P3F-5 Ultrasonic Transmission Device Based On Crossed Beam Forming

Substantial technological progresses have been achieved recently for bone quantitative ultrasound imaging using two fully connected 2-D arrays (24 times 24 elements) of transducers. However, the electronics managing the scanning of the active focusing sub-aperture is complex and remains expensive. To overcome these disadvantages, we developed a new device, based on two 1-D transducer arrays and cylindrical crossed beam forming. The intersection of two focus lines (one in emission, one in reception, oriented at 90deg one compared to the other) replaces the spherical focused beams. The number of elementary transducers of each matrix is considerably reduced as well as the complexity of driving electronics and fabrication cost. The aims of this work were two-fold (a) to validate this concept for bone quantitative assessment and (b) to evaluate the performance of the new prototype. (a) Measurements were performed in a homogeneous calibration phantom and in 22 cancellous bone specimens (human femur) with two cylindrical focused mono-element transducers (center frequency: 0.5 MHz) in a crossed configuration. A close analysis of the radiofrequency signal and its Fourier transform revealed a distortion of the signal and a reduced usable frequency bandwidth of 0.15 to 0.4 MHz for the cylindrical focused transducers arrangement compared to the reference technique, i.e. spherical focusing (0.15-0.6 MHz). This result is a consequence of diffraction and of imperfect phase matching between the transmitted cylindrical wavefront and the cylindrical receiving aperture. Nevertheless, for the bone specimens, the values of the slope of the frequency-dependent attenuation coefficient (BUA) were highly correlated with those obtained with the reference technique (r2 = 0.69). (b) The crossed beam arrays consist of fully connected 72 elements with a length of 92 mm, a pitch of 1.25 mm, a center frequency 0.47 MHz, a relative -6dB frequency bandwidth of 60%. Two identical arrays are positioned confocally. The transducers are encased in thermo regulated water balloons made of compliant silicone membranes. The performance of the matrix transducers and of the electronics was tested with a calibration phantom using several beam forming solutions (electronic or synthetic focusing) and several aperture sizes and apodization functions. Results indicate that a careful selection of the frequency bandwidth and apodization function is required to minimize the diffraction effect and to obtain a BUA value in the phantom close to the reference technique