I. Introduction
Ultrasound imaging is an inexpensive and widely used modality for the diagnosis of many diseases, it does not involve radiation, and it is noninvasive. The traditional focused transmission is regarded as a standard tool for clinical ultrasound scanning, although a long scanning time is required (leading to the low frame rate, usually less than 50 frames/s). As the requirements for medical ultrasounds expand, high frame rate ultrasounds and rapid imaging technologies have become hot research topics because the conventional focused modality cannot quite meet the temporal resolution demands of some ultrafast imaging applications such as the shear wave elasticity imaging, the heart or vascular motion imaging, and the 3-D/4-D real-time imaging [1]–[3].