# An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles
## 1 Introduction
Amplitude modulation is another multi-pulse technique, which results in a similar cancellation of the linear echoes [15]. Contrast specific imaging modalities are exploited in perfusion imaging. A rapid succession of high pressure pulses can be transmitted to destroy the microbubbles in the region of interest. Destruction still occurs well within FDA guidelines that states for cardiac applications the MI should not exceed 1.9 [16]. By suppressing the tissue response, quantitative assessment of reperfusion is possible. It may be possible to further improve quantitative perfusion imaging by using a monodisperse population of microbubbles, with higher sensitivity and more control over the acoustic response [7]. To take full advantage of a monodisperse population for such applications, it is important to characterise the population acoustically and to tailor the insonification accordingly. Stable cavitation and inertial cavitation can be exploited for therapeutics, either through attachment and delivery of a therapeutic agent [17] or through sonoporation of cells and co-administration of a therapeutic agent [18]. A number of mechanisms may contribute to sonoporation, including stable cavitation and micro-streaming [19]. Whenstreaming occurs close to a cell membrane, shear stress created by the moving fluid causes pores to appear. During more violent inertial cavitation, jet formations can occur, which can cause large pores to appear in the cell membrane [19; 20]. The microbubbles experience their maximum radial response at resonance. For sonoporation, this translates into more forces exerted on the surface of the cell [21]. Sonoporation efficiency may be increased by using a monodisperse population of microbubbles created through filtering of a polydisperse population [7] or directly formed using microfluidic flow focusing techniques [22]. Alternatively, the efficiency may be increased for a polydispersed population through a broadband excitation such as a linear-frequency-modulated pulse [23]. For delivery of a therapeutic agent, the agent can be attached to the shell in liposomal form [24] or can be contained in a thin oil layer in the microbubble [21]. The therapeutic agent can be released through inertial cavitation or through controlled lipid shedding [25]. Once again, exciting the microbubbles at the resonant frequency increases the efficiency of the payload delivery [21]. The changes in the shell parameters of the liposome-loaded microbubbles have been shown to alter their acoustic characteristics through optical [26] and acoustic measurements [27]. The addition of an oil layer in the shell also leads to changes in the microbubbles' acoustic characteristics [28]. Because the efficiency of the drug delivery is dependent on the insonification parameters, it is important to characterise these modified microbubbles when trialing their use for therapeutics applications. Examples of acoustic characterisation experiments that have been performed previously are included in Table 1. Despite the clear benefit to both imaging and therapeutics applications, there is very little consistency in the experimental setup or protocol used to investigate the acoustic characteristics of microbubbles. Variations in the parameters of the transmitters and receivers, vessel geometry, and the interrogating acoustic wave can make comparison between different research groups challenging. For attenuation measurements, the transmitter and receiver are usually placed in line with each other. A hydrophone is often used as the receiver to obtain the response from the sample over a broad range of frequencies [27; 29]. An alternative approach uses the transmit transducer as the receiver and a strong reflector positioned behind the sample [4]. Scattering can be measured by the transmit transducer [30] or by another transducer, or hydrophone, placed in a position orthogonal to the transmit transducer [27; 29].