Enhanced tissue harmonic ultrasound with stack-layer dual-frequency transducer

OBJECTIVE: Tissue harmonic imaging (THI) has been widely used to improve ultrasound imaging quality, and further enhancing its performance remains a research focus.
Approach: In this work, we propose a dual-frequency mixed harmonic imaging (DF-MHI)method based on a stack-layer dual-frequency ultrasound transducer (SL-DFUT) to further enhance the harmonic imaging signal-to-noise ratio (SNR). This method utilizes the overlapping sound fields of the SL-DFUT to generate difference and sum frequency harmonics at lower voltages. The method not only improves the imaging quality of harmonic imaging but also reduces the ultrasound system's voltage requirements. To validate the effectiveness of the DF-MHI, we developed a DF-MHI imaging platform capable of generating both DF-MHI and THI images. 
Main results: The phantom imaging results show that, under low high-frequency acoustic pressure, THI fails to generate effective harmonic signals in highly reflective regions, whereas DF-MHI successfully generates these harmonic signals. When the high-frequency acoustic pressure is five times that of the low-frequency pressure, the visual difference between THI and DF-MHI became less pronounced than at lower pressure ratios, but DF-MHI still maintained clear quantitative advantages. Specifically, the contrast ratio
(CR) increases by 46.53%, the contrast-to-noise ratio (CNR) improves by 47.26%, and thegeneralized contrast-to-noise ratio (gCNR) rises by 34.87%.
Significance: The DF-MHI provides higher image quality than conventional harmonic imaging under the same excitation voltage.