Ultrasonic velocity measurements through the calcaneus: Which velocity should be measured?

Abstract

The assessment of skeletal integrity by the measurement of ultrasonic velocity through the calcaneus has only recently become widely available and is usually made in conjunction with the measurement of broadband ultrasonic attenuation. Using data obtained with a contact ultrasonic bone analyser (CUBA) system, this report examines whether ultrasonic studies of the heel require the measurement of true velocity of sound in the calcaneus (V_bone), or whether heel velocity (V_heel, defined as the mean velocity through bone and soft tissue) or time of flight velocity (V_tof, defined as the mean velocity between the two transducers) are adequate surrogates. The populations selected for study were 15 healthy young women (group 1, mean age 26 years), 231 healthy peri- and postmenopausal women (group 2, mean age 52 years) and 33 osteoporotic women with confirmed vertebral fracture (group 3, mean age 66 years). Precision was studied by performing 10 repeated scans on the subjects in group 1 and duplicate scans on 144 women randomly selected in groups 2 and 3. Precision was expressed as the percentage coefficient of variation (CV). Both precision studies yielded similar results. The precisions (and 5% to 95% ranges) for all groups combined were: V_bone 2.71% (1465–1809 m/s); V_heel, 1.10% (1511–1646 m/s); V_tof, 0.70% (1349–1425 m/s). Although the precision data suggest V_tof should be preferred, when the range of clinical values is taken into account the smaller CV is exactly cancelled by the narrower range. To demonstrate this numerically it is proposed to introduce a new measure of precision, the standardized coefficient of variation (SCV), defined as the percentage CV divided by the percentage ratio of the range over the mean. SCV had a value of 12.7% for V_tof and 12.8% for V_bone and V_heel. For comparison a SCV of 2.2% was estimated for dual X-ray absorptiometry (DXA). Thus the measurement of ultrasonic velocity through the calcaneus compares equally in terms of CV, but not in terms of SCV. A scatter plot of V_bone against V_tof gave a significantly poorer correlation (r=0.828) than that of V_bone against V_heel (r=0.984). This latter correlation can be explained by theoretical studies demonstrating convergence of the two measurements when the velocity through the bone equals that through soft tissue (1540 m/s). Since the measurement of V_heel dispenses with the need to determine soft tissue thickness over the calcaneus, it is the optimum velocity parameter for the CUBA system.