Axial Super-Resolution Ultrasound Imaging With Quasi-Monopolar Pulses From a Dual-Frequency Transducer

In biomedical ultrasound imaging, the axial resolution is determined by the pulse length, which reaches the bottleneck for the existing piezoelectric materials. The resonance of piezoelectric materials in conventional ultrasound transducers causes relatively long pulses with multiple wavelengths, which cannot be effectively shortened for decades. The waveform with practically the shortest pulse length is a monopolar half-cycle sinusoidal wave, which is beyond the capability of traditional piezoelectric transducers. Here, we propose a method for generating quasi-monopolar ultrasound pulses by the superposition of two ultrasound signals from a stack-layer dual-frequency ultrasound transducer. In this article, we designed and fabricated a 1+ 3 MHz stack-layer dual-frequency ultrasound transducer, and validated the generation of the quasi-monopolar pulse by specific excitation of each piezoelectric layer. Imaging with the quasi-monopolar pulses on copper wire phantoms demonstrated a pulse length of
$0.17\lambda $
(85.5 ns referring to 2 MHz), which generates images with superb axial resolution and clearly differentiates the phantom wires separated with 1 mm. This design provides a practical approach for general-purpose super-resolution ultrasound imaging without a contrast agent.