title: Analysis of index modulation of doppler microembolic signals part II: in vitro discrimination.
abstract: The purpose of this study was to validate through experiments that frequency modulation (FM) of microembolic signatures was principally due to the radiation force. Several experiments were required to prove that such a frequency modulation originates from microdisplacements induced by the radiation force acting on microbubbles. The first experiment was performed to verify that the diffraction effects due to the presence of a skull did not disturb the acoustic field appreciably and to validate that a radiation force in the brain was sufficient to create a detectable microdisplacement. A second in vitro experiment using a single gate transcranial Doppler (TCD) system was conducted to show discrimination feasibility and to check that microembolic frequency modulation signatures (FMS) and frequency modulation index (FMI) were the same as those observed in vivo and those calculated by simulation. A final in vitro experiment was performed using a multigate multichannel TCD system to confirm the second experiment by directly measuring the microdisplacement induced by the radiation force. A new parameter, to be known as the position modulation index (PMI), is proposed. We showed that the radiation force is sufficient to induce detectable microdisplacements despite the presence of the skull. We also showed that the diffraction effects due to the skull induced a decrease in the ultrasound beam of 7.6 dB. Finally, we showed by using FMI and PMI that it is possible to discriminate gaseous from formed elements (<100 microns) despite the presence of the skull. The discrimination based on the FMI is an off-line technique allowing the analysis of standard TCD recordings. However, discrimination based on the PMI requires recordings obtained exclusively from a multi-gate system.
authors.affiliation: Université François Rabelais de Tours, UMRS Imaging and Brain U930 and CNRS ERL 3106, Tours, France. jean-marc.girault@univ-tours.fr
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Girault
authors.forename: Denis
authors.initials: D
authors.lastname: Kouamé
authors.forename: Sébastien
authors.initials: S
authors.lastname: Ménigot
authors.forename: Francesco
authors.initials: F
authors.lastname: Guidi
authors.forename: Grégrory
authors.initials: G
authors.lastname: Souchon
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Remenieras
mesh_terms: D002102:Cadaver; D003937:Diagnosis, Differential; D006801:Humans; D066298:In Vitro Techniques; D020766:Intracranial Embolism / Q000000981:diagnostic imaging*; D008962:Models, Theoretical; D010825:Physics; D012886:Skull / Q000000981:diagnostic imaging; D014159:Transducers; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods*