Sizing emboli in blood using pulse Doppler ultrasound. II. Effects of beam refraction

For pt.I see ibid., vol.43, no.6, p.572-80 (1996). A theoretical and numerical study of the acoustic field intensity within a curved flow conduit having (1) diameter similar to the wavelength of the interrogating frequency and (2) speed of sound mismatch with the surrounding medium is presented. The field intensity is shown to vary significantly and in a monotonic fashion across the flow conduit. The resulting insonation of emboli transiting through the Doppler sample volume is explored with a Monte Carlo study of the behavior of the embolus to blood power ratio (EBR). The numerical simulation findings are shown to be in good agreement with previously reported experimental results. A method is explored for estimating embolus diameter when this refraction artifact is present, and shown to yield excellent results when applied to experimental data. Further work toward clinical application of these results is discussed.