Experimental investigation of the subharmonic emission from microbubbles using linear and nonlinear frequency modulated signals

In medical ultrasound imaging, coded excitation is widely used to increase signal to noise ratio (SNR) and penetration without increasing the peak pressure level by using longer pulse durations. The aim of this work is to utilize chirp coded excitation and investigate the effect on subharmonic emission from contrast agents for nondestructive subharmonic imaging. The subharmonic emission of the microbubbles is measured as a function of pressure and bandwidth for linear frequency modulated (LFM) and nonlinear frequency modulated (NLFM) signals. Results indicate that for both excitation schemes, narrow band signals produced higher subharmonic level. It is also observed that for wide-band signals, NLFM excitation generated the highest subharmonic level.