Multigated Doppler ultrasound in the detection of emboli in a flow model and embolic signals in patients.

BACKGROUND AND PURPOSE: The ability to detect asymptomatic circulating cerebral emboli may contribute to the management of patients with stroke, but its clinical usefulness will depend on effective systems for automatically detecting embolic signals (ES) and differentiating them from artifact. A new method involves the use of a multidepth probe that allows recording from both distal and proximal sample volumes along the same vessel. Theoretically, an embolus should appear sequentially, with a time delay, between the two channels, whereas an artifact should appear simultaneously in the two channels.

METHODS: We evaluated this method in an in vitro flow model and in patients. In an in vitro model, with a flow pattern mimicking intracerebral flow, 181 air bubbles and 193 thrombus emboli were compared with the signals resulting from 368 episodes of artifact; a sample volume of 5 mm and a channel separation of 10 mm were used. ES from two groups of patients-those with carotid artery stenosis (141 ES) and those with mechanical prosthetic cardiac valves (125 ES)-were studied and compared with 222 episodes of artifact produced in the same patients.

RESULTS: In the model the mean (SD) time delay was 17.32 (9.94) ms for air emboli and 17.78 (10.66) ms for thrombus emboli compared with -0.01 (0.39) ms for artifact (air and thrombus emboli versus artifact, P < .0001). A sensitivity of 100% and specificity of 100% were obtained when a cutoff of > 2 ms was used for an embolus. The method allowed equally good detection of those air emboli that resulted in receiver overload and aliasing. In patients the mean (SD) time delay was 29.6 (28.2) ms for valve ES and 14.9 (15.42) for carotid ES compared with 0.00 (0.46) for artifact (carotid and valve ES versus artifact, P < .0001). Considering only those signals that were visible in both Doppler time domains resulted in a sensitivity for valve ES of 98.9% and for carotid ES of 94.0%, with a specificity of 99.0%. However, in one patient in the valve group some ES were visible only in the proximal channel, possibly because of passage of emboli down branch vessels between the two sample volumes. In addition, for the less intense carotid ES some signals were unclear or absent in one or both of the time domain signals at either depth, although visible in the post-fast Fourier transform spectra. Including those ES visible in only one channel reduced the sensitivity to 75.2% for valve ES and 92.6% for carotid ES.

CONCLUSIONS: The multigated technique offers a new method of detecting ES and differentiating them from artifact and is the first reliable method for differentiating intense ES resulting in receiver overload from artifact. Occasionally its sensitivity is reduced when ES do not appear in the distal channel, probably because they pass down a side branch; this may be reduced by reducing gate separation. Some less intense carotid ES can be difficult to detect if the amplitude increase is small compared with the amplitude of the background Doppler signal.