title: Phase-contrast velocity mapping for highly diffusive fluids: optimal bipolar gradient pulse parameters for hyperpolarized helium-3.
abstract: OBJECTIVE
  In MR-velocity phase-contrast measurements, increasing the encoding bipolar gradient, i.e., decreasing the field of speed, usually improves measurement precision. However, in gases, fast diffusion during the bipolar gradient pulses may dramatically decrease the signal-to-noise ratio, thus degrading measurement precision. These two effects are contradictory. This work aims at determining the optimal sequence parameters to improve the velocity measurement precision.
  
  
  METHODS
  This work presents the theoretical optimization of bipolar gradient parameters (duration and amplitude) to improve velocity measurement precision. An analytical approximation is given as well as a numerical optimization. It is shown that the solution depends on the diffusion coefficient and T2 *. Experimental validation using hyperpolarized (3) He diluted in various buffer gases ((4) He, N2 , and SF6 ) is presented at 1.5 Tesla (T) in a straight pipe.
  
  
  RESULTS
  Excellent agreement was found with the theoretical results for prediction of optimal field of speed and good agreement was found for the precision in measured velocity, but for SF6 buffered gas.
  
  
  CONCLUSIONS
  The theoretical predictions were validated, providing a way to optimize velocity mapping in gases.
authors.affiliation: IR4M (UMR8081) Univ Paris-Sud, CNRS, Orsay, France.
authors.forename: Lionel
authors.initials: L
authors.lastname: Martin
authors.forename: Xavier
authors.initials: X
authors.lastname: Maître
authors.forename: Ludovic
authors.initials: L
authors.lastname: de Rochefort
authors.forename: Mathieu
authors.initials: M
authors.lastname: Sarracanie
authors.forename: Marlies
authors.initials: M
authors.lastname: Friese
authors.forename: Pascal
authors.initials: P
authors.lastname: Hagot
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Durand
mesh_terms: D000465:Algorithms; D003287:Contrast Media / Q000032:analysis / Q000737:chemistry; D004058:Diffusion; D006371:Helium / Q000032:analysis* / Q000737:chemistry*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D007554:Isotopes / Q000032:analysis / Q000737:chemistry; D008279:Magnetic Resonance Imaging / Q000379:methods*; D019275:Radiopharmaceuticals / Q000032:analysis / Q000737:chemistry; D015203:Reproducibility of Results; D012212:Rheology / Q000379:methods*; D012680:Sensitivity and Specificity
chemical_list: D003287:Contrast Media; D007554:Isotopes; D019275:Radiopharmaceuticals; D006371:Helium
keywords: airflow velocity; diffusion; flow; gas; hyperpolarized helium-3; phase-contrast; phase-contrast MRI