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Multi frequency excitation for a wide range of bubble detection
D. Fouan 1
,2*, S. Mensah2
1Bf Systemes – Technopole de la Mer, 229 chemin de la Farlède, 83500 La Seyne sur Mer, France
2 Mechanics and Acoustics Laboratory, CNRS-UPR 7051, Ch Joseph Aiguier, 13402 Marseille,
France
* e-mail: damien.fouan@bf-systemes.fr
During hyperbaric decompression (diving or hyperbaric medicine), both the absolute ambient and the
absolute inspired pressures are reduced and bubbles with diameters ranging from 5 to 200 µm may
take place accordingly from pre-existing gas nuclei (the 0.1-5 µm bubbles). An accurate monitoring of
the size and of the density of the bubble population (including the nuclei) will provide a valuable
means to understand the nucleation and growth processes in various supersaturated tissues as well as
to help in sickness prevention.
In this aim, an ultrasonic characterization method based on a dual frequency technique [1] applied on a
single bubble is tested. The method consists in sending two ultrasonic waves on a stationary bubble.
One is a low frequency wave (10 kHz<fLF<90 kHz), which activates the bubble near its resonance
frequency and the other one is a high frequency wave ( fHF=1MHz) that measures the changes in the
acoustic cross-section induced by the low frequency activation. Considering both the blood flow speed
in the right ventricle (where the detection is made) and the size of the focal zone of the transducer, the
elementary characterization duration is fixed to 0.01 sec. Furthermore, embedded instrumentation and
low mechanical index constraints imply that only a low power excitation of the low frequency
transducer is used. The objective that we are looking forward is to detect and characterize bubbles
with diameters ranging from 20 to 200µm. Thus the LF-excitation frequency should vary between the
corresponding resonance frequencies (320 and 32 kHz). However, such a wide range sweep implies
that the instantaneous frequency is varying too fast to make the bubble-resonance easily detectable in
the noisy pericardial environment. Therefore, the solution proposed consists in sending non-sinusoidal
LF waves whose harmonics contribute in the bubble excitation. Afterwards, the amplitude
modulations of the various backscattered harmonic components are processed and the resonance
frequency is extracted. Using the [10-90 kHz]-chirps, this procedure leads to a sizing of stationary
bubbles accuracy up to 5% and about 5 dB signal to noise ratio. This method allows a wide range of
bubbles detection and characterization in reasonable process duration, compatible with the
supersaturated-hemodynamics constraints.
[1] J. Y. Chapelon et al., Ultrasonic measurement of bubble cloud size profile, JASA,
78(1):196-201, 1985.