title: Effects of poor bone window on the assessment of cerebral autoregulation with transcranial Doppler sonography - a source of systematic bias and strategies to avoid it.
abstract: BACKGROUND
  The consequences of poor insonation conditions on autoregulation parameters assessed with transcranial Doppler (TCD) are unclear.
  
  
  METHODS
  We present two new complementary methods to assess the quality of a TCD signal. Inserting a thin aluminium foil between TCD probe and skin makes a simple model to artificially worsen a good insonation window. Validation studies are presented. We assessed insonation quality and cerebral autoregulation parameters with transfer function analysis and cross correlation in 46 healthy volunteers with and without the aluminium foil model. The same studies were operated on 45 patients with good insonation windows, naïve, after worsening the bone window and during constant infusion of an ultrasound contrast agent. For studying reproducibility, we assessed autoregulation twice in 30 patients with poor bone windows, with and without constant contrast infusion.
  
  
  RESULTS
  Both methods to measure insonation quality are valid and reproducible. The aluminium foil model realistically simulates a natural poor bone window, reducing the signal quality (e.g. energy of the signal spectrum from 33.4+/-3.5 to 26.2+/-2.5 dB, p<0.001). Thereby, the autoregulation parameters are systematically biased (e.g. phase difference from 37.3+/-10.1 degrees to 25.9+/-15.1 degrees , p<0.001); while with the use of an ultrasound contrast agent this can be largely compensated (phase difference 35.7+/-10.7 degrees , p<0.001). The reproducibility is significantly improved (ICC from 0.76 to 0.90, p<0.05).
  
  
  CONCLUSIONS
  Poor bone windows can cause considerable bias in TCD autoregulation parameters. This bias might be avoided by the use of ultrasound contrast agents, which may greatly improve the credibility of TCD autoregulation assessment in elderly patients.
authors.affiliation: Department of Neurology, Johann Wolfgang Goethe-University, Frankfurt am Main, Germany. matthias.lorenz@em.uni-frankfurt.de
authors.forename: Matthias W
authors.initials: MW
authors.lastname: Lorenz
authors.forename: Nadine
authors.initials: N
authors.lastname: Loesel
authors.forename: Nina
authors.initials: N
authors.lastname: Thoelen
authors.forename: Marilen
authors.initials: M
authors.lastname: Gonzalez
authors.forename: Christian
authors.initials: C
authors.lastname: Lienerth
authors.forename: Florian
authors.initials: F
authors.lastname: Dvorak
authors.forename: Waltraud
authors.initials: W
authors.lastname: Rölz
authors.forename: Marek
authors.initials: M
authors.lastname: Humpich
authors.forename: Matthias
authors.initials: M
authors.lastname: Sitzer
mesh_terms: D000328:Adult; D000368:Aged; D017607:Aluminum Compounds; D001783:Blood Flow Velocity; D001927:Brain Diseases / Q000000981:diagnostic imaging; D002560:Cerebrovascular Circulation; D015331:Cohort Studies; D004453:Echoencephalography / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging*; D008954:Models, Biological; D011134:Polysaccharides; D015203:Reproducibility of Results; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods*
chemical_list: D017607:Aluminum Compounds; D011134:Polysaccharides; C059891:SHU 508