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Optimal multiple motion encoding in Phase-Contrast MRI

  • Herthum, Helge
  • Carrillo, Hugo
  • Osses, Axel
  • Uribe, Sergio
  • Sack, Ingolf
  • Bertoglio, Cristobal
Publication Date
Sep 23, 2020
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Purpose: To propose an optimal multiple motion encoding (OMME) method for Phase-Contrast MRI (PC-MRI) with application to Magnetic Resonance Elastography (MRE) for reconstructing wrap-free wave images. Theory: OMME is formulated and analyzed as an extension of [5] using an arbitrary number of measurements with different motion encoding gradients (MEG) to increase the dynamic range of motion encoding. Methods: OMME is assessed numerically for several high and low motion sensitivity ratios in terms of motion-to-noise-ratio (MNR) for different image SNR. The algorithms are then tested on MRE data from heparin phantom experiments and ultimately from in vivo human brain experiments for the most robust MEGs combination. The wrap-free motion images are further used to reconstruct shear wave speed (SWS) maps and compared to the ones obtained using conventional phase unwrapping methods. Results: For the dual encoding case, OMME performs better than the standard method, in terms of robustness to noise and effective MNR. OMME allowed to successfully combine three and four MRE wave images with different dynamic ranges in the phantom and volunteer data, respectively, leading to SWS maps which preserve important details (e.g. discontinuities in the stiffness) when compared to conventional unwrapping methods. Conclusion: In Phase-Contrast MRI, with OMME the MNR grows exponentially with the number of measurements as opposeds to standard image averaging. In MRE, this allows for great improvements in recovering tissue details in the SWS maps in contrast to conventional unwrapping methods.

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