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 stro ke, but its clinical usefulness will depend on effective systems for a utomatically detecting embolic signals (ES) and differentiating them f rom artifact. A new method involves the use of a multidepth probe that allows recording from both distal and proximal sample volumes along t he same vessel. Theoretically, an embolus should appear sequentially, with a time delay, between the two channels, whereas an artifact shoul d appear simultaneously in the two channels. Methods We evaluated this method in an in vitro flow model and in patients. In an in vitro mode l, with a flow pattern mimicking intracerebral flow, 181 air bubbles a nd 193 thrombus emboli were compared with the signals resulting from 3 68 episodes of artifact; a sample volume of 5 mm and a channel separat ion of 10 mm were used. ES from two groups of patients-those with caro tid artery stenosis (141 ES) and those with mechanical prosthetic card iac valves (125 ES)-were studied and compared with 222 episodes of art ifact 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 f or 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 emb oli that resulted in receiver overload and aliasing. In patients the m ean (SD) time delay was 29.6 (28.2) ms for valve ES and 14.9 (15.42) f or carotid ES compared with 0.00 (0.46) for artifact (carotid and valv e ES versus artifact, P<.0001). Considering only those signals that we re 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 9 9.0%. However, in one patient in the valve group some ES were visible only in the proximal channel, possibly because of passage of emboli do wn 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 t he first reliable method for differentiating intense ES resulting in r eceiver overload from artifact. Occasionally its sensitivity is reduce d when ES do not appear in the distal channel, probably because they p ass down a side branch; this may be reduced by reducing gate separatio n. Some less intense carotid ES can be difficult to detect if the ampl itude increase is small compared with the amplitude of the background Doppler signal.