# An Open Access Chamber Designed for the Acoustic Characterisation of Microbubbles
## 4 Discussion  
A statistically significant difference was found for all the reported acoustic measurements between the measured control sample, containing only deionised water, and the measured sample for both dilutions of contrast agent. Furthermore, the scattering characteristics measured are in agreement with previous studies [31] for the ultrasound contrast agent SonoVue(r). By performing the measurements in both directions across the chamber, a great degree of confidence can be taken in the measured characteristics. For all the measurements taken, the lower dilution sample resulted in a lower scattering value. This validates the efficacy of the MACC for performing scattering measurements at both the fundamental frequency and the second harmonic. The motivation behind the MACC is to provide a standardised piece of equipment for characterising microbubbles acoustically. In this work, narrowband waveforms were used to interrogate the sample at fixed frequency intervals. An alternative approach might have been to use broadband waveforms to obtain the response over a range of frequencies. The transmission parameters and digital signal processing steps required to obtain a robust acoustic characterisation methodology that can be used for differing research objectives and across research groups are yet to be determined. In this work, it was necessary to pre-distort the transmitted waveforms to obtain uniform pressures across a range of frequencies and transducers. It was also necessary to account for the sensitivity of the different transducers at different frequencies in receive. Without these steps, it would not have been possible to fuse the data from different transducers. In providing a unified piece of equipment, it is possible for researchers in different locations to reproduce results. Researchers are able to share the parameters of the interrogating acoustic waves and flow rates used with the chamber alongside results. It is envisaged that, with a more standardised apparatus, a consistent protocol can be developed and direct comparison across research groups will become possible. Due to the vast range of research objectives that may be encountered when characterising microbubbles, it is difficult to create a single piece of equipment to cover all possible scenarios. The MACC has been designed to allow a flexible configuration. The chamber, in  
Figure 8: Measured scattering at the second harmonic over the frequency range of 3.2–6.4 MHz for (**a**) the 1:1000 and (**b**) the 1:2500 dilutions of SonoVue®. The error bars reflect the standard error over the 12 experiment repetitions. its current form, allows the incorporation of absorbers and reflectors. These can be used at the expense of removing one or more of the transducers. Whilst the transducers chosen in this study offer the ability to investigate a broad range of frequencies around those typically required for microbubble characterisation, they are interchangeable, and transducers covering different frequency ranges may be used. For instance, if research objectives require the measurement of a lower fundamental frequency or sub-harmonic measurements, a pair of transducers with a central frequency of 1 MHz could be incorporated. Other research objectives may require attenuation measurements. Typically, attenuation measurements are made through a microbubble screen that is wider than the beam width. At the expense of performing the measurements in both directions across the chamber, the unfocused transducers on one side of the chamber could be replaced with focused transducers. By ensuring these focused transducers have a fixed focal depth coinciding with the sample stream, they could be used for the sole purpose of measuring attenuation. The MACC has been released to allow other researchers the ability and freedom to modify and extend the design. Future modifications might include the incorporation of optics to provide optical characterisation capabilities.