Ultrasound parametric imaging of the calcaneus: in vivo results with a new device

An ultrasound transmission scanning system was constructed to make in vivo parametric images of the acoustic properties of the heel. Broadband ultrasonic attenuation (BUA) images were obtained in transmit mode by using a pair of broadband focused transducers (center frequency 0.5 MHz, diameter 29 mm, focus 50 mm) immersed in a water bath at room temperature. With these characteristics, the theoretical beam width at the focus was approximately 5 mm. The total duration of the acquisition period was 3 minutes. Comparison of the BUA image and the radiograph of the calcaneus was possible in one case and showed that all the anatomical details could be identified. The images reported here demonstrate the wide range of BUA found in both the whole bone and within a ROI centered in the posterior part of the bone thus reinforcing the idea of tremendous heterogeneity of the acoustic properties of bone. This suggests that the accurate control of the position of the measurement site is of the utmost importance for between-subject comparison and for repetitive measurements. We proposed a new method, the likelihood image, as an efficient way of highlighting the regions of the image suspected to be subject to waveform distortion. It could be used to guide the selection of the optimal measurement site. Our results suggest that ultrasound parametric imaging has the potential for enhancing the current ultrasound technique by (1) allowing reproducible, repetitive measurements, (2) permitting the selection of similar optimal measurement sites in all subjects, and (3) avoiding accuracy errors due to waveform distortion.