Ultrasonic characterisation of inhomogeneous materials: some problems with measurements of forward scattered fields

Measurements of the complex pressure fields transmitted by scattering specimens consisting of glass beads in silicone rubber and of in vitro samples of ox myocardium, liver and kidneys have been carried out by point-by-point mapping of the amplitude and phase of the fields, using a new design of PVDF needle hydrophone in a high precision scanning tank. The results show that disruption of the phase fronts by the specimens is small and the measured values depend on the choice of measurement distance and frequency, and spatial sampling interval. As a result, there is little difference between phase sensitive and phase insensitive measurements at frequencies below 5 MHz. It is therefore unlikely that the proposed method could be applied successfully to the characterisation of such materials at low megahertz frequencies.