Effect of excitation envelope on volumetric subharmonic vibrations of single contrast agent microbubbles using an acoustical camera

Recently we have shown that a suitable design of the excitation envelope to generate a self-demodulation signal at the subharmonic (SH) frequency can enhance the SH emission of microbubble (MB) contrast agents at 10 MHz. In this study we investigated effects of the excitation envelope on volumetric SH response of individual MB excited in the frequency range of 5-8 MHz with an acoustical set up (acoustical camera) measuring relative volume variations of single MB. Using the acoustical camera, we retrieved the radial response of single BR14 MB (Bracco) to 2 μs pressure waves (25 kPa to 200 kPa peak to peak) with Gaussian and rectangular envelopes. Each single MB is excited by a sequence of 10 signals with Gaussian and rectangular envelopes and with frequencies of 5, 6, 7 and 8 MHz. The 5 MHz pair is exactly repeated at the end of the sequence to check the MB for possible deflation or disruption. The total length of the excitation waves sequence was 30 μs ensuring that the same single MB was excited with all excitation bursts. The vibration responses of the single MB to the 10 excitations are received through the nonlinear coupling with a 50 μs 30 MHz probing wave. Amplitudes of relative radial SH vibration of single MB excited with rectangular envelope bursts were in average 2 times larger than when excited with Gaussian envelope bursts. This study suggests that excitation with rectangular envelope enhances the volumetric radial SH response of a single MB compared with excitation with Gaussian envelope at low acoustic pressures (<;200 kPa peak to peak). Such an enhanced SH response can be then used for imaging the UCA.