In both in vitro and in vivo models, microscopic air bubbles and glass microspheres as small as 5–20 µm, can be detected using Doppler ultrasound and appear as short duration high intensity signals. Less echogenic common embolic materials, such as thrombus, atheroma and platelet aggregates, can also be detected with a high sensitivity, although the smallest emboli which have been introduced into models are 0.2–0.4 mm. Emboli of all types result in a characteristic frequency-focused short duration high intensity signal, although very intense signals may result in receiver overload and aliasing. Both in vitro and in animal models, a positive correlation has been found between embolus size and both relative intensity increase and duration of embolic signals. This may allow information about embolus relative size to be derived from the Doppler signal. Air bubbles result in more intense signals than similarly sized solid emboli, and thrombus and atheroma embolic signals are more intense than those produced by platelet aggregates. However, current technology does not allow differentiation between say a larger platelet embolus and a smaller thrombus embolus. The use of models has allowed the detailed study of the frequency spectrum associated with emboli and artefact, and computerised algorithms have been developed which can differentiate between the two.