Tissue harmonic images obtained with parallel transmit beamforming by means of orthogonal frequency division multiplexing

In classic pulse echo ultrasound imaging, the data acquisition rate is limited by the speed of sound. To overcome this limitation, parallel beamforming techniques in transmit (PBT) and in receive mode (PBR) have been developed. To perform harmonic imaging, PBT techniques are preferable being capable of generating stronger harmonics thanks to the possibility to use focused beams in transmission. Recently, orthogonal frequency division multiplexing (OFDM) has been explored to perform PBT. To date, only numerical studies and in-vitro experiments in water have been performed, thus neglecting the effect of frequency dependent absorption. In this paper, PBT by means of OFDM tissue harmonic images are presented. A homemade agarose tissue mimicking phantom containing water filled cylindrical cavities was utilized as test object. The ULA-OP ultrasound open platform was used in combination with the linear array probe LA533 (Esaote Italy). Starting from the available transducer bandwidth, sub-bands were allocated to each beam transmitted in parallel. Three orthogonal sub-bands were used, improving the frame rate by a factor three. A classic B-mode tissue harmonic image was then obtained with the same setup for comparison. The contrast to noise ratio (CNR) and average background brightness inside the cavities were evaluated to provide an indirect measure of the influence of interbeam interference.