title: A double-gaussian, percentile-based method for estimating maximum blood flow velocity.
abstract: OBJECTIVE
  Transcranial Doppler sonography allows for the estimation of blood flow velocity, whose maximum value, especially at systole, is often of clinical interest. Given that observed values of flow velocity are subject to noise, a useful notion of "maximum" requires a criterion for separating the signal from the noise. All commonly used criteria produce a point estimate (ie, a single value) of maximum flow velocity at any time and therefore convey no information on the distribution or uncertainty of flow velocity. This limitation has clinical consequences especially for patients in vasospasm, whose largest flow velocities can be difficult to measure. Therefore, a method for estimating flow velocity and its uncertainty is desirable.
  
  
  METHODS
  A gaussian mixture model is used to separate the noise from the signal distribution. The time series of a given percentile of the latter, then, provides a flow velocity envelope. This means of estimating the flow velocity envelope naturally allows for displaying several percentiles (e.g., 95th and 99th), thereby conveying uncertainty in the highest flow velocity.
  
  
  RESULTS
  Such envelopes were computed for 59 patients and were shown to provide reasonable and useful estimates of the largest flow velocities compared to a standard algorithm. Moreover, we found that the commonly used envelope was generally consistent with the 90th percentile of the signal distribution derived via the gaussian mixture model.
  
  
  CONCLUSIONS
  Separating the observed distribution of flow velocity into a noise component and a signal component, using a double-gaussian mixture model, allows for the percentiles of the latter to provide meaningful measures of the largest flow velocities and their uncertainty.
authors.affiliation: Department of Statistics, University of Washington, Box 354322, Seattle, WA 98195-4322 USA. marzban@stat.washington.edu.
authors.forename: Caren
authors.initials: C
authors.lastname: Marzban
authors.forename: Paul R
authors.initials: PR
authors.lastname: Illian
authors.forename: David
authors.initials: D
authors.lastname: Morison
authors.forename: Pierre D
authors.initials: PD
authors.lastname: Mourad
mesh_terms: D000465:Algorithms; D001783:Blood Flow Velocity; D001921:Brain / Q000098:blood supply / Q000502:physiology; D002560:Cerebrovascular Circulation; D003627:Data Interpretation, Statistical; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D020768:Middle Cerebral Artery / Q000502:physiology*; D016011:Normal Distribution; D015203:Reproducibility of Results; D012212:Rheology; D012680:Sensitivity and Specificity; D059629:Signal-To-Noise Ratio; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods*
keywords: blood flow; brain; head injury; noninvasive; transcranial Doppler sonography