Measurement of fluctuations in ultrasonic waves propagating in inhomogeneous media.

The past thirty years or so have seen rapid developments in the science of ultrasonics and its practical applications as an imaging and measurement modality. One aspect of ultrasound that has recently received increasing attention is the limit of validity of the widely applied theoretical simplification of plane wave propagation, particularly with regard to inhomogeneous media. The work presented in this thesis is concerned with the quantitative experimental investigation of wavefront fluctuations in tissue and other non-uniform materials. A sensitive experimental wavefront detector system was designed and used subsequently to assess the acoustic field distributions radiated by plane disc ultrasonic transducers. This data enabled the system to be further refined and direct measurements to be made of wavefronts emerging from both artificial material (containing scatterers of known size and distribution) and fresh animal tissue (beef liver). The preliminary results on animal tissue suggest that while, under certain circumstances, ultrasonic waves may indeed propagate through tissue as near perfect plane waves, in some tissue regions quite large fluctuations in amplitude and phase are induced by internal scatterers. The physical origin of the scatterers inducing these fluctuations is not obvious and this is identified as a topic for further detailed investigation