Experimental observation of subharmonic oscillations in Infoson bubbles

Abstract

This paper reports the result of an experimental study of nonlinear emission from gas-filled bubbles with particular emphasis on the subharmonic emission. The gas bubbles are Infoson, a contrast agent for use in echocardiology consisting of small gas-filled microspheres with a mean diameter of approximately 4 μm. Pulsed signals with center frequencies of 3.5 and 4 MHz were transmitted through a cloud of bubbles and the level of the subharmonic component was measured as a function of the level of the exciting signal. No sharp threshold, as expected from theory, is found. The amplitude of the subharmonic response increases, however, approximately as the cube of the driving pressure when the driving pressure is in the region of 50–100 kPa.

Ultrasound contrast agents : dynamics of coated bubbles

“…This finding confirms previous results by Bhagavatheeshwaran et al [81] and by Kimmel et al [82]. In those cases where subharmonic oscillations were observed these were initiated already at driving pressure amplitudes smaller than 40 kPa confirming the results found by another set of authors [7,[43][44][45][46][47].…”

“…5.10A the subharmonic oscillation amplitude at the maximum subharmonic response frequency is plotted as a function of the driving pressure amplitude. We observe that the threshold pressure for the initiation of subharmonic oscillations is smaller than 5 kPa, much lower than that of an uncoated gas bubble without a shell and much lower than is expected based on the additional damping introduced by the phospholipid shell of the bubble [7,[43][44][45][46][47]. For the 5 kPa driving pressure the only driving frequency showing a subharmonic response was 2.8 MHz corresponding to a resonance frequency of 1.4 MHz.…”

“…Interestingly, we observe that the subharmonic amplitude decreases for increasing driving pressure amplitudes above a pressure of 80 kPa. To investigate these results in more detail we conducted numerical simulations using three different models, an uncoated gas bubble model as described by Lotsberg et al [43], a purely linear viscoelastic shell model [63] and the model proposed by Marmottant et al [12]. The shell parameters for the model of Marmottant were taken from the best fit from Fig.…”

“…Shankar et al [44] studied the subharmonic behavior of coated bubbles following the analysis of Prosperetti [42] and confirmed, by using a purely linear viscoelastic shell model as by De Jong [63], Church [33], 5.2 THEORY or Hoff [35], that indeed the threshold for subharmonic generation is increased as a result of the increased damping. There exists, however, experimental evidence in the literature showing that for both the albumin-coated contrast agents Optison TM and Albunex and the phospholipid-coated contrast agent SonoVue , the threshold pressure to excite subharmonic behavior is lower than that of uncoated bubbles [7,[43][44][45][46][47][48][49]. Other work reports no significant change in the threshold pressure, neither for albumin-coated bubbles [81] nor for the phospholipid-coated Definity TM contrast agent microbubbles [82].…”

“…As the oscillations of coated bubbles are considerably stronger damped it has been assumed that subharmonic behavior for coated bubbles must occur at higher acoustic pressures. However, it has been shown experimentally that subharmonic behavior of coated bubbles occurs at lower acoustic pressures even lower than those for uncoated bubbles [7,[43][44][45][46][47][48][49]. The Fourier transform of the radius-time curve of an oscillating coated bubble insonified at a frequency of 2 MHz shows a strong subharmonic component at a frequency of 1 MHz, see …”