Precision control and seasonal variations in quantitative ultrasound measurement of the calcaneus

To assess the dependence of calcaneal quantitative ultrasound (QUS) indices (speed of sound [SOS] and broadband ultrasound attenuation [BUA]) on foot temperature and season, we measured these indices monthly, from August to February of the next year, in five healthy Japanese adults (three men and two women, aged 31-52 years), using a dry-type QUS device (Sahara; Hologic). The monthly QUS measurement was performed three consecutive times after the start, at 30, 60, and 120 min from the start, in a test room where the temperature was kept at 25 +/- 2degreesC. Skin temperature was monitored at the dorsum pedis throughout the measurements. A significant seasonal trend in the monthly mean Z-scores of QUS indices was observed (P < 0.001). QUS indices were higher during the winter than summer, while skin temperatures were lower during the winter. SOS showed a significant inverse association with skin temperature (r = -0.64), while BUA did not (r = 0.09). Skin temperature at the dorsum pedis slowly declined and SOS increased during the experiment. BUA decreased by 60 min from the start of the experiment and then increased slightly. The coefficient of variation (CV) of both QUS indices showed the smallest values at 60 min from the start. In conclusion, there are seasonal variations in the QUS indices, and SOS particularly depends on skin temperature. When a subject is followed up for years, the QUS measurements should be conducted in the same season. In a setting of community-based medical checkups using QUS devices, health professionals should measure the QUS indices of a subject after 60 min of rest in the examination room.