Multiple motion encoding in Phase-Contrast MRI: A general theory and application to elastography imaging
Résumé
Purpose: Phase-contrast (PC) MRI allows to encode the motion of tissue in the magnetization's phase. Yet, it remains a challenge to obtain high fidelity motion images due aliasing in the phase for high encoding efficiencies. Therefore, we propose an optimal multiple motion encoding method
(OMME) and exemplify it in Magnetic Resonance Elastography (MRE) data.
Theory: OMME is formulated as a non-convex least-squares problem for the motion using an arbitrary number of phase-contrast measurements with different motion encoding gradients (MEGs). The mathematical properties of OMME are proved in terms of standard deviation and dynamic
range of the motion's estimate for arbitrary MEGs combination which are compared using synthetically generated data.
Methods: OMME's performance is assessed on MRE data from in vivo human brain experiments and compared to dual encoding strategies. The unwrapped images are further used to reconstruct stiffness maps and compared to the ones obtained using conventional phase unwrapping methods.
Results: OMME allowed to successfully combine several MRE phase images with different MEGs. OMME outperforms dual encoding strategies in either motion-to-noise ratio (MNR) or number of successfully reconstructed voxels with good noise stability. This lead to stiffness maps with greater
resolution of details than obtained with conventional unwrapping methods.
Conclusion: The proposed OMME method allows for a flexible and noise robust increase in the dynamic range and thus provides wrap-free PC images with high MNR. In MRE, the method may be especially suitable when high resolution images with high MNR are needed.
Domaines
Traitement des images [eess.IV]
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