Evaluation of error bounds on calcaneal speed of sound caused by surrounding soft tissue

For absorptiometry measurements, soft tissue may have an impact on quantitative ultrasound (QUS) measurements. ill the present study, we focused primarily on the quantification of measurement error on speed of sound (SOS) caused by surrounding soft tissue. To meet our goal, SOS measurements were taken at the right heel using a QUS imaging device in 21 healthy subjects. Site-matched measurements of soft tissue thickness (STT) and bone width were performed using magnetic resonance imaging (MRI) of the heel. Several bone velocities were calculated either by accounting for bone width (SOSBW) only or by taking into account the exact path lengths of all major components traversed by ultrasound (V-b). Given that soft tissue composition is difficult to determine in vivo, we chose to estimate lower and upper error bounds on bone velocity (V-b (lower) and V-b (upper)) by spanning the full range of available values in the literature. Accounting for true BW only resulted in no significant difference between SOS (1533 +/- 37) and SOSBW (1531 +/- 33). By contrast, accounting for both true BW and surrounding soft tissue resulted in an increase in the calculated bone velocity and statistically significant differences between SOS and V-b (upper) (1568 +/- 36) and V-b (lower) (1542 +/- 34). Root mean square errors between SOS and the calculated velocities were 0.34, 2.32, and 0.70% for SOSBW, V-b (upper) and V-b (lower) respectively.