Diffraction and interface losses in broadband ultrasound attenuation measurements of the calcaneum

Ultrasonic investigation of bone disease most frequently involves measurements on the heel, and the parameter most often used for this is the broadband ultrasound attenuation (BUA) which is the slope of the attenuation as a function of frequency between 0.2 and 0.6 MHz. In this study, the possible losses in the BUA measurement due to diffraction and interfaces have been discussed and evaluated using existing data and a standard diffraction model. The loss due to diffraction was found to depend critically upon whether a contact or immersion technique is used. For a contact method, we estimate that the diffraction loss can be greater than 10 dB MHz-1, whereas insertion devices typically will have losses of up to 0.6 dB MHz-1, and it not always clear whether or how manufacturers have attempted to correct for this. Dispersion which is found in the os calcis has only a small effect on the diffraction loss, but it can cause a frequency dependent interface loss of about 0.2 dB MHz-1. It may be impossible to correct for these discrepancies in vivo as the necessary data for the individual components of the heel are not known. However the losses should be borne in mind as factors limiting the accuracy of the measurements and, in the case of in vitro investigations, may merit further study.