The numerical simulation of subharmonic emission from microbubbles

Ultrasound contrast agents (UCAs) have been extensively researched in order to improve and develop new imaging techniques for clinical diagnosis in recent years. In ultrasound contrast imaging, many techniques have been proposed to improve the ratio of contrast-to-tissue (CTR). They are generally based on the nonlinear oscillation motion properties of UCAs. The second harmonic imaging (SHI) is one conventional technique and is commercially available. However, SHI is associated with disadvantages, due to the second harmonic emission and cumulating from microbubbles has to compete with the tissue. Because of the lack of subharmonic emission from tissue, one attractive alternative technique is the subharmonic imaging by transmitted at fundamental frequency (F₀) and received subharmonic frequency (¹/₂F₀). Moreover, the subharmonic can provide higher CTR and much better lateral resolution than the fundamental and SHI. In this paper, we present a numerical simulation to investigate the subharmonic emission from single microbubble contrast agent based on Church model in varied parameters. The results review different aspects of subharmonic imaging, i.e., the transmit frequencies, acoustic pressure and the shape of the excitation pulse, can be maximized for subharmonic generation.