Characterization of the Ideal Speed of Sound for Plantar Soft Tissue Using Quasi-Simultaneous Ultrasound and Computed Tomography in Cadaveric Feet.

Clinical ultrasound systems typically assume a speed of sound of 1540 m/s for reconstructing images. However, the speed of sound varies between tissues, and this assumption creates distortions and errors in representing the thickness of tissue structures. Quasi-simultaneous (i.e., taken subsequently without movement of the specimen) computed tomography (CT) and ultrasound scans at various speeds of sound (1480, 1540, 1600, and 1660 m/s) were taken at the calcaneus and the second metatarsal head for seven cadaveric feet. A speed of sound of 1600 m/s demonstrated the lowest bias with a mean signed error of - 0.06 mm for the calcaneus and - 0.19 mm for the second metatarsal head, while for 1540 m/s, the calcaneus had a bias of - 0.44 mm, and the second metatarsal head had a bias of - 0.67 mm. The CT-derived distance was used to estimate the ideal speed of sound by calculating the slope of the change in distance with the change in speed of sound and was found to be 1616.6 ± 76.5 m/s for the calcaneus and 1623.9 ± 63.6 m/s for the second metatarsal head. However, limitations in the use of cadavers, CT as the reference standard, and methodological assumptions in analysis limit the extent to which these findings can be generalized to in vivo conditions. This work suggests that using an assumed speed of sound greater than 1540 m/s may be more appropriate for imaging plantar soft tissue; however, additional in vivo studies are needed to corroborate this finding.