Effect of temperature on ultrasonic properties of the calcaneus in situ.

To assess the dependence of calcaneal quantitative ultrasound (QUS) on foot temperature, a series of acoustic measurements were made in five cadaver feet in situ (all soft tissues retained) over a temperature range of 25 degrees C to 40 degrees C in steps of 5 degrees C. An implanted probe was used to measured temperature directly in the calcaneus itself. Ultrasound velocity decreased linearly with increasing temperature, with a mean thermal coefficient of -2.2 m/s/ degrees C. In contrast, broadband ultrasonic attenuation (BUA) increased with temperature with a mean thermal coefficient of +0.75 dB/MHz/ degrees C. We argue that the temperature trends in velocity are likely to be due to the influence of fat, present in the bone marrow and in the soft tissues, which has a negative thermal coefficient for acoustic velocity. The attenuation trends may arise, in part, from greater scattering losses inside the cancellous bone due to an increased acoustic impedance mismatch between trabeculae and fatty marrow at higher temperatures. These considerations suggest that the greatest temperature effects may be expected in patients with a high proportion of fat within the measured volume and/or low calcaneal bone density. Given the magnitude of the thermal coefficients observed, the clinical impact of temperature-related QUS errors is likely to be modest for diagnostic purposes but of greater significance in follow-up studies.