Crossed-Array Transducer for Real-Time Three-Dimensional Ultrasound Imaging
Joyce, Andrew Waters
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Volumetric ultrasound imaging can present numerous advantages to medical diagnostics, such as increased spatial context, better long-term patient tracking, and reduced imaging times. Creating three-dimensional ultrasound systems is complicated by the attendant increase in system complexity that is required to capture the additional data, and by pulse discrimination imposing repetition frequency limits from the fixed speed of sound in tissue. If the same principles utilized in two-dimensional cross-sectional imaging are applied directly to capture a volumetric data set, the electrical interface, system complexity, and acquisition times grow exponentially. An alternative volumetric imaging method that enables real-time three-dimensional ultrasound imaging using a crossed-array structure is presented in this thesis. The crossed-array transducer developed in this thesis makes use of careful material design and optimization to increase sensitivity and eliminate artifacts. A fine-pitch 1-3 piezocomposite has been optimized for performance in this application. This is combined with a novel method for creating intrinsic apodization within the piezoceramic material. Direct manipulation of the material sensitivity was found to be necessary to eliminate an artifact unique to the large defocused aperture being used: range secondary lobes. An acoustic stack has been designed for the transducer that optimizes sensitivity and reduces manufacturing complexity. Traces on a flexible circuit are used to define the array elements and the polyimide substrate is acoustically integrated as a matching layer. A defocusing rubber lens is used to keep the unfocused axis from experiencing near-field effects. Custom electronics are developed that incorporate a pulser and transmit beamforming system. The pulser uses a new circuit topology to create Ohmic grounding of the array elements when inactive. This is necessary in order to use the same substrate for both send and receive. A transmit beamformer and an advanced control interface have been developed that permit flexible testing using computer control, as well as stand-alone operation using focal data loaded through interchangeable cards. The system is characterized by its C-scan radiation patterns, showing confinement along the beamformed axis of 0.42mm at 75mm imaging depth. The defocused axis exhibits a smooth defocused profile with a 6dB angular spread of 45 degrees.