A comparison of four methods for distinguishing Doppler signals from gaseous and particulate emboli.

Abstract

Many reports in the medical literature have proposed methods of differentiating between gaseous and particulate emboli detected with the use of transcranial Doppler ultrasound. The purpose of this study was to compare the previously published methods with our own sample volume length (SVL) parameter to assess the accuracy of each method in classifying emboli. A pure source of gaseous and particulate emboli was obtained from in vitro and in vivo studies, respectively, and recorded onto digital audiotape for off-line analysis. In total, 100 gaseous emboli and 215 particulate emboli were analyzed to measure four embolic parameters, namely, embolic duration, embolic velocity, relative signal intensity increase (measured embolic power [MEP]), and SVL of the embolic signal (= Duration x Velocity). Receiver operator characteristic analysis was used to assess the optimum threshold for each parameter to differentiate between particulate and gaseous emboli, and levels of sensitivity and specificity were calculated. Embolic duration and velocity produced the poorest levels of sensitivity and specificity compared with the MEP and SVL parameters. The optimum thresholds for embolic duration and velocity were 35 ms and 1 m/s, respectively, which produced a sensitivity (specificity) of 85.1% (87%) and 87% (67%), respectively. The optimum MEP and SVL thresholds were 30 dB and 12.8 mm, respectively, which produced a sensitivity (specificity) of 86.5% (95%) and 93% (97%), respectively. The SVL and MEP parameters were compared statistically (chi 2) at chosen specificity values of 90%, 95%, 97%, 99%, and 100%, which showed that the SVL sensitivities were statistically greater than MEP sensitivities (P < 0.01). SVL is the best parameter for differentiating between gaseous and particulate emboli but needs to be calculated with the use of a high-temporal-resolution spectral analyzer to measure embolic duration and velocity.