Most low-frequency (<5 MHz) ultrasound imaging systems are based on multi-element transducer arrays. Transducer arrays have the ability to optimally focus the ultrasound energy at all depths. The difficulty in fabricating miniature transducer arrays for high-frequency (>30 MHz) medical ultrasound imaging is currently limiting the development of this field. Most high-frequency imaging systems are based on single-element geometrically shaped transducers. Although these transducers are relatively easy to fabricate, their performance is restricted by the inherent trade-off between the image resolution and depth of field. This compromise can be avoided by replacing the single-element transducer with an annular array.

An annular array intended for high-frequency medical ultrasound imaging has been successfully developed. The array has a 2 mm aperture, 7 equal-area elements, and operates at a centre frequency of 50 MHz. The fabrication process is straightforward and reproducible, and the resulting performance agreed with the theoretical model.

In order to synthetically focus a transducer array, an accompanying electronic device called a beamformer is required. This thesis also describes the development of a digital beamformer intended for a 50 MHz annular array. The performance of the combined system (array + beamformer) was tested by imaging wire phantoms and in-vivo laboratory mice. The axial resolution was 50 μm, and the lateral resolution varied from 80 μm to 130 μm over a 5.12 mm scan depth. The images showed superior depth of field as compared to a commercially available single-element system. This is the first successfully developed high-frequency ultrasound system that is based on a multi-element transducer.