# An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles
## 1 Introduction
In the case where the transmit transducer is used to measure backscatter, the limitations of the transducer bandwidth may prevent the measurement of sub-harmonics or second harmonics. Measurements are taken with deionised water to obtain a reference. Measurements are then taken with a known dilution of microbubbles. The experiment is usually performed with only one transmitter and one receiver, and the different acoustic measurements are normally taken separately [27; 29]. Generally, a sample vessel with acoustically transparent windows is used to constrain the solution of microbubbles in the far-field of the transmitter and receiver. The vessel dimensions used vary considerably. In [29], the volume of the vessel was 500 mL, and in [27], the vessel had a total volume of 100 mL. The dimensions of the vessel used is a trade-off between limiting the size of the microbubble sample required and minimising the interactions between the chamber walls and the ultrasound beam. The sample volume is made larger than the receiver beam-width to ensure as many microbubbles are measured in a single insonification as possible. In procedures using a needle hydrophone, the beam-width is small and the sensitivity is low compared to a transducer. Only a small subset of the microbubbles in the sample vessel are in the path of the receiver and a large percentage of the sample goes unmeasured. A magnetic stirrer is used to keep the microbubbles uniformly distributed in the ultrasound beam during the experiment. The sample vessel must be refilled with a fresh sample regularly for new measurements and measurement repetitions. This can lead to the requirement of a large quantity of microbubbles to perform the complete acoustic characterisation. The transmitter is typically excited with a short duration pulse [29] to excite the broadband response of the transducer or a linear frequency-modulated pulse [27] to obtain a broadband frequency response. By using a broadband transmission, the response of the microbubbles over a range of frequencies can be obtained in a short time. In other works, narrowband excitation is used [7; 31]. Although some of this variation can be accounted for due to the different objectives of the research, it remains to be determined which approach is the most robust in characterising a microbubble population for direct comparison. \begin{table}
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**Measurement** & **Vessel** & **Transmitter** & **Transmission** & **Receiver** & **Microbubble** & **Ref. ** \\ \hline Attenuation & Sample vessel with acoustic path length of 60 mm and a 30 \(\mu\)m-thick material at 15\({}^{\circ}\) to the acoustic axis of transducer. & & 125 ns pulse (1 and 2.25 MHz transducers) or 35 ns pulse (5 MHz transducers, 1.2, 2.5, 5, and 10 MHz transducers, (Panametrics-NDT, Waltham, MA, USA). & Transmitter used as 35 ns pulse (5 MHz transducers, 1.25 ns pulse (1 and 2.25 MHz transducers) or 35 ns pulse (5 MHz transducers). & \\ \hline Attenuation & 3 different sample vessels with acoustic path lengths of 4, 6, and 8 cm; 6 mm acoustically transparent windows. & & Narrowband 4-cycle & 3.5 MHz transducer. (V380 Panametrics-NDT, Waltham, MA, USA). Focused. & Albune\({}^{\circledown}\) (Molecular Biosystems, Inc., San Diego, CA, USA) \\ \hline \multirow{4}{*}{Scattering} & & & & \\  & & & & \\  & & & & \\  & & & & \\  & & & & & \\ \hline \multirow{4}{*}{Attention} & & & Narrowband 4-cycle & 3.5 MHz transducer. (V380 Panametrics-NDT, Waltham, MA, USA). Focused. & 3.5 MHz transducer. (V380 Panametrics-NDT, Waltham, MA, USA). Focused. & \\  & 3 different samples with acoustic path lengths of 4, 6, and 8 cm; 6 mm acoustically transparent windows. & & & & \\  & & & & \\  & & & & & \\  & & & & & \\ \hline \multirow{4}{*}{Scattering} & & & & 2-cycle Gaussian pulse.