title: Advances in real-time phase-contrast flow MRI using asymmetric radial gradient echoes.
abstract: OBJECTIVE
  To provide multidimensional velocity compensation for real-time phase-contrast flow MRI.
  
  
  METHODS
  The proposed method introduces asymmetric gradient echoes for highly undersampled radial FLASH MRI with phase-sensitive image reconstruction by regularized nonlinear inversion (NLINV). Using an adapted gradient delay correction the resulting image quality was analyzed by simulations and experimentally validated at 3 Tesla. For real-time flow MRI the reduced gradient-echo timing allowed for the incorporation of velocity-compensating waveforms for all imaging gradients at even shorter repetition times.
  
  
  RESULTS
  The results reveal a usable degree of 20% asymmetry. Real-time flow MRI with full velocity compensation eliminated signal void in a flow phantom, confirmed flow parameters in healthy subjects and demonstrated signal recovery and phase conservation in a patient with aortic valve insufficiency and stenosis. Exemplary protocols at 1.4-1.5 mm resolution and 6 mm slice thickness achieved total acquisition times of 33.3-35.7 ms for two images (7 spokes each) with and without flow-encoding gradient.
  
  
  CONCLUSIONS
  Asymmetric gradient echoes were successfully implemented for highly undersampled radial trajectories. The resulting temporal gain offers full velocity compensation for real-time phase-contrast flow MRI which minimizes false-positive contributions from complex flow and further enhances the temporal resolution compared with acquisitions with symmetric echoes.
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: Biomedizinische NMR Forschungs GmbH am Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany.
authors.forename: Zhengguo
authors.initials: Z
authors.lastname: Tan
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.|DZHK, German Center for Cardiovascular Research, partner site 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: Volkert
authors.initials: V
authors.lastname: Roeloffs
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.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schätz
authors.affiliation: Biomedizinische NMR Forschungs GmbH am Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany.|DZHK, German Center for Cardiovascular Research, partner site Göttingen, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Frahm
mesh_terms: D000465:Algorithms; D001011:Aorta / Q000000981:diagnostic imaging / Q000473:pathology; D001022:Aortic Valve Insufficiency / Q000473:pathology; D003198:Computer Simulation; D003251:Constriction, Pathologic; D003287:Contrast Media / Q000737:chemistry*; D005189:False Positive Reactions; D064368:Healthy Volunteers; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015233:Models, Statistical; D019047:Phantoms, Imaging
chemical_list: D003287:Contrast Media
keywords: asymmetric echo; cardiovascular blood flow; nonlinear inverse reconstruction; phase-contrast flow MRI; radial MRI; real-time MRI