MR Water-Fat Separation with Eddy Current Phase Correction
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Solution Overview
Problem
Existing MR imaging techniques, such as Dixon-type water/fat imaging, face challenges in high B0 field inhomogeneities, leading to phase errors that disrupt phase consistency between echo signals, especially in bipolar acquisitions, which complicates water/fat separation and requires additional calibration measurements, increasing scan time.
Innovation Solution
A method that generates multiple echo signals at different echo times, using a signal model to estimate and correct eddy current-induced phase errors through a fitting procedure, allowing for accurate separation of water and fat contributions without additional phase error corrections, by modeling phase errors alongside other parameters like magnetic field inhomogeneities and relaxation rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Dixon-type water/fat imaging is performed at high B0 fields, then imaging capability is improved, but phase errors increase due to B0 field inhomogeneities
Solution Approach 1:
The patent applies preliminary action by performing phase error estimation and correction before the water/fat separation process. A fitting procedure is used to estimate phase errors from the acquired echo signals, and these estimated phase errors are then corrected in a preliminary step before the actual Dixon water/fat separation is performed. This ensures that phase consistency is restored prior to separation, resolving the issue of phase errors at high B0 fields.
2Measurement precision
If additional calibration measurements are performed to correct phase errors, then measurement precision is improved, but scan time increases
Solution Approach 1:
The patent applies self-service by using the already-acquired echo signals from the imaging sequence itself to estimate and correct phase errors, rather than requiring separate calibration measurements. The fitting procedure extracts phase error information directly from the multi-echo Dixon data that is already being collected for water/fat separation, making the phase correction self-contained within the existing imaging protocol and eliminating additional scan time.
3Productivity
If bipolar acquisitions are used to reduce scan time, then productivity is improved, but phase errors increase due to disrupted phase consistency
Solution Approach 1:
The patent converts the harmful phase errors introduced by bipolar acquisitions into a correctable parameter. Instead of avoiding bipolar acquisitions, the method embraces them for their time efficiency and then uses a fitting procedure to estimate the specific phase errors they introduce. By modeling these errors as unknown parameters in the signal model, the method transforms the disadvantage of bipolar sequences into a solvable problem that can be corrected during reconstruction, maintaining both speed and accuracy.
4Measurement precision
If phase errors are corrected at high resolution, then measurement precision is improved, but signal-to-noise ratio decreases
Solution Approach 1:
The patent applies partial action by performing phase error estimation and correction at a reduced resolution level first. The fitting procedure is initially applied to low-resolution versions of the echo signals to obtain phase error estimates, which are then used to correct the full-resolution data. This two-stage approach allows phase correction to be performed on data with better signal-to-noise characteristics, and the correction parameters are subsequently applied to the high-resolution images without directly processing noisy high-resolution data through the fitting procedure.
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 enables unambiguous determination of chemical species contributions, particularly fat fraction, with improved signal-to-noise ratio and reduced scan time, by correcting phase errors at lower resolution and applying the corrections for high-resolution water/fat separation, thus enhancing the accuracy and efficiency of MR imaging.
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
the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time)
Implementation Method 4
switched magnetic field gradients (also referred to as 'gradient pulses') 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 5
eddy current-induced phase errors, wherein the eddy current-induced phase errors are estimated by a fitting procedure from the two or more echo signals
Data Source
AI summary
An improved method of MR imaging of at least two chemical species having different MR spectra, such as water and fat, enables a precise quantification of water and fat or derived measures, such as a fat fraction. The method includes the steps of: a) generating two or more echo signals at different echo times by subjecting a body (10) placed in the examination volume of a MR device (1) to an imaging sequence of RF pulses and switched magnetic field gradients; b) acquiring the two or more echo signals; c) separating signal contributions of the at least two chemical species to the acquired echo signals on the basis of a signal model including the MR spectrum of each of the chemical species, the spatial variation of the main magnetic field in the examination volume, the effective transverse relaxation rate, and eddy current-induced phase errors. The eddy current-induced phase errors are estimated by a fitting procedure from the two or more echo signals.