Ultrawide Bandwidth High-Frequency Ultrasonic Transducers With Gradient Acoustic Impedance Matching Layer for Biomedical Imaging

The high-frequency ultrasonic transducers with larger bandwidths yield excellent imaging performance in the biomedical field. However, achieving perfect acoustic impedance matching from the piezo-element to the target medium in the operating frequency spectrum is still a challenge. Conventional matching layers are mostly fabricated by only one or two uniform materials which are limited by their acoustic property. We propose a novel composite matching layer with gradient acoustic impedance based on a 1–3 gradient composite structure and multilevel matching theory. The proposed gradient-composite matching layer applied for ultrasonic transducer provides efficient impedance matching and ultrawide bandwidth which can significantly improve the quality of biomedical imaging. The active aperture size of the matching layer is $5\times $ 5 mm², and the overall thickness for five equivalent layers is 115 $\mu \text{m}$ . The −6-dB bandwidth and the center frequency obtained by the ultrasonic transducer equipped with the 1–3 gradient composite matching layer are 141.7% and 22.3 MHz, respectively. The exceedingly good imaging performance of the fabricated ultrasonic transducer was demonstrated by the tungsten wire phantom and study on the biological tissues of a zebrafish and porcine eyeball. The theoretical and experimental results provide a novel train of thought for improving the quality of biomedical ultrasonic imaging.

I. Introduction

Ultrasound is widely used in biomedical and industrial manufacturing, such as cardiac ultrasound, endovascular endoscopy, abdominal ultrasound, and gynecological ultrasound due to its high sensitivity, noninvasiveness, low cost, and high resolution [1]–[7]. The high resolution and sensitivity of the ultrasonic transducer will result in high-quality imaging [8], [9]. Sensitivity is reduced due to the large attenuation of high-frequency sound waves, which affects the penetration distance. The lateral resolution is limited by the diameter of piezo-element and center frequency. However, the lateral resolution can be significantly improved by applying an external lens or mechanical ball pressure, whereas the axial resolution is directly determined by the performance (bandwidth and center frequency) of the ultrasonic transducers [10], [11]. A large bandwidth tends to form narrow pulses, which will greatly improve the axial resolution [12]–[14]. Therefore, excellent performance (high frequency, large bandwidth, and high sensitivity) is the goal that researchers in ultrasonic imaging have been pursuing. In particular, how to obtain wide bandwidth ultrasonic transducers by effective methods has always been a great challenge [15]–[17].