Quantitative ultrasound imaging of the calcaneus: Precision and variationsduring a 120-day bed rest

This study reports on the precision and variation of quantitative ultrasoun d (US) parameters [broadband ultrasonic attenuation (BUA) or slope of the f requency-dependent attenuation in dB/MHz and speed of sound (SOS m/second)] after 120 days of continuous bed rest in six normal male volunteers. Quant itative US was measured at the calcaneus using a new US bone imaging scanne r. The measurements were carried out on both heels at approximately 2-week intervals. The short-term precision was 0.31% for SOS and 2.8% for BUA. The long-term precision was 0.58% for SOS, 4.7% for BUA. A significant decreas e of SOS values of -26 m/second (P < 0.0001) for the right heel and -17 m/s econd (P < 0.05) for the left heel was found at the group level. In terms o f percentage change this represents -1.7% for the right heel and -1.1% for the left heel. These percentage decrements were 3.5-5.5 times that of the s hort-term precision and 2-3 times that of the longterm precision of the tec hnique. At the individual level, the decrease of SOS was statistically sign ificant (P < 0.05) or marginally significant (P < 0.1) for four out of 6 su bjects. For 2 other subjects, similar trends were observed, but without rea ching statistical significance. BUA did not change significantly during fol low-up. These results are consistent with previous findings on changes of u ltrasonic properties from the calcaneus during aging, pregnancy, or therapy , showing that calcaneus SOS is a valuable index of bone loss. These prelim inary data suggest that prolonged exposure to simulated weightlessness may lead to a lower SOS, which then could be used for the follow-up of bone dem ineralization occurring during long-term space flights.