USE OF A DECIBEL THRESHOLD IN DETECTING DOPPLER EMBOLIC SIGNALS

Background and Purpose To improve reproducibility and reliability in t he identification of embolic signals detected with the use of Doppler ultrasound, many studies have used an intensity threshold. However, va riable thresholds between 3 and 12 dB have been used, and often the me thod of measurement of intensity is not stated. Potentially different methods of measurement could result in different intensity measurement s for the same embolic signal. We determined the effect of these diffe rences using commercial transcranial Doppler systems. Methods We analy zed 81 embolic signals recorded from the middle cerebral arteries of p atients with carotid artery disease using three different methods of m easuring intensity that had been previously used in research studies. In method 1 individual lime frames of the frequency spectra were analy zed, in method 2 a color-coded intensity scale was used, and in method 3 automated software was used. Results There was a highly significant correlation between measurements made by the different techniques (me thod 1 versus method 2: r = .68, P < .0001; method 1 versus method 3: r = .66, P < .0001; method 2 versus method 3: r = .70: P < .0001). How ever, the absolute values of intensity for the same embolic signals va ried markedly for the different methods. For example, a 4-dB threshold according to method 1 was equivalent to an approximately 7-dB thresho ld measured by method 2. These differences had major effects on the pr oportion of embolic signals detected with the use of the same decibel threshold but with intensity measured in the different ways. For examp le, using a threshold of 7 dB would result in only 4.9% of signals bei ng missed by method 2 but 42.2% and 51.4% being missed by methods 1 an d 3, respectively. Conclusions Our results demonstrate that the intens ities of the same embolic signals, recorded with the same parameters, are markedly different when analyzed in the different ways used in pre vious studies. This has important implications when a decibel threshol d is used and emphasizes that criteria developed by one investigator o n one machine cannot be used by another investigator without initial r eevaluation. This could account for some of the differences in frequen cies of embolic signals reported in previous clinical studies.