Medical ultrasonics: dynamic focusing in diagnostic imaging

The design and implementation of an ultrasonic dynamic focusing annular array system is described. The array is designed for use as a contact transducer and for eventual incorporation into the head of a real-time mechanical scanner.A solution for the continuous wave field of a focused thin ring array is used to examine the effects of the number of rings, ring arrangement and apodisation of the array aperture on its focal plane response. Other aspects of array design are also considered. A suitable method of array fabrication, based on printed circuit board techniques, is described.The design and implementation of an electronic system for the generation, detection and processing of the array signals is given. Digital techniques are used to implement the dynamic delays.The dynamic focusing capabilities of the array are confirmed experimentally. Dynamic focusing of the reception response of the array is usually combined with a fixed focus on transmission. The compromise of fixed focusing on transmission may be overcome by applying the idea of Synthetic Aperture Imaging. An effective dynamic focus is then achieved on both transmission and reception. This novel application to annular arrays is investigated experimentally, and significant advantages are confirmed, however, at the expense of a lower pulse repetition frequency