2D arrays device for calcaneus bone transmission: an alternative technological solution using crossed beam forming

In the context of manned space flight with the European Space Agency, a quantitative ultrasound device for transmission imaging through the calcaneus bone has been developed. It includes two matrix transducers of 576 elements each in order to electronically perform the scanning and the focusing of the 500 kHz ultrasonic beam. This device called the BEAM scanner, provides two parametric images of attenuation (BUA, broadband ultrasonic attenuation) and velocity (SOS, speed of sound) of the investigated skeletal site. The cost and complexity of such a device has motivated the study of an alternative solution, less demanding in terms of technology, based on a crossed beam former [H. Ermert et al., A new concept for a real-time ultrasound transmission camera, in: IEEE Ultrasonics Symposium Proceedings, 2000, pp. 1611-1614]. It consists in forming two perpendicular cylindrically focused planes, one in emission, one in reception, instead of two spherically focused apertures. The crossing line of the two planes replaces the focused beam. The 2D beam forming technological challenge is moved to a 1D simpler and cheaper architecture. In this work the two solutions have been compared for in vivo measurements. Data sets have been acquired using all spatial combinations of emission and reception single elements of the matrix. Then signals have been processed using either the cylindrical or the spherical focussing mode. For cylindrical focussing, the increased level of the side lobes caused severe artefacts. Several apodization techniques have been implemented to reduce these artefacts, resulting in encouraging results. After a brief description of this new ultrasonic method for bone quantitative assessment, several reconstructed images using both processing schemes are presented. Corresponding statistical results obtained in 29 subjects are also provided.