Roberts, Morgan; (2023) Development and feasibility assessment of open-UST: A low-cost, open-source, manufacturing framework for an ultrasound tomography research system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Widely available and re-configurable UST hardware is needed to enable the ongoing development of advanced UST imaging techniques. Having access to a physical UST transducer array in-house accelerates algorithm development, compared to relying solely on open databases of experimental data from other groups. Widely available UST hardware also allows new algorithms to be tested on real data from physical transducers, which is a more robust evaluation than using synthetic data. In this work, an open-source 256-element transducer ring array was developed (morganjroberts.github.io/open-UST) and manufactured using rapid prototyping, with a material cost of ∼£2k. This is the first example of a low cost opensource array design with a specification suitable for UST imaging, and was made possible through novel fabrication methods, resulting in 0.46° and 1.17° elevational and lateral beam axis skew angles, a 104 μm standard deviation in on-axis element position, and a ±18.9 μm deviation in matching layer thickness. The nominal acoustic performance was measured using hydrophone scans and UST watershot data, and the 61.2 dB SNR, 55.4° opening angle, 16.3 mm beamwidth and 528 kHz cutoff frequency (-40 dB) were found to be similar to existing systems, and compatible with full waveform inversion reconstruction methods. The inter-element variation in acoustic performance was typically <10% without using normalisation, suggesting that the elements can be modelled identically during image reconstruction, removing the need for individual source definitions based on hydrophone measurements. The fact that the open-UST manufacturing framework is accessible and low cost, but also has low IEV in acoustic performance, shows that the manufacturing techniques developed are resilient to the large tolerances associated with rapid prototyping, which is a key contribution of the work in this thesis. Finally, UST data was collected for an oil-saline breast mimicking phantom, and its sound speed distribution was estimated using the simultaneous algebraic reconstruction technique. This reconstruction code has been made available online as a toolbox (github.com/ucl-bug/ust-sart). Overall, these results demonstrate that the open-UST system is accessible for users, and suitable for UST imaging research. This thesis contributes to a growing body of open-source work demonstrating that rapid prototyping technologies can lower the barrier to entry for researchers manufacturing ultrasound transducers in house.

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