• Shan Gao and Ka Sing Wong.
• Department of Medicine & Therapeutics, Chinese University of Hong Kong.
• J Neuroimaging. 2003 Apr 1; 13 (2): 124-32.

Background And PurposeUsually, microembolic signal (MES) monitoring is performed for MESs arising from downstream sources. The aim of this study is to describe the special characteristics of MESs originating from nearby sources in the middle cerebral artery (MCA).MethodsWe analyzed 265 MESs detected in 5 patients (the study group) who had acute ischemic strokes within the territory of MCA stenoses and 48 MESs detected in 7 patients (the control group) from proximal sources: severe internal carotid artery stenoses and prosthetic heart valves. MESs were recorded with a 2-MHz, bigate transducer. We performed offline analyses of the recorded MESs using both post-fast Fourier transform (FFT) spectra and pre-FFT time domain signals. MESs were divided into 3 types--focused-frequency signals (FFSs), bottom-frequency signals (BFSs), and multifrequency signals (MFSs)--according to the frequencies of the signals displayed on the post-FFT spectra.ResultsIn the study group, among 265 MESs, 245 (92.5%) were MFSs, 11 (4.2%) were FFSs, and 9 (3.4%) were BFSs. In the control group, among 48 MESs, 45 (93.5%) were FFSs, 2 (4.2%) were MFSs, and 1 (2.%) was a BFS. There was significant difference in MES type between MCA stenoses and other embolic sources (P < .05). On the post-FFT spectra, MFSs recorded from MCA stenoses occupied multiple frequencies along the vertical axis, and the low-frequency parts were usually bidirectional. On the pre-FFT time domain signals, the highest frequency parts presented distorted, amplitude-modulated sine waves, and the low-frequency parts presented totally irregular waves. The amplitudes of the low-frequency parts diminished with time in the proximal channel but enlarged with time in the distal channel over a short period.ConclusionMESs recorded from MCA stenoses may have special characteristics of multiple frequencies on both post-FFT spectra and pre-FFT time domain signals. Our findings may represent rotating or vibrating emboli as they are just dislodged from the thrombus and are moving from the vessel wall to the center. Further clinical and laboratory studies are needed to confirm this hypothesis.

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