# An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles
## 1 Introduction  
Microbubbles are gas-filled bubbles encapsulated in a shell and are typically 1-10 \(\upmu\)m in diameter. Microbubbles are used as Ultrasound Contrast Agents (UCAs) because they oscillate in the presence of an acoustic field. This oscillation is due to the stiffness of the enclosed gas and the inertia of the liquid surrounding the microbubble [1, 2]. Microbubbles act as resonant systems during insonification [3]. The resonant frequency is dependent on the physical properties of the encapsulating shell, the gas core, and the surrounding medium [2, 4, 5]. An oscillating microbubble dissipates energy through re-radiation, viscous dissipation, and thermal dissipation [3, 6]. The behaviour of single microbubbles can be extended to populations provided that the concentration is sufficiently small [1]. At low concentrations, multiple scattering and the interactions between oscillating microbubbles can be ignored. Thus, for a population, both scattering and attenuation can be measured acoustically. The total scattering measured is the summation of the scattering from individual microbubbles and the total attenuation measured is the summation of the scattering and absorption from individual microbubbles [2]. Because the microbubble radius is inversely proportional to the resonant frequency [2], the population size distribution has an impact on the acoustic response. By measuring the overall acoustic response of a microbubble population, as opposed to single microbubble measurements, the number of measurements required to obtain a result that is statistically significant is reduced [7]. This can reduce the time needed to perform the characterisation experiment. A common clinical application for UCAs is in contrast-enhanced echocardiography for Left Ventricular Opacification (LVO) [8]. For this application, the contrast agent is required to generate strong backscattered signals in the presence of a pressure wave in the range of frequencies used for diagnostic cardiac ultrasound imaging. Typically, cardiac imaging has a frequency range of 2-8 MHz. The scattering power of oscillating microbubbles is the largest near to the resonant frequency. Another requirement for cardiac imaging is that microbubbles are sufficiently small (less than 8 \(\upmu\)m in diameter) to pass through the pulmonary microcirculation and stable enough to reach the left ventricle [4]. The use of gases with a high molecular weight and the encapsulation of microbubbles in a shell have improved the stability of UCAs. These advances also cause an increase in both resonant frequency and viscosity, which increases the damping [3]. Microbubbles less than 8 \(\upmu\)m in diameter have resonances within the range of frequencies of diagnostic ultrasound [9]. The acoustic properties of UCAs can induce imaging artefacts unique to contrast-enhanced ultrasound imaging. In LVO, the concentration of microbubbles administered must be kept sufficiently low to limit the shadowing artefacts that can be introduced as a result of microbubble attenuation. At the Mechanical Index (MI) typically used in conventional echocardiography, destruction can occur by fragmentation of the microbubble or diffusion of the gas [10]. For most contrast-enhanced imaging applications, the MI is typically below 0.5 or, to limit microbubble destruction further and alleviate the swirling artefact, an MI lower than 0.2 may be used [8]. When driven at resonance, microbubble oscillation can be highly non-linear, resulting in the emission of sub-harmonics or harmonics of the incident frequency [11]. This non-linear response can be exploited to improve the delineation of the endocardial border in contrast-enhanced echocardiography through contrast-specific imaging modalities. Second-harmonic imaging [12] and sub-harmonic imaging [13] are two such modalities that rely on separating the contrast signal from that of tissue based on frequency filtering. In pulse inversion imaging [14], two pulses of opposite polarity are emitted and are subsequently compounded in the receive beamformer.