Assessing the performance of 360∘ ultrasound probes designed for endoanal ultrasound

A bespoke phantom has been designed, with clinically relevant features for endoanal ultrasound (EAUS), capable of rigorously assessing the performance of 360${}^{\circ }$ ultrasound probes.

The performance of three, commercially available, anorectal probes, capable of producing both 2D and 3D images, was assessed. One of the probes was also assessed in two states: before failure and after a repair to correct a failure. For each probe the signal to noise ratio (SNR), contrast to noise ratio (CNR), penetration depth, resolution, focus depth, distance accuracy and low contrast object detectability (LCOD) were assessed at varying dynamic ranges, receive gains and operating frequencies. A Python program (SAUQA) was developed to semi-automate the analysis.

In general the measured parameters varied as expected. However, at intermediate receive gains, adjusting the receive gain resulted in the SNR, CNR, penetration depth and LCOD varying in an unexpected manner. The reason for this is not known, but because it was exhibited by all probes it is believed to be related to the ultrasound machine itself and/or an inherent characteristic of the probe design.

The quantitative results suggest that all probes tested offer an effective method of assessing the integrity of the Internal Anal Sphincter (IAS) and the repair of the probe appears to have been successful. However, differences between the probes were observed both quantitatively and qualitatively, with the original probe providing the best results for EAUS.

In light of the results, a recommendation was made, to the EAUS service at University Hospitals Dorset, to adjust the default machine start-up settings for EAUS.