I. Introduction
Ultrasound imaging is a well-established medical diagnostic tool for determining anatomical information. Recently, ultrasound technology has become progressively more widespread. Compared with general medical imaging methods, including magnetic resonance imaging (MRI), X-ray, and computed tomography, ultrasonic imaging demonstrates high efficiency, reduced radiation exposure, and noninvasive characteristics [1]–[3]. Unlike standard clinical ultrasound with a central frequency of 2–10 MHz, high-frequency ultrasound imaging (with frequency higher than 15 MHz) can achieve more acceptable image resolution with tens of micrometers due to the reduced imaging wavelength and pulse duration. Application areas of high-frequency ultrasound include cardiology, obstetrics and gynecology (OB-GYN), ophthalmology, intravascular ultrasound (IVUS) imaging, and small animal imaging [4], [5].