Transducers and signal processing techniques for simultaneous ultrasonic imaging and therapy.

摘要

Recently, high intensity focused ultrasound (HIFU) was successfully used for noninvasive treatment of the benign or malignant tissues. In ultrasound image-guided HIFU (US-gHIFU) using an integrated HIFU/imaging transducer, real-time simultaneous therapy and imaging is more desirable because it allows for tracking tissue movement and monitoring feedback induced by a treated target. However, reflected HIFU signals corrupt the quality of signals received by an imaging transducer during treatment.;In the first part of this thesis, it was demonstrated that these interference signals can be significantly reduced in the formed brightness mode (B-mode) ultrasound image by implementing coded excitation with fixed notch filtering. In the second part, short pulse excitation with adaptive noise canceling technique was proposed to optimally suppress therapeutic interference with variable amplitudes while maintaining the original image form as closely as possible. To demonstrate the performance of these techniques, a design of the integrated HIFU/imaging phased array transducer was proposed for treatment of malignant prostate tissues. The center row forms imaging signals and the two identical outer rows work together to produce HIFU signals. As a preliminary experiment, the prototype integrated HIFU/imaging transducer composed of three single elements was built and the performance of the proposed techniques was verified with a soft biological tissue specimen.;The third part investigates formation of a large tissue lesion per HIFU sonication to reduce the overall treatment time. The goal of this study is to show the feasibility of enlarging tissue lesion size with a dual-focus therapeutic ultrasound transducer by increasing the depth-of-focus (DOF). The proposed transducer for treatment of the malignant prostate tissues consists of a disc- and an annular-type element of different radii of curvatures to produce two focal zones. To compensate attenuation and to maintain uniform beamwidth of the elongated DOF, each element transmits ultrasound of a different center frequency: the inner element at a higher frequency for near field focusing and the outer element at a lower frequency for far field focusing. By activating two elements at the same time with a single transmitter capable of generating a dual-frequency mixed signal, the overall ablated region of the proposed transducer can be considerably extended. By employing this design to integrated HIFU/imaging transducer, not only real-time simultaneous therapy and imaging, but also reduced HIFU treatment time can be achieved at the same time.