In vitro study of continuous wave doppler spectral changes resulting from stenoses and bulbs

Quantitative analysis of continuous wave (CW) Doppler spectra by measurements of peak frequency and spectra broadening is an important non-invasive method for detecting disturbed flow caused by carotid arterial stenosis. It is known that severe stenoses can be detected; however, the spectral changes associated with minor or moderate stenoses may not be detected or can potentially be confused with those produced by flow disturbances in the normal carotid bulb. In order to determine if the flow disturbances in a normal bulb and those associated with a minor stenosis produce significant spectral changes, Doppler spectra were recorded from straight tubes with bulbs or stenoses in an in vitro model with steady flow rates of 400, 600, and 800 cc/min (Reynolds numbers of 1700, 2600, and 3500). Stenoses greater than approximately 30% cross-sectional area were associated with an increased peak frequency and increased spectral broadening as measured by spectral broadening index (SBI), coefficient of variation (CV), coefficient of skewedness (CS) and coefficient of kurtosis (CK). Stenoses less than 30% were not detected. With flow rates of 400 and 600 cc/min, the presence of a bulb did not affect peak frequency or the extent of spectral broadening. With a higher flow rate (800 cc/min), there was an increase in SBI, CV and CS but no increase in peak frequency. Based on the results of these in vitro steady flow experiments in straight tubes, we conclude that increased peak frequency and spectral broadening are the results of a stenosis greater than 30% cross-sectional area. Bulbs and stenoses less than 30% do not produce significant changes in the continuous wave Doppler spectrum with flow rates in the clinical range.