title: Distinguishing air from solid emboli using ultrasound: in-vitro study of the effect of Doppler carrier frequency.
abstract: OBJECTIVE
  To compare the ability of the signal relative-intensity and sample-volume-length (SVL) to discriminate air bubbles from solid spheres in an in-vitro model using two different carrier frequencies of the Doppler transducer.
  
  
  METHODS
  A gel ultrasound phantom was connected to a circuit in which blood-mimicking fluid circulated. Air bubbles (100-140 microm) and latex spheres (125 +/- 10 microm) were injected into the circuit and interrogated using 1- and 2-MHz transducers. High-intensity-transient-signals (HITS) were recorded with a dual-gated transcranial Doppler (TCD) system. Receiver-Operating-Characteristic curves determined the best cut-off points that would distinguish between embolic materials.
  
  
  RESULTS
  HITS from air bubbles had higher intensities and longer SVL than solid spheres with either transducer (P < .0001). Air bubbles (P < .0001) and microspheres (P= .049) showed higher intensities with the 1-MHz relative to the 2-MHz transducer. The intensity increase with the 1-MHz transducer was greater for air bubbles than microspheres (P < .0001). The discriminating efficacy of both the relative-intensity and SVL was similar between transducers (intensity, P= .201; SVL, P= .98).
  
  
  CONCLUSIONS
  The relative-intensity and SVL are equally effective to distinguish solid from air emboli using 1- and 2-MHz transducers. Our study indicates that using a lower carrier frequency does not improve the discrimination of air from solid emboli.
authors.affiliation: Department of Surgery, Division of Cardiac Surgery, University of Ottawa Heart Institute, Ottawa, Ontario, Canada. Rrodriguez@ottawaheart.ca
authors.forename: Rosendo A
authors.initials: RA
authors.lastname: Rodriguez
authors.forename: Carlos D
authors.initials: CD
authors.lastname: Rodriguez
authors.forename: Thierry
authors.initials: T
authors.lastname: Mesana
authors.forename: Howard J
authors.initials: HJ
authors.lastname: Nathan
mesh_terms: D000388:Air; D000704:Analysis of Variance; D004618:Embolism, Air / Q000000981:diagnostic imaging; D005782:Gels; D006801:Humans; D020766:Intracranial Embolism / Q000000981:diagnostic imaging; D008863:Microspheres; D019047:Phantoms, Imaging; D012372:ROC Curve; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D018709:Statistics, Nonparametric; D014159:Transducers; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods*
chemical_list: D005782:Gels