Abstract
The simplest source of focussed ultrasound used in medicine is the spherical cap transducer. It appeared that the theory of O'Neil, invariably used for predictions had not been previously tested comprehensively under narrowband low amplitude conditions. Extensive measurements on four weakly focussed transducers in water have shown that this theory is a poor predictor of the field actually radiated under these conditions. If the nominal geometrical parameters of a transducer are replaced in the theory by effective values derived from pressure amplitude extrema, a greatly improved fit between theory and experiment can be realised. A reliable protocol for the determination of effective parameters has been proposed. It has been shown that the effective parameters defined from measurements in water can be used with comparable success to predict the fields in castor oil. A novel method, using a spherical cap transducer and a miniature hydrophone, has been developed for the measurement of ultrasonic attenuation in penetrable media. Use of the technique for measuring the frequency dependence of the coherent scattering component of the attenuation in a suspension of polystyrene beads in a castor oil matrix, presenting refractive index variations of 35%, gave results in good agreement with the theory of Waterman and Truell. Amplitude measurements in the suspension have shown that the coherent part of the field is not defocussed. The defocussing observed in soft human tissue such as breast, by Poster and Hunt, appears to be largely due to the incoherent scattering contribution and its quantification is identified as the subject of future investigation.