DESIGN AND MANUFACTURE OF A HIGH-FREQUENCY ANNULAR ARRAY ULTRASOUND SYSTEM FOR MEDICAL IMAGING
Lay, Holly Susan
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This thesis presents the design of a high-frequency annular array ultrasound system suitable for medical imaging. To reduce the cost of the system, off-the-shelf parts were used whenever possible. The system consists of four main components; 1) a transmit beamformer, 2) a high voltage pulse generator, 3) an annular array transducer and 4) a receive beamformer. The transmit beamformer and pulser were designed for an 8-channel array but could be easily expanded for larger arrays. The pulser produces monocycle electrical pulses with centre frequencies that could be adjusted from 10-50 MHz and with amplitudes up to 90 Vpp. The annular array transducer has 12 equal area elements and a total active aperture of 6 mm. The transducer array produced pulses with a centre frequency of 20 MHz and 50% bandwidth. The resulting images had a lateral resolution of 172.5 μm at 10 mm and an axial resolution of 180 μm. A new fabrication method was developed that makes it easier to build the array. The receive beamformer was based on a commercial 8-channel analog-to-digital converter. The digital signals were transferred to a laptop where the beamforming was performed in software. This avoided the need to develop custom hardware and allowed it to be reconfigured for different transducers by simply modifying the software. The beamformer used a new interpolation method that reduced the required sampling frequency while maintaining a satisfactory radiation pattern. The system produces images at 10 frames/sec.