Performance of Echographic Equipment and Potentials for Tissue Characterization

The imaging performance of echographic equipment is greatly depenlent on the characteristics of the ultrasound transducer. The paper is confined to single element focussed transducers which are still widely employed in modern equipment. Extrapolation of the results to array transducers maybe made to a certain extent. The performance is specified by the 2 dimensional point spread function (PSF) in the focal zone of the transmitted sound field. This PS? is fixed by the bandwidth of the transducer when the axial (depth) direction is considered, and by the central frequency and the relative aperture in the lateral direction. The PSF concept applies to the imaging of specular reflections and the PSF is estimated by scanning a single reflector. In case of scattering by small inhomogeneities within parenchymal tissues the concept of “speckle” formation has to be introduced. The speckle is due to interference phenomena at reception. It can be shown that the average speckle size in the focus is proportional to the PSF above defined for specular reflections. The dependencies of the speckle size on the distance of the tissue to the transducer (beam diffraction effects) and on the density of the scatterers were explored. It is concluded that with the necessary corrections tissue characterization by statistical analysis of the image texture can be meaningful.