Release and characterization of a single bubble

Abstract

During hyperbaric decompression (diving or hyperbaric medicine), both the absolute ambient and the absolute inspired pressures are reducing; (5-100 µm) bubbles may be generated from pre-existing gas nuclei (0.1-5 µm). An accurate monitoring of the size and of the density of the bubble (including the nuclei) population will provide a valuable means to understand the nucleation and growth processes in various supersaturated tissues. In this aim, an ultrasonic characterization method based on a dual frequency technique [1] applied on a free single bubble is tested. In order to design and evaluate the accuracy of a free single bubble sizing technique operating in water, bubbles of different diameters tethered on a wire have been set free by using high amplitude pressure waves. Then, two ultrasound focused waves impinge simultaneously on each microbubble: a low frequency chirp (20 kHz<fLF<80 kHz, 10 ms duration) is used to modulate the microbubble size and a high frequency imaging pulse (fHF=1 MHz) measures the changes in the acoustic cross section induced by the low frequency activation. The amplitude modulation of the backscattered imaging signals produces sum-and-difference components that max out when the low frequency modulation encompasses the microbubble's resonance frequency. The latter is inversely proportional to its radius. By taking advantage of the time-frequency distribution (spectrogram), the chirps returned by the bubble are analyzed both from a frequency and an amplitude points of view. The prior knowledge of the low frequency modulation allows us to restrict the search of the possible resonance only at the locus, i.e. the support of the chirp in the time-frequency plane, where the scattering energy is expected. In order to improve the quality factor of the estimation, multiple amplitude modulations are extracted and then averaged. When the microbubble radii are comprised between (40 – 150 µm), an ultrasound accuracy (5% error) equivalent to that of an optical camera (2.8 micrometers/pixel) is obtained. Specific influence on the measurement induced by the use of a pulsed high frequency wave is also introduced. [1] D. Cathignol et al., Bubble sizing with high spatial resolution, IEEE UFFC, 37(1):30-37, 1990.

Full text

Release and characterization of a single bubble
D. Fouan1,2, T.Goursolle1, B. Potier1 P. Lasaygues2 and S. Mensah*2
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: mensah@lma.cnrs-mrs.fr
During hyperbaric decompression (diving or hyperbaric medicine), both the absolute ambient and the
absolute inspired pressures are reducing; (5-100 µm) bubbles may be generated from pre-existing gas
nuclei (0.1-5 µm). An accurate monitoring of the size and of the density of the bubble (including the
nuclei) population will provide a valuable means to understand the nucleation and growth processes in
various supersaturated tissues. In this aim, an ultrasonic characterization method based on a dual
frequency technique [1] applied on a free single bubble is tested.
In order to design and evaluate the accuracy of a free single bubble sizing technique operating in
water, bubbles of different diameters tethered on a wire have been set free by using high amplitude
pressure waves. Then, two ultrasound focused waves impinge simultaneously on each microbubble: a
low frequency chirp (20 kHz<fLF<80 kHz, 10 ms duration) is used to modulate the microbubble size
and a high frequency imaging pulse (fHF=1 MHz) measures the changes in the acoustic cross section
induced by the low frequency activation. The amplitude modulation of the backscattered imaging
signals produces sum-and-difference components that max out when the low frequency modulation
encompasses the microbubble’s resonance frequency. The latter is inversely proportional to its radius.
By taking advantage of the time-frequency distribution (spectrogram), the chirps returned by the
bubble are analyzed both from a frequency and an amplitude points of view. The prior knowledge of
the low frequency modulation allows us to restrict the search of the possible resonance only at the
locus, i.e. the support of the chirp in the time-frequency plane, where the scattering energy is expected.
In order to improve the quality factor of the estimation, multiple amplitude modulations are extracted
and then averaged.
When the microbubble radii are comprised between (40 – 150 µm), an ultrasound accuracy (5% error)
equivalent to that of an optical camera (2.8 micrometers/pixel) is obtained. Specific influence on the
measurement induced by the use of a pulsed high frequency wave is also introduced.
[1] D. Cathignol et al., Bubble sizing with high spatial resolution, IEEE UFFC, 37(1):30-37, 1990.