title: Robust estimation of pulse wave transit time using group delay.
abstract: OBJECTIVE
  To evaluate the efficiency of a novel transit time (Δt) estimation method from cardiovascular magnetic resonance flow curves.
  
  
  METHODS
  Flow curves were estimated from phase contrast images of 30 patients. Our method (TT-GD: transit time group delay) operates in the frequency domain and models the ascending aortic waveform as an input passing through a discrete-component "filter," producing the observed descending aortic waveform. The GD of the filter represents the average time delay (Δt) across individual frequency bands of the input. This method was compared with two previously described time-domain methods: TT-point using the half-maximum of the curves and TT-wave using cross-correlation. High temporal resolution flow images were studied at multiple downsampling rates to study the impact of differences in temporal resolution.
  
  
  RESULTS
  Mean Δts obtained with the three methods were comparable. The TT-GD method was the most robust to reduced temporal resolution. While the TT-GD and the TT-wave produced comparable results for velocity and flow waveforms, the TT-point resulted in significant shorter Δts when calculated from velocity waveforms (difference: 1.8±2.7 msec; coefficient of variability: 8.7%). The TT-GD method was the most reproducible, with an intraobserver variability of 3.4% and an interobserver variability of 3.7%.
  
  
  CONCLUSIONS
  Compared to the traditional TT-point and TT-wave methods, the TT-GD approach was more robust to the choice of temporal resolution, waveform type, and observer.
authors.affiliation: CMR Unit, Fondazione G. Monasterio CNR-Regione Toscana and Institute of Clinical Physiology, Pisa, Italy; Department of Pediatrics, Division of Cardiology, Children's Hospital Los Angeles, Los Angeles, California, USA.
authors.forename: Antonella
authors.initials: A
authors.lastname: Meloni
authors.forename: Heather
authors.initials: H
authors.lastname: Zymeski
authors.forename: Alessia
authors.initials: A
authors.lastname: Pepe
authors.forename: Massimo
authors.initials: M
authors.lastname: Lombardi
authors.forename: John C
authors.initials: JC
authors.lastname: Wood
mesh_terms: D000293:Adolescent; D000328:Adult; D000465:Algorithms; D000704:Analysis of Variance; D000755:Anemia, Sickle Cell / Q000503:physiopathology; D001013:Aorta, Thoracic; D015331:Cohort Studies; D005260:Female; D006801:Humans; D019028:Magnetic Resonance Imaging, Cine / Q000379:methods*; D008297:Male; D015588:Observer Variation; D011673:Pulsatile Flow; D063177:Pulse Wave Analysis; D012039:Regional Blood Flow / Q000502:physiology*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D013997:Time Factors; D055815:Young Adult; D017086:beta-Thalassemia / Q000503:physiopathology
keywords: aorta; group delay; phase contrast cardiovascular magnetic resonance; pulse wave velocity; transit time