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An approximate solution for the streaming velocity generated by flat and weakly focused transducers was derived by directly solving the Dirichlet boundary conditions for the Poisson equation. The theoretical calculations were verified using a purpose-designed 32 MHz pulsed Doppler unit. The applied average acoustic power was changed from 1 /spl mu/W to 6 mW. The experiments were done on 4 mm diameter flat and focused transducers. The streaming velocity was measured along the ultrasonic beam from O to 20 mm. Streaming was induced in a solution of water and corn starch. The experimental results showed that for a given acoustic power the streaming velocity was independent of the starch density in water changed from 0.3 grams to 40 grams of starch in 1 litre of distilled water. For applied acoustic powers, the streaming velocity changed linearly from 0.2 to 40 mm/s. Theoretical solutions for both plane and focused waves agreed with experimental results.

The 1-3 composite transducers are superior to their piezoceramic counterparts in terms of a good acoustic matching to the biological tissue and higher electromechanical coupling coefficient with reduced impedance with no loss of sensitivity. Numerical calculations based on finite element formulation are given. Due to the large number of degrees of freedom the homogenisation process of the fundamental composite unit cell was introduced.

Harmonic imaging at frequencies up to 15 MHz is now routinely used in clinical practice and frequencies well beyond 20 MHz are considered for diagnostic ultrasound imaging applications. However, currently available measurement tools are not fully adequate to characterize such high frequency systems, primarily due to the combined effects of limited frequency responses and spatial averaging effects. To alleviate this problems, a comprehensive wave propagation model has been developed and tested. The model can predict the linear and nonlinear acoustic wave propagation generated by differently shaped acoustic radiators at virtually any point in the field and takes into account spatial averaging effects introduced by hydrophone probes and their associated frequency responses. The applicability of the model in hydrophone probe calibration up to 100 MHz is demonstrated. Also, a novel calibration technique termed Time-Gating Frequency Analysis (TGFA) is briefly described and calibration results in the frequency range up to 60 MHz for hydrophones having effective diameters between 150 and 500 /spl mu/m are presented. Also presented are the results of the investigation that determined the effect of using hydrophone probes of different diameters and bandwidth on Spatial-Peak Pulse-Average Intensity (I/sub SPPA/). It was found that the values of I/sub SPPA/ increased with decreasing effective aperture of the hydrophone probe and its bandwidth.