Spectral composition of Doppler signals.

The spectral content of the Doppler signal (as revealed by spectral analysis) provides useful diagnostic information about local hemodynamic conditions. Changes in these conditions are used to diagnose atherosclerotic lesions in arteries accessible to ultrasound. Extremely high frequencies, as found in and distal to a tight stenosis, indicate the presence of a jet, while Doppler signals with a wide bandwidth are related to regions with wide velocity ranges and/or disturbed flow patterns. However, the random interactions of the scatterers, the dimensions of the sample volume, the velocity of the scatterers and the method of signal processing used affect the displayed spectral composition of the Doppler signal. This article reviews the basic mechanisms and cautions the investigator against a quick and superficial interpretation of the results obtained without a proper appreciation of the random fluctuations due to the methods used to record and process the signals.