EXPERIMENTAL ASPECTS OF HIGH-INTENSITY TRANSIENT SIGNALS IN THE DETECTION OF EMBOLI

Experimental studies in the 1960s and 1970s demonstrated the high sens itivity of Doppler ultrasound in detecting gaseous bubbles. More recen t studies have shown that microscopic air bubbles, as well as glass mi crospheres as small as 5 mu to 20 mu, cause characteristic high-intens ity signals. Recently it has been demonstrated that less echogenic emb olic materials such as thrombus, platelet aggregates, and atheroma can also be detected with a high sensitivity. Such ''solid,'' or formed-e lement, emboli as small as 200 mu to 400 mu can be detected; the lower size limit of detection was due to an inability to make smaller embol ic particles rather than to the sensitivity of the detection process i tself. Analysis of the Doppler signals provides some information about embolus size and composition, but accurate characterization in clinic al practice is not possible using current technology. Studies in exper imental models have allowed the detailed description of embolic signal s; they appear as a short-duration, frequency-focused increase in inte nsity, predominantly unidirectional in the direction of flow, and usua lly contained within the spectral envelope. In contrast, artifacts app ear as a bidirectional, high-intensity increase with maximum intensity at low frequencies. These differences have been exploited to develop automatic embolus detection programs, and an off-line version has been successfully validated in an experimental model. (C) 1995 John Wiley and Sons, Inc.