Selective transmission of a focused Doppler ultrasound beam through a plastic layer

Laboratory test objects are widely used in Doppler ultrasound (US). Althoug h the acoustic properties of in vitro materials are usually known, they are unlikely to match each other, or their in vivo counterparts, exactly. We c onducted theoretical and experimental studies of a focused ultrasound beam as it passes from one fluid, through an intervening plastic layer at an obl ique angle, and then into a different fluid. Dual mode propagation may occu r (i.e., both longitudinal and shear waves can propagate in the plastic lay er). Our calculations show that the power transmitted by either mode drops very rapidly to zero at certain critical angles. A range of angles of incid ence exists within a focused beam and this, combined with the highly angle- dependent power transmission behaviour, can produce major distortions of Do ppler data. These may persist even when the beam axis is not oriented exact ly at the critical angle. The total power transmitted depends on all the wa ve speeds, may involve mode conversion, and is a very complicated function of the angle of incidence. This study reports a practical method for the ca lculation of power transmission though a plastic layer, and shows how the r esulting power vs. angle graph can be used to avoid artefacts in in vitro D oppler studies. (C) 2000 World Federation for Ultrasound in Medicine & Biol ogy.