title: Measurement of systolic and diastolic arterial wall shear stress in the ascending aorta.
abstract: OBJECTIVE
  Wall shear stress (WSS) appears to contribute significantly in the initiation and progression of atherosclerotic disease. The purpose of this work is to present in vivo systolic and diastolic WSS calculations in the human ascending aorta by the application of three straightforward methodologies based on Poiseuille's theory of flow.
  
  
  METHODS
  Blood flow measurements were performed retrospectively in the ascending aorta of 20 non-atherosclerotic patients using phase-contrast MRI. WSS calculations were performed assuming Poiseuille's theory of flow based on average flow volume, average flow velocity and maximum flow velocity. Systolic and diastolic WSS values were calculated and compared with the calculated maximum and minimum values of WSS throughout the cardiac cycle.
  
  
  RESULTS
  Systolic WSS values calculated by average flow volume, average flow velocity and maximum flow velocity were similar (0.4+/-0.2N/m(2), 0.4+/-0.3N/m(2)and 0.4+/-0.2N/m(2), respectively). Diastolic WSS values calculated by maximum flow velocity were significantly higher (11.6+/-7.0x10(-2)N/m(2)) compared to values calculated by average flow volume (0.3+/-0.9x10(-2)N/m(2)) and average flow velocity (0.3+/-1.0x10(-2)N/m(2)). Comparison of systolic and diastolic WSS values with maximum and minimum WSS values showed that time instances of maximum and minimum blood flow velocities do not coincide with time instances of maximum and minimum blood flow volume.
  
  
  CONCLUSIONS
  In vivo calculation of WSS in the ascending aorta is feasible by phase-contrast MRI flow measurements and straightforward methodologies based on Poiseuille's theory of flow. However, measurements based on maximum flow velocity show larger deviations compared to measurements based on mean flow volume or mean flow velocity.
authors.affiliation: Second Department of Radiology, Medical School, National and Kapodistrian University of Athens, Athens, Greece. stathise@med.uoa.gr
authors.forename: Efstathios P
authors.initials: EP
authors.lastname: Efstathopoulos
authors.forename: George
authors.initials: G
authors.lastname: Patatoukas
authors.forename: Ioannis
authors.initials: I
authors.lastname: Pantos
authors.forename: Odysseas
authors.initials: O
authors.lastname: Benekos
authors.forename: Demosthenes
authors.initials: D
authors.lastname: Katritsis
authors.forename: Nikolaos L
authors.initials: NL
authors.lastname: Kelekis
mesh_terms: D000328:Adult; D000368:Aged; D001011:Aorta / Q000502:physiology*; D001783:Blood Flow Velocity / Q000502:physiology*; D003198:Computer Simulation; D003971:Diastole / Q000502:physiology*; D055119:Elastic Modulus; D005260:Female; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D008955:Models, Cardiovascular; D033081:Shear Strength; D013599:Systole / Q000502:physiology*; D055815:Young Adult