Basic Acoustic Properties of Microbubbles

Abstract

Small (encapsulated) gas bubbles in a contrast medium react to an external oscillating pressure field with volume pulsations. Depending on the magnitude of the ultrasound wave, the vibrations will be related either linearly or nonlinearly to the applied acoustic pressure. For low acoustic pressures, the instantaneous radius oscillates linearly in relation to the amplitude of the applied external pressure field. The oscillation of the bubble is governed by parameters such as resonance frequency, damping coefficients, and shell properties. For higher amplitudes of the external field, the pulsation of the bubbles becomes nonlinear. The spectrum of the scattered ultrasound wave also contains higher harmonics of the emitted frequency in addition to the fundamental frequency. The emitted frequency, bubble size, and nonlinear propagation effects have significant influence on the harmonic generation. For encapsulated bubbles exposed to even higher acoustic amplitudes, their scattering effectiveness increases dramatically and becomes transient. The scattered frequency spectrum broadens, containing higher harmonics. This consequence is due to rupture, disappearance, change of gas content, etc. Using these specific characteristics of the contrast bubbles will open new perspectives in imaging and analysis for medical diagnosis.

Myocardial Ischemic Memory Imaging With Molecular Echocardiography

“…Higher transducer frequencies were required to image rat myocardium than typically used in humans, which may induce less nonlinear microbubble behavior because the natural harmonic frequency of microbubbles is lower. 39 Higher acoustic intensities resulting from more microbubble resonance at the lower transducer frequencies use in human imaging could compensate for the inherent limitations of acoustic windows in clinical populations. Ultimately, ultrasound systems designed specifically for molecular imaging applications are necessary to optimize the clinical detection of adhered microbubbles.…”

Multimodality Cardiovascular Molecular Imaging, Part I

“…14,15 The ultrasound contrast agents are typically gas-filled microspheres (microbubbles) of varying chemical composition that can be induced to expand and contract (resonate) in the presence of ultrasound delivered at the resonance frequency of the microbubbles. 16 The natural resonance frequency of the microbubbles is determined predominantly by their size (typically Ͻ4 m) and shell characteristics, and ranges from 1 to 5 MHz, which is fortuitously the frequency range of transducers for clinical diagnostic echocardiography. At sufficiently high transmitted acoustic pressures, the bubbles expand and contract in nonlinear fashion, emitting signals (harmonics) that are acoustically distinct from tissue backscatter.…”

Bubble Puzzles

“…14 In that technique, a solution of micron-sized bubbles is injected into the bloodstream. Normally the bubbles are coated to avoid clustering and to prevent surface tension from dissolving the bubbles by pushing the gas out of them.…”