High-resolution emboli detection and differentiation by characteristic postembolic spectral patterns.

BACKGROUND AND PURPOSE High-intensity transient signals (HITS) detected by transcranial Doppler ultrasonography correspond to microemboli in intracranial arteries. The purpose of this study was to develop new diagnostic criteria for the differentiation of these microembolic signals from artifact, based on a high-resolution analysis of Doppler power spectra in an in vitro model. METHODS Two hundred seventy-six formed emboli, consisting of different biological and nonbiological materials and as air bubbles, were injected into a flow phantom with artificial blood vessels and perfused in a steady or a pulsatile way. Embolic passage was assessed with a modified 2.5-MHz pulsed Duplex machine and a commercial 2-MHz Doppler system. Embolic HITS were analyzed using internationally accepted criteria for the audiovisual characteristics of HITS. Doppler spectra changes associated with HITS were evaluated by means of a specially developed high-resolution analysis of Doppler raw data. RESULTS Seventy-seven percent of all embolic events could be identified using conventional audiovisual criteria for embolic HITS. Analysis of Doppler spectra showed that all injected emboli generated high-amplitude signals with a minimum of at least 3 dB above background level. In addition, using high-resolution processing, specific changes in Doppler spectral patterns could be identified after all embolic HITS caused by solid particles. These postembolic spectral patterns were always characterized by a Doppler frequency shift decreasing in time and resembling the letter lambda (lambda). Duration and appearance of the postembolic spectral patterns were mainly influenced by the size and velocity of the embolus. Similar phenomena could not be found in case of embolism by either small air bubbles or in case of provoked artifact registration. Using a commercial Doppler system specific, we documented postembolic spectral patterns in 47% of injected emboli. CONCLUSIONS In this study, highly specific changes in Doppler spectral patterns associated with microembolic HITS could be characterized, resulting in further criteria for the differentiation between microembolic signals and artifact in Doppler emboli detection. The sensitivity of the detection of these signals can be increased by high-resolution analysis of raw Doppler data.