Multi-frequency Ultrasound Transducers and Their Applications in Tissue Ablation and Intravascular Acoustic Angiography

Ultrasonic wave has been broadly applied to biomedical therapy and diagnosis as a means to deliver energy or information or both. Broadband ultrasound transducers have been increasingly important in advanced imaging and therapy. However, transducers made of high efficiency materials such as Lead zirconium titanate (PZT) or relaxor-based lead titanate (relaxor-PT) single crystals are limited in bandwidth (-6 dB fractional bandwidth less than 100%) because of the material properties and existing transducer designs. In this dissertation research, multi-layer, multi-frequency transducers were designed with the assistance of microwave theory to analyze wave propagation in the transducers, and these transducers were then fabricated and tested for both therapeutic and diagnostic applications.;Firstly, a dual frequency high intensity focused ultrasound (HIFU) transducer was designed with two identical active layers (PZT-2) bonded together, which broke the symmetry of the vibration boundary conditions and generated 1.5 MHz and 3 MHz ultrasonic waves with almost the same amplitudes. Acoustic power outputs of the single or dual frequency waves were calibrated with an acoustic power meter. The tissue ablation with 6 W acoustic power output from this transducer illustrated consistently higher heat generation efficiency with the dual frequency wave than conventional single frequency ones. The high efficiency of dual frequency HIFU tissue ablation indicated the possibility of reducing the energy deposition and ablation time for the same heat generation as in the conventional tissue ablations, which is expected to promote the safety and reduce the cost of the HIFU treatment.;Secondly, dual frequency intravascular ultrasound (IVUS) transducers were investigated with a low frequency (6.5 or 5 MHz) transmitter to excite nonlinear vibration of ultrasound contrast agent (microbubbles) and a high frequency (30 MHz) receiver to detect the super-harmonic responses from microbubbles. An anti-matching layer was sandwiched between the two active elements to suppress the aliasing echo in high frequency receiving waves and to enhance the low frequency wave propagation. Microwave theory were introduced to demonstrate the wave propagation in the multi-layer transducers. The transmitter (6.5 MHz or 5 MHz) was evaluated by a hydrophone and at least 1 MPa pressure was generated. The receiver was characterized by pulse echo experiment, and it showed the loop sensitivity of -27 dB and the -20 dB pulse length of 116 ns. Aliasing echo was suppressed to -23.7 dB compared to the target reflection signal. Imaging result elucidated the detection of a cellulose micro-tube (mimicking vasa vasorum) with diameter of 200 mum. The measured length of the pulses received from microbubbles was equivalent to about 70 mum, indicating its capability of high resolution intravascular acoustic angiography, which is promising for assessment of plaque vulnerability and diagnosis of atherosclerotic cardiovascular disease.;In conclusion, multi-frequency ultrasound exhibited unique performances which can be significant in biomedical applications for both therapy and diagnosis. The wave propagation control methods in the multi-layer transducers were successfully demonstrated for development of advanced multi-frequency ultrasound transducers. Specific dual frequency HIFU transducers were studied to achieve high efficiency heat generation for enhanced tissue ablations, and the dual frequency IVUS transducers were demonstrated for vasa vasorum imaging and acoustic angiography for vulnerable plaque identifications.