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UBC Theses and Dissertations

A hand-held probe for vibro-elastography Rivaz, Hassan


Vibro-elastography is a new medical imaging method that identifies the mechanical properties of tissue by measuring tissue motion in response to a multi-frequency external vibration source. Previous research on vibro-elastography used ultrasound to measure the tissue motion and system identification techniques to identify the tissue properties. This thesis describes a hand-held probe with a combined vibration source and ultrasound transducer which is operational in the 5-25 Hz range to cover a significant bandwidth of the tissue response. The design uses a vibration absorption system to counter-balance the reaction forces from contact with the tissue. Four different types of vibration absorption are briefly described and among them, active dynamic vibration absorption is selected for the hand-held device. A proportional integrator control is designed for the active vibration absorption and is compared with a well-established active vibration absorption method, the delayed resonator. An electromagnetic actuator is selected for active vibration absorption with a single accelerometer providing the feedback. The dynamics of the electromagnetic actuator as well as the response of the different niters are considered in the control law. The stability of the both absorber system and the combined system are considered to find the operational frequency range. The concept of tuning speed for different vibration absorbers is elaborated and is related to the vibration absorption speed. The design of the hand-held device, which includes the active vibration absorber, is presented next. The design utilizes another electromagnetic actuator which is used to vibrate the tissue and is operated with displacement feedback. The design of the vibration absorber is elaborated next. In this design the effect of different absorber parameters on the actuator stroke, control force, stability of the combined system and the tuning speed of the absorber is studied. After manufacturing the device, the absorber parameters are identified in order to optimize the vibration absorption performance. Simulation results are provided next that verify the theories presented on the stability and tuning speed. The results show that 100% vibration absorption can be achieved in steady state for both the proportional integrator and the delayed resonator controllers. They also show that nonlinearities in the absorber system decrease the amount of vibration absorption. Experimental results are presented next, showing approximately 85% vibration absorption in the operational range. The existing parameter identification methods that identify the mechanical properties of the tissue using the ultrasound are modified for the hand-held device. Simulation results are provided to validate the revised parameter identification methods. The first elastograms obtained with the hand-held device are finally presented.

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