Asymmetric-backed multi-frequency ultrasonic transducer for conformal tumor ablation

Objective: Minimally invasive ultrasound ablation transducers have been widely studied. However, conventional designs are limited by the single working frequency, restricting their conformal ablation ability (i.e. ablation size and shape controllability). Methods: New multi-frequency ultrasonic transducer design method is proposed based on the asymmetric backing layer, which divides the transducer into non-backing-layer region (i.e. front-piezoelectric region) and backing-layer region (i.e. front-piezoelectric-backing region) with multiple local thickness mode resonant frequencies. Ablation zone can be controlled by exciting the local resonance within or between the regions, and its control flexibility is further enhanced by driven under a multi-frequency modulation signal. Experiments and calculations are combined for verifying the proposal. Results: The fabricated transducer with a Y-direction asymmetric backing layer shows five resonances, with two in each region and one resonance excited in both regions. Spatial ultrasound emission is demonstrated by acoustic measurements. Tissue ablation experiments verified spatial ablation zone control, and frequency modulation driving method enables the spatial transition of ablation zone from one region to the other, generating different ablation sizes and shapes. Finally, patient-specific simulations verified the effectiveness of conformal ablation. Conclusion: The proposed transducer enables flexible control of ablation zone. Significance: This study demonstrates a new method for conformal tumor ablation.