ANALYSIS OF THE FREQUENCY-MODULATION PRESENT IN DOPPLER ULTRASOUND SIGNALS MAY ALLOW DIFFERENTIATION BETWEEN PARTICULATE AND GASEOUS CEREBRAL EMBOLI

The purpose of this study was to investigate the incidence of frequenc y modulation in Doppler signals from cerebral emboli and to seek possi ble explanations for its occurrence, Signals from 200 particulate embo li and 200 presumed gaseous emboli were studied, The Doppler signals w ere visualised in the time domain and were classified into three main types, Type I signals contained no modulation, type II signals showed gradual frequency changes and type III showed a rapid change evident i n only a small percentage of the entire signal, Type I signals were ob served from 71.5% of particulate emboli but only 19% of gaseous emboli (chi(2) = 111, p < 0.001), Type II signals were found in 28.5% of par ticulate emboli and 38% of gaseous emboli (chi(2) = 4.06, p < 0.05), T he most surprising and significant finding was that 43% of gaseous emb olic signals were categorised as type III signals compared with 0% of signals from particulate emboli (chi(2) = 109, p < 0.001), The finding that known particulate emboli appear never to produce rapid frequency modulation may provide a basis for differentiating between gaseous an d particulate emboli, (C) 1997 World Federation for Ultrasound in Medi cine & Biology.