title: Spatiotemporal phase unwrapping for real-time phase-contrast flow MRI.
abstract: OBJECTIVE
  To develop and evaluate a practical phase unwrapping method for real-time phase-contrast flow MRI using temporal and spatial continuity.
  
  
  METHODS
  Real-time phase-contrast MRI of through-plane flow was performed using highly undersampled radial FLASH with phase-sensitive reconstructions by regularized nonlinear inversion. Experiments involved flow in a phantom and the human aorta (10 healthy subjects) with and without phase wrapping for velocity encodings of 100 cm·s(-1) and 200 cm·s(-1) . Phase unwrapping was performed for each individual cardiac cycle and restricted to a region of interest automatically propagated to all time frames. The algorithm exploited temporal continuity in forward and backward direction for all pixels with a "continuous" representation of blood throughout the entire cardiac cycle (inner vessel lumen). Phase inconsistencies were corrected by a comparison with values from direct spatial neighbors. The latter approach was also applied to pixels exhibiting a discontinuous signal intensity time course due to movement-induced spatial displacements (peripheral vessel zone).
  
  
  RESULTS
  Phantom and human flow MRI data were successfully unwrapped. When halving the velocity encoding, the velocity-to-noise ratio (VNR) increased by a factor of two.
  
  
  CONCLUSIONS
  The proposed phase unwrapping method for real-time flow MRI allows for measurements with reduced velocity encoding and increased VNR.
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck Institut für biophysikalische Chemie, Göttingen, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Untenberger
authors.affiliation: Fraunhofer MEVIS Institute for Medical Image Computing, Bremen, Germany.
authors.forename: Markus
authors.initials: M
authors.lastname: Hüllebrand
authors.affiliation: Fraunhofer MEVIS Institute for Medical Image Computing, Bremen, Germany.
authors.forename: Lennart
authors.initials: L
authors.lastname: Tautz
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck Institut für biophysikalische Chemie, Göttingen, Germany.|German Center for Cardiovascular Research (DZHK), Göttingen, Germany.
authors.forename: Arun A
authors.initials: AA
authors.lastname: Joseph
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck Institut für biophysikalische Chemie, Göttingen, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Voit
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck Institut für biophysikalische Chemie, Göttingen, Germany.
authors.forename: K Dietmar
authors.initials: KD
authors.lastname: Merboldt
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck Institut für biophysikalische Chemie, Göttingen, Germany.|German Center for Cardiovascular Research (DZHK), Göttingen, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Frahm
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019047:Phantoms, Imaging
keywords: flow MRI; phase unwrapping; phase-contrast MRI; real-time MRI; velocity-encoded MRI