acquisition from the out-of-phase signal acquisition and handling them separately with appropriate bandwidth optimization, the method eliminates the conditions that generate FID artifacts while preserving the water-fat separation capability.
Solution Approach 2:
The patent changes the receive bandwidth parameter differently for in-phase versus out-of-phase echo acquisitions. By using a lower first receive bandwidth for in-phase echoes and a higher second receive bandwidth for out-of-phase echoes, the method optimizes each signal type's acquisition parameters to prevent FID artifact generation while maintaining water-fat separation accuracy.
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 enhances the SNR and sampling efficiency of MR images, enabling efficient water-fat separation with reduced blurring and scan time, and effectively suppresses FID artifacts, improving the overall quality of Dixon-type MR imaging.
Implementation Method 1
at least one main magnet coil (2) for generating an essentially uniform, static magnetic field B0 within an examination volume
Implementation Method 2
a number of gradient coils (4, 5, 6) for generating time-varying magnetic field gradients within the examination volume
Implementation Method 3
at least one body RF coil (9) for generating electromagnetic pulses within the examination volume and/or for inducing voltage signals from a precessing magnetization
Implementation Method 4
the magnetization performs a precessional motion about the z-axis
Implementation Method 5
acquiring the echo signals from the object at a first receive bandwidth using unipolar readout magnetic field gradients, subjecting the object to a second imaging sequence, which comprises a series of refocusing RF pulses, wherein a pair of echo signals is generated in each time interval between two consecutive refocusing RF pulses, acquiring the pairs of echo signals from the object at a second receive bandwidth using bipolar readout magnetic field gradients
Data Source
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Figure 3
AI summary
The invention relates to a method of Dixon-type MR imaging. The method comprises the steps of: - subjecting the object (10) to a first imaging sequence (31) comprising a series of refocusing RF pulses, wherein a single echo signal is generated in each time interval between two consecutive refocusing RF pulses, - acquiring the echo signals from the object (10) at a first receive bandwidth using unipolar readout magnetic field gradients, - subjecting the object (10) to a second imaging sequence (32), which comprises a series of refocusing RF pulses, wherein a pair of echo signals is generated in each time interval between two consecutive refocusing RF pulses, - acquiring the pairs of echo signals from the object (10) at a second receive bandwidth using bipolar readout magnetic field gradients, wherein the second receive bandwidth is higher than the first receive bandwidth, and - reconstructing a MR image from the acquired echo signals, whereby signal contributions from water protons and fat protons are separated. Moreover the invention relates to a MR device (1) and to a computer program to be run on a MR device (1).