On the Relationship Between Microbubble Fragmentation, Deflation and Broadband Superharmonic Signal Production

Abstract

Acoustic angiography imaging of microbubble contrast agents utilizes the superharmonic energy produced from excited microbubbles, and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. In order to elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, while weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when imaging populations of stationary microbubbles with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1 μm bubbles than for 4 μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in a decrease in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. While the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate in order to maintain adequate contrast during in vivo imaging.

Molecular acoustic angiography: Demonstration of in vivo feasibility for high resolution superharmonic ultrasound molecular imaging

“…Although image intensity decreased rapidly with each subsequent scan (Figure 3), superharmonic echoes were detectable on the initial scan despite the fact that microbubbles had already been subjected to multiple pulses due to the large focal size of the low frequency transducer element ( Figure 1). This partial persistence of superharmonic signals over a few pulses is consistent with our own previous results [9]. We have also observed that superharmonic molecular imaging may be enhanced by using a microbubble population with a slightly larger diameter, consistent with previous observations that larger microbubbles improve sensitivity in conventional molecular imaging [10,12] and in acoustic angiography [8].…”

“…Additionally, larger bubbles are likely to produce superharmonic signals over a greater number of pulses than smaller bubbles [9]. In order to examine the effect of microbubble diameter on superharmonic imaging in vivo, two populations of microbubbles were prepared: a 1 μm population and a 4 μm population.…”

“…Despite the promising resolution of dual-frequency superharmonic imaging, our own recent studies indicate that dual-frequency superharmonic imaging is at least partially destructive to microbubbles [9], which makes implementation of superharmonic molecular imaging a significant challenge. Because the focal spot is larger for the low frequency element than the high frequency element, a single microbubble experiences multiple insonations as the transducer is swept mechanically (Fig.…”

“…1). However, there is evidence that the same microbubbles can produce superharmonic echoes in response to multiple pulses [4,9], suggesting that it may be possible to balance production of superharmonic signals and microbubble destruction in such a way as to enable molecular imaging.…”

Acoustic angiography: a new high frequency contrast ultrasound technique for biomedical imaging

“…Microbubbles having diameters of 1-4 μm were examined. Results indicate that production of a superharmonic signal was associated with two distinct physical behaviors: 1) immediate rupture of the microbubble shell, and 2) instantaneous decrease in shell diameter (at least 10%) in response to an acoustic pulse [63]. In the case of shell rupture, the amplitude of the superharmonic signal was approximately 4x higher than in the case of decrease in diameter.…”