  Fast imaging methods are needed to promote widespread clinical adoption of
Ultrasound Tomography (UST), and more widely available UST hardware could
support the experimental validation of new measurement configurations. In this
work, an open-source 256-element transducer ring array was developed
(morganjroberts.github.io/open-UST) and manufactured using rapid prototyping,
for only {\pounds}2k. Novel manufacturing techniques were used, resulting in a
1.17$^{\circ}$ mean beam axis skew angle, a 104 $\mu$m mean element position
error, and a $\pm$13.6 $\mu$m deviation in matching layer thickness. The
nominal acoustic performance was measured using hydrophone scans and watershot
data, and the 61.2 dB SNR, 55.4$^{\circ}$ opening angle, 16.3 mm beamwidth and
54% transmit-receive bandwidth (-12 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, meaning that the elements can be modelled identically
during image reconstruction, removing the need for individual source
definitions based on hydrophone measurements. Finally, data from a phantom
experiment was successfully reconstructed. These results demonstrate that the
open-UST system is accessible for users, and suitable for UST imaging research.
