Favourable Effects of a Concave Transducer on the Ultrasono-Tomogram

Abstract

The use of a concave transducer in the ultrasono-cardiotomography has many favourable effects on the improvement of the picture quality of the tomograms. In this paper, these favourable effects of the concave transducer on the ultrasono-tomogram are discussed on the basis of the results of theoretical analysis and experimental sound field measurement.
Actual calculations of the sound field for the concave transducers are performed by using Torigai's theory which in for near and far sound fields generated by a concave transducer.
The theoretical results are compared with the experimental patterns of the sound field which are obtained by measuring the echo intensity from a small target placed in a water medium, and are found to coincide well with the experimental results. By employing a concept designated as “distance-functional directivity”, the azimuthal resolving power of the transducer is examined, and it is found that the concave transducer has favourable effects on the improvement of the azimuthal resolving power because of its narrow beam width due to the converging effect.
From the consideration on the sound field pattern, when the concave transducer is used for the ultrasonic reflection technique, it is concluded that better results should follow by the use of a transducer having such a configuration as (radius of the transducer) 2/ (radius of curvature × wave length in the medium) _??_4. By employing the transducer of this configuration, the length of the Fresnel's interference zone is made shorter that of a plane circular piston of the same diameter, and the beam width is made sufficiently narrow.
These facts markedly appeared in the picture quality of the actual tomogram of an excised dog heart taken by an immersed method. It is considered that the use of concave transducers makes the miniaturization of actual transducers and the introduction of STC (sensitivity-time-control) circuit easier.