Abstract
BackgroundThe observation of ultrasound generated cavitation bubbles deep in tissue is very difficult. The development of an imaging method capable of investigating cavitation bubbles in tissue would improve the efficiency and application of ultrasound in the clinic. Among the previous imaging modalities capable of detecting cavitation bubbles in vivo, the acoustic detection technique has the positive aspect of in vivo application. However the size of the initial cavitation bubble and the amplitude of the ultrasound that produced the cavitation bubbles, affect the timing and amplitude of the cavitation bubbles’ emissions.MethodsThe spatial distribution of cavitation bubbles, driven by 0.8835 MHz therapeutic ultrasound system at output power of 14 Watt, was studied in water using a synchrotron X-ray imaging technique, Analyzer Based Imaging (ABI). The cavitation bubble distribution was investigated by repeated application of the ultrasound and imaging the water tank. The spatial frequency of the cavitation bubble pattern was evaluated by Fourier analysis. Acoustic cavitation was imaged at four different locations through the acoustic beam in water at a fixed power level. The pattern of cavitation bubbles in water was detected by synchrotron X-ray ABI.ResultsThe spatial distribution of cavitation bubbles driven by the therapeutic ultrasound system was observed using ABI X-ray imaging technique. It was observed that the cavitation bubbles appeared in a periodic pattern. The calculated distance between intervals revealed that the distance of frequent cavitation lines (intervals) is one-half of the acoustic wave length consistent with standing waves.ConclusionThis set of experiments demonstrates the utility of synchrotron ABI for visualizing cavitation bubbles formed in water by clinical ultrasound systems working at high frequency and output powers as low as a therapeutic system.
Highlights
The observation of ultrasound generated cavitation bubbles deep in tissue is very difficult
It is essential to conduct a fundamental study for cavitation detection and cloud cavitation control to improve the safety and application of ultrasound therapy and possibly for ultrasonography which is pervasively used for neonatal imaging
The present paper addresses cavitation bubble formation in a type of ultrasound system commonly used for physical therapy applications (14 W and 0.88 MHz) where one might not expect to observe cavitation bubbles
Summary
The observation of ultrasound generated cavitation bubbles deep in tissue is very difficult. When the acoustic cavitation bubble collapses close to or on a solid surface, it can collapse asymmetrically and produce high-speed jets of liquid being driving into the surface of the solid have been observed at speeds close to 400 km/h [1] This can seriously damage the impact zone and create a newly exposed surface. High intensity focused ultrasound treatment (HIFU) in which the ultrasound is focused into a small focal zone can damage tissue as a result of the very high temperature inside the bubbles produced, the collapse that creates a shock wave and jets, and time duration of tissue exposure. Since the onset of cavitation and the resulting tissue damage is not predictable, high acoustic intensity is generally avoided in clinic, cavitation is under investigation to be used as a means to enhance HIFU ablation. It is essential to conduct a fundamental study for cavitation detection and cloud cavitation control to improve the safety and application of ultrasound therapy (such as lithotripsy and HIFU) and possibly for ultrasonography which is pervasively used for neonatal imaging
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